Indian monsoon, mechanism, onset and retreat

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The word “monsoon” comes from the Arabic word mausim, meaning season.Monsoons refer to a system of winds in the tropical regions under which the direction of winds is reversed completely between the summer and the winter seasons. Under this system, the winds blow from land to sea in winter and from sea to land in summer. Therefore, most of the rainfall in the regions influenced by the monsoons is received in the summer season while winter season is generally dry.

Due to a higher temperature over the land in summer, a low pressure area develops over the continents and the winds blow from neighbouring oceans towards the land. These winds are of maritime origin and hence cause ample rainfall in summer. On the other hand, the continents become colder than the neighboring oceans in winter. As a result a high pressure area is developed
over the continents. Therefore, winds blow from land to sea in winter. These winds, being of continental origin, are dry and do not cause rain.

Role of Jet streams, Tibet Plateau and atmosphere-ocean circulation is of prime importance for the origin of Indian Monsoon.During this summer season the vertical sun rays at tropic of cancer ,the circum polar whirl along with westerly jet stream weakened and shifted northward of the Himalayas and southern branch of this jet stream disappeared by 6-10 June. This removal of westerly jet stream to the north of Tibet plateau leads to reversal of curvature of flow of free air to the north and northwest of the subcontinent. It further leads to development of dynamic depression overlying the thermal depression already established at the surface of northwest Indian subcontinent which ultimately help to trigger the burst of Indian monsoon along with it vigorous advance over the Indian subcontinent.

As long as the position of upper air jet stream is maintained above the surface low pressure to the south of the Himalayas the dynamic anticyclonic conditions persist in the northwestern part of Indian subcontinent which obstruct the ascent of air from the surface low as the air is continuously descending from the above. This is the reason why the months of April, May and June are dry inspite of high temperature and evaporation while upper air low pressure in the east of the Himalayas over Myanmar, Assam and Bangladesh helps in ascend of air and produce rainfall.

During October and November, the high pressure over northern lndia, the winds start blowing from northeast as retreating monsoons. Clear skies and the increase in mercury northern plains is observed which is termed as ‘October heat’. At this time, the low pressure conditions shift to the Bay of Bengal giving rise to cyclonic depressions. These cyclonic depressions often results in the destruction of life and property generally on the eastern coast and the southern coast. Most of the rainfall of the Coromandel Coast is derived from depressions and cyclones.

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