Shelf-Deep Ocean Exchange in the East Australian Current

Moninya Roughan1,2, Amandine Schaeffer3, Matthew Archer4, Paulna Cetina Heredia2, Steefan Contractor2, Shane R Keating5, Colette Gabrielle Kerry6, Michael Hemming5, Neil Malan2 and Adil Siripatana2, (1)New Zealand Meteorological Service (MetService), MetOcean Division, Auckland, New Zealand, (2)University of New South Wales, Coastal and Regional Oceanography Lab, School of Maths and Statistics, Sydney, NSW, Australia, (3)University of New South Wales, School of Mathematics and Statistics, Sydney, NSW, Australia, (4)NASA Jet Propulsion Laboratory, Pasadena, CA, United States, (5)University of New South Wales, Sydney, NSW, Australia, (6)University of New South Wales, Coastal and Regional Oceanography Lab, School of Biological Earth and Environmental Sciences, UNSW, Sydney, NSW, Australia
Abstract:
The East Australian Current (EAC) transports heat, mass and biota poleward, along Australia’s most populous coastline where societal impacts are greatest. It flows poleward to ~32S where it separates and sheds large mesoscale anticyclonic eddies. Over the past decade the NSW-IMOS team have used state-of-the-art observations combined with data assimilating models to understand 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 variability strengthen in summer. However this is often masked by the mesoscale variability in the meandering of the jet at the 90-110 day timescales (also the eddy shedding timescale). The strength of EAC intrusions on the shelf varies with latitude and advective acceleration, ultimately generating surface divergence at it deviates from the coast, and leads to quasi-permanent upwelling.

Cross-shelf exchange is driven by current and wind driven upwelling and downwelling. Submesoscale ‘frontal eddies’ form on the inside edge of the EAC which can grow to large cyclonic eddies, as they are advected poleward. These disproportionately productive Frontal eddies drive cross-shelf exchange, and the entrainment, and retention of shelf larval fish populations.

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.

A temperature climatology derived from a multi decadal hydrographic sampling programme has identified the signature of seasonal upwelling and marine extremes (e.g. heatwaves). Using a multidecadal ROMS simulation we have shown the variability of jet as it flows poleward, strengthening, then deepening.

Future work includes using deep gliders to observe mesoscale eddies, and adding value to data assimilation prediction efforts. Data uptake is enhanced through data products available in a web app.