How well do CMIP6 models simulate the climatological northern boundary of the East Asian summer monsoon?

The northern boundary of the East Asian summer monsoon (EASM) lies within the transitional zone from humid to arid areas, and is under the combined impacts from the EASM and the westerlies over the mid-high latitudes. The changes in the EASM northern boundary location can exert profound impacts on the surroundings with high climate sensitivity, causing frequent occurrences of extreme natural disasters. In this study, we evaluated the skills of 45 models from the phase 6 of the Coupled Model Intercomparison Project (CMIP6) in simulating the climatological location of the EASM northern boundary, which can provide important basis for further studies on its changes in the past and future. The northern boundary of the EASM has shown to be shifted northwardly in all the models compared to the observations, with the low-skill models holding more northern locations compared to the high-skill ones. Further analysis indicates that the low-skill models have two main biases with more westward shift of the western Pacific subtropical high (WPSH) and northward movement of the EASM subtropical westerly jet comparing with the high-skill models. Both biases can induce more rainfall in the northern China, leading to northward shift of the EASM northern boundary. In addition, the northward movement of the EASM subtropical westerly jet is suggested to be driven mainly by the warming sea surface temperature (SST) in the Kuroshio/Oyashio (KOE), and the warming in the KOE and the maritime-continent and southeastern Indian Ocean (MC-SEIO) exert combined impacts on the westward movement of the WPSH. Our results suggest that the SST warmings in the KOE and MC-SEIO are the leading biases for the CMIP6 models in simulating the EASM northern boundary.