This site defines thermohaline circulation. Cold water, in general, is denser than warm water. Likewise, water with a high salinity is denser than water that contains less salt. Surface ocean currents are primarily driven by winds. Deep ocean currents, on the other hand, are mainly a result of density differences...

A significant characteristic of the large-scale North Atlantic circulation is the poleward transport of heat. Heat is transferred in a northward direction throughout the North Atlantic. This heat is absorbed by the tropical waters of the Pacific and Indian oceans as well as of the Atlantic and is then transferred to the high latitudes, where it is finally given up to the atmosphere.

Concern has been expressed that global warming could lead to massive changes in ocean currents such as the Gulf Stream. Currents in the ocean are responsible for about half the work of the climate system in redistributing heat between the equator and the poles. This redistribution makes some parts of the world warmer or cooler than they would otherwise be, and therefore changes to currents could affect some regional climates—either magnifying or partially neutralising some of the global warming changes in those places.