Role of Air-Sea Interaction in the Northeastward Summer Monsoon Progress over the Western North Pacific

Renguang WUl and Bin WANG

International Pacific Research Centerl and Department of Meteorolog/, School 0 Ocean and Earth Science and Technology,

University of Hawaii at Manoa, Honolulu, HI 96822, USA

Abstract -The summer monsoon shows distinct northeastward advance in the western North Pacific ( WNP) .This feature is shown to relate closely to air.sea interaction. In the South China Sea and WNP, the high sea surface temperature (SST) center develops first in the South China Sea before mid-May, shifts to the western Philippine Sea in mid-June, and moves to east of 140'E in late-July. From the atmospheric per- spective, the seasonal northeastward march of the warm SST center and related seasonal change of the SST gradient favors the northeastward move of the monsoon trough and extension of the high convective in- stability region. On the other hand, the monsoon induced cloud and circulation changes feedback to the seasonal SST change. The flare up of convection reduces the input of the solar radiation due to cloud block. ing. The surface latent heat flux increases following the flare up of convection. 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 short wave solar radiation. The wind speed decrease in relation to the northeastward move of the monsoon trough reduces the latent heat flux. This favors the maintenance of positive SST tendency. As a result, the warm SST center moves northeastward. Thus, the northeastward summer monsoon advance in the WNP is an air.sea interaction process.