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Marine Ecosystem Upwelling |
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Upwelling
Upwelling describes a condition in which the cooler, nutrient-rich waters of the ocean beneath the thermocline are drawn upwards to replace the warmer, surface waters which have been displaced by surface winds. It is also the vertical upward movement of a fluid due to density differences or where two fluid masses converge, displacing fluid upward. Upwellings are caused by strong seasonal winds moving surface coastal water out from the coast and leaving a space that the upwelling fills in. Many marine plants and animals live off this nutrient-rich water. The thin horizontal layer of water riding on top of the ocean that is affected by wind is the Ekman layer.
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Ekman transport causes surface waters to diverge or move away from the coast and deeper (often cold and nutrient-rich) water to be brought to the surface. Because winds blowing on the sea surface produce an Ekman layer that transports water at right angles to the wind direction, any spatial variability of the wind, or winds blowing along some coasts, can lead to upwelling as seen in the diagram
The diagram shows a sketch of Ekman transport along a coast leading to upwelling of cold water along the coast. The water transported offshore must be replaced by water upwelling from below the mixed layer North winds along a west coast in the northern hemisphere cause Ekman transports away from the shore
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The winds produce a mass transport away from the shore everywhere along the shore. The water pushed offshore can be replaced only by water from below the Ekman layer, and this is upwelling (Fig. 6.3b). Because the upwelled water is cold, the upwelling leads to a region of cold water at the surface along the coast.
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Upwelled water is colder than water normally found on the surface, and it is richer in nutrients. The nutrients fertilise phytoplankton in the mixed layer, which are eaten by zooplankton, which are id turn eaten by small fish.
The small fish are eaten by larger fish and so on. As a result, upwelling, regions are productive waters supporting the world's major fisheries. Atmospheric winds generate horizontal currents that move around the
ocean's surface. Wind can also generate vertical water motions in the processes of upwelling and downwelling. When wind blows over water, the surface water does not move directly in front of the wind but moves about 45
degrees towards the right of the wind's motion in the northern hemisphere in the diagram.
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This process is called Ekman transport and is a result of the Coriolis effect. In the southern hemisphere, surface water is deflected to the left of the wind's motion. Where winds cause the surface water to move away from a coastline or to diverge from another surface water mass, deeper water will move up to the ocean surface, creating an upwelling current. Where\ winds cause the surface water to move towards a coastline or to converge with another water mass, the surface water will try to move downward to create a downwelling current. For example, northerly winds are common in summer along the California coast. Winds moving from north to south cause surface water to move towards the west, away from the coastline. Upwelling currents are created, which bring deeper, colder water to the surface
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While surface waters are usually depleted of nutrients such as phosphates and nitrates that are crucial for plant growth, deeper waters have high concentrations of these nutrients. Upwelling replenishes the surface layers with the nutritional components necessary for biological productivity. Regions of upwelling are among the richest biological areas of the world.
The importance of upwelling can be well understood from the following facts.
· Upwelling enhances biological productivity, which feeds fisheries.
· Cold upwelled water alters local weather. The onshore weather in regions of upwelling tend to have fog, low stratus clouds, a stable stratified atmosphere, little convection, and little rain.
· Spatial variability of transports in the open ocean leads to upwelling and downwelling which in turn leads to redistribution of mass in the ocean, which leads to wind-driven geostrophic currents via Ekman pumping.
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