The El Niño-Southern Oscillation (ENSO) is one of the major drivers of prominent ocean-atmosphere coupled phenomenon over the equatorial Pacific Ocean that strongly modulates the interannual variability of Indian summer monsoon rainfall (ISMR). However, the relationship between ENSO-ISMR has gone through secular variation during different decades. This study comprises a detailed analysis of the changing relationship between the ENSO and ISMR during two recent independent 30-year periods; (i) 1961–1990 and (ii) 1991–2020, using Sea Surface Temperature (SST), rainfall and various atmospheric variables. It was observed that the negative correlation of ENSO with ISMR has significantly weakened during 1991–2020 compared to 1961–1990. It was found that the La Niña associated positive rainfall intensity over central India was reduced during 1991–2020 due to weakening of vertical wind shear as compared to 1961–1990. Similarly, the El Niño associated negative rainfall intensity was reduced over large parts of north and central India during 1991–2020 due to weaker subsidence anomaly compared to 1961–1990. However, the weakened ENSO-ISMR relation in the recent period (1991–2020) mostly contributed from the disconnection of La Niña-ISMR relationship compared to El Niño-ISMR, as seen from the large-scale dynamics including the changes in the vertical wind shear and the shift in the Walker circulation. This study highlights how the changes in the patterns and the intensity of the atmospheric as well as oceanic fields within the tropical Indo-Pacific Ocean contributed to the weakening of ENSO-ISMR relation in the recent decades.

This study attempts to understand the asymmetry in the Indian Summer Monsoon Rainfall (ISMR) response to two types of La Niña whether they evolved from El Niño or La Niña in the previous boreal winter season. It was seen that nine La Niña years during the monsoon season were preceded by El Niño (hereafter ELLA) whereas eight were preceded by La Niña (hereafter LALA) during the period 1961-2021. India received more rainfall during the ELLA years as compared to the LALA years, linked to the difference in the Sea Surface Temperature (SST) and large-scale atmospheric circulation anomalies over the tropical Pacific and Indian oceans. Based on the strength and patterns of the cold SST anomaly and shift in the Walker circulation over the equatorial Pacific Ocean, the enhanced (weakened) convection over the Indian landmass during the ELLA (LALA) years, contributed to more (less) rainfall over India.