The Hydrodynamics of Gulf St. Vincent, South Australia

Abstract

The hydrodynamics of Gulf St. Vincent (GSV) are investigated through the use of several numerical models based on the Regional Ocean Modeling Suite (ROMS). Idealised models which investigate tides, wind forcing and thermohaline forcing are undertaken, as well as a fully forced qausi-realistic hindcast model. The model domain encompasses Investigator Strait, GSV and Backstairs Passage, as well as some of the shelf to the south-east. The tidal model incorporates a new method for estimating the quadratic coefficient of drag which accounts for depth and benthic type. Results of the tidal model indicate that the presence of seagrass meadows are significant for bottom friction and the estimation of tidal elevations and currents. This is a particularly important result for GSV, as the health and distribution of seagrass meadows is a key environmental factor. The idealised wind-forced and geostrophically adjusted models indicate that the wind-stress direction and magnitude is critical in determining the connectivity of the gulf with the shelf, where thermohaline circulation is less important. Results of the idealised models indicate that during summer, when winds are dominated by south-easterlies, gulf waters are largely recirculated and salinity increases as a result. During winter the winds become dominated by south-westerlies, which induce a gulf wide clockwise circulation which draws in fresher shelf waters via Investigator Strait and expels salty gulf water via Backstairs Passage. A volume averaged salt balance for the quasi-realistic model supports this finding, indicating that salinity increases whilst under summer wind conditions and decreases for winter wind patterns. This is due to the excess of evaporation over precipitation year round. Flushing time scales for summer and winter conditions are also explored based on the results of the quasi-realistic model, and indicate that the interior of the gulf is flushed more slowly during summer than winter. Investigator Strait and Backstairs Passage exhibit little seasonal variability. Passive Lagrangian float tracer studies are also conducted, and further support the finding that summer conditions show reduced connectivity between the shelf and gulf when compared to winter. In the weather band, periods of enhanced exchange between the shelf and gulf are shown to coincide with wind-stresses from the west, while periods of reduced exchange coincide with wind-stresses from the east.