When these currents encounter land, they divide to flow north and south along the eastern borders of the continents. Figure 3 illustrates the large-scale surface currents. Notice the large clockwise gyres in the Northern Hemisphere and the counter-clockwise gyres in the Southern Hemisphere. As momentum passes downwards through ocean layers, the currents rotate further under the influence of Coriolis and moderate. The Ekman Spiral has a different direction in the Southern Hemisphere than in the Northern Hemisphere, where the rotation is towards the right.
Ocean temperature isoclines off the West Coast of North America.
Figure 4 shows the Ekman Spiral for the Northern Hemisphere. For a wind blowing southward along the coast of California, the ocean current will be seaward. The cold waters off the coast of California are clearly unusual. The cold waters off California are produced by the same process that results in cold surface waters off the western coasts of South America and Africa. The process is related to the offshore surface current driven by the prevailing winds, which blow parallel to the coast.