Mesoscale Eddies

Abstract

The oceans of the world circulate horizontally and vertically, and eddying motions accompany this general circulation. Satellite images show them to be ubiquitous, filling the world's oceans like a checkerboard while often marching slowly westward. Mesoscale eddies are typically 100km wide, with currents concentrated near the ocean surface; but when intense, their flow can extend down through the full depth of the ocean. They are analogous to the much larger atmospheric “weather” features that extend over hundreds to thousands of kilometers. Ocean eddies are often produced by instability of the larger-scale circulation, as well as directly by atmospheric forcing and flow past complex seafloor topography. Conversely, as in the atmosphere, oceanic mesoscale eddies can reshape, redistribute and intensify the currents in which they are embedded, feeding back on the general circulation. They “stir” the ocean and contribute to turbulent vertical mixing which in turn affects the ocean ecosystem, for example the distribution of primary biological productivity in the upper ocean. Mesoscale eddies are strongly nonlinear cousins of Rossby waves, which are undulations of the ocean and atmosphere, owing to the rotation and sphericity of the Earth.