Why Cyclones never happen during Southwest Monsoon?
This is one of the earliest questions that come up in the minds of weather bloggers as they discover their interest in weather & slowly start understanding the various nuances involved with tropical weather like that of Indian Sub Continent. This question becomes relevant today in the context of a possible Cyclone in Bay of Bengal that is likely to happen today. What was a cyclonic circulation about 24 hours back off the Myanmar coast has intensified into a Depression last evening & is expected to become a Deep Depression in the next few hours and possibly a Cyclone as well later tonight. If it does intensify into a Cyclone it will be named as Cyclone Gulab a name given by Pakistan.
IMD has issued a Pre Cyclone watch last evening for this depression expecting it to take a general westward track & make a landfall over North AP & South Odisha Coast sometime tomorrow evening / night and continue further on similar track over the northern parts of Peninsular India. If Cyclone Gulab happens then it would be only 3rd time a Cyclone has formed in Bay of Bengal during the month of September. Cyclone Daye happened during September 2018 over pretty much the same area while in the year 2005 Cyclone Pyarr sort off exhibited similar genesis & track to what is likely to be the track of upcoming Cyclone. Since the year 2000 only 6 times has a cyclone formed during the Southwest Monsoon Months.
So we come back to the question at the beginning. Why Cyclones never happen during Southwest Monsoon?
To answer this question we will have to go back to a phenomenon discovered by Prof. Koteswaram during the year 1958. “The Easterly Jet Stream in the Tropics” . He realized during his studies this Easterly Jet was a crucial component in the Southwest Monsoon dynamics & the created the necessary shear along with the Low Level Findlader Jet (Somali Jet) from the west necessary for a strong monsoon performance. But as Newton’s 3rd Law goes “Every Action has an equal & opposite reaction” while this Shear created by the Easterly Jet Stream on the one hand helps the overall monsoon dynamics it prevents the formation of Cyclones through the process of constant heavy shearing from the East thereby not allowing the convection to build up intensity into a cyclone. It is like how you prevent the milk vessel boiling over by blowing froth or shaving off the ice cream from the top so that the cone never tops up.