Dynamics of a tidally-forced stratified shear flow on the continental slope
My PhD focused on turbulent mixing in near-boundary flows on the continental slope where internal wave forcing is important. My research was motivated by the need to improve the description and prediction of mixing processes in hydrodynamic models both at the regional and global scale and the engineering need to design safe and reliable offshore structures and stable pipeline systems.
Accurately predicting mixing affects our ability to predict global ocean circulation, and consequently climate change, given the coupling between the atmosphere and the ocean. In the stratified-ocean, internal waves emanate from topographical features from the interaction of surface tides with local topography. When internal waves shoal and break, they modify the physical structure of the bottom boundary layer, generating localized intense shear and gust speeds; these features also alter turbulence properties with implications for turbulent exchanges (e.g., salt, nutrients, and heat) at the sediment-water interface. My field research emphasizes that generated nonlinear internal waves (e.g., bores and boluses) enhances turbulent dissipation and mixing locally in time and space in the ocean. Furthermore, the complex flow dynamics hinders the use of idealized laws for describing the size of the turbulent overturns: the basis of the two-point turbulence closure schemes that are embedded in ocean circulation models to estimate mixing rates (eddy diffusivities). Additionally, I demonstrated that mixing rates at energetic oceanic sites can be over-predicted by more than an order of magnitude when using Osborn’s model with an efficiency of 20%, which is customary by the oceanic community. The broader implication of these findings is that oceanographic numerical models need to adopt a variable mixing efficiency to predict accurately large-scale flow dynamics.
Turbulent mixing homogenizes heat, nutrients, and mass and is particularly significant near the ocean seafloor. The amount of energy dissipated from turbulent mixing also determines the strength of large-scale current circulation. These small-scale processes are parameterized in ocean circulation models given that we cannot model exactly all scales of motion (km to mm) with current computing power. The purpose of my thesis is to test and develop better parameterization of turbulent mixing using field measurements collected near the seabed, throughout the Australian North West Shelf.