A Numerical Investigation of Tidally Forced Internal Waves in the Ocean

                   

Principal Investigator

Peter Holloway

School of Geography and Oceanography

The Australian Defence Force Academy

Internal waves are a common feature of many oceanic continental margins, e.g. on the Australian North West Shelf, generated through the interaction of tidal flow moving density stratified water over steeply sloping topography. The flows associated with the waves are highly energetic and can be responsible for significant loadings on offshore structures and for scouring sediment from around seabed pipelines and platforms, for mixing nutrients from deep water into the surface layers and can contribute to the exchange of water and material between the slope waters and the shelf waters. The tidal flow generates a long internal wave which can become unstable and form internal bores, internal solitary waves and other strongly nonlinear wave forms. The project aims to investigate the formation of these waves using numerical models to simulate the generation and evolution of these different wave forms on the North West Shelf.

Progress to date has been with the use of a high spatial resolution model to investigate the generation of internal tides and their evolution into short wavelength nonlinear waves such as internal solitary waves.

     
             

Projects

g69 - VPP

       
             
                   

       
                   

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

Significant progress was made with the high spatial resolution model runs. The speed and memory available on the VPP allowed model runs, not possible on my own institutions computing resources, to be carried out. The model generates a long wavelength internal tide through the interaction of tidal flow of density stratified water over a sloping seafloor. The fine resolution showed the formation of internal bores and associated internal solitary waves forming on the leading face of the long internal tide. Similar results from primitive equation modelling had not previously been obtained. Results of this work are to be presented at the International Offshore and Polar Engineering Conference to be held in Hawaii in May, 1997.

The work has only just begun and will continue to investigate short period nonlinear wave formation. The 3-dimensional modelling will commence in the future and aim to develop a regional scale model of internal wave generation on the Australian North West Shelf.

 
                   

- Appendix B

 

   
                   

     

What computational techniques are used?

The modelling work uses a fully 3-dimensional primitive equation model of oceanic flows. The model is nonlinear, with a free surface and has imbeded a turbulence 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, a coupled set of partial differential equations, and solves the equations by stepping through time. 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.

Publications

Holloway, P.E., E. Pelinovsky, T. Talipova, B. Barnes (1997). Nonlinear models of internal tide and internal solitary wave evolution over a continental slope . Proceedings of 7th International Offshore and Polar Engineering Conference (ISOPE-97), Hawaii, 25-30 May 1997.