The suddenness, with which the southwest monsoon surges over the Indian sub-continent in June, is termed as the ‘burst of monsoon’. The onset of summer monsoon in India 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 insolational heating of the sub-continent tends to establish the southwesterly 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 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 also moves westward from about 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 very 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 of the Southeast Asia.
But over India it is delayed until 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.
According to Koteswaram, the burst of monsoon is closely related to the development of a warm-core anticyclone 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 patterns. Gradually the easterly jet covers the entire region extending from India to eastern Africa. The above mentioned upper air conditions pave the way for the advance of southwesterly monsoon current over the subcontinent.
The monsoon current thereafter prevails throughout India. The depth of the monsoon current in southern India is given as 6.5 kilometers, while over the Gangetic Plain it is only 5 kilometers. This current is overlain by a layer of easterly winds.
The abrupt and dramatic arrival of monsoon is of great climatic as well as economic significance to the inhabitants of the sub- continent. It puts an end to the dry heat and scorching hot winds known as ‘loo’ in the Gangetic Plain in India.
The entire atmosphere is surcharged with humidity. Sowing operations of kharif crops start. However, high summer temperatures combined with high relative humidity make the weather sultry which makes life miserable.
Even with slight physical exertion, people begin to perspire. Even the nights are warmer and uncomfortable. However, the maximum temperature records a sudden and substantial drop, while the minimum temperature registers a slight increase.