2009年江淮梅雨异常： （1）在正常梅雨期， 江淮梅雨区的降水量明显偏少， 有人认为是空梅或非典型梅雨； （2）在7月下旬到8月初， 江淮流域又出现了所谓“二度梅”或“晚梅雨”的现象。为什么该年的梅雨会前弱后强？本文重点分析了2009年江淮异常梅雨的大尺度环流背景， 并从季节内振荡角度探讨了异常梅雨的形成机制。结果表明： （1）亚洲夏季风的异常变化是造成6月底～7月初和7月下旬～8月初江淮区域两段梅雨降水期的环流背景。前者发生在亚洲夏季风明显偏弱的时期， 持续时间短， 降水量少； 后者发生在亚洲夏季风明显增强之后， 降水持续性强、 降水强度大， 并发生于通常的少雨时段。（2）东北冷涡异常偏强且持续时间长， 使得夏季风难以北进， 导致第一段梅雨期开始偏晚； 夏季风水汽输送影响偏弱引起该段梅雨期降水量偏少。受东北低涡增强南压影响， 副热带高压南退， 来自西太平洋的异常水汽输送到达江淮地区， 并与北方的水汽输送产生异常辐合， 对第二段梅雨期的持续性异常降水产生重要影响。（3）15～30天大气低频振荡是2009年梅雨降水异常的可能机制。与通常30～60天低频振荡北传影响的情况不同， 2009年东亚夏季风15～30天低频振荡的北传十分明显， 其与北方15～30天低频振荡的南传在江淮汇合， 对江淮两段梅雨降水期有重要作用， 且第二段梅雨期的持续性强降水与15～30天低频振荡的异常增强有关。

Meiyu precipitation in Yangtze\|Huaihe basins is abnormal in summer of 2009. During usual Meiyu period (i.e. from mid June to early July), Meiyu precipitation is too less than climatology to come up to the standard of occurrenceof Meiyu. And some people view it as empty Meiyu or untypicalMeiyu. But during the period from late July to the beginning of August, Meiyu belt retreats to Yangtze\|Huaihe basins again. The so\|called ‘secondary Meiyu’or ‘late Meiyu’has so abundant precipitation that standard of occurrenceof Meiyu is satisfied. Why does Meiyu in 2009 exhibit characteristics of former weak and later severe precipitation? To answer the question, largescale circulation background and mechanism of abnormal Meiyu from the angle of interseasonal oscillation are discussed. The results indicate that: (1) Anomalous Asian summer monsoon provides largescale condition for the occurrenceof Meiyu during the end of June to the beginning of July and from late July to the beginning of August, respectively. The former Meiyu which takes place during the period of distinctively weak Asian summer monsoon has short duration and low rainfall; while the latter occurred after the remarkable strengthening of Asian summer monsoon and meanwhile the time period usually with little rainfall, has long duration and severe precipitation. (2) Anomalously strong Northeast Cold Vortex with long duration blocks the northward advance of west Pacific subtropical high, which brings about late beginning of the first Meiyu period. And weak influences of moisturetransport by summer monsoon attributes much to the lack of precipitation during the first Meiyu period. During the second Meiyu period, due to the southward retreat of the west Pacific subtropical high under the influence of strengthening of Northeast Cold Vortex, anomalous moisture transport from the west Pacific converges with those from North China in Yangtze\|Huaihe basins, which exerts important effects on anomalously sustainable precipitation. (3) 15~30\|day atmospheric oscillation may be the possible mechanism of abnormal Meiyu in 2009. Different from the usually important influences of northward propagation of 30~60\|day low frequency oscillation (LFO) on Meiyu, the obviously northward propagation of 15~30\|day LFO of east Asian summer monsoon joins the southward propagation of 15~30\|day LFO from the North China in the Yangtze\|Huaihe basins, which plays the important roles on the occurrenceof the two Meiyu periods. In addition, the persistent and severe precipitation during the secondary Meiyu relates to the anomalously strengthening of 15~30\|day LFO.