Below the Surface: Detecting Mesoscale Eddies from Argo Density Profiles in the South Atlantic

Abstract. Mesoscale eddies play a major role in the distribution of heat, salt and tracers in the ocean, yet satellite observations alone cannot fully characterise their three-dimensional structure. Here we develop a method that detects and characterises mesoscale eddies directly from Argo vertical profiles of potential density anomaly in the South Atlantic. Eddies identified from the TOEddies altimetry atlas are used to colocate Argo profiles and to calibrate region-dependent detection thresholds over the period 2000–2025; altimetry thus serves as a reference and calibration benchmark rather than as an input to the detection itself. We evaluate the method against an independent dataset from two field campaigns, in which mesoscale eddies were located independently from ADCP velocities, comprising 12 anticyclonic and 14 cyclonic eddies. Applying the method to the full dataset reveals distinct spatial distributions of surface- and subsurface-intensified eddies, with cyclones showing a stronger tendency toward subsurface intensification. Surface-intensified eddies are more prevalent in tropical regions and the Cape Basin, while both cyclonic and anticyclonic eddies show evidence of subduction in the Cape Basin, transitioning from surface to subsurface intensification along their trajectories, a transition supported by individual eddy trajectories sampled by Argo floats. The vertical structure of eddies is also closely linked to mixed-layer dynamics and its seasonal variability, with winter associated with more surface-intensified eddies and deeper mixed layers, and summer with more subsurface-intensified eddies and shallower mixed layers. Because the detection thresholds are calibrated on altimetry-detectable eddies, the fraction of eddies found without a surface altimetric signature represents a lower bound on what altimetry misses. By resolving the vertical structure that altimetry cannot see, our approach reveals that a large fraction of South Atlantic eddies are subsurface-intensified, and that eddies detected at the surface in the Cape Basin systematically subduct into the ocean interior along their trajectories. Recovering eddies at depth after they lose their surface signature demonstrates that they have not dissipated but continue to propagate, transporting their trapped water masses and properties far from their generation site. This surface-to-subsurface transition, invisible to satellite altimetry, has direct implications for the subsurface redistribution of heat, salt and tracers across the South Atlantic.