Briefly discuss the causes of origin of the south west monsoon. (200 words)
Answers
Onset of Monsoon is a complex phenomenon whose origin is not fully understood yet. Nevertheless, the following causes do lead to the origin of South West Monsoon:
1. Northward shifting of ITCZ => Such a shift in June causes low pressure at Tibetan plateau. Tropical Easterly Jets move towards the Mascarene High and moist monsoon winds from Mascarene High move towards Tibetan plateau. The occasional merging of
second equatorial trough with ITCZ further aids the process.
2. Appearance of Somali Jet Stream in Summer => Somali Jet Streams appear in summer and intensify the Somali Ocean current. This pushes monsoon winds towards India
3. Absence of STWJ in winter => In winters, Sub Tropical Westerly Jet (STWJ) gets bifurcated and its one strand causes high pressure in India. In summers, entire STWJ is beyond Himalayas and hence low pressure is maintained over India.
4. Indian Ocean Dipole and Walker Cell => The Indian Ocean Dipole between Western Pacific Pool and Mascarene High pushes monsoon winds towards India. In an El Nino year, this push is weak whereas in La Nina year it is strong.
The cumulative effect of these factors is that a low pressure is created over the Indian subcontinent while a high pressure is formed in the Indian Ocean. This pushes the moist, rain bearing monsoon winds from the Arabian sea towards the subcontinent. The blockage of these winds by Western Ghats causes rains in the southern region while that by Himalayas leads to downpours over North India.