Ocean currents research papers report that major ocean currents fall into two broad types: surface currents and deep-ocean currents. Deep currents exist below the pynocline and, unlike the surface currents, are driven by the density of water. That density is a function of gradients in temperature and salinity. The currents which are most familiar to man, the famous ones such as the Kuroshio and the Gulf Stream, are surface currents and it is these with which we will be concerned.
Surface currents are caused by the wind. In the oceanic basins both north and south of the equator, wind driven masses of water form gyres, closed, roughly circular cells of moving water. In the northern hemisphere they move in a clockwise direction; in the southern hemisphere their direction is counter-clockwise. The edge of these gyres are the currents.
The major surface currents of the ocean are macro-phenomena that involve the displacement of huge bodies of water and the transfer of huge amounts of energy. Speaking of the Gulf Stream, Cramer states, “Together, all rivers washing into the sea from dry land do not begin to equal its surge. Coursing up from Cape Hatteras, the Gulf Stream delivers forty billion gallons of water each second to the Grand Banks….” Cramer also notes that this current carries one thousand times more water than the Amazon River.
There is another way to think of the edge of a gyre of water than the somewhat linear concept of “streams”. Kunzig notes that it is incorrect to regard the Gulf Stream as being akin to a river in the sea.