Variability and long-term trends in the shelf circulation off eastern Tasmania

This study investigates trends and interannual variability of the marine climate across the continental shelf off eastern Tasmania for 1993–2016. This region is a hotspot for global warming and biodiversity. Eastern Tasmania lies at the boundary between two ocean currents (the East Australian Current Extension, or EAC Extension, and the Zeehan Current, ZC) leading to the local marine climate exhibiting trends and variability from both boundary currents. A numerical ocean model is used to provide high‐resolution (∼2 km) estimates of the temperature, salinity and circulation for the region. Results indicate significant positive trends in temperature, salinity and southward flow over the shelf, consistent with an increasing EAC Extension. These trends are particularly strong in autumn, indicating a lengthening of the warm season. The interannual variability in the EAC Extension and ZC was quantified by a simple index, based on a modal analysis of surface circulation, indicating the relative dominance of each current. Strong EAC years were related to significantly more summertime marine heatwave days. Large‐scale, remote drivers of variability were considered and we found weak but significant links with El Niño–Southern Oscillation (ENSO) and Tasman Sea Blocking (TSB). ENSO was found to modulate the EAC Extension in summer with a El Niña leading to enhanced southward flow and warming over the shelf. TSB was found to drive enhanced southward surface flow, particularly in winter. Nonetheless, large‐scale forcing modes explain less than 25% of the total variability in the EAC‐ZC system indicating that most of the variability is internally‐generated.