Ekman pumping

Ekman pumping is the vertical movement of water in the ocean induced by the divergence or convergence of Ekman transport — the wind-driven surface flow that is deflected 90° from the wind direction by the Coriolis effect. In the tropical Pacific, easterly trade winds drive equatorial surface water poleward, creating divergence along the equator that pulls cold, nutrient-rich water upward from the thermocline. This upwelling is the primary mechanism maintaining the eastern Pacific cold tongue and the east-west sea surface temperature gradient that sustains the Walker circulation. Without Ekman pumping, the Bjerknes feedback would have no oceanic anchor, and the coupled dynamics of the tropical Pacific would collapse into a fundamentally different regime. The process is named after Swedish oceanographer Vagn Walfrid Ekman, who in 1905 derived the mathematical theory of wind-driven currents under the influence of Earth's rotation.