Standing wave-induced tidal shear in a submarine canyon in the Rockall Trough

Yuchen Ma, Raffaele Ferrari, Kurt Polzin, Matthew H Alford, Alberto C. Naveira Garabato, Gunnar Voet 

Mixing in the ocean abyss sustains the deepest branches of the global overturning circulation, yet the processes that drive deep-ocean mixing remain poorly understood. Recent field measurements in a deep submarine canyon of the Rockall Trough have revealed that intense mixing occurs during strong, vertical shear-generated overturns exceeding 200 meters. These overturning events last only a few hours and occur at different tidal phases across sections of the canyon. We investigate the origin of the shear and its dependence on along-canyon location using a high-resolution numerical simulation and theory. The tidal shear is associated with internal Kelvin waves trapped by the canyon’s geometry, whose structure varies along the slope, resulting in distinct tidal phases of shear at different canyon locations. These waves are confined to the depth of the canyon, resulting in stronger shear than the low-mode internal waves that dominate in the ocean interior. Our results suggest that the strong shear associated with standing waves in canyon may play an important role in driving abyssal ocean mixing.