After a 24% deficit monsoon rain in August, the India Meteorological Department (IMD) said on Wednesday that while rain in September — the last monsoon month — would be “above normal”, the overall June-September rainfall would be at the “lower end” of normal.
In August, the IMD forecast rainfall to be 101% of the seasonal average, but it now expects it to be around 98%, with a 4% error window.
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The spurt in September will help sowing in central India, which is seeing a large rain deficit, but would be insufficient to completely bridge the nearly 9% shortfall in all-India monsoon rainfall. All regions — northwest, central and eastern and northeast — are seeing deficits in rainfall, except for the southern peninsula, which has reported an 8% surplus.
“The rainfall averaged over the country as a whole during September 2021 is most likely to be above normal (above 110% of the long period average) which is about 170 mm,” the IMD said in a statement. “
Considering the expected above normal rainfall activity during September 2021, the current deficiency is very likely to reduce, and accumulated seasonal rainfall during 1 June to 30 September 2021 is very likely to be around lower end of the normal.”
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Had rainfall been above 100% as the IMD had forecast in April and June, this would have been a rare event with three consecutive years of normal or above normal rainfall.
Director-General, IMD, M. Mohapatra said at a news conference that the reasons for the IMD’s inability to predict the August shortfall were being looked into. “The models that we were using, while able to predict the regions where there would be a shortfall, were unable to predict the quantum of deficit. These are relatively newer models and are still being improved.”
Negative dipole
The factors that caused the August deficit were a “negative” Indian Ocean Dipole, characterised by warmer eastern Indian Ocean and colder closer to the Indian coast, which is considered unhelpful for the monsoon. Also, there were no significant depressions, that push the rain rising from the Bay of Bengal in August, unlike the two or three that were usual for the month, Mr. Mohapatra said.
The central equatorial Pacific was expected to show a “cooling trend” towards September and this could be favourable for the monsoon, Mr. Mohapatra said. But the Indian Ocean temperatures were not expected to be significantly helpful.
The IMD has formally adopted a dynamic-modelling approach, whereby the weather conditions are simulated and projected into the future by supercomputer. Though the approach is more accurate at forecasting impending changes in weather a week or two in advance, it has not proved reliable at long-range forecasting in the past three years. For instance, in 2019, the IMD forecast reduced monsoon rainfall at 96% of the LPA but India got record-breaking rainfall at 110% of the average. In 2020, it first forecast 100% of the LPA and updated it to 102%, but India again got a torrential 109% which was outside the 5% error window.