Benoit Legresy (Commonwealth Scientific and Industrial Research Organisation, TAS, Australia); Annie Foppert (Australian Antarctic Program Partnership, University of Tasmania, TAS, Australia); Beatriz Peña Molino (Commonwealth Scientific and Industrial Research Organisation, TAS, Australia); Helen Phillips (University of Tasmania, TAS, Australia)

The Southern Ocean and its main feature, the Antarctic Circumpolar Current (ACC), plays a key role in heat and carbon exchanges. Recent studies have emphasized the role of smaller-scale processes in tracer exchanges within the ACC. The new SWOT satellite enhances the capability of observing ocean dynamics down to a 15 km wavelength and will help understand the role of smaller-scale features in redistributing tracers in the ocean. SWOT data are affected by instrumental noise and unbalanced components in the observed SSH field, affecting geostrophic velocity estimation.

In this presentation, we assess the velocities derived from high-resolution SSH measurements from the SWOT satellite using Lagrangian drifters deployed during the FOCUS SWOT-validation voyage in an energetic meander of the ACC. We show that SWOT’s ability to resolve sharper SSH gradients results in more intense and accurate velocity estimates that match remarkably with in-situ observations. In the region, we find that SWOT SSH is valid to infer balanced dynamics at scales as small as 15 km, with a 25 km length scale found to be a trade-off between suppressing errors due to unbalanced SSH while preserving fine-scale balanced motions in the region. At these scales, geostrophic balance alone becomes insufficient, and nonlinear terms in the momentum balance make a significant contribution to the drifter velocities. Pair-statistics computed from drifters and virtual particles further reveal that SWOT-derived velocities accurately capture dispersion properties over the 5-200 km scale, comparing well and unprecedently to those calculated from drifters, observational evidence that balanced motions are dominant in driving dispersion at these scales.