The Great Ocean Conveyor Belt
The great ocean conveyor belt is an example of a density-driven current. These are also called thermohaline currents, because they are forced by differences in temperature or salinity, which affect the density of the water. We prefer the term density currents though because it is more direct, and doesn't sound quite so intimidating.
The great ocean conveyor belt begins as the coolest of all currents, and we mean that quite literally. But this is a conveyor belt we are talking about, so let's start just before the beginning:
The Gulf Stream delivers warm, and relatively salty surface waters north to the Norwegian Sea. There the water gives up its heat to the atmosphere, especially during the frigidly cold winters. The surface waters cool to near freezing temperatures, at which time they become denser than the waters below them and sink. This process continues making cold water so dense that it sinks all the way to the bottom of the ocean.
During this time, the Gulf Stream continues to deliver warm water to the Norwegian Sea on the surface. The water can't very well pile up in the Norwegian Sea, so the deep cold water flows southward. It continues to flow southward, passing the Equator, until it enters the bottom of the Antarctic Circumpolar current. It then drifts around Africa and Australia, until it seeps northward into the bottom of the Pacific.
The cold bottom waters eventually warm and rise towards the surface. There they flow out towards the Indian Ocean, around the tip of Africa, and back into the Atlantic. Eventually these same waters are transported northward by the Gulf Stream back to the Norwegian sea. This is what is referred to as the great ocean conveyor belt. It is the primary mechanism for flow along the bottom of the world's oceans.
Now as you can see from the map, this is a long distance to travel. And the flows are exceedingly slow. So the entire round trip may take up to 1000 years!
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