The arrival of Monsoon and its four-month journey is considered a historic occurrence every time. The onset of monsoon is a much-awaited event, packed with a mix of anxious and exciting moments. Its truant behavior attaches both, the hope of success and fear of failure, till it unfolds completely in September.
Southwest Monsoon normally arrives over the Andaman Sea around 20th May, reaches Sri Lanka by 25th, and strikes the mainland coast on 01st June. Last year, it announced its early arrival over the Andaman Sea on 18th May but held back thereafter to delay Kerala onset by one week.
In the last 10 years, the earliest arrival was on 23rd May 2009 and the most delayed was on 08th June 2016. As such, early or late arrival over Andaman Sea does not find any linkage with the Kerala onset, like last year. Also, these dates have no correlation with the further advancement and overall performance of the Monsoon rains. With the change of database from 1901-1940 to 1961-2019, there have been some amendments introduced for the arrival and departure dates of monsoon from different parts of the country. The striking changes are its delay in arrival by 3-7 days over the central and eastern parts of the country and early arrival with late withdrawal from northwest India.
Some structural changes, mostly over the oceans precede the ingress of monsoon current over the Indian region. The dominant ones are the cross-equatorial flow and displacement of an anticyclone from the Arabian Sea. Easterly trade winds from the southern hemisphere cross the equator and turn right to become southwesterly.
Consequentially the prefix 'southwest' gets attached to the seasonal monsoon. Also, the anticyclonic (clockwise) winds over the central Arabian Sea finds a push from the cross-equatorial flow to become westerly right up to the coast. All these changes take place due to internal forces associated with the monsoon dynamics. In the event of any Low Pressure or Depression forming in the Bay of Bengal or the Arabian Sea around mid- May, it can expedite the resurgence of monsoon current leading to on or before time onset.
Commencement of the second week of May is just about the right time to visualize transitional changes as a precursor to the broader monsoon circulation sweeping across the Bay of Bengal and the Arabian Sea. Notably, appreciable changes have already made a beginning with trade winds crossing the equator, though limited to 5 deg north latitude. With the likelihood of a vortex coming up in the southwest Bay of Bengal anytime around 12th May, the process could speed up, thereby enhancing the force and reach of Bay of Bengal current.
