Emerging Shift in the Indian Summer Monsoon Sensitivity to Equatorial Indian Ocean Sea Surface Temperature Anomalies: Insights from High-Resolution AGCM SST Patch Experiments

Usha K H , Sajani Surendran, Kavirajan Rajendran, Endo Hirokazu, Kitoh Akio

Indian Summer Monsoon (ISM) rainfall exhibits strong sensitivity to sea surface temperature anomalies (SSTAs) across four Indo-Pacific nodal regions: The Western and Eastern Equatorial Indian Ocean (WEIO and EEIO), the Western Pacific (WPAC), and the Niño3.4 region. Historically, positive ISM rainfall anomalies are associated with warming in WEIO and WPAC, while warming in EEIO and Niño3.4 suppresses rainfall. However, observations after 2000 reveal a marked weakening/reversal, of the ISM response to equatorial Indian Ocean SSTAs, whereas its sensitivity to WPAC and Niño3.4 remains largely unchanged. Using high-resolution Atmospheric General Circulation Model (AGCM) SST patch experiments with idealized perturbations, we isolate the influence of these regions on monsoon variability. The simulations indicate that positive SSTAs in the equatorial Indian Ocean enhance local deep convection, lower-tropospheric diabatic heating, and moisture convergence, redistributing convection away from India and weakening ISM rainfall. These findings have important implications for monsoon predictability under a warming climate.