Ocean Circulation of the Great Australian Bight

               

Principal Investigator

John F. Middleton

Mathematics

UNSW

 

Co-Investigators

Guennadi Platov

Mauro Cirano

School of Mathematics

UNSW

 

Projects

h00 - VPP

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 also act to downwell water over the continental slope.

During 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, and the numerical studies outlined should help to determine the general circulation for the slope region and pinpoint areas for future research.

The shelf within the Bight is one of the few in the world where downwelling dominates the circulation over an extended period and few studies of wind-forced downwelling have been made. An additional goal therefore is to determine the fundamental dynamics of wind-forced downwelling.

   
         
               

     
               

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

The VPP300 has been in use since December 1997 and three papers have been prepared dealing with the basic dynamics of wind-forced downwelling and upwelling along a uniform continental shelf.

In the downwelling case, (Middleton and Cirano 1998), the initial evolution of the current system is shown to be qualitatively described by linear coastal-trapped wave dynamics. After 20 days or so, the thermal-wind shear associated with the (80m) thick bottom boundary layer (BBL) was shown to be important for the existence of a strong equatorward Undercurrent.

               
- Appendix B

 
               

       

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 case of wind-forced upwelling, the existence of several original studies had led the investigator to believe that this classical and important problem was well understood. The study by Middleton (1999) has shown that this is not the case.

The dynamics cannot be explained by coastal-trapped wave dynamics, but rather by nonlinear effects and density gradients within the surface mixed layer. These effects lead to the development of an anticyclonic vortex, and downwelling over the slope.

An idealised study into the effect of Bass Strait on the Zeehan Current has also been made (Middleton and Platov 1998) and shows the current over the slope to be little affected by the presence of the strait. Farther inshore, the current is reversed leading to sub-surface upwelling into the strait.

Studies are also underway into the shelf circulation of the Great Bight region,forced by seasonal mean winds and with open boundaries specified using the output from Global Circulation Ocean Models. The slope circulation between South Australia and Tasmania is to be examined for both summer and winter conditions during 1999.

Work on the thermohaline structure to be used is underway and a parameterisation is being developed for the effects of tides and coastal-trapped waves on bottom friction.

Further details are available at:-

http://www.maths.unsw.edu.au/~jffm/GAB.html

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 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. A model of this size is simply too big for any of the computers available to us at UNSW.

Publications

J.F. Middleton and M. Cirano, Wind forced downwelling slope currents: a numerical study, J. Phys. Oceanogr., 1998, to appear .

       
Appendix B -