Diurnal Coastal Trapped Waves and Associated Deep-Ocean Mixing Near the Head of Suruga Trough

Coastal submarine canyons are expected to be sites of enhanced turbulent mixing which create productive fishing areas in coastal waters. Recent studies suggested the potential role of diurnal coastal trapped waves (CTWs) in inducing significant turbulent mixing in coastal submarine canyons poleward of 30 degrees where the diurnal tide becomes subinertial. Nevertheless, the detailed physical processes responsible for the generation and dissipation of CTWs in such submarine canyons have hardly been investigated so far. In the present study, to investigate the turbulent mixing processes associated with diurnal CTWs in submarine canyons, we carry out high-resolution (Delta x, Delta y = 1/240 degrees) non-hydrostatic three-dimensional numerical experiments focusing on Suruga Bay located at the north end of Suruga Trough, Japan. The numerical experiments show that diurnal (subinertial) tidal currents over the Izu Ridge generate CTWs which propagate cyclonically along the coast of Suruga Bay. In the middle of Suruga Bay, bottom-intensified flow due to the diurnal CTWs interacts with rough seafloor topography to excite upward propagating internal lee waves, which create mixing hotspots extending high above the seafloor. The resulting diurnal (subinertial) tidal energy dissipation in Suruga Bay is found to be much larger than that associated with semidiurnal (superinertial) internal tides. Furthermore, it is found that baroclinic energy flux based on a simple barotropic-baroclinic decomposition severely underestimates the wave energy flux in areas with high CTW activity. These results strongly indicate that the existence of CTWs might be a key factor to clarify the dissipation/mixing processes in submarine canyons.

Automated summary

Coastal submarine canyons are potential areas for fishing due to enhanced turbulent mixing, and recent studies have suggested that diurnal coastal trapped waves (CTWs) play a role in inducing turbulent mixing in these canyons. However, the detailed processes of CTWs generation and dissipation in submarine canyons have not been well investigated. In this study, numerical experiments in Suruga Bay, Japan, show that diurnal tidal currents generate cyclonic CTWs along the coast, and interactions between these CTWs and the seafloor topography lead to intensified flow and mixing in the middle of the bay.