September 28, 2010 : Ocean Gyre

Ocean Gyre

A gyre in oceanography is any large system of rotating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis Effect; planetary vorticity along with horizontal and vertical friction, which determine the circulation patterns from the wind curl (torque). The term gyre can be used to refer to any type of vortex in the air or the sea, even one that is man-made, but it is most commonly used in oceanography, to refer to the major ocean systems.

The following are the five most notable gyres: North Atlantic, South Atlantic, Indian Ocean, North Pacific and South Pacific.

Tropical gyres are less unified and tend to be mostly east-west with minor north-south extent. These include the Atlantic Equatorial Current System, Pacific Equatorial Current System and the Indian Monsoon Gyres.

The center of a subtropical gyre is a high pressure zone. Circulation around the high pressure is clockwise in the northern hemisphere and anticlockwise in the southern hemisphere, due to the Coriolis effect. The high pressure in the center is due to the westerly winds on the northern side of the gyre and easterly trade winds on the southern side of the gyre. These cause frictional surface currents towards the latitude at the center of the gyre. The build-up of water in the center of the gyre creates equatorward flow in the upper 1,000 to 2,000 m (3,300 to 6,600 ft) of the ocean, through rather complex dynamics. This equatorward flow is returned poleward in an intensified western boundary current.

The intensified western boundary current of the North Atlantic Gyre is the Gulf Stream, in the North Pacific it's the Kuroshio Current, in the South Atlantic it's the Brazil Current, in the South Pacific it's the East Australian Current, and in the Indian Ocean it's the Agulhas Current.

Subpolar gyres form at high latitudes (around 60°). Circulation of surface wind and ocean water is anticlockwise in the Northern Hemisphere, around a low-pressure area, such as the persistent Aleutian Low and the Icelandic Low. Surface currents generally move outward from the center of the system. This drives the Ekman transport, which creates an upwelling of nutrient-rich water from the lower depths. 

Subpolar circulation in the southern hemisphere is dominated by the Antarctic Circumpolar Current, due to the lack of large landmasses breaking up the Southern Ocean. There are minor gyres in the Weddell Sea and the Ross Sea, the Weddell Gyre and Ross Gyre, which circulate in a clockwise direction.