Process-oriented modeling studies of the 5500-km-long boundary flow off western and southern Australia

Abstract

While the unique character of the coastal current system off the western and southern coasts of Australia has been recognized, this vast 5500-km-long boundary flow has been studied far less than other current systems of the world. Recent observational studies from satellite altimetry and climatology are consistent with a continuous current extending from its origin at the North West Cape to the southern tip of Tasmania. To date, coastal modeling studies have focused on either the western Australian coast to Esperance or on southern Australia. There has been no process-oriented modeling study of the entire region that would allow the systematic exploration of the two independent forcing mechanisms (i.e., wind-forcing and thermohaline gradients) and their interactions that have been noted to act in a synergistic manner to maintain the longest continuous coastal current system in the world. This study uses a regional circulation model (in this case the Princeton Ocean Model (POM)) to systematically address the roles the forcing mechanisms play in generating and maintaining the major features of this continuous coastal current system. Several process-oriented experiments, arranged in order of increasing complexity, are explored. The results show that, even in the absence of bottom topography, a continuous 5500 km coastal current system can be generated by wind forcing or by thermohaline forcing. If wind forcing alone is used, coastal currents in the direction of the wind and opposing undercurrents can be generated. If thermohaline forcing alone is used, coastal currents in the opposite direction of the wind and subsurface currents similar to the Flinders Current can be generated. The addition of topography shows that topography is responsible for the currents’ shelf break locations and, for broad shelf regions, can separate the surface flow into two cores, one at the coast and one over the shelf. On the west coast, topographic beta due to the continental slope prevents currents from becoming broader and drifting offshore. The combination of wind forcing, thermohaline gradients and topography show that swift currents forced by thermohaline gradients are slowed to more realistic speeds by opposing wind and by topography. Meanders and eddies result from the opposition of surface and subsurface currents as well as from thermohaline and wind forcing. The results illustrate that the 5500-km-long current system over the shelf break can be maintained year-long due to the two independent forcing mechanisms, their interactions, and the strong trapping effect of bottom topography. The seasonal and daily wind-forcing experiments highlight both the seasonal and interannual variability of this complex current system. The Leeuwin Current along the western coast is slightly stronger in winter (July) than summer (January). There is much greater mesoscale activity in January when the opposing winds are strongest. The results also show that, although upwelling has been observed only in the summer in the Capes Current region, upwelling occurs intermittently in the 2001 winter but not in the 2001 summer. This illustrates that, depending on the strength of the forcing mechanism, such as strong equatorward winter 2001 winds, features such as upwelling on the west coast, usually thought to exist in the summer but only intermittently, can occur in different seasons. Along the southern coast, a gyre forms intermittently in the Great Australian Bight in summer, but the flow is constantly eastward across the entire shelf in winter. The production of upwelling in the Great Australian Bight during the 2000 summer but not during the 2001 summer is an indication of the importance of interannual variability. Overall, the results of this process-oriented study compare well with available observations off western and southern Australia.