Horizontal surface currents are propelled by the frictional force of wind dragging the water. The subsurface flow of deep ocean water, called thermohaline circulations, arise from differences in density in seawater. Some surface currents are transient and seasonal, a number of them flow with great persistence, setting up a circulation that continues with relatively little change throughout the year. Because of the influence of wind in creating currents in the surface-layer circulations of the ocean, there is a relationship between oceanic circulation and the general circulation of the atmosphere. A notable feature of the oceanic circulation is that it is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
Currents perform a chore that benefits the entire planet.
Oceans serve as vast heat reservoirs. They store heat in the summer and release it during the winter. Currents are the mass transit system that moves large amounts of heat, plus suspended solids and dissolved chemicals, between low and high latitudes, effectively moderating the world's climate. Major currents in the Northern Hemisphere include the Gulf Stream in the Atlantic and the Kuroshio Current in the Pacific. These currents, called western boundary currents, are important links in this heat transfer.