Tidally Induced Turbulence in the Abyssal Ocean

Abstract: It has been suggested that the upwelling branch of the abyssal overturning circulation is characterized by strong flows driven by turbulence along sloping topography. The Boundary Layer Turbulence field campaign has provided direct evidence for strong upslope flows along a deep submarine canyon of the Rockall Trough. Turbulent overturning events spanning 200 m in the vertical were observed every tidal cycle, suggesting that the strong tidal flows in the canyon periodically undergo some form of instability. However, it is shown that the flow never satisfied the classical instability condition for time-independent sheared flows in a stratified fluid commonly used in oceanographic studies to determine transition to turbulence. This study illustrates that the time dependence of the tidal flow changes the stability properties and explains the observed transition to a turbulent state. The findings suggest that turbulent mixing induced by oscillatory shear flow may be ubiquitous over sloping topography and play an important role in deep ocean mixing that supports the global overturning circulation.