Deciphering the Intermodel Spread in Projections of the Impacts of Indian Summer Monsoon on ENSO Under Global Warming

Abstract

The Indian summer monsoon (ISM) is intricately linked to the El Niño-Southern Oscillation (ENSO) on interannual timescale. Although previous studies have explored ENSO's effects on the ISM, the reverse influence, particularly under global warming, remains unclear. This study examines the projected changes in the ISM's impacts on ENSO under the SSP5-8.5 emission scenario using 34 climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) that reasonably simulate the monsoon's effects on ENSO. A significant spread is found in the projections across the models, with approximately half of the models projecting an enhancing influence of ISM on ENSO, whereas the other half indicates a weakening effect. The intermodel spread is primarily associated with the projected changes in the strength of the feedback between precipitation and low-level circulation over the tropical northwest Pacific, which is crucial for generating ISM-induced anomalous circulation over the region. Models projecting an enhanced precipitation-circulation feedback simulate larger ISM-driven rainfall and circulation anomalies over the tropical northwest Pacific in a warmer climate, leading to more pronounced zonal wind anomalies near the equator along the southern side of the anomalous circulation and vice versa. As a result, the larger zonal wind anomalies caused by abnormal monsoons exert intensified effects on the subsequent ENSO evolution by significantly suppressing or amplifying the atmosphere-ocean coupling processes related to ENSO development.