The BioSWOT-Med cruise was carried out in the northwestern Mediterranean Sea during the fast-sampling phase of the satellite SWOT mission (fixed tracks were revisited every 24h), contributing to the international efforts of studying the oceanic submesoscale to mesoscale (1 to 100 km) dynamics. This region is an ideal natural laboratory for fine-scale biogeochemistry as its dynamics sustains strong contrasts associated with high production in the north (Gulf of Lions) and oligotrophic conditions with moderate energy in the south. The BioSWOT-Med cruise used an adaptive and Lagrangian sampling strategy, determined from near-real time satellite observations analysis, combining in-situ shipborne measurements with drifters, ocean gliders and Biogeochemical (BGC)-Argo floats, to achieve high spatio-temporal resolution multidisciplinary measurements within SWOT swaths. In this work, we present the preliminary outcomes derived by autonomous platforms (BGC-Argo floats, gliders, and surface drifters) with high-frequency sampling. Drifters combined with SWOT observations have been able to disentangle distinct features such as frontal zones, cyclones, anticyclones, and filaments. BGC-Argo floats collected measurements within an anticyclonic eddy visible in SWOT images but unresolved by conventional altimetry, and within a meander of the North Balearic front. The frontal area is influenced by saltier and colder Atlantic origin Water to the north and younger Atlantic Water to the south, resulting in a strong horizontal salinity gradient (~0.4 PSU). Chlorophyll concentrations co-varied with these frontal features and showed remarkable variations both at the surface and in the Deep Chlorophyll Maximum, with strong vertical gradients. As revealed by gliders measurements, the anticyclonic eddy, located south of the front, was characterised by fresher, warmer, and less productive Atlantic Water, and exhibited a distinct horizontal gradient compared with waters north of the front. Using both optical imaging and dual-frequency active acoustics, Zooglider revealed a marked change in the community of grazing zooplankton in the eddy center. The BGC-Argo floats equipped with chlorophyll and backscatter sensors also allowed estimating ocean productivity in terms of Net Community Production (NCP) which provided new insights about the link between ocean production and physical processes. This study demonstrates the importance of performing a synergic approach combining unprecedented high resolution satellite observations from SWOT and autonomous platforms (BGC-Argo floats, gliders, and drifters) to resolve the biological and physical interactions at fine scales.

The SWOT satellite measures sea surface height at an unprecedented resolution about ten times better than conventional altimetry products. SWOT data offer a unique opportunity of observing very fine-scale (~few km) surface dynamics from space. In situ samplings, aside from complementing the 2D picture, also provide a 3D view of the observed fine scale dynamics essential for interpretation of bio-physical interaction processes. Nevertheless, exploring this regime during field experiments remains challenging due to the difficulty to precisely locate fine-scale features in real time. A shift of a few km may not be of critical importance when sampling a large structure such as an eddy with a radius of about 100 km. However a similar sampling error could obviously lead to severe misinterpretations in the case of a 10 to 20 km wide eddy. The problem is even exacerbated by the fact that the lifetime typically decreases with the size of eddies and filaments. One way to address this problem with field experiments at the SWOT scales is therefore to update and adapt the sampling location and shape, with synoptic near-real time information of the sea state provided by available high resolution remote sensing (SST and Chlorophyll), and analysis of altimetry and model assimilation. Although vulnerable to cloud coverage and/or limited in resolution, this information can be complemented by near-real time Lagrangian analysis of the surface geostrophic fields providing finer diagnostics of the sampling site dynamics. Early SWOT data also filled some gaps in terms of parameters and spatiotemporal coverage. By using the BIOSWOT-Med cruise as an example, here we review the tools offered by the SWOT AdAC Consortium to the field experiments that have been deployed during the SWOT fast-sampling phase (March-June 2023). After evaluating synergies and shortcomings with in situ platforms, we will discuss how adaptive sampling strategies may evolve in the future to assist field experiments during the SWOT Science phase.

The BioSWOT-MED cruise (https://doi.org/10.17600/18002392) was designed to study the bio-physical coupling in the region of the North Balearic Front in the Western Mediterranean Sea, an area of moderate fine scale energy level. The cruise took place during the SWOT fast-sampling phase in April-May 2023 thus providing a unique opportunity to study the daily evolution of mesoscale eddies and fine scale structures as inferred from SWOT in combination with high frequency in-situ measurements.In-situ measurements were focused on a fine scale front identified from SWOT altimetry data and Chl-a gradient from remote sensing (Sentinel-3). The front was located at the northern margin of a small mesoscale anticyclonic eddy (~30 km of diameter, too small to be detected by conventional altimetry maps), from a cyclonic area. Preliminary results revealed strong wave-eddy interactions. Two triplets of 24-h Lagrangian stations (the boat is passively advected by the current to follow the water mass sampled) were performed during the cruise with one station at the front and two on both sides. At the end of the cruise, a third 24-h station was conducted in the anticyclonic eddy.Two consecutive wind events (~25–30 kn) before the second and third sets of stations allowed us to observe and characterise the generation of near-inertial waves (NIWs) and their propagation at depth. Whereas NIWs amplitude was uniformly small during the first triplet of stations, contrasted NIWs amplitudes were observed after the wind events. A remarkable intensification of NIWs in the anticyclonic eddy was observed at the last station with amplitudes reaching up to ~0.4 m/s down to ~300 m, in strong contrast with weak NIW amplitudes in the frontal and the cyclonic area. An inertial chimney trapping NIWs can be evidenced within the anticyclonic eddy. Vertical Microstructure Profiler measurements showed that those trapped NIWs significantly enhanced turbulence and mixing activity in the anticyclonic eddy through intense shear generation. Contrasts in vertical nutrients fluxes between the inertial chimney in the anticyclonic eddy, the front and the cyclonic area are finally discussed.

During the SWOT fast sampling phase in spring 2023, the BioSWOTMed cruise (https://doi.org/10.17600/18002392) sampled for four weeks a front located on both the western and eastern swaths of the 003 SWOT pass, about 100 km northeast of Menorca, in the Western Mediterranean Sea. The front coincided to a portion of the north Balearic front. The front was modulated by the presence of eddies of small Rossby radius (of the order of 20-30km), not visible in conventional altimetry maps. The availability of cloud-free images of ocean color (OLCI) for five consecutive days also revealed the presence of various submesoscale structures and part of their life cycle. The front consisted of a strong roughly eastward meandering jet, separating cold, salty and more productive waters (modified Atlantic Water) in the north from warm, fresher and more oligotrophic waters (younger Atlantic Water) in the south. In addition to classical hydrological, glider, drifters/floats and moving vessel profiler (MVP) measurements, 3D oceanic velocities were measured by 3 ship-mounted acoustic Doppler current profilers (ADCPs), 2 lowered-ADCPs, 1 free-falling newer generation 5-beam ADCP and 2 autonomous vertical velocity profilers. The jet had horizontal velocities up to ~0.35 m.s-1 (0-300 m average), with a cross-jet distance of ~40 km and a vertical extension of 200 m. 1m-surface drifters deployed in the core of the jet traveled at a speed of ~0.5 m.s-1. Two northerly storms generated intense near-inertial waves interacting in the mesoscale field of several eddies sampled by the ship. Cyclogeostrophic velocities derived from SWOT are in good agreement with the measured ADCP (horizontal) velocities. The normalized relative vorticity provides a regional view of the complex oceanic circulation of the jet meandering between various mesoscale eddies and interacting with submesoscale structures. The high variability of vertical velocities (+/- 1.5 cm.s-1) of various origins masks the expected cross-frontal ageostrophic circulation. The finescale 3D circulations observed and well captured at the surface by SWOT are also associated with complex biological content distribution.

High-frequency radars (HFRs) provide remote information on ocean surface velocity in extended coastal areas at high resolutions in space (O(km)) and time (O(h)). They directly produce radial velocities (in the radar antenna’s direction) combined to provide total vector velocities in areas covered by at least two radars. HFRs are a key element in ocean observing systems, with several important environmental applications. Here, we provide an assessment of the HFR-TirLig network in the NW Mediterranean Sea, including results from the gap-filling open-boundary modal analysis (OMA) using in situ velocity data from drifters. While the network consists of three radars, only two were active during the assessment experiment, so the test also includes an area where the radial velocities from only one radar system were available. The results, including several metrics, both Eulerian and Lagrangian, and configurations, show that the network performance is very satisfactory and compares well with the previous results in the literature in terms of both the radial and total combined vector velocities where the coverage is adequate, i.e., in the area sampled by two radars. Regarding the OMA results, not only do they perform equally well in the area sampled by the two radars but they also provide results in the area covered by one radar only. Even though obviously deteriorated with respect to the case of adequate coverage, the OMA results can still provide information regarding the velocity structure and speed as well as virtual trajectories, which can be of some use in practical applications. A general discussion on the implications of the results for the potential of remote sensing velocity estimation in terms of HFR network configurations and complementing gap-filling analysis is provided.

The BioSWOT-MED cruise (https://doi.org/10.17600/18002392) was designed to study the bio-physical coupling in the region of the North Balearic Front in the Western Mediterranean Sea, an area of moderate fine scale energy level. The cruise took place during the SWOT fast-sampling phase in April-May 2023 thus providing a unique opportunity to study the daily evolution of mesoscale eddies and fine scale structures as inferred from SWOT in combination with high frequency in-situ measurements.In-situ measurements were focused on a fine scale front identified from SWOT altimetry data and Chl-a gradient from remote sensing (Sentinel-3). The front was located at the northern margin of a small mesoscale anticyclonic eddy (~30 km of diameter, too small to be detected by conventional altimetry maps), from a cyclonic area. Preliminary results revealed strong wave-eddy interactions. Two triplets of 24-h Lagrangian stations (the boat is passively advected by the current to follow the water mass sampled) were performed during the cruise with one station at the front and two on both sides. At the end of the cruise, a third 24-h station was conducted in the anticyclonic eddy.Two consecutive wind events (~25–30 kn) before the second and third sets of stations allowed us to observe and characterise the generation of near-inertial waves (NIWs) and their propagation at depth. Whereas NIWs amplitude was uniformly small during the first triplet of stations, contrasted NIWs amplitudes were observed after the wind events. A remarkable intensification of NIWs in the anticyclonic eddy was observed at the last station with amplitudes reaching up to ~0.4 m/s down to ~300 m, in strong contrast with weak NIW amplitudes in the frontal and the cyclonic area. An inertial chimney trapping NIWs can be evidenced within the anticyclonic eddy. Vertical Microstructure Profiler measurements showed that those trapped NIWs significantly enhanced turbulence and mixing activity in the anticyclonic eddy through intense shear generation. Contrasts in vertical nutrients fluxes between the inertial chimney in the anticyclonic eddy, the front and the cyclonic area are finally discussed.

An array of Lagrangian instruments (more than 100 drifters and a profiling float) were deployed for several days in the coastal waters of the southeastern Ligurian Sea to characterize the near-surface circulation at the submesoscale ( < 10 km). The drifters were trapped in an offshore-flowing filament and a cyclonic eddy that developed at the southwestern extremity of the filament. Drifter velocities are used to estimate differential kinematic properties (DKPs) and the relative dispersion of the near-surface currents on scales as small as 100 m. The maximum drifter speed is similar to 50 cm s (- 1) . The DKPs within the cluster exhibit considerable spatial and temporal variability, with absolute values reaching the order of magnitude of the local inertial frequency. Vorticity prevails in the core of the cyclonic eddy, while strain is dominant at the outer edge of the eddy. Significant convergence was also found in the southwestern flow of the filament. The initial relative dispersion on small scales (100-200 m) is directly related to some of the DKPs (e.g., divergence, strain and instantaneous rate of separation). The mean squared separation distance (MSSD) grows exponentially with time, and the finite-size Lyapunov exponent (FSLE) is independent of scale. After 5-10 h of drift or for initial separations greater than 500 m, the MSSD and FSLE show smaller relative dispersion that decreases slightly with scale.

<p>Supplementary Figure 1. Spearman's rank correlation coefficients (rs) for volume and area density measures, stratified by age.</p>

Vertical transport pathways in the ocean are still only partially understood despite their importance for biogeochemical, pollutant, and climate applications. Detailed measurements of a submesoscale frontal jet in the Alboran Sea (Mediterranean Sea) during a period of highly variable winds were made using cross-frontal velocity, density sections and dense arrays of surface drifters deployed across the front. The measurements show divergences as large as +/-(SIC) implying vertical velocities of order 100 m/day for a asymptotic to 20 m thick surface layer. Over the 20 hr of measurement, the divergences made nearly one complete oscillation, suggesting an important role for near-inertial oscillations. A wind-forced slab model modified by the observed background frontal structure and with initial conditions matched to the data produces divergence oscillations and pattern compatible with that observed. Significant differences, though, are found in terms of mean divergence, with the data showing a prevalence of negative, convergent values. Despite the limitations in data sampling and model uncertainties, this suggests the contribution of other dynamical processes. Turbulent boundary layer processes are discussed, as a contributor to enhance the observed convergent phase. Water mass properties suggest that symmetric instabilities might also be present but do not play a crucial role, while downward stirring along displaced isopycnals is observed.

<p>Supplementary Table 1. Age- and BMI-adjusted linear regression results for the association between participant characteristics and standardized volume and area mammographic density measures. Supplementary Table 2. Spearman's rank correlation coefficients for volume and area density measures, unadjusted and adjusted for age and body mass index.</p>

Abstract. A 2-year dataset of a stand-alone mooring, deployed in November 2020 down the Levante Canyon in the eastern Ligurian Sea, is presented. The Levante Canyon Mooring (LCM) is a deep submarine multidisciplinary observatory positioned at 608 m depth in a key ecosystem area. The Levante Canyon hosts a valuable and vulnerable ecosystem of deep-living cold-water corals (CWCs), studied and monitored since 2013 through integrated mapping of the seabed and water column. The 2-year dataset, acquired on the mooring and presented here (data from November 2020 to October 2022), includes measurements conducted with both current meters and conductivity–temperature–depth (CTD) probes and provides information about the hydrodynamics and thermohaline properties across almost the entire water column. The observatory is still ongoing, and the dataset is regularly updated. All the described data are publicly available from https://doi.org/10.17882/92236 (Borghini et al., 2022). They must therefore be preserved and are of considerable scientific interest.

This repository holds the data used in the study entitled "Inertial oscillations and frontal processes in an Alboran Sea jet: Effects on divergence and vertical transport" which is being submitted to the Journal of Geophysical Research: Oceans (first submission: August 2022). This study reports on the wind response interaction with an ocean current jet in geostrophic balance. The primary objective of the study is to better understand the potential role played by Near Inertial Oscilations (NIOs) in generating vertical transport in the upper ocean. This dataset includes near-surface drifters' tracks (CARTHE and SVP), vessel mounted ADCP (Acoustic Doppler Current Profiler) and underway CTD (Conductivity, Temperature and Depth) as well as data generated by an idealised numerical model (slab-layer type). Details of the files are provided in a README.txt and details on the data processing and analysis are provided in the manuscript to be published.

This chapter provides a concise overview of the entire monograph by assembling summaries of 10 individual chapters starting with a global review of large-scale, persistent nutrient fronts of the World Ocean followed by regional chapters on the Arctic Ocean, North Atlantic, Baltic Sea, Kuroshio Current, and the Yellow Sea, a global review of CDOM dynamics at fronts, a chapter on persistent organic pollutants and marine organisms in the Kuroshio-Oyashio frontal zone, and two chapters on marine litter and its dynamics in frontal zones.

Measuring vertical motions represent a challenge as they are typically 3-4 orders of magnitude smaller than the horizontal velocities. Here, we show that surface vertical velocities are intensified at submesoscales and are dominated by high frequency variability. We use drifter observations to calculate divergence and vertical velocities in the upper 15 m of the water column at two different horizontal scales. The drifters, deployed at the edge of a mesoscale eddy in the Alboran Sea, show an area of strong convergence (O $\mathcal{O}$(f)) associated with vertical velocities of -100 m day(-1). This study shows that a multilayered-drifter array can be an effective tool for estimating vertical velocity near the ocean surface.

The Mediterranean Sea is a prominent climate-change hot spot, with many socioeconomically vital coastal areas being the most vulnerable targets for maritime safety, diverse met-ocean hazards and marine pollution. Providing an unprecedented spatial and temporal resolution at wide coastal areas, high-frequency radars (HFRs) have been steadily gaining recognition as an effective land-based remote sensing technology for continuous monitoring of the surface circulation, increasingly waves and occasionally winds. HFR measurements have boosted the thorough scientific knowledge of coastal processes, also fostering a broad range of applications, which has promoted their integration in coastal ocean observing systems worldwide, with more than half of the European sites located in the Mediterranean coastal areas. In this work, we present a review of existing HFR data multidisciplinary science-based applications in the Mediterranean Sea, primarily focused on meeting end-user and science-driven requirements, addressing regional challenges in three main topics: (i) maritime safety, (ii) extreme hazards and (iii) environmental transport process. Additionally, the HFR observing and monitoring regional capabilities in the Mediterranean coastal areas required to underpin the underlying science and the further development of applications are also analyzed. The outcome of this assessment has allowed us to provide a set of recommendations for future improvement prospects to maximize the contribution to extending science-based HFR products into societally relevant downstream services to support blue growth in the Mediterranean coastal areas, helping to meet the UN's Decade of Ocean Science for Sustainable Development and the EU's Green Deal goals.

Due to the semi-enclosed nature of the Mediterranean Sea, natural disasters and anthropogenic activities impose stronger pressures on its coastal ecosystems than in any other sea of the world. With the aim of responding adequately to science priorities and societal challenges, littoral waters must be effectively monitored with high-frequency radar (HFR) systems. This land-based remote sensing technology can provide, in near-real time, fine-resolution maps of the surface circulation over broad coastal areas, along with reliable directional wave and wind information. The main goal of this work is to showcase the current status of the Mediterranean HFR network and the future roadmap for orchestrated actions. Ongoing collaborative efforts and recent progress of this regional alliance are not only described but also connected with other European initiatives and global frameworks, highlighting the advantages of this cost-effective instrument for the multi-parameter monitoring of the sea state. Coordinated endeavors between HFR operators from different multi-disciplinary institutions are mandatory to reach a mature stage at both national and regional levels, striving to do the following: (i) harmonize deployment and maintenance practices; (ii) standardize data, metadata, and quality control procedures; (iii) centralize data management, visualization, and access platforms; and (iv) develop practical applications of societal benefit that can be used for strategic planning and informed decision-making in the Mediterranean marine environment. Such fit-for-purpose applications can serve for search and rescue operations, safe vessel navigation, tracking of marine pollutants, the monitoring of extreme events, the investigation of transport processes, and the connectivity between offshore waters and coastal ecosystems. Finally, future prospects within the Mediterranean framework are discussed along with a wealth of socioeconomic, technical, and scientific challenges to be faced during the implementation of this integrated HFR regional network.

Horizontal velocity gradients of a flow field and the related kinematic properties (KPs) of divergence, vorticity, and strain rate can be estimated from dense drifter deployments, e.g., the spatiotemporal average divergence (and other KPs) over a triangular area defined by three drifters and over a given time interval can be computed from the initial and final areas of said triangle. Unfortunately, this computation can be subject to large errors, especially when the triangle shape is far from equilateral. Therefore, samples with small aspect ratios are generally discarded. Here we derive the thresholds on two shape metrics that optimize the balance between retention of good and removal of bad divergence estimates. The primary tool is a high-resolution regional ocean model simulation, where a baseline for the average divergence can be established, so that actual errors are available. A value of 0.2 for the scaled aspect ratio L and a value of 0.86 pi for the largest interior angle u are found to be equally effective thresholds, especially at scales of 5 km and below. While discarding samples with low L or high theta values necessarily biases the distribution of divergence estimates slightly toward positive values, this bias is small compared to (and in the opposite direction of) the Lagrangian sampling bias due to drifters preferably sampling convergence regions. Errors due to position uncertainty are suppressed by the shape-based subsampling. The subsampling also improves the identification of the areas of extreme divergence or convergence. An application to an observational dataset demonstrates that these model-derived thresholds can be effectively used on actual drifter data.

Abstract. The Mediterranean Sea is a prominent climate change hot spot, being their socio-economically vital coastal areas the most vulnerable targets for maritime safety, diverse met-ocean hazards and marine pollution. Providing an unprecedented spatial and temporal resolution at wide coastal areas, High-frequency radars (HFRs) have been steadily gaining recognition as an effective land-based remote sensing technology for a continuous monitoring of the surface circulation, increasingly waves and occasionally winds. HFR measurements have boosted the thorough scientific knowledge of coastal processes, also fostering a broad range of applications, which has promoted their integration in the Coastal Ocean Observing Systems worldwide, with more than half of the European sites located in the Mediterranean coastal areas. In this work, we present a review of existing HFR data multidisciplinary science-based applications in the Mediterranean Sea, primarily focused on meeting end-users and science-driven requirements, addressing regional challenges in three main topics: i) maritime safety; ii) extreme hazards; iii) environmental transport process. Additionally, the HFR observing and monitoring regional capabilities in the Mediterranean region required to underpin the underlying science and the further development of applications are also analyzed. The outcome of this assessment has allowed us to finally provide a set of recommendations for the future improvement prospects to maximize the contribution in extending the science-based HFR products into societal relevant downstream services to support the blue growth in the Mediterranean coastal areas, helping to meet the UN’s Decade of Ocean Science for Sustainable Development and the EU’s Green Deal goals.

Abstract. Due to the semi-enclosed nature of the Mediterranean Sea, natural disasters and anthropogenic activities impose stronger pressures on its coastal ecosystems than in any other sea of the world. With the aim of responding adequately to science priorities and societal challenges, littoral waters must be effectively monitored with High-Frequency radar (HFR) systems. This land-based remote sensing technology can provide, in near real-time, fine-resolution maps of the surface circulation over broad coastal areas, along with reliable directional wave and wind information. The main goal of this work is to showcase the current status of the Mediterranean HFR network and the future roadmap for orchestrated actions. Ongoing collaborative efforts and recent progress of this regional alliance are not only described but also connected with other European initiatives and global frameworks, highlighting the advantages of this cost-effective instrument for the multi-parameter monitoring of the sea state. Coordinated endeavours between HFR operators from different multi-disciplinary institutions are mandatory to reach a mature stage at both national and regional levels, striving to: i) harmonize deployment and maintenance practices; ii) standardize data, metadata and quality control procedures; iii) centralize data management, visualization and access platforms; iv) develop practical applications of societal benefit, that can be used for strategic planning and informed decision-making in the Mediterranean marine environment. Such fit-for-purpose applications can serve for search and rescue operations, safe vessel navigation, tracking of marine pollutants, the monitoring of extreme events or the investigation of transport processes and the connectivity between offshore waters and coastal ecosystems. Finally, future prospects within the Mediterranean framework are discussed along with a wealth of socio-economic, technical and scientific challenges to be faced during the implementation of this integrated HFR regional network.

Every year, vast quantities of plastic debris arrive at the ocean surface. Nevertheless, our understanding of plastic movements is largely incomplete and many of the processes involved with the horizontal and vertical displacement of plastics in the ocean are still basically unknown. In this chapter we review the dynamics associated with the transport of plastics and other pollutants at oceanic fronts. Fronts had been historically defined as simple barriers to exchange, but here we show that the role of these structures in influencing the transport of plastics is more complex. The tools used to investigate the occurrence of frontal structures at various spatial scales are reviewed in detail, with a particular focus on their potential applications to the study of plastic pollution. Three selected case studies are presented to better describe the role of fronts in favoring or preventing plastic exchanges: the large-scale Antarctic Circumpolar Current, a Mediterranean mesoscale front, and the submesoscale fronts in the Gulf of Mexico. Lastly, some aspects related to the vertical subduction of plastic particles at oceanic fronts are discussed as one of the most promising frontiers for future research. The accumulation of floating debris at the sea surface is mainly affected by the horizontal components of frontal dynamics. At the same time, vertical components can be relevant for the export of neutrally buoyant particles from the surface into the deep sea. Based on these evidences, we propose that submesoscale processes can provide a fast and efficient route of plastic transport within the mixed layer, while mesoscale instabilities and associated vertical velocities might be the dominant mechanism to penetrate the deeper ocean on slower but broader scales. We conclude that given the ubiquitous presence of fronts in the world's ocean, their contribution to the global plastic cycle is probably not negligible and the role of these processes in vertically displacing neutrally buoyant microplastics should be investigated in more detail.