Spatial and Temporal Variation of Summer Precipitation over the Eastern Tibetan Plateau and the North Atlantic Oscillation

The interannual variability of summer precipitation over the eastern Tibetan Plateau (ETP) was examined in relation to the Northern Hemisphere macroscale circulation patterns during the period 1961-90. Summer precipitation data for 66 stations located above 2000 m MSL are used in the analysis. Using principal component analysis, it is found that the dominant spatial pattern of interannual variability of the summer precipitation is a seesaw structure between the southern and northern parts of ETP. Correlation analysis shows that this pattern of precipitation anomalies is closely associated with the North Atlantic oscillation (NAO). Further analysis based on midtropospheric geopotential height and wind data suggests the upstream zonal flow variation associated with the NAO pattern as the major mechanism linking the regional precipitation fluctuation to macroscale circulation conditions. During the summers of low NAO index values, the westerly winds between 40 and 50N from the eastern Atlantic to Europe are intensified. The enhanced upstream westerly winds generate anomalous anticyclonic flows in the lower-latitude area to the west of the plateau and stronger dynamic bifurcation flows to the south of the plateau, which promote development of cyclonic flows to the east of the plateau. As a result, the southerly winds in the southern ETP and the northerly winds in the northern ETP are strengthened simultaneously. In this case, summer precipitation is usually above normal in the southern ETP but below normal in the northern ETP. During the summers of high NAO index values, the above processes are reversed, producing a pattern with below-normal precipitation in the southern ETP and above-normal precipitation in the northern ETP. This study suggests that the combination of the dynamic effect of large orography such as the Tibetan Plateau and the macroscale atmospheric circulation can be the determinant factor of regional climatic variability.