Lesson from 2009: An early southwest monsoon isn't always a good one

Scientists said the progress of southwest monsoon depend on many supportive factors such as frequency of Low Pressure Systems over the Bay of Bengal and Arabian Sea

The southwest monsoon arrived unusually early this year on May 24, which is eight days ahead of its usual onset date of June 1. It arrived over Northeast India even earlier — that is, around 12 days ahead of its usual onset date. The last time the rains arrived earlier than this year was back in 2009, when the onset happened on May 23. 

However, the early onset of the monsoon over Kerala is no guarantee of strong performance or uniform distribution across the country. There have been multiple instances in the past when the southwest monsoon arrived over the Kerala coast ahead of its normal onset date, but thereafter, its performance across the country was patchy. 

More than the onset, it is the timeliness and distribution of rainfall in the next four months, starting from June, that will determine the future trajectory of agricultural production and economic growth in 2025–26. 

But this time, it could be different, as the India Meteorological Department (IMD), in its April forecast, said that in 2025, not only would the rains be ‘above normal’ at 105 per cent of the long-period average (LPA), but they would also be fairly well distributed.  Also, between 2009 and now, IMD’s forecasting prowess has also considerably improved.  Scientists said the progress of the southwest monsoon depends on many supportive factors, such as the frequency of low-pressure systems over the Bay of Bengal and the Arabian Sea, rain-bearing clouds over the oceans, and the strength of winds that carry them across the mainland. In fact, the last time the southwest monsoon arrived earlier than in 2025 — that is, back in 2009 — the rains lost steam after the early onset and ended the season with almost 23 per cent deficient rainfall, which was the lowest recorded average rainfall in several decades over India. According to the IMD’s end-of-season report for 2009, the southwest monsoon that year, in the June to September months, was 77 per cent of the LPA.  Around 60 per cent of the districts in the country that year received scanty or deficient rains. The IMD said that in 2009, the monsoon set in over Kerala on May 23, one week before its normal date of June 1, but then went into a long hiatus from June 8–20. Thereafter, it revived to cover the entire country by July 3, compared to its normal date of July 15. In August 2009, the IMD said that only one low-pressure area had formed over the Bay of Bengal, which was identical in the following month of September. 

“The cumulative rainfall distribution shows that the large deficiency in rainfall during the early part of the season caused the cumulative seasonal rainfall over the country as a whole to remain below normal by 19 per cent or more during every week of the season,” the IMD had then said. It acknowledged that the operational long-range forecast for the seasonal rainfall over the country as a whole, and over four homogeneous regions except the south peninsula, was not accurate in 2009. 

Sixteen years hence the IMD has predicted that the 2025 southwest monsoon would be ‘above normal’ at 105 per cent of the LPA. Its forecast comes with a model error of plus or minus 5 per cent. Hopefully, 2009 does not get repeated in 2025.