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International audience; This paper presents the paleocurrent evolution of the Persian Gulf deep water (PGDW) during the last 5000 years using faunal (foraminiferal assemblage) and non-faunal (magnetic susceptibility, sediment grain size, number of coarse relict particles and physicochemical characteristics) indicators from a 133 cm long core retrieved from the Strait of Hormoz (SH). Our results suggest that the precession-related insolation change and solar variability induced long-term and short-term sea surface temperature (SST) rise and fall resulted in variation of PGDW outflow strength. Two periods of decreasing SST (5000-4300 and 2000-1450 cal yr BP), accompanied by wetter conditions in the Middle East, are related to the reduction of the PGDW current velocity. A combination of high sedimentation rate (0.72 mm yr-1, due to high terrigenous input) with a relatively low bottom flow energy cause the settling of finer sediments providing a favourable environment for the expansion of benthic foraminifera (dominated by infaunal taxa). On the contrary, the PGDW current velocity strengthens in the periods of SST increase (4300-2000 and 1450 cal yr BP to present) with decreasing sedimentation rate (0.15-0.34 mm yr-1, due to low river discharge). During both periods of high-velocity bottom currents, the benthic foraminifera accumulation rate decreases. However, as the PGDW brings a relatively high amount of dissolved oxygen (3.25 mg/l in summer and 4.79 mg/l in winter) and phytodetritus particles, compared to the surrounding water, some particular foraminifera (especially robust suspension feeder epifaunal taxa) evolved. SST increase, intensified by anthropically caused global warming, may lead to PGDW intensification in the future.

The Lofoten Vortex (LV), a quasi-permanent anticyclonic eddy in the Lofoten Basin of the Norwegian Sea, is investigated with an eddy-permitting primitive equation model nested into the ECCO2 ocean state estimate. The LV, as simulated by the model, extends from the sea surface to the ocean bottom at about 3000 m and has the subsurface core between 50 m and 1100 m depths. Above and below the vortex core the relative vorticity signal decreases in amplitude while the radius increases by as much as 25–30% relative to the values in the core. Analyzing the model run, we show that the vertical structure of the LV can be casted into four standard configurations, each of which forms a distinct cluster in the parameter space of potential vorticity anomalies in and above the LV core. The stability of the LV for each of the configurations is then studied with three-layer and a two-layer (in winter) quasi-geostrophic (QG) models over a flat bottom as well as over a realistic topography. The QG results show a number of common features with those of the primitive equation model. Thus, among the azimuthal modes dominating the LV instability, both the QG model and the primitive equation model show a major role the 2nd and 3rd modes. In the QG model simulations the LV is the subject of a rather strong dynamic instability, penetrating deep into the core. The results predict 50–95% volume loss from the vortex within 4–5 months. Such a drastic effect is not observed in the primitive equation model, where, for the same intensity of perturbations, only 10–30% volume loss during the same period is detected. Taking into account the gently sloping topography of the central part of the Lofoten basin and the mean flow in the QG model, brings the rate of development of instability close to that in the primitive equation model. Some remaining differences in the two models are discussed. Overall, the LV decay rate obtained in the models is slow enough for eddy mergers and convection to restore the thermodynamic properties of the LV, primarily re-building its potential energy anomaly. This justifies the quasi-permanent presence of the LV in the Lofoten Basin.

We investigate the stability of a composite cyclone representative of Arabian Sea eddies using a high resolution primitive equation model. We observe that the eddy is unstable with respect to a mixed barotropic/baroclinic instability, leading to the growth of an azimuthal mode 2 perturbation. The latter deforms the eddy, which eventually evolves into a tripole after about 4 months of simulation. The presence of a critical level for the most unstable mode generates sharp fronts in the surface mixed layer where the Rossby number is large. These fronts then become unstable, and this generates submesoscale cyclones and filaments. Near these fronts, diapycnal mixing occurs, causing the potential vorticity to change sign locally, and symmetric instability to develop in the core of the cyclonic eddy. Despite the instabilities, the eddy is not destroyed and remains a large-scale coherent structure for the last 6 months of the simulation. Looking at Sea Surface Height only, the composite eddy evolves little, and fairly represents the eddy observed in the altimetry which can live for several months. The study of this simulation thus illustrates the numerous kinds of instabilities which may occur in large cyclonic eddies but cannot be captured directly by altimetric data.

We use cross-slope sections of direct current observations together with a high resolution numerical simulation to revisit estimates of transports and entrainment in the Gulf of Cadiz. We provide a three dimensional picture of the outflow from the Mediterranean into the intermediate layers of the Atlantic. In the model, the time-averaged Mediterranean Undercurrent is characterised by two cores of zonal velocity at 8 ° 30 ′ W: one at 500 m (in the σ 1 interval 31.6–31.8 kg m−3) and another around 1100 m (∼32.2 kg m−3) with maximum westward velocity of 0.36 m s−1. A single well defined vein of saltier and warmer water (salinity maximum ∼36.9 psu, 1 psu = 1 kg salt/1000 kg seawater) is found attached to the slope, centred at 1300 m (32.2–32.4 kg m−3). The observational sections corroborate this description but instant maximum velocity reaches 0.6 m s−1 whereas salinity peaks just above 36.5 psu. Unlike what was previously thought, the velocity veins and the thermohaline anomaly cores are not co-located. At the Strait of Gibraltar, we estimate that the transport of pure Mediterranean Water (S > 38.4 psu) is about 0.48 Sv (1 Sv = 106 m3 s−1). Near the Portimao Canyon, the results for westward transport (31.6–32.6 kg m−3) computed from observations are within the range 2.6–3.6 Sv and the time-mean estimate from the numerical simulation is of 3.5 Sv. The westward salinity and heat transports are of ≲1.3 psu Sv (1 psu Sv = 103 m3 s−1) and ∼ 20 × 10 12 W , from observational and numerical data alike. By tracking water masses within a closed domain in the Gulf of Cadiz, we find that most of the North Atlantic Water entrained into the undercurrent is supplied through the south and southwest borders. After analysing volume balances per layer, we conclude that the entrainment from shallower layers is around 1.1 Sv in total: 0.32, 0.34, 0.40 and 0.04 Sv distributed by four equally spaced density intervals (0.2 kg m−3) between isopycnals 31.8 and 32.6 kg m−3.

In the Arabian Sea, mesoscale eddies are prominent features of the circulation. They influence biological activities, tracer transport, and physical and chemical properties of the seawater. In particular, the pathways of salty water masses outflowing from the marginal seas are strongly impacted by the mesoscale eddies that evolve in the western part of the basin. The impact of mesoscale eddies on the different salty water masses has been investigated in previous studies with altimetric data and in situ data for short durations. In this study, we extend this analysis to most of the mesoscale eddies detected from altimetry data for the period 2000–2015. We use Argo floats colocalized with eddies to describe the 3D impact of mesoscale eddies on the dynamical and thermohaline properties of water masses in the Arabian Sea. The ∼ 30,000 Argo stations used show a well characterized distribution of water masses in the different sub-basins of the Arabian Sea, with the Persian Gulf Water (PGW) flowing from the north and the Red Sea Outflow Water (RSOW) from the northwest. In addition, colocalizing Argo profiles in or out of these eddies allows for the extraction of temperature and salinity anomalies related to mesoscale eddies. Then, we present the vertical structure of density associated with these eddies. These results allow us to divide the Arabian Sea into areas depending on their hydrological and dynamical properties. In the Gulf of Oman and the Gulf of Aden, the water in the core of eddies is fresher than at their edge. Thus, most of eddies residing in these gulfs were generated in the Arabian Sea. In these eddies, the salinity anomalies corresponding to PGW and RSOW are on average stronger at their edge than in their core; this indicates that eddies in these gulfs impact outflow waters via advecting processes on their rim. Finally, we select the northern part of the Arabian Sea (including the Gulf of Oman), where most Argo profiles were collected. We calculate the composite 3D profiles of angular velocity and salinity anomaly for surface-intensified cyclonic eddies and subsurface anticyclonic eddies, with radii of about 100 km. The velocity profiles computed via the thermal wind equations are validated using surface velocities from altimetry and surface drifter measurements. On average, eddies in the northern Arabian Sea exhibit a clear pattern of PGW in their core. The comparison of these 3D composite structures with a particular cyclone near the Strait of Hormuz and a dipole sampled with a SeaSoar along the Omani Coast shows that they are representative of mesoscale eddies often observed in the area.

The dynamics of passive fluid particles in the vicinity of two point vortices with arbitrary intensities, embedded in a steady external deformation flow, is studied. The motion of passive fluid particles is described by a nonintegrable 1.5 degrees of freedom dynamical system. Though the external flow is stationary, the additional half degree of freedom appears because the vortices’ motion about their stationary positions is periodic. Then, this periodic motion plays the role of a periodic perturbation for the system describing the passive particle dynamics. Therefore, chaotic advection of passive fluid particles in the vicinity of these two vortices can occur. If the vortices, however, are situated at their stationary positions, they become motionless, and the dynamical system describing the passive particles’ dynamics is also stationary. In the case of motionless vortices, a classification of the phase portraits of the passive particle motion is conducted by analyzing the number of critical points. When the vortices do not lie at their stationary position, the system becomes nonstationary. In this case, the existence of impenetrable transport barriers for chaotic advection is shown. These barriers are destroyed when stochastic layers merge; these layers widen as the deviation of the vortex position from the stationary points, increases. The efficiency of chaotization is analyzed by means of Poincare sections and accumulated Lyapunov exponents.

In a two-and-a-half-layer quasi-geostrophic model, a process study is conducted on the interaction between a vortex and a zonal jet, both with constant potential vorticity. The vortex is a stable anticyclone, initially located north of the eastward jet. The potential vorticity of the jet is allowed to have various vertical structures, while the vortex is concentrated in only one layer. The flow parameters are set to values characteristic of the Azores region. First, the jet is stable. Weak vortices steadily drift north of the jet without crossing it while strong vortices can cross the jet and tear off a cyclone with which they pair as a heton (baroclinic dipole). This heton often breaks later in the shear exerted by the jet; the two vortices finally drift apart. When crossed by deep anticyclones, the jet develops meanders with 375 km wavelength. These results exhibit a noticeable similarity with the one-and-a-half-layer case studied in Part I. Secondly, the jet is allowed to be linearly unstable. In the absence of the vortex, it develops meanders with 175 km wavelength and 25-day e-folding time on the β -plane. For various vertical structures of the jet, baroclinic instability is shown to barely affect jet–vortex interaction if the linear growth rate of unstable waves is smaller than 1/(14 days). Further simulations with a linearly unstable, nonlinearly equilibrated jet evidence its strong temporal variability when crossed by a deep vortex on the β -plane. In particular, long waves can dominate the spectrum for a few months after jet crossing by the vortex. Again in this process, the deep vortex couples with a surface cyclone and both drift southwestward.

Southwest of Portugal, in situ data show that part of the mesoscale variability of the Mediterranean Water undercurrents is triggered by topographic effects (near capes and submarine canyons), and is driven by a dominantly baroclinic instability. The vertical alignment of these undercurrents observed at sites of eddy formation is indeed favorable to baroclinic instability. This instability is materialized by the formation of filaments and of small eddies in the Portimao canyon and of meddles near Cape Saint Vincent and near the Estramadura Promontory. Hydrological data and float trajectories reveal that these eddies have a baroclinic structure. Near Portimao canyon, the variation of the potential vorticity of the undercurrents again proves their sensitivity to baroclinic and also to barotropic instabilities, enhanced by the canyon. Finally, a model of stationary coastal current explains the variations of its horizontal structure over a canyon., Au sud-ouest du Portugal, des données in situ montrent qu'une partie de la variabilité méso-échelle des sous-courants d'eau méditerranéenne est déclenchée par des effets topographiques au voisinage des caps et des canyons sous-marins et est générée par une instabilité essentiellement barocline. L'alignement vertical de ces sous-courants, observé aux sites de formation de tourbillons, est en effet favorable à l'instabilité barocline. Cette instabilité est matérialisée par la formation de filaments et de petits tourbillons lenticulaires dans le canyon de Portimão, et de “meddies” au cap Saint Vincent et près du promontoire d'Estrémadure. Les données hydrologiques et les trajectoires de flotteurs lagrangiens révèlent que ces tourbillons ont une structure barocline. Près du canyon de Portimão, la variation de la vorticité potentielle des sous-courants montre leur sensibilité à l'instabilité barocline, et également barotrope, amplifiées par le canyon. Enfin, un modèle de courant côtier stationnaire permet d'expliquer ses variations de structure horizontale sur un canyon.

A new approach to assimilation of observations is proposed, which extends previous work on adaptive Kalman filtering. In the latter, the gain matrix of the filter was progressively determined without a priori explicit specification of the covariance matrices of the model or observation noise, so as to minimize the norm of the innovation vector. The new step taken here is to introduce a (relatively) low dimension parameterization of the gain matrix, thereby substantially decreasing the numerical cost of the filter. The reduced-order adaptive filter (ROAF) thus defined is tested on a simple diffusive-reactive equation, and implemented on the four-layer adiabatic Miami isopycnical coordinate ocean model (MICOM). In the latter case, the filter is used to assimilate synthetic observations of surface height. Both sets of experiments clearly show the efficiency of the proposed approach, and its superiority, in terms of the quality of the results, on Newtonian relaxation. In the case of the diffusive-reactive equation, the reduced-order adaptive filter is also superior to, and more economical than, a reduced non-adaptative Kalman filter.

A numerical algorithm is proposed for estimation of the gain matrix in a reduced-order adaptive filter. Under detectability condition, stability of a Minimum Prediction Error filter can be ensured by suitable choice of projected subspace and appropriate parametrization of filter's gain. Convergence of algorithm is provided by adaptive adjustment process of unknown parameters in the gain. Numerical experiments are performed which show the efficiency of the proposed algorithm.

Resume On propose une nouvelle methode pour traiter les problemes de conditions aux limites dans un modele de Saint-Venant barotrope linearise. Elle combine un decouplage des equations avec une methode de relaxation newtonienne. On demontre theoriquement qu'elle permet d'evacuer les ondes sortantes sans modifier l'ecoulement dans le domaine physique etudie, contrairement aux methodes habituelles. Les resultats exposes valident cette methode.

In a two-layer quasi-geostrophic model, we numerically investigate the nonlinear evolution of a baroclinically unstable geostrophic vortex. For perturbations with a moderate growth rate and a weak amplitude and weak viscosity, a regime of regular oscillation is attained. This regime has a quasi-periodic reversal of the vertical phase shift between the flow patterns in each layer, with a corresponding back-and-forth potential energy transfer between the mean vortex state and the fluctuation pattern. This oscillating vortex, averaged in a frame of reference co-rotating with the angular velocity of each layer, has an elliptical horizontal configuration with a simple relation between potential vorticity and co-rotating streamfunction. Simulations with smaller Burger or Reynolds number or larger initial perturbation amplitude show less regularity in the oscillation and eventually result in vortex breaking.