Southwest Monsoon has arrived today over South Andaman Sea, Bay Islands and Southeast Bay of Bengal. Monsoon has arrived about a week earlier than its normal date of 22ndMay. More often than not, monsoon has been making early appearance over this region, in the recent times. As per the revised dates of monsoon onset, it arrives over mainland Kerala on 01stJune, after a gap of 10 days.
However, there is no correlation at all between the arrival over Bay Islands and gate way state of Kerala. After having set in over the Andaman Sea, it could reach Kerala with military punctuality on 01stJune and get retarded or accelerated in between over Sri Lanka and Southwest Bay of Bengal.
Except in 2018, the monsoon stream mostly reached on or before time. However, further progress to arrive over Kerala had wide dispersion and included all possibilities: early, late and on time onset.
There is a time tested set objective criteria for declaring monsoon over Kerala. This include rainfall, wind pattern ( direction, speed and depth) and outgoing long wave radiations. There is no such criteria for announcing arrival over Bay Islands, the 1st entry point for the Indian sub-continent. The criteria remains largely subjective. As such, Andaman Islands and Nicobar Islands are separated by nearly 10° latitude with a 150km wide channel in between.
As the monsoon approaches, the easterly trade winds over equatorial Indian and Pacific ocean weaken and westerly zonal winds set in over the region. Also, westerly zonal flow extends up to 600 hpa ( 12,000 feet). During the onset, rains cover a wide area of the Islands after the low level cross equatorial flow gets established from the Somali coast to South Andaman Sea, across South Bay of Bengal.
Simultaneously, there is acceleration of cross equatorial flow across the Somali coast and westerly zonal flow over equatorial Indian Ocean. Some significant changes are noticed across the Islands on account of onset of monsoon and these include : East-West oriented deep convective pattern with typical cloud characteristics, sea conditions, besides the circulation pattern over the Indo-Pacific region.
While the onset of monsoon over Kerala can be faithfully determined based on the set criteria objectively, the meteorologists still struggle to find a subjective semblance for South Andaman Sea. Onset over the Bay Islands remains purely on the basis of weather pattern, realized rainfall and broad scale changes in the circulation pattern.