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The suddenness and abruptness of the Indian monsoon is known as the ‘burst of monsoonThe onset over the Indian sub-continent is abrupt and dramatic. It is always accompanied by turbulent weather. The so called burst of monsoon is associated with certain basic changes in the general upper-air circulation over southern Asia. In April and May, the insolation heating of the sub-continent tends to establish the south-westerly monsoon flow from the adjacent warm ocean, but northward surge of the same is retarded by the westerly zonal flow associated with the subtropical jet stream over northern India. However, in late May or early June, when the thermal conditions are satisfied, the jet stream disappears completely from the south of the Himalayas and shifts to a position to the north of the Himalayas and Tibet. At the same time, the upper trough low (low pressure) also moves westward from 85° E to 75°E. It may be pointed out that the jet stream does not retreat slowly. The process of this shift is rather quick. Now, with the disappearance of the jet, a definite monsoon circulation from the sea on to
the land is established. The summer monsoon generally begins in late May in most parts of south east Asia. But over India, it is delayed until the middle or late June. The change from one regime to another is abrupt. It is well to remember that the onset of monsoon occurs in several stages depending on the periodic advance and withdrawal of the equatorial convergence zone.
In the opinion of Koteswaram, the burst of monsoon is closely related to the development of a warm-core upper anticyclone (high pressure) over the extensive and lofty Tibetan Highland. This upper level anticyclone produces an easterly jet over India which is positioned at about 15°N. It is definitely a part of readjustment in the general planetary circulation pattern. Gradually, the easterly jet covers the entire region extending from India to east Africa. The above mentioned upper air condition paves the way for the advance of south-westerly monsoon current over the subcontinent. The monsoon current, therefore prevails throughout India. The depth of the monsoon in India is about 6.5 km, while over the Gangetic Plain it is only about 5 kilometres. The current is overlain by a layer of easterly winds (easterly jet stream).
The abrupt arrival of monsoon is of great climatic and social significance to the people of the subcontinent of India. The onset of monsoon puts an end to the scorching weather and the local hot winds (loo) in the northern plains of India. The relative humidity increases in the atmosphere tremendously. The arrival of monsoon is also the beginning of agricultural operations for the kharif crops in the rain-fed areas. The high temperature and high relative humidity are, however, oppressive and injurious to health. It is in the season of general rains (July to September) that people suffer from many diseases and epidemics.