Buoyancy Flux and Mixing Efficiency from Direct, Near-Bottom Turbulence Measurements in a Submarine Canyon

Turbulent kinetic energy and thermal variance dissipation rates e and x, buoyancy flux Jb, diffusivity kr, and mixing coefficient P ; Jb e21, which is simply related to the mixing efficiency Rf 5 (1 1 P21)21, are estimated from highly resolved microstructure measurements collected in a submarine canyon that has been previously shown to be experiencing near-bottom diapycnal upwelling. It is demonstrated that turbulence arises primarily from the convective instability of the internal tide. Twelve tidally resolving stations (12-48 h long) were conducted, wherein profiles were collected from between 5-15 m and 400 m above the bottom every 13-15 min using a custom turbulence vehicle. Turbulent buoyancy flux is estimated using the Osborn and Winters and D'Asaro methods, allowing direct estimation of the mixing coefficient as a function of time, temperature, and height above bottom. Turbulent dissipation and buoyancy flux generally increase toward the seafloor. The associated turbulent diapycnal diffusivity is 1024-1022 m2 s21. Observed P is ;0.2 6 0.05 near the top of our measurement range, as expected in the ocean interior, and increases to 0.3-0.7 approaching the bottom, consistent with turbulence generated by convective instability.