A study by the India Meteorological Department reveals that the southwest monsoon season in India has extended by 1.6 days per decade over the past 50 years, increasing its contribution to annual rainfall from 75% to 79%.
This extension is linked to delayed monsoon withdrawal, impacting agricultural yields and altering rainfall metrics crucial for climate research.
The duration of the southwest monsoon (SWM) season that brings 75 per cent of annual rainfall to India has extended in the past five decades, according to a research paper published by scientists from the India Meteorological Department (IMD).
The paper published in the journal Mausam highlighted that the SWM season that normally lasts from June 1 to September 30 has added 1.6 days per decade between 1970 and 2020. The scientists draw the correlation between the extension of the SWM and the delayed withdrawal of the monsoon winds from the sub-continent in this period.
The onset of the season over Kerala shows year on year variability but there is no significant change in the dates over the past 50 years, according to the paper.
The scientists also calculated the contribution of this extended monsoon season to the annual rainfall over the country and found that it increased to 79 per cent instead of the normal 75 per cent. This would likely distort the monsoon rainfall metrics that the IMD keeps from June 1 to September 30, crucial for long term research on changing rainfall patterns.
According to data from IMD analysed by Down To Earth, the beginning of the withdrawal of the SWM from India that normally begins from western Rajasthan around September 17 has been delayed by at least one day in the last 14 years.
In 2019, the SWM started withdrawing on October 9, 22 days later than the normal date. In 2021, the monsoon winds began their retreat on October 6, with a delay of 19 days.
The IMD also changed the withdrawal dates for the monsoon from various parts of the country in 2020 though the normal date of the overall withdrawal of the monsoon remained the same, October 15. If one considers the older normal date of September 1 for the start of the monsoon season, the start of withdrawal in 2019 was 40 days late.
In its press statement from April 2020 informing about the changes to the dates of withdrawal, IMD had stated climate change as the primary reason for their consideration.
The dates of complete withdrawal of the monsoon winds from India in the past decade have also been post the normal date of October 15.
The current study also found better correlation between the rainfall with the delayed withdrawal of the monsoon and the yield of Rabi crop across the country and the El Niño Southern Oscillation (ENSO) phenomenon. The yields of rapeseed and mustard were also higher when the rainfall figures from the delayed monsoon were considered, according to the paper.