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In oceanography, a gyre is any large system of circulating ocean surface 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.

Gyre can refer to any type of vortex in an atmosphere or a sea, but it is most commonly used in terrestrial oceanography to refer to the gyres that control the major ocean basin circulation.

Gyre Formation
Ocean gyres are wind-driven circulation, meaning that their locations and dynamics are controlled by the prevailing global wind patterns: easterlies at the tropics and westerlies at the midlatitudes. These wind patterns result in a wind stress curl that drives Ekman pumping in the subtropics and Ekman suction in subpolar regions. Ekman pumping results in an increased sea surface height at the center of the gyre and anticyclonic geostrophic currents in subtropical gyres. Ekman suction results in a depressed sea surface height and cyclonic geostrophic currents in subpolar gyres.

Ocean gyres are asymmetrical, with stronger flows on their western boundary and weaker flows throughout their interior. The weak flow that is typical over most of the gyre is a result of the conservation of potential vorticity. In the shallow water equations (applicable for basin-scale flow as the horizontal length scale is much greater than the vertical length scale), potential vorticity is a function of relative (local) vorticity $$\zeta$$, planetary vorticity $$f$$, and the depth $$H$$, and is conserved with respect to the material derivative :

$${D \over Dt}\left (\frac{H} \right ) = 0$$

In the case of the subtropical ocean gyre, Ekman pumping results in water piling up in the center of the gyre, compressing water parcels. This results in a decrease in $$H$$, so by the conservation of potential vorticity the numerator $$\zeta + f$$ must also decrease. It can be further simplified by realizing that, in basin-scale ocean gyres, the relative vorticity $$\zeta$$ is small, meaning that local changes in vorticity cannot account for the decrease in $$H$$. Thus, the water parcel must change its planetary vorticity $$f$$ accordingly. The only way to decrease the planetary vorticity is by moving the water parcel equatorward, so throughout the majority of subtropical gyres there is a weak equatorward flow. Harald Sverdrup quantified this phenomena in his 1947 paper, "Wind Driven Currents in a Baroclinic Ocean", in which the (depth-integrated) Sverdrup balance is defined as :

$$fV_{g}=\beta\rho{w_E}$$

Here, $$V_g$$ is the meridional mass transport (positive north), $$\beta$$ is the Rossby parameter, $$\rho$$ is the water density, and $$w_E$$ is the vertical Ekman velocity due to wind stress curl (positive up). It can be clearly seen in this equation that for a negative Ekman velocity (e.g., Ekman pumping in subtropical gyres), meridional mass transport (Sverdrup transport) is negative (south, equatorward) in the northern hemisphere ($$f>0$$). Conversely, for a positive Ekman velocity (e.g., Ekman suction in subpolar gyres), Sverdrup transport is positive (north, poleward) in the northern hemisphere.

Western Intensification
As the Sverdrup balance argues, subtropical (subpolar) ocean gyres have a weak equatorward (poleward) flow over their area. However, there must be some return flow that goes against the Sverdrup transport in order to preserve mass balance. In this respect, the Sverdrup solution is incomplete, as it has no mechanism in which to predict this return flow. Contributions by both Henry Stommel and Walter Munk resolved this issue by showing that the return flow of gyres is done through an intensified western boundary current. Stommel's solution relies on a frictional bottom boundary layer which is not necessarily physical in a stratified ocean (currents do not always extend to the bottom).

Munk's solution instead relies on friction between the return flow and the sidewall of the basin. This allows us to consider two cases: one with the return flow on the western boundary (western boundary current) and one with the return flow on the eastern boundary (eastern boundary current). A qualitative argument for the presence of western boundary current solutions over eastern boundary current solutions can be found again through the conservation of potential vorticity. In order to move northward (an increase planetary vorticity $$f$$ for the case of a subtropical gyre in the northern hemisphere), there must be a source of relative vorticity to drive the northward flow. The relative vorticity in the shallow-water system is :

$${\zeta} = {\partial v \over \partial x} - {\partial u \over \partial y}$$

Here $$v$$ is again the meridional velocity and $$u$$ is the zonal velocity. In the sense of a northward return flow, the zonal component is neglected and only the meridional velocity is important for relative vorticity. Thus, this solution requires that $${\partial v / \partial x} > 0$$ in order to increase the relative vorticity and have a valid northward return flow in the northern hemisphere subtropical gyre.

Due to friction at the boundary, the velocity of flow must go to zero at the sidewall before reaching some maximum northward velocity within the boundary layer and decaying to the southward Sverdrup transport solution far away from the boundary. Thus, the condition that $${\partial v / \partial x} > 0$$ can only be satisfied through a frictional boundary layer on the western boundary, as the eastern boundary frictional layer forces $${\partial v / \partial x} < 0$$. One can make similar arguments for subtropical gyres in the southern hemisphere and for subpolar gyres in either hemisphere and see that the result remains the same: the return flow of an ocean gyre is always in the form of a western boundary current.

The western boundary current must transport on the same order of water as the interior Sverdrup transport in a much smaller area. This means western boundary currents are much stronger than interior currents, a phenomena called "western intensification". A quantitative analysis of ocean gyres, western boundary currents, and the so-called Munk Layer can be found in numerous texts.

Subtropical Gyres
The five major subtropical gyres are :
 * Indian Ocean Gyre
 * North Atlantic Gyre
 * North Pacific Gyre
 * South Atlantic Gyre
 * South Pacific Gyre

They flow clockwise in the Northern hemisphere, and counterclockwise in the Southern hemisphere.

Subpolar 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 Weddell Sea and the Ross Sea, the Weddell Gyre and Ross Gyre, which circulate in a clockwise direction.

Life in a Gyre
Gyres are sometimes described as "ocean deserts" or more precisely "biological deserts", a concept that uses the concept of desert in the sense of an environment lacking life and not necessarily water. Other places that are called oceanic deserts are hypoxic or anoxic waters such as dead zones.

Climate change
Ocean circulation re-distributes the heat and water-resources, therefore determines the regional climate. For example, the western branches of the subtropical gyres flow from the lower latitudes towards higher latitudes, bringing relatively warm and moist air to the adjacent land, contributing to a mild and wet climate (e.g., East China, Japan). In contrast, the eastern boundary currents of the subtropical gyres streaming from the higher latitudes towards lower latitudes, corresponding to a relatively cold and dry climate (e.g., California).

Currently, the core of the subtropical gyres are around 30° in both Hemispheres. However, their positions were not always there. Satellite observational sea surface height and sea surface temperature data suggest that the world's major ocean gyres are slowly moving towards higher latitudes in the past few decades. Such feature show agreement with climate model prediction under anthropogenic global warming. Paleo-climate reconstruction also suggest that during the past cold climate intervals, i.e., ice ages, some of the 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.