Abstract: 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.