夏季索马里越赤道气流垂直结构的年代际变化

基于1950—2010年美国国家环境预报中心/国家大气研究中心（NCEP/NCAR）再分析月平均资料、哈得来中心月平均海温资料，定义夏季索马里越赤道气流（简称SMJ）垂直结构指数VS₁和VS₂，分别表征索马里越赤道气流全区一致、上下反相两种分布型。接着利用合成分析、奇异值分解方法探讨夏季索马里越赤道气流垂直结构的年代际变化，寻找可能影响其变化的环流因子和海温信号，并进一步分析垂直结构与亚洲夏季风活动的年代际关系，主要结论如下：（1）两个指数在年代际尺度上的阶段性变化特征十分显著，VS₁在20世纪80年代中期发生由负转正的年代际变化；VS₂在70年代初和90年代中期分别发生由负转正、由正转负的年代际变化。（2）合成分析表明，VS₁和VS₂的年代际变化与同期大气环流联系紧密，南极涛动可能是影响VS₁年代际变化的一个重要因子。（3）印度洋海温变化可能对夏季索马里越赤道气流两类垂直结构的年代际变化起调控指示作用。VS₁的年代际变化与夏季全印度洋海温增暖的趋势是一致的，VS₂与夏季南印度洋偶极型振荡事件关系显著。（4）高VS₁年代里，印度半岛至中南半岛夏季风偏弱，纬向水汽供应减少，降水偏少，东亚夏季风亦偏弱，使得华北地区纬向和经向水汽输送均偏弱，同时水汽辐合减弱，华北地区偏旱，而在江南地区由于水汽辐合，降水偏多；反之亦然。高VS₂年代，东亚季风区内中国西南、华南地区夏季风减弱，使得经向和纬向水汽输送均偏弱，水汽供应减少，因此华南、西南地区偏旱，日本地区夏季风偏弱，西风水汽输送亦偏弱，降水亦偏少，反之亦然。

The interdecadal variations of the vertical structure of the summer Somali cross-equatorial flow

Based on the 1950-2010 NECP/NCAR reanalysis monthly mean data and the Hadley monthly mean SST, the two indices, denoted by VS₁ and VS₂, are defined, which are able to describe the variabilities of the two vertical modes, called the consistent mode and the vertically opposite mode, of the summertime Somali cross-equatorial flow (SMJ). Using the singular value decomposition (SVD) and the composite analysis, it is mainly done in this paper to analyse the interdecadal variations of the vertical structures of SMJ and the associated atmospheric circulation features,as well as the SST signals. The relationships between the indices and Asia summer monsoon activity are also discussed. The results show that: (1) VS₁ and VS₂ exhibit obvious interdecadal variabilities. In the study time of interest, the former turns positive in the mid-1980s, the later experiences two decadal mutations, one is from positive to negative in the early 1970s and the other is from negative to positive in the late 1990s. (2) The composite analysis indicates that the interdecadal variabilities of VS₁ and VS₂ are closely related to the atmosphere circulation. It's also worth mentioning that the Antarctic Oscillation (AAO) may be an important factor which effects the interdecadal changes of VS₁. (3) The SSTA in the Indian Ocean could play the regulating and indication role in the interdecadal variations of the vertical structures of SMJ. VS₁ is found to be in line with the trend seen in SSTA of the whole Indian Ocean. VS₂ is related to the Southern Indian Ocean SST anomaly dipole oscillation. And, (4) on interdecadal timescale, when VS₁ is in a positive phase, the summer monsoon weakens, and accordingly, the zonal moisture supply over the Indian peninsula to the Indo-China Peninsula reduces with the summer rainfall tending to be less than normal. At the same time, the relatively weak East Asia summer monsoon, the related weak moisture transportation and the weak moisture convergence lead to more rainfall in North China. More rainfall occurs in the south Yangtze River area because of the enhancement of moisture convergence. The case of negative VS₁ is opposite. On interdecadal timescale, when VS₂ is in a positive phase, the summer monsoon over the southwestern China, South China and Japan declines, moisture transportation is weakened, thereby summer rainfall in these areas tends to be less, and vice versa.