A Numerical Investigation of Tidally Forced Internal Waves in the Ocean

                   

Principal Investigator

Peter Holloway

School of Geography and Oceanography

ADFA

University of New South Wales

Projects

g69 - VPP

Internal waves in the ocean are generated through the
action of the tides oscillating density stratified water
over sloping topography. This can occur over the continental margins as well as over steep topography in the deep ocean. Resulting internal waves are energetic producing strong currents and large vertical excursions of density interfaces. The motion contributes to mixing of properties in the ocean, can move sediment on the sea-floor and can be significant for the stability of offshore structures. The project aims to use numerical models to investigate the generation of internal waves by tidal flow over topography and the resulting propagation and evolution of the waves. An emphasis is placed on the Australian North West Shelf and the Hawaiian Islands and Ridge.
   
             
                   

     
                   

What are the results to date and the future of the work?

Earlier work on the implementation of a fully three-dimensional density stratified model of the semi-diurnal tide and internal tide has been extended in domain to cover the Australian North West Shelf. This regional model covers a domain 1700 by 700 km, from North West Cape to Joseph Bonaparte Gulf, using a horizontal grid size of 4 km and 51 vertical levels. Initial results show strong internal wave generation over most of the continental slope region between water depths of 100 and 500 m. Energy flux calculations show high spatial variability in the signal in response to variability in topography. The work will continue aiming to better model the observed barotropic tide, and to investigate the nature of the internal tide to forcing from other semi-diurnal and diurnal constituents and to investigate the model dependence on varying density stratification.

Work is also underway at modelling the generation of internal tides over the Hawaiian Ridge. Weak tidal currents in the deep ocean are observed to generate an energetic internal tide when flowing over the very steep topography such as that of the Hawaiian Ridge. Initial work at modelling internal tide generation by tidal flow over idealised topographic representations of seamounts ridges and islands has been completed and the work submitted for publication. Work has begun in modelling a section of the Hawaiian Islands. Strong internal tide generation is observed in various locations and shows the potential importance of this region in extracting energy from the tide and contributing to ocean mixing. Work will continue, modelling various sections of the Hawaiian Ridge, investigating the sensitivity of results to different tidal constituents and to stratification and also to the parameterisation of mixing processes.

 
                   
Appendix B -

                   

     

What computational techniques are used?

The work uses a fully 3-dimensional primitive equation model of oceanic flows. The model is nonlinear, hydrostatic, free surface and includes a turbulence sub-model for calculating sub-grid scale vertical mixing of momentum, heat and salt. The code is written in FORTRAN and uses a finite difference representation of the equations of motion, a coupled set of partial differential equations, and solves the equations by stepping through time and space. Given 3 spatial dimensions and the time dependence, the solution involves a large number of nested DO loops. Also, large arrays are used requiring significant memory. Substantial output is written to disk.

     
- Appendix B