The El Niño-Southern Oscillation (ENSO) is deemed as the most important driver of the Indian summer monsoon (ISM) variability. Co-occurring El Niños (La Niñas) are associated with anomalous deficit (surplus) in the ISM rainfall. Locations of impacts within the sub-continent are of course subject to whether the co-occurring El Niño is a Modoki or canonical.

The ENSO association with the ISM has shown a significant weakening since 1990s. It can be surmises that the apparent weakening of the ENSO impact on the monsoon observed in the recent decades may simply be due to a combination of interference from co-occurring other phenomena such as the IOD, etc., and several interannual to decadal processes. We recently found, for example, that in the recent decades a strengthened cyclonic circulation in the recent decades over the northwest Pacific may have contributed to the weakening of ENSO-Monsoon links. Nonetheless, the impact of the strong ENSOs is, in general, clear; the below normal ISM during 2015, co-occurring with a mega El Niño, bears a testimony to the typical impact of the strong El Niños.   Further, our analysis of the multi-model PMIP3 simulations for the last-millennium shows that, ENSO-ISM links go through centennial modulation. The result is the Medieval Warm Period (Little Ice Age) showing higher (lower) monsoon precipitation.

The ENSO is a lynchpin connecting the Indian and Australian summer monsoons, resulting in what is suggested as the tropical biennial variability. Therefore, any changes in one of the three players results in a decadal waxing or waning of this variability.

The talk will also touch upon various mechanisms suggested to explain the impact of the ENSO on the ISM, and questions that remain. it will briefly cover the recent progress in the monsoon prediction, among other relevant issues.