Dynamic features and formation mechanism of the uppper mixed layer in the South China Sea during 1998 summer Monsoon onset

Based on data observed during South China Sea Monsoon Experiment (SCSMEX), the depth of upper mixed layer, the upper boundary of thermocline and the thickness of the barrier layer were discussed. The thermal and dynamical features of the mixed layer and its relation with the onset of the summer monsoon of the South China Sea (SCS) were also analyzed. It was found that the variations of temperature and depth of the mixed layer could be divided into three sub-periods on both the north observation point (200N, 1160E) and the south observation point (6015’N, 1100E ) during the onset period of 1998 summer monsoon: Before the onset of the southwest monsoon, comparison of the two observation points in the North and the South SCS shows the north point has weaker wind, stronger sun radiation, higher mixed layer temperature (280C or more) and lower mixed layer depth (about 10m). During the onset of the southwest monsoon, pattern changed. The south observation point’s mixed layer has temperature increased up to 310C, and depth reduced to 10m or less. After the onset of the southwest monsoon, pattern changed again. The depth of the north observation point’s mixed layer decreases, and temperature of it reverted to higher value (280C or more).

The one dimensional turbulence kinetic (TKE) model was used to simulate and to discuss the SST and the upper mixed layer formation mechanism in the south of the SCS. The results show that the TKE model can simulate the main features of the time variation of the SST and mixed layer depth except in the middle period of May.

In the north of the SCS, daily oscillation of the SST is mainly determined by daily variation of the sun short-wave radiation and restrained by the mixed effect of the wind. Compared with sun short-wave radiation and the wind stress, the latent heat and sensible heat flux are minor factors controlling the variation of the SST in spring. In May, both the effect of the wind stress and the short-wave radiation can make the mixed layer grow on 5~10m. But in June, the short-wave radiation can only make the mixed layer shallow 1~2m. The latent heat and sensible heat flux make the mixed layer depth become more intensify about 1~2m, the depth of mixed layer is controlled mainly by wind stress.

In the south of the SCS, diurnal oscillation of the SST is mainly determined by diurnal variation of the solar short-wave radiation and restrained by the mixing effect of the wind. Compared with solar short-wave radiation and wind stress, the latent heat and sensible heat fluxes are minor factors controlling the variation of the SST and the mixed layer during summer monsoon onset in the SCS. In spring, the effect of short-wave radiation could make the SST increase 40C or less. The effect of the wind stress could make the mixed layer deepen 20~30m, the short-wave radiation could only make the mixed layer shallow 2~3m. The effect of heat flux on the mixed layer depth is smaller than that of wind stress in spring.