Convective instability in the bottom of Northern South China Sea

The unstable convection of the ocean floor has a significant influence on the energy transport and turbulent mixing at the ocean bottom. In order to estimate the contribution of the unstable convection to the energy transport, an oceanographic survey was undertaken at a site near the northeast of the Dongsha Atoll in the South China Sea (20.8953°N, 117.1808°E, ocean depth of 385 m). The characteristic of temperature structure and the variations of convection layer’s thickness in the ocean bottom boundary layer were studied, which is based on the 37-h observed hydrologic data. The contribution of unstable convection to the transportation of energy and the turbulent mixing was investigated. Research shows that, for the whole observational period, the time fraction unstable layer was about 56%, and its mean thickness was 6.17 m. During the down-slope phase, the percentage of unstable layer’s presence was about 50% with mean thickness smaller than 10 m, while during the up-slope phase, its presence percentage was about 70%, and its thickness reached as large as 40 m. The vertical heat flux introduced by unstable convection E_B and the heat transfer efficiency Nu appeared occasionally, with mean E_B as 0.41 W/m² and mean Nu as 249.91. The heat flux of convection was much higher than the bottom velocity shear flux. The results indicated that the contribution of unstable convection to the turbulent mixing decrease gradually with enhancement of the bottom turbulent mixing.