Gulf Stream - Formation and Behavior

Formation and Behavior

A river of sea water, called the Atlantic North Equatorial Current, flows westward off the coast of northern Africa. When this current interacts with the northeastern coast of South America, the current forks into two branches. One passes into the Caribbean Sea, while a second, the Antilles Current, flows north and east of the West Indies. These two branches rejoin north of the Straits of Florida, as shown on the accompanying map.

The trade winds blow westward in the tropics, and the westerlies blow eastward at mid-latitudes. This wind pattern applies a stress to the subtropical ocean surface with negative curl across the north Atlantic ocean. The resulting Sverdrup transport is equatorward. Because of conservation of potential vorticity caused by the northward-moving winds on the subtropical ridge's western periphery and the increased relative vorticity of northward moving water, transport is balanced by a narrow, accelerating poleward current, which flows along the western boundary of the ocean basin, outweighing the effects of friction with the western boundary current known as the Labrador current. The conservation of potential vorticity also causes bends along the Gulf Stream, which occasionally break off due to a shift in the Gulf Stream's position, forming separate warm and cold eddies. This overall process, known as western intensification, causes currents on the western boundary of an ocean basin, such as the Gulf Stream, to be stronger than those on the eastern boundary.

As a consequence, the resulting Gulf Stream is a strong ocean current. It transports water at a rate of 30 million cubic meters per second (30 sverdrups) through the Florida Straits. As it passes south of Newfoundland, this rate increases to 150 million cubic metres per second. The volume of the Gulf Stream dwarfs all rivers that empty into the Atlantic combined, which barely total 0.6 million cubic metres per second. It is weaker, however, than the Antarctic Circumpolar Current.

The Gulf Stream is typically 100 kilometres (62 mi) wide and 800 metres (2,600 ft) to 1,200 metres (3,900 ft) deep. The current velocity is fastest near the surface, with the maximum speed typically about 2.5 metres per second (5.6 mph). As it travels north, the warm water transported by the Gulf Stream undergoes evaporative cooling. The cooling is wind-driven: Wind moving over the water causes evaporation, cooling the water and increasing its salinity and density. When sea ice forms, salts are left out of the ice, a process known as brine exclusion. These two processes produce water that is denser and colder (or, more precisely, water that is still liquid at a lower temperature). In the North Atlantic Ocean, the water becomes so dense that it begins to sink down through less salty and less dense water. (The convective action is not unlike that of a lava lamp.) This downdraft of cold, dense water becomes a part of the North Atlantic Deep Water, a southgoing stream. Very little seaweed lies within the current, although seaweed lies in clusters to its east.