SWOT reveals fine-scale balanced motions and dispersion properties in an energetic meander of the Antarctic Circumpolar Current

The Southern Ocean (SO), dominated by the Antarctic Circumpolar Current (ACC), plays a key role in the global uptake and transport of heat and carbon. Interactions between the ACC and topographic features form standing meanders, hotspots of small-scale motions that enhance cross-frontal exchanges. In-situ observations are challenging and remain sparse in the SO, and hence progress in understanding the dynamics of eddies and their role in tracer exchanges readily relies on satellite altimetry data. An accurate representation of velocity and kinetic energy toward the smaller scales is thus needed to better understand the Southern Ocean's role in the global climate system. 

Launched in December 2022, the SWOT (Surface Water and Ocean Topography) satellite mission offers an unprecedented view of ocean dynamics at scales down to 15 km.  Ocean currents and kinetic energy budget are typically inferred by applying geostrophic balance to sea surface height (SSH) observations. However, at the small spatial scales resolved by SWOT, this balance may not hold anymore or shift to higher-order equilibrium, and validation steps are crucial before exploiting these observations in climatic studies. Using surface drifters deployed during the SWOT validation campaign FOCUS, we present the first analysis of velocities and dispersion derived from SWOT SSH, in an energetic meander of the ACC.

Introducing a fitting kernel method tailored to derive velocities from SWOT observations, we show that SWOT SSH remain primarily balanced and valid for inferring surface velocities at scales as small as 10 km in this region. At these scales, geostrophic balance alone becomes insufficient and leads to a 10-20% bias compared to drifter velocities in cyclonic eddies, which is effectively corrected by applying cyclogeostrophy to SWOT SSH. Then, we compute distance-averaged pair statistics from real drifter pairs and virtual particles and show that SWOT accurately captures dispersion properties over the 5-200 km range, unveiling distinct dispersion patterns between large and small separation scales. This suggests that balanced dynamics resolved by SWOT are still the main driver for ocean dispersion in this range. By capturing balanced dynamics with unprecedented accuracy, SWOT offers new opportunities to understand the impact of small scales on tracer exchange and better quantify the transport of heat and carbon in the Southern Ocean.