拉尼娜事件对长江中下游旱涝的影响

为研究不同强度拉尼娜事件下长江中下游旱涝差异,利用旋转经验正交函数方法,将长江中下游划分为4个区(西北部、中部、南部和东部);基于MEI指数,提取了1961—2014年的拉尼娜事件,并采用标准化降水蒸散指数来评估旱涝等级。结果表明:不同强度的拉尼娜事件可对各分区的旱涝频率、强度和时空分布产生不同的影响。在强事件持续期,一区易发严重干旱,二到四区易发生严重洪涝;在强事件结束后,一区可出现轻度洪涝,二到四区可出现中等以上干旱。在中等事件持续期,一区洪涝的频率和强度略大,二区到四区干旱的频率和强度略大;在中等事件结束后,一区洪涝的频率和强度非常大,二区到四区干旱的频率和强度非常大。弱事件对旱涝影响较小,且在南部和北部有反向变化特征,南旱北涝。旱涝频率、强度和分布差异与不同等级的拉尼娜事件影响水汽输送和垂直运动的强度有关。

Influence of La Niña events on drought and flood in middle and lower reaches of Yangtze River

In order to study the difference of drought and flood in the middle and lower reaches of the Yangtze River under different intensities of La Niña events, the research area were divided into four regions(northwest, central, south and east)by using the method of rotating empirical orthogonal function(REOF). Based on multivariate ENSO index(MEI), the La Niñ a events from 1961 to 2014 were extracted, and the standard precipitation evapotranspiration index(SPEI)was used to evaluate the drought and flood grades. The results show that different intensities of La Niña events have different influences on the frequency, intensity and spatial and temporal distribution of drought and flood in different regions. During the duration of strong events, severe drought is prone to occur in northwest region, and severe floods are prone to occur in other regions. After strong events, mild floods may occur in northwest region and moderate or more droughts may occur in other regions. During the duration of moderate events, the frequency and intensity of flood in northwest region are slightly higher than that of drought in other regions. After moderate events, the frequency and intensity of flood in northwest region are very large, and the frequency and intensity of drought in other regions are very large. Weak events have little influence on drought and flood, and have reverse change characteristics in south and north parts, while southern drought and northern flood. The reasons for the difference in frequency, intensity and distribution of drought and flood are related to the intensity of water vapor transport and vertical movement which are affected by different levels of La Niña events.