A network of surface and deep ocean currents constantly circulate ocean water from one side of the globe to another. The image above illustrates the path of the great ocean conveyor belt, also known as the thermohaline circulation. Thermohaline refers to how changes in temperature ("thermo") and salinity ("haline") cause ocean currents.

Two characteristics affect water's density: temperature and salinity. In general, colder water is more dense that warmer water and salty water is more dense than freshwater. Changes to ocean water's temperature and salinity cause the ocean currents, which occur as a result of these changes in density. Temperature and light availability decrease with depth in the world's oceans, while water density and pressure increase with depth.

The unequal distribution of solar radiation on Earth's surface is one of the most important factors in the conveyor's circulation. The Sun warms surface water near the equator. This warm water then moves as a current from the tropics toward higher latitudes, where it cools and transfers its heat to the atmosphere. One such current, known as the Gulf Stream, originates in the Gulf of Mexico and carries warm water across the northern Atlantic Ocean. The heat it releases helps to keep northwest Europe warmer than other regions at the same latitude.

Water moving northward in the conveyor cools, and as a result it becomes denser than the warmer water arriving from the tropics behind it. The formation of sea ice at high latitudes also affects the density of water. As ice forms at the ocean surface, salt ions are left behind, increasing the water's salinity, which in turn, increases the water's density. Colder and saltier, this dense water begins to sink. Warm water from the tropics moves in to replace it at the surface, and it, too, begins to cool. The cold water descends to the ocean bottom where it forms what oceanographers call the North Atlantic Deep Water, a mass of water that fills most of the deep Atlantic basin.

In the Indian Ocean, upwelling occurs as deep water warms and rises to the surface, where it warms even more and provides moisture for monsoon rains. This warm water then swings back and joins the other surface currents flowing in from the Pacific. Together, they pass the Cape of Good Hope and head north, past the west coast of Africa and Europe, toward Iceland in the North Atlantic, where the 1,000-year journey begins again. The circulation of surface and deep ocean currents circulates heat and nutrients throughout the world's oceans.

The following video explains more about thermohaline circulation and upwelling.


Source: Teachers' Domain, Great Ocean Conveyor Belt: Part I, published October 21, 2005, retrieved on December 29, 2009,
http://www.teachersdomain.org/resource/ess05.sci.ess.earthsys.convey/