中文摘要：

利用青藏高原中东部1961-2004年60个气象台站夏季（6-8月）降水资料,通过旋转经验正交函数分解发现青藏高原夏季降水存在南北反向变化的空间模态,分析表明这种变化模态与北大西洋涛动（NAO）密切相关.利用NCEP/NCAR再分析资料进行环流场分析,探讨了NAO对青藏高原这种降水空间变化模态的影响机制.结果表明：强NAO年份时,高原北部水汽输送通量强度增强,水汽辐合增强,而高原南部水汽输送通量强度减弱,此时高原切变线位置明显偏北,正是在这种水汽输送和环流形式配置下使得高原北部降水偏多而高原南部降水偏少;在弱NAO年份,上述情况基本相反.

英文摘要：

As the Third Pole,the Tibetan Plateau（TP） is the headwaters of the most important large rivers in the Asia and plays an important role in the Asia climate system and water cycle.Precipitation variation on the TP has a significant impact on the local human activities,ecosystems and water resources.The mechanism of precipitation change still need to be searched,although a lot of works have been done on the precipitation variation on the TP.Here a detailed analysis of the spatial and temporal changes in summer precipitation in the TP over the period of 1961-2004 is presented based on a developed daily precipitation dataset of 60 weather stations.The 60 stations in the central and eastern TP are selected for the rotated empirical orthogonal functions（REOF） analysis with a relatively uniform distribution of the station network.It is found that there is dramatically difference between the southern and northern TP,and even reverse.Such dipole precipitation pattern over TP is believed correlating well with the North Atlantic Oscillation（NAO）.The possible mechanism of the correlation is studied by using the NCEP/NCAR reanalysis data with analysis-by-synthesis and precipitation diagnosis approach.When NAO index is high,the moisture convergence strengthens with precipitable vapor and the water vapor flux increasing in the northern TP,but the precipitable vapor and the water vapor flux decreasing in the southern TP.However,the variation of water vapor flux divergence is relatively complex under the influence of the southwest monsoon.Meanwhile,the plateau shear line move northward obviously.As a result,the summer precipitation is usually above the normal in the northern TP but below the normal in the south TP.When NAO index is low,the above processes are almost opposite.