Above normal rainfall likely in September: IMD
There is likely to be above normal rainfall activity at over 110% of the so-called long period average or LPA in September, and while this will reduce the current monsoon rain deficiency of 9% over the country, overall monsoon rain (June 1 to September 30) will still be in the “lower end of normal category,” the India Meteorological Department (IMD) said in its monsoon monthly forecast on Wednesday. The weather office also admitted that its model got the August forecast wrong.
The LPA of rainfall during September, based on the data of 1961-2010, is about 170mm. IMD defines as normal for entire country rainfall of 96% to 104% of LPA, and its Wednesday forecast suggests that this year’s overall monsoon rains will be closer to the 4% deficient bound.
The LPA for the entire season is 880mm.
In September, according to IMD, above normal rainfall activity is likely over many parts of central India, and normal to below normal rainfall is likely over many areas of north-west, north-east and southern most parts of peninsular India.
IMD forecast normal rain in August over the country as a whole at 94% to 106% of LPA, but got this wrong. August reported a 24% deficiency due to a subdued monsoon phase till around August 17. In its long-range forecast, IMD said the south-west monsoon seasonal rain (June to September) over the country as a whole is likely to be normal (96% to 104% of LPA) at about 101% of the LPA with an error of ±4%. This assessment has now been downgraded to the lower end of the normal (up to 20% deficient) category on Wednesday.
IMD on Wednesday acknowledged that its multi-model ensemble forecasting system got its August rainfall prediction wrong. “Though the model partially picked up the spatial pattern of rains in August, it couldn’t pick up the magnitude of rain in August. We will examine this and endeavour to improve the output,” said M Mohapatra, director general, IMD. The model also couldn’t capture severe rainfall deficiency over Gujarat and Odisha.
Long periods of subdued rain in July and August have impacted agriculture. There is deficiency in sowing in many parts of central India, Mohapatra said.
The monsoon entered a break or a partial break phase three times this season, with the longest and driest phase between June 29 and July 11. It revived over northwest India on August 19, but weakened again from August 24 onwards and started reviving gradually from August 29.
According to IMD, the main reasons for the subdued monsoon in August is what is called a negative Indian Ocean Dipole (IOD) which is normally unfavourable for monsoon; and complete absence of formation of monsoon depressions in August (normally at least two depressions form in August. There was very less typhoon activity over the West Pacific. The remnants of these typhoons move westwards normally and help in creating low pressure areas over Bay of Bengal.
IOD is a phenomenon where the western Indian ocean becomes warmer than the eastern one. A positive IOD effect is associated with a better monsoon in India. Now IMD expects La Nina conditions to set in by end of monsoon season.
“There will be colder sea surface temperatures across the east equatorial Pacific Ocean which improves monsoon flow over India. At the same time we have negative IOD conditions which subdues monsoon. Together we can expect normal monsoon in September,” Mohapatra said.
La Nina, a cooling of sea surfaces near the Peru coast, affects the Indian monsoon positively.
IMD also acknowledged that monsoon rain patterns are skewed and often a lot of rain is recorded during a few hours instead of being evenly distributed during the month. “Yes this pattern can hamper agriculture. The solution is rainwater harvesting,” Mohapatra said.