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The ocean is the largest solar energy collector on Earth. The amount of heat it can store is modulated by its complex circulation, which spans a broad range of spatial scales, from metres to thousands of kilometres. In the classical paradigm, fine oceanic scales, less than 20 km in size, are thought to drive a significant downward heat transport from the surface to the ocean interior, which increases oceanic heat uptake. Here we use a combination of satellite and in situ observations in the Antarctic Circumpolar Current to diagnose oceanic vertical heat transport. The results explicitly demonstrate how deep-reaching submesoscale fronts, with a size smaller than 20 km, are generated by mesoscale eddies of size 50–300 km. In contrast to the classical paradigm, these submesoscale fronts are shown to drive an anomalous upward heat transport from the ocean interior back to the surface that is larger than other contributions to vertical heat transport and of comparable magnitude to air–sea fluxes. This effect can remarkably alter the oceanic heat uptake and will be strongest in eddy-rich regions, such as the Antarctic Circumpolar Current, the Kuroshio Extension and the Gulf Stream, all of which are key players in the climate system. Deep-reaching, small-scale oceanic fronts can drive upward heat transport from the ocean interior to the surface in eddy-rich regions, suggest satellite and in situ observations of the Antarctic Circumpolar Current.

International audience; The differentially heated rotating annulus is a laboratory experiment historically designed for modelling large-scale features of the mid-latitude atmosphere. In the present study, we investigate a modified version of the classic baroclinic experiment in which a juxtaposition of convective and motionless stratified layers is created by introducing a vertical salt stratification. The thermal convective motions are suppressed in a central region at mid-depth of the rotating tank, therefore double-diffusive convection rolls can develop only in thin layers located at top and bottom, where the salt stratification is weakest. For high enough rotation rates, the baroclinic instability destabilises the flow in the top and the bottom shallow convective layers, generating cyclonic and anticyclonic eddies separated by the stable stratified layer. Thanks to this alternation of layers resembling the convective and radiative layers of stars, the planetary's atmospheric troposphere and stratosphere or turbulent layers at the sea surface above stratified waters, this new laboratory setup is of interest for both astrophysics and geophysical sciences. More specifically, it allows to study the exchange of momentum and energy between the layers, primarly by the propagation of internal gravity waves (IGW). PIV velocity maps are used to describe the wavy flow pattern at different heights. Using a co-rotating laser and camera, the wave field is well resolved and different wave types can be found: baroclinic waves, Kelvin, and Poincaré type waves. The signature of small-scale IGW can also be observed attached to the baroclinic jet. The baroclinic waves occur at the thin convectively active layer at the surface and the bottom of the tank, though decoupled they show different manifestation of nonlinear interactions. The inertial Kelvin and Poincaré waves seem to be mechanically forced. The small-scale wave trains attached to the meandering jet point to an imbalance of the large-scale flow. For the first time, the simultaneous occurrence of differentwave types is reported in detail for a differentially heated rotating annulus experiment.

We investigate the non-linear equilibration of a two-layer quasi-geostrophic flow in a channel {with an initial eastward baroclinically unstable jet in the upper layer}, paying particular attention to the role of bottom friction. In the limit of low bottom friction, classical theory of geostrophic turbulence predicts an inverse cascade of kinetic energy in the horizontal with condensation at the domain scale and barotropization in the vertical. By contrast, in the limit of large bottom friction, the flow is dominated by ribbons of high kinetic energy in the upper layer. These ribbons correspond to meandering jets separating regions of homogenized potential vorticity. We interpret these results by taking advantage of the peculiar conservation laws satisfied by this system: the dynamics can be recast in such a way that {the initial eastward jet} in the upper layer appears as an initial source of potential vorticity levels in the upper layer. The initial baroclinic instability leads to a turbulent flow that stirs this potential vorticity field while conserving the global distribution of potential vorticity levels. Statistical mechanical theory of the 1 1/2 layer quasi-geostrophic model predicts the formation of two regions of homogenized potential vorticity separated by a minimal interface. {We explain that cascade phenomenology leads to the same result.} We then show that the dynamics of the ribbons results from a competition between a tendency to reach the equilibrium state, and baroclinic instability that induces meanders of the interface. These meanders intermittently break and induce potential vorticity mixing, but the interface remains sharp throughout the flow evolution. We show that for some parameter regimes, the ribbons act as a mixing barrier which prevents relaxation toward equilibrium, favouring the emergence of multiple zonal (eastward) jets.

International audience; High-resolution direct numerical simulations have shown the occurrence of inertia gravity waves simultaneously with baroclinic instabilities within a differentially heated rotating annulus, the "baroclinic cavity". The working fluid is characterised by a Prandtl number Pr = 16. A decomposition technique applied to the dependent variables has allowed us to separate in space and in time the contributions of the large-scale baroclinic structures from that of the small-scale fluctuations. These latter have been identified as inertia gravity waves from their dispersion relation. The present work is particularly focused on the mechanism responsible for the spontaneous generation of these waves.

International audience; The stability properties of a coastal current composed of a deep flow and of an intense counterflowing surface jet are investigated with a linear quasi-geostrophic model and with a nonlinear isopycnic shallow-water model (MICOM), both in three-layer configurations. The currents are modeled by strips of uniform potential vorticity anomaly (PVA), with opposite signs. The linear stability analysis of the current is performed with exponentially-growing modes, varying the ratio of layer thicknesses, the potential vorticity distribution (width and intensity of the PVA strips) and the perturbation wavelength. The effect of a sloping bottom is also investigated. The various nonlinear regimes are described and interpreted in terms of growth of the modal perturbations, of critical layer distributions and of interactions of PVA poles. The linear growth rates of the perturbations are essential for enough PVA of the currents to reach the critical layers before wave breaking can occur. The positions of the critical layers determine where cut-off of the PVA contours occurs. This position is shown to depend only on the lower layer thickness (the upper layer one being kept constant). The intermediate layer thickness determines only the growth rates of the perturbation. Finally, the long-time nonlinear evolutions are governed by the interaction of detached PVA poles. An oceanographic application is performed using a 4-layer configuration, representative of the Mediterranean water undercurrents flowing under the South Portugal coastal upwelling jet. The analysis of this configuration shows that even for a stable upper layer, instability-driven eddies in the lower layers can disturb the surface jet and generate large meanders and filaments, similar to the observations from south of the Iberian peninsula.

The Atlantic Water transport in the eastern Mediterranean is strongly impacted by the dynamics of mesoscale eddies. The latter are mainly produced through baroclinic instability of coastal currents. These eddies, as well as the meanders characterizing the instabilities, play an important part in the transport of water masses, heat, and nutriments between the coast and the open sea. Those transfers have to be well represented in numerical simulations used in decision-making processes facing socio-economic issues. Nevertheless, the impact of the bathymetry on the general ocean surface circulation in the Mediterranean is still poorly understood. This PhD work aims at understanding and quantifying the impact of the undersea topography on current instabilities and on the dynamics of eddies formed alongshore. Observations of those features in the eastern Mediterranean, using surface drifters and numerical simulations, confirm that they are ubiquitous and that they influence strongly the surface circulation. This study highlights also the difficulties of numerical models in reproducing those eddies. The baroclinic instability above a topographic slope is then studied by means of an idealized coastal current produced both in laboratory experiments on a turntable and in high resolution numerical simulations. The combined use of these two methodologies emphasizes the stabilizing effect of the bathymetry inducing a decrease of the unstable growth rates and the formation of smaller scale structures as the topographic slope increases. The dynamics of these features above a slope is then driven by non-linear processes yielding the formation of submesoscale structures.; Le transport de l'Eau Atlantique en Méditerranée orientale est fortement influencé par la dynamique de tourbillons méso-échelles. Ceux-ci, trouvent leur origine principalement dans l'instabilité barocline de courants côtiers. Ces tourbillons ainsi que les méandres caractéristiques des instabilités, jouent un rôle important dans le transport de masse d'eau, de chaleur, d'éléments nutritifs entre la côte et le large. Ces échanges se doivent d'être correctement reproduits dans les modèles numériques de circulation générale, qui servent souvent à la prise de décision dans des problématiques socio-économiques. Or, le rôle de la bathymétrie sur la circulation océanique de surface en Méditerranée n'est toujours pas bien compris. Ce travail de thèse cherche donc à comprendre et à quantifier l'influence de la topographie sous-marine sur les instabilités des courants de bord et sur la dynamique des structures tourbillonnaires formées le long de la côte. Une observation de ces structures en Méditerranée Orientale, à l'aide de flotteurs de surface et de simulations numériques, confirme leur omniprésence ainsi que leur importance dans la circulation générale de surface. Cette étude met également en avant les difficultés des modèles numériques à reproduire correctement ces tourbillons. L'instabilité barocline au-dessus d'une pente topographique est ensuite étudiée à l'aide d'un courant côtier idéalisé produit dans des expériences en laboratoire sur table tournante et des simulations numériques haute résolution. L'utilisation conjointe des deux approches met en lumière le caractère stabilisant de la bathymétrie qui induit une diminution des taux de croissance instables et la formation de structures de plus petites tailles lorsque la pente topographique augmente. La dynamique de ces structures au-dessus d'une pente est soumise à des processus non-linéaires conduisant à la formation de structures sous méso-échelles.

IN COASTAL AREAS, WHEN THE SUMMER STRATIFICATION SETTLES, THE BOTTOM AND SURFACE TURBULENT BOUNDARY LAYERS CAN OVERLAP AND DELINEATE THERMAL FRONTS. AN EXAMPLE IN THE IROISE SEA IS THE USHANT FRONT THAT DEVELOPS AROUND AREAS OF INTENSE MIXING (ZMP): THE "RAZ DE SEIN" AND THE AREA OF OUESSANT. ITS EXTENSION IS VARIABLE AND EXCEEDS THE AREAS OF MIXED WATERS PRODUCTION. BOTH THE EXTENSION OF THE FRONT AND ITS VARIABILITY ARE SENSITIVE TO MECHANISMS OF DISPERSION OF HOMOGENIZED WATERS. THOSE MECHANISMS OF DISPERSION ARE STUDIED USING ACADEMIC CONFIGURATIONS WHERE TIDAL MIXING IS REPRESENTED AS FIXED AREAS WHERE STRATIFIED WATERS ARE CONTINUOUSLY HOMOGENIZED. IT IS SHOWN THAT BAROCLINIC INSTABILITY DEVELOPING AT THE EDGE OF THE ZMPS REPRESENTS THE MOST EFFICIENT MECHANISM TO DISPERSE THE HOMOGENIZED WATERS. ANY PROCESS THAT MODIFIES THE CHARACTERISTICS OF BAROCLINIC INSTABILITY, OR THE AUTO-PROPAGATING EFFICIENCY OF EMERGING STRUCTURES, SIGNIFICANTLY IMPACTS THE DISPERSION RATE AND PATTERN. THE SENSITIVITY OF DISPERSIVE PROCESSES TO SEASONAL AND LOCAL PARAMETERS IS ANALYZED. Realistic configurations of the Iroise sea are then used to investigate the variability of thermal fronts when mixing processes are combined with dispersive mechanisms. Thermal gradients in the surface and subsurface layers are investigated considering extension patterns of the Ushant front during summer 2009. It is shown that thermal fronts are partly initiated by the presence of mixed waters produced in ZMPs and subducted in subsurface. Those anticyclonic structures thicken the subsurface layer, and can interact with surface turbulence, thus favoring the outcropping of cold to mild waters at the surface.; Dans les zones côtières, la turbulence des couches limites conduit parfois à l'homogénéisation totale de la colonne d'eau. Lorsque la stratification estivale se met en place, ces zones homogénéisées sont associées à des fronts thermiques. En mer d'Iroise, les fortes marées aux abords de Sein et d'Ouessant font apparaître le "front d'Ouessant". Son extension résulte de mécanismes dispersi fs des eaux homogènes, est variable et dépasse les zones de brassage où ces eaux sont produites (ZMP). Les mécanismes dispersifs sont étudiés à l'aide de configurations académiques où le mélange de marée est représenté par une zone fixe où les eaux stratifiées sont brassées en permanence. On montre que l'instabilité barocline, qui se développe aux frontières de la ZMP, est le processus le plus efficace pour disperser les eaux mélangées. Tout processus affectant l'instabilité barocline, ou la dispersion des structures émergentes, influence significativement le taux de dispersion et la forme du panache d'eaux mélangées. Ceci permet d'analyser la sensibilité du processus dispersif aux paramètres locaux et saisonniers. Des études en configuration réaliste permettent d'analyser les effets conjugués du mélange et de la dispersion sur l'extension des gradients thermiques en mer d'Iroise. Leur influence sur la variabilité des fronts en surface et subsurface est analysée sur l'été 2009. On montre que les fronts thermiques de surface sont en partie initiés par la présence d'eaux produites dans les ZMP et infiltrées dans la thermocline. Ces structures épaississent la couche intermédiaire et, par l'action de la turbulence de surface, facilitent la création d'eaux froides à tempérées en surface.

IN COASTAL AREAS, WHEN THE SUMMER STRATIFICATION SETTLES, THE BOTTOM AND SURFACE TURBULENT BOUNDARY LAYERS CAN OVERLAP AND DELINEATE THERMAL FRONTS. AN EXAMPLE IN THE IROISE SEA IS THE USHANT FRONT THAT DEVELOPS AROUND AREAS OF INTENSE MIXING (ZMP): THE "RAZ DE SEIN" AND THE AREA OF OUESSANT. ITS EXTENSION IS VARIABLE AND EXCEEDS THE AREAS OF MIXED WATERS PRODUCTION. BOTH THE EXTENSION OF THE FRONT AND ITS VARIABILITY ARE SENSITIVE TO MECHANISMS OF DISPERSION OF HOMOGENIZED WATERS. THOSE MECHANISMS OF DISPERSION ARE STUDIED USING ACADEMIC CONFIGURATIONS WHERE TIDAL MIXING IS REPRESENTED AS FIXED AREAS WHERE STRATIFIED WATERS ARE CONTINUOUSLY HOMOGENIZED. IT IS SHOWN THAT BAROCLINIC INSTABILITY DEVELOPING AT THE EDGE OF THE ZMPS REPRESENTS THE MOST EFFICIENT MECHANISM TO DISPERSE THE HOMOGENIZED WATERS. ANY PROCESS THAT MODIFIES THE CHARACTERISTICS OF BAROCLINIC INSTABILITY, OR THE AUTO-PROPAGATING EFFICIENCY OF EMERGING STRUCTURES, SIGNIFICANTLY IMPACTS THE DISPERSION RATE AND PATTERN. THE SENSITIVITY OF DISPERSIVE PROCESSES TO SEASONAL AND LOCAL PARAMETERS IS ANALYZED. Realistic configurations of the Iroise sea are then used to investigate the variability of thermal fronts when mixing processes are combined with dispersive mechanisms. Thermal gradients in the surface and subsurface layers are investigated considering extension patterns of the Ushant front during summer 2009. It is shown that thermal fronts are partly initiated by the presence of mixed waters produced in ZMPs and subducted in subsurface. Those anticyclonic structures thicken the subsurface layer, and can interact with surface turbulence, thus favoring the outcropping of cold to mild waters at the surface.; Dans les zones côtières, la turbulence des couches limites conduit parfois à l'homogénéisation totale de la colonne d'eau. Lorsque la stratification estivale se met en place, ces zones homogénéisées sont associées à des fronts thermiques. En mer d'Iroise, les fortes marées aux abords de Sein et d'Ouessant font apparaître le "front d'Ouessant". Son extension résulte de mécanismes dispersi fs des eaux homogènes, est variable et dépasse les zones de brassage où ces eaux sont produites (ZMP). Les mécanismes dispersifs sont étudiés à l'aide de configurations académiques où le mélange de marée est représenté par une zone fixe où les eaux stratifiées sont brassées en permanence. On montre que l'instabilité barocline, qui se développe aux frontières de la ZMP, est le processus le plus efficace pour disperser les eaux mélangées. Tout processus affectant l'instabilité barocline, ou la dispersion des structures émergentes, influence significativement le taux de dispersion et la forme du panache d'eaux mélangées. Ceci permet d'analyser la sensibilité du processus dispersif aux paramètres locaux et saisonniers. Des études en configuration réaliste permettent d'analyser les effets conjugués du mélange et de la dispersion sur l'extension des gradients thermiques en mer d'Iroise. Leur influence sur la variabilité des fronts en surface et subsurface est analysée sur l'été 2009. On montre que les fronts thermiques de surface sont en partie initiés par la présence d'eaux produites dans les ZMP et infiltrées dans la thermocline. Ces structures épaississent la couche intermédiaire et, par l'action de la turbulence de surface, facilitent la création d'eaux froides à tempérées en surface.

The Atlantic Water transport in the eastern Mediterranean is strongly impacted by the dynamics of mesoscale eddies. The latter are mainly produced through baroclinic instability of coastal currents. These eddies, as well as the meanders characterizing the instabilities, play an important part in the transport of water masses, heat, and nutriments between the coast and the open sea. Those transfers have to be well represented in numerical simulations used in decision-making processes facing socio-economic issues. Nevertheless, the impact of the bathymetry on the general ocean surface circulation in the Mediterranean is still poorly understood. This PhD work aims at understanding and quantifying the impact of the undersea topography on current instabilities and on the dynamics of eddies formed alongshore. Observations of those features in the eastern Mediterranean, using surface drifters and numerical simulations, confirm that they are ubiquitous and that they influence strongly the surface circulation. This study highlights also the difficulties of numerical models in reproducing those eddies. The baroclinic instability above a topographic slope is then studied by means of an idealized coastal current produced both in laboratory experiments on a turntable and in high resolution numerical simulations. The combined use of these two methodologies emphasizes the stabilizing effect of the bathymetry inducing a decrease of the unstable growth rates and the formation of smaller scale structures as the topographic slope increases. The dynamics of these features above a slope is then driven by non-linear processes yielding the formation of submesoscale structures., Le transport de l'Eau Atlantique en Méditerranée orientale est fortement influencé par la dynamique de tourbillons méso-échelles. Ceux-ci, trouvent leur origine principalement dans l'instabilité barocline de courants côtiers. Ces tourbillons ainsi que les méandres caractéristiques des instabilités, jouent un rôle important dans le transport de masse d'eau, de chaleur, d'éléments nutritifs entre la côte et le large. Ces échanges se doivent d'être correctement reproduits dans les modèles numériques de circulation générale, qui servent souvent à la prise de décision dans des problématiques socio-économiques. Or, le rôle de la bathymétrie sur la circulation océanique de surface en Méditerranée n'est toujours pas bien compris. Ce travail de thèse cherche donc à comprendre et à quantifier l'influence de la topographie sous-marine sur les instabilités des courants de bord et sur la dynamique des structures tourbillonnaires formées le long de la côte. Une observation de ces structures en Méditerranée Orientale, à l'aide de flotteurs de surface et de simulations numériques, confirme leur omniprésence ainsi que leur importance dans la circulation générale de surface. Cette étude met également en avant les difficultés des modèles numériques à reproduire correctement ces tourbillons. L'instabilité barocline au-dessus d'une pente topographique est ensuite étudiée à l'aide d'un courant côtier idéalisé produit dans des expériences en laboratoire sur table tournante et des simulations numériques haute résolution. L'utilisation conjointe des deux approches met en lumière le caractère stabilisant de la bathymétrie qui induit une diminution des taux de croissance instables et la formation de structures de plus petites tailles lorsque la pente topographique augmente. La dynamique de ces structures au-dessus d'une pente est soumise à des processus non-linéaires conduisant à la formation de structures sous méso-échelles.

The meanders of a baroclinic coastal current in the Northwestern Mediterranean Sea have already been reported in the literature. These meanders can be surrounded by vortices. Such vortices have been observed in the western part of the Gulf of Lions but the location and the mechanism of their formation are poorly documented. In this paper, we use the current measurements of a one-year experiment, which was conducted in the eastern part of the Gulf of Lions to detect and characterize the vortex activity. A vortex detection algorithm based on few velocity data was developed. Current measurements were available at the sea surface (HF radars) and in the water column from 50 to 140 m depth (four current meter moorings). SST images and hydrologic data were also used. Results focus on observations that are coherent 50 m and at the surface. Vortices are anticyclonic, of submesoscale size and present maximal velocities of 30-50 cm/s. The drift speed of the vortices is comparable to but less than the velocity of the Northern Current. These observations enable to estimate the minimum vortex occurrence in this area. The presence of vortex structures is strongly correlated with a specific sequence of wind patterns. (C) 2010 Elsevier Ltd. All rights reserved.

For the atmosphere and oceans, which are forced mostly at low frequency, there is a time-scale separation leading to a weak coupling between the slow balanced and fast unbalanced components of the dynamics. Ageostrophic instabilities may provide a route to escape this separation and couple balanced and unbalanced motions. To understand these mechanisms systematic linear stability analyses in a two-layer rotating shallow water model are provided. A focus is made on the Rossby-Kelvin (RK) instability in a rectilinear channel and in the annulus for comparison with the experiments. We demonstrate the existence of crossover regions where baroclinic and RK compete having similar growth rates. We then revisit the linear stability problem for coastal currents and study various instabilities due to resonances between vortical (Rossby-like), frontal and coastal trapped waves. The nonlinear evolution of these instabilities is then studied in the shallow-water approximation with the help of high-resolution DNS and in a continuously stratified fluid using a mesoscale model. It is shown that saturation for the RK instability and the Kelvin-frontal (KF) instability is achieved through the breaking of the Kelvin mode forming a Kelvin front and leading to inertia-gravity waves and reorganization of the mean flow through dissipative and wave-meanflow interaction effects. We also show for coastal curents how nonlinear saturation of the ageostrophic instabilities and reorganization of the potential vorticity field lead to coherent vortex structures appearing and eventually detaching from the coast., Les écoulements atmosphériques et océaniques sont habituellement séparés en une partie lente, proche de l'équilibre géostrophique, et une partie rapide qui interagissent faiblement. Les instabilités agéostrophiques procurent néanmoins un mécanisme capable de coupler les mouvements équilibrés et non-équilibrés. L'étude de tels mécanismes dans cette thèse est d'abord effectuée par des études systématiques de stabilité linéaire dans le modèle de l'eau peu profonde. L'instabilité de Rossby-Kelvin (RK), issue du couplage barocline d'une onde de Rossby (équilibrée) et d'une onde de Kelvin ou de Poincaré (non-équilibrées), est mise en évidence dans un canal. Ces résultats sont étendus à un anneau en géométrie cylindrique et montrent que dans les configurations proches des dispositifs expérimentaux il peut y avoir une compétition entre l'instabilité RK et l'instabilité barocline classique. Les instabilités des courants côtiers, caractérisés par la présence d'un bord vertical et d'un "outcropping", sont ensuite étudiées de manière exhaustive. Les instabilités dues à l'interaction d'un mode frontal et d'une onde de Kelvin (KF) ou d'une onde de Rossby (RF) sont mises en évidence. Le développement non-linéaire de ces instabilités est étudié dans le modèle de l'eau peu profonde par une méthode numérique aux volumes finis et dans un fluide continument stratifié à l'aide d'un modèle méso-échelle. Ces simulations montrent que les instabilités RK et KF ont un développement non-linéaire marqué par une croissance à amplitude finie et une saturation. Le déferlement de l'onde de Kelvin conduit alors à la formation d'un front de Kelvin, zone localisée de mélange et de dissipation, à l'émission d'ondes d'inertie-gravité et à la réorganisation de l'écoulement moyen. Dans le cas des courants côtiers cette réorganisation est suivie du développement d'une instabilité secondaire qui conduit à la formation et au détachement de vortex.

International audience; The representation of baroclinic instability in numerical models depends strongly upon the model physics and significant differences may be found depending on the vertical discretization of the governing dynamical equations. This dependency is explored in the context of the restratification of an idealized convective basin with no external forcing. A comparison is made between an isopycnic model including a mixed layer (the Miami Isopycnic Coordinate Ocean Model, MICOM), its adiabatic version (MICOM-ADIAB) in which the mixed layer physics are removed and the convective layer is described by a deep adiabatic layer outcropping at the surface instead of a thick dense mixed layer, and a z-coordinate model (OPA model).In the absence of a buoyancy source at the surface, the mixed layer geometry in MICOM prevents almost any retreat of this layer. As a result, lateral heat exchanges in the upper layers are limited while mass transfers across the outer boundary of the deep convective mixed layer result in an unrealistic outward spreading of this layer. Such a widespread deep mixed layer maintains a low level of baroclinic instability, and therefore limits lateral heat exchanges in the upper layers over most of the model domain. The behavior of the adiabatic isopycnic model and z-coordinate model is by far more satisfactory although contrasted features can be observed between the two simulations. In MICOM-ADIAB, the more baroclinic dynamics introduce a stronger contrast between the surface and the dense waters in the eddy kinetic energy and heat flux distributions. Better preservation of the density contrasts around the dense water patch maintains more persistent baroclinic instability, essentially associated with the process of dense water spreading. The OPA simulation shows a faster growth of the eddy kinetic energy in the early stages of the restratification which is attributed to more efficient baroclinic instability and leads to the most rapid buoyancy restoring in the convective area among the three simulations. Dense water spreading and warm surface capping occur on fairly similar time scales in MICOM-ADIAB although the former is more persistent that the latter. In this model, heat is mainly transported by anticyclonic eddies in the dense layer while both cyclonic and anticyclonic eddies are involved in the upper layers. In OPA, heat is mainly brought into the convective zone through the export of cold water trapped in cyclonic eddies with a strong barotropic structure. Probably the most interesting difference between the z-coordinate and the adiabatic isopycnic model is found in the temperature distribution ultimately produced by the restratification process. OPA generates a spurious volume of intermediate water which is not seen in MICOM-ADIAB where the volume of the dense water is preserved.

This is the second of three papers investigating the regime transition that occurs in numerical simulations for an idealized, equilibrium, subtropical, eastern boundary, upwelling current system similar to the California Current. The emergent upper-ocean submesoscale fronts are analyzed from phenomenological and dynamical perspectives, using a combination of composite averaging and separation of distinctive subregions of the flow. The initiating dynamical process for the transition is near-surface frontogenesis. The frontal behavior is similar to both observed meteorological surface fronts and solutions of the approximate dynamical model called surface dynamics (i.e., uniform interior potential vorticity q and diagnostic force balance) in the intensification of surface density gradients and secondary circulations in response to a mesoscale strain field. However, there are significant behavioral differences compared to the surface-dynamics model. Wind stress acts on fronts through nonlinear Ekman transport and creation and destruction of potential vorticity. The strain-induced frontogenesis is disrupted by vigorous submesoscale frontal instabilities that in turn lead to secondary frontogenesis events, submesoscale vortices, and excitation of even smaller-scale flows. Intermittent, submesoscale breakdown of geostrophic and gradient-wind force balance occurs during the intense frontogenesis and frontal-instability events.

This paper reports on an experimental investigation of the stability of a baroclinic front, as generated by the spin- up of a differentially rotating lid at the surface of a rotating two-layer fluid. In contrast to formerly reported experiments, we consider miscible fluids in a relatively wide annular tank. In the parameter space set by rotational Froude number and dissipation (i.e. ratio of spin-down time to disk rotation time), different flow regimes are observed, ranging from axisymmetric to irregular baroclinic instable flows. These regimes adjoin the regimes found for immiscible fluids by Williams et al. (2005) in a small device. Differences are the baroclinic unstable regime for lower Froude number than reported by Williams et al. (2005) suggesting Sakai Rossby-Kelvin waves and the formation of vortices. Observations suggest spontaneous emission of inertia gravity waves, which interact with the large-scale baroclinic wave and lead to intense mixing and the rapid formation of intense cyclonic vortices., Cette article représente une étude expérimentale sur l’instabilité d’un front barocline qui a été généré par la mise en rotation différentielle d’un fluide bi-couche. Contrairement à d’autres expériences précédentes de ce type, nous considérons des fluides miscibles dans une cuve relativement grande. Dans l’espace déterminée par le nombre de Froude et le nombre de dissipation, nous observons différents régimes, allant des écoulements axisymétriques aux écoulements instables et irréguliers. Ces régimes adjoignent les régimes obtenus pour un fluide bicouche immiscibles par Williams et al. (2005) dans une cuve petite. Différente est l’apparition du régime barocline pour des nombres de Froude plus bas, suggérant les ondes Rossby-Kelvin de Sakai, et la formation des tourbillons. Les observations suggèrent l’émission spontanée des ondes inertie-gravitée lesquelles interagissent avec l’onde barocline à grande échelle, et ainsi déclenche du mélange et la formation rapide des tourbillons intenses.

International audience; We examine the spin-up from rest of a stratified fluid with initial Brunt–Väisälä frequency N bound within a cylindrical container of height 2H and radius R which is set to rotate impulsively with angular speed f/2. Particular attention is given to characterizing the dependence of the form of the resulting flow on the governing parameters. Our experimental study reveals a wealth of flow behaviours and instabilities. In all experiments, the initial phase of motion is marked by the establishment of mixed axisymmetric corner regions fed by radial Ekman transport, a process detailed in Flór et al. (Flór, J.B., Ungarish, M. and Bush, J.W.M., “Spin-up from rest of a stratified fluid: boundary flows”, J. Fluid Mech., 472, 51–82 (2002).).The subsequent evolution of the central vortex depends critically on the Burger number , where is the buoyancy frequency of the central core following the establishment of the corner regions. For B > 1.0, the axisymmetry of the system is retained throughout the spin-up process: the central vortex attains a state of near solid body rotation by the diffusion of vorticity from the sidewalls. For B < 1.0, the central core becomes baroclinically unstable, and its streamlines strained from circles into ellipses. Subsequently, for Nc/f < 1 (and B < 1.0), the symmetry of the central core is broken in a manner reminiscent of the elliptical instability. For short tanks (2H/R < 1), the instability is marked by a simple tip-over of the central core in the laboratory frame that is resisted by the core stratification. For 2H/R > 1, the centreline of the stratified core is deflected into a helical form before the core breaks into a series of stacked vortices. A Burger number criterion, , for the baroclinic instability of the central core is derived and found to be consistent with the experimental observations.

International audience; We focus on the influence of the -effect on the singular modes of baroclinic instability in the Phillips model. An analytical intercomparison between normal and singular modes for the Eady problem on an f-plane has already been performed, showing that the amplification rate of the singular mode for the kinetic-energy norm after a dimensional time of three days is about 1.2 times larger than for the normal mode. We show here that, in the presence of the -effect, the maximum amplification rate of the singular mode can be 1.5 times larger than for the normal-mode case. This difference is due to the lesser stabilizing effect of on the singular modes compared with the normal modes. This result is obtained for the case of equal layer depths which is relevant for the atmospheric situation. For the oceanic situation with unequal layer depths the amplification due to is further increased leading to a factor of 1.8 in amplification rates between the two types of modes after a dimensional time of about 20 days. This behaviour is explained by the fact that only intervenes in the anti-Hermitian part of the linear dynamical operator, the main effect of which is to alter the vertical phase tilt of the perturbations. As a consequence, the structure of the bi-orthogonal mode becomes more inclined to the vertical in the presence of while, on the contrary, the structure of the unstable normal mode becomes less inclined.