Eddies

Eddies are localized swirling motions in fluids, formed when flow becomes disturbed or unstable. They appear as circular or spiral currents with their own vorticity, embedded within a larger, slower background flow. Eddies occur in water and air and are common in oceans, seas, rivers, and the atmosphere.

Eddies form when a fluid stream encounters an obstacle or topographic variation, or when shear and instabilities in jets and boundary layers generate swirling motion. Wind stress on the ocean surface, buoyancy contrasts, and interactions between currents also contribute to eddy creation. They can pinch off from larger currents and persist long enough to move with the flow, gradually dissipating energy to smaller scales.

Eddies vary widely in size and lifetime. Oceanic eddies are often mesoscale, roughly 10 to 100 kilometers in diameter, lasting days to months. They are typically categorized as cyclonic, rotating in the same sense as a low-pressure system, and anticyclonic, rotating in the opposite sense. In the Northern Hemisphere, cyclonic eddies rotate counterclockwise and anticyclonic eddies rotate clockwise; the sense is reversed in the Southern Hemisphere. Atmospheric eddies include weather-scale structures such as midlatitude cyclones and anticyclones, as well as smaller turbulent eddies in the planetary boundary layer.

Eddies play a major role in transport and mixing, moving heat, salt, nutrients, and momentum across regions. They influence climate and marine ecosystems and are studied using satellite altimetry, radar, and in situ instruments. Related concepts include turbulence, vortex, and eddy currents.

See also: turbulence, vortex, eddy current.