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Abstract
Low frequency variability is investigated in the waters of the continental shelf of south-eastern Australia using new in situ temperature and velocity observations. The East Australian Current (EAC), a strong Western boundary current (WBC) which closes the South Pacific gyre, flows adjacent to south-eastern Australia along the shelf break impacting on the shelf circulation. Previous studies in this narrow (approximately 30 km wide) shelf region have been limited to short term process studies using temporary oceanographic moorings or have made use of monthly hydrographic samples. Additionally, over-ocean wind observations are scarce in this region, hence finding an adequate over-ocean wind proxy is important to enable an investigation of wind stress on the coastal ocean. The recent (since 2008) deployment of two sustained mooring arrays through Australia's Integrated Marine Observing System, upstream and downstream of the EAC's separation zone now allow investigations into low frequency variability inshore of this dynamic WBC. Historic over-ocean wind observations from the Ocean Reference Station allow for an investigation of coastal wind proxies in the region. The results clearly show the importance of in situ long term observations. The seasonal variability in the oceanography on the shelf is identified revealing the low frequency impact of the EAC on the shelf hydrography upstream of the separation point. For the first time, multi-year observations of EAC eddy encroachment on the Sydney shelf are shown, having a direct impact on the shelf circulation with a periodicity of 110 days. Thus these results clarify the misconception that the circulation on the shelf downstream of the EAC's separation point exhibits a seasonal cycle. These results show a new link between the periodicity in the separation of the EAC and the periodicity of shelf circulation downstream of the separation point. Understanding this periodicity and its shelf impacts will assist with modelling and prediction of eddy encroachment on the continental shelf and the biological impacts. The results presented here support the continuation of sustained long term in situ observations in coastal waters, for if we are to understand the impacts of climate variability into the future, we must first understand the natural variability.