Explained: What is the onset of monsoon? How it is declared?

June 1 is regarded as the date of onset of the monsoon in India, as indicated by the arrival of the monsoon in the southernmost state of Kerala. The monsoon accounts for nearly 70% of the rainfall in India.

New Delhi, May 30: The Southwest Monsoon has set in over Kerala on Sunday, three days ahead of schedule, and is likely to reach parts of Karnataka by June 2 in what is forecast to be a normal four-month-long rainy season.

While the monsoon has advanced over Kerala and parts of Tamil Nadu, the Bay of Bengal arm of the weather system was sluggish as it continued to linger just above the Andaman islands.

The Extended Range Forecast issued by the IMD has suggested that the progress of the monsoon over Karnataka, and entire north-east India could be slow.

What is the onset of monsoon? How it is declared?

June 1 is regarded as the date of onset of the monsoon in India, as indicated by the arrival of the monsoon in the southernmost state of Kerala. The monsoon accounts for nearly 70% of the rainfall in India.

The onset of the broad-scale monsoon occurs in many stages and represents a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region.

The IMD announces the onset of monsoon over Kerala which was based on the daily rainfall of 14 stations over Kerala and the neighboring area along with wind field and Outgoing Longwave Radiation (OLR) over the southeast Arabian Sea.

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

The weather body 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".

Is early monsoon unusual?

Early monsoon is not unusual in India. For exapmle, in 2018 and 2017, the onset over Kerala occurred on May 29 and May 30 respectively.
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 set June 6 as the monsoon arrival date, however, it finally set in over Kerala on June 8, 2019.

Does early monsoon mean more rain?

Early arrival of the monsoon does not mean it would bring more rainfall. The IMD, on April 14 predicted a normal monsoon in India in the range of 96-104 percent of the Long Period Average (LPA),

A monsoon, that occurred two days in advance of the normal date, does not indicate a good monsoon season. There are instances where it rained heavily for about 10 days but the season as a whole ended with 14% less rain than normal.

The early arrival of the monsoon has raised the hopes that the output of crops like rice and oilseeds will get a boost after a deadly heatwave hit wheat production, resulting in its restrictions.
The arrival of the monsoon is critical to the country's farm output at a time when the country is battling soaring food prices.

The Russia Ukraine war has worsened the situation affecting the global food supply.