Upwellings are a flow to the surface of deep, cold, nutrient-rich waters from greater depths in the ocean. The most extensive upwellings are associated with persistent coastal currents that draw surface water away from or along the coast to be replaced by a surface-ward flow of deeper waters. The most famous of these sorts of regional upwellings are found off the west coast of southern South America and in parts of the Antarctic Ocean.
Extensive upwellings can also develop where large currents are moving in opposite directions. This occurs in parts of the Pacific Ocean where the equatorial current in the Southern Hemisphere tends to move in a southerly direction, while that in the Northern Hemisphere moves to the north. Where these divergent equatorial currents flow beside each other, they develop extensive upwellings of deep waters.
More local upwellings can be caused when currents encounter surface or subsurface obstructions to their flow which can also force deeper water to the surface. Upwellings also develop on the leeward (down-flow) side of islands that obstruct the passage of a prevalent current.
Upwellings are most common in high-latitude regions of the Arctic and Antarctic Oceans, along the equator, and in certain coastal locations on the eastern sides of oceans. The most important of the latter types of upwellings are associated with the California current, the Peru current off South America, and the Benguala current off west Africa.
Upwelling waters are relatively rich in inorganic nutrients such as nitrate and phosphate. As a result, upwelling waters can sustain a large productivity of phytoplankton when they reach the surface of the ocean where there is ample sunlight to support photosynthesis. The relatively great primary productivity of phytoplankton can in turn support a large productivity of zooplankton, which can sustain a great abundance of fish, seabirds, and marine mammals. Because of their intrinsic fertility, upwelling waters are much more ecologically productive than the open ocean, which is generally highly deficient in nutrients.
Some of the world's largest fisheries are associated with extensive upwellings. These include the cold-water fisheries of certain regions of the Antarctic and Arctic Oceans and the temperate fisheries off Peru, Chile, California, and west Africa. The world's largest and most productive populations of seabirds and marine mammals also occur in those upwelling-driven marine ecosystems.