Oceanic gyre

A gyre is any manner of swirling vortex. It is often used to describe large-scale wind or ocean currents. 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). [Heinemann, B. and the Open University (1998) "Ocean circulation", Oxford University Press: Page 98 ]

Ocean gyres

The Earth's oceans have the following major gyres: [http://www-pord.ucsd.edu/~ltalley/sio210/Wind_circulation_NAtl_SHem/lecture_natl_shem.htm PowerPoint Presentation ] ]

* North Atlantic Subpolar Gyre
* North Pacific Subpolar Gyre
** Contains the smaller Alaska Gyre
* North Atlantic Subtropical Gyre
** Gulf Stream, Labrador Current, East Greenland Current, North Atlantic Current, North Atlantic Equatorial Current
* North Pacific Subtropical Gyre also known as North Pacific Gyre

This gyre comprises most of the northern Pacific Ocean. It is located between the equator and 50º N latitude and occupies an area of approximately ten million square miles (34 million km²). The North Pacific Gyre has a clockwise circular pattern and comprises four prevailing ocean currents: the North Pacific Current to the north, the California Current to the east, the North Equatorial Current to the south, and the Kuroshio Current to the west.
An accumulation of marine debris known as the "Great Pacific Garbage Patch" is collecting in the gyre. [cite web |url=http://news.bbc.co.uk/1/hi/sci/tech/7314240.stm |title=New 'battle of Midway' over plastic |accessdate=2008-04-01 |date=26 March 2008 |publisher=BBC News ]

* South Atlantic Subtropical Gyre
** Contains the smaller Brazil Current System
* South Pacific Subtropical Gyre
** Contains the smaller East Australian Current System
* Indian Ocean Subtropical Gyre (Southern Hemisphere)
** Contains the smaller Agulhas Current System
* Antarctic Circumpolar Current
* Weddell Sea Subpolar Gyre (Southern Ocean)
* Ross Sea Subpolar Gyre (Southern Ocean)

Tropical circulations are less unified and tend to be mostly east-westwith minor north-south extent.
* Atlantic Equatorial Current System (two counter-rotating circulations)
* Pacific Equatorial/Tropical Current System
* Indian Monsoon Gyres [http://www.mar.dfo-mpo.gc.ca/science/ocean/BedfordBasin/Papers/Longhurst1998/Provinces/BGCP_table.htm] (Northern Indian Ocean, two counter-rotating circulations)

ubtropical gyres

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 force. 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 buildup of water in the center of the gyre creates equatorward flow in the upper 1000 to 2000 meters of the ocean, through rather complex dynamics. This equatorward flow is returned poleward in an intensified western boundary current (Western intensification).

The intensified western boundary current of the North Atlantic's subtropical gyre is the Gulf Stream; in the North Pacific it is the Kuroshio; in the South Atlantic, it is the Brazil Current; in the South Pacific, it is the East Australia Current; in the Indian Ocean, it is the Agulhas Current.

ubpolar gyres

Subpolar gyres form at high latitudes (around 60 degrees). Circulation of surface wind and ocean water is counterclockwise in the Northern Hemisphere, around a low pressure system (such as the persistent Aleution Low and the Icelandic Low). Surface currents generally move outward from the center of the system. This drives Ekman transport which creates an upwelling of nutrient-rich water from the lower depths. [ [http://oceanmotion.org/html/background/wind-driven-surface.htm Wind Driven Surface Currents: Gyres] ]

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 Wendall and Ross Seas, which circulate in a clockwise direction.

References

ee also

* Anticyclone
* Cyclone
* Eddy (fluid dynamics)
* Fluid dynamics
* Maelstrom
* Sargasso Sea
* Skookumchuck
* Vortex
* Whirlpool

External links

* [http://oceanmotion.org/html/background/wind-driven-surface.htm Wind Driven Surface Currents: Gyres]
* [http://www-pord.ucsd.edu/~ltalley/sio210/Global_circulation/ SIO 210: Introduction to Physical Oceanography - Global circulation]
* [http://www-pord.ucsd.edu/~ltalley/sio210/Wind_circulation_NAtl_SHem/ SIO 210: Introduction to Physical Oceanography - Wind-forced circulation notes]
* [http://sam.ucsd.edu/sio210/lect_6/lecture_6.html SIO 210: Introduction to Physical Oceanography - Lecture 6]
* [http://www.physicalgeography.net/fundamentals/8q.html Physical Geography - Surface and Subsurface Ocean Currents]
* [http://eros.eas.gatech.edu/npgo/ North Pacific Gyre Oscillation] — Georgia Institute of Technology