Winds blowing across the ocean surface often push water away from an area. When this occurs, water rises up from deep beneath the surface to replace the surface water. Upwelling occurs in the open ocean and along coastlines.
Water that rises to the surface as a result of upwelling is typically colder, rich in nutrients, and biologically productive. Therefore, good fishing grounds typically are found where upwelling is common. For example, the rich fishing grounds along the west coasts of Africa and South America are supported by year-round coastal upwelling.
Seasonal upwelling also occurs along the West Coast of the United States. During the summer, winds blow from the north to the south, and water moves offshore, resulting in upwelling along the coast. This summer upwelling produces cold coastal waters in the San Francisco area, contributing to the frequent summer fogs.