Xavier Carton

Hydrodynamical Modeling Of Oceanic Vortices

Mesoscale coherent vortices are numerous in the ocean.Though they possess various structures in temperature and salinity,they are all long-lived, fairly intense and mostly circular. Thephysical variable which best describes the rotation and the density anomaly associated with coherent vortices is potential vorticity. It is diagnostically related to velocity and pressure, when the vortex is stationary. Stationary vortices can be monopolar (circular or elliptical) or multipolar; their stability analysis shows thattransitions between the various stationary shapes are possible when they become unstable. But stable vortices can also undergo unsteady evolutions when perturbed by environmental effects, likelarge-scale shear or strain fields, β-effect or topography. Changes in vortex shapes can also result from vortex interactions. such as the pairing, merger or vertical alignment of two vortices, which depend on their relative polarities and depths. Such interactions transfer energy and enstrophy between scales, and are essential in two-dimensional and in geostrophic turbulence. Finally, in relation with the observations, we describe a few mechanisms of vortex generation.

The life-cycle of tripoles in two-dimensional incompressible flows

The mechanisms of coherent tripole formation from unstable shielded circular vortices are analysed in the context of two-dimensional incompressible flows. Three stages are identified during the transformation process : the linear growth of the initial normal mode perturbation, its nonlinear amplification and the finite-amplitude saturation under the tripolar form. We give a geometrical discussion of the mutual influence of the core vortex and of the satellites generated from the shield. The role of the angular momentum in determining the finite amplitude saturation is demonstrated using a simple elliptical model of the core vortex associated with two point-vortex satellites. The long-time asymmetric breaking of the tripole into a dipole and a monopole is shown to be driven by the erosion of the core vortex by stripping and diffusion. Finally the influence of bottom topography on tripole formation is considered, providing a rich phenomenology when the height of the topography is varied.

Meddy coupling with a deep cyclone in the Gulf of Cadiz

Hydrological measurements made in July 1999 in the Gulf of Cadiz reveal the presence of two meddies, one south of Cape Saint Vincent (meddy Christine, centered at 9°45′W–35°30′N), the other one near the Moroccan shelf (meddy Isabelle, centered at 8°30′W–34°25′N), and their interaction with a deep cyclone (centered at 8°30′W–35°15′N). These meddies are medium-scale features with thermohaline anomalies of 60–70 km diameter concentrated between 750 and 1500 m depths; these anomalies reach 2 °C and 0.5 in salinity. Meddy Christine exhibits unusually strong thermohaline gradients at its periphery. Their maximum azimuthal velocities are 0.12 m/s (meddy Isabelle) and 0.10 m/s (meddy Christine). They both have a (vertically) tripolar structure in potential vorticity anomaly. The cyclone is shallower (between 600 and 1300 m) with a weaker signature of Mediterranean water (12 °C at 800 m, 36.2 in salinity between 800 and 1200 m). Its velocity maximum (0.16 m/s) is also centered near 800 m. This cyclone appears coupled with meddy Isabelle as a baroclinic dipole. This dipole tears a long warm and salty filament away from meddy Christine at 1200 m. The long-term evolution of these eddies is described by means of deep-drogued drifting buoy trajectories: while meddy Christine drifts southwestward, the two other eddies remain close to each other and move together along the continental shelf towards Cape Saint Vincent where they separate. A three-layer quasi-geostrophic model is used to evaluate the possible origin of meddy Isabelle and of the cyclone near the Portuguese coast, and to assess if their interactions, both mutual, with the domain boundaries, and with bottom topography, can account for their motion. The interaction of this dipole with meddy Christine is also quantified in this model.

Dynamics and Evolution of a Northern Meddy

Abstract A meddy was discovered in April 1997 off the northwestern corner of Spain, near 45°N, 11°30′W. It was tracked during 18 months with Lagrangian floats and deep drogued buoys, and several cruises were set to collect further hydrological and Lowered-ADCP measurements on it. The meddy, named Ulla, was a one-core lens with maximum values of temperature and salinity of 11.5°C and 36.17 psu near 1000-m depth, yielding anomalies above 2.5°C and 0.5 psu compared to its environment. Its rotation frequency was close to 1 loop every 5 days. The maximum azimuthal velocities of 15–20 cm s−1 were reached near a 15-km radius. The meddy had a wide remote influence, notably up to the surface, and was associated with a total azimuthal volume transport of around 10 Sv, (Sv = 106 m3 s−1), of which around 2 Sv was trapped in the core. A widening of the radial structure with decreasing depth was notable in August 1997. Meddy Ulla was significantly elliptic for most of the time and, depending on the periods, the main el...

Multipolar vortices in two-dimensional incompressible flows

In a two-dimensional incompressible fluid, the barotropic instability of isolated circular vortices can lead to multipole formation. The multipoles we study here are composed of a core vortex surrounded by two or more identical satellite vortices, of opposite-sign vorticity to the core, and the total circulation is zero. First, we present the generation of multipoles from unstable piecewise-constant monopoles perturbed on a monochromatic azimuthal mode. The stationary multipoles formed by this nonlinear evolution retain the same energy, circulation and angular momentum as the original monopoles, but possess a lower enstrophy. These multipolar steady states are then compared to multipolar equilibria of the Euler equation, obtained either analytically by a perturbation expansion or numerically via a relaxation algorithm. Finally the stability of these equilibria is studied. Quadrupoles (one core vortex bound to three satellites) prove relatively robust, whether initially perturbed or not, and resist severe permanent deformations (mode-2 shears or strains of amplitude up to 0.1ζ (max) . Amplification of the mode-3 deformation proves more destructive. More complex multipoles degenerate in less than a turnover period into end-products of a lesser complexity, via vortex splitting, pairing or merging. We use the conservation of integral properties to classify the large variety of instability mechanisms along physical guidelines. To conclude, we synthetize the connections between these various vortex forms.

Hydrology and circulation in the Strait of Hormuz and the Gulf of Oman—Results from the GOGP99 Experiment: 1. Strait of Hormuz

In October and early November 1999, the GOGP99 experiment collected hydrological, currentmeter, tide recorder, thermistor and drifting buoy data near the Strait of Hormuz. Data analysis provides the water mass structure in the Strait: Persian Gulf Water (PGW) core is banked against the Omani coast, while Indian Ocean Surface Water (IOSW) lies near the Iranian coast. These water masses are most often covered by a homogeneous surface layer. Thermohaline characteristics of the PGW core decrease substantially downstream, from the Persian/Arabian Gulf to the Gulf of Oman. PGW and IOSW thermohaline characteristics and distribution also exhibit notable changes at periods shorter than a month as shown by repeated hydrological sections. The tidal signal measured south of the Strait by moored ADCP and thermistor chains has predominant semi-diurnal M2 and S2 and diurnal K1 components and possesses a complex vertical structure. Tidal intensification near the surface pycnocline is associated with noticeable internal waves. At subtidal timescale, mooring recordings confirm the water mass variability observed in the repeated hydrological sections. The mixed layer also deepens substantially during the 1-month period. Finally, trajectories of surface buoys drogued at 15 m exhibit reversals over periods characteristic of changes in wind direction.

Instability of the Mediterranean Water undercurrents southwest of Portugal: effects of baroclinicity and of topography

Abstract 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 meddies 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.

Hydrological and dynamical characterization of Meddies in the Azores region: A paradigm for baroclinic vortex dynamics

The Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) oceanographic experiment surveyed a 500 × 500 km2 domain south of the Azores from June to November 1993 and collected hydrological data, float trajectories, and current meter recordings. This data exhibited three intrathermocline eddies of Mediterranean water (Meddies), two of them being repeatedly sampled. Their hydrological and dynamical properties are quantified here by an isopycnic analysis. For the three Meddies, intense temperature and salinity anomalies (up to 4°C and 1.1 practical salinity units (psu)) are observed extending vertically over up to 1000 m and centered around 1000 m. Horizontally, these anomalies spread out to radii of 50–60 km, while the maximum azimuthal velocities (30 cm s−1, as computed by geostrophy) lie only at 35–40 km from the central axis. These Meddies followed curved trajectories, with drift velocities up to 7.5 cm s−1, under the influence of the neighboring mesoscale features (cyclonic vortices or Azores Current meanders). The three-dimensional structure of potential vorticity in and around these features evidences their complex interactions. Northwest of the domain, a Meddy was coupled to a subsurface anticyclone, forming an “aligned” vortex. It later interacted with the Azores Current, creating a large-amplitude northward meander by vertical alignment of vorticity. In the southeastern part of the domain, another Meddy was vertically aligned with an anticyclonic meander of the Azores Current and horizontally coupled with a cyclone of large vertical extent. These two features, as well as a small warm and salty fragment in their vicinity, seem to result from the southward crossing of the Meddy under the Azores Current. These observations illustrate previous theoretical studies of baroclinic vortex dynamics.

Vortex multipoles in two-layer rotating shallow-water flows

The stability of elliptically perturbed circular vortices is investigated in a two-layer shallow-water model, with constant background rotation. The fluid is bounded above and below by rigid and flat surfaces. The linear stability analysis shows that elliptical perturbations are most unstable for moderate Burger numbers and vorticity shears. Shorter waves dominate for more sheared vortices. Shallow-water and quasigeostrophic growth rates exhibit a striking similarity, except at each end of the Burger number domain. There, cyclones (anticyclones) with nite Rossby numbers are more (less) unstable than their quasi-geostrophic counterparts. A simple model gives a rst-order trend for this bias. Nonlinear model runs with initially perturbed vortices also show the similarity between the two dynamics. In these runs, elliptically deformed vortices stabilize as stationary rotating tripoles for moderate linear instability; on the other hand, strongly unstable vortices break as dipoles. During these nonlinear processes, energy transfers indicate that barotropic instability is at least as active as the baroclinic one. For tripole formation, the modal analysis of the perturbation exhibits a dominant contribution of the original wave and of the mean flow correction. The ageostrophic and divergent parts of the flow are respectively weak and negligible. The Lighthill equation proves that few internal gravity waves are generated during tripole formation or dipolar breaking. Finally, the eects of triangular perturbations on circular vortices and the formation of quadrupoles are briefly addressed.

Real-time tracking of a Galician Meddy

We describe the properties and displacement of the northernmost Meddy that has ever been studied. This Meddy was found to the northwest of the Iberian Peninsula in April 1997. Its hydrological properties suggest that it was born offshore Galicia, the northwestern corner of Spain. It was tracked in real-time for more than one year using recent types of deep drogued buoys and Lagrangian floats, and remained quasi-stationary for an unusually long period of 11 months around 45°N, 12°W.