Ocean currents are large masses of surface water that circulate in regular patterns around the oceans, as shown in the world map in Fig. 12.9.
Those that flow from equatorial regions polewards have a higher surface temperature and are warm currents. Those that flow from polar regions equatorwards have a lower surface temperature and are cold currents. Their direction of movement is indicated by the arrows. But why should they follow such a pattern? Some of the underlying causes or factors are explained below.
1) The planetary winds: Between the equator and the tropics the Trade Winds move equatorial waters polewards and westwards and warm the eastern coasts of continents. For example, the North-East Trade Winds move the North Equatorial Current and its derivatives, the Florida Current and the Gulf Stream Drift, to warm the southern and eastern coasts of Brazil as the warm Brazilian Current.
In the temperate areas, latitudes blow the Westerlies. Though they are less reliable than the Trade Winds, they result in a north-easterly flow of water in the northern hemisphere, so that the warm Gulf Stream is driven to the western coast of Europe as the North Atlantic Drift.
In a similar manner, the Westerlies of the southern hemisphere drive the West Wind Drift equatorwards as the Peruvian Current off-South America and the Benguela Current off southern Africa. The planetary winds are probably the dominant influence on the flow of ocean currents. The strongest evidence of prevailing winds on current flows is seen in the North Indian Ocean. Here the direction of the currents changes completely with the direction of the monsoon winds which come from the northeast in winter and south-west in summer.
2) Temperatures: There is much difference in the temperature of ocean waters at the equator and at the poles. As warm water is lighter and rises, and cold water is denser and sinks, warm equatorial waters move slowly along the surface polewards, while the heavier cold waters of the polar regions creep slowly along the bottom of the sea equatorwards.
3) Salinity: The salinity of ocean water varies from place to place. Waters of high salinity are denser than waters of low salinity. Hence waters of low salinity flow on the surface of waters of high salinity while waters of high salinity flow at the bottom towards waters of low salinity.
For example, in the Mediterranean regions, there is great difference in salinity between the waters of the open Atlantic and those of the partially enclosed Mediterranean Sea. The less saline water of the Atlantic flows on the surface into the Mediterranean, and this is compensated for by an outflow of denser bottom water from the Mediterranean.
4) The earth’s rotation: The earth’s rotation deflects freely moving objects, including ocean currents, to the right. In the northern hemisphere this is a clockwise direction (e.g. the circulation of the Gulf Stream Drift and the Canaries Current). In the southern hemisphere it is an anti-clockwise direction (e.g. the Brazilian Current and the West Wind Drift).
5) The shape of the land masses: Currents flowing westwards pile up against the coasts towards which they flow. This results in the current being deflected northwards and southwards. It may even divide into a number of branches; for example, the South Equatorial Current divides on reaching Malagasy and the East African coast.