Circulation, dynamics and variability in Australia's boundary currents

Abstract

Meridional heat transport around Australia is determined by Australia’s two poleward flowing boundary currents: the East Australian Current (EAC) and Leeuwin Current (LC). This thesis aims to improve our understanding of the circulation, variability and dynamics of the LC and EAC by characterising the importance of bathymetry and non-linear processes in terms of the role of eddy fluxes and forcing variability. In Part 1, a quantitative assessment of LC pathways suggests that more than half of LC source waters originate from tropical waters. The Lagrangian framework quantifies the preference for LC waters to exit the LC north of 30S; if the LC waters travel beyond this latitude, then they are more likely to continue downstream. In particular, eddy fluxes allow only limited transport (0.2 Sv) to travel the entire length of the LC into the Great Australian Bight. In Part 2, we examine the role of deep bathymetry in the EAC separation by removing New Zealand. We find that the complete removal of New Zealand leads to the EAC mean separation latitude shifting >100km southward. We remove New Zealand with a hierarchy of linear models of increasing complexity; we find that linear processes and deep bathymetry play a major role in the Tasman Front position, whereas non-linear processes are crucial for the extent of the EAC retroflection. Contrary to previous work, we find that meridional gradients in the basin-wide wind stress curl are not the sole factor determining EAC separation. Part 3 examines the EAC separation in terms of the role of local versus remote forcing variability in setting the mean state of the Tasman Sea circulation. We find that local, variable wind stress forcing, with a period shorter than ~2 months, results in a rectified Tasman Sea circulation. The increased extent of the EAC extension is characterized by increases in eddy shedding rates, southward eddy propagation and increased EAC extension transports. An energetics framework suggests that these EAC extension changes are coincident with increases in offshore, upstream eddy variance (via near-surface barotropic instability) as well as increases in subsurface mean kinetic energy along the path of the EAC.