中文摘要：

利用NCEP/NCAR再分析月平均资料,分析了1948-2009年夏季青藏高原（下称高原）600hPa位势高度场的变化趋势,发现整个高原位势高度场都出现大范围的升高,低压中心主要位于27.5°-40°N、80°-102.5°E范围内,定义该范围内位势高度场的平均值为高原夏季低压指数。采用经验正交函数分解（EOF）和小波分析等方法对近62年夏季高原低压的年际和年代际变化特征进行了分析。结果表明,近62年夏季高原低压总体呈减弱趋势,在20世纪80年代之前,夏季高原低压指数均处于低值范围,并在1962年出现最小值,在1979年左右出现最大值,之后在高值范围内上下振荡;空间分布表现为低压在高原大部分区域均为由南向北递增,呈明显的纬向分布;低压在1976年发生了一次较明显的减弱突变。小波分析表明,低压具有1～2年和13年周期。利用中国596个测站的月降水资料,采用相关分析和合成分析等方法分析了高原低压与我国夏季降水的关系,分析表明,高原低压增强时长江流域和新疆地区的降水偏多,而东北、华北和华南地区的降水则偏少。

英文摘要：

The tendency variation of 600hPa geopotential height fields over the Qinghai-Xizang Plateau（QXP）in the summer months（JJA）during 1948-2009were calculated based on the NCEP/NCAR data.The geopotential height field shows a increasing trend since 1948.The center was located on the low area over 27.5°-40°N,80°-102.5°E on 600hPa,it is defined regional average as summer low index over the QXP.Using the Empirical Orthogonal Function（EOF）and wavelet analysis,the interannual and interdecadal changes of the summer low index over the QXP are examined.The results of temporal-spatial structures reveal that：In general,the summer low over the QXP shows a decreasing trend since 1948.The low index has been in a low phase before 1980s,and it is the lowest in 1962and the highest in 1979.From the 1980sto the early 21st century the low index has been in a high phase.The spatial distribution of the low index reveals that most of the low indexes over the QXP region increase from south to north,appears obvious zonal distribution,and its expansion coefficient shows the long-term positive trend.The analysis of the expansion coefficient shows that there is a obvious weakening change of the low in 1976.Wavelet analysis of expansion coefficients indicates that there are the periods of 1～2and 13years.At last,using the precipitation data from 596stations in China,the relationship between interannual and interdecadal changes of the summer low over the QXP and the precipitation in China is explored：When the plateau low increasing,the precipitation in the Yangtze River and Xinjiang is more,precipitation in the Northeast China,North China and South China is less.