Multi-stage onset of the summer monsoon over the western North Pacific

The climatological summer monsoon onset displays a distinct step wise north eastward movement over the South China Sea and the western North Pacific (W NP) (110 degrees -160 degreesE, 10 degrees -20 degreesN). Monsoon rain comm ences over the South China Sea-Philippines region in mid-May, extends abrup tly to the southwestern Philippine Sea in early to mid-June, and finally pe netrates to the northeastern part of the domain around mid-July. In associa tion. three abrupt changes are identified in the atmospheric circulation. S pecifically, the WNP subtropical high displays a sudden eastward retreat or quick northward displacement and the monsoon trough pushes abruptly eastwa rd or northeastward at the onset of the three stages. The step wise movemen t of the onset results from the slow northeastward seasonal evolution of la rge-scale circulation and the phase-locked intraseasonal oscillation (ISO). The seasonal evolution establishes a large-scale background for the develo pment of convection and the ISO triggers deep convection. The ISO over the WNP has a dominant period of about 20-30 days. This determines up the time interval between the consecutive stages of the monsoon onset. From the atmo spheric perspective, the seasonal sea surface temperature (SST) change in t he WNP plays a critical role in the northeastward advance of the onset. The seasonal northeastward march of the warmest SST tongue (SST exceeding 29.5 degreesC) favors the northeastward movement of the monsoon trough and the high convective instability region. The seasonal SST change, in turn, is af fected by the monsoon through cloud-radiation and wind-evaporation feedback s.