LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences
The performances of 19 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) in simulating the water cycle over East China are evaluated based on observations and reanalysis data using Brubaker model in this study. The sources of model bias are also investigated. The CMIP6 multi-model ensemble (MME) can reasonably simulate the climatic distribution and annual cycle of precipitation and evaporation with pattern correlation coefficient of 0.92 and 0.87, respectively. Compared with observations, MME overestimates precipitation in North China (0.55 mm day-1) but underestimates precipitation in coastal areas of South China (-0.3 mm day-1). All 19 models overestimate evaporation with biases of 0.03-0.98 mm day-1. Thus, the differences between precipitation and evaporation simulated by most of the models are smaller than the observation and reanalysis data. The MME can well simulate the annual cycle of the contribution of each moisture source to precipitation, but underestimates the contribution of remote moisture via southern boundary, resulting in dry bias over the study region. We find that the southerly wind speed over southern boundary determines the difference of water vapor transport among CMIP6 models. The stronger the southerly wind speed is in the model, the higher water vapor flux incomes via the southern boundary, and the more precipitation the model simulates. The position of the convergence zone over the Northwest Pacific is one of the important systems affecting the southerly wind speed over the southern boundary. The eastward shift of the convergence position in the model results in weaker southerly winds, leading to weaker moisture transport to the study region and less precipitation, and vice versa. This study systematically evaluates the performance of CMIP6 in reproducing the East Asian water cycle, and demonstrates the limitation of the models in simulating the convergence zone over the Northwest Pacific and its impact on the East Asian water cycle.