CMIP6 Evaluation and Projection of Precipitation over Northern China: Further Investigation

Based on 20 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6), this article explored possible reasons for differences in simulation biases and projected changes in precipitation in northern China among the all-model ensemble (AMME), "highest-ranked " model ensemble (BMME), and "lowest-ranked " model ensemble (WMME), from the perspective of atmospheric circulations and moisture budgets. The results show that the BMME and AMME reproduce the East Asian winter circulations better than the WMME. Compared with the AMME and WMME, the BMME reduces the overestimation of evaporation, thereby improving the simulation of winter precipitation. The three ensemble simulated biases for the East Asian summer circulations are generally similar, characterized by a stronger zonal pressure gradient between the mid-latitudes of the North Pacific and East Asia and a northward displacement of the East Asian westerly jet. However, the simulated vertical moisture advection is improved in the BMME, contributing to the slightly higher performance of the BMME than the AMME and WMME on summer precipitation in North and Northeast China. Compared to the AMME and WMME, the BMME projects larger increases in precipitation in northern China during both seasons by the end of the 21st century under the Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5). One of the reasons is that the increase in evaporation projected by the BMME is larger. The projection of a greater dynamic contribution by the BMME also plays a role. In addition, larger changes in the nonlinear components in the BMME projection contribute to a larger increase in winter precipitation in northern China.