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As Earth's wind systems set ocean water in motion, circular gyres are produced. If you look at a map of winds in the North Atlantic, you will see that winds originate from the west in the north and from the east in the south. These southern winds are called trade-winds. Ekman currents always carry water to the right of the wind in the northern hemisphere as seen in the figure.
The Ekman currents push warm water to the center of the ocean. This surface convergence creates a mound of surface water that is elevated up to one meter above the equilibrium sea level.
The mound of water pushes out from the center. This is a pressure force. The water does not flow out because the Earth is rotating. Instead, it goes around the high at a speed just fast enough that the Coriolis force exactly balances the pressure force. This kind of flow is said to be geostrophic flow.
In the northern hemisphere, the flow is clockwise around highs, just as air flows clockwise around highs in the atmosphere. The flow is counterclockwise around lows in the ocean. Lows are depressions formed when Ekman currents push water away from an area in the ocean.
Notice that currents can be clockwise or counterclockwise in the northern hemisphere even though the Coriolis force is always to the right of the flow looking downstream. Coriolis force does not set the direction the flow turns. The mounds and depressions in the sea caused by Ekman currents, which are caused by the wind, set it.
North Atlantic part of the Global Conveyor Belt. (courtesy Woods Hole Oceanographic Institution)
As the water flows in the ocean, it carries heat from one ocean to another, and from the equator to near the poles. Variations in the heat transport lead to variation in weather patterns. The oceanic component of the heat-transport system is called the Global Conveyor Belt. The heat from the sun shining on the entire Atlantic Ocean is carried northward by currents. The heat warms the air in the far north Atlantic. The heat warms up Ireland, England, Norway, and the rest of northern Europe. Northern Europe is far warmer than Labrador and northern Canada especially in winter. Yet Norway is further north than Labrador. As the ocean loses heat in the far north, the water gets colder and colder. Finally it gets so cold and dense it sinks to the bottom. It flows along the bottom and gradually comes up in different parts of the ocean to complete the conveyor belt circulation.
Sometimes the Global Conveyor Belt slows down. When this happens, Europe becomes cold. Sometimes the current stops, and the northern hemispheres enter an ice age. See The Role of the Ocean in Weather for more information.
Another important variation in ocean circulation is the change in the equatorial circulation known as El Niño (See our El Niño page).
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