Ekman spiral

Wind stress pushes the ocean surface and sets the upper water in motion. Viscous and turbulent mixing transfer momentum downward, so deeper layers are driven partly by the water above them. Earth’s rotation, represented in a rotating-frame description by the Coriolis effect, deflects the currents. In the Northern Hemisphere the current direction turns progressively to the right with depth in the classical steady model; in the Southern Hemisphere it turns to the left. The speed generally diminishes downward, so a plot of the current vector through the layer traces a spiral.

The name honors Swedish oceanographer Vagn Walfrid Ekman, whose work on wind-driven currents was inspired by observations of drifting ice. The spiral is an idealized solution, not a claim that every real ocean profile has a neat, measurable spiral.

Although current direction varies with depth, the depth-integrated movement of water in the idealized Ekman layer is approximately 90 degrees to the wind: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This net movement is called Ekman transport. It is distinct from the direction of the surface current, which is usually at an angle to the wind and differs from the integrated transport.

Ekman transport helps explain large-scale ocean circulation and wind-driven coastal and open-ocean processes. Where surface waters diverge, deeper water may rise to replace them; where waters converge, water can be driven downward. These vertical motions can affect the distribution of heat, nutrients and marine organisms.

The classical spiral assumes simplified conditions, including steady wind forcing and a sufficiently uniform water column. Real oceans are influenced by changing winds, waves, stratification, turbulence, coastlines, bottom friction and other currents. These factors can change the measured profile and the depth over which wind-driven motion is important, so a single depth or spiral shape does not apply everywhere.

Oceanographers test Ekman theory using current meters, moored buoys, ships and other observations. Measurements can separate wind-driven currents from background and other motions, and comparisons with theory help reveal where its assumptions work and where improved models are needed. Modern observations also show that short-period motions and local conditions may produce behavior unlike the classical steady profile.