1991、1998和2016年三个大水年长江中下游夏季降水季节内特征的对比

针对长江中下游三个大水年1991、1998和2016年，利用NCEP/NCAR大气环流再分析资料和CMAP降水资料，对比了夏季降水的季节内特征，分析了引起降水季节内变化的大气环流季节内振荡ISO演变及源地。小波分析表明，三年季节内降水周期差异明显，分别为20~30 d、20~40 d和10~20 d。随之，以东亚季风区季节内振荡指数及热带外Rossby波活动通量，诊断了引起三年季节内活动异常的热带和中纬度ISO变率特点。结果显示影响三年季节内降水的ISO差异较大。1991年受到来自印度洋10~30 d和中纬度高层Rossby波10~30 d的ISO共同影响，造成周期为20~30 d的低频降水；1998年ISO来源路径单一，受中北太平洋30~60 d和10~30 d的ISO西传叠加作用，降水表现为20~40 d的振荡；引起2016年季节内降水异常的ISO源地较多，既有来自印度洋向东北传播30~60 d的ISO，又有来自太平洋向西北传播10~30 d的ISO，还有来自热带外10~30 d的ISO，三者在长江中下游汇合，引起降水10~20 d的振荡。研究结果对认识长江中下游夏季集中降水的形成有重要意义。 

A COMPARISON OF INTRASEASONAL RAINFALL FEATURES OVER THE MIDDLE AND LOWER REACHES OF YANGTZE RIVER DURING SUMMERS OF 1991, 1998 AND 2016

By using the NCEP/ NCAR reanalysis data and CMAP daily precipitation data, we compared intraseasonal oscillation (ISO) characteristics of rainfall during the summers of 1991, 1998 and 2016, three summers of the severest floods, over the Middle and Lower reaches of the Yangtze River (MLYR). Then we analyzed the evolution and source of atmospheric ISO. A wavelet analysis shows the difference in the periodicity of intraseasonal rainfall in these three years, which are 20~30 d, 20~40 d and 10~20 d, respectively. One diagnosis based on the boreal summer intraseasonal oscillation (BSISO) indices and the Rossby wave activity indices suggest that: (1) In 1991, two ISO, one from the tropical Indian Ocean and the other from the mid-latitudinal Eurasia, lasted for a period of 10~30 d. The ISO propagated toward northeast and southeast, and merged in the MLYR, causing a 20~30 d fluctuation in rainfall. (2) In 1998, the ISO originated from the central North Pacific. It propagated westward with two characteristic periods of 30~60 d and 10~30 d, leading to a 20~40 d oscillation in the rainfall over the MLYR. (3) In 2016, the origination of ISO was relatively more complicated. The ISO was a combination of those from the tropical Indian Ocean with a period of 30~60d, those from the tropical Pacific with a period of 10~30 d, and those from the mid latitudinal Eurasia with a period of 10~30 d. They propagated simultaneously toward the MLYR, resulting in a 10~20 d oscillation in rainfall.