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The rising of cold, usually nutrient-rich waters from the ocean depths to the warmer, sunlit zone at the surface. Upwelling usually occurs in the subtropics along the western continental coasts, where prevailing trade winds drive the surface water away from shore, drawing deeper water upward to take its place. Because of the abundance of krill and other nutrients in the colder waters, these regions are rich feeding grounds for a variety of marine and avian species. Upwelling can also occur in the middle of oceans where cyclonic circulation is relatively permanent or where southern trade winds cross the Equator.