Explained: What is Onset of Monsoon And What Does an Early Onset Over Kerala Means?

The onset of monsoon marks a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region, says IMD.

New Delhi: The India Meteorological Department (IMD) had announced on Friday that the southwest monsoon is likely to set in over Kerala on May 27, well ahead of its normal date of June 1. If it so happens, then this would be the earliest onset of the monsoon over Kerala since 2009. There can be “a model error of four days on either side”, said the IMD.

What is the onset of monsoon?

The onset of monsoon over Kerala marks the beginning of the southwest monsoon season over India through a four-month period (June-September). It brings more than 70 per cent of the country’s annual rainfall.

The onset of monsoon marks a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region, says IMD.

The IMD announces it only after certain newly defined and measurable parameters, adopted in 2016, are met. The IMD checks for the consistency of rainfall over a defined geography, its intensity, and wind speed.

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”.

The 14 enlisted stations are:

Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kasaragod, and Mangaluru.

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.

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

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. An onset is just an event that happens during the progress of the monsoon over the Indian subcontinent.

A delay or early arrival by a few days has no bearing on the quality or amount of rainfall or its regional distribution across the country.

The IMD, on April 14, 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 in the southwest monsoon season during this period was 87 cm.

Does a delayed onset mean cascading delays across the country

A delay in onset over Kerala can potentially delay the arrival of the monsoon in other parts of the country, especially in the southern states, which normally start getting rain within days of the monsoon reaching the Kerala coast. But again, a delayed onset over Kerala does not automatically or invariably mean delays in the arrival of the monsoon over the entire country.

After its onset over Kerala, the monsoon spreads over the entire country by July 15.

(With agency inputs)