The majority of the movement and circulation of water and energy throughout the ocean is driven by differences in density between adjacent water masses. Density is dependent on the salinity and temperature of the water. The thermohaline circulation, or Global Conveyor Belt, is initiated by density differences and distributes heat energy between tropical and polar regions. Rainfall is higher in the tropics, and this addition of freshwater into the ocean decreases the salinity, and, therefore, the density, of the ocean waters in these areas. The colder oceans receive less rainfall because of lower evaporation, and, thus, are saltier and denser. The higher density of colder portions of the oceans relative to the tropics creates a difference in sea height, with colder oceans depressed, and warmer oceans elevated, relative to each other.
The density of near-surface water in the tropics is further decreased by solar heating; however, its salinity is increased by evaporation. This upper, less dense, but more saline tropical water, thus moves in a poleward direction over the colder water. As it moves north, it releases heat to the atmosphere and the lower ocean and is thereby cooled. As it cools, its high salt content makes it denser relative to the water it is flowing over, and therefore sinks. This overturning circulation varies from year to year and as well as longer timescales. Changes in precipitation, runoff, ice melt, solar heating, and winds can strengthen or weaken the conveyor belt. This can affect short-term weather conditions, but can also alter climate in the long term if the circulation pattern settles into a new equilibrium.
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