The term monsoon is derived from an Arabic word ‘mausim’ or from the Malayan ‘monsim’ which means ‘season’. The word ‘monsoon’ is applied to such a circulation which reverses its direction every six months, i.e. from summer to winter and vice versa.
The term was first applied by the Arab navigators to winds over the Arabian Sea between Arab and India, which blow for 6 months from the northeast and for another 6 months from the southwest.
In meteorology, monsoon signifies the directional shifting of winds from one season to the other. In summer, there is a warm and moist wind blowing from the ocean towards land, while during the winter months a cold and dry wind originating on land blows seaward.
Monsoon circulation involves a change of 180 degrees in the direction of wind. If, however, this criterion of a seasonal wind reversal is applied strictly, then only a few regions of the world have monsoon wind systems. Here, it may be pointed out that originally this meaning of monsoon was applied with reference to what is today known as Monsoon Asia.
Undoubtedly, it is in this part of the earth that monsoonal circulation is found in the ideal form. There are other continents such as the United States of America, Northern Australia, West Africa, where only pseudo- monsoons or ‘monsoon tendencies’ are to be observed.
The phenomenon of monsoon winds and their characteristic reversing circulation used to be explained by the differential heating of land and water. It was also ascribed to the seasonal shifting of tropical and subtropical wind belts.
But there are certain peculiar aspects of the monsoon circulation such as its burst or sudden transition from dry to wet season, and the vagaries of rainfall associated with it which cannot be fully explained by the aforesaid causes alone.
Recent investigations into the upper atmospheric circulation have led meteorologists to look for a more complete explanation by examining the significant role played by the upper atmospheric wind movements and the jet streams.