Like circulation in the atmosphere, circulation of water within the oceans helps transport the excess solar energy at the equator towards the poles. Like the atmosphere, this flow of energy poleward is driven by uneven heating and is influenced by the Coriolis effect. Unlike the atmosphere, the oceans are warmed from the top down rather than from the bottom up, so heat-driven convection is not enough to cause ocean circulation.
Wind blowing along the surface can move large amounts of water by friction, but to get vertical mixing from top to bottom, some difference in density with less dense water sitting under higher density water is required. Density-driven circulation does mix the oceans, but temperature alone is not sufficient. The oceans are warmed from above by the sun.
This results in stratification with warmer, less dense water sitting on top of cooler, denser water. If this was all that was going on, then no mixing would happen. But there is another factor that influences density of seawater, salinity.
Ocean salinity, that is the concentration of salt in the water, is fairly constant over space and time, but small variations do occur due to patterns in evaporation and precipitation, which cause surface salinity to vary between regions of high and low rainfall. The zone of precipitation associated with the low pressure system around the equator causes lower salinities near the equator. Salinity then increases through the mid-latitudes due to evaporation where the lower portion of the Hadley cell moves dry air towards the equator.
Salinity then decreases again towards the poles where there is an increase in precipitation associated with the low pressure region between the ferrule and polar cells. All else being equal, the saltier water gets, the denser it becomes. This means that both temperature and salinity need to be considered when looking at how water density changes over space and time.
Due to warming from above, temperature causes stratification at low latitudes. The transition region between the top layer of warm water and cooler water below is called the thermocline. As we move poleward and the amount of solar energy reaching the surface of the Earth goes down, the thermocline breaks down and temperature from top to bottom in the ocean evens out. Close to the poles, there is little or no temperature difference from surface to bottom.
The evening out of density creates a situation where mixing from top to bottom is possible. but simple mixing is not sufficient to drive density-driven circulation, in which dense water sinks to the bottom near the poles and then flows equatorward under less dense water. In order to get density-driven circulation from high to low latitudes, changes in both temperature and salinity are required. The way this occurs near the poles on Earth is through the process of ice formation, because as salt water freezes to form ice, the ice crystals exclude the salt, increasing the salinity of the water left behind.
So in regions near the poles where significant ice formation occurs, the exclusion of salt, which ends up in the surrounding water, leaves that water cold enough and salty enough to be denser than the surrounding water, allowing it to sink. This triggers a slow but important large-scale circulation cycle. This sinking of dense water triggered by ice formation only occurs in a few places on Earth, two in the North Atlantic and in one location near Antarctica in the Southern Ocean. The sinking water in the North Atlantic spreads southward below the surface water creating a mass of cold dense deep water referred to as the North Atlantic bottom water.
The bottom water formed in the southern ocean spreads around the southern ocean moves up into other large ocean basins. This Antarctic bottom water is the densest water in the oceans and fills the bottom of much of the earth's oceans but while denser than the North Atlantic bottom water not as much of it is formed. So the bottom water formed in the North Atlantic moves all the way through the South Atlantic into the Southern Ocean, and along with Antarctic bottom water, fills the depths of all the Earth's oceans. The process by which this deep water slowly mixes with other water masses and eventually makes it to the surface is not well understood, but eventually, in regions of upwelling and through slow mixing of different water masses, this deep water reaches the surface.
Once at the surface, this water warms, and slowly completes the cycle returning from the Indian and Pacific Oceans through the Southern Ocean into the Atlantic and back to the places where deep water is formed. This is a very slow process. It takes upwards of 1,000 years to complete the cycle.
This process of dense deep water formation in three locations drives a slow, steady movement of warm surface water towards the poles, transporting heat in the process. This is called thermohaline circulation. It is also referred to as the ocean conveyor belt. and it is responsible for moving significant amounts of heat from the equator towards the poles.