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The frictional stress of equatorward wind on the ocean's surface, in
concert with the effect of the earth's rotation, causes water in the
surface layer to move away from the western coast of continental land
masses. This offshore moving water is replaced by water which upwells,
or flows toward the surface from depths of 50 to 100 meters and more.
Upwelled water is cooler and saltier than the original surface water,
and typically has much greater concentrations of nutrients such as
nitrates, phosphates and silicates that are key to sustaining biological
production. It is for this reason that marine ecosystems in the ocean's
eastern boundary currents are highly productive, and capable of
maintaining large standing crops of plankton, massive fish stocks such
as sardines and anchovies, and major populations of marine mammals and
sea birds. The major eastern boundary currents include the Canary off
the Iberian peninsula and northwestern Africa, the Benguela off
southwestern Africa, the Peru off western South America, and the
California Current System off western North America. Moreover
variations in upwelling over seasonal to interannual periods, due to
large-scale shifts in wind patterns and atmospheric systems, are
linked to variability in fish populations and other biological
components in coastal ocean ecosystems.
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