Spatial variation of bottom mixed layer in the South China Sea and a potential mechanism

Based on 201 full depth profiles of temperature-salinity and velocity, we investigated the thickness, stratification and spatial variation of the bottom mixed layer in the South China Sea. The mean values of thickness and stratification N2 of the bottom mixed layer are about 154 m and 6.2 × 10-7s−2, respectively, which is five times thicker and 100 times less stratified than the upper mixed layer. The Luzon Strait and Zhongsha Island Chain are found to be two hotspots of thick bottom mixed layer, where the mean thickness (270 m) is 2.4 times that of the other areas (114 m). Compared with the Luzon Strait where internal tides cause energetic bottom mixing and hence form a thick bottom mixed layer, the scenario in the Zhongsha Island Chain remains unclear. A 2D MITgcm numerical simulation is employed to explore the elevated mixing and associated evolution of the bottom mixed layer in the Zhongsha Island Chain, where internal tide is weak. The results indicate that the bottom current impinging on the unique topography would generate energetic lee waves. The breaking of lee waves causes elevated dissipation reaching 3.2 × 10-7Wkg−1, which forms a thick bottom mixed layer with thickness and N2 of 320 m and 2.3 × 10-7s−2, respectively, consistent with the observed values. This study implies that internal lee waves could serve as an alternative source to the energetic mixing when internal tide is weak.