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 Yan-gtze River basin has been discussed from both time and space aspects by the analysis of the precipitation, atmospher-ic 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 climato-logical 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 rairffall than normal there. In the strong monsoon years, however, only the westward propagating CISO from the western Pacific exerts impact on the Yan-gtze River basin, so it is not easy to stimulate the precipitation.%利用1979～2005年NCEP/NCAR的环流场再分析资料和降水资料,通过对季风期降水、大气环流、水汽输送及低频振荡等方面的分析,分别从时间和空间上分析了西北太平洋夏季风与中国长江流域夏季降水的联系.结果表明:(1)西北太平洋夏季风与中国长江流域夏季降水存在显著的负相关关系,在西北太平洋夏季风强盛时,副热带高压异常偏北,其西侧的偏南气流异常偏弱,使得我国长江流域形成低层异常环流及水汽输送的辐散区,从而造成长江流域夏零降水偏少;而在西北太平洋夏季风减弱的年份,西太平洋副高异常偏南偏西,在长江流域以南地区形成异常偏强的偏南风水汽输送,使得长江流域成为南、北距平风的汇合区,其上空对流活动异常活跃,非常有利于长江流域的降水.(2)东亚局地Hadley垂直环流在强、弱季风年也显著不同,在强季风年里,Hadley局地环流异常偏弱,长江流域上空出现的下沉运动距平,使得该地区降水减弱,而弱季风年则正好相反.(3)西北太平洋夏季风存在显著的气候平均的大气季节内振荡(CISO),在西北太平洋夏季风减弱时期,长江流域降水同时受到源自热带西北太平洋西传CISO和源自热带印度洋东传CISO的共同影响,可能造成了某种锁相关系,从而造成降水偏多;而在强季风年里长江流域只受由西太平洋西传的CISO的影响,不容易激发降水.
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