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Retention and Leakage of Water by Mesoscale Eddies in the East Australian Current System

Mesoscale eddies are ubiquitous in the ocean, transporting semi-isolated water masses as well as advecting tracers and biota. The extent to which eddies impact the environment depends on the time they retain water parcels. Here we quantify retention times of mesoscale eddies in a (1/10)° model of the East Australian Current and its extension along the southeast coast of Australia. We find that retention times vary widely, between 3 and 357 days, but peak around 24 and 27 days for anticyclones and cyclones, respectively. Changes in eddy shape, though not in eddy size, relate to water exchange between the eddy and the background flow. An increase in eccentricity (eddy elongation) often leads to water leakage, while a decrease is associated with water retention. Thus, the change in eddy eccentricity can be used as a diagnostic of the eddy's likelihood to exchange water with its surrounding. We find that water within a region of the eddy that is close to uniform rotation and rotating faster than uniform vorticity is more likely to be retained. Typical retention times are long enough for eddies to transport water across regions of contrasting hydrographic properties, develop a biogeochemical response, and influence connectivity patterns.