Causes of the Origin of Ocean Currents (continued from previous page)
Atmospheric Pressure. The atmospheric pressure affects the ocean currents in two ways.
(i) It is not the same at all places on the Earth. The water is pressed down when the atmospheric pressure is high and it rises up when the atmospheric pressure is low. Due to the difference in the levels of sea water caused by the differences in atmospheric pressure, water from higher level flows towards that of lower level. This originates ocean currents.
(ii) The prevailing winds also create ocean currents. The prevailing winds are those permanent winds, which blow, in the same direction throughout the year. Differences in atmospheric pressure originate them. These winds try to carry the ocean water along with them by friction. It is due to trade winds that the equatorial ocean currents flow from east to west. Due to the westerlies the Gulf Stream and the Keurosiwo ocean currents move towards east from the west.
Evaporation and Rain. More evaporation takes place in equatorial areas than in other latitudes. Evaporation reduces the amount of water in the ocean and increases salinity and density. The combined effect of all these things reduces the sea level and is the main cause of the movement of currents. This, is why the equatorial ocean currents travel towards Tropic of Cancer and that of Capricorn, Similarly ocean currents flow from polar areas towards middle latitudes. In order to maintain equilibrium ocean currents flow in opposite directions under the sea level.
The Changes of Seasons. Due to change of season the prevailing winds undergo changes in direction and in areas. It has an effect on ocean currents. When the sun is vertical in northern hemisphere, the flow areas of ocean currents shift a little towards the north. When the sun is vertical in the southern hemisphere, the areas of ocean currents shift a little towards the south.
The greatest effect of seasonal changes is experienced by the Monsoon winds. A change from summer to winter reverses the direction of the winds. Similar effect is seen on the directions of oceans currents.
The Shape of Coasts. The shape of coasts also creates ocean currents. The best example is offered by Cape San Roque. South equatorial current strikes at it and is divided into two branches (i) one branch travels north along the Brazilian coast and joins with the North Equatorial Current. (ii) The second branch flows south along the Brazil coast and is known as Brazil current.
The Gulf of Mexico has such a shape that it turns the northern branch of the South Equatorial Current into northeast and when the currents comes out of the Gulf, it is known as the Gulf Stream.
Types of Ocean Currents. Ocean currents are classified on the bases of temperature and speed.
(1) On the Bases of Speed. The ocean water current, which flows fast, is called a stream and that which flows slowly is called a drift. A good example of stream is the Gulf Stream in the Atlantic and that of drift is the West Wind Drift along Southern Australia.
(2) On the Bases of Temperature. Ocean currents, which have high temperature, are known as Warm Stream and those, which have low temperature, are called Cold Stream. Example of warm currents is the Gulf Stream Kuro Shio etc., and those of cold stream are Banguella, Labrador, etc.
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