ALBERT

Description: Coastal submarine canyons are potential sites of enhanced turbulent mixing that could make them productive fishing grounds. Recent studies suggest the crucial role of diurnal coastal trapped waves (CTWs) in inducing significant turbulent mixing in coastal submarine canyons poleward of 30º, where the diurnal tide is subinertial. However, the detailed physical processes responsible for the generation and dissipation of CTWs in such submarine canyons remain to be investigated. In this study, to investigate the turbulent mixing processes associated with diurnal CTWs in submarine canyons, we conduct high-resolution three-dimensional numerical experiments focusing on the northern end of the Suruga Trough, Japan. It is found that CTWs generated by diurnal (subinertial) tidal currents over the nearby Izu-Ogasawara Ridge propagate anticlockwise with the slope of the Suruga Trough on the right. Furthermore, the bottom-intensified flow associated with the diurnal CTWs interacts with the rough seafloor topography to excite internal lee waves that propagate upward while creating mixing hotspots that extend high above the seafloor. We also find that the baroclinic energy flux based on a simple barotropic-baroclinic decomposition severely underestimates the wave energy flux in areas of high CTW activity. These results indicate that the presence of CTWs is a key factor in elucidating the dissipation/mixing processes in submarine canyons. In the presentation, we will also compare the results of the above numerical experiments with those of direct microstructure measurements conducted in the Suruga Trough in November 2021.

Description: A series of numerical experiments are performed to revisit the physical mechanism of tide-induced "near-field mixing", which has been thought to be caused by the breaking of high- wavenumber internal tidal waves. We find that, for strong tidal currents with an excursion parameter Te (= kHU0 /ω with kH the wavenumber of the seafloor topography, U0 the amplitude of the tidal current, and ω the frequency of the tidal current) much larger than unity, it is the high-wavenumber internal lee waves excited over the seafloor topography that propagate upward while interacting with the background Garrett-Munk (GM) internal waves to induce near-field mixing. The vertical profile of the resulting mixing hotspot is classified by the steepness parameter Sp (= Nh/U0 with N the buoyancy frequency and h the amplitude of the seafloor topography). When Sp 〉 0.3, the near-inertial flow formed over the seafloor topography is enhanced by absorbing part of the energy of the internal lee waves propagating from below, while breaking the internal lee waves and inhibiting their continuous upward propagation, resulting in the formation of a "short mixing hotspot". In contrast, when Sp 〈 0.3, the near-inertial flow formed over the seafloor topography disappears, allowing the internal lee waves to continue to propagate upward while interacting with the background GM internal waves to form a "tall mixing hotspot". This tall mixing hotspot, extending from the seafloor to the main thermocline, may serve to compensate for the lack of turbulent mixing intensity to sustain the global overturning circulation.