gyre n : a round shape formed by a series of concentric circles [syn: coil, whorl, roll, curl, curlicue, ringlet, scroll]
- to whirl
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 vorcity along with horizontal and vertical friction which determine the circulation patterns from the wind curl (torque).
The Earth's oceans have the following major gyres:
- North Atlantic Subpolar Gyre
- North Pacific Subpolar Gyre
- Contains the smaller Alaska Gyre
- North Atlantic Subtropical Gyre
- North Pacific Subtropical Gyre also known as North Pacific Gyre
- 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-west with minor north-south extent.
- Atlantic Equatorial Current System (two counter-rotating circulations)
- Pacific Equatorial/Tropical Current System
- Indian Monsoon Gyres (Northern Indian Ocean, two counter-rotating circulations)
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.
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.
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.
- Wind Driven Surface Currents: Gyres
- SIO 210: Introduction to Physical Oceanography - Global circulation
- SIO 210: Introduction to Physical Oceanography - Wind-forced circulation notes
- SIO 210: Introduction to Physical Oceanography - Lecture 6
- Physical Geography - Surface and Subsurface Ocean Currents
- North Pacific Gyre Oscillation — Georgia Institute of Technology
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