SWOT Reveals Fine-Scale Balanced Motions Driving Near-Surface Currents and Dispersion in the Antarctic Circumpolar Current

Abstract

The Antarctic Circumpolar Current (ACC) is a hotspot for the generation of small-scale motions that have a key role in cross-frontal exchanges. We present the first analysis of surface currents in the ACC derived from high-resolution sea surface height (SSH) fields provided by the new Surface Water and Ocean Topography (SWOT) satellite. To mitigate the impact of noise and unbalanced SSH, we introduce a two-dimensional fitting kernel method for deriving geostrophic and cyclogeostrophic velocities at different lengthscales. These velocity estimates are evaluated against the low-pass filtered component ($>$1 day) of trajectories from 21 surface drifters that passed through the ACC meander. The SSH is found to be balanced and appropriate for inferring surface velocities at scales as small as 10 km, with an 18 km length scale identified as a trade-off between suppressing residual unbalanced waves and preserving finer-scale balanced signals in SWOT denoised SSH. At this scale, the geostrophic approximation becomes inaccurate, and higher-order terms in the momentum balance contribute up to 20% of the observed drifter velocities. Finally, distance-averaged pair statistics calculated from drifter pairs and virtual particles reveal that SWOT accurately captures dispersion properties over the 10–200 km range, providing observational evidence of the dominant role of balanced motions in particle dispersion within this range. By capturing balanced dynamics with unprecedented accuracy, SWOT SSH offers new opportunities to understand the impact of small scales on tracer exchange in the ACC and the Southern Ocean more broadly.