What Are Rising Currents of Nutrient-Rich Waters From the Bottom of the Ocean to the Surface Called?
Upwellings are areas of the ocean where cold water rises to the surface from deep below, bringing up nutrients that invigorate the marine ecosystem. Often resulting from wind action along a coast that drives surface waters out to sea, allowing colder, nutrient-rich waters to rise from the depths, upwellings are associated with some of the most productive fisheries in the world due to the flourishing populations of phytoplankton supporting a vast food web.
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Ekman Transport
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Winds blowing along the surface of the ocean transfer energy to the water, creating a surface current. Due to the Coriolis effect, the direction of the current rotates to a different direction from the actual wind, termed Ekman transport. The magnitude of the direction change increases with depth, but the effect is limited to the first 500 feet of depth. The net effect of this change in direction produces a current that flows perpendicular to the direction of the wind. Ekman transport produces a current 90 degrees to the right of the wind direction in the Northern Hemisphere and 90 degrees to the left in the southern hemisphere.
Tropical Oceans
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With some notable exceptions, much of the tropical ocean displays low biological productivity. As with terrestrial ecosystems, the food webs of the surface ocean are driven by the productivity of photosynthesizing organisms, in particular phytoplankton-autotrophic organisms that produce food through photosynthesis, similar to terrestrial plants. Although tropical oceans have abundant sunlight, a strong, persistent thermocline -- or thin layer in which temperature changes rapidly -- prevents nutrients from deep in the ocean from reaching the phytoplankton. Even in temperate oceans this thermocline produces a similar effect during summer months.
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Upwelling Ecology
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In certain coastal regions of the ocean, a wind blowing along the coast creates a current that drives the warm surface waters out to sea due to Ekman transport. Cold waters from deep below the surface rise in these areas, bringing with them an abundance of nutrients. This surge of nutrients, combined with the abundant sunlight, allows phytoplankton populations to prosper. The abundance of phytoplankton, in turn, supports an increased number of larger animals, including fish, marine mammals and seabirds. However, sometimes the thermocline prevents the rush of accompanying nutrients. This greatly reduces the productivity of an area and is exemplified by El Nino events off the Pacific coast of South America.
Downwellings
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If a wind blowing in one particular direction along a coast creates a current away from shore, then it would seem a wind in the opposite direction would create a current toward the shore. Indeed, this is the scenario that creates a downwelling. In a downwelling, the warm surface waters are forced deeper into the ocean. Although downwellings and upwellings are often most noticeable near landmasses, they can also occur in the open ocean, such as near the Equator.
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References
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