Ocean currents are large-scale permanent or semipermanent movements of water at or beneath the surface of the oceans. Currents may be divided into those caused by the winds and those caused by differences in density of seawater. In the former case, friction between the prevailing wind and the water surface causes horizontal motion, and this motion is both modified by and in part transferred to to deeper layers by further friction. Density variations may result from temperature differences, different salinities, etc. The direction of flow of all currents is affected by the Coriolis effect. Best known, perhaps, are the Gulf Stream and Humboldt current. See also tide.