Uncertainty in projected changes of Indian Summer Monsoon Rainfall by CMIP6 models

A robust and trustworthy rainfall projection over the Indian landmass is vital for devising climate adaptation strategies. However, past studies show large inter-model spread in Indian Summer monsoon (ISM) rainfall projections thus calling for more detailed investigations on the underlying process. In the present study, we investigate this aspect using Coupled Model Intercomparison Project Phase 6 (CMIP6) model projections (Shared Socioeconomic Pathways, SSP5–8.5) and historical simulations. The Multi-Model Ensemble mean (MME) results show intensification of ISM rainfall at the end of the 21st century with ISM rainfall increasing by 1.6 ± 0.8 mm/day under SSP5–8.5 scenario. A moisture budget analysis for the MME further infers that the thermodynamic effect (TH) due to global warming plays a dominant role in enhancing ISM rainfall in the projections, with its dynamic counterpart (DY) assuming an additional contribution. It is also revealed that both DY and TH terms contribute to the inter-model uncertainty in ISM rainfall, but with DY dominating over the other this time. The inter-model uncertainty in DY and ISM rainfall changes is linked to inter-model spread in interhemispheric thermal contrast which in-turn depends on the diversity in Equilibrium Climate Sensitivity (ECS) and Global Mean Temperature (GMT) among the models. Intriguingly, when we remove the inter-model diversity in ECS through a GMT scaling, an Atlantic meridional surface temperature gradient, involving both land and ocean, emerges as a crucial driver in controlling the uncertainty in both DY and ISM rainfall changes, and drives large-scale monsoon circulation changes over African and the Indian subcontinents.