Using the National Center for Enviromental Prediction/National Center for Atmospheric Research (NCEP/NCAR)reanalysis data and NOAA satellite-observed outgoing long-wave radiation (OLR) data, the development of the South China Sea (SCS) summer monsoon and intraseasonal (30～60 d) oscillation (ISO) have been examined. The results show that there exists obvious interannual variability of intraseasonal oscillaiton. Using the 16 a time series of filtered OLR averaged over the SCS, an index is defined to define 'onset events' over the SCS on the ISO time scales.Of the 16 a examined here, 10 shows a strong ISO signal in the onset of monsoon convection over the SCS. In these cases, the ISO initially suppresses the seasonal development of southwesterly and cyclonic circulation over the SCS before the ISO onset. As the ISO propagates northeastward, the low frequency cyclonic circulation anomaly occurs in the SCS and the low frequency southwesterly wind and convection over here dramatically intensify. The northeast progression of the ISO anomaleis plays a role in the initial suppression and then acceleration of the seasonal cycle of the SCS summer monsoon.
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