| The thesis consists of two parts. The first part deals with influences of the South China Sea and its adjacent oceans on onset of South China Sea summer monsoon (SCSSM). In the second part, an ocean model POM with higher resolution is used to simulate the ocean circulation in the South China Sea.In the first part, atmospheric and oceanic datasets are used to analyse the large-scale circulation background during SCSSM set up and the evolutionary characteristics of atmospheric circulation before/after summer monsoon onset in the South China Sea. The relationship between the interannual variability of SCSSM onset and SST anomaly (SSTA) in the tropical oceans is also discussed. Observational data in situ taken in SCSMEX98 are used to analyse the variation of SST and surface heat flux at (6(15'N,110(E) during the onset period of SCSSM in 1998. Numerical study is also performed with a regional climate model (NCAR/ReGcm2) to study the impact of the South China Sea on SCSSM onset. The results obtained are as follows: 1. Before SCSSM onset, latent heat flux released by convection and precipitation over the Bay of Bangle and Indo-China Peninsula made the South Asian High at 200hPa shift to the northwest of Indo-China peninsula, which may trigger onset of the SCSSM. 2. A close relationship exists between SCSSM onset and the atmospheric circulation anomalies in spring, which basically control the early/late onset of the SCSSM. The impacts of the anomalies of equatorial westerly over the Indian Ocean and cross-equator flow on the SCSSM are significant.3. SSTA in the adjacent tropical oceans can affect SCSSM onset, by the way of affecting the atmospheric circulation over the monsoon region. One possible process is as follows: positive (negative) SST anomaly in the key regions gives rise to positive (negative) convection anomalies, inducing anomalies of the two divergent cells (longitudinal/latitudinal) of the Asian monsoon, with the result of earlier (later) onset of the SCSSM. The key regions close correlating to onset of SCSSM are always in the ascending or descending areas of the two components of the monsoon circulation. The mid- and eastern Pacific can affect onset of SCSSM through changing the Walker circulation.4. Composite analysis shows that the pattern with positive SSTA in the north of SCS and negative in the south corresponds to earlier onset of SCSSM, while the pattern with positive SSTA in the whole SCS basin is associated with later onset of SCSSM. But there is no significant correlation between the previous (April) SSTAand the onset time of the SCSSM from correlation analysis, only when 4 pentads before onset of SCSSM appears prominent positive correlation over the south of the SCS. This positive correlation may be a response signal and does not mean a leading role for SCSSM onset. 5. Before onset of the SCSSM in 1998, SST at the southern station (6°15'N,110°E) increased abruptly, and decreased continuously after that. There are two main reasons for the increasing of SST before SCSSM onset. One is due to decrease of latent heat flux transition induced by weaker winds. The other reason is owing to weakening of oceanic mixing process as a result of wind speed decrease. The change of wind speed plays an important role in SST increasing. The decreasing of SST after SCSSM onset is mainly because the increased cloud leads to the depression of the short wave radiation largely. Before (after) monsoon onset, sea surface gains (loses) heat by transition of latent heat flux. During the observation period, the net heat flux through sea surface plays a leading role in variation of SST, but other components play different roles in SST change. In details, the latent heat made significant contribution to SST change before SCSSM onset, and the short wave radiation played a secondary role. After monsoon onset, the short wave radiation contributed more to the variation of SST than latent heat. In the process of ocean dynamics, advection term is too small and negligible. Entrainment process also plays...
|
PDF Full Text Request