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Downwelling is the vertical movement of a fluid parcel, and its associated properties, from a higher to a lower (deeper) position within the fluid. The term is used to describe a variety of Earth phenomena, including oceanic processes, mantle dynamics, atmospheric movement, and freshwater systems (large lakes). The remainder of this article will concentrate on oceanic downwelling and the important implications it has for ocean circulation, biogeochemical cycles, and the Earth’s climate. There are three primary mechanisms by which ocean surface waters can be transported downward:  buoyancy forcing, wind-driven Ekman transport (Ekman pumping), and the interaction of currents with the ocean topography. Downwelling events are accompanied by decreasing nutrient content in the surface ocean.

Mechanisms
Buoyancy-forced downwelling is the deepening of a water parcel by a change in the density of that parcel. Density changes in the surface ocean are primarily the result of evaporation, precipitation, heating, cooling, or the introduction and mixing of an alternate water or salinity source, such as river input or brine rejection, respectively. For a water parcel to move downward, the density of that parcel must increase; therefore, evaporation, cooling, and brine rejection are the processes that control buoyancy-forced downwelling.

Variability
Downwelling trends can be associated with variations in wind strength, changing seasons, and different latitudes (vertical structure of the water, variations in the bottom bathymetry, and instabilities in the currents?).

In some areas, downwelling is a seasonal event pushing nutrient-depleted waters towards the coast. Coastal winds are highly variable, generated by temperature differences between dry land air and cool, dense sea air. The relaxation or reversal of upwelling-favorable winds creates periods of downwelling as waters pile up along the coast. Temperature differences vary greatly with the seasons in temperate latitudes, creating highly variable upwelling and downwelling conditions.

For example, low levels of primary production are common in fall and winter along the Pacific Northwest coast in the United States. Southerly winds in the Gulf of Alaska and California Current system create downwelling-favorable conditions, transporting offshore water from the south and west towards the coast. Downwelling events tend to last for days at a time and can be associated with winter storms. The spring transition at the end of the downwelling season and the beginning of the upwelling season is marked by the presence of cold, nutrient-rich, upwelled water at the coast. In contrast to seasonally variable temperate regions, downwelling is relatively steady at the poles as cold air decreases the temperature of salty water transported by gyres from the tropics.

During the neutral and La Niña phases of the El Niño Southern Oscillation (ENSO), steady easterly trade winds can cause water pileup associated with increased sea surface temperature and coastal downwelling in the western Pacific. During Northern Hemisphere spring, the weakening of easterly winds in the tropical Pacific can create downwelling Kelvin waves propagating along the equator in the eastern Pacific. Series of Kelvin waves associated with increases in sea surface temperature in the eastern Pacific can be a predecessor to an El Niño event.