The onset of Southwest Monsoon is an emotional event as it may cause anxiety if delayed or shoot excitement for stepping in early. Though it is a seasonal feature but always at variance from the previous few. Kerala sees the arrival of monsoon around June 01 with a standard deviation of 05 days. The onset phase is attached to various grades like weak, mild, moderate, strong, and explosive. The extremes get mostly filtered out and onset invariably revolves around being mild/soft or strong.
There are some factors that are responsible for the onset and they keep changing from year to year. The onset needs a trigger in the form of a weather system in the proximity of the coastline. These are ocean born phenomena which accentuate the monsoon surge around the normal time of onset.
The first one is the low-pressure area or depression in the Bay of Bengal during the last days of May or the beginning of June. This is an ideal situation for speedy onset and progress as well as phase-1 over the South Peninsula.
Similarly, there are such systems in the Arabian Sea as well around the same time which results in onset over the mainland. But these systems tend to move away from the coast towards central and western parts of the Arabian Sea. They may successfully usher the onset current but impedes its further progress while over deep waters of the Arabian Sea. Some of these do move to brace the west coast for few days and stretch the monsoon rapidly over Coastal Karnataka and Goa.
The third factor is ‘Cyclonic Vortex’ which appears in the Southeast Arabian Sea, off Kerala and Lakshadweep region. They also shift along the west coast to push the monsoon current.
The fourth factor is the formation of ‘trough’ off the west coast due to temperature differential between land and sea. This situation could be for a mild start and weak progress.
Lastly the cross-equatorial flow, wherein the trade winds from the Southern Hemisphere crossover to the Northern Hemisphere can bring a strong monsoon surge towards the Indian mainland.
Presently the conditions are building up and weather activity is picking up over the Arabian Sea. Monsoon vortex or ‘Low Pressure’ likely to emerge shortly and infuse the monsoon surge over Kerala.