Impact of soil moisture on simulation of interannual variability of summer climate in northwestern China

The potential role of surface soil moisture(SSM) in improving the simulation of interannual summer climate in the northwestern China was investigated with a coupled land-atmosphere climate model.Two ensemble simulations for 22 boreal-summer seasons(1979~2000) were conducted by the Community Atmosphere Model version 3(CAM3) with specified observed sea surface temperature(SST).The impact of SSM on the simulation of summer climate in the northwestern China was investigated by the model with interannually varying SSM(ISSM) and climatological SSM(CSSM) respectively.Results show that under the specified observed SST,the interannual variability of summer temperature and precipitation simulated by the CAM3 with ISSM is better than that with CSSM over the northwestern China.The correlation coefficient of temperature between the simulations with CSSM(ISSM) and the ERA40 is 0.60(0.65) over the northwestern China.The correlation coefficients resulting from the comparison of the observed and the simulated summer precipitation over the northwestern China increase from 0.29 in the CSSM to 0.48 in the ISSM.The possible effect mechanisms of SSM on simulating the precipitation anomaly in the northwestern China were explored.Firstly,the interannual variability of the ground evaporation anomaly can be simulated by CAM3 with ISSM better than with CSSM in the western China,resulting in the anomalous precipitation simulated by CAM3 with ISSM more closely to the CMAP in the northwestern China,where there is an obvious positive correlation between the observed precipitation and the observed local ground evaporation anomaly.Secondly,the interannual variability of summer precipitation anomaly in the northwestern China is significantly influenced by the upstream atmospheric circulation over the mid-and high-latitudes of Eurasia,which can be simulated by the CAM3 with ISSM better than with CSSM,leading to the interannual variability of summer precipitation anomaly in the northwestern China better reproduced by the model with ISSM.