Friday, 12 December 2014 04:49




Monsoon comes from an Arabic word ‘MAUSAM’ which means season. Thus monsoon are seasonal winds which reverse their direction of flow with the change of season. They flow from sea to land during the summer and from land to sea during winter.

The theories regarding the monsoons are generally divided into following two broad categories:

1. Classical theory: Halley explained the monsoon phenomenon that occurs due to thermal contrasts between continents and oceans due to their differential heating.

In summer the sun shines vertically over the tropic of cancer resulting in high temperature and low pressure in central Asia while the pressure is still sufficiently high over Arabian Sea and Bay of Bengal. This induces air flow from sea to land and brings heavy rainfall to India and her neighbouring countries.

In winter the sun shines vertically over the tropic of Capricorn. The north western part of India grows colder than Arabian Sea and Bay of Bengal and the flow of the monsoon is reversed.

It was lacking in the physical ingredient of the effect of rotation of the earth.

2. Modern theory:

a) Role of ITCZ; According to modern theory of monsoon is only the normal seasonal migration of planetary winds following the sun. According to this the existence of Asian monsoon is not due to contrast between land and sea but mainly due to the annual migration of thermally produced planetary winds and pressure belts under continental influence. The southeast trade winds of the southern hemisphere cross the equator and start flowing from southwest to northeast direction under the effect of Coriolis force. These displaced trade winds are called south west monsoon and bring monsoon to the region.

b) Role of jet streams; M.T.Yin had given this concept stating that the burst of monsoon depends upon the upper air circulation. Two prominent jet streams effect the monsoon winds

• The sub tropical westerly jet stream, this jet stream dominates in winter time in upper troposphere circulation of the northern latitudes. It has a global extent between latitudes 25-32 ° N and can be located over south Asia at an elevation of about 12 km. The jet stream is split owing to the presence of Himalayan mountain system in its path. The winds tend to descend over north-western part of India resulting in atmospheric stability.

• Equatorial easterly jet stream, this jet is a prominent feature of the upper air circulation during the Indian monsoon season appearing as a band of strong easterlies extending from south East Asia across the Indian Ocean and Africa to the Atlantic.

The western and eastern jet streams flow in the north and south of the Himalayas respectively. The eastern jet becomes powerful and stationed and this results in more active south west monsoon.

c) Role of Tibetan plateau, the Tibetan plateau is located more than 4500 km above sea level with a length of 2000 km and with a width of 600 km in the west and 1000 in the east. This plateau is considered to be one of the key factors in the development of monsoon. The Tibetan plateau exerts its influence as a mechanical barrier as well as high heat plateau. An anticyclone appears in upper troposphere due to latent heating over the Tibetan plateau. It generates an area of rising air, during its ascent the air spreads outwards and gradually sinks over the equatorial part of the Indian Ocean. It picks up moisture from the Indian Ocean and causes rainfall in India and adjoining countries.




Last Updated on Friday, 12 December 2014 05:22

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