currents are driven by two main forces, wind and density differences. The friction of winds blowing over the surface of the ocean drives surface currents, whose direction is rotated by
Coriolis force. The rotational effects continue down into the water generating a phenomenon known as an Ekman spiral. The density of sea water is determined by hydrostatic pressure (i.e. depth) and the water's temperature and
salinity, and so changes markedly across any
thermocline.
Latitudinal variations in the sun's energy that reaches the surface, and rainfall, result in the surface waters tending to be buoyant in equatorial regions where there is high rainfall, heavier in the hot and arid
horse latitudes, and to vary seasonally, not only at temperate latitudes but also at polar latitudes where the sea
ice formation and melting directly effects salinity. These influences result in each ocean basin having a similar large-scale circulation pattern. Along the equator there is a narrow band of current flowing from west to east. There is a subtropical clockwise current gyre with a narrow fast current flowing towards the poles along the western boundary, driven partly by the
trade winds and partly by the density differences. In the North Atlantic this boundary current is the
Gulf Stream, in the North Pacific it is the
Kuroshio, and in the Southern Indian Ocean it is the
Agulhas. These boundary currents feed transoceanic drifts, which are broader and slower. They, in turn, feed into flows towards the equator along the eastern boundaries, and on into return flows either side of the equator that flow counter to the fast equatorial current.
Similar gyres, or giant eddies, occur at northern sub-polar and polar latitudes, but in the Southern Ocean the circumpolar current, bounded by the
Antarctic Convergence, flows eastwards unimpeded around the globe. These currents have a major influence on climate by redistributing heat from equatorial regions towards the poles. For example, the mean annual temperature at Iqaluit in the North-West Territories of Canada is -9.1 °C (15.6 °F equivalent), whereas at Trondheim, at a similar latitude of the Atlantic margin, it is +4.8 °C (40.6 °F), because of the heat transported by the Gulf Stream and its extension, the
North Atlantic Drift.
Knowledge of ocean currents has considerable value to navies as the book and film
The Hunt for Red October illustrates, by showing how, during the Cold War, Russian nuclear
submarines escaped detection when entering the Atlantic. They did so by silently riding the fast flows of cold water that gush through the deep channels in the submarine ridge between the Norwegian Sea and the Atlantic.
M. V. Angel
