IMD forecast foiled by 24% rain shortfall

It is unlikely that good rainfall in September will wipe out the deficit, say meteorologists

Updated - September 01, 2021 01:07 pm IST

Published - August 31, 2021 10:16 pm IST - NEW DELHI

Far from the forecast of a “normal” monsoon in August, India ended the month with a 24% shortfall, according to data from the India Meteorological Department (IMD).

This brings India’s overall monsoon rainfall deficit to 9%, just a percentage-point shy of what would be considered “deficient” rainfall.

Meteorologists said that because of the shortfall in August, which normally receives the second highest rainfall in the four monsoon months, it was unlikely that rainfall in September, even if substantial, would be enough to wipe out the deficit, and India could well end up with “below normal” rainfall, which is characterised by rainfall being 90-96% of the long period average of 88 cm.

In June, the IMD, as part of its forecast, had also said that rainfall in northwest, south, east and central India would be “normal” or within an 8% error window of their historical average. This forecast too has been significantly off the mark, with Northwest India and Central India registering a 14% shortfall.

The key reason for the August monsoon failure has been an extended break in rainfall from August 8 to 18. “We expected it to last three or four days, but a week or a 10-day break is quite difficult to recover from,” said Mahesh Palawat, meteorologist at Skymet, a private weather forecasting company. It too, as the IMD, expected August rain to be “normal”.

M. Mohapatra, Director-General, IMD, declined to comment to The Hindu , saying that the organisation would be addressing the forecasting failure at a press conference shortly.


Depressions on decline

Given the influence of the ocean surface temperatures on the monsoon, meteorological forecasts in June were of “neutral conditions” in the central Pacific, implying that these would not have a bearing on the monsoon. The Indian Ocean too was not expected to contribute. Wind-bearing depressions in the Bay of Bengal, coupled with moisture from the Arabian Sea, usually inject surges of rain over central India. But this did not happen and did not salvage the break-like conditions, according to Mr. Palawat. He said that September, though contributing only 17 cm of rainfall unlike 25 cm in August, would see a few more depressions.

Other meteorologists said that large effects of global warming were impacting monsoon rainfall. The number of rain-bearing depressions in the Bay of Bengal was declining and pre-monsoon cyclones, such as Tauktae that veered very close to Mumbai, possibly altered heat distribution patterns over the landmass, influencing moisture distribution and thereby causing erratic rainfall, said Roxy Mathew Koll, climate scientist at the Indian Institute of Tropical Meteorology, Pune.

“A single month’s failure isn’t climate change, but the predicted pattern of long dry spells with bursts of heavy rain is one of the consequences of warming,” he told The Hindu .

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