In oceanography, a gyre (//) is any large system of circulating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction determine the circulatory patterns from the wind stress curl (torque).
The following are the five most notable ocean gyres:
- 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.
Satellite observational sea surface height and sea surface temperature data suggest that the world eight major ocean gyres are moving towards poles in the past few decades. Such feature shows agreement with the climate model prediction under anthropogenic global warming. Paleo-climate reconstruction also suggest that during the past cold climate intervals some western boundary currents (western branches of the subtropical ocean gyres) are closer to the equator than their modern positions. These evidence implies that global warming is very likely to push the large-scale ocean gyres towards higher latitudes.
During 1992-2011, stronger winds, especially the subtropical trade winds in the Pacific ocean have provided a mechanism for vertical heat distribution. The effects are changes in the ocean currents, increasing the subtropical overturning, which are also related to the El Niño and La Niña phenomena. 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
A garbage patch is a gyre of marine debris particles caused by the effects of ocean currents and increasing plastic pollution by human populations. These human-caused collections of plastic and other debris, cause ecosystem and environmental problems that effect marine life, contaminate oceans with toxic chemicals, and contribute to greenhouse gas emissions.
The best known of these is the Great Pacific garbage patch which has the highest density of marine debris and plastic, visible from space in certain weather conditions. Other identified patches include the North Atlantic garbage patch between North America and Africa, the South Atlantic garbage patch located between eastern South America and the tip of Africa, the South Pacific garbage patch located west of South America, and the Indian Ocean garbage patch found east of south Africa listed in order of decreasing size.Garbage patches are rapidly growing because of widespread loss of plastic from human trash collection systems. It is estimated that approximately "100 million tons of plastic are generated [globally] each year", and about 10% of that plastic ends up in the oceans. The United Nations Environmental Program recently estimated that "for every square mile of ocean" there are about "46,000 pieces of plastic."
- Ecosystem of the North Pacific Subtropical Gyre
- Fluid dynamics
- Volta do mar
- 5 Gyres – Understanding Plastic Marine Pollution
- 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