Uncertainty in projections of the South Asian summer monsoon under global warming by CMIP6 models: Role of tropospheric meridional thermal contrast

Driven by the increase in CO2 concentration, climate models reach a consensus that the large-scale circulation of the South Asian summer monsoon (SASM) becomes weakened but with different magnitudes. This study in-vestigates the major uncertainty sources of the SASM response to an abrupt quadrupling of CO2 (abrupt-4xCO(2)) in 18 models of phase 6 of the Coupled Model Intercomparison Project. The projected weakening of the SASM indicated by both zonal and meridional monsoon circulation indices is closely linked to decreases in the merid-ional gradient of upper-tropospheric temperature between Eurasia and the Indian Ocean (EUTT-IUTT). A climate feedback-response analysis method is applied to linearly decompose the uncertainty of changes in EUTT-IUTT into the partial changes due to external forcing and internal processes of the earth-atmosphere column. Results show that the uncertainty of changes in EUTT-IUTT is contributed positively by the dominant atmospheric dy-namic process, followed by the cloud shortwave radiative effect, and negatively by the surface latent heat flux and cloud longwave radiative effect. Contributions from CO2 forcing and other internal processes including albedo and water vapor feedbacks, oceanic heat storage, and sensible heat flux are found to be minor.