Global Ocean Circulation and the North Atlantic
The interaction of unequal temperatures, surface winds, and density differences in the ocean influence the flow of water through Earth’s ocean basins. Ocean water moves in many ways, including surface currents, gyres, eddies, and meridional overturning circulation cells. Recall that surface currents are influenced by surface winds pulling on the ocean’s surface. Gyres are giant, permanent circular currents that flow around entire ocean basins. Eddies are smaller, temporary swirling currents only tens of kilometers wide. Meridional overturning circulation describes the movement of water at different latitudes (north/south) and ocean depths (surface/deep) averaged over long periods of time. (Meridional means along a longitude line, or along the north–south axis.) These patterns of water movement combine to form a complex system of global ocean circulation that distributes energy from the equator to the poles, and around the planet.
In the Atlantic, meridional overturning circulation spans the northern and southern hemispheres. The warm surface waters make up the upper portion of the overturning circulation. As warm surface waters move northward from equatorial regions to the high northern latitudes, they cool and increase in salinity and density. There, in the Nordic and Labrador Seas (near Iceland and Greenland), the cooled, dense waters sink. This dense, deep water mass remains quite cold as it spreads southward, forming the bottom portion of the overturning circulation at depths of approximately 1,500 meters (almost 5,000 feet) and greater. In the Southern Ocean, the deep, dense waters rise and return to the surface.
The North Atlantic region in particular plays a key role in global ocean circulation because of its unique geography. Compared to the Pacific basin, the Atlantic basin is smaller and saltier due to less input of freshwater. Thus, the sinking of very salty, extremely dense masses of water occurs predominantly in the North Atlantic.
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