Prolonged Seasonal Drought Events Over Northern China And Their Possible Causes

The drought events over Northern China exhibit a high probability of persisting over multiple seasons,and such events may exert more catastrophic impacts on agriculture,ecosystems and socio-economic development than the single-season drought events.In-depth and comprehensive understanding of the prolonged seasonal drought(PSD)events as well as the linkage with the possible factors are necessary.In this study,we first explored the frequency,duration,and intensity of the PSD events over China during 1960-2012,based on the multiscale standardized precipitation evapotranspiration indices(SPEI),which are obtained from in-situ observations(SPEI-obs)and Climatic Research Unit(CRU)high resolution gridded data set(SPEI-CRU),respectively.Secondly,the features and possible causes of PSD events over Northern China,ie.,North and Northeast China(NC;32.5°-55°N,105°-136°E)and Northwest China(NW;32.5°-50°N,75°-105°E),during 1960-2012 are explored by using Japanese 55-year reanalysis(JRA-55)datasets and the sea surface temperature dataset from the National Oceanic and Atmospheric Administration(NOAA).Finally,the capability of regional climate models from Coordinate Regional Climate Downscaling Experiment(CORDEX)in simulating precipitation,potential evapotranspiration(PET),and drought during 1981-2005 are evaluated.Further,the variabilities of drought/flood during 2006-2049 are projected under representative concentration pathways(RCPs)RCP8.5 and RCP4.5.The main conclusions are summed up as follows:(1)Contrasting to the self-calibrated Palmer drought severity index(scPDSI)and standardized precipitation index(SPI),SPEI is more suitable for exploring the PSD.In particular,the PSD events can be well reproduced by both SPEI-obs and SPEI-CRU,which are calculated by observations and CRU gridded datasets,respectively.Based on the SPEI-obs and SPEI-CRU,higher frequency,longer duration,and stronger intensity of PSD are shown in NC and NW than the other regions over China.Furthermore,the features of the strength and extent of SPEI03 generally exhibit interannual variabilities,however,the frequency show interdecadal features.More specifically for NC,more(less)droughts happens in 1960s,1990s,and 2000s(1970s and 1980s).In NW,more droughts occurs before 1980s,whereas drought decrease after 1980s.(2)PSD events(i.e.,drought events with a time scale of 9 months or less)have had a relatively high probability of occurrence from spring to autumn during the past five decades.The results of the empirical orthogonal function(EOF)analysis show that the leading mode of the SPEI09 in autumn reflects a broad pattern of drought in NC,which confirms the existence of the PSD events.The timeseries of the first principal component(PC1)reflect interannual variability with an increasing tendency of PSD events after the late 1990s.Furthermore,we conclude that two factors,teleconnection patterns in the midlatitudes of Eurasia and transitions from La Nina to El Nino,are mainly responsible for the PSD events.On the one hand,the associated teleconnection patterns in the upper-level troposphere show an Eurasia(EU)-like pattern in spring,the Silk Road(SR)pattern or the circumglobal teleconnection(CGT)pattern in summer,and a Scandinavia(SCA)-like pattern in autumn,respectively.Associated with these teleconnection patterns,descending anomalies are detected in the whole atmosphere over NC in all seasons.On the other hand,the transition from La Nina(winter)to El Nino(the following autumn)also exerts effects on the PSD through modulating the Walker circulation and the meridional vertical circulation along the coast of East Asia.In addition,physical connections likely exist among these factors,and their combined effects,together with the insufficient moisture supply,regulate the PSD events over NC.Moreover,the increasing PET contributes to the increasing PSD events after the late 1990s.(3)Besides the background of drought that lasts more than four seasons over NW,the PSD(i.e.,drought events with a time scale of 9 months or less)also have had a relatively high probability of occurrence from spring to autumn just like that over NC.The results of the EOF analysis show that the leading mode of the SPEI09 in autumn reflects a broad pattern of drought in NW,which confirms the existence of the PSD events.The PC1 reflect interannual variability with a slight decreasing(wetting)tendency.The PSD events over NW are associated with decreased precipitation and enhanced PET.From the perspective of precipitation in each season,the vertical motion anomalies and water vapor transportation over NW are affected by the persist cooling in the mid-latitude of North Atlantic,and result in decreased precipitation.Although strong thermal conditions in summer and autumn contribute to the convergence of water vapor,these water vapor are not enough to increase the precipitation.From the perspective of PET,when PSD happens,the PET exhibit strong positive values in each season,which is favorable for drought.(4)The regional climate models participating in the CORDEX-East Asia project(HadGEM3-RA,SNU-MM5,SNU-WRF,and YSU-RSM)could well reproduce the historical precipitation,PET,and primary mode of PSD over NC and NW.The projections of regional drought/flood show inconsistent tendency under different RCPs,and the relative contributions of precipitation and PET is also different.The increasing precipitation and weakening PET together lead to wetting tendency over Hetao under RCP4.5.Under RCP8.5,the wetting trends over North China and South China are primary due to the increasing precipitation,and the drying trend over NW is mainly contributed by enhanced PET.