Ekmantransport

Ekman transport, also called Ekman transport, is the net movement of water in the ocean’s upper layer caused by wind-driven shear in the Ekman layer. In this layer, the balance between surface wind stress, Coriolis forces, and friction causes the flow to tilt with depth, producing a cumulative transport that is approximately perpendicular to the wind.

The physical mechanism relies on the Coriolis effect: in the Northern Hemisphere the surface current deflects to the right of the wind, and deeper layers are deflected progressively more, producing a spiral. When the layers are integrated with depth, the resulting Ekman transport points about 90 degrees to the wind. In the Southern Hemisphere, the net transport is about 90 degrees to the left of the wind.

Mathematically, the depth-integrated Ekman transport M_E is given by M_E = (tau_y, -tau_x) / (rho f), where tau_x and tau_y are the surface wind stress components, rho is seawater density, and f is the Coriolis parameter (f = 2 Omega sin(phi)). The magnitude is |M_E| = |tau| / (rho f). This transport is measured per unit width and has units of square meters per second.

Ekman transport has important coastal and climatic implications. Divergence or convergence of M_E near coastlines drives coastal upwelling or downwelling, influencing nutrient supply and fisheries. Offshore, wind patterns and the curl of wind stress influence larger-scale circulation through its interaction with topography and stratification, contributing to the formation of gyres via Sverdrup transport.

Named after V. Walfrid Ekman, who described the mechanism in the early 20th century, Ekman transport remains a foundational concept in physical oceanography.