Mechanism Of The WNPAC Simulation Bias Of Atmospheric Background Field Regulation Anomaly In Climate Model

The anomalous western North Pacific anticyclone(anomalous WNPAC,hereafter this text will be abbreviated as WNPAC)plays a crucial role in linking ENSO and climate in East Asia,and has significant impacts on the precipitation anomalies in eastern China in different seasons during El Ni(?)o.There are still some biases and large uncertainties in the simulation of WNPAC by climate models.Understanding the source of the model biases will help improve the ability of models to predict the El Ni(?)o induced interannual climate anomalies in East Asia.Using the output results of CMIP5/6 models,combined with diagnostic analysis and numerical experiments,this paper reveals the regulation of the mean state biases on the WNPAC biases and uncertainties in the models,and expounds the physical mechanisms behind the influence of the mean state precipitation and moisture,the mean state static stability and the mean state vorticity on the simulation results.The main conclusions are as follows:(1)In the CMIP5 atmospheric experiments driven by historical sea surface temperature(AMIP),the WNPAC in multi-model ensemble mean(MME)is weaker than that observed during El Ni(?)o decaying summer,and there are large inter-model uncertainties.The sources of simulation biases and uncertainties are different in EP and CP El Ni(?)o events.The bias in MME arises from underestimated mean precipitation in the monsoon trough area and overestimated mean stability in the western North Pacific.A diagnostic analysis indicates that the former plays a major role.The northward shift of WNPAC in EP events can be explained by the northward shift of monsoon trough and corresponding rain belt.In contrast,the bias of position is inconspicuous in CP events.The inter-model uncertainties of WNPAC are mainly arise from the enhancement of local circulation-convection feedback caused by the mean precipitation.The mean precipitation in the western North Pacific is important in EP events,while that in the tropical Indian Ocean is important in CP events.The mean state vorticity has a little contribution to the uncertainty in EP events,but has no contribution in CP events.The mean state stability has no significant contribution in both of the events.(2)During El Ni(?)o mature winter,the WNPAC in AMIP models is stronger than that observed,and there also exists large uncertainties.The sources of simulation biases and uncertainties are different from those in the decaying summer,but they are basically consistent in EP and CP El Ni(?)o events.The bias in MME arises from overestimated mean precipitation and underestimated mean stability in the western North Pacific.A diagnostic analysis indicates that the former plays a major role.There are large inter-model uncertainties in both intensity and position of the WNPAC.The numerical experiments indicate that the mean precipitation in the Central Pacific and the western North Pacific play major roles in uncertainty of WNPAC intensity,and the contributions of mean stability and mean vorticity are relatively small.In contrast,the main contribution to the position uncertainty comes from mean state vorticity.The uncertainty of the meridional gradient of mean state vorticity causes the position uncertainty of WNPAC through the Rossby wave adjustment mechanism.The anomalous heat source in the Central Pacific also has a little contribution to the position uncertainty,while mean state stability has no contribution.In the CMIP6 AMIP simulations,the relationship between the mean states and the inter-model uncertainties of WNPAC during El Ni(?)o mature winter are consistent with CMIP5.(3)In the CMIP6 historical climate experiments,the atmospheric mean state can also regulate the inter-model uncertainties of WNPAC,although there are biases in El Ni(?)o related SSTA.The role of atmospheric mean state and SSTA is relatively independent,thus the influence of atmospheric mean state is also important in coupled models.Regarding the uncertainty of WNPAC intensity,SSTA in the Ni(?)o-3.4 region can affect the anomalous precipitation and circulation responses directly,while average precipitation and moisture in the Central Pacific affect the sensitivity of atmosphere to SSTA forcing,and thus affect the anomalous precipitation and circulation response.The WNPAC position uncertainty arises from the difference in westward extension of the warm SSTA and the meridional gradient of mean state vorticity in the western North Pacific.