Shelf Ocean Exchange in the East Australian Current System

Abstract

The East Australian Current (EAC) transports heat, mass and biota poleward, along Australia’s most populous coastline from ~27 - 44 S where societal impacts are greatest.  It flows poleward to ~32 S where it separates and sheds large mesoscale anticyclonic eddies. Flow continues poleward in the EAC southern extension as an eddy train.

Over the past decade through Australia’s Integrated Marine Observing System (IMOS) the we have deployed a range of modern technologies to observe the length and breadth of the EAC System.  These state-of-the-art observations include High Frequency coastal radar, autonomous underwater gliders, and a network of coastal and deep water moorings.  The data from which are freely available. Combined with data assimilating models the data are have shed insight into a range of processes including ocean-shelf exchange.

Upstream of the EAC separation region continental shelf circulation is dominated by the EAC jet.  HF radar observations show that the EAC transport and eddy kinetic energy variability strengthen in summer. However this is often masked by the mesoscale variability associated with the meandering of the jet at the 90-110 day timescales (also the eddy shedding timescale).  EAC intrusions over the narrow shelf drive upwelling and the strength of the intrusions on the shelf vary with latitude and advective acceleration.  Surface divergence is generated where the EAC deviates from the coast, which also leads to quasi-permanent upwelling. These processes are in addition to cross-shelf exchange by wind driven upwelling and downwelling.

Submesoscale cyclonic ‘frontal eddies’ form on the inside edge of the EAC which can grow to large cyclonic eddies, as they are advected poleward.  These frontal eddies drive cross-shelf exchange through the entrainment of coastal waters and advection offshore. With retention of shelf larval fish populations they are shown to be disproportionately productive.

Downstream of the EAC separation region cross-shelf exchange is driven by the mesoscale eddy field.  At ~33S onshore flow is a maximum driven by a quasi-steady eddy dipole associated with jet separation. The frontal zone between eddy dipoles can generate subduction of properties, submesoscale symmetric instabilities and the generation of coherent vortices. Mesoscale EAC eddies have been shown to either retain or leak water depending on their eccentricity, contributing to watermass exchange.

All of these processes have biogeochemical consequences in what is an oligotrophic western boundary current system.

Biography

Moninya Roughan is a professor of physical oceanography at UNSW Sydney, with expertise in the dynamics of coastal ocean circulation.  Her research focuses on improving understanding of the East Australian Current, cross shelf exchange, coastal ocean circulation, upwelling, marine heatwaves and the biological implications, using a combination of modern ocean observations and numerical models. 

Over the past decade, Roughan has been instrumental in the design, deployment and ongoing development of one of the most comprehensive ocean observing systems in the world. Through her leadership role in Australia’s Integrated Marine Observing System she has built vast ocean observational datasets critical for understanding coastal processes and improving ocean prediction along the length and breadth of the East Australian Current System.

Venue

This seminar will be held using the video conferencing software Zoom. If you would like the link, please email sees.seminars@uq.edu.au