Modelling Coastal Ocean Processes
Jason H Middleton
School of Mathemetics
University of New South Wales
Ann-Marie M Wong
Peter R Oke
Renato D Ghisolfi
School of Mathematics
Univerity of New South Wales
h02 - VPP
|This project involves the study of various coastal
regions including the coast off Wilkes Land,
Antarctica, the east coast of NSW, Australia, and the coast off southern Brazil, from extensive research in these regions and CTD (conductivity, temperature and density) data measured in these regions. We have used the Princeton Ocean Model (POM) to model these regions in order to study them.
What are the results to date and the future of the work?
Antarctic Bottom Water (AABW) is a dense water mass found at the bottom of the world's oceans and plays an important role in the heat budget of the ocean-atmosphere system. It is believed that a significant proportion of this dense water is formed off the coast of Wilkes Land, Antarctica. A model of the region with a salinity flux at the surface results in the formation of dense water on the shelf, some of this dense water spills over the shelf break and down the slope. The inclusion of a wind or coastal trapped wave in the model increases the amount of dense water transported off shore. Future modelling studies in this region hope to look at other factors such as topography and other physical processes which lead to AABW formation.
The effects of the East Australia Current (EAC) on the nearshore zone have been investiaged. It is shown that thermal fronts dominate the nearshore dynamics. To study the fronts observed off the east coast of Australia, a numerical study was undertaken and it was shown that the Lagrangian acceleration of the flow over the narrowing continental shelf caused a frictional interaction between the EAC and the shelf topography, driving an upwelling bottom boundary layer flow. This mechanism provides an explanation for nutrient enrichment and sediment entrainment into the shelf waters of the EAC region.
- Appendix B
The 2-dimensional POM (Princton Ocean Model) model was used to investigate the effects of the wind stress on a top to bottom shelfbreak density front off southern Brazil. Upwelling and downwelling favorable winds forced the structure. Upwelling favorable winds stretched the front with its top being advected offshore while entrainment of deep water occured below with the major circulation being restricted to the bottom boundary layer. The presence of a front and the bottom boundary layer due to the wind may be responsible for the upwelling detected offshore. Downwelling favorable winds produced an inverse circulation and a consequent steepness of the front as a whole. Despite the insights obtained using the 2-d POM version some effects are still yet to be investigated such as the bouyancy influx and the alongshore bottom topography variation. Such factors can only be fully investigated in a 3-d version and so this is the next step in this study.
What computational techniques are used?
POM divides the ocean domain into a finite grid, the grids differ in size and resolution depending on the study. The model solves equations for variables such as temperature, salinity, velocity, turbulence etc. in each grid point in finite time steps, dependent on the grid. The model contains large two and three dimensional arrays which need to be solved repetitively. To run a model for one year of grid size 86 x 124 x 16, with a time step of 800 seconds, requires 7.2 CPU hours and 130 Mb of memory. The code is highly vectorised (98%).
P. R. Oke and J. H. Middleton, 1998: Topographically Induced Upwelling off Eastern Australia. Journal of Physical Oceanography, (submitted).
P. R. Oke and J. H. Middleton, 1998: Nutrient Enrichment off Port Stephens: the Role of the East Australian Current.Continental Shelf Research, (submitted).
Appendix B -