Upwelling

Upwelling is a physical oceanographic process in which deeper, usually colder, water rises toward the surface, displacing warmer surface water. This vertical transfer brings nutrients such as nitrates, phosphates, and silicates into the sunlit layer, supporting high biological productivity.

There are two main forms: coastal upwelling and equatorial upwelling. Coastal upwelling occurs when prevailing winds drive surface water away from a coastline via Ekman transport, typically along western continental margins. Deep water then rises to replace it. This pattern is common in the California Current, the Humboldt (Peru) Current, the Canary, Namibian systems, and around Somalia. Equatorial upwelling occurs along the equator, where surface currents on opposite sides of the equator diverge due to the Coriolis effect, pulling cold, nutrient-rich water upward near the equator.

Ecological and climatic significance: the nutrient input sustains high phytoplankton productivity and supports large fisheries. Regions with persistent upwelling are among the most productive marine ecosystems. Upwelling intensity is influenced by seasonal winds and climate modes such as ENSO; El Niño events weaken wind-driven upwelling and reduce productivity, while La Niña can enhance it. Upwelled waters can also be oxygen-poor at depth in some regions, contributing to coastal hypoxia during strong upwelling or related events.