The Dynamic Ocean Study Guide
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.
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