Ocean currents are like river flow in oceans. They represent a regular volume of water in a definite path and direction. Ocean currents are influenced by two types of forces.
- Primary forces that initiate the movement of water
- Secondary forces that influence the currents to flow
The primary forces that influence the currents are:
- Heating by solar energy
- Coriolis force
Heating by solar energy causes the water to expand. That is why, near the equator the ocean water is about 8 cm higher in level than in the middle latitudes. This causes a very slight gradient and water tends to flow down the slope. Wind blowing on the surface of the ocean pushes the water to move. Friction between the wind and the water surface affects the movement of the water body in its course. Gravity tends to pull the water down the pile and create gradient variation. Coriolis force intervenes and causes the water to move to the right in the northern hemisphere and to the left in the southern hemisphere.
Types of Ocean Currents
Based on Depth
- Surface currents constitute about 10% of all the water in the ocean. These waters are the upper 400 m of the ocean
- Deep water currents make up the other 90% per cent of the ocean water. These waters move around the ocean basins due to variations in the density and gravity.
Based on Temperature
- Cold currents bring cold water into warm water areas. These currents are usually found on the west coast of the continents in the low and middle latitudes (both hemispheres) and on the east coast in the higher latitudes in the Northern Hemisphere
- Warm currents bring warm water into cold water areas and are usually observed on the east coast of continents in the low and middle latitudes (both hemispheres). In the northern hemisphere they are found on the west coasts of continents in high latitudes.
Major Ocean Currents
Currents of Atlantic Ocean
To the north and south of equator there are two westward moving currents - the north and south equatorial currents. The South Equatorial Current bifurcates into two branches near the Cape De Sao Roque in Brazil. Its northern branch joins the North Equatorial Current. This combined current enters the Caribbean Sea and the Gulf of Mexico, while the remaining current passes along the eastern side of the West Indies as the Antilles Current.
The part of the current which enters the Gulf of Mexico, comes out from the Florida strait and joins the Antilles Current. This combined current moves along the south eastern coast of U.S.A.. It is known as Florida Current upto cape of Hatteras. Beyond the Cape Hatteras, upto the Grand Banks, off New Foundland, it is called the Gulf Stream.
From the Grand Banks, the Gulf Stream is deflected eastwards under the combined influence of the westerlies and the rotation of the earth. It crosses the Atlantic Ocean as North Atlantic Drift. The North Atlantic Drift bifurcates into two branches on reaching the eastern part of the ocean. The northern branch continues as North Atlantic Drift; reaches the British Isles from where it flows along the coast of Norway as the Norwegian Current and enters the Arctic Ocean. The southern branch flows between Spain and Azores Island as the cold Canaries Current.
The Canaries Current finally joins the North Equatorial Current and completes the circuit in the North Atlantic Ocean.
Apart from the clockwise circulation of the currents in the North Atlantic Ocean, there are also two cold currents - the East Greenland Current and the Labrador Current which flow from the Arctic Ocean into the Atlantic Ocean. The Labrador Current flows along the eastern coast of Canada and meets the warm Gulf Stream. The confluence of these two currents, one cold and the other hot, produces fog around Newfoundland and makes it the most important fishing ground of the world. East Greenland current flows between Iceland and Greenland and cools the North Atlantic Drift at the point of their confluence.
South Equatorial Current splits into two branches near Cape De Sao Roque (Brazil). The northern branch joins the North Equatorial Current, whereas the southern branch turns south and flows along the eastern coast of South America as Brazil Current. At about 35° south latitude the influence of the westerlies and the rotation of the earth propel the current eastward to merge with the West Wind Drift.
Near the Cape of Good Hope, the South Atlantic Current is diverted northward as the cold Benguela Current. It finally joins the South Equatorial Currents thus completing the circuit. Another cold current, known as the Falkland Current, flows along the South eastern coast of south America from south to north.
Currents of Pacific Ocean
In the Equatorial belt of the Pacific Ocean, two streams of equatorial currents flow across the ocean from the Central American Coast - the North Equatorial Current and the South Equatorial Current.
The North Equatorial Current turns northwards and flows along the Philippines Islands, Taiwan and Japan to form the warm Kuroshio current. From the southeast coast of Japan, the current comes under the influence of westerlies and flows right across the ocean as North Pacific Current. After reaching the west coast of North America, it bifurcates into two branches.
The northern branch flows anti clockwise along the coast of British Columbia and Alaska and is known as the Alaska Current. The other branch of the North Pacific Current moves southward along the coast of California as the Cold Californian Current. It eventually joins the North Equatorial Current to complete its circuit.
In the northern part of the Pacific Ocean two cold currents also flow. These are the Oya Siwo Current and Okhotsk Current. The cold Oya Siwo Current flows along the coast of the Kamchatka Peninsula. Another cold current, Okhotsk Current flows past Sakhalin to merge with the Oya Siwo Current near Hokkaido Island. It later merges with Kuro Siwo Current and sinks beneath the warm waters of the North Pacific Currents.
In the South Pacific Ocean, the South Equatorial Current flows towards west and turns southwards as the East Australian Current. It then meets near Tasmania the cold South Pacific Current which flows from west to east. On reaching the South Western Coasts of South America, it turns north wards as the cold Peru Current. It then meets the South Equatorial Current and completes the circuit.
Currents of Indian Ocean
During winters Srilanka divides the currents of the Arabian sea from those of the Bay of Bengal. The North Equatorial Current flows westward just south of Srilanka. At this time in the northern section, the whole of Bay of Bengal and Arabian sea is under the influence of North East Monsoon. The North East Monsoon drives the water of Bay of Bengal and Arabian Sea west wards to circulate in an anti clockwise direction. This current is known as North East Monsoon Drift.
In summers, the northern section comes under the influences of South West Monsoon. There is an easterly movement of water in the Bay of Bengal and Arabian Sea and produces a clockwise circulation. This current is known as South West Monsoon Drift.
In the southern Indian Ocean, the South Equatorial Current, strengthened by its corresponding current of the Pacific Ocean, flows from east to west. It turns south-wards along the Coast of Mozambique in Africa. A part of this current which flows between the mainland and the Madagascar Island is known as warm Mozambique Current. After the confluence of these two currents, it is called Aghulas Current. It then turn eastwards and merges with the West Wind Drift.
A branch of this stream turns north to flow along the western coast of Australia as cold West Australian Current. West Australian Current later joins the South Equatorial Current to complete the circuit.