 Surface ocean currents are driven by surface wind patterns, for example, the trade winds in the tropics and the westerlies in the middle attitudes. The trade winds in the tropics drive surface currents from the east towards the west, and in return, the westerlies drive surface currents from the west back towards the east. In addition, the Coriolis force results in gyres, rotational systems in each of the ocean basins that are clockwise in the northern hemisphere, for example, the North Atlantic gyre, and counterclockwise in the southern hemisphere, for example, the South Atlantic gyre. These gyres move warm waters from the south towards the north, and in addition, they move cool waters from the north towards the south. Each gyre has a major effect on ocean circulation in that part of the ocean basin.