This paper investigates the variability of abnormally wet and dry conditions during the past 60 years in the eastern part of northwestern China (ENWC).Daily precipitation,temperature and compound index (CI) of meteorological drought data provided by the China National Climate Center are used,along with monthly reanalysis data from NCEP/NCAR,for the period 1951-2012.The methods of rotated empirical orthogonal function (REOF),correlation analysis,composite analysis and dynamical diagnosis are employed.Additionally,the possible underlying physical mechanism of extreme summer drought events is revealed through analys is of atmospheric circulation anomalies,such as the East Asian summer monsoon,as well as vapor transportation and vertical motion.In particular,the relationship between the upper-level subtropical jet and the upper-level anomalous divergence field is explored.The results can be summarized as follows:The trend of change in CI index is similar to that of precipitation;there are obvious interannual and interdecadal variations of CI index and precipitation in summer over ENWC.Meanwhile,a positive anomaly of summer temperature has continuously maintained over recent decades.More seriously,an obvious decrease in precipitation after the mid-1990s is apparent;and consequently,a warmer and dryer trend began and continues today over ENWC.The extreme drought centers in summer are mainly located in the Hetao Plain and the southeast of ENWC,representing a southwest-northeast trend.Moreover,this spatial distribution tendency and associated features are almost the same as those of wet events over ENWC,and are very similar to those of the northern marginal zone of the East Asian summer monsoon in the same period.Evidence shows that the CI over ENWC is significantly and positively correlated with the 500-hPa geopotential height anomaly in the Ural Mountains,and significantly and negatively correlated with that over Lake Baikal and the north Okhotsk Sea.It is also found that the upper-level subtropical jet's axis tilts (it is flat during extreme wet events),and,as a result,causes large-scale anomalous convergence during extreme summer drought events.At the same time,due to the weak intensity of the upper-level subtropical jet,abnormal easterly flows exist in the upper levels over ENWC,indirectly bringing about a weak circulation on the right of the subtropical jet exit area,which is not conducive to the monsoon moving northward.Moreover,the low-level anomalous southwest winds from Northwest China and the plateau regions indicate the East Asian summer monsoon is weak.Besides,the variation in the location of the ridge line and westward extent of the subtropical high over the western Pacific Ocean is weak,and one of the important reasons for extreme summer drought may result from the divergence of water vapor and descending motion in summer over ENWC.Previous studies have also confirmed that the changes in dry and wet conditions over ENWC are opposite to those over the middle and lower reaches of the Yangtze River.In summary,the main cause of extreme drought over ENWC in summer is the responses to the tilt of the upper-level subtropical jet's axis and its weak intensity.Specifically,a wide range of convergence in circulation may appear,which in turn causes abnormal divergence in circulation in the low levels,and thus a deficit in the water vapor budget throughout the whole troposphere.In addition,the East Asian summer monsoon is weak.Therefore,the combination of the above higher and lower level systems and the abnormal atmospheric circulation is unfavorable for precipitation,leading to extreme drought.