Southwest Monsoon over the Indian subcontinent is an annual event but has historic significance for its high stakes. Sometimes it walks in with military punctuality and on other occasions keeps loitering leaving anxious moments for the agriculture and economic sector. Though the monsoon current keeps making advances over the Indian seas anytime around mid-May or so but the most interesting and eagerly awaited advance is marked by onset over mainland Kerala. The normal date of onset is taken as 01st June with a standard deviation of one week.
Its first stop over the Indian waters is over the Andaman Sea around 22nd May. But onset over Bay Islands is poorly correlated and therefore does not find any strong connection with its arrival over the mainland. Last year the monsoon current had advanced to South Andaman Sea on 17th May but still managed to throng Kerala coast on 01st June, dot on time. The dates of onset and withdrawal have got revised since last year based on the data of the last 50 years up to 2019. There is no change in the onset mark of 01st June for Kerala but the complete onset cycle for the country has been truncated by one week so as to complete the process by 08th July. The commencement of withdrawal from West Rajasthan has been delayed by nearly 2 weeks and is expected to begin around 17th September, a significant deviation from the earlier date of 01st September.
One really does not find any sanctity in the arrival of the monsoon date. The past record shows it marching to Kerala 2 weeks before the due date and sometimes observe laxity in arriving with a similar delay. Since 2015, the monsoon has arrived either a shade early or a delay of not more than 1 week.
Along with other factors like wind direction, wind speed, and quantum of outgoing longwave radiations over specific areas in the equatorial Arabian Sea, the main criterion remains the rainfall over the designated stations. Rainfall if after 10th May, over 60% of the available 14 stations ( Minicoy, Amini Devi, Thiruvananthapuram, Punalur, Kollam, Kudulu, Alapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur and Mangalore) is reported 2.5mm or more for 2 consecutive days, the onset over Kerala be declared on the 2nd day.
There are some barriers in the wind field specially over the Arabian Sea which need to be overcome for smooth sail of monsoon southwesterly winds up to Kerala coast. There is an anticylone invariably shifting position over the central and adjoining southern parts of the Arabian Sea, mostly seen till middle of May. This feature needs dilution and ultimate extinction before the arrival of monsoon stream. This feature explicitly seems to be softening and making way for conducive conditions. Also the cross equatorial flow (winds from south of equator turning and crossing over to north) has to establish for speedy gush towards Southeast Arabian Sea, Lakshadweep area and Kerala coast. Appearance of some weather disturbance in the equatorial region either in the Arabian Sea or Bay of Bengal also expedite strengthening of monsoon stream. It happened during last year in the onset phase and conditions are shaping up for a rerun, albeit a little early to commit at this stage. Picture will be clear very soon and a final call be taken next week.