Monsoon sets in over Kerala and northeast earlier than forecast

Sub: Geography

Sec: Climatology

Tags : Onset of monsoon

Context:

The Southwest monsoon arrived in Kerala a day earlier than forecasted by the India Meteorological Department (IMD), which had predicted a May 31 onset.

Details:

Typically, the monsoon begins around June 1 and gradually covers the entire country by mid-July.
All criteria for the monsoon’s declaration were met, with significant rainfall over Kerala and surrounding areas and appropriate Outgoing Longwave Radiation and wind patterns.
Kerala usually receives an average of 2018.7 mm of rain during the four-month monsoon season, with June typically getting around 648.3 mm and July being the wettest month with 653.4 mm on average.
The IMD has forecasted above-normal rainfall for Kerala and the entire country this year, partly due to the expected development of La Nina.
Additionally, Cyclone Remal, which affected West Bengal and Bangladesh, has influenced the monsoon’s early onset over the northeast regions, which usually experience it by June 5.

“Onset of the Monsoon”:

A monsoon is a seasonal reversing wind accompanied by corresponding changes in precipitation and associated with the annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ).
The onset of the monsoon over Kerala signals the beginning of the four-month (June-September) southwest monsoon season, during which India gets more than 70% of its annual rainfall.
The monsoon covers the entire country by July 15.
Contrary to what is sometimes assumed, the onset does not mean the first rain of the season.
- For example, the Andaman and Nicobar Islands usually start receiving monsoon rainfall between May 15 and May 20, and it starts raining along the Kerala coast in the last week of May.
However, ‘onset’ is a technical expression with a specific definition and the IMD does not officially declare onset until certain prescribed conditions are met.

Conditions Determining the Onset of 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.
Wind field: 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 (passes through Kochi), and from longitude 55ºE to 80ºE.
Heat: The measure of the energy emitted to space by the Earth’s surface, oceans, and atmosphere should be below 200 watts per sq m (wm2) in the area between the 5ºN and 10ºN latitudes, and 70ºE and 75ºE longitudes.

Source: TH