Downwelling
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.
On annual cycles, widespread cooling begins in the fall, and convective mixed layer deepening can reach hundreds of meters into the ocean interior.
This movement piles up water, creating a high-pressure zone in the center of the gyre, low pressure on the borders, and deepens the mixed layer.
[3][4] Langmuir circulation develops from the wind, which, through Ekman transport, creates alternating zones of convergence and divergence at the ocean surface.
In convergent zones, marked by long strips of floating debris accumulation, such as the Great Pacific Garbage Patch, coherent vortices transport surface waters to the base of the mixed layer develop.
[7] The high central hydrostatic pressure maintained by this rotation causes the downwelling of water and the depression of isopycnals - surfaces of constant density (see Eddy pumping) at scales of hundreds of meters per year.
[13] Although compounds such as oxygen are transported into the deep ocean, there is an observed decrease in carbon export in warm-core eddies due to intensified stratification at their center.
They can develop at regions of freshwater input marked by horizontal density gradients due to salinity and temperature differences or the stretching and elongation of rotating flows.
Two primary mechanisms transport surface waters to depth: the adiabatic tilting and relaxation of these isopycnals, and along-isopycnal flow or subduction.
[20] Temperature differences and wind patterns are seasonal in temperate latitudes, creating highly variable upwelling and downwelling conditions.
[22] 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.
[23] During the neutral and La Niña phases of the El Niño Southern Oscillation (ENSO), steady easterly trade winds in equatorial regions can cause water to pile up in the western Pacific.
[24] Series of Kelvin waves associated with anomalously warm sea surface temperatures in the eastern Pacific can be a predecessor to an El Niño event.
[25] During the El Niño phase of ENSO, the disruption of trade winds causes ocean water to pile up off the western coast of South America.
[29] The Southern Ocean alone has been shown to be the most important high-latitude region controlling pre-industrial atmospheric carbon dioxide by general circulation model simulations.
Circulation of water into the Antarctic deep-water formation region is one of the main factors drawing carbon dioxide into the surface oceans.
The other is the biological pump, which is typically limited by iron in the Southern Ocean in areas with high nutrients and low chlorophyll (HNLC).
Sustained periods of upwelling can cause deoxygenation which is relieved by a downwelling event transporting dissolved oxygen back down to depths.
The accumulation of dinoflagellates and other forms of biomass nearshore due to downwelling will eventually cause nutrient depletion and mortality of organisms.
