Criteria-Onset Of Southwest Monsoon Over Indian Sub-Continent
Representational Image: AI-Skymet
Key Takeaways:
- Southwest Monsoon reached the Andaman Sea nearly a week ahead of schedule.
- Early arrival over Andaman Sea does not guarantee an early Kerala onset.
- Specific rainfall, wind, and OLR conditions are required for declaring monsoon onset over Kerala.
- The normal monsoon onset date over Kerala remains 01st June.
The Southwest Monsoon arrived earlier than normal over the Andaman Sea. It reached the South Andaman Sea and Andaman & Nicobar Islands on 16th May 2026, about a week in advance. It has progressed further over some more parts of the South and Southeast Bay of Bengal and the North Andaman Sea. There is a gap of about ten days for the stream to reach Kerala. But, quite often, it does not follow the laid-out timelines. There is no robust correlation between its arrival over the Andaman Sea and mainland Kerala. Also, the arrival date has no bearing on the overall performance of the monsoon over the country.
While there is no strict criteria laid out for the Bay Islands, the mainland arrival is subject to fulfilling stipulated guidelines. The arrival of the monsoon over the mainland is not attempted before 10th May. If, after this date, 60% of the available 14 stations, namely Minicoy, Amini Devi, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu, and Mangalore, report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala is declared on the second day, provided the following criteria are also in concurrence:
Wind Field:
The depth of westerly winds should be maintained up to about 12,000 feet in the box from the equator to latitude 10°N and longitude 55°E-80°E.
The zonal wind speed over the area bounded by 5°-10°N and longitude 70°E-80°E should be of the order of 15-20 knots at a height of about 3,000 feet.
Outgoing Longwave Radiation (OLR):
The OLR value should be below 200 W/m² in the box confined by latitude 5°-10°N and longitude 70°E-75°E.
The onset of the monsoon over Kerala happens with the strengthening of cross-equatorial flow across the Arabian Sea side. Earlier, the Andaman Sea monsoon remains under the influence of the Bay of Bengal branch. It is somewhat harder for the Arabian Sea branch, as compared to the Bay of Bengal arm, to become strong and pushy enough for ushering the monsoon over Kerala. The onset factor is mainly a monsoon system like a low-pressure area forming over the Bay of Bengal. This strengthens the monsoon surge over the Southeast Arabian Sea and Lakshadweep region, resulting in rainfall over Kerala and Coastal Karnataka. Another trigger could be a vortex forming over the Lakshadweep region, speeding up the equatorial stream. Another accelerator, albeit rarely, is the offshore trough in proximity to Kerala and Coastal Karnataka. The bigger share of these is claimed by the Bay of Bengal, which also responds to a sustained onset phase rather than a transitory event.
It is a little early to identify the main catalyst for the onset this season. The normal date of onset is 01st June, with a standard deviation of +/- seven days. The onset date has no further correlation with the time frame for advancement over other parts. The normal date for the monsoon to cover the entire country remains 08th July.
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