Abstract: In early summer,a relatively stationary 500hPa trough over India and Burma,the southern wave of the mid-latitude westerly,moves out of the Bay of Bengal to affect the Indochina and South China. Extensive surveys and case studies show that the shift is closely associated with the formation and enhancement of the cross equatorial flow. Their inter-relation can be accounted for by the twist and vertical transfer terms in the equation of complete divergence.