南海北部底层对流不稳定性研究

Convective instability in the bottom of Northern South China Sea

底层对流不稳定对海洋底层能量输运和湍流混合有重要影响。为评估底层对流不稳定对能量输运的贡献,基于 2018 年 9 月 20 日南海东北部东沙群岛海域(20.895 3 °N,117.180 8 °E,水深 385 m)37 h 锚定潜标高时空分辨率观测数据,对观测站位海洋底层水体的温度结构特征进行了分析,研究了对流底层厚度变化,及其对海洋底层能量输运和湍流混合的贡献。结果表明,观测站位对流底层出现的概率约为总观测时间的 56%,其厚度平均值为 6.17 m。底层平流为下坡流时,对流底层出现的概率约为 50%,其厚度相对较小,均小于 10 m;而为上坡流时,对流底层出现的概率约为70%,其厚度显著增大,可达到 40 m。对流不稳定驱动的垂向热通量 E_(B)和传热效率 Nu 具有间歇性特征,观测时间内E_(B)平均值为 0.41 W/m^(2),Nu 平均值为 249.91,且对流热通量远高于底层流速剪切通量。对流底层湍流热扩散率和涡扩散率的评估结果表明,随着底层湍流混合强度的增强,对流不稳定对底层湍流混合的贡献率逐渐减小。

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 (20.895 3°N, 117.180 8°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/m^(2) 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.