Mixing and transport processes in Shark Bay, Western Australia

[Truncated abstract] In Shark Bay, a large semi-enclosed bay in Western Australia, the net loss of fresh water due to high rates of evaporation and low rainfall causes hypersalinity in the isolated inner regions leading to vertical and horizontal stratification throughout the Bay. The high salinity causes Shark Bay to be denser than the ocean, and allows for its classification as an inverse estuary. Although the distribution of salinity in the Bay has been documented, the dynamics of the circulation and exchange between the Bay and ocean remain poorly understood. In particular, the circulation driven by density gradients and modulated by winds and tides is unknown. Through a combination of field measurements and numerical modelling, this thesis examines the dominant physical processes that control circulation, mixing and exchange of water and waterborne material between Shark Bay and the ocean. The influence of hydrodynamic processes on the larval dispersal of the saucer scallop (Amusium balloti)—a commercially important species that has experienced dramatic inter-annual variability in recruitment—is also examined. Field measurements revealed a typical residual flow pattern for an inverse estuary with nearbed outflow of water and inflow at the surface. The bottom outflows were enhanced during periods of low tidal mixing which occurred during the transition from semi-diurnal to diurnal tidal stages and was therefore a regular, predictable, occurrence. Winds had the capability to vertically mix the water column and control the outflows. The northern entrance (Geographe Channel) had a more pronounced outflow when compared to the western entrance (Naturaliste Channel). A three-dimensional baroclinic ocean circulation model (the General Estuarine Transport Model, GETM) was then applied to examine the detailed dynamics of the residual circulation and to investigate the role of wind and tidal mixing for control of stratification. In Shark Bay, the contribution of wind and tidal mixing energy for controlling the vertical stratification were of similar magnitude. In the shallow regions (<15 m) either wind or tide was able to fully mix the water column. In contrast, a combination of both wind and tide was required to mix the deeper channels (> 15 m). The gravitational circulation was intensified during stratified periods when tidal and wind mixing were low...