Modeling studies of the coastal/littoral current system off Southern Australia

Abstract

Both theoretical and numerical modeling studies of the current system off western and southern Australia are conducted to characterize the features of the current system, their temporal variability, and their impact on the sound speed structure. The theoretical study examines why boundary current separation occurs off Cape Leeuwin creating an area of enhanced eddy generation. It is shown that the beta effect, vortex stretching, and streamline curvature all act to decelerate the current and to thus enhance separation. The current is then turned left under the influence of Coriolis force and subsequently forms meanders which then detach from the current as eddies. The model results, using the Princeton Ocean Model (POM), reproduce the main features of the current system. They also provide insight into the generation of the main features. In particular, the current direction is caused by the thermohaline gradient, while topography is responsible for the location of the current along the shelf break. Current speed results from a combination of thermohaline gradient, the opposing wind, and topography, while meanders and eddies result from the opposition of the thermohaline and wind forcing. The gyre and upwelling in the Great Australian Bight are caused by the change in wind direction in summer. Daily wind experiments are shown to capture the seasonal variability of the current system with the Leeuwin Current along the western coast stronger in winter than in summer and mesoscale activity highest in summer. Seasonal and interannual variability are highlighted with the gyre and upwelling in the bight and along Kangaroo Island appearing intermittently but always in summer. Lastly mesoscale features in the current system, advection of water by the surface current and undercurrent, eddies, and upwelling are all shown to cause significant changes in sound speed which can adversely affect sonar operations.