The Earth's climate

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Thermohaline circulation

At tropical latitudes, the water at the surface of the oceans receives a considerable supply of heat that enables it to reach temperatures between 25 and 30 degrees Celsius. In the polar regions, surface water reaches the freezing point of seawater, which is about –2 degrees Celsius.
At depths comprised between a few dozen and several hundred meters, a layer of water called the "Mixed Layer" has a uniform temperature, close to that of the surface due to the action of the wind and waves. Beneath this layer, under the thermocline that designates a zone of wide temperature variation, exchanges of mass and energy are sharply reduced. Therefore, most of the ocean is made up of cold, high-density water. This water originates from the zones where surface water, having become cold and salty and thus very dense, is pulled by gravity under the warm, less saline water to a depth where it balances the density of the surrounding water (about 2,000 meters in the Subarctic region, 4,000 meters in the Antarctic, or 1,000 meters in the Western Mediterranean). The depths of all the oceans are filled with these waters, whose temperature, overaging 0 to 4 degrees Celsius, varies only slightly from the poles to the Equator. These so-called "deep convection zones" are few and far between. They are located in the higher latitudes, mainly in the Labrador Sea and off the coasts of Greenland and Norway, and to a lesser extent in the Weddell Sea. But they are also found in the Mediterranean, where the succession of Mistral winds in late winter can generate very high-density water.
The slow mixing of the ocean waters has been diagramed in the form of a global-scale "conveyor belt." The deep waters, which mainly form in the North Atlantic, flow in the direction of the South Atlantic. At about 60 degrees South, they join the Antarctic Circumpolar Current, in which they move west to east, gradually rising towards the thermocline and spreading throughout the South Atlantic, Pacific and Indian Oceans. The return of this massive flow to the North Atlantic takes place through warm currents near the surface, whose circulation is linked to the atmospheric currents. The entire process can take from several hundred to a thousand years.
The intensity of this circulation and the location of deep water formation sites vary between glacial and interglacial periods. During a glacial period, the presence of pack ice in the higher latitudes prevents the formation of deep water.