Reed from image by J. Wallace and S. Download image jpg, 43 KB. When the wind blows parallel to a coastline, surface waters are pushed offshore and water is drawn from below to replace the water that has been pushed away. The upward movement of this deep, colder water is called upwelling.
Upwelling regions are often measured by their productivity due to the influx of nutrients to the surface mixed layer and euphotic zone sunlit layer by upwelling currents. This drives photosynthesis of phytoplankton tiny alga , which form the base of the ocean food web. Some of the most important upwelling regions are along the coasts of continents.
In these coastal upwelling regions, surface winds push water away from the shore and create a divergence at the coast, which is replaced by water from depth. For coastal upwelling to occur, the wind must be parallel to the coast because water is deflected to the left of the wind in the southern hemisphere, and the right of the wind in the northern hemisphere.
This deflection is due to the Coriolis force which causes objects travelling in a straight line appear to curve or deflect due to the rotation of the earth. Patterns of Circulation. Vertical Structure. Wind-Driven Currents and Ekman Transport. Wind-Driven Surface Currents. Western Boundary Currents.
Ocean Conveyer Belt. Where Ekman transport moves surface waters away from the coast, surface waters are replaced by water that wells up from below in the process known as upwelling. This example is from the Northern Hemisphere. In some coastal areas of the ocean and large lakes such as the North American Great Lakes , the combination of persistent winds, Earth's rotation the Coriolis effect , and restrictions on lateral movements of water caused by shorelines and shallow bottoms induces upward and downward water movements.
As explained above, the Coriolis effect plus the frictional coupling of wind and water Ekman transport cause net movement of surface water at about 90 degrees to the right of the wind direction in the Northern Hemisphere and to the left of the wind direction in the Southern Hemisphere.
Coastal upwelling occurs where Ekman transport moves surface waters away from the coast; surface waters are replaced by water that wells up from below.
Where Ekman transport moves surface waters toward the coast, the water piles up and sinks in the process known as downwelling. Surface water moving away from land leads to upwelling, while downwelling occurs when surface water moves towards the land. Historically, some of the most productive commercial fishing grounds have been associated with coastal upwelling.
Along the coast of California, the local prevailing winds blow towards the south. Ekman transport moves the surface layer 90 o to the right of the wind, meaning the net Ekman transport is in an offshore direction. The water displaced near the coast is replaced by cold, nutrient-rich deeper water that is brought to the surface through upwelling, leading to high productivity Figure 9.
Winds along the Peruvian coast blow towards the north, and since Peru is in the Southern Hemisphere, the Ekman transport is 90 o to the left of the wind, which causes the surface water to move offshore and leads to upwelling and productivity. In any coastal upwelling location, if the winds reverse, surface water moves towards the shore and downwelling is the result. Upwelling can also occur due to geological features of the ocean floor.
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