South Asian summer rainfall from CMIP3 to CMIP6 models: biases and improvements

As a new generation of global climate models, monsoon simulation in Coupled Model Intercomparison Project (CMIP) Phase 6 models is of great concern to climate modeling community. Using 21 CMIP3 models, 28 CMIP5 models and 38 CMIP6 models, we show evidence that the long-standing dry biases in South Asia (SA) are resulted from less rainfall with both less frequency and intensity in a shortened monsoon season. By evaluating several key metrics, we identify that the monsoon rainfall simulation in CMIP6 models has improved in both of the multimodel ensemble mean (MME) and individual models, consistent with the improvements in monsoon annual cycle and rainfall characteristics. Further analyses and sensitivity experiments show that the cold SST biases all year round over the northern Indian Ocean (NIO) are important sources for the persistent dry biases in the CMIPs’ models. The cooling effect of SST biases on the tropospheric temperature becomes increasingly prominent since the boreal spring, weakening the baroclinity of monsoon circulation via the thermal wind relationship and eventually resulting in insufficient monsoon rainfall. Comparison across the three generation CMIP models also confirms that the improvement of SA summer rainfall simulation in CMIP6 MME benefits from the reduction of NIO SST biases. This study highlights the importance of improving SST simulation in reducing the monsoon rainfall biases.