downwelling

Downwelling is the vertical transport of surface water downward into the ocean interior. It occurs when surface waters converge or become denser, causing them to sink. There are two main contexts: coastal downwelling, driven by wind-induced Ekman transport that pushes surface water toward the shore and creates sinking near the coast; and open-ocean downwelling, which occurs at convergence zones such as the centers of subtropical gyres or where strong surface convergence forces water downward. In polar regions, convective downwelling can occur when cooling and increased salinity make surface water dense enough to sink.

Downwelling transports heat, dissolved gases, and anthropogenic carbon from the surface into deeper layers, contributing to the ocean's role in climate and carbon cycling. By removing surface water, it can suppress nutrient renewal at the surface, reducing biological productivity in downwelling regions, while upwelling regions gain nutrients. The pattern and strength of downwelling are important components of the global overturning circulation and regional climate.

Observation and study rely on satellite altimetry to detect sea-surface height anomalies associated with accumulated water, hydrographic surveys, and autonomous floats such as Argo to profile density and velocity. Classic examples include coastal downwelling in certain wind conditions along western ocean boundaries and open-ocean downwelling at gyre centers and in polar convection zones.