Meridional Heat and Salt Transport Across the Subantarctic Front by Cold‐Core Eddies

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 × 1020 J and −2.1 ± 0.4 × 1012 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.

Some observations on the biogeochemical cycling of zinc in the Australian sector of the Southern Ocean: a dedication to Keith Hunter

“…5) and nitrate and silicate concentrations were 22 and silicate was 3.3 mM respectively, all typical of polar waters located further south of the northern extension of the SAF (Moreau et al 2017;Patel et al 2019). Satellite analysis revealed that the eddy had formed in early February 2016 and moved northward ,1.28 of latitude at the time of sampling (Patel et al 2019). Satellite analysis of the eddy also revealed that it had a lower chlorophyll concentration than surrounding waters, consistent with its development further south in less-productive polar waters (Fig.…”

“…The eddy was ,190 km in diameter and was a stable feature that had formed in mid-February 2016, ,1.28 further south of the sampling site near the SAF (Moreau et al 2017). After formation, the eddy moved slowly northward across the northern extension of the SAF with a rotation speed of 0.527 rad day À1 (Moreau et al 2017;Patel et al 2019). The biogeochemical properties of this eddy differ from external waters (Moreau et al 2017;Patel et al 2019).…”

“…However, the surface water temperature was 7.28C, salinity was 33.8 ( Fig. 5) and nitrate and silicate concentrations were 22 and silicate was 3.3 mM respectively, all typical of polar waters located further south of the northern extension of the SAF (Moreau et al 2017;Patel et al 2019). Satellite analysis revealed that the eddy had formed in early February 2016 and moved northward ,1.28 of latitude at the time of sampling (Patel et al 2019).…”

“…After formation, the eddy moved slowly northward across the northern extension of the SAF with a rotation speed of 0.527 rad day À1 (Moreau et al 2017;Patel et al 2019). The biogeochemical properties of this eddy differ from external waters (Moreau et al 2017;Patel et al 2019). Because this eddy became 'structurally closed' after development, its ability to entrain and detrain nutrients, dissolved iron, phytoplankton and zooplankton was restricted.…”

Air‐Sea Heat and Momentum Fluxes in the Southern Ocean

“…These observations of large fluxes over the SO mesoscale eddies might be governed by the sea surface temperature anomaly arising from the ocean polar front in the SO as well as lifetime and evolution stage of the ocean eddy. During the sampling, the cold eddy had reached past its mature stage and had entered its decay phase impacting its amplitude (Patel et al, ). The warm eddy, however, was a short‐lived eddy and shifted its center quickly as per the altimetry observations.…”