Important Factors for the Development of the Asian–Northwest Pacific Summer Monsoon*

The Asian and northwest (NW) Pacific summer monsoons exhibit stepwise transitions with rapid changes in precipitation at intervals of roughly 1 month from mid-May through mid-July. A new method is developed to evaluate the effects of sea surface temperature (SST) and other changes on these rapid monsoon transitions. The latter changes include solar radiation, land memory, and atmospheric transient (SLAT) effects. The method compares two sets of atmospheric general circulation model (GCM) simulations, forced with observed seasonally varying and piecewise constant SST, respectively. The results indicate that the SLAT effects dominate all of the major transitions, except during mid-June when the SST cooling induced by the strong monsoon westerlies is a significant negative feedback resisting the intensification and northward advance of monsoon convection. The final regional onset of the monsoon system takes place in mid-July over the subtropical NW Pacific characterized by the abrupt enhancement of deep convection there. Despite a weak SST effect from the GCM assessment herein, major changes in convection and circulation are confined to the ocean east of the Philippines during the mid-July transition, suggesting the importance of transient atmospheric adjustments. Intense convection over other regions induces subsidence over the subtropical northwest Pacific during June, contributing to the delayed onset there. Satellite observations reveal a slow buildup of free-tropospheric moisture over the NW Pacific, leading to an abrupt intensification of convective precipitation in mid-July, suggesting a possibility that the gradual tropospheric moistening eventually triggers a threshold transition.

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