What is the Indian Ocean Dipole (IOD)?
It is an atmosphere-ocean coupled phenomenon in the tropical Indian Ocean (like the El Nino is in the tropical Pacific), characterised by a difference in sea-surface temperatures. A ‘positive IOD’ — or simply ‘IOD’ — is associated with cooler than normal sea-surface temperatures in the eastern equatorial Indian Ocean and warmer than normal sea-surface temperatures in the western tropical Indian Ocean. The opposite phenomenon is called a ‘negative IOD’, and is characterised by warmer than normal SSTs in the eastern equatorial Indian Ocean and cooler than normal SSTs in the western tropical Indian Ocean.
There is no established correlation between Indian summer monsoon rainfall and IOD; however, studies have shown that a positive IOD year sees more than normal rainfall over central India. This happened in 1994 and 1997. The indicated connection is between below-normal SST in the eastern Indian Ocean and above-normal rain over central India, and vice versa.
What is the IOD situation this year?
Current observations show neutral IOD conditions over the Indian Ocean. But most parts of the tropical Indian Ocean, particularly the southern tropical Indian Ocean, are warmer. Studies have linked conditions like these to below-normal rainfall over India.
So what does this mean for the monsoon?
Almost all global models have predicted the El Nino will get stronger. Considering the evolving El Nino and negative or neutral IOD in the Indian Ocean, the chances of a bad monsoon are very high.
Since 1997. Forecasts have improved in recent years. Particularly after launching the Monsoon Mission, our long-range predictions have become more reliable. The IOD is one of the parameters in the dynamic model of monsoon forecasting, even though a lot of research still remains to be done.
What studies have been done at IITM?
Indian scientists collaborated with Japanese researchers in the discovery of the oscillation of SSTs. Several studies have been carried out in India since, and research at IITM has improved the understanding of the IOD phenomenon. Most of the associations seen between the IOD and the southwest monsoon have in fact been generated at IITM. Some early studies showed a link, for example, between a positive IOD or La Nina conditions in the Pacific with a delayed withdrawal of the monsoon, and a shorter rainy season in case of a negative IOD or El Nino.
Has a connection been seen between a drought year and IOD?
Some drought years such as 2002 and 2009 have been negative IOD years.