Processes for the Northeastward Advance of the Summer Monsoon over the Western North Pacific

Abstract

In the South China Sea (SCS) and western North Pacific (WNP), climatological summer monsoon sets in first in the SCS in mid-May, extends into the southwestern Philippine Sea in mid-June, and jumps suddenly northeastward to around 20°N, 150°E in late-July. The processes leading to this distinct northeastward advance of the summer monsoon are investigated using NCEP/NCAR reanalysis data for the period of 1979-1995. It is found that the cloud-radiation and wind-evaporation feedback plays an important role for the seasonal change in sea surface temperature (SST) in the WNP. Analysis shows that monsoon-induced changes in cloudiness and surface wind produce contrasting changes in surface short wave radiation, and latent heat flux between the convection and pre-convection region. The resultant SST tendency difference turns around the SST gradient east of the convection region in about one month, and induces the northeastward shift of the highest SST center. Following the ocean surface warming in the pre-convection region, the convective instability and low-level moisture convergence increase. East of the convection region, the reversal of the SST gradient tends to be associated with the transition of low-level winds from easterly to westerly. The results suggest that the SST change induced by the monsoon onset facilitates the northeastward extension of convection. It is speculated that the summer monsoon advance over the WNP may result from air-sea interactions.