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 Tibeta n Plateau (ETP) was examined in relation to the Northern Hemisphere macrosc ale circulation patterns during the period 1961-90. Summer precipitation da ta for 66 stations located above 2000 m MSL are used in the analysis. Using principal component analysis, it is found that the dominant spatial patter n of interannual variability of the summer precipitation is a seesaw struct ure between the southern and northern parts of ETP. Correlation analysis sh ows that this pattern of precipitation anomalies is closely associated with the North Atlantic oscillation (NAO). Further analysis based on midtroposp heric geopotential height and wind data suggests the upstream zonal flow va riation associated with the NAO pattern as the major mechanism linking the regional precipitation fluctuation to macroscale circulation conditions. Du ring the summers of low NAO index values, the westerly winds between 40 deg rees and 50 degreesN from the eastern Atlantic to Europe are intensified. T he enhanced upstream westerly winds generate anomalous anticyclonic flows i n the lower-latitude area to the west of the plateau and stronger dynamic b ifurcation 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 streng thened 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, produc ing a pattern with below-normal precipitation in the southern ETP and above -normal precipitation in the northern ETP. This study suggests that the com bination of the dynamic effect of large orography such as the Tibetan Plate au and the macroscale atmospheric circulation can be the determinant factor of regional climatic variability.