An
ocean current is continuous, directed movement of
ocean water. Ocean currents are rivers of hot or cold water within the ocean. The currents are generated from the forces acting upon the water like the planet rotation, the wind, the
temperature and
salinity (hence
isopycnal) differences and the
gravitation of the moon. The
depth contours, the
shoreline and other currents influence the current's direction and strength.
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 (sub-tropical) than the tropical latitudes in which they are located because of the California Current.
Surface ocean currents are generally wind driven and develop their typical clockwise spirals in the northern hemisphere and counter-clockwise rotation in the southern hemisphere because of the imposed wind stresses. In wind driven currents, the Ekman spiral effect results in the currents flowing at an angle to the driving winds. The areas of surface ocean currents move somewhat with the seasons; this is most notable in equatorial currents.
Deep ocean currents are driven by density and temperature gradients. Thermohaline circulation, also known as the ocean's conveyor belt, refers to the deep ocean density-driven ocean basin currents. These currents, which flow under the surface of the ocean and are thus hidden from immediate detection, are called submarine rivers. These are currently being researched by a fleet of underwater robots called Argo. Upwelling and downwelling areas in the oceans are areas where significant vertical movement of ocean water is observed.
