Observational Analysis And Numerical Simulation Of The Decadal Change In Late Spring Climate Over East China In The Last Four Decades

East China climate in spring has experienced significant decadal changein the late 1970s, which has received less attention than that in winter andsummer. This study aims to investigate the decadal change of East Chinaclimate in late spring, and discuss its cause in relation to the atmospheric cir-culation change, using the observational data of 560 meteorological stationsin China and the ERA-40 reanalysis data during 1958–2000. Numerical ex-periments are further carried out to validate the observational result. Thisstudy also explores the possible mechanism for the impact of winter NAO onthe tropospheric temperature in East Asia, which is proposed to be related tothe Tibetan Plateau (TP) snowfall.Analysis with pentad-mean precipitation data shows that late spring(April 21–May 20) rainfall over southeastern China (26?–31?N, 110?–122?E)has undergone a significant decrease since about 1978. Mean rainfall during1980–2000 is 1.8mm/day less than that during 1958–1977. Such change of therainfall is closely related to the decadal cooling in the upper troposphere overcentral China (30?–40?N, 90?–125?E) with the mean temperature in the lattertwo decades lower about 1?C than in the former two decades. The upper-level cooling is associated with an anomalous meridional cell with descendingmotions in the latitudes 26?–35?N and low-level northerly winds over south-ern China (22?–30?N, 110?–125?E), causing deficient rainfall over southeasternChina. Such circulation changes are also main factors for the decadal decreaseof the total cloud amount over central East China and the decadal increase ofthe surface air temperature in the Jianghuai basin.The decadal cooling in the middle-upper troposphere, the associatedmeridional cell and the decadal change of the surface climate in East China,all move southward from mid-April to mid-May. This may be caused by the pos-itive feedback between the upper-level cooling and the drought. Due to thewind anomalies associated with the upper-level cooling, the drought occurs tothe south of the cooling center. In the drought region, the latent heat ?uxis reduced, which in turn cools the tropospheric atmosphere. Such feedbackprocess can partly explain the persistence and southward movement of theupper-level cooling signal.The cause of the upper-level cooling in late spring over East Asia is fur-ther explored, which is found to be correlated with the enhanced winter NAOindex in the preceding winter. During winters with a positive NAO index, theAsian subtropical westerly jet is intensified, which strengthens the westerlyover the TP, and also deepens India-Burma trough because of the de?ectionof the TP. Both processes enhance the water vapor transport, and thus induceconvergence of water vapor ?ux over the TP. As a result, there is deeper snowover the TP. On the other hand, winter snow depth over the TP is negativelycorrelated with late spring tropospheric temperature in East Asia. So it issuggested that the decadal increase of winter snow depth over the TP is animportant factor linking the strengthened winter NAO index and the subse-quent late spring cooling in the middle-upper troposphere over East Asia afterthe late 1970s.The general circulation model LMDZ4 is used in this study, which is de-veloped at the Laboratoire de M′et′eorologie Dynamique (LMD). The modelis zoomed over East Asia with the atmospheric circulation outside the zoomdomain nudged by the ERA-40 reanalysis data. Numerical simulations arecarried out from March 1 to May 31 in each year of 1958–2000. Results showthat the model not only reproduces the decadal change of the atmosphericcirculation over East Asia, including the upper-level cooling and the anoma-lous meridional cell, but also captures the decadal drought over southeasternChina, the decadal decrease of the total cloud amount in central East China and the decadal increase of the surface air temperature in the JiangHuai basin.This proves that the upper-level cooling and the anomalous meridional cell arethe main atmospheric circulation factors for the decadal change of East Asiaclimate in late spring.To quantify the role of the upper-level cooling, sensitivity experimentis carried out with the simulated cooling signal imposed in the model. Inresponse to the prescribed upper-level cooling, the anomalies of late spring at-mospheric circulation bear remarkable resemblance with the decadal-changesignals in the observation, which show an anomalous meridonal cell in thesoutheast ?ank of the cooling region. The drought over southeastern China,the decrease of the total cloud amount in central East China and the increaseof the surface air temperature in the JiangHuai basin are also reproduced inthe simulation. This proves that the upper-level cooling can in?uence the sur-face climate through modulating the atmospheric circulation. The simulatedresults show that the latent heating is a main factor for the upper-level coolingin late spring. This is validated in another sensitivity experiment. When thelatent heating term is halted, the simulated cooling signal is weaker than theoriginal one. The above results from the sensitivity experiments help us tounderstand the impact of the decadal middle-upper tropospheric cooling onEast China climate change in the last four decades, as well as the persistenceof the cooling anomaly in late spring.