The current month is set to end as the driest September in 17 years, with rainfall almost a third below normal until Thursday (The Indian Express, September 20). Over the last couple of days, central and western India have finally been getting some good rainfall, but the month is still set to end up being the driest September since 2001. Until Sunday, September has seen a rainfall deficiency of 22.3 per cent, and weather scientists say not much rain is expected in the remainder of the month after the current spell ends in two to three days. That could well leave September with a deficiency of over 30%. Never since 2001, with a 35.8% shortfall, has September been so dry. What is the cause, and what does it mean to the overall southwest monsoon?
Not enough depressions
Though the deviation is large, it doesn’t seem to be a result of any major atmospheric abnormality. “We have not seen many sufficiently strong low pressure systems (LPS) developing. In fact, this has been true of this entire monsoon season. If you look at IMD (India Meteorological Department) information, one or two LPS are formed almost every day but most of these are not strong enough to bring in rain. We now have one strong LPS that developed two days ago, and we are seeing good rainfall in central India because of that. The lack of strong LPS activity is likely the only cause of low rainfall in this month, and also this season,” said Govindasamy Bala, professor at the Centre for Atmospheric and Oceanic Sciences at Indian Institute of Science in Bengaluru.
According to the IMD, there were only six significant depressions this season, including the one that brought the ongoing rain over central India. South India has been dry this September, with a deficiency of over 46%.
The shortfall during September has meant that there has been a 9% deficiency in the overall rainfall this entire season, starting from June. Eastern and northeastern India has a deficiency of almost 25% over the season.
Once the current spell gets over, there is very less likelihood of any amount of substantial rainfall happening in the rest of the month. On Thursday, the IMD had said that the likelihood of another cyclonic storm activity in this month was “quite feeble”.
D Sivananda Pai, head of climate prediction at IMD, said the June-September monsoon season was most likely to end with at least 9-10% deficiency. “In the first week of September, we had good rainfall in certain areas. But after that, there has been a lull for almost 15 days. One of the reasons why we did not see a strong low pressure system developing over the Bay of Bengal side, is that around the same time, some cyclonic activity in the west Pacific region was taking place. Japan had some good rainfall and so did Philippines. The moisture over the Bay of Bengal got sucked in by those systems,” Pai said.
‘Not El Niño’
J Srinivasan of the Divecha Centre for Climate Change at Indian Institute of Science, Bengaluru, said there might not be any “external cause” behind the low rainfall in this month. He ruled out suggestions that a developing weak El Niño in the Pacific Ocean might have forced a suppression of rainfall.
“This dry spell has nothing to do with El Niño. People forget that in natural systems like monsoon, a drought can occur once in a few years without any external cause. This year, the (overall) monsoon (rainfall) may be 10 per cent below normal… If the depression which had come on September 20 (the one that is causing current spell of rains) a few days earlier, and stayed over land for a few more days, this year would have been a normal monsoon. If you compare the all-India daily rainfall in 2017 and 2018, they look very similar except for September. We must learn to live with the natural variability of the monsoon and not be looking for an external cause every time,” he said.