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

Abstract Turbulent kinetic energy and thermal variance dissipation rates ϵ and χ , buoyancy flux J b , diffusivity κ ρ , and mixing coefficient , which is simply related to the mixing efficiency , 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 10 −4 –10 −2 m 2 s −1 . Observed is ∼0.2 ± 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. Significance Statement We report detailed direct turbulence measurements very close to the seafloor in a submarine canyon that are well resolved enough in time to confidently understand the processes responsible for the turbulence. We compute the fraction of turbulent power that goes toward mixing, which is an important quantity to understand the role of turbulence in supporting the ocean’s large-scale circulation.