中国东部夏季暴雨的年代际跃变及其大尺度环流背景

本文利用1960~2011年中国东部地面测站的逐日降水资料和JRA-55再分析资料探讨了夏季暴雨分布的年代际跃变及其相关联的大尺度环流异常特征。基于暴雨频数和占比(夏季暴雨占比是指5~8月暴雨降水量对总降水量的贡献百分比)的分析结果表明:中国东部夏季暴雨分布在20世纪70年代末和90年代初经历两次反相的经向"三极子"跃变。中国东部夏季暴雨的年代际演变过程可分为三个时段:1960~1979年为华南和华北暴雨偏多、江淮流域暴雨偏少的经向"三极子"分布;1980~1991年为南方和华北暴雨偏少、江淮流域暴雨偏多的经向三极子"分布;1992~2011年为南方暴雨显著偏多、华北暴雨持续偏少,逐渐形成经向"偶极子"分布,并导致近十多年我国夏季"南涝北旱"的整体格局。1970年代末(1990年代初)跃变相关联的大尺度环流异常配置:东亚夏季风的减弱(增强),西太平洋副高的增强西伸但南撤(北抬),南亚高压的减弱南缩(增强东扩),以及蒙古高原中低层的气旋式(反气旋式)环流异常。与此同时,低层局地环流也发生调整:华北和黄淮地区以及华南和江南地区均为反气旋式(气旋式)环流异常,而江淮流域和四川盆地受控于风场切变式辐合(辐散)异常;涡度场发生相应变化,南北方大部分地区的负(正)涡度异常不(有)利于低涡的发展,而江淮流域和四川盆地的正(负)涡度异常有(不)利于低涡的发展,进而引发江南和华南暴雨减少(增加)、江淮流域和四川盆地暴雨增加(减少)、黄淮和华北暴雨减少(增加)的经向"三极子"跃变。

The Vertical Characteristics of Temperature and Humidity Inside Convective and Stratiform Precipitating Clouds in the East Asian Summer Monsoon Region and Indian Summer Monsoon Region

Based on the daily precipitation dataset of 395 gauge stations in eastern China and the JRA-55 reanalysis rainfall (HR) and their associated large-scale circulation anomalies were investigated. The mutational analysis of summer HR occurrence frequency and contributing percentage showed that there were two significant changes of summertime HR-one around the late 1970s and another in the early 1990s, both exhibiting meridional three-cell (tripole for short) anomalies in spatial terms. The period from 1960 to 1979 is referred to as P1, from 1980 to 1991 as P2, and from 1992 to 2011 as P3. A positive (negative) tripole pattern can be defined as follows:negative (positive) anomalous signs in southern China;positive (negative) anomalous signs in the Sichuan Basin and Yangtze-Huaihe River basin;negative (positive) anomalous signs in the Huanghe-Huaihe valleys and North China. In other words, the interdecadal change of summertime HR around the late 1970s (early 1990s) was characterized by a positive (negative) tripole distribution in the average difference of summertime HR frequency or percentage between the periods P2 and P1(P3 and P2), and the two interdecadal changes were almost exactly opposite. Correspondingly, the interdecadal evolution of summertime HR anomalies can be divided into three phases, as follows:a negative tripole distribution in the P1 phase;a positive tripole distribution in the P2 phase;and a dipole distribution-known as the ‘southern flood/northern drought’ pattern-in the P3 phase. Further results show that the factors responsible for the positive (negative) tripole changes of summertime HR in eastern China around the late 1970s (early 1990s) might have been associated with anomalous large-scale circulation, as follows:a weakening (strengthening) of the East Asian summer monsoon;a southwards retreat (northwards march) of the western Pacific subtropical high, which continually strengthens and extends westwards;a southwards shrinking (eastwards expansion) and weakening (strengthening) of the South Asia high;and strong cyclonic (anticyclonic) circulation anomalies in the mid-low troposphere over the Mongol highlands.