Mesoscale eddy characteristics in the interior subtropical southeast Indian Ocean, tracked from the Leeuwin Current system

Two oppositely rotating mesoscale eddies, originating from the Leeuwin Current, were captured during a research cruise in the interior southeast Indian Ocean in August 2012, a few hundred kilometers off the west coast of Australia. The two eddies had different vertical structures: the anticyclonic eddy had stronger vertical shear above 200m and a less stratified thermocline below the mixed layer, while the cyclonic eddy had a more barotropic structure in the upper 500 m. Compared with the same types of eddies sampled in 2003 close to the Australian coast, the 2012 anticyclonic eddy had a larger radius, deeper isothermal depths, and higher peak velocity; whereas the 2012 cyclonic eddy had a similar structure with the 2003 survey. We use an Argo float and three surface drifters deployed and trapped in the anticyclonic eddy to track its evolution over a 4-month period. The eddy was observed to propagate 1000 km further to the west, indicating that the Leeuwin Current properties of the eddy may be preserved for over 2000 km off the west coast of Australia. The eddy appeared to have experienced radial excursions relative to the eddy center, with periodicity of 4–6 days and 15 days, during their anticlockwise motions. The radial movements of the drifters appeared to be constrained by the radial momentum balance within the eddy. The importance of eddy distortions to ocean primary production in anticyclonic eddies in the southeast Indian Ocean warrants further studies.