Upwelling
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'''Upwelling''' is an [[physical oceanography|oceanographic]] phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. There are at least five types of upwelling: coastal upwelling, large-scale wind-driven upwelling in the ocean interior, upwelling associated with eddies, topographically-associated upwelling, and broad-diffusive upwelling in the ocean interior.
[[Image:Upwelling1.jpeg|250px|right]] [[Image:Upwelling2.jpeg|250px|right]] [[Image:Upwelling3.jpeg|250px|right]]
=== Coastal upwelling ===
Coastal upwelling is the best known type of upwelling, and the most closely related to human activities as it supports some of the most productive fisheries in the world, like small pelagics (sardines, anchovies, etc.). Deep waters are rich in nutrients which include [[nitrate]] and [[phosphate]], themselves the result of [[decomposition]] of sinking organic matter (dead/detrital plankton) from surface waters. When brought to the surface, these nutrients are utilized by [[phytoplankton]], along with dissolved [[CO2|CO<small>2</small>]] (carbon dioxide) and light energy from the sun, to produce [[organic compound]]s, through the process of [[photosynthesis]]. Upwelling regions therefore result in very high levels of primary production (the amount of carbon fixed by [[phytoplankton]]) in comparison to other areas of the ocean. High primary production propagates up the food chain because [[phytoplankton]] are at the base of the oceanic food chain. Regions of upwelling include coastal [[Peru]], [[Chile]], [[Arabian Sea]], western [[South Africa]], eastern [[New Zealand]] and the [[California]] coast.
The food chain follows the course of:
* Phytoplankton --> Zooplankton --> Predatory zooplankton --> Filter feeders --> Predatory fish
Due to this being a food chain, this means that every species is a key species within the upwelling zone.
The key piece of physics that gives rise to coastal upwelling is the [[Coriolis effect]], by which wind-driven currents tend to be driven to the right of the winds in the Northern Hemisphere and to the left of the winds in the Southern Hemisphere. For example, in the northern Hemisphere, when winds blow either equatorward along an eastern ocean boundary or poleward along a western ocean boundary, surface waters are driven away from the coasts ([[Ekman transport]] or [[Ekman spiral]]) and replaced by denser waters from below.
=== Equatorial upwelling ===
A related phenomenon is found at the equator, or more precise. Whatever its location, this results in a divergence, with denser, nutrient-rich water being upwelled from below, and results in the remarkable fact that the equatorial region in the Pacific can be detected from space as a broad line of high phytoplankton concentration.
=== Southern Ocean upwelling ===
Large-scale upwelling is also found in the Southern Ocean. Here, strong westerly (eastward) winds blow around Antarctica,driving a significant flow of water northwards. This is actually a type of coastal upwelling. Since there are no continents in a band of open latitudes between South America and the tip of the Antarctic Peninsula, some of this water is drawn up from great depths. In many numerical models and observational syntheses, the Southern Ocean upwelling represents the primary means by which deep dense water is brought to the surface. Shallower, wind-driven upwelling is also found in off the west coasts of North and South America, northwest and southwest Africa, and southwest Australia, all associated with oceanic subtropical high pressure circulations (see coastal upwelling above).
Some models of the ocean circulation suggest that broad-scale upwelling occurs in the tropics, as pressure driven flows converge water toward the low latitudes where it is diffusively warmed from above. The required diffusion coefficients, however, appear to be larger than are observed in the real ocean. Nonetheless, some diffusive upwelling does probably occur.
Localized upwelling may be due to deflection of deep currents by a [[seamount]] providing a nutrient rich island in otherwise low productivity ocean areas. These provide islands of life in such areas and are important to migrating species and human fishing.
=== Tropical cyclone upwelling ===
Upwelling can also occur when a [[tropical cyclone]] transits an area, usually when moving at speeds of less than 5 mph (8 km/h). The churning of a cyclone eventually draws up cooler water from lower layers of the ocean. This causes the cyclone to weaken.
=== Non-oceanic upwelling ===
Upwellings also occur in other fluid environments, such as the [[magma]] in Earth's [[earth's mantle|mantle]] or the [[Plasma (physics)|plasma]] within a [[star]]. They are often a result of [[convection]].
== External links ==
* [http://oceanmotion.org/html/background/upwelling-and-downwelling.htm Wind Driven Surface Currents: Upwelling and Downwelling]
* [http://www.classzone.com/books/earth_science/terc/content/visualizations/es2405/es2405page01.cfm?chapter_no=visualization Animation demonstrates the upwelling process.]
{{fisheries and fishing}}
[[Category:Aquatic ecology]]
[[Category:Oceanography]]
[[Category:Fisheries science]]
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