Cold-core anomalies at the subantarctic front, south of Tasmania

Eight years of altimetry and hydrographic data are used to examine the formation, propagation and vertical characteristics of cold-core rings formed north of the Subantarctic Front, SAF, in the region south of Tasmania. Altimetry allows us to follow the spatial–temporal evolution of these cold, low sea level anomalies. Most of these cold-core rings are formed from unstable meanders of the SAF, and the interaction of the meanders with bathymetry appears to influence the eddy spawning. The interaction of a westward propagating cold-core eddy with an eastward propagating equatorward meander can also trigger a cold-core eddy spawning event. Hydrographic sections show that these eddies have cool, low-salinity cores reaching to at least 1500 m depth. During summer, their surface temperature signal is eliminated only 2–3 weeks after spawning, however, their surface salinity signature is maintained for 2–3 months. We estimate that these cold-core eddies could contribute an annual heat deficit of −3.8×1019 J, and an annual salt deficit of −1.6×1012 kg, and contribute to cooling and freshening the region north of the SAF where mode waters form. The salt deficit is equivalent to that introduced by the northward Ekman transport, suggesting that eddies may play an important role in transporting low-salinity water into the Subantarctic Zone.