Cross-shelf exchange driven by dipole eddy structures in the East Australian Current.

Neil Malan1, Matthew Archer2, Moninya Roughan3, Paulna Cetina Heredia1, Amandine Schaeffer4, Michael Hemming5, Eduardo Vitarelli5 and Carlos Rocha5, (1)University of New South Wales, Coastal and Regional Oceanography Lab, School of Maths and Statistics, Sydney, NSW, Australia, (2)NASA Jet Propulsion Laboratory, Pasadena, CA, United States, (3)University of New South Wales, Biological Earth and Environmental Sciences, UNSW Sydney, NSW, Australia, (4)University of New South Wales, School of Mathematics and Statistics, Sydney, NSW, Australia, (5)University of New South Wales, Sydney, NSW, Australia
In western boundary current systems, sharp velocity gradients between the poleward flowing jet and coastal waters act to inhibit cross-shelf exchange. Downstream of jet separation, dynamic mesocale eddies dominate the flow. In the East Australian Current (EAC) System, counter-rotating eddy dipoles are often present, which have the potential to drive cross-shelf transport, impacting the biophysical properties of shelf waters.

However, this eddy dipole mode is poorly understood in the context of cross-shelf exchange and the effect of these structures on shelf waters is uncertain. Using 25 years of satellite altimetry, as well as in-situ sampling of a typical dipole event we investigate the characteristics of eddy-driven cross-shelf exchange. We show that the maximum onshore velocity is driven by an eddy dipole structure and occurs in a defined latitudinal band between 33°S and 34°S more than 50% of the time. We sample a typical eddy dipole and find a strong onshore jet, 37km wide, with velocities up to 1.8m.s-1 and a transport of at least 16Sv. Hydrographic data from an autonomous underwater glider shows that this jet manifests on the shelf as a subsurface intrusion of warm salty water extending from offshore up to the midshelf. In the light of climatic changes in western boundary current transport and the increase in their eddy kinetic energy, understanding eddy-driven cross-shelf exchange is important to predict future changes to the shelf water-mass.