South Asian monsoon connections to ENSO in present-day and future climate base states

Given the socioeconomic importance of the South Asian monsoon to the lives of millions of people living across that region, it is critical to understand the processes that produce fluctuations in the amplitude and patterns of monsoon precipitation. It is only then that credible monsoon predictions are possible for seasonal to decadal timescales, as well as longer term projections beyond decadal. Two Earth system models, CESM2 and E3SMv2, are analyzed to investigate how differences in base state tropical SSTs and ENSO amplitude affect the processes associated with present-day and future South Asian monsoon simulations. Monsoon-ENSO teleconnections through the Walker Circulation are reduced in E3SMv2 compared to CESM2 due to cooler mean tropical SSTs, along with ENSO amplitude in E3SMv2 that is half that in CESM2. The warmer present-day tropical Pacific SSTs in CESM2 lead to future SST warming with a stronger Bjerknes feedback and a larger El Niño-like SST response. The consequent eastward shift of precipitation and upper-level divergence in the tropical Pacific in CESM2, along with the larger amplitude ENSO in E3SMv2, produce anomalies through the Walker Circulation that increase the future strength of the monsoon-ENSO connection in E3SMv2 compared to CESM2. These results point to the importance of differences in the base state in present and future climate which affect the Walker Circulation and monsoon-ENSO connections.