中文摘要：

利用黄河上中游地区45个气象站1961年-2011年逐月降水资料及NCEP/NCAR再分析资料,分析了其盛夏降水的时空变化及其相关的环流异常特征。结果表明：①该区盛夏降水总体呈下降趋势,1960年代初、1970年代中后期、1990年代为偏涝期,其它时期为偏旱期;②该区盛夏降水的前3种空间分布模态为：全区一致型、南北反向型、东西反向型。根据REOF进一步将该区盛夏降水异常细分为高原东部区、渭河上游区、河套北部区、渭河下游区和黄河源区,各区降水趋势及周期变化存在明显差异;③该区盛夏旱涝变化与500hPa高度场上西太平洋副热带高压、印度低槽、鄂霍次克海高压、中高纬环流形势及100hPa高度场上南亚高压强度和东伸脊点位置密切相关。鄂霍次克海高压偏强与南亚高压东伸脊点偏东可能是导致黄河上中游区盛夏偏旱重要原因。

英文摘要：

The monthly precipitation data at 45 stations from 1961 to 2011 over the upper and middle reaches of the Yellow River（UMRYR） and NCEP/NCAR reanalysis data were analyzed to determine the temporal and spatial variation in midsummer precipitation,and anomalies in the UMRYR circulation.Analysis was undertaken using EOF,REOF,composite analysis,moving average and trend analysis.The results show that：（1） The spatial distribution of mean rainfall was not uniform,with rainfall levels rapidly decreasing from southeast to northwest.Temporally the levels of midsummer precipitation in the UMRYR had a downward trend.There were 3 obvious cycles from 1961 to 2011：at the beginning of the 1960＇s,at the end of the 1970＇s and 1990＇s which were all flood periods,while the other periods were drought.（2） The midsummer precipitation can be divided into three kinds of distributed modes：an entire regional distribution mode,north and south opposite distribution mode,and west and east opposite distribution mode.The frequency of the first three EOF modes accounts for 60.8%,27.4% and 11.8%,respectively,and can be separated from the remaining mode.The entire regional distribution mode was the main distributed mode-reflecting that the levels of midsummer precipitation consistently respond to annual variation in the summer monsoon.（3） With REOF the midsummer precipitation can be further divided into 5 areas：east of Qinghai plateau area,the upper reaches of Weihe river area,north of Hetao area,the lower reaches of Weihe river area and source area of the Yellow River.There were significant differences in the precipitation trend and periodic variation in each area.Over time precipitation levels in the source area of the Yellow River had an increasing trend,while the other areas had a downward trend,especially in the lower reaches of Weihe River area.Cycles in inter-annual variation existed in both the periods of 11~15 years and 2~3 years in each areas,but with the former predominant.（4） The variation in droughts an