Downwelling

Downwelling is the sinking of surface water into the ocean interior, driven by surface convergence or by wind stress that forces water toward a coast or against a density gradient. Coastal downwelling occurs when alongshore winds drive Ekman transport onshore, piling water against the coast and depressing isopycnals. Open-ocean downwelling under anticyclonic wind stress curl supplies the subtropical gyres through Ekman pumping. Downwelling carries oxygen and heat downward but suppresses nutrient supply to the surface, so downwelling coasts have lower primary production than upwelling coasts. It is the counterpart of upwelling in the wind-driven vertical circulation.