No abstract

Recent observations reveal the existence ofa southward flowing baroclinic current over the continental slope of western Bass Strait. This current appears to be a permanent feature (though of variable strength), and some drifting-buoy observations suggest that it may extend to the southern tip of Tasmania.

The initial stages of the development of a numerical model to investigate the effects of wind and tidal forcing on the sea-surface height distribution and currents within Bass Strait are described. The hydrodynamical model is based on depth-averaged equations and incorporates the effects of the earth's rotation and variations in bottom topography. Numerical experiments have been performed with the model to determine the response of Bass Strait to two separate forcing mechanisms: stationary fields of uniform wind stress suddenly imposed over the sea surface, and tidal heights specified along the open-sea boundaries of the model. The results of these experiments are presented and discussed, the principal conclusions being that the maximum tidal currents generated are generally larger than those generated by the wind, and that variable bathymetry is a major influence on the flow. Furthermore the tidal motion may be characterized by two Kelvin waves travelling in opposite directions through the strait.

Earlier models of the circulation in Bass Strait have been extended to include vertical structure. Time- dependent circulation fields in Bass Strait, induced by wind driving at the surface and tidal oscillations along open-sea boundaries, are computed at a number of selected depths. The original two-dimensional model is combined with an analytical solution of the Ekman equations, which at each grid point provides an expression for the time-dependent flow at any depth in terms of a convolution integral over the sea- surface slope and wind stress. This model should be applicable to winter conditions when the strait is well mixed vertically and hence the dynamical effects of density stratification negligible. The predicted wind-induced circulation fields are highly depth dependent, with equilibrium surface currents in the central Bass Strait basin flowing in a direction approximately 45� to the left of the wind. At lower levels, currents are controlled by pressure gradient forces due to the sea-surface slope and friction. Significant upwelling and downwelling motions along the Victorian and Tasmanian coastlines can be inferred from these circulation fields. In the deep water off the continental shelf, currents in the upper 100 m are dominated by the (Ekman) drift current which rotates in an anticlockwise direction with increasing depth, such that the wind drift at the surface is accompanied by a measure of return flow at depth. Tidal currents are predicted in the absence of wind stress, but include the effects of bottom topography. Considerable variation with depth is found and the distinctive features are explained in terms of the relative importance of Coriolis force, bottom friction, and water depth. Comparison with the few existing observations reveals that the present model is producing realistic results.

The dynamical equations governing the wind-induced barotropic motion of the shallow water in the Bass Strait basin with variable bathymetry are discussed qualitatively and solved numerically with a view of explaining and describing the circulation and sea-surface elevation fields. Immediately after the onset of a steady uniform wind, fluid is accelerated in the direction of the wind, and the sea-surface elevation gradient builds up at coastal boundaries to satisfy the condition of zero normal flow. During the intermediate stages toward steady state the effects of topography, the earth's rotation, and bottom friction come into play, and simple arguments based on the momentum and vorticity balances are used to describe their relative importance. A numerical model of the hydrodynamics of the Bass Strait waters is presented and the results interpreted in terms of the above theoretical ideas. Of special significance is the dominance of the topographic effect which acts to produce barotropic currents, which are much larger than those in a flat bottom basin and which flow in a direction along the depth contours. It is shown that these currents are controlled by the component of wind stress acting along the isobaths, which is balanced by the bottom friction. The energy balance within the strait is discussed and shown to be highly dependent on the wind stress distribution. The degree of mixing by the wind as estimated by the stratification index H/u 3, is shown to be small. A comparison with the very limited number of observations validates some of the more prominent features of the model results.