Influence of Tropical Ocean Basins on the Interannual Variability of Indian Summer Monsoon Rainfall during three recent epochs

This study examines the relationship between Indian Summer Monsoon Rainfall (ISMR: June- September Season) and climate drivers across the three tropical basins, namely the Pacific Ocean, Indian Ocean, and Atlantic Ocean, using observation and reanalysis data for the period 1961-2023. Although Sea Surface temperature (SST) conditions over the tropical Pacific Ocean have a major influence on the interannual variability of ISMR, their interactions with the Indian Ocean and the Atlantic Ocean also play a role. The interannual relationship between El Nino Southern Oscillation (ENSO) and ISMR is well-established, but its epochal variations and impact across different homogeneous regions of India (the north-west, north-east, central, and south peninsular India) is not yet fully understood. To understand this epochal variation in the ENSO-ISMR relationship and the role of the tropical basin interaction, this study considered three different periods (P-I: 1961-1980, P-II: 1981-2000, P-III: 2001-2023). The lead-lag correlation analysis showed that the SST conditions over the north Indian Ocean (NIO) and the south tropical Atlantic Ocean (STAO) during the preceding winter (October-January) season has a significant role on ISMR. The analysis showed a significant (insignificant) negative simultaneous correlation between ENSO and ISMR for P-I and P-III (P-II). Regression analysis reveals that in P-I, the SST conditions over the NIO as well as STAO had insignificant influence on the ISMR. In P-II, the role of the NIO became significant, particularly over the south peninsular India. In P-III, the influence from the NIO has reduced (although still significant), at the same time the role of STAO became significant. The thermodynamical analysis is performed to understand the mechanism relating the role of NIO and STAO in modifying the ENSO-ISMR teleconnection in the three periods. This study highlights the importance of the changes in the large-scale patterns of the oceanic as well as atmospheric fields and the interactions between the tropical Indian-Pacific-Atlantic Oceans, to the performance of ISMR.