<P> 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 currents . </P> <P> In some areas, upwelling is a seasonal event leading to periodic bursts of productivity similar to spring 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 . </P> <P> 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 . </P> <P> 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 . </P>

Ocean movement along the western side of continents is characterized by