Although some ocean currents result from density and salinity variations of water, the major ocean currents are wind-driven currents. Surface currents make up about 10% of all the water in the ocean. These waters are the upper 400 meters of the ocean. Deep water currents make up the other 90% of the ocean; these waters move around the ocean basins by density driven forces and gravity. Density difference is a function of different temperatures and salinity.
Ocean currents can flow for thousands of kilometers. They are very important in determining the climates of the continents, especially those regions bordering on the ocean. Perhaps the most striking example is the Gulf Stream, which makes northwest Europe much more temperate than any other region at the same latitude. Another example is the Hawaiian Islands, where the climate is cooler than the tropical latitudes in which they are located because of the California Current.