The Dynamic Ocean Study Guide (page 2)
The oceans are not stagnant giant bathtubs of salty water. They are continually in motion. Indeed, the oceans are fluid. Forces upon them make them swirl at the surface and mix into their most abyssal depths. Here we will look at the various mixing processes in the world's oceans.
Surface Currents and Gyres
The ocean's tides are caused primarily by the moon and, to a lesser extent, by the sun. As the tides rise and fall, the ocean's water is stirred. But as you know from playing in the surf at the beach, the winds are another potent factor in stirring the sea.
Everywhere on Earth, the winds blow across the ocean's surface, sometimes softly, sometimes with violent atmospheric storms such as in hurricanes. The winds stir the water, like when you blow across a cup of coffee to which you just added cream.
But the winds are not powerful enough to mix the ocean all the way down. The ocean is too simply too deep. How deep can the winds stir the sea into a uniform layer? The depth of intense mixing by the wind is called the mixed layer. On average, the mixed layer is about 100 meters in depth. It gets deeper when the winds are stronger and shallower during weaker winds. Scientists, of course, want to know the exact relationship and so they study the mathematics of how the depth of the mixed layer varies with wind speed. Very roughly, doubling the speed of the wind doubles the depth of the mixed layer.
Because the water within the mixed layer moves up and down during its stirring, tiny creatures that live in the mixed layer also go up and down, in and out of more intense light. The mixed layer is roughly the same as the layer where light is available to phytoplankton in the ocean. This surface zone has the technical name of pelagic zone. Most of the ocean's living things stay in the pelagic zone; for creatures that do not photosynthesize, feeding on those that do is important, and the photosynthesizers are found in the pelagic zone.
The creation of the mixed layer is one major effect of the winds on the stirring of the seas. For a second effect, we turn to the large-scale wind patterns of the atmosphere, specifically the tropical easterlies and the midlatitude westerlies. The combination of these two kinds of winds (which exist, you will recall, in both hemispheres), make huge whirlpool-like circulations in the ocean called gyres. Think of the ocean waters as fluids that are brushed around by the winds, like the way you could make water move with a broom. Just as one might start a floating inner tube spinning by pushing one side with the left hand and pulling the otherside with the right, the easterly winds in the tropics and the westerlies in the midlatitudes combine to propel gigantic surface gyres in the ocean basins.
Consider the situation of the Atlantic Ocean in the Northern Hemisphere, which sits between the combined mass of Africa and Europe to the east and North America to the west. On the southern side are the tropical easterlies, which push the south side of the region of the Atlantic from east to west. In the midlatitudes, the winds are westerlies. They push the northern edge of the zone of the Atlantic Ocean from the west to the east. Thus, the northern edge is pushed from west to east, the southern edge from east to west. Both pushes reinforce each other to make the ocean start to spin in a clockwise direction. It's just like using your two hands in opposite directions of opposite sides on the inner tube to start the inner tube spinning. These clockwise gyres exist in both the North Atlantic and North Pacific.
The Southern Hemisphere presents a different situation. Think of the South Atlantic zone. It has east erlies to the north (in the tropics) and westerlies to its south (in the southern midlatitudes). Thus, the South Atlantic is pushed east to west on its northern edge and pushed west to east on its southern edge. When looking at a globe, that means right to left on the upper edge, and left to right on the lower edge. The resulting spin of the ocean gyre is counterclockwise. That is exactly the situation in the South Pacific, South Atlantic, and Indian Ocean (which is mostly in the Southern Hemisphere).
Because the easterly and westerly winds are results of the earth's Coriolis effect, we can say that the ocean gyres are also a result of the earth's Coriolis effect, which of course comes from Earth's spin.
For the most part, the ocean gyres turn rather slowly. You couldn't watch them move before your eyes, for example. They are much slower than the winds, which might take only days to travel across the oceans. The time frames of the ocean gyres are measured in months or even years.
However, in certain regions, parts of the gyres can become concentrated and truly flow as currents. The most famous example familiar to us in the United States is the Gulf Stream. But if you lived in Japan, you would more readily recognize the name of the Kuroshio Current. And you would know what it is. The Kuroshio Current is an intense giant current of water that flows north, along the eastern coast of Japan. The Kuroshio Current and the Gulf Stream (which flows from the Caribbean, along Florida, then goes further offshore and all the way to the far northern Atlantic) have been called rivers within the sea. The Southern Hemisphere has its own rivers within the sea, also, such as the Brazil Current off Brazil.
These rivers within the sea, for dynamical reasons, occur in the western portions of the ocean gyres. Thus, they are sometimes called western boundary currents. They do not occur along the southern, northern, or eastern potions of the ocean gyres, only along the west.
Effects of currents such as the Gulf Stream (see Figure 14.1) can be dramatic. The Gulf Stream effectively carries warm tropical water up into more northern, cooler waters. The Gulf Stream does mix somewhat with the cooler waters it encounters, but because it is so powerful and coherent, much of the Gulf Stream's water stays within the stream itself. Sometimes as far as New York City, tropical fish are found, having traveled in the warm Gulf Stream and then spun off into the cooler waters, searching for and never reaching their homes a thousand miles away.
The contact between the Gulf Stream and cooler waters to the north can cause smaller gyres, or eddies, called rings. Like the gyres, the rings are whirlpools. But unlike the gyres, the cause of the rings is not the winds, but simply the rubbing or friction between the Gulf Stream and the cooler waters it encounters. When the whirlpool-like rings are formed from the cold water, they are called cold rings. When the rings are spinoffs of the Gulf Stream itself, the rings are called warm rings.
The warm waters of the Gulf Stream have an effect on climate. Just north of Florida, around Cape Hatteras, North Carolina, the Gulf Stream leaves the coast of the United States and travels in water further offshore. The Gulf Stream, in fact, heads over in the direction of England, bringing warmth to the English and Scandinavian climates that those areas would never otherwise have. Some scientists are concerned that because of global warming, the Gulf Stream might weaken or shift course. Thus, it is possible that Northern Europe could grow cooler even while much of the rest of the world grows warmer, if the Gulf Stream were to change dramatically.
How the Deep Ocean Mixes
The ocean also has a second, very different kind of circulation that has nothing to do with the winds and would exist even in the absence of wind. The technical name for this circulation is the thermohaline circulation. The word is a combination of "thermo" for temperature and "haline" for salt, because these factors determine the density of water.
When water gets cold, say in winter at high latitudes, it becomes denser and tends to sink. When sea ice forms, also in winter at high latitudes, the freezing of fresh water into ice leaves the remaining ocean water saltier. At the same temperature, saltier water is heavier water and therefore tends to sink.
In summary, the driving factor behind the thermohaline circulation is the fact that denser water will tend to sink. Both of the processes of cooling water and increasing its salinity cause it to have increased density, and both processes occur at the high latitudes during winter.
These two factors create the densest water at certain high latitude regions, which are called sites for deep water formation or bottom water formation. One site is in the far North Atlantic, around Greenland and Labrador. The especially salty water in the North Atlantic comes not only from the freezing out of fresher water into sea ice in winter, but also from the fact that water from the relatively salty Mediterranean Sea also manages to contribute (it mixes into the north Atlantic). You see the situation is complex, and oceanographers are always working on refining their understanding of the intricacies of ocean circulation. But certain facts, which we will now review, have become well established.
Water of the far North Atlantic, during winter, becomes very dense and sinks to tremendous depths, almost all the way to the bottom of the ocean (but not quite, for reasons that will become clear in a moment). This is called North Atlantic Deep Water. The water travels south, deep under the ocean's surface, pushed by new deep water behind it, and unable to move upward because its heavy density holds it at a certain depth in the ocean. It even crosses the equator at a depth of 3 kilometers under the surface and enters the Southern Hemisphere and passes into the South Atlantic Ocean.
Then the North Atlantic Deep Water gets mixed into the Antarctic Circumpolar Ocean water. Around the continent of Antarctica is the second major place on Earth where deep or bottom water forms. This region is bitter cold in winter, and the waters around Antarctica become the coldest waters in the ocean. The Antarctic Circumpolar Current (recall this region also goes by that name) is so well mixed that virtually the entire water column for many hundreds of meters could be considered a mixed layer; and the vertical mixing is quite vigorous almost all the way to the bottom.
Some of the North Atlantic Deep Water that enters the Antarctic Circumpolar Ocean is spun up to the surface where it cools during winter to become not only the coldest water on Earth, but also the most dense water on Earth. It sinks downward and then is known as Antarctic Bottom Water. Because it is the densest water on Earth, it dominates the sinking process and reaches deeper than does the North Atlantic Deep Water. In fact, some of the Antarctic Bottom Water travels north into the Atlantic Ocean underneath the North Atlantic Deep Water.
The Antarctic Bottom Water also travels into the Pacific and Indian Oceans. It goes all the way along the bottom in a wide swath a kilometer or so in vertical extent, reaching as far as the North Pacific.
The northern part of the Indian Ocean is still in the tropics (near southern India), so its surface waters never get cold enough to become deep water. But what about the North Pacific? It also experiences intense winters (think Alaska). Well, the deep and bottom waters of the world are, in a sense, in competition with each other. The densest waters make it to the bottom and become the bottom water. The next densest waters make it not quite as far and become the deep water. The waters of the North Pacific get not quite as dense as those that become either the North Atlantic Deep Water or the Antarctic Bottom Water. It is possible that, at other times in Earth's history, when conditions of climate, rivers, and continental positions were different, the North Pacific did produce deep or bottom water.
Consider the geometry of the ocean. Its average depth is about 4 kilometers, or about 2.5 miles. And the typical width of an ocean is many thousands of miles. The distance from the places that form the North Atlantic Deep Water to the equator is about 4,000 miles or more. Consider now the ratio between the ocean's depth and its length, using . That's a ratio of 0.000625, or . The ocean is not deep at all, compared to its length! And yet the ocean's layers of density cause it to be so stratified that the deep and bottom waters can travel thousands of kilometers and stay at their depths, confined within those layers of density. Like the solid earth, the ocean is sratified by its density.
A dramatic demonstration of this stratification can be seen by oceanographers who pull up samples of deep water from high-tech buckets that can be lowered down into the ocean deep, then closed to trap the water they encounter. When these insulated buckets are hoisted to the surface and opened up, the water inside is very cold, just a couple of degrees above freezing. This happens even at the equator, where some of the warmest water on Earth is at the surface!
We've now discussed the deep and bottom waters of the world, traveling around way under the ocean. But that situation can't go on. New deep and bottom water has to come from the surface and that requires taking as well as replacing surface water. How do the deep and bottom waters of the world circulate back to the surface? The answer is little by little, gradually here and there. The amount of bottom water that reaches the North Pacific, for example, is less than the bottom water that entered the South Pacific from the Antarctic Circumpolar Ocean. As the bottom water travels northward into the Pacific, it sheds some of its mass upward toward the surface. This happens because of mixing processes, similar to the way that the Gulf Stream loses some of its flow as it mixes during its travels.
So the return flow to the surface of the deep and bottom waters occurs everywhere. This is different from the situations that form the deep and bottom waters, which happen only in special local places in the North Atlantic and around Antarctica. The return flow to the surface is distributed throughout the world's oceans and is given the term upwelling.
Recall that the levels of nutrientions phosphate and nitrate are almost zero at the ocean's surface but are high down deep in the ocean. How do the nutrients get back to the surface from which they were removed by photosynthesis? The answer is via the ocean's upwelling. The upwelling of the deep waters, which happens everywhere, carries the phosphate and nitrate nutrients back to the surface to be used again by the phytoplankton.
The thermohaline circulation is very powerful, but the ocean is huge. It takes, on average, about a thousand years for the thermohaline circulation to make a complete cycle around the ocean. The stirring time of the world ocean is thus about 1,000 years. In that time period, the entire ocean, from its surface to its deep abyss, is mixed. That seems like a long time to us, but it's short compared to the time scales of geology. In fact, that time is almost instantaneous.
Practice problems of this concept can be found at: The Dynamic Ocean Practice Questions
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