THE ORIGIN AND MECHANISM OF INDIAN MONSOON
Monsoon is the phenomena of seasonal reversal of wind along with corresponding change in the precipitation. In Indian context, the summer monsoon (June to September) causes rainfall across the country, whereas the other phase i.e. the retreating monsoon, causes rainfall in south-eastern coast of India during winter. There are various factors behind the origin of Indian Monsoon-
The differential heating and cooling of land and water creates low pressure on the landmass of India while the seas around experience comparatively high pressure.
The shift of the position of Inter Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain (this is the equatorial trough normally positioned about 5°N of the equator). Presence of the high-pressure area in the east of Madagascar
Intense heating of the Tibetan plateau during summer causing strong vertical currents.
Movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer.
Mechanism of Indian monsoon: The seasonal reversal of wind is due to the shifting of Inter-Tropical Convergence Zone (ITCZ), the low-pressure region near the equator. As a result, In July, when the ITCZ is located in the north, it creates the Monsoon Trough.
This encourages the development of thermal low over the North and Northwest India. Due to this shift of the ITCZ, trade winds of the southern hemisphere cross the equator between 40oE and 60oE longitudes and start blowing from southwest to northeast due to the Coriolis force. It becomes Southwest monsoon.
In winter, the ITCZ moves southward, and so the reversal of winds, from the northeast to south and southwest takes place thus leading to Northeast monsoon.
The moisture is picked up from water bodies, mostly the Arabian Sea and the Bay of Bengal. The shape of the landmass and mountains (Western Ghats in the south and Himalayas in the North) guide the winds’ direction.
Uneven distribution of monsoonal rainfall in India
Temporal distribution- The monsoon may burst in the first week of June in the coastal areas of Kerala, Karnataka, Goa and Maharashtra while in the interior parts of the country; it may be delayed to the first week of July. Monsoonal rainfall is largely governed by relief or topography and rainfall has a declining trend with increasing distance from the sea. By the end of September, the monsoon becomes weak in response to the southward march of the sun.
Spatial distribution- In terms of intensity, maximum rainfall (above 200 cm) occurs on the western coast
and north-eastern hills. Moderate rainfall (100-200cm) occurs in some parts of the Western Ghats and West Bengal. Inadequate rainfall (Less than 60cm) occurs in western part of Rajasthan and Gujarat, Ladakh and south-central part receives rainfall of less than 20cm.
Factors behind this uneven distribution-
Distance from the sea (Continentality): As the distance from the sea increases, the moisture content of the wind decreases. E.g. Patna receives more rainfall than Delhi from the Bay of Bengal branch of monsoon winds.
Relief: The windward side of mountains and hills receive more rainfall than leeward side. E.g. Western
Ghats as compared to Eastern Ghats.
Physical obstacles- Those areas which have no mountains to check the rain bearing winds get scanty
rainfall. E.g. Aravalli hills standing parallel to monsoon bearing winds.
El-Nino: This warming of tropical Pacific waters affects the global pattern of pressure and wind systems. Strong El Nino causes dry conditions and deficient monsoon in Indian continent.
Indian Ocean Dipole: Also called Indian Nino, it is a condition where sea surface temperature of the western region of Indian Ocean becomes abnormally colder (negative) and hotter (positive) than the eastern region in alternative phases. The positive’ IOD phase brings more rain during monsoon and vice versa. Further, the phenomenon of Madden Julian Oscillation (MJO) also contributes in a good monsoon.
Cyclonic disturbances: such as cyclonic depressions over the Andaman Sea causing rainfall over the Eastern coast. Over recent decades the uneven nature of the monsoon or rainfall pattern has aggravated causing unprecedented rainfall. Of late, climatic changes because of global warming have resulted in
unpredictable rainfall pattern all over the world including India.
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