Sinking water does most of the driving in the oceans.
Warm water rises, cold and salty water sinks, and that loop carries tropical warmth toward the poles.
Kurzgesagt – In a Nutshell · The Gulf Stream ExplainedIt is easy to picture the oceans moving the way air moves, pushed around by wind. Wind does move water in part, but differences in density are primarily responsible for the movement. The main motor is the weight of the water itself.
Oceanographers call the system the thermohaline circulation. Thermo means temperature, and haline means salt content, and together the two decide how dense, and therefore how heavy, a patch of water is. Warm water is lighter and rises. Cold water is heavier and sinks. Saltier water is also heavier, because dissolved salt adds density.
That difference sets the whole ocean in motion. Near the equator, the sun beats down especially hard. Heat evaporates huge amounts of water, leaving the salt behind, so the remaining surface water gets warmer and saltier. That warm water starts to flow away from the tropics toward the poles, carrying heat with it. As it travels north, it cools, gets denser, and eventually sinks into the deep ocean, while colder water rises elsewhere to take its place.
Strung together, those risings and sinkings form what scientists call the ocean conveyor belt. It is not one current but a combination of many currents that link four of the five global oceans into a single worldwide loop.
The loop helps power the global climate system. Along with wind systems, it carries tropical warmth toward polar regions, and in doing so helps set the weather, the climate, and much of what depends on them.
Questions people ask
If warm water rises, why does warm, salty water in the tropics eventually sink?
Temperature and salt pull in opposite directions. Warm water is light, but very salty water is heavy. Near the equator, evaporation leaves extra salt behind. As that warm, salty water flows toward the poles it cools, and the combination of cold and salt makes it dense enough to sink.
How long does it take water to go all the way around the conveyor belt?
Estimates vary by path, but a commonly cited figure is around 1,000 years for a full loop. Surface currents move relatively fast, while deep water creeps along at only a few centimeters per second. So water sinking today in the North Atlantic will not return to the surface for centuries.
Which ocean is left out of the conveyor belt?
The Arctic Ocean is the one usually excluded when scientists say four of the five oceans are connected. It exchanges water with the Atlantic and Pacific through narrow straits, but it is largely enclosed by land and sea ice. The Atlantic, Pacific, Indian, and Southern oceans form the main loop.