UNIT 6 THE BLUE PLANET: DYNAMICS OF THE OCEANS

A. INTRODUCTION

Overview

Ocean currents reflect the influences of surface winds and density differences and are measured by a variety of techniques. The oceans are turning in space along with the Earth as it spins on its axis, and this results in distinctive behavior that is referred to as the Coriolis Effect. You will have the opportunity to investigate it at first hand in an exercise that requires a phonograph turntable. Prevailing winds and Earth's rotation cause great gyres, or rotary motions, within each ocean basin. The Gulf Stream is an example of the tendency of ocean currents to intensify along the western edge of the gyre. This moving mass of water has a vital influence on the climate of much of Europe. The oceans in fact are an important influence on global climate, serving as a huge reservoir of thermal energy in close interaction with the atmosphere. Vertical mixing of ocean waters is limited in most places, but in localities where deep nutrient-rich water upwells, fish and fisherman prosper. The phenomenon of El Niño involves a shifting in the trade winds and the equatorial currents that suppress upwelling off the west coast of South America. When a strong El Niño occurs, the local fishing industry is hard hit. Worse, global climate reacts to the change in ways that can bring disaster to widespread areas of the planet. A new tool for the oceanographer is the satellite, from which a global view of the oceans may be obtained. Measurements from space tell us of the temperature and plankton content of seawater, of the flow of currents, and of the shape of the ocean floor. Circular eddies or ring currents have been observed to be a significant part of oceanic circulation, and their peculiar effects are just beginning to be understood. Marine life has been the subject of study for centuries, but until recently only that life in the uppermost few meters of the sea could be investigated in detail. Now marine biologists are extending their observations to greater depths, including the floor of the deep ocean. This has been the locale for a major new discovery in recent years -- of communities of creatures cut off totally from the Sun and dependent on the chemistry of hot springs in the spreading ridges of the sea floor.

B. WAVES

1. Wind Driven Waves

When you arrive at the seashore, one of the first things you note is the state of the sea; whether it is rough or whether it is calm. It is rather obvious that on a windy day the waves are higher than when there is a gentle breeze. You may also notice that the waves are rather smooth out to sea, forming swells, and that as they come on shore they take the form of turbulent breakers.

The first thing we need to do is to make the term wave, which we have just used, a bit more precise. The more specialized term we would like to use is progressive water wave. This term is most easily explained using Figure 6-1 and is most applicable to the smooth swell. A technical description of breaking waves is more complex and does not need to be attempted here.

• Figure 6-1 Elements of a Progressive Water Wave

  

The highest portion of the wave is called the crest and the lowest portion is called the trough. The horizontal distance between two crests or between two troughs is the wavelength and the vertical distance between the trough and the crest is the wave height or wave amplitude.

• Figure 6-2 Orbital Motion in a Progressive Water Wave

  

  HM 6-1: A breaking wave.

The pattern described in Figure 6-1 travels over the surface of the sea at the wave velocity. The wave velocity depends in a rather complicated way upon a number of factors, the most important being wavelength and water depth. When the water is deep, the wave velocity is proportional to the square root of the wavelength rising to a maximum in excess of 200 meters per second (greater than 450 miles per hour). At such wave velocities, the wavelength becomes so long that, to the wave, the water no longer appears deep and in this case another relation applies.

The rule for shallow water is that the wave velocity is proportional to the square root of the water depth. Since all oceans are of finite depth, velocities of the above magnitude are the highest observed. The reason that such rapidly progressing patterns are not commonly seen, is that the wave height is so small they are not noticed except by sensitive scientific measuring instruments. Only great earthquakes produce visually observable waves that move at such high speed.

Even though the wave pattern may move rapidly in such cases, the actual volumes of water do not travel very far. If you were to go beyond the line of breaking waves at the seashore and watch the swell as it goes by, the little bits of seaweed and other debris floating in the water at different depths would rise and fall as the wave passes by. On careful observation, you would see that they move shoreward as the crest passes and seaward as the trough passes.

Figure 6-2 shows the orbital motion of small volumes of water (flagged by debris). Note that this motion dies away rather rapidly with depth, almost disappearing by the time the depth equals one wavelength. Note that the wavelength of the wave can also be described by the period of orbital motion associated with it. This decrease of water motion with depth has two practical applications. The first is that when a large swell comes in and is about to break, a swimmer may dive below the wave and experience much less commotion of the water. The second is that submarines can ride out the most violent of storms while submerged only 20 meters or so. Since water motion is confined largely to the surface, wave effects will have little influence on the sea bottom except near the shoreline. The principal water forces acting on the bottom of the deep ocean will be oceanic currents, to be described later in this unit.

Now that we have described wave patterns and motion, we should look into the causes of waves themselves. Most progressive water waves are driven by the wind, gradually picking up energy from the wind as it travels over the surface of the water. The longer the wind blows steadily, the more energy the waves acquire and the larger they become. Since the Pacific Ocean is larger than the Atlantic Ocean, this is the first reason why surfing is more successful in the Pacific; the waves are larger! A second reason, particularly applicable for North America, is that the prevailing wind direction is from the west. The highest sea waves generated during storms are estimated to be about 25 m (80 ft) in height. The relationship between the velocity of wind and the resulting waves have been studied by mariners for thousands of years. The Beaufort Scale of Wind Force is still commonly used .

• EXERCISE

  You can do a simple experiment to demonstrate the rapid decrease of wave energy with depth. All you need are a liter (quart) bottle, an empty tin can at least 15 cm (6 inches) in height, a kitchen sink or bathtub, and a towel (to wipe up afterwards). Cut both ends out of the tin can.

  Fill the sink two-thirds full of water and move the liter bottle rapidly up and down in the water to get a series of waves moving away from the point of application. Rapid motion is necessary so that the whole sinkful of water is not put into sympathetic oscillations. As the waves move away from the moving bottle they will decrease in amplitude.

  The next step is to insert the can with your other hand to a depth of 10 cm (4 inches) and hold it still. As the waves arrive in the vicinity of the can they will move around and beyond it, but the water in the interior of the can will hardly move at all. You might say to yourself, the rigid metal of the can is stopping the wave action. You would be right, but the wave energy should be able to enter the interior through the open bottom. According to the argument above, however, there will be little energy at depth.

  Now raise the can slowly so that less and less of it is beneath the surface of the water. As you do so, you will see the water on the interior of the metal surface become more and more disturbed until at the point where the can just grazes the surface (but never leaves it), the motion of the water inside the can will be nearly as large as outside the can.

2. Waves from Other Sources

Less frequent, but often spectacular, causes of ocean waves are great earthquakes that take place in the deep trenches of the ocean floor. Often, a dangerous seismic sea wave is triggered by the earthquake and moves out rapidly over the surface of the ocean. Since so much energy is released by the earthquake, even a small part of it can cause a large seismic sea wave, which is sometimes called a tsunami -- from the Japanese word for the phenomenon. The word tidal wave is a misnomer since the effect has nothing to do with tides. The rapid progress of the seismic sea waves associated with the 1960 Chilean earthquake is shown in Figure 6-3. This seismic sea wave traveled with wave velocities in excess of 200 meter per sec (450 miles per hour), crossing the entire Pacific Ocean in one day.

As rapidly as seismic sea waves move, the P waves sent out by the earthquake move even faster. From a quantitative examination of seismograph records it is possible to estimate whether such a wave will be excited by a great earthquake taking place somewhere around the edge of the Pacific Plate. It is then possible to send out radio warnings of possible damaging waves so that harbor and coastal areas may be evacuated and/or made secure. Several nations lying around the Pacific Basin have cooperated to form into a warning service to minimize possible damage and loss of life. However, people sometimes respond strangely to warnings. Once, when warned of an approaching seismic sea wave, many people did not evacuate but went down to the shore to watch it come in. Fortunately, the arriving wave was large, but not disastrous.

Far worse were the tsunamis caused by an earthquake off the west coast of the Japanese island of Hokkaido in 1993. These monster waves built to more than 30 meters (100 feet) high as they came onshore and were responsible for many of deaths caused by that earthquake. The tsunami from the 1986 Sanriku (Honshu, Japan) earthquake produced a 24 meter wave in which 26,000 people drowned. One of the most deadly events of this kind were the tsunami waves generated by the 1755 earthquake which occurred offshore of Portugal. This earthquake event caused considerable damage to the city of Lisbon, which was then devastated by a series of 5 meter tsunami waves. Including the additional destruction of the fire which followed, approximately 60,000 were killed.

Earthquakes cause another type of water disturbance known as a seiche. This is found in smaller bodies of water such as lakes. The water is set into motion as a whole without a wave pattern being developed. As the water moves toward one end of the lake, the water level rises while falling at the other end. A half-cycle later the water has gone the other way, increasing the level at the other end of the lake. During the Alaskan earthquake of 1964, the ice in several small lakes near the source region was cracked and showed from which direction the earthquake energy had come. Even more spectacular, though, was the setting into motion of the entire body of water comprising the Gulf of Mexico. This can only happen in the greatest of earthquakes.

Lastly, wave patterns can be caused by changing atmospheric pressure. High pressure depresses the ocean surface, while low pressure raises it. In a major storm, we may get a storm surge. The wind can pile up water against the shore in a storm surge, raising the level of the sea. If we combine high winds, low pressures, and high tides (as with a passing hurricane or typhoon), we may get exceptionally high levels of water with very large waves. The combination is capable of doing considerable damage. As a great storm approaches, all these effects are taken into account in the warnings put out by the weather service.

• Figure 6-3 Tsunami

  The curved lines show 5-hour intervals in the progress of a large seismic sea wave generated by a large earthquake in Chile.

  

3. Waves and the Shoreline

As the swell approaches the shoreline, two processes begin to act to change the fundamental nature of the wave pattern. As the water depth becomes less, the waves slow down in accordance with the rule governing the wave velocity in shallow water. That is, the wave velocity is proportional to the square root of the depth. Shallow, in this case, is defined as those areas where the water depth is less than one-half of the wavelength of the pattern of approaching waves. There is also a slowing force on the bottom comprised in part by friction and in part by returning water from the waves breaking on the shoreline. Since the same number of wave crests have to pass any given point during a certain time interval, we find that as they slow down, the distance between crests (the wavelength) must decrease. Since the wave energy is more concentrated, the wave height must increase. For a while the wave pattern endures, the swell growing in amplitude and the crests and troughs more closely following each other. This is shown near the center of Figure 6-4.

Figure 6-4 Changes in Wave Pattern as Shoreline is Met

As the swell comes even nearer the shore, the water cannot pile higher and higher. Water volumes in the crest tend to travel faster than water volumes in the trough and the crests then overtake the troughs, spilling over into them. If the sandy bottom is nearly level, the crests fill the troughs with a big rush of foam. If the bottom is somewhat steeper, we find breakers perfect for surfing. For very steep bottoms, the waves break right onto the sand itself. As waves break, the violently churning water can exert tremendous forces both upon the sand of the bottom and upon solid rock in the form of sea cliffs or isolated rocks lying above the sea surface. Some of the strongest erosional forces found in nature are generated at this energetic shoreline. However, it can be seen that as the shoreline is eroded away and the sand is distributed by the action of the breaking waves, the beach will become shallower and shallower. After a time, the incoming surf will no longer pound but merely break into foam. This foaming wave action is far less energetic and a wide shallow beach will be formed that is relatively stable. If all ocean waves approached straight in upon the shoreline, this would be all that happened. We might note at this point that the beaches of the eastern United States are of a more gently sloping nature compared to those of the western coastline where they are generally steep. Thus we have a third reason for better surfing on the west coast; breaking waves instead of foaming ones.

But the waves approach from the direction of the swell's source as modified by the local prevailing winds. This direction of approach may not be from directly out to sea. As the swell approaches the shoreline the early arriving crests begin to slow, allowing the following ones to catch up causing a bending of the wave crests. This process is known as wave refraction and is illustrated in Figure 6-5.

• Figure 6-5 Wave Refraction and the Longshore Current

  

Even though the principal effect of refraction is to make the swell approach the shoreline in a more nearly straight-on direction, the figure shows that there will still be a component of wave velocity parallel to the shoreline. This gives rise to a longshore current that, together with the breaking waves, can move beach sand parallel to the shore. If to the right of the figure there were no source of sand (say that the shore was formed from a seacliff of extremely resistant rock) then the longshore current would tend, over a period of time, to remove much sand from the area of the figure and move it out the left-hand side. Then the beach would narrow and be less desirable from many viewpoints.

The major source of beach sand is not the erosion of sea cliffs and shorelines but the influx of sand from rivers and streams as they enter the sea (or lake). Two human actions (in densely populated countries) have tended to reduce this influx of sand in recent decades. People have extracted even more water from shorebound rivers for metropolitan water sources. This water is often impounded by dams far from the shore where the load of sand and silt that the river carries is deposited in the bottom of a reservoir. Dams are also constructed to control flood waters, but the flood control basins they create have a similar effect. This tremendous load of sand and silt never reaches the sea. Thus on many shorelines, lakes as well as oceans, we see a net loss of sandy beach. Efforts to stop this longshore transport by the erection of rock or steel barriers merely slows this flow, temporarily saving "your" shoreline while depriving your "neighbor" of his source of sand and depleting his shoreline even faster. It is in problem areas of this type that understanding and cooperation are necessary to create the most beneficial environment while considering the competing needs of water supply, flood control, recreation, and scenic beauty.

C. TIDES

1. Tidal Forces

The earliest sailors knew that the Sun and the Moon were intimately linked to the tides but they did not have the theoretical basis for understanding their interactions in detail. Yet tides were so important to navigation that meticulous records of the tides were kept in every port so that ships would know when to set sail. The word "tide" appears in many current expressions in the sense of the best or safest time to do something. Today, with all our current scientific sophistication, there are still many questions to be answered about why tides have the heights and timing that they do. On the other hand, modern computers can predict (on the basis of previous observations) what the tides will do at any given port far into the future. Our first task will be to see how our nearest heavenly neighbors influence the tides.

Tides are caused by the gravitational attraction of the Moon and of the Sun in combination with the movement of the Earth. As the Moon revolves about the Earth in its orbit, the Earth does not remain still, but revolves itself about the center of mass of the Earth-Moon system. Because the Earth is so much more massive than the Moon, this point is within the volume of the Earth itself.

In either case, the pull of gravity is matched by the acceleration (change from a straight line path) needed to keep the Earth or Moon in nearly circular orbits. This acceleration is known as centripetal (center seeking) acceleration and is described by Newton's laws of motion. For a rigid body such as the Earth, this centripetal acceleration as it moves in its orbit will be the same everywhere within the body (accelerations from rotational motion are taken into account elsewhere).

It may not seem that this should be so, but a simple experiment with a piece of cardboard pierced by three pieces of chalk will demonstrate the point. If the cardboard is moved, as in Figure 6-6, about a blackboard in any path (not just a circular one) but is not rotated, then the three pieces of chalk will all trace out the same path. Since the centripetal acceleration is related to changes in direction of motion, it will be the same at all points of the moving cardboard. So it is with the Earth.

• Figure 6-6 Rigid Body Paths

  

Even though the centripetal acceleration of a moving planetary body is the same everywhere within its rigid portion, this is not true of the gravitational attraction experienced by that body. Gravity lessens with distance, so that those portions of the Earth more distant from the Moon will experience slightly lessened gravitational attraction. For the Moon to maintain its (almost) circular orbit about the Earth, the centripetal acceleration it experiences will exactly match the Earth's gravitational attraction at the center of mass of the Moon. Likewise for the Earth.

The centripetal acceleration necessary to follow its small orbit about the center of mass of the Earth-Moon system will equal the gravitational pull of the Moon only at the center of the Earth. On the side of the Earth nearer to the Moon the centripetal acceleration will be less than the pull of the Moon's gravity, while on the far side of the Earth the centripetal acceleration will be greater than the pull of the Moon. The effect is described in Figure 6-7.

2. Tidal Bulges

If gravitational attraction and centripetal acceleration match at the Earth's center, there will be a mismatch at the surfaces nearest and farthest from the Moon. On the side nearest to the Moon, gravity will be the stronger and it will tend to pull the water of the oceans toward that part of the Earth. On the farther side, the rigid Earth will tend to pull out from under the fluid water and leave it behind. The net effect of this is to have not one, but two tidal bulges on the surface of the Earth. This double tidal bulge will remain in fixed relation to the direction of the Moon, while the Earth rotates under it once each 25 hours. The extra hour arises because the Moon is moving in its orbit. During the course of this rotation any given point will see two high tides and two low tides. We call this a diurnal (twice daily) tide.

• Figure 6-7 Gravitational Attraction vs. Centripetal Acceleration

  

The Sun also has a tidal influence, though it is only 46% as great as that of the Moon. Although the Sun is much more massive than the Moon, it lies at a much greater distance and so its gravitational influence is much reduced. The net solar tide-producing force is thus less than the lunar tide-producing force. The motions of the Sun and Moon are such that the Sun appears overhead once every 24 hours while the Moon takes approximately 25 hours to return to an overhead position. Thus the Moon will appear to move in the sky relative to the Sun. So also will the tidal bulges produced by the Moon move in relation to the those produced by the Sun.

The two twin bulges will be superimposed twice each month when the Moon is in the same portion of the sky as the Sun or when the Moon is directly opposite to the Sun. When the Sun and Moon are at right angles to each other, the twin tidal bulges will not coincide. When the tidal bulges do coincide, we have extra large tides known as spring tides while the smaller ones produced by lack of coincidence are known as neap tides. The two situations are shown in Figure 6-9.

• Figure 6-8 Tidal Bulges from the Moon and the Sun

  

The tidal pattern now becomes more complicated. The Earth rotates once daily beneath a tidal pattern that changes twice a month. The pattern that would be seen at the tidal equator would be as in Figure 6-9 and is known as the equilibrium tide. In it we see the diurnal tide slowly growing and lessening during a twice monthly cycle.

It would be nice if this were sufficient to fully describe oceanic tides. However we have left a lot of things out. First of all, the Earth's Equator is tilted by 23.5° with respect to the orbits of the Moon and Sun. This makes every other high tide higher than the ones in between. In addition, the rise and fall of the level of the oceans requires that great masses of water must race across the Earth's surface. But, as we have seen in the previous section, there is a limiting velocity to the water's movement. Consequently the tides lag behind the tide-producing forces in various amounts according to the depth of the water. Land masses such as peninsulas and large islands also get in the way of the moving water and add further time delays.

Not only is the physical mechanism of massive water movement incompletely understood, but the geometry is also very complicated. Since all these factors cannot be taken into account, we proceed from the knowledge that the Moon and Sun have a very regular (periodic) appearance overhead. If we can determine the local tides by accurate measurements, then they will always maintain the same amplitude and time delays relative to the lunar and solar positions and thus may be predicted. This makes accurate measurements of tides an important task in harbors and bays around the world.

3. Tidal Measurements

Measurements of the tides would be fairly easy if we did not have to contend with the ups and downs of the water level due to wave action. But, we can use what we have learned earlier to make a simple tide gauge that will largely eliminate the wave effect. One merely needs to insert a long pipe into the water so that (as in the experiment of the preceding section) the water surface inside the pipe will remain calm. The level of the water surface inside the pipe will thus record quite accurately the level of the tide. A sketch is shown in Figure 6-10.

• Figure 6-9 The Equatorial Equilibrium Tide

  

4. Other Tidal Effects

The tidal forces described previously also act upon the fluid atmosphere and upon the solid Earth itself. The atmosphere experiences tides but they have no noticeable physical manifestations as do oceanic tides. However, the motions of the atmospheric tides can be detected in several ways and are useful in studying atmospheric properties.

In putting together our model of tide-producing forces, we have treated the Earth as a rigid body. This is largely correct, but there is a slight deformation of the solid Earth as well as of the oceans. This earth tide, as it is called, is much less than the oceanic tide, but again it is definitely measurable using precise instruments.

However, the existence of earth tides may have an important consequence. It has long been known that the rotational rate of the Earth has been slowing down due to frictional effects of the tides. For a long time it was supposed that this frictional effect was produced as ocean water poured through narrow channels on the Earth's surface. On re-investigation, it has been found difficult to make a case for ocean tides as the source of friction. This has led investigators to look elsewhere for a source of frictional loss. One possibility that has been suggested lies in the plastic zone, or asthenosphere, of Earth's rocky interior. Not only can such a partially melted zone dissipate energy, but the dissipation itself will provide part of the heat to keep it in a partially melted state. However, this remains a hypothesis and we need to know the properties of the asthenosphere much better before this idea can be accepted.

• Figure 6-10 A Simple Tide Gauge

  

D. OCEAN CURRENTS

Early seafarers found that they had two different forces to contend with in their explorations: wind and ocean currents. Their large sailing ships did not tack well into the wind, and so they had to chart the prevailing wind directions and plot routes that would take best advantage of them. Ocean currents -- massive streams of moving water -- could speed them on their way if they took advantage of currents moving the way they wanted to go; or they could impede their progress if the current opposed them.

Fortunately, surface currents and winds tend to move in the same direction. There are many variations from this general scheme, however, as Benjamin Franklin discovered from his Nantucket sea captain acquaintance. The boundaries of ocean currents can be quite sharp and the wise sea captain pays close attention to their whereabouts.

This of course led to an early interest in charting ocean currents that persists to the present time. Our interest now extends not only to the currents that exist near the ocean surface, but also to those that ply the depths of the sea. A number of methods have been devised to measure ocean currents, and these may be divided into two classes: direct and indirect.

In direct measurements, the motion of the water is monitored either by a device that is fixed with respect to the ocean bottom or by a device that floats with the current. In the former, a buoy may be anchored to the bottom and the flow of water past it is measured and recorded. Given a sufficient number of such buoys (or the relocation of the buoy to a number of different sites), the measured directions of water flow can be plotted and compiled into a general picture of the extent and direction of the current.

Floating buoys go where the currents take them, and if their motion is monitored over a period of time, their path provides a direct map of at least a portion of the current. The simplest such device is a note in a sealed bottle, cast from a ship. The note directs the finder to contact the researcher with information of where and when the bottle was found. This provides rough information on where the current ends up, but little detail on how it got there. Even so, this is a very inexpensive way of gaining a broad-brush view of oceanic circulation. More sophisticated floating devices use radio transmitters to allow them to be tracked during their travels and later retrieved for reuse. They may also contain devices that permit the measurement of temperature and salinity.

The currents near the surface tell us only part of the story, however. In many places, currents at depth travel in different directions and at different speeds than those above. These may be of little interest to the sea captain, but they are of great importance to the oceanographer. One way of measuring them directly is through the use of buoys that are designed to float at a specified depth. This is accomplished by carefully adjusting the density of the float.

An object whose density is greater than that of the surrounding water will sink, while one whose density is less than that of the surrounding water will rise. Because the density of water increases with depth, all that needs to be done is to adjust the density of the float to exactly equal that of the water at the desired depth. This can be accomplished by adding weights to the interior of the sealed, hollow float. The buoy will then sink until it reaches the depth at which its density matches that of the water and there it will stay, moving horizontally with the deep currents.

There are also indirect methods for measuring ocean currents. Because of its dissolved salts, seawater conducts electricity. Whenever a conductor of electricity moves within a magnetic field, electrical currents are generated. This, in fact, is the way in which all electrical generators operate to supply our homes and cars with electricity. In a similar manner, the motion of masses of salt water in the ocean within the Earth's magnetic field also generates electrical currents within the water. These may be detected by metal plates towed behind a ship and the motion of the water deduced from the currents.

Still another indirect method for measuring ocean currents utilizes the measurement of density changes within the ocean. Once the dynamics of ocean currents is understood, maps of density variations may be used to calculate the resulting currents at depth. As you saw in the previous unit, the density of seawater is determined mostly by the salinity and temperature. The measurement of salinity and temperature profiles can be used to construct maps of density variations, which in turn serve to indicate the directions and speed of currents within the body of the ocean.

For many years, this was the principal method of measuring deep ocean currents and was accomplished through the use of a simple but ingenious device, called a Nansen bottle, for bringing seawater samples up from the depths. The Nansen bottle is lowered on a cable to the desired depth in an inverted position at which point the bottle is flipped over, trapping the water sample inside. A thermal sensor on the bottle records the temperature of the water when it is captured. The sample bottle is then retrieved and the seawater is analyzed in a chemical laboratory to determine its salinity. Because of its simplicity, reliability, and low cost, the Nansen bottle is still in use today.

E. OCEANIC CIRCULATION

Waves on the ocean surface are caused by the wind. In addition, the waves produce a rough surface on the sea upon which the wind can get a purchase. This is sufficient to set in motion the surface layers of the ocean, and provides the essential driving force for the major surface currents that are diagrammed in Figure 6-11.

HM 6-3 (A-B): Satellite images of Sea Surface Temperatures (SST). A) Satellite sensed sea surface temperature (image from NOAA). Although the first-order pattern of warmer water within the equatorial region is simple, highly complex horizontal flow occurs. If you include the earth's atmosphere the overall system is something akin to a gigantic heat engine, moving heat from the equatorial regions to the poles.

B) The sea surface temperature anomaly is defined when the image above is subtracted from an "average" for this time of year. Note the El Nino event off the western coast of northern South America. (Image from NOAA).

The connection between the winds and the currents is not so simple as it may seem, however. For one thing, the major currents such as the Gulf Stream and the Kuroshio Current off Japan contain water flows that are far too concentrated and rapid to be explained by the direct driving force of the wind.

What the winds do accomplish is the establishment of large circular vortexes, called gyres in the ocean basins. In the tropics, the trade winds tend to blow toward the west, while at mid-latitudes, the westerlies on average blow toward the east. Nearer the poles, the prevailing winds tend to blow once again toward the west. Note that the gyres nearest the equator rotate in a clockwise sense north of the equator and in a counterclockwise sense south of the equator, but that small gyres nearer the poles (such as the one in the north Pacific) tend to rotate in the opposite sense.

• Figure 6-11 Major Surface Ocean Currents

  

To gain further understanding of the operation of currents such as the Gulf Stream, it is necessary to understand the influence that the Earth's rotation has on the circulation of the oceans. This is called the Coriolis Effect.

1. The Coriolis Effect

The Coriolis Effect describes the tendency of any moving mass on the surface of the Earth to be deflected from its path to the right in the northern hemisphere and to the left in the southern hemisphere.

The Coriolis Effect may seem at first like some kind of mysterious force, but in fact it is simply the result of the tendency of moving bodies to want to move in straight lines. Look at Figure 6-12a, which looks down on a rotating platform from directly above. You might visualize the device as similar to the rotating platforms or merry-go-rounds that are often found in children's playgrounds. The platform is rotating in a counterclockwise direction as shown by the circular arrows.

• Figure 6-12 A Demonstration of the Coriolis Effect

  

Imagine that you are standing on point (Y) and you throw a ball directly toward a friend who is at point (F). The direction of the ball is shown by the heavy straight arrow. From the time that the ball leaves your hand it will move in a straight line. In the meantime, however, you will have moved with the platform to point Y' in Figure 6-12b, and your friend will have moved to point F'. The ball will arrive where your friend used to be at point F, but now it is clearly going to miss its intended target. The ball will travel far to the right of your friend (as seen by you), in spite of the fact that you initially threw it directly toward him. From your point of view, the ball appeared to curve to the right.

From a vantage point above the rotating platform, it is clear that the ball did not curve to the right; both you and your friend traveled in curved paths to the left while the ball traveled straight ahead. But to someone on the platform (especially if they are not aware that they are in a rotating frame of reference), all moving objects will appear to veer off to the right of their intended paths.

The Coriolis Effect operates in a very similar way on the rotating Earth. Because the Earth rotates from west to east, the effect in the northern hemisphere is the same as that in Figure 6-12. A moving water or air mass appears to want to veer off to the right as seen by someone fixed on the surface of the rotating Earth.

Because the Earth rotates only once in 24 hours, we are not aware of being in a rotating frame of reference. Only the wheeling of the Sun, Moon, and stars through the vault of the heavens serves to remind us that we are living on a spinning ball. From our point of view, the Coriolis Effect acts like a curious kind of force that bends moving objects off to one side of their intended path. For this reason, it is sometimes referred to as a fictitious "Coriolis Force."

If you understand the concept of the Coriolis Effect, you should be able to see from Figure 6-12 that it is less severe when the motion of the object is very fast compared to the speed with which the platform turns. If you throw the ball faster, it will miss your friend by a lesser amount. On the other hand, if the platform is turning very quickly and you throw the ball slowly, you will miss him by a greater margin. For this reason, the Coriolis Effect is most noticeable for slow-moving objects such as water and air masses, and less noticeable for fast-moving objects such as bullets (though it is still present to a slight extent).

To see that the Coriolis Effect works to veer moving objects to the left of their paths in the southern hemisphere, we would only need to reverse the direction of the rotating platform in Figure 6-12. Here is a simple exercise that you can try for yourself to see how it works. As seen from directly above the South Pole, the Earth now appears to be rotating in a clockwise direction. The Earth always rotates from west to east, but seen from the South Pole, east is to your right, while seen from the North Pole, east is to your left. Hence the appearance of an opposite sense of rotation.

Phonograph turntables rotate in a clockwise sense, and so we can use one to demonstrate the Coriolis Effect in the southern hemisphere. With a pencil, make a small hole in the center of a sheet of paper and place it on the turntable with the spindle through the hole. If your phonograph has a stationary spindle, you may have to make the hole large enough so that the paper will turn freely. Next trim the edge of the paper so it does not overlap the turntable edge -- you don't want the paper corners to strike the pickup stylus when the turntable is in motion. As a final step, you may need to tape the paper to the turntable in one or two places to keep it from slipping. Start the turntable, making certain that the phono arm does not contact the paper. If you have an automatic changer, you may have to intercept the arm as it drops toward the paper and gently return it to its rest. If you don't have a phonograph (CD players won't work for this!), use a lazy susan, bar stool, or any other rotating platform turned by hand. Be sure you are rotating the platform in a clockwise direction.

Now take a marking pen, a crayon, or soft pencil, and while the turntable is moving, try to draw a straight line on the paper. That is, move the pen in a straight line across the paper. Do this in a variety of directions: move the pen toward the left, toward the right, toward the spindle and away from it. Try to note which is the beginning and end of each line. Stop the turntable and examine the marks on the paper. If you have trouble determining which is the beginning and which is the end of each mark, the beginning probably starts more abruptly, with the mark disappearing more gradually at its end.

Remove the paper from the turntable and put an arrow head on each line showing the direction that the pen moved, pointing from the beginning toward the end of the line. Note that in every case, the line bends toward the left, corresponding to the Coriolis Effect in the southern hemisphere.

2. The Gulf Stream and Other Major Ocean Currents

Now let us see how the Coriolis Effect influences the ocean currents. Consider the case where surface winds are blowing to the north in the northern hemisphere. Surface waters will be dragged along, but will be deflected to the right (east) by the Coriolis Force. Water at progressively greater depths is dragged along by the layer immediately above. But the Coriolis Force always acts to the right of the direction of motion, so water is progressively deflected farther and farther to the right as we go to greater depth. In fact, at some point, we will find that the water at depth is flowing in the opposite direction to that of the wind. Due to frictional losses, however, the return flow is much less than the forward flow near the surface. Even so, the net flow of water in the moving layer turns out to be to the right at roughly a right angle to the wind direction.

The layer of water that is set in motion is called the Ekman layer, after the Swedish oceanographer (V. W. Ekman) who described it in 1902, and the net flow of water to the right of the surface winds is called the Ekman-layer flow. Its action greatly complicates the motion of ocean water in response to the driving forces of the persistent winds. We shall only outline the process here without going into much detail.

We may use the North Atlantic gyre as an example, which is shown schematically in Figure 6-13. The westerlies blow from west toward east in the mid-latitudes, while the trade winds blow toward the southwest. To show only the essential processes, we show them as blowing toward the west. The Ekman-layer flow is always to the right of the wind direction, with the result that water is forced toward the center of the gyre. The accumulating water actually forms a broad mound one or two meters high in the center of the gyre, but the most important effect is that water is forced down beneath the mound. This downwelling current descends and spreads out.

HM 6-4 (A-B): Satellite images of Sea Surface Temperatures (SST) showing Gulf Stream. A) Sea surface temperature (SST) derived from satellite observations. Note the present of the Gulf Stream current. (Images from NOAA).

B) Anomaly in sea surface temperature, note the strong El Nino event visible along the left-bottom portion of the image. The land bridge between North and South America has a profound effect on global oceanic circulation and climate, in this case notice how the temperature anomaly in the central Pacific is completely blocked by this land mass and cannot spread into the Atlantic ocean.

To proceed farther, you need to understand what is meant by the horizontal component of the Earth's rotation. This refers to the extent to which a horizontal plane somewhere on the Earth's surface rotates about an axis that is perpendicular to the plane. You may think that a plane anywhere on Earth is rotating along with the Earth, and that is true, but at most places, only a portion of that rotation is contained within the horizontal plane. If you are standing on a horizontal plane at the North Pole, then it is easy to see that the plane is rotating horizontally, like a Frisbee in flight. If you are standing on a horizontal plane on the equator, you and the plane both are traveling with the rest of the Earth in a circle, but the plane of that circle is perpendicular to the plane upon which you are standing. For this reason, there is no horizontal component of rotation of the plane.

This, by the way, explains why the Coriolis Effect disappears at the equator. If the surface of the Earth is not rotating horizontally there, then there will be no tendency for a moving object to veer off to either the right or the left. As we travel from the equator toward either pole, the horizontal component of rotation increases and so does the Coriolis Effect. Take a moment and try to picture how this works in your mind's eye. Go back over the preceding two paragraphs if necessary.

As a result of this effect, any body of water that is traveling with the Earth has a particular rate of horizontal rotation that depends on its latitude, with the rate increasing from zero at the equator to a maximum at the poles. In the northern hemisphere, that rotation is counterclockwise, the same as that of the Earth itself.

• Figure 6-13 Converging Water in the Center of a Gyre

  Because Ekman-layer flow always moves to the right of the wind direction, the prevailing winds cause a convergence and downwelling in the Sargasso Sea in the North Atlantic gyre.

  

A device on display in many science museums demonstrates the effect directly. A long pendulum (usually several stories high) is set in motion along a particular line in the morning. As the day goes on, the line along which the pendulum is observed to swing appears to rotate clockwise (in the northern hemisphere) with respect to the floor of the building. Actually, the line of the pendulum's swing is staying as fixed as it can, and the floor of the building is rotating counterclockwise beneath it. The device, called a Foucault pendulum, would be pointless to install in a museum on the equator, since it would show no rotation at all.

Now let us return to the water that is sinking beneath the center of the North Atlantic gyre. That water shares the horizontal component of the Earth's rotation that is appropriate to its latitude. As it sinks and spreads out, however, the rotation is slowed, just as a twirling ice skater slows her spinning by extending her arms outward. Because of this, the water mass is now rotating slower than the ocean floor below it. As a result, the body of water migrates south to where its slowed rotation rate fits that of the solid Earth. Recall that the horizontal rotation rate decreases as you move toward the equator.

This effect causes the entire center of the gyre to slowly move toward the equator (Figure 6-14). To avoid emptying the northern oceans, however, this southward flow must be balanced somewhere by an equal northward flow. Any water moving away from the equator, however, must increase its counterclockwise rotation rate in order to stay in sync with that of the solid Earth. This can be accomplished by water moving north along the east coast of North America, where friction with the coast to its left provides the necessary torque to give the moving water its required counterclockwise spin.

• Figure 6-14 Operation of the Gulf Stream

  Based on R.W. Stewart, "The Atmosphere and the Oceans," Scientific American Vol. 221, p. 84, © September 1969. All rights reserved. Included with permission.

  

At last we have the reason for the narrow and powerful Gulf Stream. All the water that has moved south throughout the entire area of the North Atlantic gyre must now be funneled along the coast of North America in order to provide it with the additional rotation that it needs as it moves north.

The explanation is neither obvious nor easy to understand, so we shall summarize its most critical features: The prevailing winds create a vorticity or gyre that, combined with the Coriolis Effect, creates an Ekman-layer flow that causes downwelling in the center of the gyre. The descending water spreads out at depth and slows its rotation, causing the water to move toward the equator. The return path to the north is taken along the western margin of the ocean basin, where friction with the continent slows the western edge of the current. This imparts to the water the additional counterclockwise rotation that it needs in order to move north.

The last step in this process is not at all obvious from Figure 6-14, but may become clearer if you hold the page between thumb and forefinger at a point at the top center of the page (directly above Figure 6-14) and rotate the page counterclockwise about this point. This is roughly how the Earth rotates, and perhaps this will help you to see why water moving toward the north (that is, toward your fingers) will need to pick up additional counterclockwise rotation.

The same western intensification of ocean currents occurs in the Pacific, giving rise to the powerful Kuroshio Current off Japan. In the southern hemisphere, the large gyres circulate in the opposite sense, that is, in a counterclockwise direction. But because the Coriolis Effect and the resulting Ekman-layer flow are directed to the left in the southern hemisphere, the convergence and downwelling in the center of the gyre occurs once again. Going through the whole process again, we would find that the current would be intensified on the western edge, just as in the case for the northern hemisphere.

Of all these great currents, the Gulf Stream is the best known. From the Gulf of Mexico, it flows northeasterly off the coasts of Georgia and the Carolinas, casting out to sea at Cape Hatteras. Because the Gulf Stream carries warm tropical waters, beaches south of Cape Hatteras are noted for their warm water, but beaches just to the north of the cape have water temperatures that are distinctly cooler.

From Cape Hatteras, the Gulf Stream sweeps across the Atlantic Ocean and fans out against Europe. Perhaps you have noted that much of Europe stands at rather high latitudes compared to comparable climatic zones in North America or Asia. London is farther north than the northern tip of Newfoundland in Canada; northern Scotland stands at the same latitude as Juneau, Alaska; and Scandinavia shares the same latitude range as Siberia (see Figure II-5). The habitability of the northern tier of western Europe may well be due to the Gulf Stream, bringing warm water from the tropics to its shores. A competing theory, however, holds that cold currents moving south through the Denmark Strait between Iceland and Greenland create a situation in which warm water from the Mediterranean is drawn to the north, along the coasts of Spain, France, and England. Whichever is the case, northern Europeans can thank the prevailing patterns of oceanic circulation for a hospitable climate.

Traveling at a speed of a few kilometers per hour (as noted by Franklin's sea captain friend), the Gulf Stream is 50 to 75 kilometers wide but less than two kilometers deep. Even so, the volume of its flow is more than 100 times the combined flow of all the rivers of the world.

The Sargasso Sea, in the center of the Gulf Stream gyre, results from the downwelling waters described earlier. It got its name from masses of floating seaweed that accumulate over the converging currents. Early seamen told stories of ships that became hopelessly entangled in the seaweed, but in fact the ships were merely becalmed in the dead center of the gyre, between the regions of east- and west-flowing prevailing

3. Upwelling and Downwelling Currents

Compared to the horizontal motions of the surface and deep currents, vertical motion of seawater is surprisingly restrained. When we recall from the previous unit that a scale model of the oceans would resemble a sheet of paper, this seems even more curious. This behavior results from a stable arrangement in which the stratification or layering of the ocean is based on density, with the densest layers on the bottom. We have already seen earlier that the bottom waters are cold and saline, making them very dense. In most places, it would require an enormous amount of energy to raise these dense waters to the surface. In some places vertical mixing does occur, however, and these upwelling and downwelling currents have a number of important effects.

We have already discussed the downwelling that occurs in the center of the large oceanic gyres. Similarly, upwelling occurs in the center of the smaller gyres that go in the opposite direction, such as the Alaskan gyre in the northern Pacific Ocean (see Figure 6-11).

Along shorelines, we find that the Coriolis Effect and the Ekman-layer flow are important influences. Consider what can happen in the northern hemisphere when a persistent wind blows parallel to a shoreline, as shown in Figure 6-15. In the left diagram, the action of the wind is to force the surface water in the Ekman layer against the shore where it piles up and sinks, producing a downwelling current. In the right diagram, an upwelling current is formed.

• Figure 6-15 Downwelling and Upwelling Currents at a Shoreline

  The situation shown is for the northern hemisphere. In the Southern Hemisphere, reverse the wind directions. The straight arrows depict the movement of water in the Ekman layer, not that of water right at the surface of the ocean. In the northern hemisphere, Ekman-layer flow is to the right of the wind direction.

  

It turns out to be possible to measure the residence time of water in the deeper layers of the ocean. The method used is radiocarbon dating, a radioisotope technique similar to those discussed in Unit 1, A Sense of Time. The radioactive parent isotope is Carbon-14 (C¹⁴), which decays with a half-life of 5730 years. Carbon-14 is produced in the upper atmosphere by the bombardment of cosmic rays from outer space. The carbon dioxide in Earth's atmosphere is in equilibrium with respect to Carbon-14, with just as many being formed as are decaying in a given unit of time. Hence the concentration of Carbon-14 in atmospheric carbon dioxide (CO₂) is essentially constant. The most common isotope of carbon is Carbon-12, which is not radioactive. It serves as a standard against which to measure the concentration of Carbon-14.

So long as the CO₂ gas is in the well-mixed atmosphere, its C¹⁴/ C¹² ratio remains constant. As C¹⁴ atoms decay, they are replaced by new ones created in the upper atmosphere. Atmospheric CO₂ gas is dissolved in the ocean's surface waters, keeping the ratio in the upper layer nearly what it is in the atmosphere. However, once the water sinks into the deeper layers, it is cut off from the new supply of Carbon-14 in the atmosphere, and the C¹⁴/ C¹² ratio begins to decrease. In this way, it is possible to determine how long water has been away from the surface layer.

Radiocarbon dating of CO₂ in deep water yields ages on the order of 1000 years. This, then, gives us an indication of the time necessary for surface waters to circulate through the deep waters and to find their way back to the surface once again.

F. CLIMATIC EFFECTS

1. Interactions between the Oceans and the Atmosphere

In the previous unit we noted that because of its polarized nature, the water molecule has a number of unusual properties. One of these is that it has a very high heat capacity. Heat capacity is the amount of heat that must be put into one gram of a material in order to raise its temperature by one degree Celsius. Similarly, we might think of it as the amount of heat that escapes from one gram of a material when it cools by one degree Celsius. As its name implies, the heat capacity is a measure of the ability of a material to store heat energy when its temperature is raised and to release that same amount of heat energy when it cools.

Because of its high heat capacity, the water in the world oceans is an enormous reservoir for thermal energy. It serves to temper world climate, absorbing heat from the Sun in the tropics and redistributing it to more northerly regions via the great ocean currents. For this reason, the Gulf Stream is able to bring its warmth thousands of kilometers from the tropics to the European shores before cooling to the temperatures of Arctic waters.

Contrariwise, regions in warm latitudes that are visited by cold ocean currents enjoy a natural form of air conditioning. The delightful climate of the seacoast of California is due to this effect. The circum-Antarctic current (see Figure 6-11) travels from west to east, completely encircling the continent of Antarctica with a cold current and forming a partial barrier to the transfer of heat from lower latitudes to the frigid Antarctic landmass.

A full discussion of the influence of the oceans on world climate must wait until we have learned something of the workings of the atmosphere as well. We will do this in the next unit. For now, we look at one particular case where the nature of the ocean currents is of prime importance in determining climatic impact.

2. El Niño

The El Niño phenomenon occurs in the Pacific Ocean and usually its effects are felt most noticeably at the coasts of Ecuador and Peru in South America. In Spanish, the name means "The Child," an allusion to the Christ child. It is usually around Christmas time that fisherman notice the warming of the ocean surface that is its first sign.

Historically, Ecuadorians and Chileans welcomed El Niño. Its altered patterns of ocean currents and winds brought them gifts from the tropics such as floating coconuts. It was only after the fishing industry became highly developed in the 1940's and 1950's that the occurrence of El Niño took on a more sinister connotation.

The coastal waters off Ecuador and Peru are normally the site of an upwelling of deep bottom waters, bringing cold water to the surface. When these are replaced by warm waters during an El Niño event, the local anchovy fishing industry is dealt a massive blow as catches decline to near-vanishing levels. More important than this local effect are the the widespread climatic changes that can accompany a particularly severe El Niño. Such was the case with The Child's visit in late 1982. In its wake, torrential rains and flooding struck desert areas; parching droughts visited parts of every continent save Europe and Antarctica; tropical cyclones or hurricanes were spawned in record numbers in the Pacific; whole populations of sea birds were decimated; and even coral colonies on the sea floor were ravaged by the altered conditions. In a single event, a substantial number of the world's ecosystems were seriously disturbed.

To understand what happens during an El Niño event, it is necessary first to understand the normal situation just south of the equator in the Pacific Basin. Figure 6-16 presents a schematic view of the most important elements. The prevailing winds off the coast of South America are from the southeast. An upwelling of cold deep-ocean water is produced by the mechanism diagrammed in Figure 6-15, but note that the effect is reversed here. Off the coast of Peru we are in the southern hemisphere, and so a wind blowing from the south creates an offshore current and an upwelling, just as a wind blowing from the north would in the northern hemisphere. Don't forget that the Coriolis Effect works to deflect moving objects toward the left in the southern hemisphere.

• Figure 6-16 Schematic Cross Section of Pacific Basin in So. Hemisphere

  The diagram is not to scale, and greatly exaggerates the depth of the ocean.

  

In the previous unit, we mentioned that ocean bottom water is rich in dissolved oxygen and in nutrients such as phosphates, derived from decaying organic matter that has fallen from the surface layers above. Because of the relative scarcity of marine life at these depths, these life-sustaining resources are not consumed, and remain in the bottom waters. When upwelling occurs, however, the introduction of nutrient-rich water into the sunlit surface zone produces an explosion of life.

The process begins with plankton, marine organisms that simply float with the water and are not able to swim rapidly. Plankton range in size and variety from microscopic plants to jellyfish, and they form the base of the food web in the ocean. In the upwelling off Peru, anchovies are among the principal beneficiaries of planktonic abundance. The Peruvian fishing industries are based on the anchovy, with much of the catch going toward the production of high-protein fish meal. A portion of it, of course, goes to supply pizza-lovers all over the world.

At these latitudes, we are not far from the equator, and the Coriolis Effect is fairly weak. In fact, the Coriolis Effect goes to zero at the equator, because this is where the tendency to veer to one side switches from right to left. The westward movement of surface waters is aided substantially by the presence of the Trade Winds, which blow to the west across the entire width of the Pacific Ocean. The persistence of these winds causes warm surface water to pile up against the western edge of the basin, greatly thickening the surface layer there.

What we have drawn is a picture of the "normal" behavior of the ocean in this region, extraordinary though it may seem. The onset of El Niño is caused by a faltering of the persistent winds that drive the process. Once this happens, upwelling of cold water off the coast of South America ceases and the slanting dotted line in Figure 6-16 is no longer a stable configuration. Winds at the equator relax or blow eastward and surface currents actually reverse direction as the warm water piled up in the western part of the basin flows back toward South America. The dotted line separating warm surface waters from cold bottom waters becomes more horizontal, greatly thickening the surface water layer off the coast of Peru. In the process, ocean surface temperatures in the east may rise by as much as 8°C (14°F).

With a thick layer of warm water near South America, upwelling is contained entirely within the warm water. Nutrients from the bottom water no longer make it to the surface and the food web collapses. Each time a severe El Niño strikes, the fishing industry of Peru is crippled.

HM 6-6 (A-G): Satellite derived sea surface temperature anomalies  (El Nino) (Images from NOAA)	Date
	A) April 21, 1997
	B) May 1, 1997
	C) May 11, 1997
	D) May 21, 1997
	E) June 19, 1997
	F) July 9, 1997
	G) July 19, 1997
	TOPEX Satellite data

El Niño's effects can range far wider, however. During the 1982-83 occurrence, ocean surface temperatures were elevated in a band nearly a thousand kilometers wide stretching across much of the Pacific Ocean. The climatic repercussions were many and severe. The warm surface waters gave rise to heavy rains in Ecuador and the entire central Pacific region. Arid regions that normally see only centimeters of precipitation were deluged by more than three meters (nine feet) of rainfall, leading to widespread flooding and landslides. Thousands of homes were destroyed in Ecuador, and damages to crops and property amounted to more than 400 million dollars in a nation that could ill afford it.

Farther from the source, climatic effects were felt in many parts of the world. Flooding in the lower Mississippi valley was laid at El Niño's door. So too, were violent storms that brought a combination of flooding and severe wave damage to the west coast of the United States.

During El Niño, the tropical rain belt shifts from the eastern Pacific to the central Pacific. Thus, severe droughts afflicted portions of Australia, Indonesia, India, southern Africa, and Central America. Australia was particularly hard hit. The worst drought of the century turned much of eastern Australia to tinder and bushfires raged in many places, killing 75 people. Gigantic dust storms moved millions of tons of topsoil and the toll of livestock was staggering. Relief, when it finally came, was excessive. Heavy rains brought floods, severe soil erosion, and further decimation of the weakened herds.

In the Pacific, Hawaii's garden isle of Kauai was pummeled by a rare hurricane, and six major storms in five months raged through French Polynesia, which had not seen a single hurricane in the previous 75 years. Yet, surprisingly, it would appear that hurricane formation in the Caribbean region was actually suppressed. It is an ill wind that blows no good, and El Niño was no exception. Even as the anchovy population of the South American seas was being ravaged, other species moved in and flourished in the altered conditions. Shrimp, tuna, bonita, and dolphins multiplied and expanded their ranges, while on land, deserts burst into unaccustomed bloom. But there were many trade-offs. Vast areas of coral reefs perished in the soaring ocean temperatures, leaving only bleached and lifeless remains. In many places sea birds virtually disappeared as populations of fish upon which they fed shifted.

Life on Earth is extremely adaptable. When the environment changes abruptly, some species suffer and others prosper, taking over altered and vacated ecological niches. Mankind, too, has adapted his lifestyle and survival techniques to individual environments, and sudden change is almost always a hardship. Disruption of the status quo, whether done at the hands of man or by the whims of nature, more often than not brings disaster in its wake. By the time the 1982-83 El Niño had spent its fury, the toll stood at 3,000 dead and nearly nine billion dollars in damage.

What causes El Niño and when can it be expected? On average, they occur every four or five years, though not with any regular pattern. The previous one occurred in 1976-77 and, though not so severe, brought to the eastern United States a record cold winter and to California its worst drought. Clearly its effects are not always the same, since the 1982-83 visit brought the California coast storms and deluges. A later El Niño in the late 1980's was very mild, but the El Niño of 1991-93 was somewhat more pronounced and unusually long in duration. At this writing, it is still not clear if this El Niño contributed to the catastrophic flooding along the Missouri and Mississippi Rivers during the summer of 1993. What is clear from satellite observations is that another El Niño is developing and will strike during the fall of 1996.

HM 6-7: SST showing a very from El Nino event. A) Sea surface temperature (SST-NCEP) anomaly sensed from satellite. Note the very strong El Niño event present offshore of northwestern South America. (Image from NOAA).

The exact cause of El Niño is still not known. An immediate precursor to El Niño is a shift of a major persistent low-pressure cell normally found over northern Australia and Indonesia eastward into the central Pacific, a phenomenon called the Southern Oscillation. Its onset is apparently tied to the shifting of the monsoons of the Indian Ocean. There the winds and ocean currents change direction twice each year, alternately bringing drought and torrential rains to India.

The 1982-83 El Niño, perhaps the most severe of this century, prompted scientific investigations of international scope. The hope is to determine its causes sufficiently to allow prediction of its occurrence and severity. A major ten-year program was launched under the acronym TOGA (Tropical Ocean and Global Atmosphere). Should another El Niño occur during this period, it will be monitored by environmental satellites, research vessels, and a variety of instruments designed to sense oceanic and atmospheric conditions.

G. THE DRYING OF THE MEDITERRANEAN

As we learn more about the present-day oceans and their workings, it seems only natural to inquire about their ancient history. Plate tectonic theory assigns a very mobile nature to the ocean floor; continental drift implies that the very shape of the ocean basins has changed throughout geologic time. The reconstructions in Unit 3, Plate Tectonics, allow us to see what shapes the oceans have had at various times; our knowledge of the principles of ocean dynamics should allow us to reconstruct the ancient current systems within the oceans. This should also help us to model ancient climates as well, and we will look at efforts to do so later on, in Unit 8, Climates of Earth.

In a few cases we do not need to rely on theoretical models in order to determine the ancient state of the oceans. Direct evidence tells us of radically different oceanic conditions in the geological past. The Mediterranean Sea is a particularly interesting example .

Not every "sea" is an ocean. The North Sea, for instance, is a shallow body of water that covers continental rather than oceanic crust. Its waters communicate with the Arctic and Atlantic Oceans, but it seldom reaches depths greater than 100 meters (330 ft). The fact that oil has been discovered beneath its waters is a sure indication that we are dealing with continental and not oceanic crust. In essence, the North Sea is flooded continental shelf on Europe's northern border.

The Mediterranean Sea, on the other hand, is a genuine ocean basin. Its waters reach depths as great as 4,300 meters (14,000 ft), indicating that the crust beneath it is thin and oceanic in nature. Take a moment now and look at its situation and outline on the map in Figure II-3. Disregarding the presence of the Suez Canal, note that its sole communication with the world oceans lies through the narrow Strait of Gibraltar, separating Spain from Morocco by only 15 kilometers (9 mi).

The Mediterranean is a very warm sea, with a hot and sunny climate. As a result, evaporation is very high. Even though a number of major rivers such as the Rhone in France and the Nile in Egypt pour fresh water into it, more water is evaporated than the rivers can supply and the Mediterranean is consequently very salty. At the Strait of Gibraltar there is a net inflow of Atlantic Ocean water to replace the evaporative losses in the Mediterranean Basin. Actually, surface water flows into the Mediterranean, but dense saline waters flow out at depth into the Atlantic.

When the Aswan High Dam was being built on the Nile River, boreholes were sunk to determine the nature of the foundation for the dam. To the astonishment of the engineers, it was found that there was a deep canyon cut into the bedrock that was now filled almost completely with sediment (Figure 6-17). In addition, the lower portion of the sediments contained fossils of marine (sea-dwelling) organisms. When this discovery was first made, no explanation was known, and the mystery joined that of a similar deep gorge cut in bedrock below the Rhone River in France.

The mystery was solved when oceanographic research discovered that the floor of the Mediterranean held thick layers of sediment containing salt and gypsum deposits up to one kilometer thick. The only way in which these deposits could form is by the evaporation of seawater -- hence their generic name evaporite. To form evaporite beds of such thickness, it would have required the evaporation of the entire Mediterranean Sea. Earlier research based on sudden changes in the fossil record and on geological indications of a sudden change in local climate had hinted at such a possibility. Now proof was at hand and the great canyons of the Nile and Rhone Rivers could be explained.

The African Plate is gradually moving to the north, encroaching on the European Plate. The Mediterranean Sea is a closing ocean, perhaps one of the last remnants of the ancient oceans that predated Pangea. Its very name means "sea in the middle of land", and with a lower-case "m" the term might well be applied to other oceanic fragments like the Black and Caspian Seas. Approximately eight million years ago, the jostling between the African and European Plates apparently folded up a ridge of land at the western end of the Mediterranean Basin, blocking the Strait of Gibraltar. Cut off from its supply of ocean water, the Mediterranean would have dried up in only a thousand years, leaving behind a bed of evaporites on its floor.

• Figure 6-17 Cross Section of the Nile River Valley Gorge

  

During its time, this great dried-up ocean basin would have presented a geological spectacle not to be seen anywhere on Earth today. With its floor standing five kilometers (3 mi) below sea level, it would have been three times the depth of the Grand Canyon of Arizona and vastly larger in area. From many places on its "rim" (the present shoreline), the opposite side could not be seen, and temperatures well in excess of those found in Death Valley would have presented a formidable barrier to travel by any but the hardiest of creatures.

Rivers such as the Nile and Rhone that emptied into the basin would do so in spectacular waterfalls, their waters spreading out and evaporating completely on the baking sea floor. A huge waterfall of this type would have great powers of erosion, and it would soon migrate upriver, carving a deep canyon in the riverbed as it went, much as Niagara Falls is cutting a gorge as it retreats up the Niagara River. As far upstream as the Aswan Dam site, the bottom of the Nile gorge cut in solid granite stands at 210 meters (700 ft) below sea level.

About 5.5 million years ago, the great dam at Gibraltar was breached. This may have been accomplished by the normal processes of erosion in the newly uplifted land, or it may have been aided by earthquake activity along the plate boundary, which here cuts through the area of the strait. Whatever the cause, once a flow of water from the ocean was established, it quickly became a torrent. For a brief period this grandest of waterfalls fed the refilling of the Mediterranean, a process that may have lasted only a hundred years. Ten kilometers (6 mi) or more in width, with a flow a thousand times that of Niagara, it should have presented a truly impressive view. One wonders whether any ape-like ancestors to Homo sapiens that might have come across the scene had yet developed the capacity to appreciate it in an aesthetic sense.

H. OCEANOGRAPHY FROM SPACE

Consider for a moment the relative human population densities for the land and the ocean. Except within the most heavily traveled trade routes, a ship can travel for days and not see another vessel clear out to an unobstructed horizon. In the open oceans, the major shipping lanes are concentrated in a relatively narrow band of latitudes from N 30° to N 60°, and so it is only within these zones that frequent observations of phenomena such as wind and current speed can be carried out more or less continuously.

On the other hand, a single orbiting satellite may make several trips around the world each day, logging almost continuous records of a wide variety of observations. Seasat, the first satellite designed specifically for ocean measurements, failed prematurely after only 100 days. But during that time it produced a wealth of observations that have opened up whole new areas of study. For example, during its 100 days of operation, Seasat made approximately as many measurements of wind speed and direction as during the previous century of shipboard observations.

Another consideration has made satellite observation timely. In 1983 an international Law of the Sea Treaty was drawn up and signed by a number of nations (though not by the United States). The treaty restricts access to many coastal waters, a situation that ship-bound oceanographers find distressing. Satellite observations can help to alleviate this loss to some extent.

Oceanography from space developed somewhat indirectly at first, utilizing instruments on board satellites that were designed primarily for other purposes. Results from Landsat (land resources), Nimbus (weather), and the manned Skylab (space habitability and general science) satellites showed the potential for space-based ocean studies and led to the brief but highly successful Seasat mission.

Satellites are able to do far more than just take photographs of the ocean surface. Let us look briefly at some of the types of measurements that can be made from space. The temperature of the surface waters can be monitored by measuring radiation emitted from the ocean surface. All bodies emit electromagnetic radiation at wavelengths that depend on the temperature of the body. Measurements taken of radiation ranging from the infrared to the microwave region (see Figure 6I-3) make it possible to determine the ocean surface temperature to an accuracy of better than 1°C (2°F).

• Figure 6-18 Average Elevation of Sea Surface in NW Atlantic Ocean

  This is a contour map of the surface of the ocean, where each wavy line shows all points on the sea surface that stand at the same elevation. Adapted from R. W. Steward, "The atmosphere and the oceans", Scientific American. Included with permission.

  

  HM 6-9 (A-B): TOPEX sea surface topography. A) TOPEX sea surface topography image. New images are available daily (image from NOAA).

  B) Oblique perspective three dimensional representation of sea surface topography, refer to scale bar above.

Photographs and measurements taken in visible light show that the color of the sea surface conveys quite a bit of useful information. Clear deep water appears as a dark blue, while murky waters appear lighter. The presence of phytoplankton lends a distinct greenish cast to the ocean surface. Phytoplankton are microscopic marine plants that are the starting point for much of the oceanic food web; they are so incredibly numerous that they alone account for half of total global photosynthesis. By mapping the worldwide abundances of phytoplankton, we can obtain a global perspective of biological productivity in the oceans.

One of the most useful and versatile measurements that can be made from space is that of the exact distance from the satellite to the ocean surface. This is accomplished by using radar or laser ranging, in which a pulse of radiation is sent down from the satellite and bounced off the ocean surface. The time taken for the round-trip from satellite to ocean and back again is measured and this allows the distance to be determined to within 10 - 20 centimeters (4 - 8 inches). In the previous unit you learned that the sea surface mounds up over areas of high gravity, reflecting the presence of extra mass on the sea floor. In this way, massive structures on the sea floor such as volcanoes, plateaus, and the continental shelf can be seen clearly in maps that show the elevation of the sea surface.

Figure 6-18 shows the kind of detail that can be obtained by this method, where the edge of the continental shelf off the east coast of the United States and the undersea mountains supporting Bermuda show up as places where the contour lines appear very close together. This represents a relatively steep slope in the ocean surface.

When you stand on a beach and look out to the ocean on a calm day, you may find it hard to believe that the surface of the ocean is anything other than flat. Yet, from your first understanding of what a globe represents, you have known that the world is roughly spherical in shape, and that the ocean surface must share that shape. Now you can take your understanding a step further with the realization that the sea surface also contains undulations that reflect the mass distribution on the ocean floor. These undulations are not great, generally amounting to no more than a few centimeters of elevation change per kilometer of horizontal distance. Even so, in the roughly 2,400 kilometer (1,500 mi) horizontal distance shown in Figure 6-18, the total elevation change of the ocean surface amounts to more than 50 meters (160 ft).

The shape of the Earth as defined by the mean ocean surface is called the geoid. The shape of the geoid is determined by the distribution of mass within the Earth along with its rotation, and the gentle undulations that we have been discussing are determined mostly by the structure of the oceanic crust. There is, however, another but smaller effect on the elevation of the ocean surface and this is due to the currents. Currents produce bulges in the sea surface on the order of one meter (three ft), and the measurement of these small distortions allows oceanographers to deduce the presence of currents.

Because this effect is nearly 100 times smaller than that due to the geoid, the contribution of the geoid must be subtracted from the total measurement before the effect of the current can be seen. Fortunately, many ocean currents change with time, while the geoid does not, and this helps to separate the two effects.

Wave height may also be measured by radar ranging, providing information of use to mariners. Equally useful are satellite measurements of wind speed, obtained indirectly by measuring the roughness of the sea. The tracking of cloud movement with time also allows measurement of wind speed and direction, though not directly at the sea surface.

In views from space, ice floating in the water reflects sunlight brilliantly and stands out starkly against the darker open-ocean water. Measuring the extent of sea ice is useful not only for the tracking of icebergs that might be hazardous to shipping, but as a climatic influence and as a sensitive indicator of global warming. Having available up-to-date maps of sea-ice coverage is an important new aid to any ships that venture into the polar regions.

The availability of satellite-based oceanographic observations has raised a formidable problem for the modern oceanographer: that of dealing with all the data. The voyage of H.M.S. Challenger. M. S. ;cited at the beginning of the previous unit may put things into perspective. The results of that single voyage required 20 years to analyze and publish. Now we have satellites taking measurements continuously, filling the airwaves with data radioed back to laboratories on Earth. Stored in digital form, the data are available for scientists of many nations to study and interpret. Without the aid of computers, the task would be an impossible one.

Within the next decade, the quality and quantity of data from satellites should increase substantially. TOPEX, a joint United States-France effort, has built upon the initial success of Seasat and nowhas a measures the elevation of the sea surface, wind speed, significant wave height and water vapor. Multi-spectral band global imaging from the AVHRR satellite is allowing mapping of chlorophyll and plankton at a resolution of 1.1 km per picture element (pixel). Two new satellites are on the drawing boards for launches within the next decade. OCI will look at ocean color and map distributions of chlorophyll and the important plankton. Finally, GRM will map the geoid in great detail, providing a foundation for the interpretation of data from the TOPEX mission.

Practical applications for all this data are easy to find. We have already mentioned several examples of aid to shipping and yachting. The day-to-day operations of the merchant marine and armed forces naval vessels are influenced by the availability of up-to-date information on currents and sea-surface conditions.

The view from space of plankton distribution provides new information of critical concern to the fishing industry. Waters that are rich in plankton are also likely to be rich in fish. Combined with data on seawater temperature, plankton maps can be used by commercial fishing fleets to increase the efficiency of their operations, with a significant reduction of costs.

Perhaps the greatest beneficiary of oceanography by satellite will be climatic studies. The ability to predict oceanic changes such as El Niño would pay handsome dividends in lives saved and property damage averted. The prediction of long- and short-term weather also stands to gain much from satellite observations of the oceans because the interactions between the oceans and the atmosphere are so important in understanding the causes of weather. We shall return to this subject in the next unit.

• Figure 6-19 The Generation of Ring Currents by the Gulf Stream

  

Not all the observations from space are being made by robot scanners. Paul Scully-Power became the first oceanographer to make observations from space in 1984 when he rode the space shuttle Challenger into orbit and observed the world's oceans through the cockpit windows. Among the features that he reported seeing were intricate circular eddies in the Mediterranean and elsewhere. During the past few years, large rotating bodies of water have captured the interest of oceanographers as major features of the world oceans. Because they are large and yet occur on a scale smaller than that of the great oceanic gyres that occupy most of the major ocean basins, these are called mesoscale eddies, or middle-scale eddies.

The Gulf Stream is a prolific producer of these eddies, and most studies so far have been concentrated in the northeastern Atlantic. Though their existence has been known for 40 years, detailed studies have received considerable impetus in recent years because of the large-scale view afforded by satellite observations in the infrared, which clearly show eddies. Figure 6-19 diagrams the creation of eddies on both sides of the Gulf Stream.

In (a), as the Gulf Stream pulls away from the continental shelf and moves to the northeast across the Atlantic, it sometimes develops a meander that intensifies. The loop can become cut off (b) and take off on its own (c) as a ring current. Note that in (a) and (b) the core of the ring consists of water that has been brought over from the other side of the Gulf Stream. In this part of the Atlantic the water on the continental slope is very cold, while the water to the east of the Gulf Stream is part of the warm Sargasso Sea. As a result, the eddies consist of three distinctly different waters. In the case of the ring that appears south of the Gulf Stream in (c), the core of the ring consists of cold slope water, hence it is called a cold-core ring. The ring current itself is water that previously flowed in the Gulf Stream, while outside the ring current are the warm waters of the Sargasso Sea.

Now consider the ring that broke off to the north of the Gulf Stream. Its core waters came from the Sargasso Sea and are warm; hence it is called a warm-core ring. Once again, it is surrounded by a ring current of Gulf Stream water. Note that because of the way that they form, warm-core rings rotate in a clockwise direction, while cold-core rings rotate counterclockwise.

Cold-core rings range from 150 - 300 km (95 - 190 mi) across, while warm-core rings tend to be a bit smaller. One of these rings may rotate once every two to five days, and its total lifetime may range from months to a year or two. The temperature difference between the center of a cold-core ring and the surrounding Sargasso Sea can be substantial, amounting to 10°C (18°F ). Interestingly, the core of a cold-core ring extends right down to the ocean floor, while that of a warm-core ring extends less than halfway to the ocean floor. Each year, between five and eight rings are produced on either side of the Gulf Stream.

Marine biologists have found the mesoscale eddies to be ideal natural laboratories in which to study the effects of environmental changes on complex ecosystems. The cold slope water contains a high oxygen level and many nutrients, supporting abundant phytoplankton and a large population of organisms, though the total number of species represented is not great. On the other hand, the Sargasso Sea supports a great diversity of tropical species, but because its waters are low in oxygen and nutrients, the total biomass is much lower.

When a ring current is formed, populations of organisms from the one ecosystem are transported into the other and cut off from their own. For a while, they feel no ill effects, since the core of the ring current serves as a closed aquarium preserving the original ecosystem on the other side of the Gulf Stream. But as the ring current weakens and begins to die, mixing occurs and the ecosystem begins to change. One change that was noted in the cold-core rings was that as warm water began to invade, cold-water species of a shrimp-like crustacean migrated to greater depths in an attempt to find colder water. Eventually, however, the trapped population of cold-water organisms began to starve in the nutrient-poor water of the Sargasso Sea.

Mesoscale eddies undoubtedly exist in conjunction with the other great currents of the world. It has been estimated that a sizable fraction of the total energy of world oceanic circulation is tied up in them. The physics, chemistry, and biology of mesoscale eddies are just beginning to be worked out, and questions about them are far easier to come by than are answers. Nonetheless, they are a major feature of the oceans and oceanographers will be devoting considerable attention to them in coming years.

I. THE EXPLORATION OF INNER SPACE

Just as mesoscale eddies have become a fascinating new line of investigation for the marine scientist, so has the study of midwater organisms for the marine biologist. Until recently, marine biologists were restricted to the uppermost 30 meters of the open sea that is accessible to the scientist equipped with SCUBA gear. That, certainly, is the region with the richest and most diverse collection of life, and the one that is by far the most familiar to us. It is the region dominated by sunlight, chlorophyll, and phytoplankton. What was known of life in the greater depths was obtained by towing nets behind ships. Few creatures could survive such collection intact, let alone alive, especially the fragile life forms that tend to inhabit the midwaters.

Now marine biologists, using submersible research vessels, are venturing into the realm of the midwater to study at firsthand the life that inhabits the largest environment on Earth. Because of its depth as well as it breadth, the volume of the midwater is more than 200 times greater than the habitable space on all the landmasses put together. Richard Harbison, of the Woods Hole Oceanographic Institution, eloquently explains why he has chosen to study midwater organisms:

• …the midwater environment is so different from the one we know that it is the best comparative system we biologists have until life is discovered on another planet. Imagine, if you can, what it must be like to live in a world without sun, where the only light is produced by other living things. A world without boundaries, where you can move in any direction, where there is virtually no turbulence, current, or motion, where there is no need to struggle against the force of gravity, where there are no seasons. That's why I study midwater animals -- to try to understand what is possible for life in this most alien of all the environments on this planet. A better understanding of how animals live in midwater cannot help but teach us more about how animals live in general, and how they interact with their environment.

Because of the absence of sunlight, in the middle depths of the ocean there are no living phytoplankton -- chlorophyll is useless in the absence of light. There are, however, a wide variety of zooplankton -- animals that drift with the water -- ranging from microscopic creatures to jellyfish and including some fish and squids. The majority are transparent and gelatinous whose body tissues are mostly water. These fragile creatures are well adapted to feeding in the quiet, stable midwaters of the world oceans.

At present we know very little of these organisms. New species are being discovered with almost every dive, and knowledge of their behavior is almost totally lacking. We do not know how they capture prey and avoid predators, what their mating rituals are, or even how long they live. The use of research submersible vehicles such as Alvin now allows the biologist to visit them in their own habitat, but dive time is expensive and scarce. Marine biologists look forward to the possibility of constructing a deep ocean habitat from which observers can operate for extended periods of time. Without such a facility, a sizable part of the marine population will remain shrou