Onset and the evolution of the summer monsoon over the South China Sea during SCSMEX field experiment in 1998

The South China Sea Monsoon Experiment (SCSMEX), which was carried out from 1 May to 31 August 1998, is a multi-endeavor that is closely linked. to and coordinated with activities of national weather services and oceanographic bodies of different nations and regions. The scientific goal of the SCSMEX is to provide a better understanding of the keg physical processes for the onset, maintenance and variability of the summer monsoon over Southeast Asia and the South China Sea (SCS), leading to improved predictions of monsoons. Based on the datasets obtained from the SCSMEX Intensive Observation Period (IOP), the characteristics of upper and low-level circulation, outgoing long-wave radiation (OLR) and precipitation patterns have been analyzed. The obtained results have shown that (1) the onset of the SCS summer monsoon consists of two stages: the first onset on the northern SCS and the full onset ever the whole SCS. The summer monsoon onset ever the northern part of the SCS occurred in the fourth pentad of May while for the whole area of the SCS the onset occur-red in the fifth pentad of May. The date of the onset of the SCS summer monsoon for 1998 was overall about 1-2 pentads later than normal condition (May 15); (2) the monsoon onset over the northern part of the SCS was not a localized phenomenon. It broke out simultaneously with the monsoon over the Bay of Bengal and Indo-China Peninsula. From a viewpoint of synoptic process, its onset is related to the early rapid development of a twin cyclone to the east of Sri Lanka and subsequently a monsoon depression over the Bay of Bengal, two stepwise significant retreats of the subtropical high from the SCS and possible triggering effect of a strong cold wave from mid-latitude; (3) activities of the SCS monsoon were mainly affected by activities of the Indian monsoon, and its enhancement and northward advances were greatly influenced by acceleration and eastward extending of the near equatorial westerly wind from the Indian Ocean.