Ocean Gyres are large scale systems of rotating ocean circulation that occupy much of the major ocean basins. They are produced primarily by the interaction of global wind patterns, the Coriolis effect, the geometry of ocean basins and the conservation of potential vorticity. The major subtropical gyres occur in the North and South Atlantic, North and South Pacific and Indian Ocean, while high latitude circulation systems have different characteristics.
Subtropical gyres generally rotate clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. This direction results from the combination of prevailing winds and the sign of the Coriolis effect. Gyres are not single circular currents but broad circulation systems containing several distinct currents, including western and eastern boundary currents and weaker interior flows.
The wind patterns that help create Ocean Gyres are associated with global atmospheric circulation. Trade winds generally blow from east to west in the tropics, while westerlies generally blow from west to east at mid latitudes. These winds impart momentum to the ocean surface. Because Earth's rotation deflects moving water, the resulting Ekman transport differs from the direct wind direction.
The convergence of surface transport in subtropical regions causes water to accumulate and move downward, contributing to a raised sea surface and depressed thermocline near gyre centres. The resulting pressure gradients interact with the Coriolis effect to produce geostrophic circulation. Thus gyres arise from the interaction of atmosphere, rotation, density structure and basin geometry rather than from wind alone.
The five major subtropical gyres have distinct geographical settings. The North Atlantic Gyre includes the Gulf Stream system, North Atlantic Current, Canary Current and North Equatorial Current. The North Pacific Gyre includes the Kuroshio, North Pacific Current, California Current and North Equatorial Current. The South Pacific and South Atlantic gyres have broadly analogous structures with opposite rotational direction because they lie in the Southern Hemisphere. The Indian Ocean differs because monsoon winds reverse seasonally, causing circulation patterns that are more strongly seasonal than those of the other major subtropical gyres. These systems redistribute heat, freshwater, nutrients, floating material and pollutants over enormous distances.
Ocean Gyres create characteristic patterns of sea surface height and thermocline depth. Convergent subtropical gyres tend to have elevated sea surface height near their centres, while the thermocline is deeper there than around the edges. The horizontal pressure gradient associated with sea surface height is balanced by the Coriolis force in approximately geostrophic flow. This balance explains why large scale ocean currents can flow at substantial speeds even when the direct wind forcing is relatively weak. Satellite altimetry can measure variations in sea surface height and therefore provide valuable information about geostrophic circulation. The physical structure of gyres is consequently measurable through a combination of satellite and in situ observations.
Ocean Gyres have important biological and chemical characteristics. Subtropical gyres are generally nutrient poor because persistent surface convergence and downwelling inhibit the delivery of deep nutrients to the sunlit surface. Their central regions are therefore often oligotrophic, meaning that nutrient concentrations and biological productivity are relatively low. In contrast, boundary regions, fronts, upwelling areas and other parts of the circulation can support much greater productivity.
Gyres also influence the movement of floating debris and pollutants because convergent surface circulation can concentrate persistent materials in broad accumulation zones. These accumulation areas are sometimes referred to as garbage patches, although they are generally diffuse regions containing dispersed particles rather than solid islands of rubbish.
Ocean Gyres influence climate by transporting heat between low and high latitudes. Western boundary currents commonly carry warm tropical water toward higher latitudes, while eastern boundary currents return cooler water toward lower latitudes. The resulting heat redistribution affects coastal climates, atmospheric pressure patterns and storm development. Gyres also exchange heat and moisture with the atmosphere and influence evaporation and precipitation. Changes in wind patterns, ocean temperature and climate variability can alter gyre strength and position. Long term monitoring is therefore important for distinguishing natural variability from persistent changes in circulation.
The Indian Ocean demonstrates why Ocean Gyres cannot be reduced to a single permanent pattern. The monsoon system reverses direction seasonally, causing major changes in surface currents, especially in the northern Indian Ocean. In other basins, subtropical gyres remain more stable in their overall orientation because the prevailing wind systems are less seasonally reversible. Polar and subpolar circulation systems also have different structures because of their latitude, sea ice and density driven processes. Thus a gyre is best understood as a basin scale circulation pattern whose precise structure depends on atmospheric forcing, planetary rotation, boundaries, stratification and seasonal conditions.
A common misconception is that Ocean Gyres are perfectly circular whirlpools in which water travels around a fixed centre at constant speed. Real gyres are irregular, three dimensional and variable. Their constituent currents differ greatly in width, depth and velocity, and eddies continually detach and modify the circulation. The strongest currents are usually concentrated near western boundaries, while interior circulation is broader and slower. Gyres also interact with deep circulation, although the surface and deep components cannot be treated as one simple rotating wheel. Understanding Ocean Gyres therefore requires considering atmospheric forcing, geostrophic balance, boundary currents, vertical structure, eddies and seasonal and interannual variability.
