Effects Of Intraseasonal Oscillation On South China Sea Summer Monsoon Onset

Understanding of South China Sea(SCS)summer monsoon(SCSSM)onset response to tropical intraseasonal oscillation(ISO)is critical for extended-range prediction of SCSSM onset.In this study,we investigate the effect of ISO on SCSSM onset for the period of 1980-2013.The 34 onset cases are classified into three groups,early onsets around May 6th,normal onsets around May 21st and late onsets around June 8th,and the late onsets are even later than the Indian monsoon onsets.Before each onset,the SCS experiences a dry ISO phase to precondition the convective energy due to the easterly wind anomalies of the wet ISO phase over the tropical Indian Ocean for the group of early onsets,over the southern Bay of Bengal monsoon region for the group of normal onsets and over the southern Indian monsoon region for the group of late onsets.After each onset,the SCSSM is supported by the westerly wind anomalies of the dry ISO phase over these associated regions.Each early SCSSM onset is triggered by the northwestward propagating Rossby wave of the wet ISO in the western Pacific which comes from the Indian Ocean.For each normal(late)onset,the SCSSM is triggered by synoptic-scale low-level westerlies in conjunction with seasonal low-level westerlies when the wet ISO moves to the northern Bay of Bengal region(Indian monsoon region),since this convection to the north of 1O°N cannot excite the easterly wind anomalies of the Kelvin wave responses over the SCS to suppress the convection.The mechanisms explaining the mean state-controlled ISO-SCSSM onset relationship are also discussed.Around May 6th,in the early onset years,positive convective instability anomaly,warm(sea surface temperature anomaly,SSTa),positive moisture and convergence anomalies over the western North Pacific is favorable for the growth of the ISO.Around May 21st,the mean states experience strong negative convective instability anomaly,cold SSTa,negative moisture anomaly,and strong divergence anomaly over the northern Indian Ocean and western North Pacific in late onset years suppressed and hardly affects the SCSSM onset.Around June 8th,strong northward gradient of the background convective instability with maximum convective instability over the land and easterly vertical shear over the Arabian Sea both prefer the northward propagation of the ISO,to trigger the Indian summer monsoon.By analyzing the 10-25-day quasi-bi-weekly oscillation and the 30-60-day oscillation modes,we find in all three types of onsets,the 30-60-day intraseasonal oscillation modes have more important influence on the SCSSM onset.The role of the10-25-day mode is weak.Finally,by compositing the SSTa evolution in the early onset and late onset,it was found that during La Nina decay years,the SCSSM onset occur earlier,while in El Nino decay year,the SCSSM onset occur later.Therefore,according to the SSTa pattern in the previous winter,the type of SCSSM onset can be determined.Combined with the prediction of ISO characteristics which initialed from the Indian Ocean,it can provide a theoretical basis for the extended-range forecast of the SCSSM onset.