Air-sea interaction in the summer monsoon onset over the western North Pacific

The summer monsoon advance displays notable eastward component over the western North Pacific. The plausible role of local air-sea interaction in leading to this distinct monsoon advance is investigated using NCEP/NCAR reanalysis during 1979-1995. Before the western North Pacific summer monsoon onset, a warm SST tongue (SST over 29°C) is observed to extend/move northeastward in the Philippine Sea. The warm SST preconditions the atmosphere for the flare up of deep convection by destabilizing the atmosphere. The warm SST center shifts from the South China Sea to the Philippine Sea in mid-June and displaces northeastward through late-July. The associated SST gradient change induces the penetration of westerlies to the Philippine Sea. The role of the seasonal SST change is through surface heat fluxes and moisture convergence. On the other hand, the seasonal SST change involves important feedback from the monsoon-induced cloud and surface wind changes. The flare up of convection reduces the input of solar radiation due to cloud blocking. The surface latent heat flux, oceanic mixing, and upwelling increase following the invasion of monsoon disturbances and development of cyclonic winds. These changes turn around the local SST tendency. To the east or northeast of the convection region, the compensating downward motion enhances or maintains the solar radiation. The wind speed decrease in relation to the northeastward move of the monsoon trough reduces the latent heat flux and oceanic mixing. These favor the maintenance of positive SST tendency. As a result, the warm SST center moves northeastward. Thus, the northeastward summer monsoon advance in the western North Pacific may be an air-sea interaction process. The relative contributions of cloudiness and surface wind changes to the SST tendency will be discussed.