A study on the onset and its active and break periods of summer monsoon over the south china sea

Based on ECMWF reanalysis data of 1979-1993 and TBB data of GMS, an analysis on the onset and its activities of the summer monsoon over the South China Sea (SCS) is made.It is found that the mean onset date is around 16 May, the latest onset is at beginning of June.There is a marked interannual variation of the onset date.There occurs marked southwesterly wind over the SCS in the last dekad of April in some years, but it only persists for a short period, and then, the easterly flow appears again.In the beginning of June, the true onset occurs.From the end of April to the middle of May prior to the onset, strong southwesterly wind appears to the north of 15N over the SCS, which could not be treated as the onset of the SCS summer monsoon.These strong westerlies are due to the southward propagation of cold front or the strengthening of the subtropical high in the SCS.After the onset, there are active and break periods of the southwesterly summer monsoon, the average break period is about 15 days in one year (from onset of summer monsoon to 20 August).