title: Physical and biogeochemical structure of Southern Ocean eddies
Creators: name: Patel, RS
Publisher: name: University of Tasmania
Year: 2021

description: The Antarctic Circumpolar Current (ACC) is the dominant feature of the Southern Ocean and is an integral part of the global meridional overturning circulation. Importantly, it acts as a barrier to the cross-frontal exchange of both physical and biogeochemical tracers and preserves the cold-climate of Antarctica. Understanding the exchange of tracers across the ACC remains a key open question, critical to understanding the past and projecting future changes. Mesoscale eddies have been identified as one mechanism which facilitates this cross-frontal transport. However, observations of individual eddies are rare. Only a few studies have examined the subsurface structure of the physical properties of eddies and their contribution to heat and salt transport. Furthermore, as of yet no studies have explored the subsurface biogeochemical structure of Southern Ocean eddies. The goal of this thesis is to analyse in situ observations to characterise the subsurface physical and biogeochemical structure of cold-core eddies, to enable estimation of the cross-frontal exchanges of heat, salt, and nutrient. In our analysis, we also incorporate satellite observations and eddy tracking software, and an eddy-resolving general ocean circulation model to extend our analysis in space and time.to heat and salt transport. We present novel in situ observations that allow characterization of cold-core eddy’s the threedimensional physical and biogeochemical structure. The sampled eddy was generated at the Subantarctic Front south of Tasmania, Australia – recognised as a hotspot for cross-frontal transport. Automated eddy detection software was used in conjunction with satellite altimetry to track the eddy through the Subantarctic Zone. Our results demonstrate strong transformation in physical and biogeochemical properties over its lifespan. We also showed that the eddy carried cold, fresh and nutrient rich water in its core, with respect to surrounding water. Relative to surrounding Subantarctic Zone waters, the eddy carried heat and salt content anomalies of -0.5 ± 0.1 x 10\(^{20}\) J and -2.1 ± 0.4 x 10\(^{12 }\) kg, respectively. In comparison with previous Southern Ocean eddies studies, this heat and salt contents were larger by a factor of 2 to 3, especially compared to typical values south of Tasmania. We demonstrated that this underestimate of the contents was due to limited observations or incomplete sampling of the eddies. To understand the impact of this underestimate on our understanding of the role of eddies in the meridional transport, we developed a methodology which allowed us to infer the subsurface content of an eddy using their surface expression. Our results indicated that about 21% of the heat carried across the Subantarctic Front south of Tasmania is achieved by cold-core eddies entering the Subantarctic Zone. Further, we demonstrated using an eddy-resolving general circulation model (Ocean Forecasting Australia Model 3) that this meridional transport of heat is consistent when Eulerian and Lagrangian approaches are reconciled. The freshwater contribution to the Subantarctic Zone by long-lived cold-core eddies is of the same order of magnitude as the Ekman salt transport in this region and is about 76% of the total transient transport in the simulation. Moreover, we demonstrated that relative to Subantarctic Zone waters, long-lived cold-core eddies carry nitrate anomalies of 1.6 ± 0.2 x 10\(^{10}\) moles and silicate anomalies of -5.5 ± 0.7 x 10\(^{10}\) moles across the fronts each year. This cross-frontal transport of nutrients has a negligible impact on Subantarctic Zone productivity; however, it has the potential to modify the nutrient content of mode waters that are exported from the Southern Ocean to lower latitudes. This thesis provided the first robust estimate of meridional transport of heat and salt by discrete eddies from in situ observations and uses a numerical simulation to both verify the assumptions made and extend our analysis beyond the single observed eddy. Moreover, this thesis provided the first-ever characterization of the vertical structure of nutrients in cold-core eddies and quantified the meridional transport of nutrients by these eddies. This detailed assessment of the physical and biogeochemical structure of eddies and new estimates of the meridional transport by eddies are critical for parameterizing eddy processes in Earth system models and improving our knowledge of the role of Southern Ocean eddies in global heat and freshwater budgets and nutrient cycles.