What is the ‘onset’ of the monsoon, and why the delayed onset is not necessarily bad news

Source: The post is based on the article “What is the ‘onset’ of the monsoon, and why the delayed onset is not necessarily bad news” published in the Indian Express on 9th June 2023

What is the News?

According to the India Meteorological Department (IMD), the southwest monsoon onset over Kerala this year will be delayed.

What is meant by the “onset of the monsoon” over the Kerala coast?

The onset of the monsoon over Kerala signals the beginning of the four-month (June-September) southwest monsoon season. During the southwest monsoon, India gets more than 70% of its annual rainfall. It is an important day in the economic calendar of the country.

‘Onset’ is a technical expression with a specific definition and the IMD does not officially declare onset until certain prescribed conditions are met. The onset does not mean the first rain of the season. That can start happening in certain places even before the onset is declared.

The normal date of the onset of the monsoon over the Kerala coast is June 1.  The monsoon covers the entire country by July 15. Neither an early nor a late onset of the monsoon is unusual.

What are the IMD defined conditions for the onset of the monsoon?

According to the IMD, the onset of the monsoon happens when there is a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region. The IMD essentially looks at the consistency of rainfall over a defined geography, the intensity of the rainfall, and the wind speed.

Rainfall: The onset is declared 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 onset over Kerala is declared on the second day, as long as specific wind and temperature criteria are also fulfilled.

Wind field: The IMD says that the depth of westerlies should be up to 600 hectopascal (1 hPa is equal to 1 millibar of pressure) in the area that is bound by the equator to 10ºN latitude, and from longitude 55ºE to 80ºE.

Heat: The INSAT-derived Outgoing Longwave Radiation (OLR) value should be below 200 watt per sq m (wm2) in the area between the 5ºN and 10ºN latitudes, and 70ºE and 75ºE longitudes.

Note: Outgoing Longwave Radiation (OLR) value is a measure of the energy emitted to space by the Earth’s surface, oceans, and atmosphere.

What are the impacts of the delayed onset of the monsoon?

A delay in the onset over Kerala can potentially delay the arrival of the monsoon in other parts of the country, especially in the southern states. However, the delayed onset over Kerala does not automatically or invariably mean delays in the arrival of the monsoon over other parts of the country.

This is because the northward progression of the monsoon after it has hit the Kerala coast is not uniform and depends on local factors, including the creation of low-pressure areas.

A delay or an early arrival has no bearing on the quality or amount of rainfall, or its regional distribution across the country.