Interannual circulation variability along Western Australia’s continental shelf

On the Western Australian coast two current systems control along-shelf transport and distribution of water-borne material: the poleward-flowing Leeuwin Current, and wind-driven coastal currents inshore, such as the Capes Current, that tend to flow northward driven by prevailing winds. Distinct temperatures and salinities characterize the two regimes, with a complex field of eddies and variability across many time and space scales. A better understanding of transport along the coast, in particular how this influences the recruitment and settlement of economically important species (rock lobsters, crabs, snapper) is of high priority. Observations of coastal ocean surface currents with high spatial and temporal resolution are challenging to obtain yet are needed to address this concern. Fortunately, High-Frequency Radar (HFR) systems that rely on Doppler backscatter observations in the HF (radio wave) portion of the electromagnetic spectrum (3-50 MHz) are able to map surface currents on hourly timescales up to several hundred kilometres offshore. In southwest Western Australia, the IMOS (Integrated Marine Observing System) coastal HF radar network has nearly a decade of archived ongoing near-realtime surface current observations covering several hundred kilometres of the continental shelf north of Perth. In addition, The University of Western Australia has run a high-resolution ROMS circulation model hindcast (OzROMS 2000-2016) for all of Australia that captures the dominant circulation features and variability. Utilising these two sources of data as well as satellite sea surface temperature data, we documented the interannual and shorter-term variability of along-shelf transport in the region with the aim of developing a better understanding of the physical drivers of recruitment variability of important fisheries. In particular we explored how the HFR data can be used to monitor the strength of the Capes and Leeuwin Currents, and how these data can complimented by the numerical model to extend the analysis in time and space.