Surface current variability in the East Australian Current from long-term HF radar observations

Abstract. The East Australian Current (EAC) exhibits significant variability across a wide range of spatial and temporal scales, from mesoscale eddies and meanders to seasonal, interannual, and decadal fluctuations in its intensity, pathway, and influence on the continental shelf circulation. Understanding and monitoring this variability is crucial because the EAC plays an important role in controlling shelf dynamics, regional circulation, coastal weather and global climate patterns. As such, two high-frequency (HF) coastal radar systems have been deployed on the east coast of Australia to measure surface currents upstream and downstream of the East Australian Current (EAC) separation point. The multi-year radar dataset (spanning 4–8 years) is presented here and its use is demonstrated to assess the spatial and temporal variability of the EAC and the adjacent continental shelf circulation, ranging from seasonal to interannual scales. The dataset is gap-filled using a 2dVar approach (after rigorous comparison with the traditional unweighted Least-squares fit (LS) method). Additionally, we explore the representation depth variability of the observations by comparing the data with surface Lagrangian drifter velocities (with and without depth drogues). The multi-year radar-derived surface current dataset, which was validated using short-term drifter and long-term current meter observations, revealed that the local upstream circulation is strongly dominated by the EAC’s annual cycle, peaking in the austral summer. The analysis using 8 years of upstream data revealed the period of the EAC intensification at around 3–5 years. The interannual variability of the poleward transport downstream was driven by the intrinsic variability of the jet. This dataset which continues to be collected, complemented by numerical simulations and in-situ measurements, will provide a comprehensive view of the EAC’s variability and its impact on the broader regional circulation dynamics which can be used for a range of dynamical investigations.