Internal tide-induced turbulent mixing and suspended sediment transport at the bottom boundary layer of the South China Sea slope

Abstract

Internal tides produce significant vertical and horizontal flows in the ocean and cause strong turbulent mixing that is essential for the energy cascade of multiscale dynamic processes. However, the effects of internal tides on turbulent mixing and sediment transport in the ocean bottom boundary layer remain unclear due to limited observations. In this study, a seabed-based monitoring platform was placed on the continental slope of the South China Sea (118° 09.80′E, 22° 03.78′N, water depth: 1145 m) to observe the internal tides and their influence on turbulent mixing and sediment transport at the bottom boundary layer for the first time. A primarily east-west direction of movement was observed with a maximum velocity of 0.41 m/s, which is much higher than regular barotropic tidal velocity (approximately 0.08 m/s) at the observation site. Influenced by the internal tides, the shear stress and the turbulent kinetic energy dissipation rate of the bottom boundary layer increased from 0.01 to 0.41 Pa and from 7.6 ×10⁻⁶ to 5.6 ×10⁻⁴ W/kg, respectively. The enhanced shear stress and turbulent mixing led to the resuspension of seabed sediment, and the bottom suspended sediment concentration increased from 3.6 to 12.7 mg/L. In the process of upward propagation, the internal tides also carried the bottom sediment to a height of more than 300 m from the seabed. The sediment transport flux caused by internal tides reached 2.4 g/s/m².