Indian seas have predominantly two stormy seasons, Pre Monsoon (March-May) and Post Monsoon (October-December). Respective peaks during the stormy seasons are in May and November. Pre Monsoon storms of May on either side of the coastline are consequential for the onset of monsoon, subject to the timings of these systems.
Monsoon onset dates were revised in 2020 based on 60 years of data from 1960 to 2019. The onset date for Port Blair (Andaman Nicobar) is 22nd May, Sri Lanka 26th May and the mainland (Kerala) 01st June. Pre Monsoon cyclones of May in the Indian seas have unique characteristics and differ from basin to basin.
Monsoon circulation is a huge synoptic scale global phenomenon, largely confined to the tropics. It is predominantly wind-driven, yet gets modified by ocean thermals and freshwater fluxes. Prominent limbs as part of the monsoon circulation are 'Low-Level Jet' (LLJ) and 'Tropical Easterly Jet' (TEJ). LLJ are the strong westerly winds off and along the Kerala coast in the lower levels of the atmosphere. TEJ are the high-speed easterly winds in the higher atmospheric levels over South Peninsular India. Both have a crucial and outstanding role during the onset phase of monsoon. Monsoon circulation brings about a typical transition of weather patterns over Kerala, easily discernible to the weatherman.
Pre Monsoon cyclones forming around the 2nd and 3rd week of May can bring about large-scale changes in the circulation pattern. It may even disrupt the ocean-atmosphere coupling and corrupt the moisture flux. Storms forming within 10 days prior to the onset of monsoon over mainland Kerala become extremely crucial and decisive for the timely or delayed onset. It also dictates the pace of the monsoon current to become sluggish or brisk. Track of these storms has serious consequences for advancement of monsoon.
Arabian Sea cyclones invariably move away from the West Coast and frequently head for Oman, Yemen or the Somali coast. These cyclones pull the moisture towards the centre of the storm, away from the coastline. It also erodes the LLJ which in turn changes the ocean surface temperature profile. Therefore, it can derange the thermodynamic structure and impact adversely the onset phase of the Indian Summer Monsoon (ISM).
Bay of Bengal(BoB) cyclones originating during this period influence favourably the wind pattern. It accelerates the LLJ and TEJ. These storms also stimulate the cross-equatorial flow, considered positive for the monsoon build-up. However, the track of the storm becomes consequential. Passage of these storms as such depletes the heat energy of the ocean and will need a decent time frame for recouping. Tropical storms heading for Myanmar and adjoining Bangladesh do not trigger the monsoon stream to facilitate timely onset. Cyclones heading for West Bengal and Odisha after 15th May, may set the ball rolling and expedite early/timely onset.
Let us keep in mind: Cyclones are known to be mysterious. These monsters defy timelines, track and intensity. Change of goalpost at short notice is not very rare. Never to be taken for granted.