Shelf and slope processes offshore Fremantle, Western Australia

The continental shelf and slope region off Fremantle, Western Australia, is unique compared with other continental shelves. The anomalous Leeuwin current dominates the circulation at the shelf edge and offshore, and transports warm, nutrient-poor water poleward. On the continental shelf, the wind forces the circulation, with coastally trapped waves also contributing. The presence of offshore islands and a submarine canyon make the system more complex. Thus an ocean observation system to monitor this unique physical/biological oceanographic system would provide exciting opportunities for undertaking process studies. This observation system includes HF radar systems, autonomous ocean gliders and moored instruments.
In this presentation, integrated data from these systems will be used to examine the upwelling/downwelling processes in the presence of topography. The Leeuwin current creates a downwelling environment along the shelf break; however, the topography (Perth submarine canyon and Rottnest Island) and strong summer winds cause localized upwelling. In autumn and winter, higher density inner shelf waters, resulting from high evaporation during the summer and then cooling exit the shelf as higher density bottom plumes. A major feature revealed in the ocean glider data, and visible in satellite images, was the presence of vertically mixed water at the shelf edge, which increased the primary production. Thus the observation system (‘pre- operational system’) provides a great opportunity to gain a deeper understanding of the physical processes and their relationship to biological processes.