Influence of the Western North Pacific Summer Monsoon on Summer Rainfall over the Yangtze River Basin
西北太平洋夏季风对中国长江流域夏季降水的影响

Based on the NCEP/NCAR reanalysis circulation data and the precipitation data from 1979 to 2005, the relationship between the western North Pacific summer monsoon (WNPSM) and the summer rainfall over the Yangtze River basin has been discussed from both time and space aspects by the analysis of the precipitation, atmospheric circulations, water vapor transport and intraseasonal oscillations in the summer monsoon period. The results show that: (1) There is a remarkable negative correlation between WNPSM and the precipitation over the Yangtze River basin. When the WNPSM strengthens, the western Pacific subtropical high (WPSH) is abnormally northern and the southerly current along the western flank of WPSH is weaker than normal, which induces divergence of the low-level circulations anomalies and the water vapor transport anomalies over the Yangtze River basin, as a result, the precipitation over the Yangtze River basin is reduced. While in the weak WNPSM years, the WPSH is abnormally southern and western, and the strong southerly water vapor transport anomalies form in the Yangtze River basin and its southern area, which induces convergence of the southern and northern wind anomalies over the Yangtze River basin, the atmospheric ascending motion over it is exceptionally active, which is favorable for the precipitation over the Yangtze River basin.(2) The East Asian local Hadley circulation also displays obvious distinctions in the strong and weak monsoon years. When the WNPSM is strong, the local Hadley circulation in East Asia is quite weak, with the anomalous subsidence occurring over the Yangtze River Basin, which makes the rainfall over the Yangtze River basin decline, while the situation is just the opposite in the weak monsoon years.(3) There is a significant climatological intraseasonal oscillation (CISO) in the WNPSM region. In the period of the weak WNPSM, the Yangtze River rainfall is simultaneously influenced by the westerly CISO from the tropical western Pacific and the easterly CISO from the tropical Indian Ocean together, which gives rise to more rainfall than normal there. In the strong monsoon years, however, only the westward propagating CISO from the western Pacific exerts impact on the Yangtze River basin, so it is not easy to stimulate the precipitation.