SO-Eddy: Front-Affiliated Mesoscale Eddies in the Southern Ocean

• Project by: AntoninoIan Ferola
• December 5, 2025

Antonino Ian Ferola, Dr Internal Research Fellow, @Esrin Science Hub

Rationale

Mesoscale eddies are rotating vortices that arise from flow instabilities and play a key role in maintaining the dynamical balance of the Antarctic Circumpolar Current (ACC). The ACC is organised into several fronts, each separating water masses with distinct properties and origins. Eddies detach from these fronts, transporting heat, freshwater, carbon and tracers across strong gradients. In the Southern Ocean, harsh conditions limit in situ observations, making it difficult to characterise eddy dynamics and their vertical structure. Satellite observations, however, provide continuous, basin-scale coverage and, when combined with targeted in situ data, open the door to a more systematic description of eddy properties and their role in Southern Ocean variability and change.

Objectives

The main goal of this project is to build a front-aware catalogue of Southern Ocean eddies by:

• Classifying mesoscale eddies according to their ACC front of origin, using the TOEddy atlas and front-detection methods.
• Describing their vertical structure (Temperature, Salinity, Density, Ocean Heat Content, Chlorophyll, Nitrate etc.) from co-located in situ profiles, and deriving key integrated quantities such as heat content, eddy kinetic energy and meridional heat transport.
• Assess how eddy properties vary with front, season, region and eddy type (cyclonic vs anticyclonic), and how they have evolved over recent decades, with a particular focus on their connection to recent Southern Ocean salinity changes and shifts in the sea-ice regime.

Data

The core of the project is leveraging on the TOEddy Global Mesoscale Eddy Atlas colocated with Argo profiles, derived from daily gridded Absolute Dynamic Topography (1993–2023). TOEddy provides eddy positions, radius, amplitude and complex interactions (splitting/merging), together with 23 years of co-located Argo profiles (2000–2023), including BGC and Deep floats where available.

Methods

Robust, observation-based characterisation of front-affiliated eddies:

1. Co-located eddies:
   Start from the eddy–Argo co-located dataset.
   For each eddy, extract the time and position of the first observation (eddy “birth”).
   Output: eddy birth dates and coordinates.
   
2. Frontal field
   Around each birth date, compute a short temporal mean SSH (e.g. 7-day average).
   Detect ACC fronts using a contour-based method on the mean SSH field.
   Output: frontal contours for each birth window.
   
3. Eddy–Front association
   For each eddy, compute the distance to the detected fronts at its birth time.
   Assign the nearest front within a chosen distance; otherwise flag as unaffiliated.
   Output: eddy–front affiliation flags.
   
4. Sensitivity analysis on Eddy-Front association criteria

Results

The following figure shows the mean sea surface height (SSH) over the Southern Ocean for 2000–2023, with the main ACC fronts depicted as four colour-coded contour sets. On top of these, coloured dots indicate the birth locations of eddies that were co-located at least once with an Argo profile during their lifetime, using matching colours to denote their affiliated front. The number of Argo profiles greatly exceeds the number of eddy birth points, as several profiles can be associated with the same eddy. This front-affiliated eddy–Argo subset provides the basis for the subsequent analysis of eddy vertical structure.

As a preliminary result, the next figure presents the mean vertical structure of anticyclonic eddies associated with the Southern Subantarctic Front (SSAF), shown as composite “cylinders” of temperature and salinity. These composites illustrate the typical thermohaline properties in the eddy core and its vertical extent:

Emerging Research Directions

Building on this effort, the project opens several avenues for future research:

• Eddy-driven CO₂ fluxes
  Linking warm- and cold-core eddies to CO₂ source–sink behaviour by combining eddy properties with surface and subsurface carbon datasets
• Ozone depletion, winds and ACC eddy field
  Using CCI-Ozone products to explore how changes in stratospheric ozone and westerly winds have influenced ACC structure and eddy activity, and how this relates to Southern Ocean heat and carbon uptake.

These directions aim to connect eddy statistics to broader climate processes, from carbon cycling to atmospheric forcing and water-mass transformation.