SEMINAR: Dynamics of a tidally-forced stratified shear flow on the continental slope

The energy contained in large-scale ocean flows is dissipated in small-scale turbulent motions and these control the rate at which heat, momentum, chemicals, nutrients, and biological matter are stirred in the ocean. On the global scale, a large proportion of the mechanical energy contained in the ocean’s tides is converted to internal wave energy that can propagate large distances before ultimately dissipating particularly near bottom boundaries where enhanced mixing occurs due to bottom-induced friction. These complexities make both the study of turbulence in the ocean and the development of mixing models for the global ocean particularly challenging.

The first part of my presentation examines how to rigorously estimate turbulence properties, in particular, the rate of dissipation of turbulent kinetic energy, from moored field observations. I developed a methodology that takes into consideration the sampling program, the instruments’ capabilities, and the flow characteristics. Notably, the method considers both the effects of mean flow shear and density stratification on turbulence spectral properties in a systematic and robust way, making the method applicable for a vast range of environmental flows.

The second part of my presentation examines the results of applying this methodology to near-bottom observations from the continental slope on Australia’s North-West Shelf. Internal bores propagate up slope through the site, generating strong shear and intensified near-bed currents (>6 times background tidal currents) in a highly unsteady environment. These bores are associated with large isotherm displacements and enhanced turbulent dissipation. The observations of the mean and turbulent flows demonstrate that idealized laws, often used in ocean mixing models, cannot describe the vertical extent of turbulent overturns and the induced mixing.

In the final part of my presentation, I use the turbulence properties and mixing rates derived from the above field observations to assess various mixing models. Despite the high mixing rates observed (>200-300 times molecular rates), the mixing efficiency was of the order of 1%. This analysis demonstrates that using a constant mixing efficiency of 20%, which is customary in the oceanic community, over-predicts the mixing rate by more than an order of magnitude. I demonstrate that mixing rate predictions are improved when the reduction in mixing efficiency with increasing turbulence intensity is taken into account, implying that oceanographic numerical models need to adopt a variable mixing efficiency to accurately predict flow dynamics in energetic regions.