The Annual Change Of Scs Summer Monsoon Onset Mechanism

The NCEP/NCAR-2 reanalysis data and NOAA OLR data are employed to composite and numerical diagnostic studies on the interannual variations and related physical mechanisms of the onset of South China Sea Summer Monsoon (SCSSM). The study reveals that the onset of the SCSSM has a close connection with the preceding anomalous circulation over mid and high latitude areas in northern hemisphere, the variation of local meridional vertical circulation over eastern Asia and the activities of 30-60d low-frequency oscillation. The main conclusions are as follows:1. Before the onset of SCSSM, as for climatological normals, there exist distinct differences in the variation of atmospheric heat sources and moisture sinks between over Indo-China Peninsula (ICP) and over South China Sea (SCS). The atmospheric heat sources and moisture sinks enhanced rapidly over the ICP, and increased slowly over SCS. The rapid development of the atmospheric heat sources and moisture sinks over ICP is closely related to the northward movement of heat sources and moisture sinks in Sumatran. While the slow increase of heat sources and moisture sinks over SCS is also associated with northward (southward) movement of atmospheric heat sources and moisture sinks in Kalimantan (in southeastern part of China). Further analysis also reveals that the zonal differences of meridional temperature gradient reversion in mid-upper troposphere over ICP and SCS are closely related to the variations of atmospheric heat sources and moisture sinks over the corresponding places.2. During the first half month of May, anomalies of goepotential height and horizontal circulation in troposphere are quite different in early and late monsoon years. In early monsoon years, there exists a anomalous wave train with negative anomalies of geopotential height field (anomalous cyclone) over Ural-mountain and its western region and along the seacoast of eastern Asia, the positive anomaly (anomalous anticyclone) over middle latitude continent and Okhotsk Sea. However, the phases of anomalous wave train is opposite in late monsoon year. What's more, the zonal differences of meridional averaged (over 50-60oN) geopotential height anomalies are very obvious from the beginning of April. This signal may play an important role in prediction of the onset of SCSSM.3. The early onset and late onset of SCSSM are affected by the strength and position alteration of heat source and cumulus convection over ICP and west Pacific, the southward movement of those in southeastern part of South China and the northward propagation and intensity of those in Sumatra and"land bridge"region. In the first half month of May, if the convection activities over ICP and Philippine sea strengthen, which will result in a negative anomaly of geopotential height and a anomalous cyclone at 850hPa over Bay of Bengal, central and northern part of SCS and subtropical western Pacific, then the onset of SCSSM will happen earlier. Whereas reverse. 4. The local meridional circulation anomaly located in eastern Asia is very important for the establishment of SCSSM. The physical mechanism of meridional circulation variation is...