Coastal upwelling

Winds blowing roughly parallel to a coastline can move surface water away from shore. In many settings, the resulting divergence is replaced by water rising from below; this vertical movement is upwelling. The direction and strength of the wind, coastline orientation, Earth's rotation, and local ocean circulation influence the transport. Upwelling also occurs in the open ocean, but coastal upwelling is especially associated with alongshore winds. Downwelling is a related but opposite process in which surface water converges and sinks.

Water brought upward from depth is commonly colder and contains nutrients that have accumulated below the sunlit layer. When these nutrients become available to phytoplankton, they can stimulate primary production, supporting zooplankton and the animals that feed on them. This process helps sustain productive fisheries in some coastal regions. The outcome depends on local conditions: nutrients alone do not determine ecosystem productivity, and strong offshore transport may move organisms as well as nutrients.

Major eastern-boundary upwelling systems are associated with the California, Humboldt, Canary–Iberian and Benguela current systems. They are shaped by interactions between atmospheric winds and ocean circulation, and each region has its own seasonal patterns and natural variability. Upwelling can also affect larval dispersal: microscopic larvae of coastal species may drift offshore with surface water, potentially reducing their return to suitable nearshore habitat. Thus the same circulation that fuels food webs can create ecological trade-offs.

The timing and intensity of upwelling vary with winds, seasons and broader climate patterns. Reviews of eastern-boundary systems report observed and projected regional changes, including indications of stronger upwelling-favourable winds in some poleward areas. However, future ecosystem consequences are difficult to predict: changes in wind-driven nutrient supply interact with warming, water-column stratification, source-water chemistry and local ecology. It is therefore not accurate to assume that upwelling will strengthen or weaken everywhere, or that stronger upwelling automatically means greater ecosystem productivity.