The record-breaking rainfall this monsoon season, particularly during August and September, has left weather scientists confounded. After a more than 30% shortfall in June, the season ended with 10% excess rainfall, the first time such a thing has happened since 1931. The September rainfall (152% of long period average, or LPA) was the highest since 1917, the August rainfall (115% of LPA) was the highest since 1996, and the overall seasonal rainfall (110% of LPA) was the highest since 1994.
Search for answers
As late as the first week of September, the India Meteorological Department maintained that the seasonal rainfall was going to be normal (in the 96-104% range). With an influencer like El Niño Southern Oscillation (ENSO) in the Pacific remaining largely neutral this year, scientists are trying to pin down the exact reason for the unusual rainfall.
In the search for answers, one phenomenon attracting some attention is the Indian Ocean Dipole or IOD, an ocean-atmosphere interaction similar to El Niño, but in the Indian Ocean. IOD is a measure of the difference in the sea-surface temperatures of the western Indian Ocean (Arabian Sea) and the eastern Indian Ocean, south of the Indonesian coast. When the western waters are warmer than the eastern, IOD is said to be positive; in the opposite state, IOD is negative.
Like ENSO in the equatorial Pacific Ocean, IOD too influences weather and climate events, though its impact is weaker because the Indian Ocean is considerably smaller, and shallower, than the Pacific. The IOD has an impact on the Indian monsoon: a positive IOD is understood to aid monsoon rainfall while negative IOD is known to suppress it.
This year’s IOD, which began developing around June and grew strong after August, has been one of the strongest on record. IOD records are not very old. Accurate measurements are available only since 1960, according to the Australian Bureau of Meteorology (ACB). “The current positive Indian Ocean Dipole event has strengthened significantly over the past month. The latest weekly value of +2.15°C is the strongest positive weekly value since at least 2001 (when the Bureau’s weekly dataset commenced), and possibly since 1997, when strong monthly values were recorded,” the ACB said in its latest bulletin on October 15.
This has led to scientists looking at IOD for possible clues to this year’s bumper rainfall, especially since such strong IOD events in previous years, too, were associated with high monsoon rainfall.
“In previous years, we have had very strong IOD events in 1997 and 2006. In both those years, the southwest monsoon rainfall over India was around 100% of normal. 1997 also happened to be a strong El Niño year (El Niño suppresses monsoon rainfall), but thanks to the positive IOD, the monsoon rainfall was normal that year,” said Sridhar Balasubramanian, associate professor of mechanical engineering and an adjunct faculty member at IDP Climate Studies at IIT Bombay. “This year the positive IOD started strengthening from July, and by September it evolved into the strongest positive IOD ever recorded in the history of Indian summer monsoon.”
Beyond the correlation, scientists are careful not to directly blame the IOD for this year’s rains. That is because IOD’s link with the Indian summer monsoon is tenuous at best. It is only one of several factors that impact the monsoon, and not the most dominant.
In fact, the IOD’s influence on the monsoon is not fully understood. It is known to have a much weaker influence than ENSO, though. IOD’s relationship with the Indian summer monsoon is also much less studied compared to that of ENSO, said J Srinivasan, distinguished scientist with the Divecha Centre for Climate Change at IISc, Bengaluru.
Besides, it is not clear if the IOD influences the monsoon or if it is the other way round. The IOD generally takes shape towards the latter half of the summer monsoon, in August and September, and scientists do not rule out the possibility that the monsoon could play some role in its emergence.
“It is critical to remember that IOD usually peaks in September-October-November, and its impacts on monsoon are not very robust. It is unclear if monsoon itself plays a critical role in forcing the IOD,” said Raghu Murtugudde of the University of Maryland, US. “The problem with using IOD as an explanation is that its definition is not really solid. It is defined as a gradient of east-west SST (sea surface temperature) changes, but the action is all in the east,” Murtugudde said.
This year, earlier years
This absence of ‘action’ in the western Indian Ocean was evident this year too, Srinivasan pointed out. “This year there was strong cooling south of Sumatra (in the east Indian Ocean) but the western Indian Ocean did not show a large warming,” Srinivasan said.
Data from the Australian Bureau of Meteorology show that since 1960, there have been only 10 strongly positive IOD events before this year. Summer monsoon rainfall was deficient on four of those occasions, more 100% on four others, and normal on the remaining two.
The fact that IOD could have played a role in bringing excess rains in August and September can not be ruled out, but the extent of its influence is something that still needs to be studied.
“The high rainfall in August and September this year was a record, and as of now, it would not be wrong to say that we do not understand the reasons for it,” Srinivasan said.
Don’t miss from Explained: What fish do amid climate change