THE oceans of the world are in perpetual motion. Apart altogether from the movement of the surface waves, great bodies of water are transported from place to place by means of currents. These may be wind currents, which are ephemeral movements of the surface waters brought about by local winds, or ocean currents - large scale permanent or semi permanent marine rivers like the Gulf Stream that transport large masses of water over great distances.
Like most disciplines, oceanography has its own special terminology to describe these movements. An oceanographer, for example, does not speak of the speed of an ocean current, or of its "direction", as a meteorologist might in the case of wind; he or she refers to its set and drift. The set of a current is the direction - towards which it moves thee opposite convention to that - used for specifying wind direction; the drift of a current is its speed of movement.
Surface ocean currents tend to be associated with prevailing winds, but their mechanism is complex. The trade winds of both the northern and southern hemispheres drive the water westwards in the low latitudes, generating a surface current flowing from east to west in equatorial regions. If unimpeded, these equatorial currents would circle the entire globe. But their path is blocked by the continents, and the water is forced to turn, forming circulating cells called gyres. The gyres are centred near the semipermanent subtropical regions of high atmospheric pressure around which warm currents flow polewards on the western sides of the oceans in relatively narrow bands of briskly moving water, while the broader and more slow moving cold currents drift equator wards along the eastern sides.
Currents deep within the oceans, on the other hand, are driven largely by differences in temperature and salinity, and hence are known as thermohaline currents. Surface water moving northwards along the western flank of an ocean ultimately arrives at a latitude where sea ice is inclined to form. The freezing of some of the water in this way leaves a greater concentration of salts in any water that has not turned to ice, and this increased salinity allows the remaining water to continue to cool well below zero while still remaining liquid. This "extra cold" water is very dense and heavy; it therefore sinks and begins a return trip southwards along the ocean floor, sliding along beneath the warmer, lighter water it encounters on its journey at lower latitudes.