Regional Differences In Land-Atmosphere Coupling And Possible Impacts Of Soil Moisture Over Eastern China

Land-atmosphere coupling is a key process of the climate system,and various coupling mechanisms have been proposed before based on observational and numerical analyses.The impact of soil moisture(SM)on weather/climate is the main part of such coupling.Using the ERA-Interim reanalysis and GLDAS(Global Land Data Assimilation System)V2.0 SM datasets,this study explored the land-atmosphere coupling over eastern China.Firstly,the spatial features and seasonality of land-atmosphere coupling over this region were analyzed.Secondly,the effects of SM on local thermal conditions and non-local precipitation in different key areas and their mechanisms were discussed.Finally,the potential indication of SM to persistent drought in Southwest China was further explored.The main results of this study are as follows:1.The influence of SM on evapotranspiration(ET)and hence the surface temperature(ST)anomaly is an important aspect of land-atmosphere coupling.By analyzing the relationships among SM,ET and ST,it is found that two diagnostics of the coupling(i.e.,SM-ET correlation and ST-ET correlation)are highly dependent on the climatic states of SM and ST,with spatial variation and seasonal evolution.In North China,SM is relatively low throughout the year,and there is enough energy to evaporate the water from the soil to the atmosphere only in the warmer months,so that the intensity of the coupling changes with the temperature,which leads to the strongest coupling in summer.However,in Southwest China,ST is relatively high throughout the year,but SM decreases significantly during the dry season and reaches the lowest in spring,resulting in a strong coupling in spring.2.This study quantitatively analyze the SM feedbacks on the local thermal states in summer over North China.Our results show that SM exhibits a downward trend with the decadal warming in North China,which results in enhanced negative SM feedbacks on local thermal conditions and even extreme hot events.The SM feedback contributes 6%of the total air temperature variation during 1961-2012,while it reaches 36%after the regional warming during 1994-2012.Such SM affecting temperature is mainly reflected in its feedback on daily maximum temperature,which is also intensified during the warm period.The decadal intensification is also found in SM feedback on hot extremes.Further analyses show that the abnormal changes of the surface energy fluxes caused by the SM anomaly are the main reasons that affect the thermal conditions.Besides,with the decreasing SM,the upward latent heat flux shows a significant reduction,and thus more energy is partitioned into sensible heat.The Bowen ratio increases rapidly after 1990s,indicating that the ability of land surface heating atmosphere is intensified,which leads to the decadal intensification of the local thermal feedback of SM in summer over North China.3.The effect of SM on non-local precipitation is another important aspect of land-atmosphere coupling.Through the analysis of assimilation SM and observed precipitation data,it is found that the spring SM in Southwest China is significantly correlated with the summer rainfall in eastern China during the past 52 years(1961-2012).Especially,if the soil is dryer than normal over Southwest China,the rainfall is relatively higher over the middle and lower reaches of the Yangtze River basin,and vice versa.More importantly,this negative relationship exists an inter-decadal change in the early 1990s,with a significant correlation before 1991 and an evident decrease after then.Further analysis shows that the dry(wet)soil condition in spring over Southwest China causes the surface temperature abnormally higher(lower),heating(cooling)the atmosphere,and then increases(decreases)the local geopotential height.This situation will attract(exclude)the West Pacific Subtropical High(WPSH)in summer,causing its westward extension(eastern back),which leads to less(more)water vapor transferred to the north and more(less)precipitation over the Yangtze River basin.In recent years,due to the global warming,the large-scale circulation system exhibits a tremendous variation:The WPSH is obviously increased and extended westward,which has certain impacts on China's climate states.Correspondingly,the land-atmosphere coupling strength decreases over Southwest China,indicating the influence of SM on the circulation system is weakened.Therefore,the relationship between spring SM over Southwest China and summer rainfall over eastern China is weakened and even disappeared in recent two decades.4.During the last decade,several high intensity and long duration droughts happened in Southwest China and resulted in tremendous socioeconomic losses.SM is a direct drought index and plays a key role in weather/climate predictions.Thus,a general analysis of SM is beneficial to drought research and prediction over this region.Results show that SM exhibits a strong and season-dependent persistence.Particularly,the autumn anomaly shows the strongest memory that can be sustained to the next spring.In addition,the most significant soil drying trend is exhibited in autumn accompanied by an evident abrupt change in 1991.Correspondingly,the autumn SM memory has extended from 3 months before the abrupt change and 6 months in recent.Furthermore,we use the Standardized Precipitation Evapotranspiration Index(SPEI)with multiple time scales to identify the droughts in different seasons,and the inter-annual change patterns of autumn SM and SPEIs are generally in agreement with each other,which confirms that SM is suitable for indicating the droughts over Southwest China,especially those multi-season persistent drought events started in autumn.