Ocean mixing in a shelf sea driven by energetic internal waves

We collected observations of ocean mixing from three moorings placed at the 330m, 200m, and 150m isobaths on a pelagic ridge on the Australian North West Shelf (NWS). The region is subject to energetic surface and internal tides, non-linear internal waves, flow-topography interactions, and episodic intense wind events (i.e., tropical cyclones) that collectively drive energetic diapycnal mixing. We identified five dominant internal wave categories: both low (time scales from double the buoyancy period to 4 hours) and high-frequency (time scales between buoyancy period and double the buoyancy period) mode-1 waves, mode-2 waves, internal bores, and internal hydraulic jumps. A small number of turbulent mixing events dominated the total vertical heat flux at each mooring, with 15% of estimates accounting for as much as 90% of the total observed heat flux. These turbulent mixing events often occurred during the passage of internal wave events, with the internal wave events accounting for as much as 60% of the total heat flux in some locations. High-frequency mode-1 waves were the most significant contributors to the total vertical heat flux (∼ 20%). Internal bores made significant but localized contributions to mixing, accounting for up to ∼ 50% of the total vertical heat flux in some regions but with a negligible influence elsewhere. The contributions of the different internal wave categories to the total flux became more heterogeneous at shallower sites, indicating an increasingly complicated relationship between the forcing internal wave field and the mixing.