Abstract

The East Australian Current (EAC) in the western Pacific Ocean is Australia’s strongest and most important boundary current, supporting Australia’s temperate coastlines by transporting heat poleward. The large variability of the latitude at which the EAC separates has an important impact on local climate and cross-shelf marine ecosystems. For example, in the summer of 2015, south-east Australian waters observed record temperatures as compared to the previous two decades; this event is (mostly) attributed to an increase in the southward penetration of the EAC . Thus, understanding the mechanisms that control the variability in the EAC extension is important for improving future climate projections.

Here, using NEMO and a NEMO -WRF regional coupled simulations, we present a hierarchy of sensitivity experiments designed to expose the factors that influence the EAC ’s separation. In Part 1, we look at the role of New Zealand and nonlinear processes in the partial separation of the EAC . Contrary to previous work, we find that meridional gradients in the basin-wide wind stress curl are not the sole factor determining EAC separation. In Part 2, the role of forcing variability in influencing the mean state of the Tasman Sea circulation is investigated. These simulations show an EAC with high intrinsic variability and stochastic eddy shedding. We further show that local atmospheric variability leads to increases in eddy shedding rates and southward eddy propagation, and this leads to an increased extent of the EAC . Given the possible relevance of changes in forcing variability (from Part 2) for future climate attribution studies, Part 3 uses a coupled NEMO -WRF simulation forced with CMIP5 RCP85 anomalies to extend our understanding of what is driving the southward shift of the EAC in future climate model projections.