Onset of the Western North Pacific Summer Monsoon

The mechanisms of monthly and pentad western North Pacific summer monsoon (WNPSM) were discussed by many previous works. The present study, onset and withdrawal of the WNPSM using defined more detailed daily outgoing longwave radiation (OLR) and examined the physical mechanisms at the onset. To define the onset of WNPSM, we investigated the slowly evolving annual cycle (L-mode) and the rapidly evolving annual cycle (S-mode) of the area mean OLR in the region of WNPSM, with the use of pentad means and the difference between pentad means. This definition indicates that L-mode largely affects the onset of WNPSM, and make the onset days somewhat earlier than those of previous works. The comparison based on the onset day clearly shows that the environments are largely changed beginning of through the WNPSM. Sea surface temperature (SST) decreases after beginning of WNPSM in the western North Pacific (WNP). The warm SST condition before the onset generates and maintains weak atmospheric stability. The northward shift of the subtropical pacific high weakens the descent motion in the WNP. As a result, the convective activity is activated there, which is the onset of WNPSM. The onset of WNPSM and the convective activity are thus influenced by seasonal movement of subtropical pacific high with the effect of suppression for the upward motion. More detailed structures will be shown in December.