gyre

Gyre is a term used in physical oceanography, meteorology, and limnology to describe a circular or spiral flow in a fluid. The word derives from the Greek gyros, meaning circle or ring. In common usage, gyres describe the large-scale, rotating patterns of ocean surface currents or atmospheric circulation.

In the oceans, subtropical gyres are the dominant large-scale circulation systems in each basin. They form under the influence of steady winds: the trade winds push surface water westward, while the westerlies push it eastward at mid-latitudes. The Coriolis effect deflects moving water, producing a circular pattern. The result is a clockwise gyre in the Northern Hemisphere and a counterclockwise one in the Southern Hemisphere. Each gyre contains a warm western boundary current (for example, the Gulf Stream in the North Atlantic and the Kuroshio in the North Pacific) and a cooler eastern boundary current (such as the Canary, California, and Humboldt Currents).

Five major subtropical gyres are typically recognized: the North Atlantic Gyre, South Atlantic Gyre, North Pacific Gyre, South Pacific Gyre, and Indian Ocean Gyre. The Antarctic Circumpolar Current around Antarctica forms a planet-wide, circumpolar flow that links these basins. Gyres transport heat and nutrients, influence weather patterns, and shape marine ecosystems.

Because surface convergence within gyres concentrates floating debris, gyres are often associated with pollution accumulations, including large patches of microplastics, most famously the so-called Great Pacific Garbage Patch.

Outside oceanography, the term gyre is also applied to large-scale atmospheric circulations that can resemble a rotating system, although it is most commonly used in discussions of oceanic currents.