Maximo Garcia-Jove

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.

Mesoscale and submesoscale features play a critical role in transporting heat and biogeochemical tracers from the surface ocean to depths below the mixed layer, by driving vertical motions across density gradients. In the winter of 2022, strong mesoscale and submesoscale features were observed in the Western Mediterranean Sea during the ONR CALYPSO oceanographic campaign. This multidisciplinary experiment combined multiplatform in-situ observations with high-resolution numerical simulations to observe and predict small-scale ocean variability. In particular, a mesoscale density front associated with a vortex dipole was observed using CALYPSO observations and satellite imagery. A 650m resolution model simulation is used here to understand the evolution of the front and the energy transfer to submesoscale cyclonic eddies. The simulation properly reproduces the intense, narrow, and elongated frontal convergence structure and a dense cyclonic ridge linked to the vortex dipole. The evolution of the front is characterized by: i) an intensification through frontogenesis, and ii) a decay due to favorable conditions for overturning instabilities during a down-front wind event. The frontogenesis and instabilities processes enhance vertical motion via an across-front ageostrophic secondary circulation and contribute to the restratifying effect. The front decayed within days, interacting with the mesoscale ridge to break down into smaller structures and generate submesoscale cyclonic eddies (SCEs) at its edges. The formation of SCEs is associated with the frontal decay, as well as centrifugal and gravitational instabilities, which transfer energy from the mesoscale front to the SCEs. The SCE structure reveals a 3D helical-spiral recirculation pattern that transports parcels vertically. Observations of oxygen and chlorophyll confirm the enhancement of the vertical transport of tracers from the surface to the ocean interior. Submesoscale eddy-induced frontogenesis mechanism and instability processes drove subduction along outcropping isopycnals at the periphery of the SCE.

Mesoscale features and the corresponding submesoscale structures can vertically transport heat, freshwater, and biogeochemical tracers (i.e., phytoplankton, oxygen, and carbon) from the surface to the interior. These structures may grow, decay, and transfer energy through various processes. This study examines the small (~20 km) mesoscale eddy evolution and the associated energy transfers from a four-dimensional, three-month-long glider fleet survey in the Western Mediterranean Sea. The combined glider fleet covered nearly 15978 km over the ground, performing 704 glider days while doing over 4837 dives to as deep as 700 m, measuring physical and biochemical parameters. The sources of eddy kinetic energy are examined and compared with numerical eddy-resolving simulations (2-km grid-scale). The comparison allows us to identify whether the energy exchange is local or has a broader interaction between the mean and eddy flow. We study the redistribution of energy during the eddy merging and splitting processes, and how these processes relate to changes in the flow divergence and vertical velocities. During the eddy merging, the vertical velocity reaches up to 20 m/day. However, we observed a reduction in the areas where significant vertical motion occurs, which was associated with a decrease in frontogenesis in the periphery of the eddy and a redistribution of kinetic energy across the merging eddy. During the eddy splitting, the vertical velocity was significantly reduced (less than 10 m/day) by a frontolytic event in the northern eddy. Eddy splitting caused a significant reduction of the positive and negative divergence, and the energy of the two newly formed cyclonic eddies (CEs) decayed (vertical velocities decreased from ~20 m/day to ~10 m/day). The eddy merging event can be considered as a large-scale energy pump in the regions where an inverse energy cascade occurs. The observed imbalance in the transfer of EKE during eddy splitting suggests that the northern CE decays quicker and maintains less kinetic energy than the southern one. We examine the energy transfer terms of the baroclinic and barotropic components, taking into account both the horizontal and vertical energy transfer (baroclinic horizontal term, the baroclinic vertical term, and the barotropic term), which provides a better understanding of the instability processes responsible for the eddy formation.

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.

This article analyses the Balearic Islands wind-driven coastal upwelling in response to the intense and long-lasting Storm Blas, which affected the western Mediterranean Sea in November 2021. The storm was associated with a pronounced pressure low, generating heavy rains and intense winds and showing some characteristics of a tropical cyclone. The Balearic Islands area was particularly affected since the core of the storm was moving over a 1-week-long period from the south-west of this area to just above the islands of Menorca and Mallorca. High-resolution regional forecast models indicated that the intense north-easterly winds blowing over the region during the first days of the storm led to the development of intense upwellings along the north-western coasts of the islands of Mallorca and Ibiza together with a reversal of the surface current. While the clouds associated with the storm prevented the radiometers on board satellites to precisely observe the evolution of the sea surface signature of the upwelling, signals of enhanced chlorophyll concentration were still detected in the upwelling region. The high-resolution Western Mediterranean Operational Prediction System model, which downscales the Copernicus Marine Service Mediterranean predictions, is used to describe the characteristics of this intense coastal-upwelling event, as well as to analyse its singularity over the past-9-year time series through the comparison of different coastal-upwelling indices. The upwelling event is found to have a duration of 3 d (considering its effects on the sea surface temperature) with a spatial offshore extension of around 20 km. It was characterized by intense cold coastal sea surface anomalies of around 6 ^∘C. While it was the most intense event over the past 9 years in terms of local cross-shore sea surface temperature gradients, it is ranked second in terms of the intensity of cross-shelf transports, behind the Storm Gloria upwelling event in January 2020. This study demonstrates the benefits of operational oceanography for the characterization of extreme events through the provision of time series of high-resolution modelling results in the coastal area.

The CALYPSO research program, funded by the Office of Naval Research, aims to understand the coherent pathways from the surface ocean to the interior. In February and March 2022, the CALYPSO campaign surveyed the upper layers of the Balearic Sea (Western Mediterranean), providing high-resolution observations of mesoscale and submesoscale structures. This included submesoscale density fronts, stretching regions and small coherent eddies. In this work, we analyze the formation, evolution, and impact of these submesoscale structures in the development of vertical velocities and in the determination of three-dimensional pathways from the surface to the ocean interior. We combine multi-platform in-situ observations with high-resolution numerical simulations both in free-run and data-assimilative modes. In particular, the WMOP reanalysis assimilates remote sensing observations of temperature and sea level anomaly as well as vertical profiles of temperature and salinity from floats, underway CTD and gliders obtained during the CALYPSO campaign. The simulations are shown to realistically reproduce the main characteristics of the Balearic front and some of the associated submesoscale patterns, providing an appropriate tool to analyze the processes responsible for vertical velocities development. Particle tracking analyses indicate pathways for vertical exchange of water between the surface and the ocean interior in the frontal region. Finally, the process of frontogenesis and the submesoscale structures play an important role in the development of the vertical velocities and the energy transfers.

Mesoscale features and their corresponding submesoscale structures may generate a substantial vertical transport of carbon and biogeochemical tracers from the surface to the interior. The baroclinic instabilities formed from the Northern and Balearic Currents are associated with vigorous mesoscale eddy activity in the Balearic Sea. In the CALYPSO 2022 experiment, eight gliders were programmed to dive as deep as 700 m from 25 March until 21 June 2022. The glider fleet measured temperature, salinity, velocity, chlorophyll fluorescence, oxygen, and acoustic backscatter. The glider data was mapped utilizing objective mapping of the across-front, along-front, and time on 10 m vertical levels. The geostrophic velocity was inferred using a variational approach. We estimate the vertical and ageostrophic horizontal velocities using the omega equation. The glider observations provide a complete description of the evolution of the eddy field. The analysis of the uplifted isopycnal surface as the eddy formed showed consistency between the movement of the dynamic and biogeochemical tracers. Glider data show the evolution of a cyclonic eddy (20-30 km) in the area, where vertical velocities tend to be downward. As the eddy developed, its axis shifted westward. The isopycnal uplift inside the pycnocline can provide nutrients to the euphotic zone. While the initial cyclonic feature has dissipated, a second shoaling of the 28.9 isopycnal occurred in the east toward the end of April 2022, where the biggest relative vorticity (~0.5 ζ/f) was observed in the region. The eddy extended during its growing phase, showing a westward shift of the eddy axis. The observed peak downward vertical velocities were near 30 m day-1 during the eddy intensification, with the size of the cyclonic eddy varying between 20-30 km. The new cyclonic feature has been spreading in the area for a few days before dividing into two separate cyclonic eddies (15km) around the beginning of May. The two smaller cyclonic eddies moved north and west until they vanished from our study area. However, an anticyclonic structure (20km) was developed within their separation. The vertical velocity tended to be downward on the dense side of the front and upward on the light side, flattening the eddy characteristic. The glider observations reveal horizontal density gradients up to 0.5 kg m-3 over ~10 km. The obtained maximum velocities were up to 30 cm/s in the region. Upwelling and downwelling were also detected by chlorophyll fluorescence, oxygen, and acoustic backscatter near the frontal interface. Glider observations were integrated with remote sensing and modeling simulations to evaluate mesoscale and submesoscale variability in developing vertical velocities in the Balearic Sea and their impact on biological carbon storage.

The Orust-Tjörn fjord system is located on the west coast of Sweden and consists of several fjords with many small islands. The fjord system has more than one connection to the open water, enabling a generally counterclockwise circulation through the fjords. It is home to nature reservoirs, different industries on land and aquaculture farms. It is threatened for example by hypoxic areas, invasive species, water pollution and algae blooms with the water quality strongly been influenced by the water exchange in the fjord system. Therefore it is important to understand the circulation, the state and the exchange of w­ater between the fjords system and the open water outside the fjord. Not much knowledge of the circulation inside the fjords and the drivers of the water exchange between the fjords and the open water exists. The reasons are that observations are spatially sparse and the resolution of regional ocean circulation models are generally too coarse to resolve this complex fjord system with its shallow and narrow straits between the different fjords. Therefore, we developed a setup of a coastal ocean circulation model with a horizontal resolution of 50 m to study the drivers of water exchange of the Orust-Tjörn fjord system. Within a sensitivity study a set of simulations are performed using (a) temporally constant wind forcing, (b) temporally and spatially constant sea level at the open boundaries (c) no tides, and (d) constant offshore density, i.e. temporally and spatially constant temperature and salinity profiles at the open boundaries. The simulation period is September 2016 – February 2017, which includes a high saline water inflow in October 2016. Model results compare well with observations from moored high-frequency velocity, temperature and salinity instruments as well as regular monitoring data. Results show for example that the density difference between the southern and northern entrance of the Orust-Tjörn fjord system highly influences the water exchange between the fjords and open water. This is because with constant offshore density applied, near surface velocities of the Baltic current as well as the circulation above sill level within the fjord system weakens. The timing and strength of the high-saline inflow event is most sensitive to the cases of constant winds and constant offshore densities. In general, we will present a detailed analysis of the main drivers of water exchange above sill level.

This article analyzes the Balearic Islands wind-driven coastal upwelling in response to the intense and long-lasting storm Blas which affected the Western Mediterranean Sea in November 2021. The storm was associated with a pronounced pressure low, generating heavy rains and intense winds and showing some characteristics of a tropical cyclone. The Balearic Islands area was particularly affected since the core of the storm was moving over a one-week-long period from the south-west of this area to just above Menorca and Mallorca Islands. High-resolution regional forecast models indicated that the intense northeasterly winds blowing over the region during the first days of the storm led to the development of intense upwellings along the northwestern coasts of Mallorca and Ibiza Islands together with a reversal of the surface current. While the clouds associated with the storm prevented the radiometers onboard satellites to precisely observe the evolution of the sea surface signature of the upwelling, signals of enhanced chlorophyll concentration were still detected in the upwelling region. The high-resolution Western Mediterranean Operational model, which downscales the Copernicus Marine Service Mediterranean predictions, is used to describe the characteristics of this intense coastal upwelling event, as well as to analyze its singularity over the past 9-year time series through the comparison of different coastal upwelling indices. The upwelling event is found to have a duration of three days with a spatial off-shore extension of around 20 km. It was characterized by intense cold coastal sea surface anomalies of around 6 °C. While it was the most intense event over the past 9 years in terms of local cross-shore sea surface temperature gradients, it is ranked second in terms of the intensity of cross-shelf transports, following the Gloria episode in January 2020.