Abstract: Tides, turbulence and internal waves



Authors: Greg Ivey

Internal waves play an important role in conveying energy from typically localized generation regions throughout the ocean. In turn, these internal waves drive turbulent mixing in regions often far removed from the original generation locations. Much research has been focused on the energy loss from the internal wave field when it impacts on topographic boundaries and creates turbulent motions, and on quantifying the resulting vertical turbulent mixing rates in the ocean. Relatively little is known about the role of turbulence in the generation of internal waves, however, and this is the focus of the present work.

The work is particularly motivated by oceanic observations on the Australian North West Shelf (NWS), a topographically complex region with strong density stratification all year round, very strong tidal forcing, and a very energetic tidally-driven internal wave field. When barotropic tides with frequency omega force a fluid with a density stratification characterized by a buoyancy frequency N then, when the internal wave slope (determined by the ratio of omega and N) matches the local bottom slope, internal wave beams can be generated.  These then propagate away from the bottom region into the interior of the ocean. Recent laboratory experiments have demonstrated that, while this is a necessary condition to generate internal wave beams, the generation process is also very sensitive to the strength of the tidal forcing and the near bottom turbulence generated by the barotropic flow itself.  In the extreme limit when the forcing is strong and the near-bottom turbulence is very energetic, all energy is dissipated locally and no internal waves form at all.

This process has also been examined with the aid of recent numerical modeling and a dedicated bottom boundary layer experiment conducted in the ocean at an internal wave generation location in 420 m of water on the NWS. A high frequency sampling array measured the complex turbulent bottom boundary layer dynamics over the bottom 30 m over a spring-neap cycle in October and November of 2008. The talk will integrate the field, laboratory and numerical modeling work in order to describe the mechanisms that are operating and the intimate links between the tides, the turbulence and the internal waves.