Aspects of the mean wintertime circulation along Australia’s southern shelves: numerical studies

Abstract

A high-resolution numerical model is used to study the mean wintertime shelf-slope circulation between the Gulfs of South Australia and the west coast of Tasmania. The mean downwelling-favorable winds for the region result in a continuous eastward coastal current (CC) extending from Cape Leeuwin to the eastern coast of Tasmania. The magnitude of the CC is generally largest over the shelf break, has speeds of more than 40 cm s21 [transport 2.4 Sv (Sv [ 106 m3 s21)], and is generally in good agreement with observations. The relative importance of the Leeuwin Current, zonal winds, and density gradients to the shelf circulation is established. Within Bass Strait, the eastward circulation is in good agreement with observations and simple models show that about 30% of the transport arises from local winds and the remainder from the CC. The total transport through the strait (0.95 Sv) is shown to be controlled by sea level on the east coast of Tasmania. A current of up to 10 cm s21 is also found to flow from Tasmania to Esperance in the far west (transport 6.8 Sv) and to be centered within the permanent thermocline (depth 600 m) and near the shelf slope. This Flinders Current arises from the equatorward Sverdrup transport but is also fed by the ‘‘Tasman outflow’’ that enters the region off of the southern tip of Tasmania. A sensitivity study shows that dense water formation occurs off of Spencer Gulf and the cascade to depths of 250 m is in qualitative agreement with observations. Dense water formed near the coast can enhance the CC by up to 5 cm s21. Suggestions for future modeling and observational studies are made.