A gyre in oceanography is any large system of circulating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity along with horizontal and vertical friction, 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.
- Indian Ocean Gyre (generally flowing counter-clockwise)
- North Atlantic Gyre
- North Pacific Gyre
- South Atlantic Gyre
- South Pacific Gyre
Tropical gyres are less unified and tend to be mostly east-west with minor north-south extent.
- Atlantic Equatorial Current System (two counter-rotating circulations)
- Pacific Equatorial Current System
- Indian Monsoon Gyres (two counter-rotating circulations in northern Indian Ocean)
The center of a subtropical gyre is a high pressure zone. Circulation around the high pressure is clockwise in the northern hemisphere and counterclockwise 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. These cause frictional surface currents towards the latitude at the center of the gyre.
This build-up of water in the center creates flow towards the equator in the upper 1,000 to 2,000 m (3,300 to 6,600 ft) of the ocean, through rather complex dynamics. This flow is returned towards the pole in an intensified western boundary current. The boundary current of the North Atlantic Gyre is the Gulf Stream, of the North Pacific Gyre the Kuroshio Current, of the South Atlantic Gyre the Brazil Current, of the South Pacific Gyre the East Australian Current, and of the Indian Ocean Gyre the Agulhas Current.
Subpolar gyres form at high latitudes (around 60°). Circulation of surface wind and ocean water is counterclockwise 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.
Recently stronger winds, especially the subtropical trade winds in the Pacific ocean have provided a mechanism for vertical heat distribution. The effect are changes in the ocean currents, increasing the subtropical overturning, which are also related to the El Niño and La Niña phenomenon. Depending on natural variability, during La Niña years around 30% more heat from the upper ocean layer is transported into the deeper ocean. Several studies in recent years, found a multidecadal increase in OHC of the deep and upper ocean regions and attribute the heat uptake to anthropogenic warming.
The influence of the Coriolis effect on westward intensification
Because the Coriolis effect is strongest near the poles, water flowing eastward at high latitudes turns sooner and is "short circuiting" the gyre. At the equator the Coriolis effect is nonexistent and the water flows westward until it encounters a blocking continent. Westward boundary currents are therefore faster and deeper than eastern boundary currents, and the geostrophic hill is offset to the west.
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