High Resolution Circulation of the Hawkesbury Shelf Region, SE Australia: Mean, Variability and Transport Pathways

Abstract

The exchange between the open ocean and coastal waters within continental shelf systems is driven by complex circulation processes making it hard to measure and observe. They are highly productive regions, sustaining marine ecosystems and are often nursery areas for many organisms, providing the basis for a variety of natural resources of high economic importance. Along southeastern Australia, the shelf circulation has been studied through in situ temperature and velocity observations, coarse resolution satellite data and modelling studies. However, due to the lack of high spatial resolution data over continued time periods, the circulation over the shelf as a system has not yet been explored. The general aims of this thesis are to assess the mean and time varying circulation over the Hawkesbury Shelf (HS, 31.5-34.5oS) to understand the influence of the EAC on the shelf circulation variability; and to identify water sources of an offshore artificial reef off Sydney. Towards these aims, a high-resolution version of the Regional Ocean Modelling System (ROMS) is configured to model the circulation on the HS for 2012 and 2013. Robust model validation against high-resolution observations shows that the model achieves a reliable representation of the circulation on the shelf. Mean along and across-shelf transports are quantified to identify major regions of ocean/shelf exchange and its spatial and temporal variability. Throughout the year, maximum shelf-water export is associated with the eastward separation of the EAC jet between 31 and 32.5oS. Downstream of the EAC separation, transports become more variable and weaker in magnitude. While the EAC separation creates distinct circulation regimes outside the 200 m isobath, the inner shelf remains unaffected by the EAC variability as indicated by high correlations of along-shelf transports along the coast. Finally, the model is used to understand the movement of water over the inner shelf, with particular focus on the source of water arriving at an offshore artificial reef (OAR) off Sydney. Using Lagrangian particle tracking, the source of water reaching the continental shelf is identified to discern regions that are more likely to be productive. Backward water parcel trajectories reveal that deep offshore waters come mainly from middle shelf regions between 33 and 33.5oS and also from the south, downstream of the typical EAC separation latitudes. In contrast, waters found in inner shelf regions come from the north by poleward flowing inshore waters. The results suggest that seasonal variability in source water location is low; however, the region where the source waters come from is narrow in summer and broader in autumn and winter. Results presented in this thesis aid in understanding the influence of mesoscale variability on the coastal circulation and transport of waters to the inner shelf.