Upwelling areas are often located at the eastern boundary of an ocean. If the wind blows parallel to the coast and toward the equator at the eastern boundary of an ocean, water of the upper layer (the so-called Ekman layer) moves away from the coast in either hemisphere and is replaced by water upwelling from below this layer. The upwelled water is cooler than the original surface water and a band of low temperature surface water develops close to the coast.
Often this band has a patchy structure. Usually it has a larger concentration of nutrients (phosphates, nitrates, silicates, etc.) than the original surface water. Thus upwelling areas are usually areas of high biological productivity with a high fish population. This implies that in these regions the amount of surface active substances secreted by marine plants and animals, which float to the sea surface and create there a surface film, are greatly enhanced.
Upwelling areas of this kind are found off the northwest and southwest African coast, off the coast of Peru, and off the North American coast from British Columbia to California.
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