Baptiste Mourre

Vertical velocities (w) are essential for understanding upper-ocean tracer transport, but remain difficult to observe and quantify directly. Here we exploit the synergy between high-resolution Sea Surface Height observations from the Surface Water and Ocean Topography (SWOT) mission and in-situ glider density profiles to reconstruct the three dimensional density, geostrophic velocity, and quasigeostrophic vertical velocity fields of an intrathermocline eddy in the western Mediterranean Sea. The reconstruction is independently validated against ship-based CTD profiles and shipboard Acoustic Doppler Current Profiler (ADCP) velocities and against an Observing System Simulation Experiment based on a regional numerical model. At 300 m depth, the eddy’s geostrophic flow reaches Rossby numbers of |Ro| ∼ 0.5, and the diagnosed w field exhibits a multipolar pattern of upwelling and downwelling cells at the eddy periphery, up to ±40 m/day. An equivalent reconstruction from DUACS conventional gridded altimetry yields |Ro| ≲ 0.2 and w ∼ ±5 m/day. A natural-coordinate decomposition of the frontogenetic Q-vector forcing identifies curvature-induced frontogenesis as the dominant organizer of the diagnosed vertical motion. Independent glider measurements of dissolved oxygen and spice show an antisymmetric arrangement across the eddy flanks consistent with the diagnosed circulation. Combining SWOT altimetry with targeted in-situ profiling in this way allows us to resolve the three-dimensional dynamics of small-scale ocean features.

The wide-swath altimeter SWOT provides unprecedented two-dimensional sea-level observations, whose ability to capture upper-ocean dynamics requires assessment. The dynamically balanced signal and noise contributions in SWOT-KaRIn Level-3 (L3) sea level products are here originally quantified and contrasted with those from Level-4 (L4) gridded nadir-only products, combining sea level data, 137 trajectories from drifters deployed in the Western Mediterranean, ERA5 winds, and the framework of Demol et al. (2025). The filtered L3-2km product, or interestingly the unfiltered L3-2km product with a 25 km Gaussian filter, offers the best compromise for fine-scale studies, though residual noise still accounts for about one-third of total variance. L4 products contain less balanced signal but are noise-free and better suited for large-scale analyses. SWOT KaRIn adds value mainly at scales smaller than ∼100 km and shorter than ∼10 days. This studies provides a benchmark for global sea-level assessments.

Near‐inertial waves (NIWs) are an important source of turbulence for the ocean interior. Mesoscale anticyclonic eddies are known to facilitate their propagation at depth while trapping them. However, in situ observations have so far focused on large ( km radius), energetic eddies, whereas most of the ocean is populated by smaller, moderately energetic fine‐scale structures. Are these smaller structures efficient to trap NIWs and enhance turbulence? Here, we present in situ observations from the BioSWOT‐Med 2023 cruise addressing this issue by surveying a fine‐scale frontal area of the North Balearic front in the Mediterranean Sea, assisted by the first high‐resolution Sea Surface Height images of the new Surface Water and Ocean Topography (SWOT) satellite mission during its Calibration/Validation phase. We explore how fine scales modulate the evolution of turbulence below the mixed layer after experiencing two consecutive strong wind events. We show that turbulence remains low in the front and its cyclonic side, while being greatly enhanced in the anticyclonic side. The latter side is dominated by a fine‐scale anticyclone (12.8 km of radius, Rossby number of 0.5) that trapped NIWs, increasing turbulent dissipation level to several W . The NIW‐induced vertical kinetic energy flux reach up to 5.1 mW below the pycnocline and represent % of the wind power input into inertial motions, higher or similar to previous estimations outside and inside mesoscale anticyclones. Future work is needed to investigate whether these results extend to fine scales elsewhere in the world ocean, especially in regions with larger baroclinic Rossby radius of deformation.

Coastal mesoscale eddies, which are essential for the redistribution of heat, nutrients, and momentum, often propagate along the coastline. Monitoring the propagation of these coastal eddies requires high spatio-temporal resolution, which has made their observation particularly challenging using conventional gridded altimetry. The initial fast-sampling phase of the Surface Water and Ocean Topography (SWOT) wide-swath mission offers an unprecedented opportunity to resolve these features at fine spatio-temporal scales. This study analyzes the alongshore propagation of a coastal anticyclonic eddy in the Algerian Basin, comparing the capability of SWOT with that of conventional altimetry. SWOT measurements better capture the eddy’s structure, deformation and trajectory, particularly during its interaction with coastal topography, leading to refined estimates of the eddy’s alongshore propagation speed. These results confirm SWOT’s added value in observing coastal mesoscale dynamics, and highlight new opportunities for understanding the interaction between small mesoscale eddies and coastal topography.

Mesoscale and submesoscale features with Rossby and Richardson numbers near unity indicate a breakdown of geostrophic balance. This gives rise to ageostrophic flows that drive circulation across density gradients and produce vertical motions, transporting heat and biogeochemical tracers below the mixed layer. During winter 2022, high-resolution multiplatform in situ observations and realistic numerical simulations captured the evolution of mesoscale and submesoscale features in the northwestern Mediterranean Sea. A mesoscale front in the Balearic Sea was observed progressing from intensification to decay, culminating in the formation of two submesoscale cyclonic frontal eddies (SCEs). These formed as the front elongated and interacted with a mesoscale ridge, illustrating the dynamic interplay between mesoscale and submesoscale processes. The front intensified due to strain-induced frontogenesis. A strong down-front wind event triggered submesoscale instabilities and the nonlinear Ekman effect, enhancing vertical motions through an ageostrophic secondary circulation and contributing to restratification. As the front weakened, isopycnal slopes flattened, and energy cascaded toward smaller scales, forming the SCEs. This energy transfer was primarily driven by submesoscale instabilities, with additional contributions from centrifugal and gravitational instabilities. A Lagrangian analysis revealed that horizontal parcel transport was dominated by mesoscale circulation, while vertical displacements were controlled by submesoscale processes. The evolving SCEs exhibited a three-dimensional helical-spiral recirculation pattern, promoting vertical transport. Submesoscale eddy-induced frontogenesis drove subduction into the mixed layer, intensified by submesoscale instabilities and guided by downward-sloping isopycnal surfaces at the eddy periphery.

Ocean currents are crucial in regulating Earth’s climate, with a significant impact in the distribution of ocean properties. During the Calibration/Validation phase of the Surface Water and Ocean Topography (SWOT) satellite mission, we performed a high-resolution, multi-platform experiment to evaluate SWOT’s ability to resolve small-scale features, focusing on a ~25 km-radius anticyclonic eddy in the Western Mediterranean Sea. Acoustic Doppler Current Profiler (ADCP) recorded maximum velocities of 30 cm/s at 155 m depth and underwater glider data identified biconvex isopycnals, classifying the eddy as intrathermocline. SWOT successfully captured the sea level signal and surface geostrophic currents of the eddy, showing notable error reduction over conventional altimetry: 24% in sea level representation compared to glider observations, and 35% and 31% in horizontal velocity magnitude compared to ADCP and drifter measurements, respectively. This study highlights SWOT’s potential in resolving small-scale ocean dynamics.

Near-inertial waves (NIWs) are an important source of turbulence for the ocean interior. Mesoscale anticyclonic eddies are known to facilitate their propagation at depth while trapping them. However, in-situ observations have so far focused on large (>50 km radius), energetic eddies, whereas most of the ocean is populated by smaller, moderately energetic fine-scale structures. Are these smaller structures efficient to trap NIWs and enhance turbulence? Here, we present in-situ observations from the BioSWOT-Med 2023 cruise addressing this issue by surveying a fine-scale frontal area of the North Balearic front, assisted by the first high-resolution Sea Surface Height images of the new SWOT satellite mission during its Calibration/validation phase. We explore how fine scales modulate the evolution of turbulence below the mixed layer after experiencing two consecutive strong wind events. We show that turbulence remains low in the front and its cyclonic side, while being greatly enhanced in the anticyclonic side. The latter side is dominated by a fine-scale anticyclone (12.8 km of radius, Rossby number of 0.5) that trapped NIWs down to 300 m depth, increasing turbulent dissipation level to several 10-8 W/kg. Estimations of the NIW-induced vertical kinetic energy flux are about 1 to 10 times stronger than previous estimations outside and inside mesoscale anticyclones (5-10 mW/m2 vs 0.5-10 mW/m2). These results suggest that moderately energetic fine-scale structures are as important to structure flows and turbulence as large and intense structures found in western boundary currents and upwelling systems, addressing new challenges for their parameterization in Earth system models.

The FaSt-SWOT sea trial experiments, conducted in the Balearic Sea (Western Mediterranean Sea) between 25-28 April and 7-10 May 2023, aimed at collecting multi-platform in-situ observations of meso- and submesoscale ocean structures in the area covered by the SWOT satellite during its initial fast-sampling phase. The general objectives of the FaSt-SWOT project are twofold: 1) participate with these data to the satellite cal/val activities, and 2) improve the characterization and understanding of the fine-scale dynamics by combining in-situ multi-platform and satellite data with high-resolution numerical models and machine-learning-based computational techniques. The experiments consisted in 2 phases both using multi-scale ship-based instruments (CTD, Moving Vessel Profiler, thermosalinograph, ADCP and GoPros), autonomous platforms (surface drifters and gliders), and satellite observations (SST, ocean color and altimetry). In addition, 2km-resolution data-assimilative modelling simulations were produced to provide a complementary view of the fine-scale ocean variability. Finally, machine-learning-based optimization algorithms were also tested to define adaptive sampling strategies during the experiment. The sampling first focused on a 20km-diameter anticyclonic eddy detected under the swath of the satellite thanks to satellite imagery and drifter trajectories. Several cross-sections of the Moving Vessel Profiler and underwater gliders provided insights into the vertical structure of temperature and salinity fields and the associated signals in chlorophyll and dissolved oxygen. Two gliders were programmed to perform back-and-forth sections during a 3-week time with a 1-day delay between them, allowing to evaluate the temporal variability of the ocean fields at the period of repetitivity of the satellite. The second phase started 9 days after the end of the first one. A 48-hour dense radiator-like pattern was performed by R/V SOCIB, allowing to characterize the evolution of the small eddy observed during the first leg. A total of 45 surface drifters were deployed during the two phases to evaluate in-situ surface currents and their associated convergence and divergence in the vicinity of the eddy. While conventional altimetry was not able to properly represent the sea level signature of the observed eddy, initial SWOT measurements indicate an improved detection capability by the new satellite. In addition, high-resolution numerical simulations reproduce a small anticyclonic eddy with similar characteristics as that of the observed eddy. These simulations are used to provide a more general understanding of the situation, indicate the origin of the eddy in the frontal area between recent and modified Atlantic waters, and provide insights into the vertical extension of the small mesoscale structure. We provide here an overview of the whole FaSt-SWOT dataset, including both observing and modelling components. A more detailed analysis of the measurements is provided in a companion presentation.

Coastal habitats are amongst the most dynamic on Earth, due to their simultaneous exposure to terrestrial, oceanic and atmospheric processes. Coastal taxa are therefore often ecologically specialised and adapted to withstand frequent shifts in sea level, wave exposure, temperature or salinity. This specialisation often resulting in significant cryptic diversity. Previous molecular studies have suggested that genetic differentiation in non-marine coastal organisms may be influenced by oceanic currents and fronts, but the extent to which such processes affect dispersal and evolution of such taxa remains unclear. Here we explore whether population genetic structure in two supralittoral rockpool beetle species (genus Ochthebius) can be predicted from the general circulation pattern of the marine currents and associated oceanic fronts. We simulated dispersal using a Lagrangian particle tracking model and compared this with population genetic structure inferred from COI (mitochondrial) and wingless (nuclear) genes applying linear models and Mantel tests. We show that a biophysical model based on oceanic currents and fronts in the western Mediterranean Sea is a much better predictor of observed population genetic structure than isolation by distance in both species. Our results show that oceanic processes, besides shaping contemporary population connectivity in fully marine organisms, also exert a meaningful influence on terrestrially-derived coastal taxa such as supralittoral rockpool beetles — the first time this mode of dispersal has been demonstrated in an insect.