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Deep ocean currents

Like gigantic conveyor belts, the major oceanic currents flow at various depths. So-called thermohaline circulation is the driving engine of the deep currents: Cold, salt-laden water is heavier than warm water and sinks into the depths. To replace it, warmer water masses flow in. These then cool and follow the sinking waters.

 

The largest-scale "pump" of this kind is found in the North Atlantic. The Gulf Stream, which moves five hundred times as much water as the Amazon, is part of the worldwide system of currents. Warmed by the Caribbean sun, it flows northwards along the North American coastline until it reaches subpolar waters. Between Greenland and Norway, arctic winds cool the salt-rich water masses from the tropics almost to the freezing point.

 

The high level of salinity of this water makes it so heavy that it sinks to the depths - at a rate of flow of about 17 million cubic metres per second in winter, twenty times the flow volume of all the rivers on earth. At a depth of two to three kilometres, the cold water then begins its trip around the globe: southwards through the Western Atlantic Basin to the Antarctic Circumpolar Current and from there into the Indian and Pacific Oceans. In upwellings, for instance off the coasts of Peru and California, seawater that sank to the depths centuries before rises once again to the surface.

 

Whereas winds mix the surface layer of the oceans and can push them around at consisiderable speeds - the Gulf Stream moves at up to 9 km/h - deepwater currents move very slowly, at most 0.36 km/h, so it takes about 1,000 years for water to circulate around the entire globe on these global conveyer belts.