Internal solitary waves enhancing turbulent mixing in the bottom boundary layer of continental slope

Abstract

The importance of internal solitary waves (ISWs) to the energy and material transport in the upper ocean has been confirmed, but how ISWs affect turbulent mixing in the bottom boundary layer is lack of direct field observation. To reveal the characteristics of ISWs and the turbulent mixing they cause in the bottom boundary layer, a 36-h fine-scale observation of ISWs and their influence on turbulent mixing was conducted at the South China Sea slope (water depth: 659 m). During the observation, a group of ISWs passed by with horizontal and vertical velocities of 0.2 and 0.04 m/s at 0.1 m above bottom (mab) respectively, resulting in strong velocity shear exceeding 0.5 m/s per meter. When the ISWs passed through the slope, although they did not deform and break near the seabed, they increased the bottom shear stress, turbulent kinetic energy production rate and turbulent kinetic energy dissipation rate (ε) at 0.6 mab several times. The ISWs induced ε reached O (10⁻⁶) W/kg, which was one order of magnitude higher than the previously observed ε in the upper ocean of the South China Sea slope. At the same observation site, the internal tides induced ε in the bottom boundary layer was O (10⁻⁵) W/kg, which indicated that the internal tides played a more important role than the ISWs in enhancing bottom turbulent mixing.