Ekman Transport

Ekman transport is the net movement of water at a 90-degree angle to the surface wind direction, caused by the balance between the wind stress and the Coriolis force. It is named after the Swedish oceanographer Vagn Walfrid Ekman, who derived the theory in 1905 from observations of drifting ice.

The mechanism is simple but counterintuitive. Wind drags the surface layer, which drags the layer below it, which drags the layer below that. Each layer experiences the Coriolis force, which deflects it to the right (Northern Hemisphere) or left (Southern Hemisphere). The deflection increases with depth because the Coriolis force acts on each layer's velocity relative to the rotating Earth. The net integrated transport is perpendicular to the wind — to the right in the north, to the left in the south.

Ekman transport is responsible for coastal upwelling and downwelling, the convergence and divergence of surface waters in the centres of ocean gyres, and the coastal currents that shape marine ecosystems. It is not a minor correction to wind-driven flow; it is the fundamental mechanism by which the atmosphere communicates mechanical energy to the ocean, and a key component of ocean circulation dynamics.

The theory assumes steady-state conditions and ignores vertical mixing and nonlinear effects, which limits its accuracy in regions of strong turbulence or rapidly varying winds. Coriolis Force is the essential physical ingredient, and Wind Stress is the driving boundary condition.