This is one of the webpages of Libarid A. Maljian at the Department of Physics at CSLA at NJIT.

 

 

 

New Jersey Institute of Technology

College of Science and Liberal Arts

Department of Physics

The Earth in Space

Phys 203

Spring 2023

Third Examination lecture notes

 

 

 

Introduction to Oceanology/Oceanography

 

The sum total of all water on Earth is called the Earth’s hydrosphere.  The study of the Earth’s hydrosphere is most commonly called oceanography, and persons who study the Earth’s hydrosphere most commonly call themselves oceanographers.  The study of the Earth’s hydrosphere should more properly be called oceanology, and persons who study the Earth’s hydrosphere should more properly call themselves oceanologists.  Although more and more textbooks are renaming this subject oceanology and although more and more persons who study this subject are calling themselves oceanologists, most textbooks still nevertheless refer to this subject as oceanography and most persons who study this subject still nevertheless call themselves oceanographers.  In this course, we will use the terms oceanology and oceanography interchangeably, and we will use the words oceanologist and oceanographer interchangeably as well.  Roughly seventy percent of the world is covered with oceans, while only thirty percent of the world is covered with continents.  This is reason enough to devote a significant amount of time in this course to oceanology/oceanography.  Moreover, most of the geologic activity of planet Earth is submarine (underwater), simply because most of planet Earth is covered with water.  Most seismic activity such as earthquakes occur at the bottom of the ocean.  Most igneous activity such as lava eruptions occur at the bottom of the ocean.  Most mountains are at the bottom of the ocean.  Even most landslides occur at the bottom of the ocean.  For all of these reasons, we must devote a significant amount of time in this course to oceanology/oceanography if we wish to study our planet Earth.

 

The measurement of ocean depths below sea level is called bathymetry, while the measurement of continental elevations above sea level is called hypsometry.  Nearly everyone refers to maps with contour lines of elevation as topographical maps, but these are more correctly called hypsometric maps.  For thousands of years, bathymetric measurements (ocean depths below sea level) were performed using a technique called sounding.  A heavy object such as an anchor was lowered into the ocean with a very long chain until the heavy object hit the seabed (the ocean floor).  We can be certain that the anchor has hit the seabed if the chain becomes slack.  Then the chain was carefully pulled out of the ocean until the anchor was retrieved.  The ocean depth was concluded to be the length of the retrieved chain.  As the chain was pulled out of the ocean, sailors would count the number of times they pulled the chain while their arms were outstretched.  For this reason, ocean depth has been and still continues to be measured in units called fathoms.  The word fathom is derived from another word meaning outstretched arms.  However, different sailors have different arms of different lengths.  Therefore, different sailors will measure a different number of fathoms of ocean depth at the same location.  Eventually, oceanographers/oceanologists realized that a fathom must be redefined as a precise unit of ocean depth.  The modern definition of the fathom is precisely six feet (two yards) of ocean depth.  In other words, if we know the depth of the ocean in feet, we would divide by six to determine the depth of the ocean in fathoms.  If we know the depth of the ocean in yards, we would divide by two to determine the depth of the ocean in fathoms.  If we know the depth of the ocean in fathoms, we would multiply by six to determine the depth of the ocean in feet or multiply by two to determine the depth of the ocean in yards.  Caution: it is incorrect to use the fathom for any other measurement of length.  We would never ever express the height of a building or the elevation of a hill or mountain or the width of a classroom or the height of a ceiling in fathoms.  Only ocean depth is measured in fathoms.

 

Beginning in the twentieth century, bathymetric measurements began to be performed using a technique called echo sounding.  A ship would use a speaker to emit a sound into the ocean.  The sound would propagate to the seabed, and the reflected sound wave would propagate back from the seabed.  A reflected sound wave is called an echo, which is why this technique is called echo sounding.  The ship would detect the echo with a microphone.  We know when the sound was emitted, and we know when the echo was detected.  If we know the speed of sound in water, we would use the equation speed equals distance divided by time to determine the depth of the ocean.  Unfortunately, the speed of sound in the ocean depends upon many variables such as temperature, pressure, and even salt content.  To roughly estimate the ocean depth, we can use the average speed of sound in the ocean, which is roughly fifteen hundred meters per second or almost one mile per second.  As a simple example, suppose we emit a sound in the ocean, and suppose we detect the echo four seconds later.  If the average speed of sound in the ocean is roughly one mile per second and if we detect the echo four seconds later, then the depth of the ocean is roughly two miles.  Caution: we must remember to take half of the time because it takes half of the time for the sound to propagate downward to the seabed and it takes the other half of the time for the echo to propagate upward back to the ship.  In the previous example, half of four seconds is two seconds, making the ocean depth two miles if the average speed of sound in the ocean is roughly one mile per second.  If a whale happens to be swimming under the ship when we emit a sound into the ocean, we may detect the echo from the whale, and our calculation will grossly underestimate the ocean depth.  Although it is improbable that a whale just happens to be swimming under the ship when we emit a sound into the ocean, even a school of fish swimming under the ship will cause an echo that the ship will detect.  Again, our calculation will grossly underestimate the ocean depth.  Therefore, a more accurate form of echo sounding was devised called SONAR, which is an acronym for SOund Navigation And Ranging.  With this technique, a ship emits many sounds into the ocean, and the ship detects many echoes.  Then, a computer is used to analyze these data, which accurately calculates the depth of the ocean.

 

After decades of bathymetric measurements, we have concluded that ocean depths are much deeper below sea level than continental elevations above sea level.  The maximum ocean depth below sea level is roughly eleven kilometers, but the maximum elevation of the continents above sea level is less than nine kilometers.  The maximum elevation of the continents above sea level is Mount Everest in the Himalayas in Asia, while the maximum ocean depth below sea level is Challenger Deep in the Mariana Trench in the Pacific Ocean.  Admittedly, this is a comparison of extreme values above and below sea level, but a comparison of average values above and below sea level reveals that the difference is even more extreme.  The average ocean depth below sea level is roughly 3.7 kilometers, but the average elevation of the continents above sea level is roughly 840 meters, less than one kilometer!  In other words, the average depth of the ocean is between four and five times deeper below sea level than the elevation of the continents above sea level.  Unfortunately, even after decades of bathymetric measurements, only roughly five percent of the ocean floor has been mapped in detail.  There are still large areas of the ocean floor that have not been mapped at all.  Sadly, we know the topography of other worlds such as Venus and Mars and the Earth’s Moon more accurately than we know the topography of our own ocean floor.

 

Geological Oceanology/Oceanography

 

As we discussed earlier in the course, research conducted by the United States Navy beginning immediately after the end of the Second World War led directly to the formulation of the Theory of Plate Tectonics, the fundamental theory of the geology of our planet Earth.  The United States Navy continues to this day to explore the ocean floor.  For example, the Deep Sea Drilling Project (DSDP) begun in 1968 drilled and retrieved rocks from the ocean floor.  The Deep Sea Drilling Project (DSDP) was eventually replaced by the Ocean Drilling Program (ODP) begun in 1983, which could not only drill rocks at the ocean floor but could drill more than two kilometers deeper than the ocean floor!  The Ocean Drilling Program (ODP) used the ship the JOIDES Resolution to drill submarine rocks.  JOIDES is an acronym for Joint Oceanographic Institutions for Deep Earth Sampling.  On this ship, a large drill bit would be lowered into the ocean, and a piece would be attached to that drill bit.  Both were lowered further into the ocean, and another piece would be attached.  As the entire drill was lowered, more and more pieces were attached until the entire drill was long enough to reach the ocean floor.  Further pieces could be attached to drill more than two kilometers deeper than the seabed.  After drilling was completed, the entire drill was slowly lifted while piece by piece was removed until the drill bit was returned to the ship.  In brief, the entire drill was assembled and disassembled wherever the JOIDES Resolution planned to drill submarine rocks.  The Ocean Drilling Program (ODP) was replaced by the Integrated Ocean Drilling Program (IODP) in 2003.  This mission was renamed the International Ocean Discovery Program (IODP) in 2013, which is still in operation.  This mission is able to drill more than seven kilometers deeper than the ocean floor!

 

The continental margin is the edge of the continent, where the continent meets the ocean.  Note that the word margin means edge.  All continental margins can be divided into two categories: active continental margins and passive continental margins.  There is an abundance of geologic activity at active continental margins, such as igneous activity and seismic activity.  This geologic activity is caused by either a transform fault or a subduction zone at the active continental margin.  There is very little geologic activity at passive continental margins.  Examples of passive continental margins include the east coast of North America, the east coast of South America, and the west coast of Africa.  Examples of active continental margins include the west coast of North America and the west coast of South America.  When studying oceanology/oceanography, we use the term trench for subduction zones.  As we discussed earlier in the course, there are two different types of subduction zones: oceanic-oceanic subduction and oceanic-continental subduction.  If the term trench is another word for the term subduction zone and if there are two different types of subduction zones, then there are two different types of trenches.  An oceanic-oceanic subduction zone is called an oceanic trench, while an oceanic-continental subduction zone is called a continental trench.  Caution: both types of trenches are at the bottom of the ocean, even though one of these types is called a continental trench.  In summary, we find continental trenches and transform faults at active continental margins.

 

The general topography of a passive continental margin is as follows.  First, the continent extends far into the ocean, and this extension of the continent into the ocean is called the continental shelf.  We emphasize that the continental shelf is a part of the continent; the continental shelf is the part of the continent that happens to be submerged under the ocean.  The continental shelf is composed of the same felsic igneous rock that composes the rest of the continent.  This is strong evidence that the continental shelf is indeed a part of the continent.  If sea levels happened to be higher, more of the continent would be submerged, making the continental shelf even larger.  If sea levels happened to be lower, less of the continent would be submerged, making the continental shelf smaller.  There is an abundance of ocean life on the continental shelf since sunlight is able to penetrate well into these shallow waters, as we will discuss.  Beyond the continental shelf, the seabed steepens to deeper depths below sea level.  This is called the continental slope, and beyond the continental slope where the seabed begins to level out is called the continental rise.  Beyond the continental rise is the ocean basin, also called the abyssal plain.  The ocean basins (abyssal plains) are the largest, flattest parts of the surface of the entire world.  Caution: it is a common misconception that only the continents comprise the surface of the world.  Actually, the continents together with the seabed comprise the surface of the world.  The seabed just happens to be the part of the surface of the world that is submerged under the ocean waters.  In fact, most of the surface of the world is the seabed.  The ocean basins (abyssal plains) are the largest, flattest parts of the surface of the world, even though they are submerged at the bottom of the ocean.  The edge of the ocean basin (abyssal plain) is the edge of the tectonic plate where we find oceanic ridges, which are mountain ranges built from submarine igneous activity, as we discussed earlier in the course.

 

There are submarine canyons within the continental slope.  These submarine canyons are not carved out by rivers from the continental interior.  As we will discuss later in the course, rivers are bodies of running water on the continents that carve out channels for themselves on the continents.  When river waters arrive at the ocean, they do not continue to carve into the rock to form the submarine canyons.  If submarine canyons were carved out by rivers, then every submarine canyon would be located where the mouth of a river meets the ocean, but this is not the case.  Moreover, submarine canyons would be built from the continent toward the ocean if they were carved out by rivers.  In fact, the reverse is true: the shape of submarine canyons reveals that they are built from the ocean toward the continent.  Submarine canyons are actually caused by turbidity currents, which are submarine landslides.  Gravity pulls rocks and sediments downward, causing a turbidity current.  These turbidity currents carve submarine canyons into the continental slope.  A turbidity current is actually a mixture of rocks, sediments, mud, and water pulled downward by gravity.  This is why they are called turbidity currents; the word turbid means heterogeneous, translucent, or thick.  The first historical evidence that submarine canyons are caused by turbidity currents was the disruption of transatlantic telegraph cables in the early twentieth century.  Before humans used mobile telephones for rapid communication, they used landline telephones.  Before humans used landline telephones for rapid communication, they used telegraphs.  The telegraph was invented in the 1830s by American inventor Samuel Morse, for whom the Morse code was named.  Telegraph cables were laid down between many of the major cities of the United States during the 1850s.  These telegraphs played an important role during the American Civil War from 1861 to 1865.  Giant telegraph cables were also dropped to the bottom of the Atlantic Ocean during the 1850s, permitting fairly rapid communication between the United States and Europe.  For centuries, it took months for a ship to cross the Atlantic Ocean.  Therefore, anything that occurred in the United States would not be known by Europeans until months later, and anything that occurred in Europe would not be known by Americans until months later.  After these transatlantic telegraph cables were dropped to the bottom of the Atlantic Ocean, relatively rapid information was communicated across thousands of kilometers of ocean for the first time in human history.  The Grand Banks earthquake in the year 1929 caused a turbidity current that not only carved out a submarine canyon but also disrupted several transatlantic telegraph cables.  The discovery of these damaged telegraph cables revealed that turbidity currents carve out submarine canyons in the continental slope.

 

The continental rise is also built by turbidity currents.  Each time a turbidity current occurs, rocks and sediments slide down and land at the base of the continental slope.  After many such turbidity currents, the result is an accumulation (a pile) of rocks and sediments at the base of the continental slope.  This is the continental rise.  Each of these accumulations of rocks and sediments that compose the continental rise is called a deep sea fan or a submarine fan.  Similar processes occur on the continents, as we will discuss later in the course.  Whenever a continental landslide occurs, rocks and sediments slide down and land at the base of the hill or mountain.  After many such landslides, the result is an accumulation (a pile) of rocks and sediments at the base of the hill or mountain.

 

The ocean basins or abyssal plains are the largest, flattest parts of the surface of the world.  These ocean basins (abyssal plains) are also covered with thousands of mountains.  Nearest the oceanic ridge are seamounts, the tallest mountains in the ocean.  Roughly halfway from the oceanic ridges to the continents we find medium-height mountains called tablemounts or guyots.  These medium-height mountains are called tablemounts because they often have flat tops like a table.  This is also why they are called guyots; there is a flat-roofed building at Princeton University in New Jersey named Guyot Hall, named for the Swiss-American geographer Arnold Henry Guyot.  Closest to the continent we find the shortest mountains called abyssal hills or seaknolls.  These seamounts, tablemounts (guyots), and abyssal hills (seaknolls) provide strong evidence for the Theory of Plate Tectonics.  Molten rock at divergent plate boundaries cools and crystallizes to form seamounts nearest the oceanic ridges.  Over millions of years, the tectonic plate moves while natural forces degrade the seamounts, making them shorter and shorter until they have become tablemounts (guyots) roughly halfway from the oceanic ridges to the continents.  Over many more millions of years, the tectonic plate continues to move while natural forces continue to degrade the tablemounts (guyots), making them shorter and shorter until they have become abyssal hills (seaknolls).  Furthermore, the rock that composes the seamounts is youngest, the rock that composes the tablemounts (guyots) is middle-aged, and the rock that composes the abyssal hills (seaknolls) is oldest, all consistent with the predictions of the Theory of Plate Tectonics.  This continuum of rock ages was discovered by the seafloor-drilling programs of the United States Navy, as we discussed earlier in the course.

 

As we discussed, the ocean basins (abyssal plains) are the largest, flattest parts of the surface of the world.  As we also discussed, the ocean basins (abyssal plains) are covered with thousands of mountains.  Are we not contradicting ourselves?  How can an ocean basin (abyssal plain) be flat while at the same time be covered with mountains?  The resolution of this paradox is as follows.  All of these mountains on the ocean basins (abyssal plains) are themselves covered with sediments.  Since the rock closest to the continents is oldest, there has been more time for sediments to accumulate, resulting in thicker sediment.  Since the rock closest to the oceanic ridges is youngest, there has been less time for sediments to accumulate, resulting in thinner sediment.  The thickest sediment covers the shortest abyssal hills (seaknolls), while the thinnest sediment covers the tallest seamounts.  Hence, the entire ocean basin (abyssal plain) levels out to a fairly flat topography.  This continuum of sediment thicknesses was discovered by the seafloor-drilling programs of the United States Navy, as we discussed earlier in the course.

 

The sedimentary rock covering ocean basins (abyssal plains) is primarily biogenic sedimentary rock, and the composition of this biogenic sedimentary rock closely matches the composition of the lifeforms living near the surface of the ocean directly on top of the rock.  At first glance, this would seem to be logical.  When marine life dies, their carcasses should sink to the bottom of the ocean, and layer upon layer upon layer of carcasses should accumulate and eventually lithify forming biogenic sedimentary rock.  However, this argument is too simplistic.  Perhaps this argument is correct for the largest fish or whales when they die, but most fish can be easily carried by ocean currents a far distance from the seabed rocks that share their composition.  The microscopic marine lifeforms that live near the surface of the ocean can certainly be carried a far distance by ocean currents.  Yet, the seabed rocks share their composition as well.  Part of the resolution of this paradox is that small fish eat these microscopic organisms.  Then, medium fish eat the small fish.  Large fish then eat the medium fish.  Whales may eat the microscopic lifeforms.  Eventually, the large fish and the whales die, and their carcasses sink to the bottom of the ocean, accumulate, and eventually lithify to form these biogenic sedimentary rocks.  Although this argument correctly accounts for some of the formation of the biogenic sedimentary rock at the ocean floor, this argument does not correctly explain the formation of most of this rock.  The primary resolution of this paradox is that most ocean life excrete fecal pellets that are dense enough to sink to the ocean floor.  Layer upon layer upon layer of this excrement accumulates on the seabed and eventually lithifies to form biogenic sedimentary rock.  The vulgar word for feces or excrement is sh!#$&*t.  To summarize, if we were able to walk upon the ocean floor, we would be walking upon biogenic sedimentary rock that has been lithified from layer upon layer upon layer of heavy, hard, dense sh!#$&*t.

 

Chemical Oceanology/Oceanography

 

The water molecule is composed of one oxygen atom covalently bonded to two hydrogen atoms.  The oxygen atom shares one pair of electrons with one of the hydrogen atoms, and the oxygen atom shares another pair of electrons with the other hydrogen atom.  However, the oxygen atom also has two other pairs of electrons that it does not share with the hydrogen atoms.  The shape of a molecule while ignoring all unshared pairs of electrons is called its molecular geometry, but the shape of a molecule while including all unshared pairs of electrons is called its electronic geometry.  The electronic geometry (including the two unshared pairs) of the water molecule is a distorted tetrahedron.  Since the two unshared pairs of electrons are on the oxygen side of the water molecule, this side of the water molecule carries a significant negative charge.  The other side of the water molecule where the two hydrogen atoms reside carries a significant positive charge.  Of course, the entire water molecule is neutral overall.  We conclude that the water molecule is electrically dipolar.  A molecule is electrically dipolar if the entire molecule is neutral overall but one side of the molecule is positively charged while the other side of the molecule is negatively charged.  The water molecule is quite strongly electrically dipolar, and this property of the water molecule gives rise to several extraordinary properties of water.  For example, water molecules can attract and repel other water molecules.  As we discussed toward the beginning of the course, like electrical charges repel, and unlike electrical charges attract.  Thus, if two water molecules have their hydrogen sides facing each other, they will repel each other, since positive and positive repel.  Similarly, if two water molecules have their oxygen sides facing each other, they will also repel each other, since negative and negative repel.  However, if the hydrogen side of one water molecule is facing the oxygen side of another water molecule, they will attract each other since positive and negative attract.  The ability of water molecules to attract other water molecules is called cohesion.  The surface tension of water is caused by water molecules cohesively attracting each other.  Water molecules can even attract other molecules besides water; this is called adhesion.  Adhesion together with cohesion is responsible for the meniscus of water within a test tube.  The water molecules adhesively attract the molecules of glass or plastic of the test tube and then cohesively attract other water molecules to form the meniscus.  Adhesion together with cohesion is also responsible for how the leaves of a tree obtain water.  The leaves of a tree do not obtain water directly from rain.  Instead, rainwater first seeps into the ground.  Due to adhesion, this water sticks to the tree roots.  Then, this water cohesively attracts other water molecules within the soil.  All these water molecules are adhesively attracted further up the tree root, and they cohesively attract other water molecules with them.  Gradually, the water defies gravity by moving up the tree roots, up the tree trunk, up the tree branches, and eventually to the leaves of the tree.  This process by which trees obtain water is called capillary action, which would not occur if water were not cohesive and adhesive due to the water molecule being strongly electrically dipolar.

 

If we spread chemicals upon a table, they will not react.  Molecules must be within a liquid so that they can float within the liquid and meet each other so that they will react.  However, chemicals will still not react if they are free to float and meet each other within most liquids.  The liquid must dissociate (rip apart) the molecules so that they are able to react with each other.  As a concrete example, suppose we wish for sodium chloride NaCl to react with magnesium sulfate MgSO4 to yield sodium sulfate Na2SO4 and magnesium chloride MgCl2.  This properly balanced reaction is written 2 NaCl + MgSO4Na2SO4 + MgCl2.  If we pour sodium chloride NaCl and magnesium sulfate MgSO4 upon a table, they will not react.  If we immerse sodium chloride NaCl and magnesium sulfate MgSO4 within a liquid where they are free to float and find each other, they may still not react.  The liquid must dissociate (rip apart) the sodium chloride NaCl molecules into sodium cations Na1+ and chlorine anions Cl1–, and the liquid must also dissociate (rip apart) the magnesium sulfate MgSO4 molecules into magnesium cations Mg2+ and sulfate anions SO42–.  This is necessary so that the sodium cations Na1+ are free to bond with the sulfate anions SO42– to form sodium sulfate Na2SO4 molecules and so that the magnesium cations Mg2+ are free to bond with the chlorine anions Cl1– to form magnesium chloride MgCl2 molecules.  We conclude that in order for a chemical reaction to occur, the molecules must be within a liquid not only so that they are free float and find each other, but the liquid must also dissociate (rip apart) the molecules so that they will react with each other.  The molecules we wish to react is called the solute, the liquid they are immersed within that will dissociate them is called the solvent, and the solute together with the solvent is called the solution.  In the previous example, we may be able to find a liquid that is a good solvent for sodium chloride NaCl, but that liquid may not be a good solvent for magnesium sulfate MgSO4.  Perhaps we can find another liquid that is a good solvent for magnesium sulfate MgSO4, but this other liquid may not be a good solvent for sodium chloride NaCl.  Perhaps we may be fortunate enough to find a liquid that is a good solvent for both sodium chloride NaCl and magnesium sulfate MgSO4, but that liquid may not serve as a good solvent for other molecules.  However, the water molecule is so strongly electrically dipolar that nearly any molecule we immerse into water will be dissociated (ripped apart).  This is because the oxygen side (the negative side) of the water molecule attacks one side of almost any molecule, while the hydrogen side (the positive side) of the water molecule attacks the other side of almost any molecule.  This results in the molecular dissociation of nearly every molecule immersed within water.  For this reason, chemists consider water to be the universal solvent.  Nearly all chemical reactions will proceed within water.  Although there are some molecules where not even water serves as a good solvent since water does not dissociate those molecules, such cases are rare.  The water molecule is so strongly electrically dipolar that nearly any chemical reaction will occur within water.

 

A lifeform is anything that is born, that grows and develops, that is able to move, that ingests substances necessary for its survival, that excretes substances harmful to its survival, that is able to reproduce, and that must eventually die.  All of these actions of life require energy.  Every second of every day of our lives, we are expending energy.  Even while asleep, our brains continue to expend energy keeping our hearts beating and our lungs breathing, which themselves expend energy as they function.  Death is when a lifeform ceases to expend energy.  All lifeforms derive the continuous energy that they must expend to remain alive from chemical reactions, but chemical reactions must occur within a solvent.  We now realize why all lifeforms are composed of mostly water.  The human body is mostly water for example.  All life continuously expends energy, and all lifeforms derive their energy from chemical reactions.  Chemical reactions must occur within a solvent, and water is the universal solvent.  Therefore, all lifeforms must be composed of mostly water.  Imagine a plastic bag filled with water with some reacting chemicals immersed within the water.  This is what a lifeform is essentially, including humans.  A human is essentially a plastic bag of water with some reacting chemicals immersed within the water.  This also explains why life began within the oceans and why for most of the history of planet Earth all life was in the oceans.  To the present day, most of the biomass of our planet is within the oceans, as we will discuss.

 

The heat capacity of any substance is the amount of heat that we must add to make it hotter by a certain temperature.  The heat capacity is also the amount of heat that we must extract from the substance to make it colder by a certain temperature.  More plainly, the heat capacity of any substance is how difficult it is to change its temperature.  If we must add much heat to make something hotter or if we must extract much heat to make something colder, this means that it is difficult to change its temperature.  If we must add only a small amount of heat to make something hotter or if we must extract only a small amount of heat to make something colder, this means that it is easy to change its temperature.  Water has a large heat capacity.  That is, we must add much heat to water to make it hotter, and we must extract much heat from water to make it colder.  In other words, it is difficult to change the temperature of water.  More plainly, hot water tends to remain hot, and cold water tends to remain cold.  This is one reason why humans have consumed soup for thousands of years.  Hot water tends to remain hot.  Thus, food placed within hot water will remain hot longer.  We can demonstrate this with a simple experiment.  Consider two identical bowls of hot chicken soup for example.  Suppose we scoop all of the broth out of one of the bowls, and suppose we wait for only a couple of minutes.  The chicken that remained in the broth-empty bowl has already become rather cold, while in the other bowl the chicken that remained within the broth is still quite hot.  Our planet Earth is mostly covered with ocean.  Since water has a large heat capacity, it is difficult to change the temperature of water.  Therefore, all of the water that covers planet Earth acts to stabilize temperatures throughout the entire planet.  For example, all of the water that covers planet Earth moderates the temperature difference between daytime and nighttime.  The daytime side of any planet faces toward the Sun, while nighttime side of any planet faces away from the Sun.  For most planets, nighttime is much colder than daytime, but the nighttime side of planet Earth is only slightly cooler than its daytime side, thanks to the stabilizing effect of all the water that covers most of the planet.  Extraordinarily, the nighttime side of planet Earth may at times become warmer than the daytime side depending upon weather patterns, as we will discuss later in the course.  As another illustration of how water stabilizes temperatures, desert nighttimes are significantly colder than desert daytimes because of a scarcity of water, which would have moderated the extreme difference in temperature between nighttime and daytime.  As another example of how water stabilizes temperatures, other planets have north poles and south poles that are much colder than their equators.  Although the Earth’s poles are cold and the Earth’s equator is hot by human standards, the difference in temperature is nevertheless moderate as compared with other planets.  Without the abundance of water that covers our planet Earth, our poles would be too cold and our equator would be too hot to be habitable for life.  The moderation of temperatures between the poles and the equator is also caused by ocean currents, as we will discuss.  As yet another example of how water stabilizes temperatures, continental coasts enjoy mild winters and mild summers due to the nearby ocean, where again the abundance of water stabilizes temperature differences.  This is called the marine effect.  In other words, the marine effect causes both mild summers and mild winters near the ocean as compared with severely hot summers and severely cold winters far from the ocean in the middle of continents, which is called the continental effect.  In other words, the continental effect causes severe seasons (both hot summers and cold winters) far from the ocean.  We can extend this logic globally, across the entire planet Earth.  The southern hemisphere is covered with much more ocean as compared with the northern hemisphere.  In fact, the southern hemisphere’s nickname is the water hemisphere, and the northern hemisphere’s nickname is the land hemisphere.  Since the southern hemisphere is the water hemisphere, temperatures are more stabilized in the southern hemisphere, thus causing more mild seasons.  Since the northern hemisphere is the land hemisphere, temperatures are more extreme in the northern hemisphere, thus causing more severe seasons.  In other words, if we compare two cities, one in the northern hemisphere and one in the southern hemisphere, we expect the northern-hemisphere city to suffer from hotter summers and colder winters, while we expect the southern-hemisphere city to enjoy mild summers and mild winters.  This assumes that all other circumstances are equal between the two cities, such as their degrees of latitude from the equator, their location on their respective continents relative to oceans, and the location of nearby mountain ranges and lakes.  Often, even exceptions to this general difference between the two hemispheres can be explained with the marine effect versus the continental effect.  Although both the North Pole and the South Pole are cold throughout the entire year and although northern hemisphere winters are generally colder than southern hemisphere winters, it is in fact the South Pole that is colder than the North Pole.  At the North Pole, there happens to be an ocean, the Arctic Ocean, and hence the marine effect moderates the cold temperatures, making the North Pole less cold.  At the South Pole, there happens to be a continent, Antarctica, and hence the continental effect intensifies the cold temperatures, making the South Pole more cold.  The cold temperatures at the South Pole are further intensified by the Antarctic Circumpolar Current that surrounds and hence isolates Antarctica from the rest of the world, as we will discuss.

 

The oceans of planet Earth are not pure water.  There are a variety of different salts dissolved within the ocean.  The most abundant salt in the Earth’s ocean is sodium chloride NaCl, which is ordinary table salt.  Since water is the universal solvent, the salts dissolved within the ocean are dissociated into positive ions (cations) and negative ions (anions).  Therefore, it is more correct to say that the most abundant salt in the Earth’s ocean is sodium chloride NaCl, which is dissociated into sodium cations Na1+ and chlorine anions Cl1–.  The salt in the ocean has three sources: rivers from continents, turbidity currents, and volcanic activity.  As rivers flow on the continent, the running water degrades the continental rock, ultimately breaking the rock down into sediments and minerals.  Among these minerals are salts.  The running water eventually delivers these salts to the ocean.  Turbidity currents (submarine landslides) liberate trapped rocks, sediments, and minerals within the continental slope into the ocean, and among these liberated minerals are salts.  Whenever any igneous eruption occurs, gases, rocks, sediments, and minerals are ejected from the eruption in addition to lava.  Some of these minerals are salts.  Most igneous activity is submarine, as we discussed.  Therefore, whenever a submarine volcanic eruption occurs, salts are sprayed into the ocean.  Even continental igneous eruptions add salt to the ocean.  An igneous eruption on a continent will eject salts into the atmosphere that may drift for hundreds of kilometers before finally landing on the ocean.  Since all of these mechanisms continuously add salt to the ocean, we might incorrectly conclude that the ocean thus becomes saltier and saltier.  This is not the case; the overall saltiness of the Earth’s oceans remains roughly constant.  If the overall saltiness of the Earth’s oceans remains roughly constant, then salt must be subtracted from the ocean at the same rate that salt is added to the ocean.  Salt is removed from the ocean by both sedimentation and biological processes.  Firstly, the salt in the ocean slowly sinks to the seabed, eventually lithifying to become sedimentary rock.  Secondly, lifeforms in the ocean extract salts and other minerals from the ocean for their own purposes, such as making hard shells for example.  In summary, salt is added to the ocean at the same overall rate that salt is subtracted from the ocean.  Hence, the overall saltiness of the ocean remains roughly constant.

 

The correct term for the saltiness of water is the salinity of the water.  More precisely, the salinity of water is defined as the amount of salt in the water as a fraction of the total amount of water.  More plainly, salinity is directly related to the amount of salt and is inversely related to the amount of water.  The first half of this definition is obvious.  If salinity is directly related to the amount of salt, then greater salt increases the salinity and lesser salt decreases the salinity.  It is almost a waste of time to try the following experiment, since the result is easy to predict.  Suppose we taste the saltiness of a cup of salt water.  If we add more salt and if we taste it again, it will taste more salty.  If we scoop out some of the salt and if we taste it yet again, it will taste less salty.  However, the second half of this definition of salinity is less obvious.  If salinity is also inversely related to the amount of water, then greater water decreases the salinity and lesser water increases the salinity.  This can be demonstrated with the following experiment.  Suppose we taste the saltiness of a cup of salt water.  If we add more water and if we taste it again, it will taste less salty.  Some students argue that this is obvious, since adding water dilutes the amount of salt in the cup, thus making it less salty.  This is essentially correct, but it is still remarkable.  We have not subtracted any salt from the cup; we have only added water.  Nevertheless, we have succeeded in decreasing the salinity, without subtracting any salt!  Conversely, suppose we taste the saltiness of a cup of salt water, and suppose we leave the cup upon the kitchen counter for a few hours, thus permitting some of the water to evaporate.  If we taste it again, it will taste more salty.  Some students argue that this is obvious, since evaporation subtracts water, thus concentrating the amount of salt in the cup making it more salty.  This is essentially correct, but it is still remarkable.  We have not added any salt to the cup; we have only subtracted water.  Nevertheless, we have succeeded in increasing the salinity, without adding any salt!  To summarize, salinity is directly related to the amount of salt, and salinity is also inversely related to the amount of water.  We emphasize again the second half of this definition of salinity.  It is possible to change the salinity of water without changing the amount of salt but instead by changing the amount of water.  In fact, this is the primary way the salinity of the ocean water changes: by the addition or subtraction of water, not by the addition or subtraction of salt.

 

Let us review the basic mathematics of fractions.  Consider the fraction four-sevenths, often written 4/7.  What is the meaning of this fraction?  What is the meaning of the four, and what is the meaning of the seven?  Most students respond that the four is the numerator while the seven is the denominator.  These terms numerator and denominator are just words.  Students who regurgitate such terms actually reveal a lack of understanding of fractions.  What is the meaning of the numerator, and what is the meaning of the denominator?  The denominator of any fraction is the number of equal pieces of the whole or total, and the numerator of a fraction is the part of that whole or total.  In other words, a fraction is a part of a whole or a part of a total.  This is the meaning of the fraction four-sevenths for example.  How would a student eat four-sevenths of a pizza pie?  First, we divide the whole pizza pie into seven equal slices (pieces).  Then, the student would eat four of those seven slices.  A percent is a particular type fraction where the whole or the total is always divided into one hundred equal pieces.  This is why it is called percent.  The word root cent always means one hundred.  There are one hundred cents in a dollar.  There are one hundred centimeters in a meter.  There are one hundred years in a century.  There are many other examples of words with the root cent meaning one hundred.  The percent symbol is written %.  For example, consider two percent, often written 2%.  The meaning of two percent is two parts out of a whole or total of one hundred equal pieces.  This means that two percent is actually two-hundredths, which could be written 2/100.  The whole or total is divided into one hundred equal pieces (as always with percent), and we take two of those one hundred equal pieces.

 

Some parts of the Earth’s oceans have greater salinity (are more salty), while other parts of the Earth’s oceans have lesser salinity (are less salty).  The average salinity of the Earth’s oceans is the total amount of salt in the ocean as a fraction of the total amount of water in the ocean.  This is an averaging of all the different salinities of all the different parts of the ocean.  This average salinity of the Earth’s oceans is roughly 3.5%.  In other words, for every one hundred pounds we pull out of the ocean, 3.5 pounds will be salt and the remaining 96.5 pounds will be water.  Note that this is on average.  Some parts of the Earth’s ocean have greater salinity, while other parts of the Earth’s ocean have lesser salinity.  In science, we always use the metric system.  Although our calculation using pounds is technically correct, we should instead use metric units such as kilograms.  So, for every one hundred kilograms we pull out of the ocean, 3.5 kilograms will be salt and the remaining 96.5 kilograms will be water.  Again, this is on average.

 

The denominator of a fraction need not be one hundred.  We can choose the denominator of a fraction to be one thousand for example.  The symbol % is commonly regarded as the percent symbol, but it is also called the parts-per-hundred symbol.  For example, 2% is most often read two percent, but 2% may also be read two parts-per-hundred.  The new symbol ‰ is called the parts-per-thousand symbol.  This symbol looks like the parts-per-hundred (percent) symbol except for an extra zero in the denominator.  As an example of using this new symbol, 9‰ is read nine parts-per-thousand, meaning 9/1000 or nine-thousandths or nine out of a whole or total of one thousand equal pieces.  How do we convert from parts-per-hundred (percent) to parts-per-thousand?  We will use 2% as a concrete example.  First, 2% equals 2/100, but we are free to multiply the numerator and the denominator of this fraction by ten yielding 20/1000, which is 20‰.  Thus, 2% becomes 20‰ by simply multiplying two by ten to yield twenty.  In conclusion, we convert parts-per-hundred (percent) to parts-per-thousand by multiplying by ten.  This stands to reason: two out of one hundred is the same part of the whole as twenty out of one thousand.  We can demonstrate this with the following concrete example.  Suppose there are one hundred persons who each have ten dollars.  The total amount of money is one thousand dollars, since one hundred persons times ten dollars each equals one thousand dollars.  If we select two of these persons, we have also selected twenty dollars, since two persons times ten dollars each equals twenty dollars.  Notice that when we selected two persons out of one hundred persons, we have also selected twenty dollars out of one thousand dollars.  This makes it clear that two out of one hundred is the same fraction (the same part of the whole) as twenty out of one thousand.

 

In oceanology/oceanography, we almost always measure salinity in parts-per-thousand, for reasons we will make clear shortly.  The average salinity of the ocean is 3.5%, which we now realize is 35‰, since 3.5 multiplied by ten is thirty-five.  In other words, for every one thousand pounds we pull out of the ocean, thirty-five pounds will be salt and the remaining 965 pounds will be water.  This is on average of course.  Again, although this calculation using pounds is valid, we should instead use metric units such as kilograms.  For every one thousand kilograms we pull out of the ocean, thirty-five kilograms will be salt and the remaining 965 kilograms will be water.  Again, this is on average.  We almost always measure salinity in parts-per-thousand in oceanology/oceanography because there are one thousand grams in one kilogram.  Therefore, for every one kilogram (one thousand grams) we pull out of the ocean, thirty-five grams will be salt and the remaining 965 grams will be water, again on average.

 

Some parts of the ocean have a salinity greater than 35‰, other parts of the ocean have a salinity less than 35‰, and still other parts of the ocean have a salinity roughly equal to 35‰.  Whereas both the addition and subtraction of salt and the addition and subtraction of water play roles in the varying salinity of the ocean, the addition or subtraction of water much more commonly determines the salinity of the ocean.  Although ocean water with a salinity greater than 35‰ could result from the addition of salt, it more commonly occurs through the subtraction of water.  Although ocean water with a salinity less than 35‰ could result from the subtraction of salt, it more commonly occurs through the addition of water.  Water that has a salinity significantly greater than 35‰ is called hypersaline water.  The word root hyper means over or too much, and the word root saline means salt.  Water that has a salinity significantly less than 35‰ is called brackish water.  The word brackish means salty-ish or not as salty as expected.  Water that has a salinity roughly equal to 35‰ is called seawater.  Since the addition or subtraction of water is the more important variable determining salinity in the ocean, we can draw general conclusions about these different types of water.  We expect brackish water in the ocean where there is greater precipitation (rain or snow from the atmosphere) than evaporation from the ocean.  We expect hypersaline water in the ocean where there is greater evaporation from the ocean than precipitation (rain or snow from the atmosphere).  Finally, we expect seawater where precipitation and evaporation are roughly in balance.

 

Water on the continents such as in lakes and in rivers and running out of our faucets is commonly known as freshwater, but this is a misleading word.  The term freshwater implies that there are no salts within the water, but there are always salts within water.  The salt could be such a tiny amount that we do not taste the salts, but the salinity is nevertheless not zero.  There is actually no such thing as absolutely pure water.  The proper term for water on the continents is limnological water.  The study of water on the continents is called limnology, commonly known as the study of freshwater using this misleading term.  Someone who studies water on the continents is called a limnologist, commonly known as someone who studies freshwater, again using this misleading term.

 

We measure salinity with a device called a salinometer.  A salinometer is built using the principle of electrical conduction.  Salt within water makes the entire solution a good electrical conductor, and the electrical conductivity of the solution is directly related to the salinity.  In other words, greater salinity results in greater electric conductivity, and lesser salinity results in lesser electric conductivity.  To build a primitive salinometer, we apply a fixed voltage to a sample of water, and we measure the electric current flowing through the solution.  If we measure a greater electric current, then the electric conductivity must be greater, which reveals that the salinity must be greater.  If we measure a lesser electric current, then the electric conductivity must be less, which reveals that the salinity must be less.  It is a common misconception that supposedly the same applied voltage always results in the same electric current, but this is false.  The electric current is determined not only by the applied voltage but also by the electric conductivity of the material.  In other words, if we apply a certain voltage across one object with a lesser electric conductivity, that voltage will push less electric current through that object.  However, if we apply that same voltage across another object with a greater electric conductivity, that same voltage will push more electric current through that second object.  To measure the salinity of ocean water, a ship lowers two electrodes into the ocean and always applies the same voltage across those two electrodes, regardless where the ship is located in the ocean.  At some locations, the ship will measure greater electric current in the water between the electrodes, which implies the electric conductivity is greater and therefore the ocean water has greater salinity.  At other locations, the ship will measure less electric current in the water between the electrodes, which implies the electric conductivity is less and therefore the ocean water has less salinity.  Another common misconception is that water is itself a good electric conductor.  This is false; pure water is in fact a poor electric conductor.  However, the salts within water do make the entire solution a good electric conductor.  Since there are always some salts in water, this makes the entire solution a good electric conductor.  Never ever use an electric appliance or even touch a light switch while wet.  We may not taste salts within the water running out of our faucets, but the salinity is nevertheless not zero.  Even tiny amounts of salt within water results in a good electric conductivity.

 

The deep ocean water mixes with itself very well, for reasons that we will make clear shortly.  If the deep ocean water mixes with itself very well, then its salinity must average out to roughly 35‰.  If the surface ocean water is hypersaline water, then its salinity is greater than 35‰.  If we descend into the deep ocean, the salinity must therefore decrease to 35‰.  If the surface ocean water is brackish water, then its salinity is less than 35‰.  If we descend into the deep ocean, the salinity must therefore increase to 35‰.  Whether the surface ocean water is hypersaline water or brackish water, we conclude that the salinity must steeply change as we descend into deep ocean water.  This steep change in salinity is called the halocline.  The word root cline means steeply changing, and the word root halo means salt.  If the surface ocean water is hypersaline water, we must have a decreasing halocline as we descend into the deep ocean water.  If the surface water is brackish water, we must have an increasing halocline as we descend into the deep ocean water.  If the surface water is seawater, its salinity is roughly equal to 35‰.  As we descend into the deep ocean, the salinity must therefore remain roughly equal to 35‰.  Therefore, there is no halocline if the surface water is seawater.  These circumstances are called isohaline.  The word root iso means not changing or steady or constant, the opposite of the word root cline.  Again, the word root halo means salt.  If the surface ocean water is seawater, then there is no halocline as we descend into the deep ocean.  The salinity remains isohaline.

 

Again, the deep ocean water mixes with itself very well, for reasons that we will make clear shortly.  If the deep ocean water mixes with itself very well, then its temperature must be roughly uniform just as its salinity is roughly uniform.  The temperature of the deep water is cold, roughly equal to the freezing temperature of water.  This is because sunlight cannot penetrate into the deep water, as we will discuss.  At the equator, the surface water is warm, since the equator is hot all year long.  If we descend into the deep ocean, the temperature must therefore decrease to roughly the freezing temperature.  We conclude that the temperature must steeply change as we descend into deep ocean water.  This steep change in temperature is called the thermocline.  Again, the word root cline means steeply changing, and the word root thermo means temperature.  At the equator, we must have a decreasing thermocline.  At the poles, the surface ocean water is cold, roughly equal to the freezing temperature of water, since the poles are cold all year long.  As we descend into the deep ocean, the temperature must therefore remain roughly equal to the freezing temperature of water.  Therefore, there is no thermocline.  These circumstances are called isothermal.  Again, the word root iso means not changing or steady or constant, the opposite of the word root cline.  Again, the word root thermo means temperature.  There is no thermocline at the poles; the temperature remains isothermal as we descend into the deep ocean.  At the midlatitudes, the temperature of the surface ocean water depends upon the season.  It is hot in the summertime, and it is cold in the wintertime.  In the summertime, the surface ocean water is warm, but the deep ocean water is still cold.  Therefore, we must have a decreasing thermocline in the summertime, just as at the equator all year long.  In the wintertime, the surface ocean water is cold, but the deep ocean water is also cold.  Therefore, we do not have a thermocline; the temperature is isothermal in the wintertime, just as at the poles all year long.

 

The salinity and the temperature together determine the density of water.  The density of water is directly related to the salinity, and the density of water is inversely related to the temperature.  Since the density is directly related to the salinity, a greater salinity results in more dense water, and a lesser salinity results in less dense water.  This stands to reason.  We expect a greater quantity of salt to make the water heavier, and we expect a lesser quantity of salt to make the water lighter.  However, the density of water is also inversely related to the temperature; hot water is less dense, and cold water is more dense.  If hot water is less dense, it will be buoyed up by the surrounding water.  Therefore, hot water must rise.  This is true for any other liquid as well as any gas such as air.  In other words, this is true for any fluid: hot fluids rise because they are less dense.  If cold water is more dense, it will be pulled down into the surrounding water.  Therefore, cold water must sink.  This is also true for any other liquid as well as any gas such as air.  In other words, this is true for any fluid: cold fluids sink because they are more dense.  Some parts of the ocean have a greater density, while other parts of the ocean have a lesser density.  Whereas both salinity and temperature play roles in the varying density of the ocean, temperature is the more important variable.  Although ocean water that is more dense could result from a greater salinity, it more commonly occurs because of colder temperatures.  Although ocean water that is less dense could result from a lesser salinity, it more commonly occurs because of warmer temperatures.  Since the temperature is the more important variable that determines density, we can deduce the density of the ocean water from its temperature.  At the equator, the surface ocean water is warm; therefore, it is less dense.  The deep ocean water is cold; therefore, it is more dense.  We conclude that the density must steeply change as we descend into the deep ocean water.  This steep change in density is called the pycnocline.  Again, the word root cline means steeply changing, and the word root pycno means density.  At the equator, we must have an increasing pycnocline.  At the poles, the surface ocean water is cold, and the deep ocean water is cold.  There is no thermocline (the temperature is isothermal); therefore, there is no pycnocline.  These circumstances are called isopycnal.  Again, the word root iso means not changing or steady or constant, the opposite of the word root cline.  Again, the word root pycno means density.  There is no pycnocline at the poles; the density remains isopycnal as we descend into the deep ocean.  At the midlatitudes, the temperature of the surface ocean water depends upon the season; therefore, the density must also depend upon the season.  It is hot in the summertime, and it is cold in the wintertime.  In the summertime, the surface ocean water is warm, but the deep ocean water is still cold.  Therefore, the surface ocean water is less dense, and the deep ocean water is more dense.  Therefore, we must have an increasing pycnocline in the summertime, just as at the equator all year long.  In the wintertime, the surface ocean water is cold, but the deep ocean water is also cold.  Therefore, we do not have a thermocline; the temperature is isothermal in the wintertime, just as at the poles all year long.  Thus, we do not have a pycnocline; the density is isopycnal in the wintertime, just as at the poles all year long.  The pycnocline at the equator all year long and at the midlatitudes in the summertime is actually a cutoff between the surface ocean water and the deep ocean water.  If the surface ocean water is hot, then it is less dense and hence it must rise.  If the deep ocean water is cold, then it is more dense and hence it must sink.  Notice therefore that the surface ocean water and the deep ocean water are not able to mix with each other.  If the density at the poles all year long and at the midlatitudes in the wintertime is isopycnal, then there is no pycnocline and thus there is no cutoff between the surface ocean water and the deep ocean water.  In other words, the surface ocean water and the deep ocean water mix with each other well when there is no pycnocline, when the density of the ocean water is isopycnal.

 

Biological Oceanology/Oceanography

 

Sunlight is only able to penetrate well into the uppermost layer of the ocean.  This uppermost layer is called the euphotic zone.  The word euphotic literally means good light, since the word root photo means light in words such as photograph, photographer, and photon for example, and the prefix eu- means good or well in words such as euphoria or euphemism for example.  The euphotic zone only reaches depths of roughly one hundred meters below sea level.  Deeper than the euphotic zone is the disphotic zone, where there is less light than the euphotic zone.  The word disphotic literally means bad light, since the word root photo again means light, and the prefix dis- means bad in words such as disgrace or disintegrate for example.  The disphotic zone only reaches depths of roughly one kilometer below sea level.  Actually, there is a fair amount of light at the top of the disphotic zone, which gradually transitions to poor light at the bottom of the disphotic zone.  For this reason, the disphotic zone is also called the twilight zone.  Deeper than the disphotic zone is the aphotic zone, where there is no light.  The word aphotic literally means no light, since the word root photo again means light, and the prefix a- means no or not in words such as apathy or asynchronous for example.  Most of the ocean is the aphotic zone, since it extends from the bottom of the disphotic zone all the way down to the ocean floor.

 

As we discussed, all life was in the oceans for most of the history of planet Earth.  To the present day, most of the biomass of planet Earth is in the oceans, meaning that the total amount of marine (oceanic) life weighs much more than the total amount of terrestrial (continental) life.  All marine life can be divided into three categories: the plankton, the nekton, and the benthos.  The plankton are microscopic marine life that cannot generate their own locomotion; they cannot swim.  The plankton can only float or drift in the ocean.  The nekton are marine life that can generate their own locomotion; they can swim.  Fish and whales and dolphins are classified as nekton.  The benthos are the marine life that live at the seafloor, such as clams, lobsters, and crabs.  There are some textbooks that claim a human at the beach who happens to be within the ocean is now a form of marine life; they are after all a lifeform in the ocean!  By this interpretation, a human swimming in the ocean becomes a nekton.  Also by this interpretation, a human floating in the ocean would become a plankton; this is a rather large plankton!  We will not adopt this extreme interpretation in this course.  The plankton can themselves be subdivided into four subcategories: the phytoplankton, the zooplankton, the bacterioplankton, and the virioplankton.  The bacterioplankton are obviously bacteria floating in the ocean, and the virioplankton are obviously viruses floating in the ocean.  We will concentrate our discussion on the phytoplankton and the zooplankton.  The phytoplankton use the energy of sunlight to synthesize their own food using photosynthesis.  Again, the word root photo means light.  In other words, phytoplankton are essentially microscopic plants floating in the ocean, since terrestrial (continental) plants also use the energy of sunlight to synthesize their own food using photosynthesis.  Zooplankton are essentially microscopic animals floating in the ocean.  The prefix zoo- means animal.  For example, zoology is the study of animals, a zoologist is someone who studies animals, and we visit animals at the zoo.  Some species of zooplankton eat various species of phytoplankton.  These zooplankton are analogous to terrestrial herbivorous animals that eat terrestrial plants.  Other species of zooplankton eat various other species of zooplankton.  These zooplankton are analogous to terrestrial carnivorous animals that eat other terrestrial animals.

 

Terrestrial plants use the energy of sunlight to synthesize their own food using photosynthesis.  An herbivorous animal may eat those plants.  A carnivorous animal may then eat that herbivorous animal.  Perhaps another carnivorous animal eats that carnivorous animal, and so on and so forth until we arrive at a carnivorous animal that no other animal eats, such as a lion or a tiger for example.  This is the terrestrial food chain.  The carnivorous animal that no other animal eats is said to be at the top of the food chain, while plants are said to be at the bottom of the food chain.  Any animal closer to the top of the food chain is said to be higher on the food chain, while any animal closer to the bottom of the food chain is said to be lower on the food chain.  The use of the term food chain implies that if one animal becomes extinct, then all other animals higher on the food chain will starve and also become extinct.  In general, this is not the case.  A carnivorous animal may eat a variety of different animals for example.  If a particular animal becomes extinct, most of the animals higher on the food chain will adapt their diets by eating other animals instead.  For this reason, the term food web is more correct than the term food chain.  However, notice that if all terrestrial plants were to become extinct, then the herbivorous animals that eat them will starve, the carnivorous animals that eat those herbivorous animals will starve, the carnivorous animals that eat those carnivorous animals will starve, and so on and so forth all the way up to the top of the food web.  Hence, if all terrestrial plants were to become extinct, all the rest of terrestrial life would become extinct as well.  Instead of saying plants are at the bottom of the terrestrial food web, it is more correct to say that plants are the foundation of the terrestrial food web.  The marine (oceanic) food web operates similarly.  We begin with phytoplankton that use the energy of sunlight to synthesize their own food using photosynthesis.  Some species of zooplankton may eat those phytoplankton.  Another species of zooplankton may then eat that zooplankton.  Perhaps a small fish eats that zooplankton.  Perhaps a medium fish eats that small fish.  Perhaps a large fish eats that medium fish, and so on and so forth until we arrive at an animal that no other animal eats, such as a shark for example.  This is the marine food chain.  The animal that no other animal eats is said to be at the top of the food chain, while the phytoplankton are said to be at the bottom of the food chain.  Any animal closer to the top of the food chain is said to be higher on the food chain, while any animal closer to the bottom of the food chain is said to be lower on the food chain.  It is usually many steps from the bottom of the marine food chain to the top of the marine food chain, but not always.  For example, whales eat plankton.  This is an example of jumping from the bottom of the food chain all the way to the top of the food chain in a single step.  Again, the use of the term food chain implies that if one animal becomes extinct, then all other animals higher on the food chain will starve and also become extinct.  In general, this is not the case.  A fish may eat a variety of other fish for example.  If a particular fish becomes extinct, most of the fish higher on the food chain will adapt their diets by eating other fish instead.  For this reason, the term food web is more correct than the term food chain.  However, notice that if all phytoplankton were to become extinct, then the zooplankton that eat them will starve, the zooplankton that eat those zooplankton will starve, the small fish that eat those zooplankton will starve, and so on and so forth all the way up to the top of the food web.  Hence, if all phytoplankton were to become extinct, all the rest of marine life would become extinct as well.  Even various species of aquatic birds would suffer from such an extinction of marine life.  Instead of saying phytoplankton are at the bottom of the marine food web, it is more correct to say that phytoplankton are the foundation of the marine food web.

 

Anything whatsoever that occurs in the biosphere is an example of biological productivity, such as one organism eating another organism or an organism reproducing and so on and so forth.  Even the death of an organism contributes to biological productivity.  There are two fundamental variables that determine biological productivity.  The first is the availability of sunlight.  If there is abundant sunlight, the phytoplankton will use that abundance of sunlight to synthesize an abundance of food causing them to proliferate.  The zooplankton that eat those phytoplankton will also proliferate.  The zooplankton that eat those zooplankton will also proliferate.  The small fish that eat those zooplankton will also proliferate, and so on and so forth.  Thus, biological productivity at all levels of the food web should be high.  However, if there is little sunlight, the phytoplankton do not have sufficient energy to synthesize an abundance of food, and they will be scarce in number.  The zooplankton that eat those phytoplankton will also be scarce.  The zooplankton that eat those zooplankton will also be scarce.  The small fish that eat those zooplankton will also be scarce, and so on and so forth.  Thus, biological productivity at all levels of the food web should be low.  However, the other variable that determines biological productivity is the availability of nutrients.  Phytoplankton may have all the sunlight they wish, but phytoplankton cannot synthesize food out of thin air.  The phytoplankton require nutrients as the raw materials to synthesize their food, just as terrestrial plants cannot create food out of thin air even if they had all the sunlight they wish.  Terrestrial plants need soil from which they extract the minerals they use as the raw materials to synthesize their food.  These two variables that determine biological productivity, availability of sunlight and availability of nutrients, are equally important.  Without nutrients to synthesize food at the foundation of the food web, an abundance of sunlight is insufficient to drive biological productivity.  Without sunlight to provide the energy to synthesize food at the foundation of the food web, an abundance of nutrients is insufficient to drive biological productivity.

 

We might conjecture that biological productivity is high in the equatorial oceans, since there is an abundance of sunlight.  Although this abundance of sunlight should make biological productivity high, this abundance of sunlight causes something else that makes biological productivity low: a thermocline that causes a pycnocline, as we discussed.  The sunlight warms the surface water making it less dense.  The sunlight cannot penetrate into the deep water, and therefore the deep water is cold, making it more dense.  The less dense surface water is buoyed upward, while the more dense deep water is pulled downward.  The pycnocline is a cutoff between the surface water and the deep water.  Most marine life lives near the surface of the ocean.  After all, the phytoplankton must live in the euphotic zone to access sunlight.  It follows that the zooplankton that eat those phytoplankton must also live in the euphotic zone, the zooplankton that eat other zooplankton must also live in the euphotic zone, and so on and so forth.  Fish that are able to swim into deep water must occasionally swim to the surface to eat, and aquatic mammals such as dolphins and whales that are able to swim into deep water must occasionally swim to the surface to not only eat but to breathe air as well.  Since the phytoplankton in the euphotic zone are cut off from the deep water by the pycnocline, they cannot access the tremendous abundance of nutrients that fills the entire ocean.  Even though there is an abundance of sunlight at the equatorial oceans, the biological productivity is low due to a lack of nutrients.  The biological productivity is so low at the equatorial oceans that oceanologists/oceanographers consider them to be marine deserts, analogous to the terrestrial deserts where there is a scarcity of water and hence a scarcity of life.

 

There is insufficient sunlight at the polar oceans.  Although this insufficient sunlight should make biological productivity low, this insufficient sunlight causes something else that should make biological productivity high: isothermal water that causes isopycnal water, as we discussed.  The insufficient sunlight keeps surface water cold.  The sunlight cannot penetrate into the deep water, and therefore the deep water is also cold.  There is no thermocline, and thus there is no pycnocline.  There is no cutoff between the surface water and the deep water.  Therefore, there is good mixing between surface water and deep water.  Thus, the phytoplankton in the euphotic zone are not cut off from the deep water, and they are able to access the tremendous abundance of nutrients that fills the entire ocean.  Nevertheless, there is insufficient sunlight at the polar oceans, making the biological productivity low, even though there is an abundance of nutrients.

 

Notice that availability of sunlight and availability of nutrients are inversely related to one another: abundant sunlight causes insufficient nutrients, and insufficient sunlight causes an abundance of nutrients!  We are tempted to sadly conclude that nowhere in the oceans is biological productivity high, but we have yet to analyze the midlatitude oceans.  During the wintertime at the midlatitude oceans, there is insufficient sunlight, which should make biological productivity low, but this insufficient sunlight causes something else that should make biological productivity high: isothermal water that causes isopycnal water.  There is no cutoff between the surface water and the deep water.  Therefore, there is good mixing between surface water and deep water.  Thus, the phytoplankton in the euphotic zone are not cut off from the deep water, and they are able to access the tremendous abundance of nutrients that fills the entire ocean.  Nevertheless, there is insufficient sunlight, making the biological productivity low even though there is an abundance of nutrients.  However, springtime causes a miracle.  There is more and more sunlight, which should make biological productivity high.  Moreover, water has a large heat capacity, meaning that it is difficult to change the temperature of water.  Thus, the surface water does not warm yet.  In other words, the thermocline has not yet formed, and thus the pycnocline has not yet formed.  The surface water is still not cut off from the deep water.  There is still good mixing between surface water and deep water, and hence there is still an abundance of nutrients.  We finally have both abundant sunlight and abundant nutrients; hence, the biological productivity is high.  Terrestrial biological productivity is similar.  Terrestrial life hibernates in the wintertime and comes out of hibernation in the springtime.  This is called the spring bloom.  We will use these same words for marine life.  Both equatorial marine life and polar marine life hibernate all year long, although for completely opposite reasons.  Equatorial marine life hibernates all year long due to insufficient nutrients, while polar marine life hibernates all year long due to insufficient sunlight.  Midlatitude marine life hibernates in the wintertime and enjoys a spring bloom in the springtime.  Whereas terrestrial life and midlatitude marine life both hibernate in the wintertime and both enjoy a spring bloom in the springtime, their biological productivities become opposite to each other in the summertime and the autumntime.  In the summertime, there is even more sunlight than springtime, which should make biological productivity high, but this abundance of sunlight causes something else that makes biological productivity low: the surface water has now warmed while the deep water remains cold.  The thermocline is established, which causes a pycnocline that is a cutoff between the surface water and the deep water.  Thus, there is a lack of nutrients, and the biological productivity is low.  Midlatitude marine life goes back into hibernation in the summertime!  This is quite different from terrestrial life.  Terrestrial life hibernates in the wintertime and then enjoys a spring bloom; terrestrial life continues to enjoy high biological productivity in the summertime, and then biological productivity decreases in the autumntime until the terrestrial life hibernates again in the wintertime.  Midlatitude ocean life hibernates in the wintertime and then enjoys a spring bloom; this is the same as terrestrial life.  However, midlatitude marine life goes back into hibernation in the summertime; this is different from terrestrial life.  In the autumntime, we have another difference between terrestrial life and midlatitude marine life.  Although there is less and less sunlight, there is still more sunlight as compared with the wintertime.  This decreasing sunlight causes the surface water to cool.  This destroys the thermocline, which destroys the pycnocline.  There is more and more mixing between the surface water and the deep water.  Thus, the availability of nutrients becomes greater and greater.  With more and more nutrients and with a satisfactory abundance of sunlight, the biological productivity is high.  This is called the autumn bloom.  Finally, midlatitude marine life returns to hibernation in the wintertime because of a scarcity of sunlight even though there is an even greater abundance of nutrients.  To summarize, midlatitude marine life goes through the following annual cycle: low biological productivity in the wintertime (hibernation), high biological productivity in the springtime (spring bloom), low biological productivity again in the summertime (hibernation), and high biological productivity again in the autumntime (autumn bloom) before returning to wintertime hibernation.  The majority of the biomass of planet Earth is marine (ocean life).  Therefore, we may assert that the biosphere of planet Earth hibernates in the winter and the summer and blooms in the spring and the autumn.  Although only midlatitude marine life follows this annual cycle, the terrestrial life comprises only a small part of the biomass of planet Earth.  Note that there are exceptions to this analysis even for marine life, as we will discuss.

 

A coral reef is a vast community of marine life living in close proximity with one another.  It is obvious how new organisms join a coral reef: the organisms simply join the community that has already been established.  However, how does the coral reef begin in the first place?  It is improbable for a large number of marine organisms to come together at the same place at the same time to form a community.  The theory of the formation of a coral reef was formulated by the British scientist Charles Darwin, who also formulated the theory of biological evolution.  First, marine organisms attach themselves to a seamount; this is called a fringing coral reef.  More and more organisms join this small community until the fringing coral reef becomes a barrier coral reef.  As more and more organisms join the barrier coral reef, it eventually becomes a coral atoll.  If this theory is correct, then we should find a seamount/guyot in the deep waters beneath a coral atoll, since the seamount that the fringing coral reef attached itself to in the first place would have been degraded by natural forces over time.  This is indeed the case.  In the deep waters beneath coral atolls, we find tablemounts/guyots, which were presumably once the seamounts the marine life attached themselves to when the fringing coral reef first formed.

 

Hydrothermal vents are sources of geothermal energy at the ocean floor.  There are entire communities of organisms living around these hydrothermal vents.  The foundation of the food web of these communities cannot be photosynthesis, since most of the ocean is the aphotic zone where it is completely dark.  Some of the organisms in these communities are actually using the Earth’s geothermal energy to synthesize their own food; this is called chemosynthesis.  Other organisms in these communities may eat those organisms, and still other organisms in these communities may eat those organisms, and so on and so forth.  This is the food web of these communities that live around hydrothermal vents at the ocean floor.  The foundation of the food web for these communities is chemosynthesis.  This is remarkable; biologists formerly believed that all life required sunlight, either directly or indirectly.  Even a carnivorous animal requires sunlight at least indirectly, since such an animal eats another animal lower on the food web which itself eats another animal even lower on the food web that eats another animal which eventually eats plants that require sunlight for photosynthesis.  Nevertheless, we now realize that all life does not require sunlight.  Whereas photosynthesis serves as the foundation of the food web for most of the biosphere of planet Earth, it is chemosynthesis that serves as the foundation of the food web for the communities at the ocean floor living around hydrothermal vents.  To our knowledge, all life absolutely requires two things: liquid water (for reasons we have already discussed) and a source of energy.  The Sun is the source of energy for most life in the biosphere on planet Earth, but even the Sun is not the source of energy for all life on planet Earth.  On our own planet Earth, geothermal energy serves as the source of energy for some of the lifeforms of our planet’s biosphere.

 

Physical Oceanology/Oceanography: Ocean Currents

 

Ocean currents are flowing bodies of water in the ocean, rather like rivers are flowing bodies of water on the continents.  All ocean currents can be divided into two categories: surface currents and deep currents.  Surface currents flow horizontally, along the surface of the ocean.  Deep currents flow vertically, within the deep ocean.  Surface currents are caused primarily by the prevailing winds, while deep currents are caused by density differences.  We have tremendous knowledge about surface currents, and so we will discuss surface currents in detail.  Not much is known about deep currents, and so we will only discuss deep currents briefly.

 

The velocity (both the speed and the direction) of surface currents is measured with a device called a drift meter, which is essentially a plastic bottle.  To measure the velocity (both the speed and the direction) of a surface current, we drop a drift meter into the ocean at one location, and we retrieve the drift meter somewhere else in the ocean.  We know where we dropped and where we retrieved the drift meter.  We also know when we dropped and when we retrieved the drift meter.  Using the equation speed equals distance divided by time, we can calculate the speed of the surface current that carried the drift meter.  We can even determine the direction of the surface current, since we know where we retrieved the drift meter relative to where we dropped the drift meter.  If we drop one drift meter into the ocean, we will most likely never find it again.  If we drop ten drift meters into the ocean, we will most probably never find even one of them.  We predict the same unfortunate outcome if we drop one hundred drift meters into the ocean.  If we drop one thousand drift meters into the ocean, perhaps we will retrieve one of them, but this would provide insufficient data to perform a reliable calculation.  If we drop ten thousand drift meters into the ocean, perhaps we will retrieve ten of them, but this still provides insufficient data to perform a reliable calculation.  If we drop one hundred thousand drift meters into the ocean, perhaps we will retrieve one hundred of them which would provide sufficient data to perform a reliable calculation, but this would also be too expensive.  Amusingly, among the most accurate measurements of the motion of surface currents resulted from accidentally spilling objects into the ocean.  For example, in May 1990 almost sixty-two thousand Nike sneakers were accidentally dropped into the North Pacific Ocean.  As these Nike sneakers washed up along the shore of the west coast of North America, oceanologists/oceanographers were able to reliably calculate surface currents in the North Pacific Ocean.  In the 1980s, Nike sneakers were a huge fad or craze; people actually murdered each other to steal the Nike sneakers that they were wearing.  Because of this fad or craze, people immediately snatched up these Nike sneakers that washed up on the west coast of North America.  Few persons would be lucky enough to find a matching pair.  Without an internet to find the match to an unmatched sneaker, people attended flea markets to either pay top dollar if they found the match to their sneaker or to trade one unmatched sneaker for another matching sneaker.  In January 1992, one hundred thousand plastic bathtub toys (such as rubber duckies) were accidentally dropped into the North Pacific Ocean, again enabling oceanologists/oceanographers to reliably calculate surface currents in the North Pacific Ocean.  The largest such accidental spill in history was the February 1997 dropping of roughly half a million Lego pieces into the North Atlantic Ocean.  This enabled oceanologists/oceanographers to reliably calculate surface currents in the North Atlantic Ocean.  In March 2001, more than a quarter of a million plastic soap dispensers were accidentally dropped in the North Atlantic Ocean, again enabling oceanologists/oceanographers to reliably calculate surface currents in the North Atlantic Ocean.  Although these accurate measurements of surface currents have occurred only in recent decades, we have had knowledge of surface currents stretching back roughly five hundred years, at least in the North Atlantic Ocean.

 

Surface currents are caused primarily by the prevailing winds.  We will discuss the prevailing winds in detail later in the course.  For now, winds generally blow in certain particular directions.  We are not saying that winds always blow in certain particular directions.  We are saying that winds blow much more often in certain particular directions as compared with other directions.  These predominant directions are called the prevailing winds.  These prevailing winds push on the surface waters, forcing the surface waters to move in those same directions.  Hence, surface currents generally flow in the direction of the prevailing winds.  The prevailing winds blow from east to west near the equator, from west to east near the midlatitudes, and from east to west again near the poles.  We will discuss why this is the case later in the course.  These prevailing winds push the surface waters in these same directions.  Thus, surface currents generally flow from east to west near the equator, from west to east near the midlatitudes, and from east to west again near the poles.

 

Consider a surface current near the equator flowing from east to west, and suppose this surface current runs into the east coast of a continent.  The flowing water will not spread out equally in both directions along the east coast of the continent because of the Coriolis force.  We will discuss the Coriolis force in detail later in the course.  For now, the Coriolis force is caused by the rotation of planet Earth and causes deflections to the right in the northern hemisphere and deflections to the left in the southern hemisphere.  We will discuss why this is the case later in the course.  The Coriolis force pushes on the prevailing winds causing them to deflect, and these prevailing winds push on the surface waters deflecting them as well.  In addition, the Coriolis force pushes directly on the surface currents, thus also directly contributing to their deflection.  Hence, when a surface current near the equator runs into the east coast of a continent, the flowing water north of the equator is deflected to the right (which is in fact northward), while the flowing water south of the equator is deflected to the left (which is in fact southward).  The result is surface currents flowing from the equator toward the poles along the east coast of the continent.  When the surface current reaches the midlatitudes, the prevailing winds will push the flowing water back into the ocean from west to east.  If this surface current runs into the west coast of another continent, the Coriolis force will deflect the midlatitude current in the northern hemisphere to the right (which is in fact southward) and will deflect the midlatitude current in the southern hemisphere to the left (which is in fact northward).  The result is surface currents flowing from the poles toward the equator along the west coast of this continent.  The flowing water may then arrive at the equator, where they will be pushed by the prevailing winds from east to west again.  We conclude that there are giant circles of surface currents in the ocean.  These are called oceanic gyres.  The word gyre means circle or wheel, and this word root is found in other words such as gyrate and gyroscope for example.  Actually, the oceanic gyres we have discussed are the subtropical oceanic gyres, since they are near the equator.  There are subpolar oceanic gyres near the poles.  Subtropical oceanic gyres north of the equator circulate clockwise, while subtropical oceanic gyres south of the equator circulate counterclockwise.  Subpolar oceanic gyres near the North Pole circulate counterclockwise, while subpolar oceanic gyres near the South Pole circulate clockwise.  Although we will discuss subtropical oceanic gyres in detail, we will only discuss subpolar oceanic gyres briefly, for reasons that we will make clear shortly.

 

The surface current that flows along the east coast of a continent from the equator toward the poles is called the western boundary current of the subtropical oceanic gyre.  The surface current that flows along the west coast of a continent from the poles toward the equator is called the eastern boundary current of the subtropical oceanic gyre.  This nomenclature is some cause of confusion.  Nevertheless, every continent ends where it meets an ocean, and every ocean ends where it meets a continent.  Therefore, the western margin of every ocean is the east coast of some continent, and the eastern margin of every ocean is the west coast of some continent.  If City A is to the north of City B, then City B is to the south of City A!  If one student sits to another student’s left, then the second student must be sitting to the first student’s right!  The western boundary current of a subtropical oceanic gyre flows from the equator toward the poles; hence, western boundary currents deliver warm water to the east coast of a continent.  The eastern boundary current of a subtropical oceanic gyre flows from the poles toward the equator; hence, eastern boundary currents deliver cool water along the west coast of a continent.  These boundary currents of subtropical oceanic gyres help to moderate temperatures on planet Earth.  Western boundary currents deliver warm water to the polar latitudes, moderating the cold temperatures at these latitudes.  Eastern boundary currents deliver cool water to the equatorial latitudes, moderating the warm temperatures at these latitudes.  Although the equatorial latitudes are still hot by human standards and the polar latitudes are still cold by human standards, the temperature difference is nevertheless moderate as compared with other planets, where differences between equatorial temperatures and polar temperatures are extreme.  The abundance of water covering planet Earth together with these boundary currents as well as the water vapor in the atmosphere (as we will discuss later in the course) all together moderate temperature differences on planet Earth.  Without all of these effects, temperature differences between daytime and nighttime and temperature differences between equator and poles would be too extreme for life to exist on planet Earth.

 

As we will discuss later in the course, the Coriolis force is weak near the equator, and the Coriolis force becomes stronger as we move away from the equator toward the poles.  Thus, the strong Coriolis force pulls much of the water off of midlatitude surface currents as compared with surface currents near the equator.  This leaves very little water by the time midlatitude surface currents reach eastern boundary currents; thus, the eastern boundary current of a subtropical oceanic gyre is weak and slow and shallow.  The weak Coriolis force pulls very little water off of the surface currents near the equator as compared with the midlatitude surface currents.  This leaves most of the water by the time surface currents near the equator reach western boundary currents; thus, the western boundary current of a subtropical oceanic gyre is strong and fast and deep.  Actually, the Coriolis force does succeed in deflecting some water all around the subtropical gyre.  If deflections are to the right in the northern hemisphere where subtropical oceanic gyres circulate clockwise, and if deflections are to the left in the southern hemisphere where subtropical oceanic gyres circulate counterclockwise, the result is inward deflections directed toward the center of subtropical oceanic gyres in both hemispheres.  Thus, there will be a higher elevation of sea level as water accumulates toward the center of the subtropical oceanic gyre.  We deduce that there are varying elevations of sea level in the ocean, literally mountains and valleys of water in the ocean!  As another example of these varying elevations of sea level, consider surface currents near the equator that flow from east to west.  This will cause a mountain of water on the western margin of the ocean as water accumulates at the east coast of some continent.  There will be a valley of water on the eastern margin of the ocean at the west coast of some other continent.  Since water must flow downhill, there will be a weak surface current flowing from west to east along the equator.  This is called the equatorial countercurrent.  This surface current flows from west to east, against the direction of the surface currents near the equator that flow from east to west.  The prefix counter- means against in words such as counterintuitive or counterclockwise for example.  Although the prevailing winds near the equator blow from east to west, the prevailing winds at the equator itself are virtually nonexistent, as we will discuss later in the course.  Therefore, the equatorial countercurrent can flow virtually unhindered against the direction of the surface currents near the equator.  The equatorial countercurrent is warm since it flows along the equator.  The equatorial countercurrent is also weak and slow and shallow.  Caution: this discussion of the equatorial countercurrent is only correct under normal circumstances; there are abnormal circumstances that we will discuss shortly.

 

The North Atlantic Subtropical Gyre circulates clockwise in the North Atlantic Ocean.  The western boundary current of this gyre is the Gulf Stream, which is strong and fast and deep and delivers warm water to the east coast of North America.  The Gulf Stream is named for the Gulf of Mexico.  The eastern boundary current of the North Atlantic Subtropical Gyre is the Canary Current, which is weak and slow and shallow and delivers cool water to the west coast of Africa.  The Canary Current is named for the Canary Islands off the coast of Morocco in northwestern Africa.  We have had some knowledge of the North Atlantic Subtropical Gyre for roughly five hundred years.  More than five centuries ago when Christopher Columbus was planning his famous voyage across the Atlantic Ocean, he noticed that midlatitude ocean currents come in from the Atlantic Ocean toward Europe while ocean currents near the equator flow out away from Africa into the Atlantic Ocean.  Therefore, when Christopher Columbus set sail from Spain with his three ships the Niña, the Pinta, and the Santa María, he first let the Canary Current carry the ships south.  Then, the surface currents near the equator carried the ships west into the Atlantic Ocean.  Finally, the Gulf Stream carried the ships north until they landed probably at one of the islands of the Bahamas, although historians are not certain precisely where they landed.  Almost three centuries after Christopher Columbus made his famous voyage (more than two centuries before today), Benjamin Franklin mapped the North Atlantic Subtropical Gyre.  Benjamin Franklin was one of the greatest founding fathers of the United States; he signed both the Declaration of Independence and the Constitution of the United States for example.  Benjamin Franklin was also a brilliant scientist; he discovered lightning is electricity and invented the lightning rod and bifocal glasses for example.  The United States is currently composed of fifty states, but the United States was born with only thirteen states along the Atlantic coast (the east coast) of North America.  Before these thirteen states were thirteen states, they were thirteen colonies of the British Empire.  In other words, England was the mother country of these thirteen American colonies.  Benjamin Franklin noticed that when sailing across the Atlantic Ocean from the colonies to the mother country, the trip took a shorter time as compared to sailing across the Atlantic Ocean from the mother country back to the colonies, which took a longer time.  This is because when sailing from Europe toward North America, we travel from east to west against the midlatitude surface current, which slows us down.  When sailing from North America toward Europe, we travel from west to east along with the midlatitude surface current, which speeds us up.  Benjamin Franklin actually proposed establishing a highway system in the North Atlantic Ocean.  Rather like always remaining on the right side of a street or a highway while riding horses (or driving cars today), Benjamin Franklin suggested that ships would only sail directly across the Atlantic Ocean when traveling from the colonies to the mother country (from west to east).  When sailing from the mother country back to the colonies, Benjamin Franklin suggested that ships should first let the Canary Current carry them south, then let surface currents near the equator carry them west into the Atlantic Ocean, and then let the Gulf Stream carry them north to arrive at the colonies.  Although such a proposed trip from the mother country to the colonies would be a longer distance, it would take less time since surface currents would carry ships faster as compared with sailing directly across the Atlantic Ocean against the midlatitude surface current.  Today, we also understand that the North Atlantic Subtropical Gyre brings warm water from the equator not only along the east coast of North America but across the Atlantic Ocean to Europe.  This explains why Europe is not as cold as Canada even though they are equally close to the North Pole.

 

The South Atlantic Subtropical Gyre circulates counterclockwise in the South Atlantic Ocean.  The western boundary current of this gyre is the Brazil Current, which is strong and fast and deep and delivers warm water to the east coast of South America.  The Brazil Current is named for the country Brazil in northeastern South America.  The eastern boundary current of the South Atlantic Subtropical Gyre is the Benguela Current, which is weak and slow and shallow and delivers cool water to the west coast of Africa.  The Benguela Current is named for the city Benguela in the country Angola in southwestern Africa.  The southern boundary current of the South Atlantic Subtropical Gyre is also important, as we will discuss.

 

Flowing from west to east along the equator in the Atlantic Ocean is the Atlantic Equatorial Countercurrent.  This current is weak and slow and shallow and takes warm water from the east coast of South America and brings it to the west coast of Africa.  Caution: this is only correct under normal circumstances; there are abnormal circumstances that we will discuss shortly.

 

The North Pacific Subtropical Gyre circulates clockwise in the North Pacific Ocean.  The western boundary current of this gyre is the Japan Current or the Kuroshio Current, which is strong and fast and deep and delivers warm water to far-east Asia.  This current is named for both the country Japan in far-east Asia and for the dark color of the water.  The eastern boundary current of the North Pacific Subtropical Gyre is the California Current, which is weak and slow and shallow and delivers cool water to the west coast of North America.  The California Current is named for the State of California on the west coast of North America.

 

The South Pacific Subtropical Gyre circulates counterclockwise in the South Pacific Ocean.  The western boundary current of this gyre is the East Australia Current, which is strong and fast and deep and delivers warm water to the east coast of Australia.  The eastern boundary current of the South Pacific Subtropical Gyre is the Peru Current or the Humboldt Current, which is weak and slow and shallow and delivers cool water to the west coast of South America.  This current is named for both the country Peru in northwestern South America and for the Prussian explorer Alexander von Humboldt.  The southern boundary current of the South Pacific Subtropical Gyre is also important, as we will discuss.

 

Flowing from west to east along the equator in the Pacific Ocean is the Pacific Equatorial Countercurrent.  This current is weak and slow and shallow and takes warm water from Indonesia and brings it to the west coast of South America.  Caution: this is only correct under normal circumstances; there are abnormal circumstances that we will discuss shortly.

 

The Indian Subtropical Gyre circulates counterclockwise in the Indian Ocean.  The western boundary current of this gyre is strong and fast and deep and delivers warm water to the east coast of Africa.  The eastern boundary current of this gyre is the West Australia Current, which is weak and slow and shallow and delivers cool water to the west coast of Australia.  The southern boundary current of the Indian Subtropical Gyre is also important, as we will discuss.

 

Flowing from west to east along the equator in the Indian Ocean is the Indian Equatorial Countercurrent.  This current is weak and slow and shallow and takes warm water from the east coast of Africa and brings it to Indonesia.  Caution: this is only correct under normal circumstances; there are abnormal circumstances that we will discuss shortly.

 

There is a misconception among North Americans and South Americans that supposedly the Atlantic Ocean is warm and the Pacific Ocean is cold.  This misconception is forgivable.  Whenever North Americans or South Americans go swimming in the Atlantic Ocean, they are swimming in the Gulf Stream or the Brazil Current, as the case may be.  These are western boundary currents that bring warm water along the east coast of these continents.  Whenever North Americans or South Americans go swimming in the Pacific Ocean, they are swimming in the California Current or the Peru/Humboldt Current, as the case may be.  These are eastern boundary currents that bring cool water along the west coast of these continents.  North Americans and South Americans on the Atlantic coast (the east coast) are accustomed to swimming in pleasantly-warm ocean temperatures due to western boundary currents; they are unpleasantly surprised when they visit the Pacific coast (the west coast) and try to go swimming in the unpleasantly-cold ocean temperatures due to eastern boundary currents.  Note that far-east Asians and Australians have the opposite misconception: they believe that the Pacific Ocean is warm!  Again, this misconception is forgivable.  Whenever far-east Asians or Australians go swimming in the Pacific Ocean, they are swimming in the Japan/Kuroshio Current or the East Australia Current, as the case may be.  These are western boundary currents that bring warm water along the east coast of these continents.  Africans have a similar misconception: they believe that the Atlantic Ocean is cold!  This is because whenever Africans go swimming in the Atlantic Ocean, they are swimming in the Canary Current or the Benguela Current, as the case may be.  These are eastern boundary currents that bring cool water along the west coast of Africa.  Here is another example of such misconceptions: Africans believe that the Indian Ocean is warm, while Australians believe that the Indian Ocean is cold.  Whenever Africans go swimming in the Indian Ocean, they are swimming in a western boundary current that brings warm water along the east coast of Africa.  Whenever Australians go swimming in the Indian Ocean, they are swimming in an eastern boundary current that brings cool water along the west coast of Australia.  In summary, different peoples living on different continents have different misconceptions about the temperature of various oceans.  In actuality, they are only experiencing the temperatures of the particular surface currents that flow along their continental coastlines.

 

There are presently only five large gyres in the entire ocean: the North Atlantic Subtropical Gyre, the South Atlantic Subtropical Gyre, the North Pacific Subtropical Gyre, the South Pacific Subtropical Gyre, and the Indian Subtropical Gyre.  There are several small subtropical gyres such as the Arabian Gyre and the Bengal Gyre.  There are also several small subpolar gyres such as the Alaska Gyre and the Bering Gyre (both near the North Pole) and the Ross Gyre and the Weddell Gyre (both near the South Pole).  There happens to be a continent at the South Pole: Antarctica.  Therefore, there is insufficient room to establish a large subpolar gyre near the South Pole.  The continents North America, Europe, and Asia as well as the microcontinent Greenland leave insufficient room to establish a large subpolar gyre near the North Pole.  Thus, there are presently five large subtropical gyres but no large subpolar gyres.  Again, there are several small subtropical gyres and several small subpolar gyres.  Due to the motion of the tectonic plates, the shape of the continents and the oceans changes over millions of years.  Perhaps millions of years ago or perhaps millions of years from now, all of the continents were or will be at the equator leaving insufficient room to to establish large subtropical gyres.  There would only be large subpolar gyres during such eras of Earth’s history.  Perhaps there were or perhaps there will be other eras of Earth’s history when the continents do leave sufficient room to establish a couple of large subtropical gyres as well as a couple of large subpolar gyres.  We happen to be alive during an era of Earth’s history when there are five large subtropical gyres and no large subpolar gyres, but again several small subtropical gyres and several small subpolar gyres.  The most simplistic definition of an ocean is an enormous body of water, but a more advanced definition of an ocean is an enormous body of water with a large gyre circulating within it.  By this more advanced definition, there are presently five oceans: the North Atlantic Ocean, the South Atlantic Ocean, the North Pacific Ocean, the South Pacific Ocean, and the Indian Ocean.  Note that this more advanced definition of an ocean forces us to count the Atlantic Ocean twice and to count the Pacific Ocean twice.  Also notice that we are not counting the so-called Arctic Ocean at all.  Indeed, the so-called Arctic Ocean is very small and very shallow as compared with the other oceans.  For all of these reasons, the so-called Arctic Ocean should not be regarded as an ocean and should really be renamed the Arctic Sea or the North Pole Sea.  During other eras of Earth’s history, perhaps there were or perhaps there will be a different number of oceans with completely different shapes from the five present-day oceans.

 

All oceanic gyres are caused by a combination of the prevailing winds and the Coriolis force pushing on the surface waters as well as the configuration of the continents interrupting the flow of surface currents.  There is only one latitude where we could actually sail all the way around the world without any continents interrupting our journey: the midlatitudes of the southern hemisphere.  Without any continents to interrupt their flow, the southern boundary current of the South Pacific Subtropical Gyre and the southern boundary current of the South Atlantic Subtropical Gyre and the southern boundary current of the Indian Subtropical Gyre merge to create the strongest current in the entire world: the Antarctic Circumpolar Current.  This current surrounds the continent Antarctica, which is at the South Pole.  The prefix circum- means around in words such as circumference and circumscribe for example; hence, the word circumpolar literally means around the pole.  The Antarctic Circumpolar Current is the strongest current in the entire world and is responsible for the rough waters at the midlatitudes of the southern hemisphere.  These rough waters are commonly known as the Roaring Forties, the Furious Fifties, and the Screaming Sixties.  The words forties, fifties, and sixties refer to degrees south latitude.  These rough waters were first encountered by Ferdinand Magellan who led the first crew to circumnavigate the entire world.  As Magellan and his crew sailed into the South Atlantic Ocean, they encountered these rough waters.  After they sailed around the southern tip of South America through the strait that would later be named for Magellan himself, they sailed north out of these rough waters and into more calm waters.  Hence, Magellan called this new ocean the Pacific Ocean, which literally means peaceful ocean.  Other words such as pacifist and pacify share the same word root.  In actuality, the entire Pacific Ocean is not peaceful.  Magellan and his crew happened to be sailing into the South Atlantic Ocean while entering the rough waters of the midlatitudes of the southern hemisphere, and they happened to be sailing into the South Pacific Ocean while leaving the rough waters of the midlatitudes of the southern hemisphere.  These rough waters were also encountered by Francis Drake, the first Englishman to circumnavigate the entire world.  As Drake and his crew sailed into the South Atlantic Ocean, they encountered these rough waters.  After they sailed around the southern tip of South America through the passage that would later be named for Drake himself, they sailed north out of these rough waters into more calm waters.  If the first person to ever circumnavigate the world happened to be from far-east Asia instead of from Europe, he would have first encountered the Roaring Forties, the Furious Fifties, and the Screaming Sixties in the South Pacific Ocean.  Perhaps after sailing around the southern tip of South America through the strait or through the passage that perhaps would later be named for this hypothetical explorer, his hypothetical crew would have then sailed north out of these rough waters and into more calm waters in the South Atlantic Ocean.  If this had happened, the Atlantic Ocean would have been named the peaceful ocean instead.

 

The Antarctic Circumpolar Current surrounds Antarctica and therefore isolates Antarctica from the rest of the world.  As a result, Antarctica and the surrounding waters are very cold.  In addition, there is a continental effect at the South Pole; after all, Antarctica is a continent!  There is a marine effect at the North Pole, since the so-called Arctic Ocean is a body of water.  For all of these reasons, temperatures at the North Pole are mildly cold as compared with temperatures at the South Pole, which are more severely cold.  Other continents were connected to Antarctica hundreds of millions of years ago forming a supercontinent.  Beginning roughly two hundred million years ago, the other continents began to rip off of Antarctica.  The final continent to rip off of Antarctica was South America, which explains why South America is the closest continent to Antarctica today.  The Drake Passage between Antarctica and South America is only roughly one thousand kilometers wide.  When South America ripped off of Antarctica roughly thirty million years ago, Antarctica became isolated at the South Pole.  With no further continental obstructions at the midlatitudes of the southern hemisphere, the Antarctic Circumpolar Current was established, further isolating Antarctica.  Hence, Antarctica became extremely cold, and our entire planet Earth plunged into what is called the Current Ice Age.  The Current Ice Age began roughly thirty million years ago and continues to the present day.  The Current Ice Age will continue for many more millions of years as long as Antarctica is isolated at the South Pole surrounded by the Antarctic Circumpolar Current that further isolates Antarctica.  There are only two scenarios that can end the Current Ice Age.  In one scenario, Antarctica may move off of the South Pole, which would make it less cold.  This would also interrupt the Antarctic Circumpolar Current, which would contribute to warming temperatures.  In the other scenario, another continent may move to the South Pole and collide with Antarctica.  This would end the isolation of Antarctica, making it less cold.  This would also interrupt the Antarctic Circumpolar Current, again contributing to warming temperatures.  Whether Antarctica moves away from the South Pole or another continent moves toward the South Pole, it takes millions of years for tectonic plates to move significantly, as we discussed earlier in the course.  Therefore, the Current Ice Age will continue for millions of more years to come.  Perhaps during other eras of Earth’s history millions of years ago or millions of years from now, the configuration of the continents and the oceans was or will be such that other continents were or will be isolated at one or both poles surrounded by surface currents that further isolated those continents.  In other words, perhaps there were or perhaps there will be other ice ages in Earth’s history.  We will discuss these other ice ages in Earth’s history later in the course.

 

Asia is the largest continent in the world; therefore, Asia experiences a strong continental effect.  In other words, Asia becomes extremely hot in the summertime and extremely cold in the wintertime.  If Asia is very hot in the summertime, the air above Asia is hot as compared with the air above the Indian Ocean.  As we will discuss later in the course, hot air is at a low pressure.  So, the hot air above Asia is at a lower pressure as compared with the air above the Indian Ocean which is at a comparatively higher pressure.  As we will also discuss later in the course, wind blows from high pressure toward low pressure.  Therefore, summertime winds tend to blow from the Indian Ocean (higher air pressure) toward Asia (lower air pressure).  These winds are humid, since water evaporates from the Indian Ocean to the overlying air.  Therefore, summertime humid winds blow from the Indian Ocean toward Asia.  These humid winds bring heavy rains and perhaps even floods to India.  This is called the southwest monsoon, since the humid winds are blowing from the southwest (toward the northeast).  The reverse occurs in the wintertime.  Since Asia is very cold in the wintertime, the air above Asia is cold as compared with the air above the Indian Ocean.  As we will discuss later in the course, cold air is at a high pressure.  So, the cold air above Asia is at a higher pressure as compared with the air above the Indian Ocean, which is at a comparatively lower pressure.  Again, wind blows from high pressure toward low pressure.  Therefore, wintertime winds tend to blow from Asia (higher air pressure) toward the Indian Ocean (lower air pressure).  These winds are dry, since there is much less water on a continent to evaporate to the overlying air.  Therefore, wintertime dry winds blow from Asia toward the Indian Ocean.  These dry winds bring droughts to India.  This is called the northeast monsoon, since the dry winds are blowing from the northeast (toward the southwest).  It is a common misconception that the word monsoon means stormy weather such as rain, but this is not correct.  The wet season and the dry season are both monsoons.  The word monsoon is derived from an Arabic word that simply means season or time of year.  At the midlatitudes, the seasons are generally regarded as summertime and wintertime.  In India, the seasons are instead these two monsoons: the southwest (wet) monsoon and the northeast (dry) monsoon.  The southwest (wet) monsoon is analogous to midlatitude summertime, and the northeast (dry) monsoon is analogous to midlatitude wintertime.

 

Something analogous to the monsoons in the Indian Ocean also occurs in the Pacific Ocean.  Under normal circumstances, the prevailing winds push the surface currents near the equator from east to west.  This brings humid winds and moderate rain to Indonesia, leaving moderately dry conditions in Ecuador and Peru.  Eventually so much water accumulates in Indonesia that all of this water begins to flow back to the Pacific Ocean, from west to east.  The water may flow so strongly that the Pacific Equatorial Countercurrent intensifies and pushes the other surface currents in this wrong direction as well.  The result is rain in Ecuador and Peru and dry conditions in Indonesia.  People living in Ecuador and Peru have noticed for centuries that on occasion ocean currents come in from the Pacific Ocean bringing rains and perhaps floods.  This always seemed to occur around Christmastime; therefore, people living in Ecuador and Peru named this El Niño, which is Spanish for the child as in the Christ Child, because Christmas is the celebration of the birth of Jesus Christ.  These people believed that El Niño was a small, local occurrence.  Today we realize that El Niño is actually occurring across the entire Pacific Ocean.  Since the Pacific Ocean is so enormous, El Niño is actually a global phenomenon.  So much water may build up in Ecuador and Peru during an El Niño that the water is forced to flow back to the Pacific Ocean, from east to west.  This is the direction of surface currents near the equator under normal circumstances, but these conditions are beyond normal or extreme normal.  The surface currents near the equator in the Pacific Ocean become so strong from east to west that the Pacific Equatorial Countercurrent may vanish or perhaps even flow in the correct east-to-west direction.  Under normal circumstances, there is moderate rain in Indonesia and moderately dry conditions in Ecuador and Peru.  During these extreme-normal circumstances, there are extreme floods in Indonesia and extreme droughts in Ecuador and Peru.  Since these extreme-normal circumstances are the opposite of an El Niño, oceanologists/oceanographers have named it La Niña, since El Niño is actually Spanish for the little boy and La Niña is Spanish for the little girl.  La Niña is just as much a global phenomenon as El Niño.  An El Niño or a La Niña may last as short as a few months, and either an El Niño or a La Niña can last as long as a couple of years.  Ironically, normal circumstances are actually the most rare circumstances; normal conditions only occur briefly while switching from an El Niño to a La Niña or the other way around.

 

For reasons we do not understand, there are decades of El Niño domination followed by decades of La Niña domination and then back again.  This alternation between El Niño domination and La Niña domination is called the Pacific Decadal Oscillation (PDO).  During decades of El Niño domination, although there are still La Niñas, there are more El Niños and they are stronger and they last longer.  During decades of La Niña domination, although there are still El Niños, there are more La Niñas and they are stronger and they last longer.  Again, we do not understand how or why the Pacific Decadal Oscillation (PDO) occurs.  Several years ago, we began a period of La Niña domination preceded by a period of El Niño domination.  This may explain why hurricane seasons were so horrific during the last decade of the previous century and the first decade of the current century, since those decades were during a period of El Niño domination.  This may also explain why hurricane seasons have been relatively mild during the last several years, since we are currently in a period of La Niña domination.  Caution: we may still have powerful hurricanes during a period of La Niña domination, just as we may still have mild hurricanes during a period of El Niño domination.  Another oscillation similar to the Pacific Decadal Oscillation (PDO) occurs in the Atlantic Ocean called the Atlantic Multidecadal Oscillation (AMO).  Direct observations of the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) only stretch back several decades.  However, measurements of the radioactive isotope carbon-fourteen  within trees have revealed that the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) each undergo on average roughly sixty-year cycles, where one complete Pacific Decadal Oscillation (PDO) for example consists of on average roughly three decades of El Niño domination followed by on average roughly three decades of La Niña domination.  The Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) together strongly influence the global climate of our planet Earth, as we will discuss later in the course.

 

Eastern boundary currents bring cool water from the poles to the equator along the west coast of continents.  This cool water destroys the thermocline, which destroys the pycnocline, thus causing good mixing of surface water with deep water at the equator.  This results in an abundance of nutrients in these particular equatorial waters.  There is certainly abundant sunlight at the equator.  Abundant sunlight and abundant nutrients together result in high biological productivity in these particular equatorial waters.  This is an exception to the marine deserts at the equatorial waters that we discussed.  For example, there is a thriving caviar industry off the coast of Ecuador and Peru.  Caution: this analysis is only correct under normal circumstances.  Even this Ecuadorian-Peruvian caviar industry is disrupted by occasional El Niños.

 

Deep currents are vertical flows of water within the deep ocean caused by density differences.  Low density water will rise and flow upward, while high density water will sink and flow downward.  The poles are cold throughout the entire year.  As we discussed, the South Pole is colder than the North Pole due to the continental effect together with the Antarctic Circumpolar Current surrounding and isolating Antarctica.  Hence, the waters around Antarctica are so cold that they sink all the way to the bottom of the ocean and flow along the ocean floor.  This is called Antarctic Bottom Water, and it is the coldest liquid water in the entire world.  The water at the so-called Arctic Ocean at the North Pole is also cold, and so it sinks almost to the ocean floor and flows on top of the Antarctic Bottom Water.  This is called Arctic Deep Water, and it is the second coldest liquid water in the entire world.  These waters flow along the ocean floor for perhaps hundreds of years or perhaps thousands of years; we are not certain.  These waters may eventually warm through perhaps igneous activity.  These warmer waters will rise to the surface of the ocean where the prevailing winds and the Coriolis force push them to become surface currents.  Eventually, these surface waters may end up at either the North Pole or the South Pole where they will become sufficiently cold to sink again, completing the entire cycle.  Again, we are not certain how long it takes ocean water to complete this entire cycle: perhaps centuries or perhaps millennia.  Not much more is known about deep currents.

 

Physical Oceanology/Oceanography: Ocean Waves

 

Ocean currents are actually flowing bodies of water in the ocean, rather like rivers are flowing bodies of water on the continents.  However, ocean waves are much more abstract.  A wave is a propagating (traveling) disturbance.  This means that when a water wave propagates (travels), the water does not necessarily actually move in the direction that the wave is propagating (traveling).  The water could be moving in the direction of the wave, but not necessarily.  The following example will make this more clear.  Imagine water flowing in a river downstream, and suppose we throw a rock or pebble into the river.  When the rock or pebble lands in the river water, the impact causes waves that propagate (travel) outward in all directions from where the rock or pebble plunged into the water.  These waves propagate (travel) in all directions.  Therefore, although some of these waves propagate downstream in the direction of the flowing water, some of these waves actually propagate (travel) upstream in the opposite direction of the flowing water!  This reveals how abstract waves are.  Ocean currents are clear: they are flowing bodies of water in the ocean.  Ocean waves are abstract: they are propagating (traveling) disturbances within the ocean.  Let us discuss waves in general while always keeping in mind that we will eventually apply everything we have discussed about waves in general to ocean waves in particular.

 

A wave is a propagating (traveling) disturbance.  This implies that a wave requires a medium through which to propagate, since we cannot have a propagating disturbance if there is nothing to disturb!  A transverse wave is a wave where the direction of the disturbance is perpendicular to the direction of propagation, while a longitudinal wave is a wave where the direction of the disturbance is parallel and antiparallel to the direction of propagation.  Light is a real-life example of a transverse wave, while sound is a real-life example of a longitudinal wave.  A wave can have a component of its disturbance perpendicular to the direction of propagation and another component parallel and antiparallel to the direction of propagation.  In other words, a wave can be both transverse and longitudinal.  Water waves in the ocean are a real-life example of a wave that is both transverse and longitudinal.  The maximum magnitude of a wave’s disturbance is called the wave amplitude, and these amplitudes occur at what are called crests (maximum positive disturbance) and troughs (maximum negative disturbance).  The wave height is the amount of disturbance between a crest and a trough.  In other words, the wave height is double the wave amplitude (or the wave amplitude is half of the wave height).  The distance from one crest to the next crest (which is also the distance from one trough to the next trough) is called the wavelength of the wave and is always given the symbol λ (the lowercase Greek letter lambda).  Caution: the word wavelength is misleading, since it may lead us to conclude that the wavelength is the length of the entire wave, which it is not.  The wavelength of a wave is the length of only one cycle of the wave.  The frequency of a wave is the number of crests passing a point every second as well as the number of troughs passing a point every second.  We may also interpret the frequency of a wave as how many cycles or oscillations or vibrations the wave executes every second.  In other words, the frequency of a wave is how frequently the wave is vibrating or oscillating, which is why it is called the frequency!  A high-frequency wave is oscillating (vibrating) many cycles every second, while a low-frequency wave is oscillating (vibrating) a small number of cycles every second.  We will use the symbol f for frequency, and its units are cycles per second or vibrations per second or oscillations per second.  This unit is called a hertz with the abbreviation Hz.  In other words, one hertz (Hz) is one cycle per second or one vibration per second or one oscillation per second.  A kilohertz is one thousand hertz or one thousand cycles per second, since the metric prefix kilo- always means thousand.  For example, one kilometer is one thousand meters, and one kilogram is one thousand grams.  A megahertz is one million hertz or one million cycles per second, since the metric prefix mega- always means million.  On the amplitude-modulation radio band (AM radio), the radio-station numbers are kilohertz, while on the frequency-modulation radio band (FM radio), the radio-station numbers are megahertz.

 

The speed of a wave is a function of the properties of the medium through which the wave propagates.  For example, the speed of sound is some speed through gases such as air, a faster speed through liquids, and an even faster speed through solids.  The speed of sound through air is not even fixed; the speed of sound through air actually changes as the temperature of the air changes.  As another example, the speed of light is some speed through gases such as air, a slower speed through liquids such as water, and an even slower speed through solids such as glass.  The speed of any wave with wavelength λ and frequency f is determined by the equation v = f λ, where v is the speed (the velocity) of propagation of the wave.  If we solve this equation for the frequency, we deduce that f = v / λ.  Therefore, frequency and wavelength are inversely proportional to each other.  Waves with higher frequencies have shorter wavelengths, while waves with lower frequencies have longer wavelengths.  In addition to the properties of the medium of propagation, the speed of a wave is almost always also a function of the wave’s own wavelength or frequency thus resulting in dispersion, as we will discuss.

 

The amplitude of any wave determines its energy.  In particular, the energy of a wave is directly proportional to the square of its amplitude.  Therefore, a wave with a larger amplitude has more energy, while a wave with a smaller amplitude has less energy.  For example, the amplitude of a sound wave determines its loudness.  A sound wave with a larger amplitude is more loud, while a sound wave with a smaller amplitude is less loud (more quiet).  As another example, the amplitude of a light wave determines its brightness.  A light wave with a larger amplitude is more bright, while a light wave with a smaller amplitude is less bright (more dim).  The frequency of a wave is difficult to interpret physically, and so we will interpret the frequency of a wave on a case-by-case basis.  For example, the frequency of a sound wave is its pitch, meaning that a sound wave with a higher frequency has a higher pitch while a sound wave with a lower frequency has a lower pitch.  As another example, the frequency of a visible light wave is its color.  In particular, a visible light wave with a high frequency is blue or violet, a visible light wave with a low frequency is red or orange, and a visible light wave with a middle frequency is yellow or green.  In order starting from the lowest frequency (which is also the longest wavelength), the colors of visible light are red, orange, yellow, green, blue, indigo, and violet at the highest frequency (which is also the shortest wavelength).  This is why the colors of the rainbow are in this order; a rainbow reveals the correct sequence of colors as determined by either the frequency or the wavelength.  We can memorize this sequence of colors with a mnemonic.  Using the letters r for red, o for orange, y for yellow, g for green, b for blue, i for indigo, and v for violet, we construct a fanciful name of an imaginary person: Roy G. Biv.

 

Whereas the frequency and the wavelength of a wave are constrained to one another through the equation v = f λ, no universal equation constrains the amplitude with the frequency.  Therefore, a wave can have a large amplitude and a high frequency, a wave can have a large amplitude and a low frequency, a wave can have a small amplitude and a high frequency, and a wave can have a small amplitude and a low frequency.  In other words, all of these combinations are physically possible.  For example, a sound wave with a large amplitude and a high frequency is a loud high-pitch sound, a sound wave with a large amplitude and a low frequency is a loud low-pitch sound, a sound wave with a small amplitude and a high frequency is a quiet high-pitch sound, and a sound wave with a small amplitude and a low frequency is a quiet low-pitch sound.  As another example, a visible light wave with a large amplitude and a high frequency is bright blue, a visible light wave with a large amplitude and a low frequency is bright red, a visible light wave with a small amplitude and a high frequency is dim blue, and a visible light wave with a small amplitude and a low frequency is dim red.

 

Consider any wave propagating in a certain medium that encounters a second medium.  This wave is called the incident wave.  At the boundary between the two media, a part of the wave will bounce back into the first medium while the rest of the wave will be transmitted into the second medium.  The wave that bounces back into the first medium is called the reflected wave, while the wave that is transmitted into the second medium is called the refracted wave.  We will make clear the meanings of the words reflection and refraction shortly.  Any line perpendicular to the boundary between the two media is called the normal to the boundary, since the word normal in physics and engineering means perpendicular.  The angle between the incident wave and the normal is called the angle of incidence with the symbol θ1.  The angle between the reflected wave and the normal is called the angle of reflection with the symbol θ3.  The angle between the refracted wave and the normal is called the angle of refraction with the symbol θ2.  The Law of Reflection states that θ1 = θ3 in all cases.  In other words, the angle of incidence is equal to the angle of reflection in all cases for all waves.  Reflection is the bouncing of a part of a wave off of another medium with no change in angle with respect to the normal.  The Law of Refraction states sin(θ1)/v1 = sin(θ2)/v2, where v1 is the speed of the wave in the first medium and v2 is the speed of the refracted wave in the second medium.  Refraction is the bending of a wave due to a change in speed of the wave.  According to the Law of Refraction sin(θ1)/v1 = sin(θ2)/v2, a wave is refracted (bent) toward the normal if v2 < v1 (if the transmitted wave propagates slower than the incident wave); conversely, a wave is refracted (bent) away from the normal if v2 > v1 (if the transmitted wave propagates faster than the incident wave).

 

When two or more waves of the same nature (the same type) occupy the same space at the same time, they add together to become a combined wave.  This is called interference.  Caution: sometimes addition is indeed an addition, but sometimes addition is actually a subtraction.  For example, positive five added with positive three yields positive eight, and negative five added with negative three yields negative eight.  However, positive five added with negative three yields positive two.  Notice that two is actually the difference between five and three.  Again, whereas sometimes addition is indeed an addition, sometimes addition is actually a subtraction.  When two waves of the same nature (the same type) occupy the same space at the same time, crest and crest may meet, trough and trough may meet, or crest and trough may meet.  The crests of a wave are locations of maximum positive disturbance, and the troughs of a wave are locations of maximum negative disturbance.  When crest and crest meet, the waves interfere with one another to become a combined wave with an even greater positive disturbance, since adding positive numbers together yields even greater positive numbers.  When trough and trough meet, the waves interfere with one another to become a combined wave with an even greater negative disturbance, since adding negative numbers together yields even greater negative numbers (in magnitude).  Either of these scenarios is called constructive interference, since the combined wave has a larger amplitude than the individual waves that interfered to form the combined wave.  When crest and trough meet, the waves interfere with one another to become a combined wave with a smaller disturbance, since adding a positive number with a negative number actually results in subtraction, such as adding positive five with negative three to yield positive two.  This scenario is called destructive interference, since the combined wave has a smaller amplitude than some of the individual waves that interfered to form the combined wave.  Any type of interference between these two extremes is called mixed interference.

 

Consider many waves with many different wavelengths (or frequencies) that interfere with one another to form a combined wave.  The speed of the individual waves is called the phase speed, while the speed of the combined wave is called the group speed.  Suppose all the individual waves move at the same speed.  So, these individual waves will all move together.  The resulting combined wave will then move at the same speed as the individual waves.  In other words, the group speed and the phase are equal to each other.  Now suppose instead that the individual waves all move at different speeds.  The combined wave will then spread as faster-moving waves pull out ahead of slower-moving waves that lag behind.  This is called dispersion.  The verb to disperse means to spread out.  The combined wave spreads because the individual waves all move at different speeds.  The combined wave will also move at a different speed.  In other words, dispersion is the spreading of a wave because the phase speed and the group speed are different from each other.  When the individual waves all move at the same speed, they will all move together, resulting in the combined wave also moving at that same speed.  The combined wave does not spread, since all the individual waves move together.  This is called no dispersion.  A rainbow results from the dispersion of light within water or glass.  White light is a combination of individual light waves with different wavelengths or frequencies (different colors).  When white light propagates within water or glass, the different colors all propagate at different speeds.  This also causes these colors to refract (to bend) at different angles which separates the individual colors from each other, thus forming a rainbow.

 

Diffraction is the bending of a wave, but this is a different bending than the refraction of a wave.  Refraction is the bending of a wave due to a change in speed of the wave, but diffraction is the bending of a wave without involving a change in speed of the wave.  Diffraction is the bending of a wave around obstacles.  However, if the wavelength of a wave is small compared with the size of the obstacles, this diffractive bending will be negligible, and the wave will seem to propagate in straight lines.  This is why light from a flashlight or even more so from a laser pointer appears to travel a straight path.  The wavelength of visible light waves is so small compared with the sizes of everyday obstacles around us that the diffraction of visible light is not noticeable.  However, the wavelength of sound waves is not small compared with the sizes of everyday obstacles around us, such as hallways and doorways.  Thus, the diffraction of sound is quite severe.  This is why we can hear someone speaking who is nevertheless standing around a corner.  The wavelength of sound is not small compared to the size of the hallway; therefore, the sound diffracts around the corner instead of traveling a simple straight path.

 

All water waves can be divided into three categories: deep-water waves, shallow-water waves, and transitional waves.  Deep-water waves are water waves in such deep water that the seabed is deeper than the bottom of the wave, called the wave base.  Therefore, the phase speed of deep-water waves is independent of the ocean depth; the phase speed of deep-water waves is only a function of the wavelength of the wave.  For these deep-water waves, the group speed is half of the phase speed (or the phase speed is double the group speed).  This can be demonstrated with the following experiment.  Suppose we throw a pebble into a lake or pond.  The plunging of the pebble into the water causes deep-water waves that propagate outward in all directions from where the pebble collided with the water.  Although there are many minor ripples propagating outward, there is often only one major ripple that propagates outward.  The major ripple propagates twice as slow as the minor ripples (or the minor ripples propagate twice as fast as the major ripple).  This reveals that the group speed is half of the phase speed (or the phase speed is double the group speed) for deep-water waves.  Shallow-water waves are waves in such shallow water that the phase speed is only a function of the ocean depth; the phase speed is independent of the wavelength of the wave.  Transitional waves propagate where the ocean is not too deep and not too shallow.  Thus, the phase speed is a function of both the wavelength of the wave and the ocean depth.  These transitional waves are the most difficult water waves to analyze mathematically, since both the ocean depth and the wavelength of the wave remain in the wave equation that describes the wave.  We will discuss why these are called transitional waves shortly.

 

Ocean waves are formed when winds cause small vibrations of the surface water.  These are commonly known as ripples, but they are more correctly called capillary waves, since it is the surface tension or more correctly the capillarity that restores the vibrations to a steady level.  Now suppose the wind speed picks up, and suppose the wind blows for a longer duration of time.  Also, suppose the wind blows for a further distance; the distance that the wind blows is called the wind fetch.  The wind adds more and more energy to the water waves, making the amplitude larger and larger.  Eventually, the amplitude is large enough that gravity becomes the restoring force instead of the capillarity or surface tension.  The amplitude may continue to grow as winds continue to add energy to the water waves.  When the amplitude becomes too large, the wave becomes unstable, and the wave breaks.  Caution: these are not the breakers we see at the beach.  We are discussing breakers in the middle of the ocean, not breakers near the continent.  In order to avoid confusion between these two types of breakers, we will use two different terms for these two different types of breakers.  Breakers in the middle of the ocean are called whitecaps.  We will reveal the term for breakers near the continent that we can see at the beach shortly.  The formation of whitecaps (the breaking of ocean waves) does not prevent winds from continuing to blow thus forming new ocean waves, which themselves may soon break as whitecaps.  Eventually, an equilibrium is established between forming waves and whitecaps.  These are called fully-developed sea waves.  These fully-developed sea waves may leave the winds that created them in the first place and propagate on their own.  It is a common misconception that we must continuously push a wave to keep it propagating, but this is false.  We may need a force to create a wave in the first place, but we do not need to continuously apply that force to keep the wave propagating.  This can be demonstrated with waves on a long spring.  We may need to wiggle the spring once to create a wave on the spring, but we do not need to continuously wiggle the spring to keep the wave propagating; the wave our first wiggle created propagates along the spring on its own without any additional force pushing it.  Any wave that propagates on its own without any force pushing it is called a free wave.  Free waves in the ocean are called swells.  To summarize, first we have capillary waves or ripples which then become fully-developed sea waves which then become swells.

 

A rogue wave or monster wave is an ocean wave with a giant crest (positive amplitude) that appears suddenly with no warning and disappears just as suddenly as it appeared.  These rogue waves or monster waves have been legends for thousands of years, but direct observations using laser sensors and marine buoys starting in the year 1995 have proven their existence.  Rogue waves or monster waves form completely accidentally.  There are thousands of waves in the ocean propagating in many different directions with different speeds and different wavelengths.  As they pass through each other, they interfere with each other.  On most occasions, they suffer from mixed interference, although sometimes they suffer from either constructive interference or destructive interference.  Suppose a large number of ocean waves happen to pass through one another in such a way that all their crests happen to be at the same place at the same time.  We would then have constructive interference from all of these waves, thus forming a single combined wave with a giant crest (giant positive amplitude).  This is a monster wave or rogue wave.  The individual waves will pass through each other, and so all of those crests will no longer be at the same place at the same time.  The giant positive amplitude quickly decreases, and the rogue wave or monster wave disappears just as suddenly as it appeared.  Many small ships are lost at sea without a trace every year.  On occasion, a large ship is lost at sea without a trace.  These missing ships are probably swallowed by monster waves or rogue waves.

 

As a deep-water wave approaches the continent, the ocean depth shallows.  Eventually, the ocean depth is less deep but not particularly shallow yet either.  So, the deep-water wave becomes a transitional wave.  As the wave continues to approach the continent, the ocean depth becomes sufficiently shallow that the wave becomes a shallow-water wave.  We now understand why transitional waves are called as such; they occur as ocean waves are transitioning from deep-water waves to shallow-water waves.  As the ocean depth decreases, the ocean waves slow down.  This permits other ocean waves to catch up to them.  When the waves are at the same place at the same time, interference occurs.  Although the result is usually mixed interference, either constructive interference or destructive interference may also occur.  With constructive interference, the combined wave has a larger crest (larger positive amplitude).  If the crest becomes too large, the wave becomes unstable, and the wave breaks.  We see these near-continent breakers while at the beach.  Whereas breakers in the middle of the ocean are called whitecaps, breakers near the continent are called surf.  Humans have invented a sport where they ride surf; this sport is called surfing, which we will discuss shortly.

 

There are several examples of words that we may believe should exist in the English language, but nevertheless they do not exist in the English language.  For example, there is no word readable in English; the correct word is legible.  There is no word eatable in English; the correct word is edible.  There is no word drinkable in English; the correct word is potable.  Relevant to oceanology/oceanography, there is no word shallowing in English; the correct word is shoaling.  As waves approach the continent, the ocean depth shallows.  If the shoaling is gradual, the ocean waves gradually slow down, and other ocean waves gradually catch up to them.  Gradual interference results, with on occasion gradual constructive interference, causing the waves to break gently.  This is the weakest type of surf, called spilling breakers.  If the shoaling is abrupt, the ocean waves suddenly slow down, and other ocean waves suddenly catch up to them.  Sudden interference results, with on occasion sudden constructive interference, causing the waves to break strongly.  This is the strongest type of surf, called surging breakers.  In between these two extremes are plunging breakers caused by shoaling that is not particularly abrupt and yet not particularly gradual.

 

The surf on the Atlantic coast (the east coast) of the United States is predominantly spilling breakers, while the surf on the Pacific coast (the west coast) of the United States is predominantly surging breakers.  This is for three reasons.  Firstly, the east coast of North America is a passive continental margin, making the shoaling gradual.  Ocean waves coming in from the Atlantic Ocean first encounter the continental rise and then the continental slope and then the continental shelf before reaching the shore.  The west coast of North America is an active continental margin, making the shoaling abrupt.  Secondly, the United States is at the midlatitudes, and the midlatitude prevailing winds blow from west to east.  Therefore, these prevailing winds blow with ocean waves that come in from the Pacific Ocean, making these ocean waves stronger.  However, these prevailing winds blow against ocean waves that come in from the Atlantic Ocean, making these waves weaker.  Thirdly, even if winds happened to be blowing from the east to the west in the Atlantic Ocean to reinforce ocean waves propagating toward the east coast of North America, the Atlantic Ocean is a much smaller ocean as compared with the Pacific Ocean.  Therefore, the wind fetch is smaller, and hence ocean waves cannot be made as strong.  The Pacific Ocean is a much larger ocean as compared with the Atlantic Ocean.  Therefore, the wind fetch can be much greater, and hence ocean waves can be made stronger.  All three of these factors cooperate to make the surf predominantly surging breakers on the west coast of the United States and predominantly spilling breakers on the east coast of the United States.  If we regard any sport as more exciting when it is more dangerous, then we would regard surfing as better on the west coast of the United States as compared with surfing on the east coast of the United States, where it is lame.  Surfers from the west coast (the Pacific coast) of the United States who attempt to go surfing on the east coast (the Atlantic coast) of the United States always think that they are on a kiddie ride, whereas surfers on the east coast (the Atlantic coast) of the United States who attempt to go surfing on the west coast (the Pacific coast) of the United States always get wiped out!

 

Tsunami are the second longest wavelength waves in the entire ocean.  As we will discuss, the tides are the longest wavelength waves in the entire ocean.  Any submarine disturbance powerful enough can cause a tsunami, such as a submarine landslide or a submarine volcanic eruption.  However, the most likely way a tsunami forms is from a submarine earthquake.  More precisely, the most likely way a tsunami forms is from a submarine dip-slip earthquake.  As we discussed earlier in the course, a dip-slip fault is a vertical break in rock, while a strike-slip fault is a horizontal break in rock.  A submarine dip-slip earthquake causes a vertical fault on the ocean floor.  This causes all of the water on top of the upward-thrusting rock to be thrust upward as well.  Also, all of the water on top of the downward-thrusting rock falls downward.  These vertical motions create a tsunami, an ocean wave with an enormous wavelength.  The part of the ocean water thrust upward is the initial crest of the tsunami, and the part of the ocean water that falls downward is the initial trough of the tsunami.  If a tsunami crest arrives at a continent before a tsunami trough, then sea level will rise first, and the ocean will first intrude into the continent.  If a tsunami trough arrives at a continent before a tsunami crest, then sea level will lower first, and the ocean will first recede away from the continent.  Regardless whether the crest or the trough arrives first, a tsunami is a wave.  So, there will be many alternating crests and troughs that arrive to the continent.  Every crest causes sea level to rise, causing the ocean to intrude into the continent.  Every trough causes sea level to lower, causing the ocean to recede away from the continent.  Again, any submarine disturbance powerful enough could theoretically cause a tsunami, such as a submarine landslide, a submarine volcanic eruption, and even a submarine strike-slip earthquake.  However, the most likely way a tsunami forms is from a submarine dip-slip earthquake.  Vertical motions of rock at the ocean floor are most likely to create crests and troughs at the ocean surface.  Since the wavelength of tsunami is very long, even the deepest parts of the ocean are shallow as far as tsunami are concerned.  Therefore, a tsunami is born a shallow-water wave.  As the tsunami approaches the continent, the depth of the ocean only becomes even more shallow, and so the tsunami remains a shallow-water wave.  Therefore, tsunami are always shallow-water waves.  This means that tsunami do not transition from deep-water waves to transitional waves.  Tsunami are born shallow-water waves, and tsunami remain shallow-water waves.  If tsunami remain shallow-water waves as they approach a continent, then different parts of a tsunami wave cannot catch up to each other to cause interference.  In particular, there is no constructive interference, which means there is no instability that would result in a breaker.  In other words, tsunami do not break at the continents as commonly depicted in disaster movies.  As we will discuss, tides do not break either.  Both tsunami and tides are always shallow-water waves that do not break.  Caution: this is the only thing that tsunami and tides share in common with each other.  The fact that they share this in common is the reason why the old term for tsunami was tidal waves.  Nevertheless, the term tidal wave is misleading, since it implies that tsunami and tides are caused by the same mechanism.  This is false; tides are caused by a completely different process from submarine disturbances, as we will discuss.  Therefore, we will always use the term tsunami, not tidal waves, to emphasize that these waves are completely different from tides.  The term tsunami is a Japanese word that simply means harbor wave.

 

If a tsunami crest arrives at a continent before a tsunami trough, then sea level will rise first, and the ocean will first intrude into the continent.  This can kill thousands of people.  If a tsunami trough arrives at a continent before a tsunami crest, then sea level will lower first, and the ocean will first recede away from the continent.  Ironically, even more people are often killed when this occurs.  If the ocean first recedes away from the continent, people who happen to be at the beach become intrigued and wander out toward the receding ocean to explore the naked and exposed continental shelf.  Then, the first tsunami crest arrives and kills all of those people in addition to people living inland when the ocean intrudes into the continent.  If we ever witness the ocean recede away from the continent thus exposing a naked continental shelf, we must leave the beach immediately.  The first trough of a tsunami has just arrived, and the first crest of the tsunami is on its way.  Tsunami are rare in the Atlantic Ocean, since the Atlantic Ocean is surrounded by passive continental margins.  However, tsunami are common in the Pacific Ocean, since the Pacific Ocean is surrounded by active continental margins, as we discussed earlier in the course.  Whenever seismologists trilaterate an earthquake to be somewhere in the Pacific Ocean, a tsunami warning is issued ordering everyone to leave the shores and travel inland.  This warning is issued throughout the west coast of South America, the west coast of North America, the east coast of Asia, the east coast of Australia, and even the Hawaiian Islands.  Unfortunately, there was no tsunami-warning system in the Indian Ocean until more than two hundred thousand people were killed from a tsunami in December 2004.  After this tragedy, a tsunami-warning system was established in the Indian Ocean.  Although tsunami will continue to occur, hopefully such a tragic loss of human life as a result of a tsunami will now be avoided.

 

The tides are by far the longest wavelength waves in the entire ocean.  We will reveal the wavelength of a tide shortly.  All of us have some familiarity with the tides.  Sometimes it is high tide, more correctly called the flood tide.  Other times it is low tide, more correctly called the ebb tide.  The origin of the tides was explained roughly three centuries ago by Isaac Newton, among the most brilliant persons who have ever lived.  Newton discovered calculus (advanced mathematics) and invented physics (the mathematical study of the equations that describe the universe) through his discovery of three universal Laws of Motion and the law of Universal Gravitation.  According to Newton’s law of Universal Gravitation, everything in the universe attracts everything else in the universe.  For example, the Earth attracts the Moon gravitationally, and the Moon attracts the Earth gravitationally.  The Earth attracts the Sun gravitationally, and the Sun attracts the Earth gravitationally.  Isaac Newton proved mathematically that any object exerts different gravitational attractions across any other object due to the varying distances between different parts of the two objects from each other.  Parts of the two objects that are closer to each other feel stronger gravitational attractions, while parts of the two objects that are further from each other feel weaker gravitational attractions.  The differences in the gravitational attractions across an object are called tidal forces, because they cause the tides in the ocean.  The Moon and the Sun each exert roughly equal tidal forces on the Earth’s oceans causing them to bulge, resulting in flood tides and ebb tides every day.  When the Earth, the Moon, and the Sun happen to form a nearly straight line (this occurs during New Moon or Full Moon), the lunar tidal force and the solar tidal force reinforce each other.  If we interpret the tides as waves, then during this straight-line configuration, the lunar tidal crests and the solar tidal crests meet each other, while the lunar tidal troughs and the solar tidal troughs also meet each other.  This is constructive interference, resulting in severely high flood tides and severely low ebb tides.  These are called the spring tides.  When the Earth, the Moon, and the Sun happen to form a nearly right angle (this occurs during First Quarter Moon or Third Quarter Moon), the lunar tidal force and the solar tidal force counteract each other.  If we interpret the tides as waves, then during this right-angle configuration, the lunar tidal crests and the solar tidal troughs meet each other, while the lunar tidal troughs and the solar tidal crests meet each other.  This is destructive interference, resulting in modest flood tides and modest ebb tides.  These are called the neap tides.  The tidal range is the wave height of the tides, the difference between the flood tide and the ebb tide.  The spring tides have the largest tidal range, since the flood tide is very high and the ebb tide is very low, resulting in a large difference between them.  The neap tides have the smallest tidal range, since the flood tide is modestly high and the ebb tide is modestly low, resulting in a small difference between them.  The Moon’s orbital period around the Earth is roughly one month.  In fact, the word month is derived from the word moon.  If we take the word month, remove the suffix -th, and insert an extra letter o, we obtain the word moon!  One month is roughly four weeks.  If today is New Moon, we will have spring tides with the largest tidal range (severely high flood tides and severely low ebb tides).  Roughly one week later will be First Quarter Moon, and we will have neap tides with the smallest tidal range (modest flood tides and modest ebb tides).  Roughly one week later will be Full Moon, and we will have spring tides again with the largest tidal range (severely high flood tides and severely low ebb tides).  Roughly one week later will be Third Quarter Moon, and we will have neap tides again with the smallest tidal range (modest flood tides and modest ebb tides).  Roughly one week later, we have returned to New Moon, roughly four weeks since the previous New Moon.  For thousands of years, humans already noticed that there is a correlation between the changing appearance of the Moon in the sky and the changing tidal range in the ocean, but it was Isaac Newton who explained mathematically why this is the case.  The lunar tidal force and the solar tidal force not only cause the Earth’s oceans to bulge, but they also cause the shape of the geosphere (the solid Earth) itself to bulge.  The shape of the geosphere (the solid Earth) itself suffers flood tides and ebb tides every day.  When the geosphere (the solid Earth) itself suffers a flood tide, we are slightly further from the center of the Earth.  Later when the geosphere (the solid Earth) itself suffers an ebb tide, we are slightly closer to the center of the Earth.  Each and every day of our lives, we move up and down roughly one meter, even while we believe ourselves to be remaining still!

 

Most coastlines experience two unequal tides every day.  In other words, there are two flood tides with one higher than the other and two ebb tides with one lower than the other.  This is called the mixed tidal pattern, the most common tidal pattern.  Some coastlines experience two equal tides every day.  In other words, there are two flood tides that are equally high and two ebb tides that are equally low.  This is called the semidiurnal tidal pattern.  The most rare tidal pattern is the diurnal tidal pattern with only one flood tide and only one ebb tide every day.  These different tidal patterns are caused by the configuration of the continents and the shape of coastal inlets such as bays and gulfs.  Since the most common tidal pattern is two tides every day, this means that the crest of a tide is on one side of planet Earth, and the other crest of a tide is on the other side of planet Earth.  Thus, the wavelength of a tide is half of the circumference of planet Earth, since the wavelength of any wave is the distance from one crest to the next crest, which is also the distance from one trough to the next trough.  The circumference of planet earth is roughly forty thousand kilometers.  Therefore, the wavelength of a tide is roughly twenty thousand kilometers!  We now understand that the tides are by far the longest wavelength waves in the entire ocean.  Since the wavelength of the tides is so incredibly long, even the deepest parts of the ocean that are only roughly eleven kilometers below sea level are extremely shallow as far as the tides are concerned.  Therefore, tides are always shallow-water waves.  As a tide approaches the continent, the depth of the ocean only becomes even more shallow, and so the tide remains a shallow-water wave.  The tides do not transition from deep-water waves to transitional waves.  Tides are born shallow-water waves, and tides remain shallow-water waves.  If tides remain shallow-water waves as they approach a continent, then different parts of a tide cannot catch up to each other to cause interference.  In particular, there is no constructive interference, which means there is no instability that would result in a breaker.  In other words, tides do not break.  We know this from common experience.  Beginning with the ebb tide, sea level simply gradually rises up to the flood tide, and then sea level gradually drops down to the ebb tide.  This is the only thing that tides share in common with tsunami: neither tides nor tsunami break since both tides and tsunami are always shallow-water waves due to their extremely long wavelengths.

 

Tides may propagate up a river causing flood tides and ebb tides upstream, not just at the shore.  This is a wave that is truly caused by the tides, and these waves should properly be called tidal waves.  Unfortunately, if we refer to these waves as tidal waves, everyone will incorrectly assume that we are referring to tsunami.  Therefore, we must give these waves a different name.  We will call a wave caused by the tides that propagates up a river a tidal bore.  These tidal bores are the only waves that have the right to be correctly called tidal waves.

 

The Bay of Fundy in Canada between Nova Scotia and New Brunswick experiences the largest tidal range in the entire world.  This is because the shape of the Bay of Fundy happens to curve to the right with the same curvature as the Coriolis force.  This permits the Coriolis force to reinforce the flood tide going into the bay every day.  During ebb tide, all of the water drains out of the bay lowering all boats until they hit the bottom of the bay.  With the next flood tide, the entire bay floods, lifting all the boats off of the bottom of the bay.  Any other bay in the northern hemisphere that happened to curve to the right with the same curvature as the Coriolis force would suffer from the same enormous tidal range.  If there happened to be a bay in the southern hemisphere that happened to curve to the left with the same curvature as the Coriolis force, it would also experience the same enormous tidal range as the Bay of Fundy.

 

Transitional Landscapes/Environments: Coasts and Shores

 

All landscapes/environments on planet Earth can be divided into three categories: terrestrial landscapes/environments, marine landscapes/environments, and transitional landscapes/environments.  Terrestrial landscapes/environments are on the continents.  Marine landscapes/environments are at the ocean floor.  Transitional landscapes/environments are where oceans and continents meet each other.  We have discussed marine landscapes/environments, and we will discuss terrestrial landscapes/environments toward the end of the course.  So, we now devote some discussion to transitional landscapes/environments.

 

Transitional landscapes/environments are where oceans and continents meet each other, commonly known as shores and coasts.  In colloquial English, the words coast and shore are often used interchangeably.  However, in science we give terms strict definitions.  In oceanology/oceanography, the words coast and shore are given precise definitions.  By these strict definitions, the coast and the shore are actually two different things.  The shore is defined as where the continent and the ocean meet each other.  The coast is defined as the part of the continent inland that is directly affected by the ocean.  The distance inland that the coast extends varies tremendously.  Some coasts extend inland for dozens of kilometers, while other coasts extend inland for less than one kilometer.  The boundary between the coast and the shore is called the coastline.  The shore extends from the coastline all the way to the edge of the continental shelf.  The precise boundary where the continent and the ocean meet each other is called the shoreline, but this shoreline moves every day due to the tides.  During the flood tide, the shoreline moves furthest inland, while during the ebb tide, the shoreline moves furthest from the continent.  The backshore is the part of the shore from the coastline to the flood-tide shoreline.  Therefore, the backshore is never submerged underwater, even during the flood tide.  The foreshore is the part of the shore between the flood-tide shoreline and the ebb-tide shoreline.  Therefore, sometimes the foreshore is submerged, while at other times the foreshore is not submerged.  The foreshore is completely submerged during the flood tide, while the foreshore is completely above sea level (completely not submerged) during the ebb tide.  Most of the time, the foreshore is partially submerged and partially not submerged.  The nearshore is the part of the shore from the ebb-tide shoreline to the breakerline, where ocean waves first break as surf.  The offshore is the part of the shore from the breakerline out to the edge of the continental shelf.  Note that both the nearshore and the offshore are always completely submerged, even during the ebb tide.  The part of the shore humans commonly call the beach is usually the foreshore.

 

Ocean waves that propagate toward the continent will eventually splash onto the shore; this is called the swash.  The water then spills back into the ocean; this is called the backwash.  A backwash often has the next swash splashing over it.  Deep water rapidly flowing away from the continent with overlying surface water rapidly flowing toward the continent is called a rip current.  Humans who find themselves within a rip current thus feel a double torque: deep water pulls the feet away from the continent while surface water pushes the torso toward the continent.  The swimmer is almost always knocked over then pulled out into the ocean by the rip current.  Many swimmers die every year from rip currents.  Attempting to swim back toward the shoreline while being pulled by a rip current expends energy, since we would be swimming against the rip current.  We must swim perpendicular to the rip current, which is parallel or antiparallel to the shoreline, to successfully escape the rip current.

 

As an ocean wave approaches the continent, the ocean wave slows due to the shoaling of the ocean.  Refraction thus causes the ocean wave to bend toward the normal; hence, the direction of the ocean wave becomes more perpendicular to the shoreline.  Nevertheless, the direction of the ocean wave is still not perfectly perpendicular to the shoreline.  In other words, the direction of the ocean wave can be broken into two components: one component is perpendicular to the shoreline while the other component is along (parallel or antiparallel to) the shoreline.  The component of an ocean wave along the shoreline is called the longshore current, and this longshore current pushes sand along the shoreline in the direction of the longshore current.  Actually, each grain of sand executes a zigzag trajectory as each swash pushes the sand grain toward the continent by a small amount and then each backwash pulls the sand grain back into the ocean by a small amount.  Even if there were no wind pushing the sand and even if there were no animals burrowing through the sand and even if there were no humans building sandcastles, every grain of sand at the beach is slowly but surely being pushed along the shoreline in the direction of the longshore current.  It is for this reason that oceanologists/oceanographers regard beaches to literally be rivers of sand.

 

Natural forces continuously destroy landscapes/environments on planet Earth; this is called degradation.  Natural forces also continuously build new landscapes/environments on planet Earth; this is called aggradation.  Both degradation and aggradation almost always occur simultaneously.  Nevertheless, a shore where there is much more degradation with significantly less aggradation is called a degradational shore, while a shore where there is much more aggradation with significantly less degradation is called an aggradational shore.  At degradational shores, ocean waves may carve out a sea cave within the continent.  Two sea caves may merge, thus forming a sea arch over them.  The sea arch may eventually collapse from its own weight, leaving a sea stack in the ocean that is divorced from the continent where there is now a sea cliff.  At aggradational shores, ocean waves may create a long sand deposit that is curved on its end.  Butchers use long rods with hooks on their ends in meat lockers; these hooked long rods used by butchers are called spits.  Since long sand deposits with curved ends at aggradational shores resemble spits used by butchers, these long sand deposits with curved ends at aggradational shores are also called spits.  The curved end of an aggradational sand spit is called a hook, such as Sandy Hook in New Jersey.  The sand deposit may entirely cut off a coastal inlet from the rest of the ocean, thus forming a bay barrier.  Ocean waves may push sand to create a bridge of sand that connects the continent to a sea stack; this is called a tombolo.  Entire islands of sand parallel to the coastline may also be built at aggradational shores; these are called barrier islands, such as Long Beach Island in New Jersey.  As some rivers approach the ocean, the river water may deposit sand, forcing the river to break into many distributaries.  This aggradational feature is called a delta.

 

Since the sea level actually changes every day due to the tides, we should strictly use the term mean sea level instead of sea level.  The mean sea level is the sea level averaged over many days, but even the mean sea level may change for three possible reasons: tectonic, isostatic, or eustatic.  A tectonic change in mean sea level is caused by a dip-slip motion of the continent.  If the continent is thrust vertically upward, mean sea level relative to that continent becomes lower.  If the continent is thrust vertically downward, mean sea level relative to that continent becomes higher.  An isostatic change in mean sea level results from the buoyant motion of a tectonic plate.  If a tectonic plate sinks into the asthenosphere, mean sea level relative to that continent becomes higher.  If a tectonic plate rises out of the asthenosphere, mean sea level relative to that continent become lower.  Note that both tectonic and isostatic changes in mean sea level are actually changes relative to the continent.  It is the continent that is actually moving; it is not the ocean that is actually changing.  However, a eustatic change in mean sea level is a global adjustment in mean sea level due to global climate change.  For example, if planet Earth becomes colder such as during a glacial period of an ice age, we expect mean sea level to become lower for two reasons.  Firstly, the water that evaporates from the oceans precipitates onto the continent forming enormous ice sheets that do not return to the ocean as rivers.  Hence, water is subtracted from the ocean without being added back to the ocean.  Secondly, the water remaining in the ocean thermally contracts due to the colder global temperatures.  When the glacial period of an ice age ends, we expect mean sea level to become higher for the same two reasons.  Firstly, the enormous ice sheets that cover the continents melt into liquid water and returns to the ocean, thus replenishing the water that was once subtracted.  Secondly, the water in the ocean thermally expands due to the warmer global temperatures.  Isostatic and eustatic changes in mean sea level are not only comparable with one another but also compensate for one another, resulting in only small changes in the mean sea level.  For example, if planet Earth becomes colder such as during a glacial period of an ice age, we have argued that there will be a eustatic drop in mean sea level.  However, the enormous ice sheets that form upon the continents will push the continental plates into the asthenosphere, and hence there will be an isostatic rise in mean sea level relative to the sinking continents!  The eustatic drop in mean sea level is compensated by the isostatic rise in mean sea level relative to the sinking continental plates.  Hence, the actual change in mean sea level is small.  Conversely, if planet Earth becomes warmer such as at the end of a glacial period of an ice age, we have argued that there will be a eustatic rise in mean sea level.  However, the melting ice sheets remove weight off of the continents permitting the continental plates to rise out of the asthenosphere, and hence there will be an isostatic drop in mean sea level relative to the rising continents!  The eustatic rise in mean sea level is compensated by the isostatic drop in mean sea level relative to the rising continental plates.  Hence, the actual change in mean sea level is small.

 

Whether the mean sea level changes tectonically, isostatically, eustatically, or some combination thereof, we can classify shores as either emergent shores or submergent shores based upon the change in mean sea level.  If the mean sea level is dropping, then we call the shore an emergent shore.  If the mean sea level is rising, then we call the shore a submergent shore.  Entire beaches can be drowned in submergent shores.  A drowned river valley in a submergent shore results in estuaries where the mouth of a river opens wide to meet the ocean.  Ocean water has a higher salinity than river water.  As a result, ocean water is more dense, while river water is less dense.  At an estuary, the more dense ocean water sinks, while the less dense river water rises.  Therefore, the limnological river water sits on top of the seawater at estuaries.  Between the more dense deep ocean water and the less dense surface river water, the seawater mixes with the limnological water to form brackish water.

 

Oil spills are commonly considered a form of ocean pollution, but this is false.  Firstly, liquid petroleum is naturally leaked into the ocean from fissures on the ocean floor.  In fact, the total amount of oil naturally leaked into the ocean from fissures every year is roughly double the total amount of oil spilled into the ocean by oil tankers every year.  Since petroleum is a hydrocarbon, petroleum that leaks into the ocean is consumed by the plankton that live in the ocean.  The plankton then proliferate, the small fish that eat the plankton thus proliferate, and so on and so forth.  In summary, the leaking of petroleum into the oceans enhances marine biological productivity.  Human-caused oil spills are not an exception; after all, petroleum is petroleum!  As evidence of the benefits of oil spills to marine life, let us discuss the Exxon Valdez oil spill as a case study.  In March 1989, the oil tanker Exxon Valdez accidentally collided with Bligh Island Reef and spilled between eleven million and twelve million gallons of oil into Prince William Sound in Alaska.  As a result of this oil spill, the ecology of Prince William Sound was enhanced in numerous ways.  Firstly, the population of the cormorant (an aquatic bird) was in decline before the spill, but the population of the cormorant doubled over the next decade immediately after the spill!  Secondly, the population of the bald eagle was also positively affected.  The bald eagle is the national bird of the United States.  The bald eagle was an endangered species several decades ago; it would be a disgrace if the national bird of the United States were to become extinct.  Fortunately, the population of the bald eagle increased so much over the several years immediately after the Exxon Valdez oil spill that the United States Fish and Wildlife Service removed it from the endangered species list as a direct result of the spill!  Thirdly, the population of the harbor seal was declining by roughly twelve percent every year before the spill, but then the population of the harbor seal declined by only six percent every year over the next several years after the spill.  If more oil was spilled, perhaps the population of the harbor seal would have increased!  Fourthly, Exxon spent more than two billion dollars immediately after the spill and an additional nine hundred million dollars over the next few years cleaning the spilled oil.  However, oceanological/oceanographic surveys of Prince William Sound over the next decade showed that the areas not cleaned by Exxon thrived ecologically while the areas that were cleaned by Exxon did not thrive ecologically!  In other words, Exxon should not have cleaned any of the oil that was spilled!  The population of the pacific herring remained unchanged for a few years after the spill.  Then, the population suddenly dropped by roughly eighty percent.  Is this a negative consequence of the Exxon Valdez oil spill?  No, since most oceanologists/oceanographers agree that this was caused by disease unrelated to the spill.  Indeed, the population of the pacific herring slowly increased after the disease ended.  To summarize, oil spills should not be considered a form of pollution, since oil spills are actually beneficial to marine ecology.  Instead of punishing oil companies that accidentally spill oil into the ocean, governments should reward oil companies that spill oil into the ocean for enhancing the marine ecology of our planet Earth.

 

 

 

Links

 

Libarid A. Maljian homepage at the Department of Physics at CSLA at NJIT

Libarid A. Maljian profile at the Department of Physics at CSLA at NJIT

Department of Physics at CSLA at NJIT

College of Science and Liberal Arts at NJIT

New Jersey Institute of Technology

 

 

 

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