Besides atmospheric circulation, ocean water circulation also contributes to the continuous re-distribution of excess heat of the equatorial zone. This thermohaline ocean circulation is density driven and redistributes not only heat, but salt and dissolved gases as well. It is sometimes called the great or ocean conveyor belt. In a simplistic view of the great conveyor belt (see Fig. 4.7), warm, salty surface water is chilled in the North Atlantic and eventually sinks to flow south towards Antarctica. There, it is cooled further to flow outward at the bottom of the oceans into the Atlantic, Indian, and Pacific basins. After upwelling primarily in the Pacific and Indian Oceans, the water returns as surface flow to the North Atlantic, therewith again supplying heat to the polar zones. The surface flow is primarily wind-driven and is confined to a layer of typically 50 m to 100 m of well-mixed water.