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.

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The first  three REOFs of summer precipitation on the Tibetan Plateau