Dynamic Behaviors of Convective Instability in the Bottom Ocean: Observation in the Continental Slope of the Northern South China Sea

Convective flow plays a crucial role in estuaries, lakes, and oceans. In the bottom boundary layer (BBL) of estuaries and lakes, convective instability is a likely cause of turbulent mixing. However, due to limited observations, the quantitative implications of this process remain less understood in the deep ocean. This study utilizes 37-hour high-resolution temperature measurements collected at the ocean bottom, up to a depth of 70 meters on the continental shelf of the northern South China Sea (21°N, 117°E, ocean depth of 385 m). We examined the temporal variability in bottom water structures and characterized the convective flow in bottom turbulence. Key findings include: 1. Contribution to Mixing: Convection significantly enhances bottom mixing under mild mixing conditions, while its influence diminishes during intense mixing events. 2. Temperature Spectrum: Water in the bottom mixed layer (BML) alternately experiences weak stratification and convective states. The Bolgiano-Obukhov (BO)-like scaling (exponent a=−1.34± 0.10) is observed in the temperature spectrum in a weakly stable stratified and strongly sheared condition. In contrast, in the unstably stratified convective turbulence of the BML, the scaling a = −1.76± 0.10 clearly deviated from the BO theory but approached the classical Kolmogorov −5/3 scaling in isotropic turbulence. The work deepens our understanding of the role of convective instability in the ocean and the scaling laws of ocean turbulence.