The thermohaline circulation (Great Ocean Conveyor) in the ocean is the flow of surface and deep ocean water currents driven by temperature and density differences. The density of sea water depends on variations in temperature and salinity. Fresh water fluxes coming from rain, ice-melt and river runoff decrease salinity. Ocean water evaporation and the formation of ice in the ocean increase salinity. As opposed to the currents driven by the wind, the thermohaline circulation does not occur in the ocean's surface, but consists of a great overturning of the ocean's water.
Two driving forces
The circulation of the water in the ocean is typically driven by two geological factors: the wind-driven circulation affecting the upper ocean's currents, and a circulation driven by water density variations affecting the deep water currents. The thermohaline circulation includes an overturning of the ocean currents consisting of a flow of warm water near the equator to the poles which, upon reaching either north or south pole, is turned into cold water, sinking and flowing back to the equator as deep water currents in the deep ocean basins.
While the driving force on the surface of the ocean is mainly the wind, in the deep ocean, the driving force differences in temperature and salinity. Colder and more saline water tends to be denser. The density of ocean water is not homogeneous; therefore, in order to gain stable density values, it has to flow. This is what provides a driving force to deep ocean currents. The thermohaline circulation owes its creation to the formation of deep-water masses in the North Atlantic and the southern oceans.
Deep water mass formations
The dense water masses forming the thermohaline circulation form in only two ocean regions. The water mass sinking off Greenland is known as the North Atlantic Deep Water (NADW), and the water mass sinking off Antarctica is known as the Antarctic Bottom Water (AABW).
In these regions, surface ocean currents are cooled by the polar wind. Evaporation of sea water increases the salinity of water, making it more dense. Since colder and more saline water is more dense, it sinks down into the deep ocean, flowing southward through the submarine basins connecting Greenland, Iceland and Great Britain and then flowing along the abyssal plains of the Atlantic in a southward direction around the tip of Africa, where the flow is broken into two flows: one flowing into the indian ocean and the other into the Pacific ocean through Australia.
The undersea currents of saltier and colder water in the Atlantic make the sea level of the Atlantic slightly lower than the Pacific. This produces a slow flow of sea water from the Pacific Ocean into the Atlantic ocean. The deep water masses sinking into the ocean have to displace other less dense masses of water. It is thought that an upwelling has to occur somewhere in the ocean. Some Scientists believe that the upwelling has to occur in the North Pacific, and others think that deep water upwelling occurs in the southern ocean. While there is not a definite consensus, there is further research that needs to be done in this regard.
The Antarctic Bottom Water
Another major region of deep-water formation is the coast of Antarctica, where deep-water masses known as the Circumpolar Deep Water (CDW), 500 meters below the fresh-water ice sheets, sinks deeply into the continental shelves of Antarctica, where it is cooled down below zero degrees C. The deep water formed in Antarctica is denser and sinks below the NADW. The mixture of Antarctic shelf water and CDW becomes the Antarctic Bottom Water (AABW), flowing in the deepest regions of the ocean and into the Atlantic, Indian and Pacific Oceans.
The thermohaline circulation is key to regulating the amount of sea ice at the poles by supplying heat. It is believed that global warming can affect the thermohaline circulation by reducing the density of ocean water at the poles and inhibiting the formation of deep-water currents. According to cdiac.ornl.gov, an increase in CO2 concentration would increase rainfall at the North Atlantic, increasing the freshwater on the surface of the ocean, thus preventing the sinking of the surface water and shutting down the thermohaline circulation.