Editing Upwelling (section)

==Variations==
[[File:Sstanom 199711 krig.jpg|thumb|300px|right|{{center|Unusually strong winds from the east push warm (red) surface water towards Africa, allowing cold (blue) water to upwell along the Sumatran coast}}]]

Upwelling intensity depends on wind strength and seasonal variability, as well as the vertical structure of the [[water]], variations in the bottom [[bathymetry]], and instabilities in the [[current (fluid)|currents]].

In some areas, upwelling is a [[seasonal]] event leading to periodic bursts of productivity similar to spring [[Phytoplankton bloom|blooms]] in coastal waters.  Wind-induced upwelling is generated by temperature differences between the warm, light air above the land and the cooler denser air over the sea.  In [[temperate]] [[latitudes]], the temperature contrast is greatly seasonably variable, creating periods of strong upwelling in the spring and summer, to weak or no upwelling in the winter.  For example, off the coast of Oregon, there are four or five strong upwelling events separated by periods of little to no upwelling during the six-month season of upwelling.  In contrast, [[tropical]] [[latitudes]] have a more constant temperature contrast, creating constant upwelling throughout the year.  The Peruvian upwelling, for instance, occurs throughout most of the year, resulting in one of the world's largest marine fisheries for [[sardines]] and [[anchovies]].<ref name="Mann"/>

In anomalous years when the [[trade winds]] weaken or reverse, the water that is upwelled is much warmer and low in nutrients, resulting in a sharp reduction in the biomass and  [[phytoplankton]] productivity.  This event is known as the [[El Nino-Southern Oscillation]] (ENSO) event.  The Peruvian upwelling system is particularly vulnerable to ENSO events, and can cause extreme interannual variability in productivity.<ref name="Mann"/>

Changes in bathymetry can affect the strength of an upwelling.  For example, a submarine [[ridge]] that extends out from the coast will produce more favorable upwelling conditions than neighboring regions.  Upwelling typically begins at such ridges and remains strongest at the ridge even after developing in other locations.<ref name="Mann"/>