Diagnosing Submesoscale Divergence From Sea Surface Height

Uncovering Ocean Mixed Layer Dynamics From the Sea Surface State

Submesoscale dynamics strongly influence the upper ocean, regulating mixing, air–sea exchange, and vertical heat transport. The recent Surface Water and Ocean Topography mission provides unprecedented high‐resolution observations of sea surface height (SSH), yet linking these surface measurements to subsurface ocean dynamics remains challenging. We develop a theoretical framework for diagnosing key mixed layer (ML) properties from surface‐observable states. We show that horizontal density anomalies induced by mixed layer eddies produce surface imprints that can be effectively captured by spatially filtered SSH. The filtered SSH is integrated into the ML Eddy parameterization to infer the effects of submesoscale restratification. A potential energy budget accounting for the mixing–restratification competition in the ML is diagnosed from surface buoyancy flux, wind stress, and the SSH gradient, enabling reconstruction of the mixed‐layer depth. Vertical eddy heat flux can be further reconstructed from the SSH gradient. This framework offers a promising approach for diagnosing interior submesoscale processes using surface observations.

Estimating the Geostrophic–Ageostrophic Transition Scale in Ocean Currents From Wind Generated Waves

SWOT resolves sea surface height at spatial scales where geostrophic balance may break down and ageostrophic motions become significant, making identification of the transition scale essential. We test whether wind generated waves provide an independent diagnostic of this transition. WAVEWATCH III was forced with high resolution surface currents decomposed into geostrophic and ageostrophic components for two contrasting regimes: an energetic Brazil Current region and a calmer South Atlantic interior. Wavenumber spectra of significant wave height yield crossover scales consistent with those inferred from current KE spectra. We also examine the wave action flux after decomposition into rotational and divergent transport components. In the Brazil Current, the flux crossover aligns with the KE transition scale, whereas coherent swell weakens the correspondence in the interior South Atlantic.

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.

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.

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

Utilizing two-dimensional sea level anomalies from the Surface Water and Ocean Topography (SWOT) wide-swath altimeter, we conduct a global census of fine-scale eddies (with equivalent radii < 30 km) that are beyond the resolution capacity of traditional altimeters. We find that these eddies are concentrated in regions with energetic mesoscale currents such as western boundary current extensions, the Antarctic Circumpolar Current and prominent interior frontal zones. While these eddies are smaller than classical mesoscale eddies (O(100) km), they exhibit dynamics distinct from the mesoscale, including a pronounced winter peak in occurrence frequency and a striking dominance of cyclonic over anticyclonic polarities. A global composite reveals a net enhancing effect of these fine-scale eddies on surface chlorophyll-a (approximately +7% within eddy cores relative to surrounding waters). Our findings shed light on the dynamical characteristics and surface chlorophyll-a signatures of fine-scale ocean eddies, necessitating their adequate representation in next-generation ocean models.

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