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A gyre is any large system of ocean currents moving in a circle, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect: because the Earth is constantly rotating, ocean currents in the northern hemisphere tend to move in a clockwise direction and currents in the southern hemisphere in an anti-clockwise direction. The Coriolis force acts most strongly on the wind, and the wind then creates a torque that tends to spin the ocean currents.
The Earth rotates from the west towards the east, and the Coriolis effect becomes more intense toward the poles. Because of this, gyres form especially strong currents on the western side of oceans (so that they are pushed against the eastern coast of the continents). The vorticity (that is, the strength of the torque force) is balanced out by frictional surface currents, which normally act against the spin of the water. However, in the middle of the ocean, the effects of friction are very weak.
The following are the five largest gyres:
Tropical gyres are not as unified and tend to be mostly east-west with minor north-south extent.
The centre of a subtropical gyre is a high pressure area, which the ocean moves around. The high pressure in the centre is due to the westerly winds on the northern side of the gyre and easterly winds on the southern side of the gyre. These cause frictional surface currents towards the latitude at the centre of the gyre.
Subpolar gyres form at high latitudes (around 60°). The ocean moves around a low pressure area. Surface currents generally move outward from the centre of the system.
- "Gyres and the Coriolis effect". Blue Planet: Infobursts. British Broadcasting Corporation. http://www.bbc.co.uk/nature/blueplanet/infobursts/gyres_bg.shtml. Retrieved 6 October 2013.
- Geoffrey K. Vallis. Climate and the Oceans. pp. 83–85. http://books.google.com.au/books?id=F4rYhTgM344C&pg=PA84.
- B. Heinemann; Open University (1998). Ocean circulation. Oxford University Press. p. 98.
- Lynne Talley, Physical Oceanography Research Division. "The five most notable gyres" (PowerPoint Presentation). University of California, San Diego. http://www-pord.ucsd.edu/~ltalley/sio210/Wind_circulation_NAtl_SHem/lecture_natl_shem.htm. Retrieved 6 October 2013.