The record-breaking rainfall this monsoon season, significantly throughout August and September, has left climate scientists confounded. After a greater than 30% shortfall in June, the season ended with 10% extra rainfall, the primary time such a factor has occurred since 1931. The September rainfall (152% of lengthy interval common, or LPA) was the best since 1917, the August rainfall (115% of LPA) was the best since 1996, and the general seasonal rainfall (110% of LPA) was the best since 1994.
Search for solutions
As late as the primary week of September, the India Meteorological Department maintained that the seasonal rainfall was going to be regular (in the 96-104% vary). With an influencer like El Niño Southern Oscillation (ENSO) in the Pacific remaining largely impartial this yr, scientists try to pin down the precise cause for the bizarre rainfall.
In the search for solutions, one phenomenon attracting some consideration is the Indian Ocean Dipole or IOD, an ocean-atmosphere interplay much like El Niño, however in the Indian Ocean. IOD is a measure of the distinction in the sea-surface temperatures of the western Indian Ocean (Arabian Sea) and the jap Indian Ocean, south of the Indonesian coast. When the western waters are hotter than the jap, IOD is claimed to be optimistic; in the other state, IOD is damaging.
Like ENSO in the equatorial Pacific Ocean, IOD too influences climate and local weather occasions, although its impression is weaker as a result of the Indian Ocean is significantly smaller, and shallower, than the Pacific. The IOD has an impression on the Indian monsoon: a optimistic IOD is known to assist monsoon rainfall whereas damaging IOD is thought to suppress it.
This yr’s IOD, which started creating round June and grew robust after August, has been one of many strongest on file. IOD data are usually not very outdated. Accurate measurements can be found solely since 1960, in response 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 mentioned in its newest bulletin on October 15.
This has led to scientists taking a look at IOD for attainable clues to this yr’s bumper rainfall, particularly since such robust IOD occasions in earlier years, too, have been related with excessive 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,” mentioned Sridhar Balasubramanian, affiliate professor of mechanical engineering and an adjunct college 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 cautious to not immediately blame the IOD for this yr’s rains. That is as a result of IOD’s hyperlink with the Indian summer time monsoon is tenuous at greatest. It is just one of a number of components that impression the monsoon, and never probably the most dominant.
In reality, the IOD’s affect on the monsoon shouldn’t be absolutely understood. It is thought to have a much weaker affect than ENSO, although. IOD’s relationship with the Indian summer time monsoon can also be much much less studied in comparison with that of ENSO, mentioned J Srinivasan, distinguished scientist with the Divecha Centre for Climate Change at IISc, Bengaluru.
Besides, it’s not clear if the IOD influences the monsoon or if it’s the different means spherical. The IOD typically takes form in the direction of the latter half of the summer time monsoon, in August and September, and scientists don’t rule out the likelihood that the monsoon might play some position 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,” mentioned 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 mentioned.
This yr, earlier years
This absence of ‘action’ in the western Indian Ocean was evident this yr too, Srinivasan identified. “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 mentioned.
Data from the Australian Bureau of Meteorology present that since 1960, there have been solely 10 strongly optimistic IOD occasions earlier than this yr. Summer monsoon rainfall was poor on 4 of these events, extra 100% on 4 others, and regular on the remaining two.
The indisputable fact that IOD might have performed a task in bringing extra rains in August and September can’t be dominated out, however the extent of its affect is one thing that also must 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 mentioned.