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Funding: We thank LOPS and UBO for a M.Sc internship grant which allowed the first author to carry out this work. The merger of two surface quasi-geostrophic vortices is examined in detail. As the two vortices collapse towards each other in the merging process, they trap their external fronts between them; these fronts are inserted into the final merged vortex, where they form a central, nearly parallel, sheared velocity strip, sensitive to barotropic instability. As a result, this strip breaks up into an alley of small vortices. Subsequently, these small vortices may undergo merger and grow in size in the core of the large merged vortex. The number of small trapped vortices decreases correspondingly. Finally, a single or two small vortices remain. These processes are analysed using a numerical model of the surface quasi-geostrophic equations. The sensitivity of this process to the initial vortex characteristics is explored. A parallel is drawn between this problem and the instability of a rectilinear strip of temperature with a central gap. The application of this problem to the Ocean is discussed. Publisher PDF

Taking into account the coupling of the ocean with the atmosphere is essential to properly describe vortex dynamics in the ocean. The forcing of a circular eddy with the relative wind stress curl leads to an Ekman pumping with a nonzero area integral. This in turn creates a source or a sink in the eddy. We revisit the two point vortex-source interaction, now coupled with an unsteady wind, leading to a time-varying circulation and source strength. Firstly, we recover the various fixed points of the two vortex-source system, and we calculate their stability. Then we show the effect of a weak amplitude, subharmonic, or harmonic time variation of the wind, leading to a similar variation of the circulation and the source strength of the vortex sources. We use a multiple time scale expansion of the variables to calculate the long time variation of these vortex trajectories around neutral fixed points. We study the amplitude equation and obtain its solution. We compute numerically the unstable evolution of the vortex sources when the source and circulation have a finite periodic variation. We also assess the influence of this time variation on the dispersion of a passive tracer near these vortex sources.

The stability of two superposed buoyancy vortices is studied linearly in a two-level Surface Quasi-Geostrophic (SQG) model. The basic flow is chosen as two circular vortices with uniform buoyancy, coaxial, and the same radius. A perturbation with a single angular mode is added to this mean flow. The SQG equations linearized in perturbation around this basic flow form a two-dimensional ODE for which the normal and singular mode solutions are numerically computed. The instability of these two vortices depends on several parameters. The parameters varied here are: the vertical distance between the two levels and the two values of the vortex buoyancies (called vortex intensity hereafter); the other parameters remain fixed. For normal modes, the system is stable if the levels are sufficiently far from each other vertically, to prevent vertical interactions of the buoyancy patches. Stability is also reached if the layers are close to each other, but if the vortices have very different intensities, again preventing the resonance of Rossby waves around their contours. The system is unstable if the vortex intensities are similar and if the two levels are close to each other. The growth rates of the normal modes increase with the angular wave-number, also corresponding to shorter vertical distances. The growth rates of the singular modes depend more on the distance between the levels than on the ratio of the vortex intensities, at a short time; as expected, they converge towards the growth rates of the normal modes. This study remaining linear does not predict the final evolution of such unstable vortices. This nonlinear evolution will be studied in a sequel of this work.

The Eastern Kamchatka Current (EKC) is the western boundary current of the North Pacific subpolar gyre. Southeast of the Kamchatka Peninsula lies a large anticyclonic eddy, the Kamchatka Eddy (KE). This eddy is quasi-stationary. More generally, the oceanic region east of the EKC contains many eddies, several of them large and long lasting. Using surface currents derived from altimetry, particle tracking and a simple two-dimensional numerical model of fluid flow, we investigate the variability of this eddy field, the generation of eddies in the bays of Kamchatka by the EKC and fluxes of water to and from these bays. Firstly, we recover in our analysis of long-lasting eddies, the main eddies of the region. Among strong eddies, the parity bias favors anticyclones. Our numerical simulations give a possible explanation for the process of eddy creation in the bays of the peninsula and show that the northernmost bay produces most anticyclones. Then, we track forward the water particles from these bays and we determine their fate in the open ocean; southeastward and southwestward trajectories are the most frequent. We also track water particles backward from the KE site; they often drift near the Kamchatka coast, but others drift south of this site and remain there, a priori trapped in other eddies. This study confirms the complexity of mesoscale motions and water exchanges in this region.

The ocean is densely populated with energetic coherent vortices of different sizes. Mesoscale and submesoscale vortices contribute to stirring of the ocean, transporting and redistributing water masses and tracers (active and passive), affecting ventilation pathways and thus impacting the large-scale circulation. Submesoscale Coherent Vortices (SCVs), i.e. vortices with radii between 1-30 km have been detected via satellite and in-situ measurements at surface or at depth (usually not more than ~2000 m depth). They are found to be of different shapes and sizes depending upon latitude and place of origin. Previous studies mostly describe the surface mesoscale and submesoscale eddies rather than the deep SCVs (> 2000 m). This study focuses on SCVs below the mixed layer along four different isopycnal surfaces: 26.60, 27.60, 27.80, and 27.86, which lie in the depth range of 10-500 m, 200-2000 m, 1200-3000 m, and 1800-4500 m, respectively. We aim to quantify their physical characteristics (radius, thickness, bias in polarity: cyclones versus anticyclones) in different parts of the Atlantic ocean, and analyze the dynamics involved in the generation and destruction of the SCVs throughout their life-cycle. We use the Coastal and Regional Ocean COmmunity model (CROCO) ocean model in a high resolution setup (3 km) of the Atlantic Ocean. The detection of SCVs are done every 12 hr using the Okubo-Weiss parameter along the isopycnal surfaces using the eddy-tracking algorithm by Mason et al., 2014. We consider only structures living for more than 21 days. The census of SCVs shows that there are in total more cyclonic than anticyclonic SCV detections. However cyclones are on average smaller and shorter lived, such that there is a dominance of anticyclones while considering long-lived and larger distance travelling SCVs. We concentrate on the strongest and longest lived SCVs among which meddies that we compare to previous in-situ observations. This study is the first step in the understanding of the formation, occurrences and structure of SCVs in the Atlantic Ocean, and their impact on the large-scale ocean circulation.

Early brain tumor resection can effectively improve the patient’s survival rate (Jakola et al. 2012). The proposed challenge intends to solve key issue in ultrasound-guided brain tumor resection, which is to outline the tumor boundary in intra-operative ultrasound. The tumor segmentation will (1) assist the surgeon in image interpretation, especially after brain shift has occurred and the MRI is no longer reliable and (2) segmented structures could be used to drive registration algorithms to compensate for brain shift. Intra-operative ultrasound (iUS) is a robust and relatively inexpensive technique to track intra-operative tissue shift and surgical tools. Automatic algorithms for brain tissue segmentation in iUS, especially brain tumors and resection cavity can greatly facilitate the robustness and accuracy of brain shift correction through image registration, and allow easy interpretation of the iUS. This has the potential to improve surgical outcomes and patient survival rate (Xiao et al. 2020). The challenge is an extension to the previous CuRIOUS 2018&2019 Challenge that focused on image registration algorithms. It will provide a snapshot of the current and new techniques for iUS segmentation which may further enhance the usefulness of ultrasound in surgical guidance by assisting the neurosurgeon in ultrasound image interpretation as well as input to image registration algorithms. The challenge will provide the opportunity to benchmark the methods on the newly released dataset of iUS brain tumor and resection cavity segmentation. References Jakola et al. 2012: Comparison of a strategy favoring early surgical resection vs a strategy favoring watchful waiting in low-grade gliomas. JAMA, 2012 Gerard et al. 2017: Brain shift in neuronavigation of brain tumors: A review, Medical Image Analysis, 2017. Munkvold et al. 2017: Tumor volume assessment in low-grade gliomas: A comparison of preoperative magnetic resonance imaging to coregistered 3-dimensional ultrasound recordings, Neurosurgery, Aug 2017. Xiao et al. 2017: Retrospective evaluation of cerebral Tumors (RESECT): A clinical database of pre-operative MRI and intra-operative ultrasound in low-grade glioma surgeries, Medical Physics, Apr. 2017 Maier-Hein et al. 2018: Why rankings of biomedical image analysis competitions should be interpreted with care, Nature Communications, 2018 Xiao et al. 2020: Evaluation of MRI to Ultrasound Registration Methods for Brain Shift Correction: The CuRIOUS 2018 Challenge, IEEE TMI, 2020 Wiesenfarth et al. 2021: Methods and open-source toolkit for analyzing and visualizing challenge results, Scientific reports, 2021 Carton et al. (2020): Automatic segmentation of brain tumor resections in intraoperative ultrasound images using U-Net, Journal of Medical Imaging 7(3); doi: 10.1117/1.JMI.7.3.031503].

The Red Sea Water enters the Gulf of Aden through the Strait of Bab El Mandeb as a density current. The Red Sea Water subsequently spreads into the Gulf of Aden under the influence of surface mesoscale eddies, which dominate the surface flow, of topographic features such as rift and capes, and of the monsoon regimes. The dynamics of a bottom density current overflowing in a semi-enclosed basin, as the Red Sea Water outflows in the Gulf of Aden, is investigated by performing idealised numerical simulations, at submesoscale resolution, in which we progressively add topographic and dynamical elements. The rift and cape play an important role, respectively, on the vertical and the horizontal mixing as well as baroclinic and barotropic instabilities undergone by the bottom density current. Mesoscale and submesoscale eddies are generated depending on the model configuration. In the presence of surface mesoscale eddies, the bottom density current water is mainly advected at their periphery. In winter, both mesoscale and submesoscale eddies are generated, while in summer only submesoscale eddies are present. Finally, to put our results based on idealised numerical simulations and Lagrangian experiments in perspective, we analyse the trajectories of three Argo floats, deployed in the Rift of Tadjurah. Clues of submesoscale eddies generation at capes are observed which is in agreement with our idealised numerical simulations.

The Red Sea Water is a warm and salty water produced in the Red Sea by evaporation induced by strong solar radiation. This dense water mass exits the Red Sea through the Strait of Bab El Mandeb, and enters the Gulf of Aden as a density current between 400 and 1000 metre depth. In the Gulf of Aden, in situ and satellites observations have shown the impact of the deeply reaching eddies dominating the mesoscale dynamics, on the spreading of the Red Sea Water. In this paper, we study the life cycle of these mesoscale eddies in the Gulf of Aden by using a regional primitive equation model at mesoscale resolution, and an eddy-tracking algorithm. The mesoscale anticyclonic eddies are formed at the mouth of the Gulf of Aden, and subsequently drift westward into the gulf. Mesoscale anticyclones are long-lived compared to the cyclones. The cyclones result from the interaction of anticyclones with the coast and the sloping topography. The wind stress, the bathymetry and the surrounding eddy field drive the life cycle of eddies. Finally, Kelvin and internal waves are triggered along the northern and southern coasts.

We investigate the strong interaction between two baroclinic tripolar eddies in a three-dimensional, rapidly- rotating, continuously stratified flow under the quasi-geostrophic approximation. Each tripolar eddy consists of an anticyclonic central vortex with two oblate cyclonic vortices located above and below the anticyclone. The interaction depends on the vertical and horizontal offsets between the two tripolar eddies. For small and low PV oblate cyclones, each tripolar eddy alone is only weakly unstable to a baroclinic mode. The instability puts the three vortices out of alignment. Most of the eddy however survives the instability. When two tripolar eddies interact, their constituent vortices may merge. Merger occurs when the eddies are close enough together, and shows similarities with the merger of monopolar vortices. Vertically separated eddies do not align vertically. This suggests the importance of an external flow for the alignment, observed in the oceans, to occur. We finally show that the interaction between two tripolar eddies with intense oblate cyclones is very different and show similarities with the dynamics of dipolar baroclinic eddies known as hetons. Postprint

The Lagrangian and Eulerian surface current signatures of a low-mode internal tide propagating through a turbulent balanced flow are compared in idealized numerical simulations. Lagrangian and Eulerian total (i.e., coherent plus incoherent) tidal amplitudes are found to be similar. Compared to Eulerian diagnostics, the Lagrangian tidal signal is more incoherent with comparable or smaller incoherence time scales and larger incoherent amplitudes. The larger level of incoherence in Lagrangian data is proposed to result from the deformation of an Eulerian internal tide signal induced by drifter displacements. Based on the latter hypothesis, a theoretical model successfully predicts Lagrangian autocovariances by relating Lagrangian and Eulerian autocovariances and the properties of the internal tides and jet. These results have implications for the separation of balanced flow and internal tides signals in the sea level data collected by the future Surface Water and Ocean Topography (SWOT) satellite mission.

Coastal upwellings, due to offshore Ekman transport, are more energetic at the western boundaries of the oceans, where they are intensified by incoming Rossby waves, than at the eastern boundaries. Western boundary upwellings are often accompanied by a local vortex field. The instability of a developed upwelling front and its interaction with an external vortex field is studied here with a three-dimensional numerical model of the hydrostatic rotating Navier-Stokes equations (the primitive equations). The baroclinic instability of the front leads to the growth of meanders with 100-200 km wavelength, in the absence of external vortex. On the $$f$$ -plane, these waves can break into a row of vortices when the instability is intense. The $$\beta$$ -effect is stabilizing and strongly decreases the amplitude of meanders. Simulations are then performed with a front initially accompanied by one or several external vortices. The evolutions in this case are compared with those of the unstable jet alone. On the $$f$$ -plane, when an external vortex is close to the front, this latter sheds a long filament which wraps up around the vortex. This occurs over a period similar to that of the instability of the isolated front. Cyclones are more efficient in tearing such filaments offshore than anticyclones. On the $$\beta$$ -plane, the filaments are short and turbulence is confined to the vicinity of the front. At long times, waves propagate along the front, thus extending turbulence alongshore. The initial presence of a vortex alley leads to a stronger destabilization of the front and to a larger cross-shore flux than for a single vortex, with many filaments and small vortices pushed far offshore. In the ocean, this cross-shore exchange has important consequences on the local biological activity.

The Florida Current (FC) flows in the Straits of Florida (SoF) and connects the Loop Current in the Gulf of Mexico to the Gulf Stream (GS) in the Western Atlantic Ocean. Its journey through the SoF is at time characterized by the formation and presence of mesoscale but mostly submesoscale frontal eddies on the cyclonic side of the current. The formation of those frontal eddies was investigated in a very high resolution two-way nested simulation using the Regional Oceanic Modeling System (ROMS). Frontal eddies were either locally formed or originated from outside the SoF. The northern front of the incoming eddies was susceptible to superinertial shear instability over the shelf slope when the eddies were pushed up against the slope by the FC. Otherwise, incoming eddies could be advected relatively unaffected by the current, when in the southern part of the straits. In absence of incoming eddies, submesoscale eddies were locally formed by the roll-up of superinertial barotropically unstable vorticity filaments when the FC was pushed up against the shelf slope. The vorticity filaments were intensified by the friction-induced bottom layer vorticity flux as previously demonstrated by Gula et al. (2015b) in the GS. When the FC retreated further south, negative vorticity West Florida Shelf waters overflowed into the SOF and led to the formation of submesocale eddies by baroclinic instability. The instability regimes, hence, the submesoscale frontal eddies formation appear to be controlled by the lateral ‘sloshing’ of the FC in the SoF.

In February 2020, a 120km‐wide freshwater plume was documented by satellite and in situ observations near the Demerara Rise (7°N/54‐56°W). It was initially stratified in the upper 10 m with a freshwater content of 2 to 3 meters of Amazon water distributed down to 40 m. On February 2nd, ship transects indicate an inhomogeneous shelf structure with a propagating front in its midst, whereas minimum salinity close to 30 pss was observed close to the shelf break on February 5th. The salinity minimum eroded in time but was still observed 13 to 16 days later with 33.3 pss minimum value up to 400 km from the shelf break. At this time, the mixed layer depth was close to 20 m. The off‐shelf flow lasted 10 days, contributing to a plume area extending over 100000 km2 and associated with a 0.15 106 m3 s‐1 freshwater transport. The off‐shelf plume was steered northward by a North Brazil Current ring up to 12°N and then extended westward toward the Caribbean Sea. Its occurrence followed 3 days of favourable wind direction closer to the Amazon estuary, which contributed to north‐westward freshwater transport on the shelf. Other such events of freshwater transport in January‐March are documented since 2010 in salinity satellite products in 7 out of 10 years, and in 6 of those years, they were preceded by a change in wind direction between the Amazon estuary and the Guianas favouring the north‐westward freshwater transport toward the shelf break. Plain Language Summary This paper documents how freshwater from the Amazon reaches the deep ocean up to 12°N in the northwest tropical Atlantic in January‐March. The classical view is that the water is channelled along the shelf to the Caribbean Sea. Here, we document a freshwater plume from in situ and satellite observations during the EUREC4A‐OA/ATOMIC program in 2020. This plume separated from the shelf near 55°W north of French Guiana on February 2‐5. This fresher water was stirred by a North Brazil Current ring up to 12°N before mostly spreading westward. The near‐surface water was initially very stratified at least until 10‐m from the surface. More than 14 days later and 400 km farther north, salinity as low as 33.3 pss with mixing depths on the order of only 20‐m was still encountered. The total area of the freshwater plume reached 100000 km2 with a flow of freshwater on the order of 0.15 Sv (106 m3 s‐1) during 10 days. This phenomenon seems to be triggered by changes in the wind direction on the shelf closer to the equator, and has also been observed in satellite products in 7 out of 10 years since 2010.

In the Indian Ocean, salty water masses from the Persian Gulf and Red Sea are important sources of salt, heat, and nutrients. Across the Arabian Sea these outflows impact human and biological activities, their thermohaline characteristics and shapes exhibiting important spatial and seasonal variability. Knowledge of the water masses properties is important to validate realistic simulations of the Indian Ocean. A classical approach to study these water masses is to track them on specific isopycnal levels. Nevertheless, their peaking thermohaline characteristics are not always found at a specific density but rather spread over a range. Here, we develop a detection algorithm able to capture the full vertical structure of the outflows, that we applied to a dataset of about 126,000 vertical profiles. We are thus able to quantify the changes in their thermohaline signatures and in their vertical structures, characterized here by the intensity of the salinity peaks of the water masses and lateral injection of fresh and salty waters, and describe their spatial variability. Across the northwestern Indian Ocean, the salty outflows undergo several changes, diminishing their thermohaline signatures and peaks and layering. In their early stages in the narrow Gulf of Oman and Aden, the outflows present configurations indicative of diapycnal mixing. In the same regions and along the western edge of the Arabian Sea, these water masses are subject to lateral mixing. All over the Arabian Sea salt fingering conditions are met for lower layers of the outflows. Plain Language Summary In the northwestern Indian Ocean, water masses with different characteristics are measured. Two of them are some of the most salty found globally, namely Persian Gulf Water and Red Sea Water. In the Arabian Sea, these outflows transport salt and heat, affecting the biological activity. Furthermore, knowledge of such quantities is necessary to simulate the circulation of the basin. A classical approach to analyze the characteristics of these outflows is to measure them at specific densities. It has been observed that peaks in salinity are more often found over a range of densities, rather than at specific values. In this study, we develop a detection algorithm able to capture the full vertical structure of the outflows. Thus, we quantify the temperature and salinity regional distribution of the outflows, and describe their vertical configurations. Along their paths, the salty outflows undergo several changes in their vertical structures, with a diminution of their peaks in salinity and lateral injections of saltier and fresher waters, particularly near their straits of exit. Finally, all over the Arabian Sea, the outflows are in salt fingering conditions, losing salt at smaller vertical scales.

In this study, we investigate the mesoscale flow field and how it enables energy to propagate vertically in form of near-inertial waves. As part of the EURAC4A-OA campaign the research vessels RV Maria S. Merian and NO L’Atalante simultaneously surveyed mesoscale eddy fronts in the western tropical North Atlantic. From velocity profile data, measured by a shipboard Acoustic Doppler Current Profiler (sADCP), we reconstruct eddies in the upper 1000m of the surveyed area, by fitting a Rankine Vortex model. The model derives an idealized velocity structure of the eddy as well as the location of its centre. Multiple occurrences of stacked eddies are identified and often surrounded by current shear structures associated with near-inertial waves. Using data from ship sections, where both research vessels operated less than 1nm apart, the vertical component of the relative vorticity (zeta) is calculated using different methods (single ship, two ships)[Shcherbina et al. 2013]. It is found that in particular zeta outside of the eddy cores is sensitive to the way the vorticity is calculated and may even change sign. Furthermore, the resulting zeta sections and its impact on the ability of near-inertial waves propagating vertically below the mixed layer is discussed.

In January-February 2020, the EUREC4A-OA/ATOMIC experiment took place in the Northwest Tropical Atlantic Ocean with the overall objective of understanding the role of fine scale processes in the internal ocean dynamics and air-sea interaction. Four oceanographic vessels, the French Atalante, German Maria S Merian and Meteor, and the American Ron Brown, closely coordinated with air-borne observations and autonomous ocean platforms (gliders, saildrones, and drifters) to simultaneously measure the ocean and atmosphere east of the island of Barbados and the coast of Guyana in the western Tropical Atlantic. A whole battery of instruments measuring the thermohaline and dynamic characteristics of the region was launched. The fixed CTD stations, reaching great depths while measuring salinity, temperature, and oxygen concentrations, serve as a reference to calibrate and validate other devices, in particular, shallower uCTD, TSG, and MVP, acquired during ship transits, and autonomous gliders and saidrones. Combined, these datasets increase the horizontal resolution and thus the description of structures ranging from mesoscale to fine scale.The Northwest Tropical Atlantic Ocean is a dynamical region filled with mesoscale eddies of different origins and transporting various water masses across the region. These eddies have rich and diverse characteristics ranging from shallow cyclonic and anticyclonic eddies to the deep reaching North Brazil Current (NBC) Rings. On the surface, down to 200 m depth, the signatures of shallow cyclones and anticyclones (NBC rings) were measured. The shallow mesoscale eddies, with core centered around a density of 25.5 kg m-3, advect highly saline and warm waters, with low oxygen concentrations compared to the surrounding water masses. Below, evolving at density around 26.7 kg m-3, thick anticyclones were observed, characterized by low temperature and salinity but with high values of oxygen, indicative of a South Atlantic origin. One was observed drifting slowly northward and another one at the NBC retroflection. Similarly, mesoscale cyclonic eddies were also observed both at the surface and at depth. Surface and subsurface eddies are not aligned vertically and they seem to evolve independently. The large number and diversity (ship-mounted or autonomous) of observing platforms implemented in the project made made it possible to innovatively sample the upper-ocean frontal scales and stratification. It has been found that the interaction between the particularly fresh waters from the Amazon River, flowing northward along the shelf-break, and NBC rings create a rich variety of submesoscale fronts and a strong barrier layer, leading to interleaving. With the high vertical and horizontal resolutions, we quantify the layering and mixing processes at play.

International audience; The simulation of realistic ultrasound (US) images has many applications in image-guided surgery such as image registration, data augmentation, or educational purposes. In this paper we simulated intraoperative US images of the brain after tumor resection surgery. In a first stage, a Generative Adversarial Networks generated an US image with resection from a resection cavity map. While the cavity texture can be realistic, surrounding structures are usually not anatomically coherent. Thus, a second stage blended the generated cavity texture into a real patient-specific US image acquired before resection. A validation study on 68 images of 21 cases showed that three raters correctly identified 64% of all images. In particular, two neurosurgeons correctly labelled only 56% and 53% of the simulated images, which indicate that these synthesized images are hardly distinguishable from real post-resection US images.

The science guiding the EUREC4A campaign and its measurements is presented. EUREC4A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EUREC4A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement.

International audience; The Ras al Hadd oceanic dipole is a recurrent association of a cyclone (to the northeast) and of an anticyclone (to the southwest), which forms in summer and breaks up at the end of autumn. It lies near the Ras al Hadd cape, southeast of the Arabian peninsula. Its size is on the order of 100 km. Along the axis of this dipole flows an intense jet, the Ras al Had jet. Using altimetric data and an eddy detection and tracking algorithm (AMEDA: Angular Momentum Eddy Detection and tracking Algorithm), we describe the life cycle of this oceanic dipole over a year (2014–2015). We also use the results of a numerical model (HYCOM, the HYbrid Coordinate Ocean Model) simulation, and hydrological data from ARGO profilers, to characterize the vertical structure of the two eddies composing the dipole, and their variability over a 15 year period. We show that (1) before the dipole is formed, the two eddies that will compose it, come from different locations to join near Ras al Hadd, (2) the dipole remains near Ras al Hadd during summer and fall while the wind stress (due to the summer monsoon wind) intensifies the cyclone, (3) both the anticyclone and the cyclone reach the depth of the Persian Gulf Water outflow, and (4) their horizontal radial velocity profile is often close to Gaussian but it can vary as the dipole interacts with neighboring eddies. As a conclusion, further work with a process model is recommended to quantify the interaction of this dipole with surrounding eddies and with the atmosphere.

The three-layer version of the contour dynamics/surgery method is used to study the interaction mechanisms of a large-scale surface vortex with a smaller vortex/vortices of the middle layer (prototypes of intrathermocline vortices in the ocean) belonging to the middle layer of a three-layer rotating fluid. The lower layer is assumed to be dynamically passive. The piecewise constant vertical density distribution approximates the average long-term profile for the North Atlantic, where intrathermocline eddies are observed most often at depths of 300&ndash, 1600 m. Numerical experiments were carried out with different initial configurations of vortices, to evaluate several effects. Firstly, the stability of the vortex compound was evaluated. Most often, it remains compact, but when unstable, it can break as vertically coupled vortex dipoles (called hetons). Secondly, we studied the interaction between a vertically tilted cyclone and lenses. Then, the lenses first undergo anticlockwise rotation determined by the surface cyclone. The lenses can induce alignment or coupling with cyclonic vorticity above them. Only very weak lenses are destroyed by the shear stress exerted by the surface cyclone. Thirdly, under the influence of lens dipoles, the surface cyclone can be torn apart. In particular, the shedding of rapidly moving vortex pairs at the surface reflects the presence of lens dipoles below. More slowly moving small eddies can also be torn away from the main surface cyclone. In this case, they do not appear to be coupled with middle layer vortices. They are the result of large shear-induced deformation. Common and differing features of the vortex interaction, modeled in the framework of the theory of point and finite-core vortices, are noted.

We have investigated the surface and subsurface submesoscale dynamics in the Gulf of Aden and the Gulf of Oman. Our results are based on the analyses of regional numerical simulations performed with a primitive equation model (HYCOM) at submesoscale permitting horizontal resolution. A model zoom for each gulf was embedded in a regional mesoscale-resolving simulation. In the Gulf of Aden and the Gulf of Oman, the interactions of mesoscale structures and fronts instabilities form submesoscale eddies and filaments. Rotational energy spectra show that the Gulf of Aden has a higher ratio of submesoscale to mesocale energy than the Gulf of Oman. Fast waves (internal gravity waves, tidal waves, Kelvin waves) and slow waves (Rossby waves) were characterized via energy spectra of the divergent velocity. Local upwelling systems which shed cold filaments, coastal current instabilities at the surface, and baroclinic instability at capes in subsurface were identified as generators of submesocale structures. In particular, the Ras al Hamra and Ras al Hadd capes in the Gulf of Oman, and the Cape of Berbera in the Gulf of Aden, are loci of submesoscale eddy generation. To determine the instability mechanisms involved in these generations, we diagnosed the Ertel potential vorticity and the energy conversion terms: the horizontal and vertical Reynolds stresses and the vertical buoyancy flux. Finally, the impacts of the subsurface submesoscale eddy production at capes on the diffusion and fate of the Red Sea Water (in the Gulf of Aden) and the Persian Gulf Water (in the Gulf of Oman) are highlighted.

International audience; Submesoscale coherent vortices (SCVs) are numerous in high-resolution numerical simulations, but their observations are scarce. Among the few in situ available measurements of SCVs, a vast majority concern anticyclones. No cyclonic SCV with large dynamical Rossby number (|ζ/f| > 1) has ever been sampled. This suggested that such small cyclones may lack robustness. Here, we present in situ measurements of an intense cyclonic SCV in the Arabian Sea. This eddy lay at 600 m depth, with a Rossby number Ro=O>(1>) and a dynamical Rossby number |ζ/f| > 1.5. This cyclone was most likely generated at the mouth of the Gulf of Aden. It trapped and advected Red Sea Water, from there on. This highlights the role of deep SCVs in the spreading of salty waters across the Arabian Sea.

To compensate for the intraoperative brain tissue deformation, computer-assisted intervention methods have been used to register preoperative magnetic resonance images with intraoperative images. In order to model the deformation due to tissue resection, the resection cavity needs to be segmented in intraoperative images. We present an automatic method to segment the resection cavity in intraoperative ultrasound (iUS) images. We trained and evaluated two-dimensional (2-D) and three-dimensional (3-D) U-Net networks on two datasets of 37 and 13 cases that contain images acquired from different ultrasound systems. The best overall performing method was the 3-D network, which resulted in a 0.72 mean and 0.88 median Dice score over the whole dataset. The 2-D network also had good results with less computation time, with a median Dice score over 0.8. We also evaluated the sensitivity of network performance to training and testing with images from different ultrasound systems and image field of view. In this application, we found specialized networks to be more accurate for processing similar images than a general network trained with all the data. Overall, promising results were obtained for both datasets using specialized networks. This motivates further studies with additional clinical data, to enable training and validation of a clinically viable deep-learning model for automated delineation of the tumor resection cavity in iUS images.

The Omani Coastal Current (OCC) flowing northward along the southern coast of Oman during the summer monsoon is associated with an upwelling system. The mesoscale circulation of the western Arabian Sea is dominated by energetic mesoscale eddies down to about 1000 m depth. They drive the pathways of the upwelling water masses and the Persian Gulf Outflow water. This paper focuses on the sub-mesoscale frontal dynamics in the OCC by analyzing the results from a regional realistic numerical simulation performed with a primitive equation model. Off the Omani coast, the interaction between the upwelling fronts and the mesoscale eddies triggers the frontogenesis at play in the surface mixed layer during the summer monsoon. In spring, sub-mesoscale eddies are generated at the Cape of Ra&rsquo, s al Hadd due to the horizontal shear instabilities undergone by the OCC. The OCC also drives and elongates Peddies formed during the Summer monsoon and located below the thermocline. Finally, the interaction between mesoscale eddies and the upwelling system leads to the formation of sub-mesoscale eddies at depth through baroclinic instabilities.

In recent years, the oceanographic community has devoted considerable interest to the study of SCVs (Submesoscale Coherent Vortices, i.e. vortices with radii between 2-30 km, below the first internal radius of deformation); indeed, both mesoscale and submesoscale eddies contribute to the transport and mixing of water masses and of tracers (active and passive), affecting the heat transport, the ventilation pathways and thus having an impact on the large scale circulation.In different areas of the ocean, SCVs have been detected, via satellite or in-situ measurements, at the surface or at depth. From these data, SCVs were found to be of different shapes and sizes depending on their place of origin and on their location. Here, we will concentrate rather on the SCVs at depth.In this study, we use a high resolution simulation of the North Atlantic ocean with the ROMS-CROCO model. In this simulation, we also identify the SCVs at different depths and densities; we analyse their site and mechanism of generation, their drift, the physical processes conducting to this drift and their interactions with the surrounding flows. We also quantify their physical characteristics (radius, thickness, intensity/vorticity, bias in polarity: cyclones versus anticyclones). We provide averages for these characteristics and standard deviations. We compare the model results with the observational data, in particular temperature and salinity profiles from Argo floats and velocity data from currentmeter recordings. This study is a first step in the understanding of the formation, occurrences and structure of SCVs in the North Atlantic Ocean, of help to improve their in-situ sampling.