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.