BENGALURU: Nearly half of the droughts occurring during Indian summer monsoon in the past century may have been driven by atmospheric disturbances from the North Atlantic region, according to a study by the Indian Institute of Science (IISc). The study, which has been published in Science, was carried out by researchers at the Centre for Atmospheric and Oceanic Sciences (CAOS), IISc.
More than a billion people depend on the annual Indian summer monsoon, which brings copious rain to large swathes of India between June and September. When it fails, and most of the country is plunged into drought. The usual suspect is El Niño, a recurring climate event during which abnormally warm equatorial Pacific waters pull moisture-laden clouds away from the Indian subcontinent. However, 10 out of 23 droughts that India faced in the past century have occurred during years when El Niño was absent.
The IISc study shows that these droughts were a consequence of a sudden and steep drop in rainfall in late August. This drop was linked to an atmospheric disturbance in the mid-latitude region over North Atlantic Ocean, creating a pattern of atmospheric currents that swoop in over the subcontinent and “derail” the monsoon, an official statement from IISc said on Friday.
“As early as the 1980s, people looked at these droughts individually. They have not collated and pooled them together, and deduced that these droughts may all have a different type of evolution than El Niño droughts, as well as a common cause, which is this mid-latitude influence,” says V Venugopal, associate professor at CAOS, and a senior author of the paper.
The research team looked closely at the daily rainfall during both El Niño and non-El Niño drought years, and noticed stark differences in their patterns between June and September. The droughts that happen during an El Niño year follow a standard pattern. The rainfall deficit — departure from a long-term average — sets in early around mid-June and becomes progressively worse. By mid-August, the deficit spreads across the country and there is no going back from a drought.
Surprisingly, the droughts during the non-El Niño years, when analysed together, also seemed to follow a common pattern. First, there was a moderate slump in June. Then, during mid-July to mid-August — the peak of the season — monsoon showed signs of recovery and the rainfall increased. However, around the third week of August, there was a sudden steep decline in rainfall, which resulted in drought conditions.
“The question was: why does the break occur this late in August?” says Jai Sukhatme, associate professor at CAOS and another author. “We tried to see if we could trace it back to a forcing agent or system that influences the behaviour over India. We looked at the winds that were prevalent in non-El Niño drought years.”
That was when the researchers noticed an unusual atmospheric disturbance in the mid-latitudes. It emerged from winds in the upper atmosphere interacting with a deep cyclonic circulation above abnormally cold North Atlantic waters. This resulted in a wave of air currents, called a Rossby wave, which curved down from the North Atlantic — squeezed in by the Tibetan plateau — and hit the Indian subcontinent around mid-August, suppressing rainfall and throwing off the monsoon that was trying to recover from the June slump. The wave’s usual course is to go from west to east, but not towards the equator, explains Sukhatme. “This inward curving was the peculiar thing that we noticed during these years.”