Fine-Scale Eddies Revealed by the Surface Water and Ocean Topography (SWOT) Mission

A New Approach to Estimate Ocean Surface Velocity from High-Resolution Surface Water and Ocean Topography (SWOT) Data

This study presents a novel cyclostrophic balance correction method for estimating submesoscale ocean surface currents in the Northern Arabian Sea using surface water and ocean topography (SWOT) mission altimetry. High-resolution Ocean Color Monitor (OCM-3) data from the EOS-06 satellite reveal fine-scale eddies and filaments with high chlorophyll-a concentrations (>0.4 mg m−3), spatially coherent with geostrophic current patterns from SWOT. At these scales, the geostrophic assumption is invalid; therefore, we introduce a curvature-based cyclostrophic correction that accounts for enhanced centripetal accelerations. Validation against high-resolution model simulations shows that our approach is in better agreement with model outputs than uncorrected and previously published corrected fields, particularly in regions with strong vorticity and strain. When applied to SWOT data, the corrected velocities demonstrate spatial correspondence with chlorophyll patterns and suppress spurious gradients. Probability density functions of normalized vorticity and strain also match theoretical expectations, emphasizing the potential of SWOT for advancing submesoscale ocean dynamics.

Enhancing SWOT Geoid Resolution in Regions Characterized by Complex Oceanic Dynamics via the mrCOSTS Method

The advanced 2-D sea surface height (SSH) observations from the surface water and ocean topography (SWOT) satellite have achieved a global average geoid resolution better than 13 km, as estimated from spectral analysis of adjacent single-cycle observations. However, in regions with complex oceanic dynamics (the ocean off southwestern Argentina and east of Japan), SWOT observations show significant temporal variability across cycles. This variability greatly hampers the satellite’s ability to detect and precisely measure steady-state marine geoid signals. Even with multicycle data stacking, resolution remains poor in some regions, sometimes underperforming conventional satellite altimetry. To solve this issue, this study employs a multiresolution coherent spatio-temporal scale separation (mrCOSTS) method to analyze sea surface height anomaly (SSHA) data from 26 SWOT cycles, aiming to isolate stable signal components and reconstruct the geoid. The approach notably improves geoid resolution in regions with intricate ocean features, with the maximum enhancement reaching up to 76%. This resolution reflects the spatial scales of features reproducibly resolved by two independent SWOT geoid estimates, rather than a direct improvement in absolute marine geoid accuracy. The method effectively suppresses ocean dynamic signals while preserving the static oceanic component, and it also contributes to improving the recovery of the marine gravity field in regions characterized by complex ocean dynamics. Furthermore, in regions with relatively high resolution, the method achieves additional subtle improvements, which are of considerable significance for obtaining a stable static field.

Assessing Dynamical Contributions to SWOT Sea Surface Height in an Internal Tide Hotspot

The Surface Water and Ocean Topography (SWOT) mission has revealed high‐frequency sea surface height anomaly (SSHA) variability associated with internal gravity waves, yet comparison with in situ observations remains limited. Here, we analyze a mooring array beneath the swaths of SWOT Cal/Val south of New Caledonia, an internal tide (IT) hotspot in the southwestern tropical Pacific. SWOT agrees closely with mooring‐derived SSHA, exhibiting three times larger variance than gridded SSHA from nadir altimetry. This excess variance is mainly driven by coherent ITs, explicitly extracted from SWOT swaths, with amplitudes exceeding 10 cm. Comparison with mooring‐based estimates demonstrates centimeter‐level accuracy in the retrieved IT signal. SWOT‐derived ITs reveal substantial errors in empirical IT atlases, routinely used to correct SWOT. Particularly, absolute (relative) errors in SSHA‐derived current velocities exceed 15 cm s−1 ${\mathrm{s}}^{-1}$ (80%) in 10% of cases during SWOT Cal/Val, calling for improved methods to separate wave‐like and balanced motions.

Mesoscale Eddies Reorganize Subsurface Oxygen Minimum Layer in the Subtropical North Atlantic

Mesoscale eddies play a key role in regulating oceanic physical and biogeochemical environments. However, their influence on vertical oxygen distribution remains poorly constrained by observations. Using 32 years (1993–2024) of in situ oxygen observations from the Bermuda Atlantic Time‐series Study site combined with satellite‐tracked eddies, we show that eddy‐induced oxygen anomalies are substantially stronger in the mesopelagic than at the surface. Anticyclonic eddies deepen the subsurface oxygen minimum layer (OML) by ∼100 m and reduce its thickness by ∼30%, with increases of ∼3%–6% in mean and minimum oxygen. Cyclonic eddies induce a similar but weaker thinning, accompanied by an upward displacement of about 100 m. More than 80% of the variance is explained by isopycnal heaving, with additional contributions from biogeochemical processes. Our results underscore that both eddy types systematically alleviate low‐oxygen conditions within the OML, with important implications for subsurface oxygen habitat variability.

Observed poleward heat transport by mesoscale eddies in the deep Southern Ocean

Mesoscale eddies in the Southern Ocean extend from the surface to the deep ocean and contribute to regional and global ocean heat budgets. However, sparse in situ observations have limited estimates of eddy meridional heat transport (EMHT) to the near surface, leaving deep-ocean processes poorly understood. Here we quantify deep eddy kinetic energy (EKE) and EMHT using Argo observations. We find that deep EMHT reaches at least the same order of magnitude as surface transport, despite deep EKE being only one-fifth of surface levels. Float observations and sensitivity experiments reveal that, relative to mean-flow transport alone, deep EMHT extends the meridional movement of subtropical warm waters towards Antarctica by over ten degrees of latitude. About 60% of this deep transport originates from the Indian Ocean sector. These findings provide observational evidence that mesoscale eddies can modulate deep Southern Ocean heat redistribution, with potential implications for Antarctic climate.

Wide-swath satellite altimetry and novel subsurface temperature observations improve predictions in a dynamic western boundary current: System optimization and performance