From UPSC perspective, the following things are important :
Prelims level : Indian Ocean Dipole (IOD)
Mains level : Role of IOD in Indian Monsoon
Record-breaking rainfall this year
- 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 was the highest since 1996, and the overall seasonal rainfall was the highest since 1994.
Indian Ocean Dipole (IOD)
- 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.
Strongest ever IOD
- 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.
- 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 due to positive IOD the monsoon rainfall was normal that year.
- 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.
- 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.