A synoptic analysis is made of the low-latitude pentad mean circulation at 100, 500, and 1000 mb from May through July of 1979, with the conclusion that the seasonal transformation of the low-latitude atmospheric circulation from winter to summer of that year occurred in the time between the second pentad and the fourth one of June. The second characteristic vector obtained from the solution of the arrays of the pentad mean flow field in the diffenent layers by the method of natural orthogonal expansion is an important component characterizing the seasonal transformation of the circulation and is called the monsoon component. The statistical analysis of the heating effect of the Qinghai-Xizang Plateau and the time coefficient of the second characteristic vector shows that there exists an obvious correlation between them. And it is found from the synoptic analysis of the diffusivity field and vertical motion field of the second vector that the correlation is due to the vertical circulation cells over the region between the Plateau and its neighbourhood, and the low latitudes.