Meridional Heat and Salt Transport Across the Subantarctic Front by Cold‐Core Eddies. Issue 2 (12th February 2019) - Patel, Ramkrushnbhai S.
Abstract: Mesoscale eddies are ubiquitous in the Southern Ocean particularly in regions where the Antarctic Circumpolar Current interacts with topography. Eddies play a critical role in the meridional transport of heat and salt across the Antarctic Circumpolar Current. However, observations of the heat and salt content of eddies are extremely rare. Here we present new observations that characterize the three‐dimensional structure of a cyclonic (cold‐core) eddy generated at the Subantarctic Front south of Tasmania. Automated detection software is used with satellite altimetry to follow the eddy through the Subantarctic Zone. The physical properties at the center of the eddy are substantially modified from those near the formation region, indicating a strong transformation during the eddy's lifetime. The eddy carried heat and salt content anomalies relative to surrounding Subantarctic Zone waters of −0.5 ± 0.1 × 10 20 J and −2.1 ± 0.4 × 10 12 kg, respectively. Previous studies have not captured the full heat and salt content of eddies due to limited observations and have underestimated their content by a factor of 2 to 3 south of Tasmania. Applying the observed correlation between eddy‐elevated volume and eddy heat and salt content to the history of cyclonic eddies in the satellite altimeter record leads us to propose that about 21% of the heat carried across the Subantarctic Front south of Tasmania is achieved by cyclonic eddies entering the Subantarctic Zone. The freshwater contribution to the Subantarctic Zone by long‐lived cold‐core eddies is of the same order of magnitude as the Ekman flux in this region. Plain Language Summary: Eddies are rotating bodies of water with diameters between 10 and 100 km that live from a week to months in the ocean. They are known to carry heat and salt across the Antarctic Circumpolar Current. Because of extremely limited direct observations of these eddies, it is difficult to calculate the actual amount of heat and salt that these eddies carry. Here we present new observations of a cold‐core eddy that we sampled during a voyage in the Southern Ocean south of Tasmania. The eddy was born in the Subantarctic Front and traveled into the Subantarctic Zone where it stayed for 2 months before returning to the Subantarctic Front and disappearing. The observations showed that the amount of heat carried into the Subantarctic Zone is 2.6 times higher than previously reported, and the amount of salt is 2.5 times larger for south of Tasmania. Based on our in situ observations, combined with satellite measurements of sea surface height and an eddy‐tracking software, we propose that 21% of the heat carried across the Subantarctic Front south of Tasmania is achieved by long‐lived, cold‐core eddies entering the Subantarctic Zone. Key Points: We characterize for the first time the physical structure of a Subantarctic Front cold‐core eddy, south of Tasmania in the Southern Ocean Long‐lived cold‐core eddies transport about 21% of the required meridional heat flux across the Subantarctic Front Previous studies have underestimated the total heat and salt content transport by cold‐core eddies due to limited observations