Solange Coadou-Chaventon

The Surface Water and Ocean Topography (SWOT) satellite provides a groundbreaking view of sea surface height across a 120 km-wide swath, opening new opportunities to reconstruct vertical velocities (w). Vertical motions play a central role in redistributing properties, influencing climate and ecosystems. Using the effective surface quasigeostrophic framework in the eddy-rich Agulhas Current Retroflection region, we reconstruct w down to 1,000 m. We assess its realism against the 1/60° (~2 km) INALT60 ocean model, finding a spatial correlation of 0.6. SWOT-derived w reveal pronounced high-frequency variability, with events lasting several days to one week during which w variance more than doubles. The Agulhas Retroflection emerges as a hotspot of enhanced w (~300 m day-1), driven by strong horizontal strain. This work provides the first quantitative reconstruction of w from SWOT in the region and highlights the mission’s potential to quantify vertical exchanges linking the ocean surface and interior.

As a cutting‐edge altimetry product, the Surface Water and Ocean Topography (SWOT) satellite observations require validation against in situ measurements and existing satellite data sets. During the fast‐repeat phase of the mission, daily SWOT altimetry retrievals capture submesoscale ocean eddies (30 km) amidst larger mesoscale (100–200 km) structures in the dynamic Agulhas Current Retroflection region. Our results reveal that SWOT significantly enhances the resolution of the Agulhas Current's frontal features, producing sea surface height gradients that are 28% sharper compared to conventional altimetry products. Comparisons with in situ velocity observations from an underwater glider and surface drifters, further demonstrate SWOT's unparalleled capability to detect the strongest velocities and velocity gradients. These findings mark a pivotal step forward in resolving fine‐scale ocean circulation from satellite altimetry, with promising implications for unveiling horizontal and vertical dynamics in western boundary currents and beyond.

As a cutting-edge altimetry product, the Surface Water and Ocean Topography (SWOT) satellite observations require validation against in situ measurements and existing satellite datasets. During the fast-repeat phase of the mission, daily SWOT altimetry retrievals capture submesoscale ocean eddies (~30 km) amidst larger mesoscale (~100–200 km) structures in the dynamic Agulhas Current Retroflection region. Our results reveal that SWOT significantly enhances the resolution of the Agulhas Current’s frontal features, producing sea surface height gradients that are 20% sharper compared to conventional altimetry products. Comparisons with in situ velocity observations from an underwater glider and surface drifters, further demonstrate SWOT’s unparalleled capability to detect the strongest velocities and velocity gradients. These findings mark a pivotal step forward in resolving fine-scale ocean circulation from satellite altimetry, with promising implications for unveiling horizontal and vertical dynamics in western boundary currents and beyond.

Upper ocean fronts are dynamically active features of the global ocean playing a key role in the air‐sea exchanges of properties and their transport in the ocean interior. With scales ranging from the submesoscale (0.1–10 km) to the mesoscale (10–100s km) and a temporal variability from hours to months, collecting in situ observations of these structures is challenging and this has limited our understanding of their associated processes and impacts. During the EUREC4A‐OA/ATOMIC field experiment, which took place in the northwest tropical Atlantic in January–February 2020, a large number of uncrewed platforms, including five Saildrones, were deployed to provide a detailed picture of the upper‐ocean fine‐scale variability. This region is strongly influenced by the outflow of the Amazon River, even in winter, which is the minimum outflow season. Here, the generation of fine‐scale horizontal thermohaline gradients is driven by the stirring of this freshwater river input by large anticyclonic eddies, the so‐called North Brazil Current Rings. Vertical shear estimates using the Saildrones ADCP show that partial temperature compensation occurs along restratifying submesoscale salinity‐dominated fronts. The distribution of surface along‐track gradients, as sampled by different horizontal length‐scales, reveals the prevalence of submesoscale fronts. This is supported by a flattening of the spectral slopes of surface density at the submesoscale. This study emphasizes the need to resolve the upper ocean at high spatial resolution to understand its impact on the broader circulation and to properly represent air‐sea interactions.

Upper-ocean fronts are dynamically active features of the global ocean that have significant implications for air-sea interactions, vertical mass and heat transfers, stratification and phytoplankton production and export. They have a large range of spatial scales from submesoscale (1 – 10 km) to mesoscale (10 – 100s km) characterized by temporal variability from days to months. The small dimensions and short duration of these structures have limited our capacity of observing, modelling and understanding fully these processes and their impact.The EUREC4A-OA/ATOMIC field experiment, that took place during January-February 2020 in the Northwest Tropical Atlantic, has tried to address this challenge. In particular, five Saildrones, which are uncrewed platforms instrumented to measure the air-sea interface, have been deployed. This region showed to be a well-suited laboratory to investigate horizontal density surface gradients over a wide range of scales. Strongly affected by the outflow of the Amazon River, the generation of fine-scale horizontal thermohaline gradients is favored by the stirring of this freshwater input by large anticyclonic eddies (a.k.a. North Brazil Current Rings). The distribution of these frontal structures highlights the presence of very intense gradients, including at the smaller spatial scales. The coherence of temperature and salinity fronts was estimated by a wavelet transform analysis. It reveals that large-scale density fronts are primarily controlled by horizontal variations in salinity but with increasing temperature-salinity coherence at the small scales range of the spectrum (O (0.1 km)) for strong gradients whereas they are poorly correlated for weaker fronts.Our study shows that processes such as the mixed layer depth, the diurnal cycle, and air-sea exchanges are strongly affected by these small-scale frontal regimes. The parallel and quasi synchronous tracks of a 4-Saildrone formation provide a detailed picture of the upper ocean vorticity, divergence, and strain from their ADCP current measurements. Overall the methodology that has been developed could be extended on other datasets in order to assess the phenomenology of fine-scale structures in other dynamical regions.

The mixed layer is the uppermost layer of the ocean, connecting the atmosphere to the subsurface ocean through atmospheric fluxes. It is subject to pronounced seasonal variations: it deepens in winter due to buoyancy loss and shallows in spring while heat flux increases and restratifies the water column. A mixed-layer depth (MLD) modulation over this seasonal cycle has been observed within mesoscale eddies. Taking advantage of the numerous Argo floats deployed and trapped within large Mediterranean anticyclones over the last decades, we reveal for the first time this modulation at a 10 d temporal scale, free of the smoothing effect of composite approaches. The analysis of 16 continuous MLD time series inside 13 long-lived anticyclones at a fine temporal scale brings to light the importance of the eddy pre-existing vertical structure in setting the MLD modulation by mesoscale eddies. Extreme MLD anomalies of up to 330 m are observed when the winter mixed layer connects with a pre-existing subsurface anticyclonic core, greatly accelerating mixed-layer deepening. The winter MLD sometimes does not achieve such connection but homogenizes another subsurface layer, then forming a multi-core anticyclone with spring restratification. An MLD restratification delay is always observed, reaching more than 2 months in 3 out the 16 MLD time series. The water column starts to restratify outside anticyclones, while the mixed layer keeps deepening and cooling at the eddy core for a longer time. These new elements provide new keys for understanding anticyclone vertical-structure formation and evolution.