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Updated: Oct 18, 2019 02:30 IST

The southwest monsoon has finally withdrawn entirely from the country and the northeast monsoon has commenced over peninsular India, according to the India Meteorological Department (IMD).

This year’s monsoon season was prolonged by 39 days and the withdrawal took place in a mere nine days against a normal withdrawal period of 45 days.

The strange behaviour of the monsoon has taken IMD scientists by surprise. They have linked the extreme rainfall episodes in Maharashtra, Kerala and Bihar to climate change.

Both IMD and private weather forecaster Skymet Weather were off the mark in forecasting the performance of the southwest monsoon. IMD had predicted a near normal monsoon at 96% of LPA (long period average) in May while Skymet Weather predicted a below normal monsoon at 93% of LPA. The season ended with cumulative rainfall of 110% of LPA.

With the withdrawal of the southwest monsoon, pollution is expected to spike in Delhi and parts of the national capital region ( NCR) and the Indo-Gangetic plains due to stable atmospheric conditions. IMD has flagged heavy rains in Tamil Nadu, Karnataka and Kerala in the next couple of days.

After a delayed onset of the southwest monsoon on June 8 and very little rain in that month due to presence of a weak El Nino effect, there were fears among meteorologists that it could be a drought year. June ended with a deficit of 33% but July, August and September received 105%, 115% and 152% of their LPA respectively.

The monsoon continued well into October, beginning its withdrawal only on October 9. After 1997, this is the first time that monsoon rain ended above normal in an El Nino year according to Skymet Weather. In 1997 there was a strong El Nino and yet monsoon rainfall was 102% of LPA.

El Nino is a climate pattern characterised by above-average sea surface temperatures in the tropical Pacific Ocean . El Nino years are linked to below normal monsoon rains in India and a higher-than-normal frequency of heat waves.

Above-normal rains in September of 259 mm against a normal of 170 mm were also a distinctive feature of this monsoon. Central, peninsular and northwest India received 107% and 48.8% excess rains in September while east and northeast India recorded a deficit of 13.5% in September.

“We were closer to reality when we forecast a normal monsoon. But we never expected that rains will be 10% excess over normal. This happened mainly due to so many low pressure systems which brought a lot of rain to central and peninsular India. El Nino had weakened to neutral by July-end and the Indian Ocean Dipole (IOD) was very strong which led to more rainfall,” said DS Pai, head, climate research services at IMD, Pune, adding that “extreme rain events are rising. It’s clear this year too. This trend is an imprint of climate change. The overall excess rains cannot be linked directly to climate change.”

“Usually in El Nino years there is less rainfall. When June ended with a deficit of 33% we didn’t imagine that the deficit would be covered in the next couple of months. I think it’s the first time in a hundred years that such a deficiency was covered and excess monsoon recorded. The Indian Ocean Dipole was also the strongest on record according to the Australian Bureau of Meteorology. In future forecasts I think we have give more weightage to the impact of IOD,” said Mahesh Palawat, vice president, climate and meteorology, Skymet Weather.

IOD is the change in the difference between sea surface temperatures of the tropical western and eastern Indian Ocean. A positive IOD or warmer temperatures on the western end is favourable for the Indian monsoon as it causes a kind of barrier in the eastern Indian Ocean and the south-westerly winds blow towards the Indian sub-continent .

With the onset of northeast monsoon in peninsular India, heavy rains are expected in Tamil Nadu on Friday. There is also a low pressure area over the south-east Arabian Sea and the adjoining Lakshadweep area. A trough of low pressure is also running from cyclonic circulation over south-east Arabian Sea to Telangana across Kerala, South Interior Karnataka and Rayalaseema. “Under the influence of the above systems, heavy rainfall is likely to occur over major parts of peninsular India during next three days,”an IMD bulletin said.

But extremely poor air quality is likely to impact Delhi-National Capital Region (NCR) in the next few days with a gradual spike October 20 onward. “Air quality is likely to remain very poor despite easterly winds at lower level till October 20. But after that, northwesterly winds will bring pollutants from Haryana and Punjab,” said Palawat, adding that stable atmospheric conditions and accumulation of local pollutants were affecting NCR. Winter inversion like conditions are expected by month-end when pollutants are trapped closer to the surface, he added.