North East Monsoon & South West Monsoon
The Monsoons of the Indian Ocean:
Over the centres of large oceans, there is a permanent low over the equator called the doldrums (around 1012 mb) and a permanent high at about 30°N and 30°S called the sub-tropical high or oceanic
high (about 1020 to 1025 mb). The trade winds blow from the oceanic highs, of 30°N & 30°S, towards the doldrums, being deflected by Coriolis force. They are thus NE trade winds in the Northern Hemisphere and SE trade winds in the Southern Hemisphere.
Over large landmasses, the atmospheric pressure is low during summer and high during winter. This seasonal change of atmospheric pressure over large landmasses results in seasonal winds, of which the Monsoons of the Indian Ocean are a classic example.
1) South West Monsoon:
During northern summer, the continent of Asia gets very warm and the resultant low pressure over it centres over the Thar Desert (NW part of the Indian sub-continent) with a pressure of about 994 mb. This low is considerably lower than the equatorial low of 1012 mb and hence a pressure gradient exists from the equator towards NW India. The SE Trade winds, blowing from the oceanic high of 30°S towards the
equatorial low, cross over the equator and blow, as a strong SW wind called the SW Monsoon, towards the low over NW India. The SW direction is the result of gradient force and Coriolis force. The SW Monsoon blows from June to October and brings heavy rain to the West Coast of India.
2) The North East Monsoon:
During northern winter, the continent of Asia gets cold and the resultant high pressure over it centres over Siberia with a pressure of about 1036 mb. The equatorial low of 1012 mb, being oceanic, remains practically unaffected by the change of season.The anticyclonic winds, around the Siberian high, reach the Bay of Bengal and Arabian Sea as the NE Monsoon with heavy rain falls on the East Coast of India. The NE Monsoon blows from December to April.
Ref : Marine Meteorology by Capt. H Subramaniam
