The regular movement of water from one part of the ocean to another is called ocean currents. The ocean currents are mainly caused by the difference in density of sea water due to variations in temperature and salinity. The prevailing winds push them onwards. The position of the land masses and shape and depth of the ocean basins also have some influence.
The origin and nature of the movement of the currents are related to four sets of factors:
(i) Factors related to earth's rotation: Gravitational or centrifugal force and force of deflection.
(ii) Factors originating outside the sea: Atmospheric pressure belts and planetary winds.
(iii) Factors within the sea: Causing variation in the density of sea water due to salinity, temperature and pressure etc.
(iv) Factors modifying the ocean currents: Direction and shape of the coast, seasonal changes and bottom topography.
As a result of interplay of these factors, the currents reveal the following characteristic:
1. Due to earth rotation currents move to their right in the northern hemisphere and their left in the southern hemisphere.
2. The atmospheric pressure creates a difference in the level of the sea water and water from higher level flow towards lower level. The prevailing winds carry oceanic water with them by friction. Due to trade winds equatorial currents flow from east and west and due to westerlies, Gulf Stream and Kuroshio moves towards east from West.
3. Due to temperature differences, warm water, which is lighter and so higher in level floats from the equator towards the poles. The cool and heavy polar water sinks and flows towards the equator under the surface.
The saline water is heavy and therefore subsides. To take is placeless saline water rushes towards this area e.g., the Mediterranean sea more saline than that of the Atlantic Ocean. As a result a current from the Atlantic flows towards the Mediterranean and another current from the Mediterranean flows to the Atlantic below the surface.
4. The shape of coasts also creates ocean currents. The best examples offered by Cape San Roque. South Equatorial Current strikes as it and is divided into two branches:
(1) One branch travels north along the Brazilian coasts and joins with the North Equatorial Current.
(2) The second branch flows south along the Brazil coast and is known as Brazil Current.
