Monsoon theories

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MONSOONMonsoon is a complex meteorological and atmospheric circulation phenomenon. Earlier, it was considered to be a simple atmospheric circulation But over a period of time experts through researches at various levels try to prove this complex phenomenon more accurately.


THERMAL THEORYThis classical theory of Indian monsoon proposed and put forth by Admand Hally in 1686Monsoonal winds are land and sea breezes produced by unequal and differential heating of continental and oceanic areas.

NORTH EAST MONSOONDuring the northern winter when sun rays fall vertically over tropic of Capricorn the land mass of Asia cools rapidly than the ocean generating a high pressure area over Asia while there is low pressure Centre on the Indian ocean. As a result of this there is an out flow of air from high pressure to low pressure, consequently the surface winds started to flow from land to sea. This wind pattern is commonly known as north-east monsoonOften dry due to lack of moisture and hence do not precipitate.

SOUTH WEST MONSOONDuring the northern summer season sun rays fall vertical on tropic of Cancer Due to excess heating of huge land mass of Asia a low pressure area develop over it. Conversely, high pressure Centre is developed in the Indian Ocean due comparative low thermal conditions. As a result of this winds started flowing from high pressure area over ocean to low pressure area over Indian subcontinent. This pattern of wind flow is popularly known as southwest monsoon in India.Due to on shore nature of southwest monsoon it bears and produces rainfall wherever it is obstructed by various topographical barriers.

DISADVANTAGESThe thermal and classical theory proposed got wide popularity and support of the scholars throughout the world due its simplicity to explain the Indian monsoon.This concept fails the explanations of onset, breaks and spatial distribution of rainfall over the Indian subcontinent.

JET STREAM THEORYThe jet streams are a narrow belt of high altitude westerly winds in the troposphereIts speed varies from about 110km/h in summer to about 184 km/h in winter. The main types are the polar jets, the subtropical westerly jets and the less common tropical easterly jets.


TIBETIAN PLATEAUIt is located in a altitude of 5000 meters It spread over about 2 lakh square kilometers. The Tibet plateau act as a heat engine .

ROLE OF TIBETIAN PLATEAUIt acts as a mechanical barrier for subtropical westerly jet stream to completely withdraw from India which helps burst of Indian monsoon. It produces a thermal anticyclone in mid troposphere over this region. This anticyclone weakens the western subtropical jet stream to the south of Himalayas but produces tropical easterly jet on the southern side of the anticyclone

SOUTH WEST MONSOONThe high temperature over the Tibetan Plateau, as well as over Central Asia in general, during the summer leads to the formation of the tropical easterly jet over India in summer. This tropical jets first develop in longitudes east of India and then extends westward across India, over the Arabian Sea and to eastern Africa.Air under this jet stream blowing along Kolkata-Bangalore axis, the air descends over Indian Ocean and southwest monsoon moves towards Indian subcontinent.

CONTDThe onset of SW Monsoon is driven by the shift of the subtropical westerly jet northwards towards the Tibetan Plateau. As the Tibetan Plateau heats up, the low pressure created over it pulls the westerly jet northwards.The westerly jet is inhibited from moving northwards by Himalayas. However, with continuous heating, sufficient force for the movement of the westerly jet across the Himalayas is created at a significant level. As such, the shift of the jet is sudden and abrupt causing the bursting of SW Monsoon rains onto the Indian plains.


The westerly jet causes high pressure over northern parts of the subcontinent during the winter. This results in the north to south flow of the winds in the form of the NE Monsoon.

Ocean Bodies and Air CirculationsEl Nino- It is a narrow warm current which appears off the coast of Peru in December. when the surface temperature goes up in the southern Pacific Ocean, India receives deficient rainfall.Southern Oscillation- It is a curious phenomena of sea-saw pattern of meteorological changes observed between the Pacific and Indian oceans.

REFERENCEConcept, Mechanism and Behaviour of Indian Monsoon. Pawan Kumar. SHRINKHALA : VOL-II * ISSUE-II*