- Classcial Theories:
- Halley’s Thermal concept: Indian monsoon is because of contrast between Indian Subcontinent and adjoining Indian Ocean. Halley’s theory, suggested in 1686, considers the summer monsoon to be a regional phenomenon.
- Aerological concept: This was suggested by R. Scherhag in 1948. According to this theory, monsoon circulation develops due to changes in air temperature at all levels over the Indian Subcontinent and adjoining Indian Ocean.
- Modern theories:
- Dynamic Theory: Flohn (1951) according to Flohn, the monsoon is a global phenomenon due to global shift in pressure belts. The shift in pressure belts bring the ITC much more into the northern margin is around 30 degrees north over the Indian subcontinent. This brings the SE trades over the Indian subcontinent as south westerly monsoons. Similarly, the shift in pressure belts globally when it is winter for the N. Hemisphere pushes the ITC to a little more south of the equator. This brings the sub- tropical high on to the southern slopes of Himalayas and hence the NE trades blow from northeast to southwest as northeast monsoons.
- Tibet and Easterly Jet: P. Koteshwaram and Flohn concluded that heating of Tibet in summer strengthens the monsoonal circulation with an average height of 4Kms above the MSL, Tibetan surface is warmed in summer and generates ascending warm air. The air turns to its right and sinks over the Arabian sea and joins the southwesterly winds there by strengthening the monsoon. This circulation is part of the tropical easterly jet stream.
- Role of Sub- Tropical westerly Jet: The sub-tropical westerly jet normally located on the south slopes of Himalayas and the northeast plains disintegrates in summer due to intense heating of the northern plains and the global shift in pressure belts to the north in summer of the northern hemisphere. This facilities the onset of the monsoon over india by facilitating the development of the monsoon trough.
- The Somali Jet: The offshore areas of Somalia develop clod waters due to upwelling giving birth to the Somali current. The cold waters led to low temperature along Somali coast ( 15 degrees centigrade) whereas along Mumbai, the temperature is 30 degrees cent centigrade. This thermal gradient leads to development of the Somali jet stream blowing from western Arabian sea to the eastern Arabian sea. This adds moisture bearing winds to the southwest monsoon.
- The Monsoon trough: This is the ITC which forms as a huge gash of low pressure from the desert of Arabian to Bay of Bengal. This covers the northern plains and induces the SE trades to cross the equator and give birth the south westerly winds.
- El Nino: The development of El Nino conditions influence the monsoon. During El Nino, the southern oscillation (i.e., the differences in pressure between port Darwin and Tahiti, French Polynesia). If the southern oscillation is negative, Darwin pressure is higher than Tahiti due to high pressure over Australia, western pacific and adjacent Indian ocean. The high pressure conditions also cover the Indian ocean and the landmass of peninsular India. This prevents the movement of southeast trades towards the landmass of India. If the southern oscillation is positive, Tahiti pressure is more than that of Darwin. Low pressure is over North Australia and over adjacent Indian Ocean. This induces northward movement of southeast trades and hence strengthens monsoonal circulation. The combination of El Nino current (which appears as a warm saline current along the west coast of S. America replacing the cold Humboldt current during a negative El Nino) and the southern oscillation is caused ENSO.
Onset of Monsoon
The summer monsoon over the Indian subcontinent first arrives over Kerala situated at the southern tip of the Indian Peninsula around 1st June with a standard deviation of about 7 days. The arrival of the monsoon over the region is noticed by wide spread persistent and heavy rainfall replacing the occasional pre-monsoon rains.
Retreat Of Monsoon
This lasts between September and end of December. This is the season of retreating monsoons. The monsoon trough over the northern plains weakens and begins to shift southward. The retreating monsoon brings rain to coastal Tamil Nadu, Krishna- Godavari delta, region west of Aravallis and to northeast India. The retreat of the monsoon begins on 1st September (i.e., in Rajasthan), 15th September in Punjab- Haryana plains, 10th October from Ganges plains, 1st December from Tamil Nadu and by 15th December, the southwest monsoon withdraws from india completely. The retreating monsoon brings rainfall to AP south of Krishna delta and Tamil Nadu. The rainfall in Tamil Nadu in October/November, is 38.25 cms which is about 39% of its annual rainfall while for coastal Tamil Nadu it is 50% of the annual.
This is the season for tropical cyclones that affect the Indian landmass. Cyclones that affect the east coast and the west coast from in the southern portions of the Bay of Bengal, particularly during the retreating monsoon season. Maximum number of that form in the Bay of Bengal form in November. More number of cyclones form in the bay of Bengal than the Arabian sea.