Ocean Circulation of the Great Australian Bight

Principal Investigator

John Middleton

School of Mathematics

University of New South Wales

A number of limited observational studies have pointed to the existence of a wintertime poleward
current, that is trapped to the continental slope, and which may flow 2,500 km from Cape Leeuwin to the west coast of Tasmania. The zonal winds in the Bight may play a major role in the generation of this current, and during the summer when the winds reverse, evidence suggests that the poleward current collapses and cold, nutrient rich water is upwelled near the coast leading to enhanced biological activity.
In this proposal, a sequence of numerical studies will be made to determine the nature of the slope circulation during both winter and summer. The relative importance of winds, density gradients, shelf bathymetry and the role of the bottom boundary and surface mixed layers will be considered in determining the nature of the slope circulation. Where possible, the results will be compared with data from the region.

While little data has been collected for the region, the numerical studies outlined should help to determine the general circulation for the slope region and pinpoint areas for future research.

Co-Investigators

Guennadi Platov

Mauro Cirano

School of Mathematics

University of New South Wales

Projects

h00 - VPP

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

While this project was begun in January 1997, the VPP300 has only been in use since December. However, two papers have been prepared dealing with the basic dynamics of wind-forced downwelling and upwelling along a uniform continental shelf. In the downwelling case, the thermal wind shear within the (80m) thick bottom boundary layer (BBL) was shown to be important for the existence of a strong equatorward Undercurrent. In contrast to results obtained without bottom drag, bottom friction and transport within the BBL were shown to lead to a 23 fold increase in the cross-shelf transport, downwelling and thermal wind shear.

In the upwelling study, where the sign of the wind is simply reversed, the dynamics are quite different. In this case, the BBL plays only a minor role in the structure of the slope currents, which are driven by nonlinear advection and density gradients within the surface mixed layer.

- Appendix B

The VPP300 was used in this study and also in a preliminary study of slope circulation using realistic bathymetry for the Great Bight region.

In the latter case, the model domain extends from Cape Leeuwin to Sydney and stable solutions for the wind-forced circulation have been obtained that are reasonably insensitive to the open boundary conditions.

This model is to be used to examine the slope circulation between South Australia and Tasmania for both summer and winter conditions during 1998. Work is proceeding on the thermohaline structure to be used as well as a parameterisation for the effects of tides on friction. Globally assimilated tidal data is being used to force a barotropic model and the results here will be extended to examine the details of the barotropic and internal tide for the region.

The bathymetry of the region is characterised by deep glaciated canyons and an idealised study of the resultant effects on both upwelling and downwelling flows is invisaged.

What computational techniques are used?

The numerical model used is the Princeton Ocean Model (POM) which solves the primitive equations for oceanic flow as an initial value problem. It is second order in space, leapfrog in time and implicit in the vertical. It was originally vectorised for a CYBER 205, is used by more than 300 research workers world wide and more information can be obtained from http://www.aos.princeton.edu/WWWPUBLIC/htdocs.pom/

The version of the model used employs 150X50 cells in the horizontal and 40 levels in the vertical and occupies about 120Mb of memory. With a 10 second time step, one model day takes about 1.5CPU hours on the VPP300. We plan to double the model domain to 150X100 cells for some of the studies above so as to confirm the validity of the open boundary conditions. A model of this size is simply too big for any of the computers available to us at UNSW.

Appendix B -