Explained: What is the ‘onset’ of the monsoon, and what does an early onset over Kerala mean?

The southwest monsoon is likely to set in over Kerala on May 27, well ahead of its normal date of June 1, the India Meteorological Department (IMD) announced on Friday (May 13). If the forecast turns out to be accurate, this will be the earliest onset of the monsoon over Kerala since at least 2009. There can be “a model error of four days on either side”, the IMD said.

What does the “onset of monsoon” mean?

The onset of the monsoon over Kerala marks the beginning of the four-month, June-September southwest monsoon season over India, which brings more than 70 per cent of the country’s annual rainfall. The onset of the monsoon is a significant day in India’s economic calendar.

According to the IMD, the onset of the monsoon marks a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region, and the Department announces it only after certain newly defined and measurable parameters, adopted in 2016, are met. Broadly, the IMD checks for the consistency of rainfall over a defined geography, its intensity, and wind speed.

* Rainfall: The IMD declares the onset of the monsoon if at least 60% of 14 designated meteorological stations in Kerala and Lakshadweep record at least 2.5 mm of rain for two consecutive days at any time after May 10. In such a situation, the onset over Kerala is declared on the second day, provided specific wind and temperature criteria are also fulfilled.

The 14 enlisted stations are: Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kasaragod, and Mangaluru.

* Wind field: The depth of westerlies should be upto 600 hectopascal (1 hPa is equal to 1 millibar of pressure) in the area bound by the equator to 10ºN latitude, and from longitude 55ºE to 80ºE. The zonal wind speed over the area bound by 5-10ºN latitude and 70-80ºE longitude should be of the order of 15-20 knots (28-37 kph) at 925 hPa.

*Heat: According to IMD, the INSAT-derived Outgoing Longwave Radiation (OLR) value (a measure of the energy emitted to space by the Earth’s surface, oceans, and atmosphere) should be below 200 watt per sq m (wm2) in the box confined by 5-10ºN latitude and 70-75ºE latitude.

In general, the Andaman and Nicobar Islands start receiving monsoon rainfall between May 15 and May 20 every year, and it usually starts raining along the Kerala coast in the last week of May. However, the onset is not officially declared until the prescribed conditions (above) are met.

Is it unusual for the monsoon to hit the Kerala coast early?

Neither early nor late onset of the monsoon is unusual, even though the forecast for this year is for earlier than would be usually expected.

In 2018 and 2017, the onset over Kerala occurred on May 29 and May 30 respectively. In 2010, onset was realised on May 31. In 2020 and 2013, the monsoon was exactly on time, hitting the Kerala coast on June 1.

In the rest of the years going back to 2010, the onset was delayed. In 2019, the IMD had announced a delay of six days, and predicted the onset for June 6. The monsoon finally set in over Kerala on June 8, 2019.

Does an early onset foretell a good monsoon?

No, it does not — just as a delay does not foretell a poor monsoon. The onset is just an event that happens during the progress of the monsoon over the Indian subcontinent.

A delay of a few days, or perhaps the monsoon arriving a few days early, has no bearing on the quality or amount of rainfall, or its regional distribution across the country, during the four-month monsoon season. In a recent year, the onset of the monsoon occurred two days in advance of the normal date, and it rained heavily for about 10 days after that — however, the season as a whole ended with 14% less rain than normal.

On April 14 this year, the IMD released its first Long Range Forecast (LRF) for this year, in which it predicted a “normal” monsoon — which means rainfall is likely to be in the range of 96% to 104% of the long period average (LPA) of the 1971-2020 period. The average annual rainfall for the country as a whole in the southwest monsoon season during this period was 87 cm.

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