South Asia - Summer monsoon
Monsoon and regional disparities
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Overview
The difference in the warming of sea and land brings about a monsoon tropical airflow with its dimension and seasonal change of direction of decisive importance for the vegetation and human life in South Asia.
Pre-monsoon and summer monsoon
From March, elements of atmospheric circulation shift abruptly, with changes in the Sun to the north. Only the southern branch of the west wind jets remains cold, due to the rugged Tibetan plateau south of the Himalayas. The weather becomes hot and dry in northern India due to the high irradiance. This is known as the "pre-monsoon phase". Interaction of cold air with humid and unstable air masses of tropical disturbances results in low pressure atmosphere "squall lines" on the ground in the Ganges delta. In the Ganges delta, the overlapping of cold air at high altitudes and warm, moist, unstable tropical air masses at ground level produces the first low-pressure disturbances, which bring higher precipitation. This marks the beginning of the monsoon in Bengal, Bangladesh, Assam, and Buam.
In early summer, an easterly current develops over India due to the heating of the Central Asian plateau at high altitude. The formation of this current opens the way for the south-west flow of the actual summer monsoon to India. This flows towards the Intertropical Convergence Zone, whose low-pressure trough moves as far north as the subcontinent in summer. As a result, the monsoon rainfall initially begins in southwest India at the end of May. The precipitation is particularly heavy where the monsoon currents meet the Western Ghats. Convection clouds form there and lead to increased monsoon precipitation in the coastal mountain backwater. With the northward migration of the rain front or the inner-tropical convergence zone, the area of maximum precipitation shifts northwards from May to July, only to retreat back towards the equator as the solar maximum recedes. Between July and November, the monsoon front with its precipitation therefore crosses the Indian subcontinent a second time.
However, the monsoon rainfall over India derives not only from the south-west monsoon current, but also from so-called monsoon depressions that develop over the warm Gulf of Bengal. With a horizontal extension of over 1000 kilometres, these are integrated into the tropical easterly current and move from the Bay of Bengal to the west or northwest. Over land, they lead to extremely high precipitation, especially in eastern India and Bangladesh. There, in the Himalayan backwater, the highest rainfall totals on earth are achieved.
Between the low pressure vortices and the southwestern monsoon flow from the Indian Ocean, the hinterland in southern and western India (Deccan Plateau), located in the lee of the coastal mountains, remains comparatively low in precipitation almost all year round.
