As stated earlier, the currents in the North Indian Ocean are influenced by the monsoon winds. North of 10°S latitude, there are seasonal changes in the direction of currents. In the northern part of the Indian Ocean, the following currents are produced by the summer and winter monsoons:
The Winter Monsoon Drift:
The Winter Monsoon Drift is also called the North-east Monsoon Drift. In the northern winter the north-east trade winds blow from the vast land mass of Asia towards the Indian Ocean.
During the months of February and March the North- East Drift starts flowing from the south-east of Sri Lanka towards the west in the fully developed form.
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On the other hand, in the Bay of Bengal and the Arabian Sea the ocean current flows from the northeast along with the prevailing north-east trade winds.
Thus, in winter there is a counter- clockwise circulation of currents in the Bay of Bengal and the Arabian Sea. During this season the Northeast Monsoon Drift carries with it enormous quantity of water and piles it up towards the coast of Eastern Africa.
During the period of winter monsoon there is the maximum development of the North Equatorial Current due to which the Equatorial Counter Current originates at about 7°S latitude.
This counter-current flows from the west towards the east between 2° and 8°S latitude. This current starts from Zanzibar on the east coast of Africa and continues up to the Island of Sumatra.
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Average velocity of this counter-current is 40 cm/sec. The North-east Monsoon Drift supplies water to this current. The N.E. Monsoon Drift bend southward near the east coast of Africa and converges with the Equatorial Counter Current.
Surface currents in the Arabian Sea are under the direct influence of the monsoon winds that blow from the northeast from November until March.
During the northeast or winter monsoon period the surface current moves south along the coast of Peninsular India and turns towards the west at about 10°N latitude.
A branch of these current moves towards the Gulf of Aden and the remaining part bends towards south along the Somali coast to join the North Equatorial Current.
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The South-west Monsoon Drift and the Somali Current;
With the beginning of the southwest or summer monsoon the North Equatorial Current disappears and is replaced by the Monsoon Current which flows from west to east.
Along the coast of East Africa a portion of the South Equatorial Current flows north. Since it flows along the Somali Coast it is called the Somali Current.
This is a very strong seasonal current with velocities in excess of 11 km/h. It continues along the coast of Arabia and India in a clock-wise pattern until it reaches 10°N latitude. Here it converges with the Southwest Monsoon Drift that has replaced the North Equatorial Current.
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In August and September, considerable upwelling takes place off the Somali Coast. It is due to the alignment of winds relative to the African and Arabian coasts. The currents in the Bay of Bengal are also influenced by the monsoon wind system.
During the southwest monsoon, there is a clockwise rotation in the bay. At the same time there is considerable upwelling along the east coast of Indian Peninsula.
With the retreat of the summer monsoon and the development of the winter or northeast monsoon in the month of November, the pattern of circulation undergoes a radical change. Now there is an anticlockwise rotation of currents in this bay.