过去300a长江中下游异常丰梅事件变化与洪涝灾害

2016年6—7月,长江流域发生重大洪涝灾害,给社会生产和人民生活带来严重损失。而该时段内梅雨因其强降水过程频繁、持续时间长、雨量集中等特点成为主要致灾因子。论文利用历史文献和观测资料,构建了1736年以来异常丰梅事件年表,分析了其长期变化特征及与El Ni?o事件的统计关系,并诊断了El Ni?o造成异常丰梅事件的大气环流背景。结果表明:1736—2016年间共发生44次异常丰梅事件(含21次特大梅雨事件),其中1900年代、1910年代、1990年代是异常丰梅事件最为频繁的3个时段;与2016年梅雨特征相近的特大梅雨事件有21次。过去300 a间,异常丰梅事件与El Ni?o存在较好的对应关系,44次异常丰梅事件中37次发生在El Ni?o的当年或次年;在21次特大梅雨事件中16次伴随El Ni?o出现。伴随El Ni?o事件而发生的异常丰梅事件环流特点是中高纬经向环流偏强,我国境内水汽输送通道偏西、偏北,大量水汽滞留在长江流域,并与南下的冷空气交汇,从而形成持续性降水;而在El Ni?o次年,低纬存在较强且稳定的副热带高压,水汽输送路径偏北,长江中下游水汽输送通量显著增强,更容易导致异常丰梅事件的发生。

Variations of Extreme Meiyu Events and Flood Disasters over the Mid-lower Reaches of the Yangtze River in the Past 300 Years

The flood disasters caused a great loss of life and property in Yangtze River Basin during June-July of 2016. Meiyu became one of the main causes of flood disasters due to its high frequency, long duration and high intensity. In this study, chronology of extreme Meiyu events since 1736 was derived from Chinese historical documents and the observational data. Furthermore, and the long-term variation characteristics of extreme Meiyu events and relationships between extreme Meiyu and El Niño events were analyzed. The general circulation patterns were diagnosed to explain how El Niño influenced extreme Meiyu. The results show that there were 44 extreme Meiyu events (including 21 mega-Meiyu events) from 1736 to 2016. The most frequent occurrences of extreme Meiyu events were detected during 1901-1920 and 1991-2000. There were 21 mega-Meiyu events over the past 300 years, whose intensities were similar to that in 2016. Closely corresponding relationships was found between extreme Meiyu events and El Niño in this period. For example, 37 of total 44 extreme Meiyu events occurred in the El Niño episodes or their following years, and 16 of total 21 mega-Meiyu events occurred in the El Niño episodes. In the El Niño years, the meridional circulations were intensified over the mid-latitudes. The water vapor transported westerly and northerly to the mainland of China. A great amount of water vapor from south intersected with the cold air from high latitudes in the mid-lower reaches of the Yangtze River, which led to continuous precipitation. Moreover, in the El Niño following years, there was a steady subtropical high system at low latitudes. The transportation path of vapor was northerly, and then the vapor fluxes increased in Yangtze River Basin. It tends to cause extreme Meiyu events under this circulation background.