Propagation and Dissipation of Internal Tides on the Northwest Shelf of Australia from Microstructure Observations and Numerical Simulations

The coastal shelf in Northwest Australia is a region of strong nonlinear internal wave generation and dissipation that results from strong tidal flows. This study focuses on the analysis of direct dissipation measurements from the first ever microstructure profile observations collected in the region. We are assessing the propagation, energy, evolution and mixing rates of internal tide wave packets crossing the shelf. A total of 309 microstructure profiles were collected between April 5-11, 2012 in a domain spanning both offshore and inshore locations and on both broad and narrow parts of the shelf, where internal tide energy ranges from strong to moderate. On the broad central portion of the shelf, where internal tide energy is stronger, microstructure observations from a 24 hour station show the passage of two internal tide packets that are out of phase with the local barotropic tidal oscillations in the region. The passing of the waves first produce thermocline deepening and high dissipation regions near the surface and subsequently the transport of cold water to the surface as the thermocline is shallowed. In a southern location with weaker internal tide energy, a microstructure section survey reveals that changes in the pycnocline slope are associated with a gap in internal tide packet propagation onshore. Concurrent observations of low-frequency flow in the area show that strengthening and weakening of a southward flowing coastal current is associated with these changes and thus demonstrates the control that mesoscale phenomena exhibits on shelf propagation of internal tides in the region. Model results from the relocatable-Navy Coastal Ocean Model (RELO-NCOM) for the microstructure study period will be analyzed both to provide context to the microstructure observations and to evaluate model turbulence closure solutions.