Abstract:In the view of data analyses, it is indicated that the variation of general circulation systems such as the subtropical westerly jet in both Hemispheres, the easterly jet, the Tibetan Plateau high and the Mexican high is greatly different between the period that the summer monsoon is in activity in the South China Sea (SCS) and the period that the SCS monsoon is in inactivity. The results are also shown that the activity or inactivity of the summer monsoon in the SCS is determined by the different phase of the northward propagating low frequency oscillation (LFO) of wind divergence field from the Southern Hemisphere and the phase of southward propagating LFO of westerly wind field from the Northern Hemisphere. And the phase locked of different type LFO in the SCS is the determinant factor in controlling the timing of activity of the SCS monsoon.