Processes controlling the position of frontal systems in Shark Bay, Western Australia

Abstract

Shark Bay is a large inverse estuary on the west coast of Australia. Analytical and numerical approaches were used to predict the locations and causes of fronts in Shark Bay, and the results were compared with Sea Surface Temperature and field data. The study first applies an analytical theory, widely used in studies of traditional estuaries, to predict the location of fronts, and then applies a 3-D, baroclinic hydrodynamic model to analyze advection of temperature and salinity, and their ultimate influence on density fronts within the Bay. Analytical theory defines a front as a transitional region between mixed and stratified conditions, and it postulates that the location of fronts may be predicted through the balance of stratifying and de-stratifying energy input to the water column, ‘the stratification parameter’. In Shark Bay, fronts are predicted where the stratification parameter, S = 3.0. Furthermore, the distribution of the mean tidal velocity magnitude was determined to correlate to regions of high bathymetric gradients, showing that changes in water depth influence the local tidal currents, the value of the stratification parameter and by extension, the location of the fronts. However, calculation of the balance of all major energy inputs reveals a balance between evaporation-driven gravitational flow and tidal mixing in Shark Bay. A numerical model was then used to investigate the hydrodynamic processes contributing to frontal dynamics in Shark Bay. Predicted residual velocities revealed a two-layer flow regime and temperature and salinity simulations accurately reproduced the major frontal features observed at the entrances of the Bay using only tidal forcing and gravitational circulation. However, it was found that wind forcing clearly influenced the distribution of salinity, defining the shape of the major frontal feature inside the Bay.