Gyres
Video: Gyres (1:04)
Surface ocean currents are driven by surface wind patterns. For example, the trade winds in the tropics and the westerlies in the mid-latitudes. 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.
As surface winds push the surface layer of the ocean with them, the surface wind gyres result in surface ocean current gyres. Along coastlines, the direction of movement of a gyre has a significant impact on continental climate. For example, a current moving from south to north in the Northern Hemisphere, or north to south in the Southern Hemisphere, will generally deliver warmer water to the coastal region, whereas a current moving from the north to south in the Northern Hemisphere or south to north in the Southern Hemisphere will generally deliver colder water. The flow of warm water will generally cause a larger moderating influence on coastal climate than will the flow of cold water. Take, for example, the Gulf Stream in the North Atlantic. This warm current has a major heating effect on the shores of Great Britain and other parts of Northern Europe, keeping these regions relatively balmy compared to locations at comparable latitudes. After it bathes the shores of Britain, the North Atlantic gyre bends towards the south, thus bringing relatively cold waters to the shores of Spain, Portugal, and Morocco further to the south, keeping these areas cooler than areas not influenced by the currents.
This NASA video provides an excellent summary of surface ocean currents: