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Xavier Carton

Brest, France


Duarte R., Carton X., Poulin F. J.
The Dynamics of a Meandering Coastal Jet in the Lee of a Cape
2016, vol. 21, no. 3, pp.  274-290
In a barotropic model of an oceanic channel, bounded to the north by a straight coast indented by a Gaussian cape, the evolution of a coastal jet is studied numerically. In the absence of the cape, the barotropic instability of the jet is determined. In the presence of the cape, a regular row of meanders develops downstream of this feature, and becomes stationary for a particular range of parameters. The relevant parameters are the velocity and width of the jet, size of the cape, and beta effect. The formation of meanders occurs first via the instability of the jet, then via the generation of vorticity anomalies at the cape, which are advected both downstream by the flow and offshore by the radiation of Rossby waves. Once the meanders are established, they remain stationary features if the propagation velocity of the meanders (due to the dipolar effect at the coast) opposes the jet velocity and the phase speed of the wave on the vorticity front. Finally, a steady state of a regular row of meanders is also obtained via a matrix method and is similar to that obtained in the time-dependent case.
Keywords: two-dimensional fluid, vorticity equation, nonlinear dynamics, effect of boundaries, Rossby waves, numerical modeling, steady state
Citation: Duarte R., Carton X., Poulin F. J.,  The Dynamics of a Meandering Coastal Jet in the Lee of a Cape, Regular and Chaotic Dynamics, 2016, vol. 21, no. 3, pp. 274-290
Duarte R., Carton X., Capet X., Cherubin L.
Trapped Instability and Vortex Formation by an Unstable Coastal Current
2011, vol. 16, no. 6, pp.  577-601
This paper addresses the instability of a two-layer coastal current in a quasigeostrophic model; the potential vorticity (PV) structure of this current consists in two uniform cores, located at different depths, with finite width and horizontally shifted. This shift allows both barotropic and baroclinic instability for this current. The PV cores can be like-signed or opposite-signed, leading to their vertical alignment or to their hetonic coupling. These two aspects are novel compared to previous studies. For narrow vorticity cores, short waves dominate, associated with barotropic instability; for wider cores, longer waves are more unstable and are associated with baroclinic processes. Numerical experiments were performed on the $f$−plane with a finite-difference model. When both cores have like-signed PV, trapped instability develops during the nonlinear evolution: vertical alignment of the structures is observed. For narrow cores, short wave breaking occurs close to the coast; for wider cores, substantial turbulence results from the entrainment of ambient fluid into the coastal jet. When the two cores have opposite-signed PV, the nonlinear regimes range from short wave breaking to the ejection of dipoles or tripoles, via a regime of dipole oscillation near the wall. The Fourier analysis of the perturbed flow is appropriate to distinguish the regimes of short wave breaking, of dipole formation, and of turbulence, but not the differences between regimes involving only vortex pairs. To explain more precisely the regimes where two vortices (and their wall images) interact, a point vortex model is appropriate.
Keywords: stability and instability of geophysical and astrophysical flows, vortex flows, rotating fluids, stability problems, applications to physics
Citation: Duarte R., Carton X., Capet X., Cherubin L.,  Trapped Instability and Vortex Formation by an Unstable Coastal Current, Regular and Chaotic Dynamics, 2011, vol. 16, no. 6, pp. 577-601
Perrot X., Reinaud J. N., Carton X., Dritschel D. G.
Homostrophic vortex interaction under external strain, in a coupled QG-SQG model
2010, vol. 15, no. 1, pp.  66-83
The interaction between two co-rotating vortices, embedded in a steady external strain field, is studied in a coupled Quasi-Geostrophic – Surface Quasi-Geostrophic (hereafter referred to as QG-SQG) model. One vortex is an anomaly of surface density, and the other is an anomaly of internal potential vorticity. The equilibria of singular point vortices and their stability are presented first. The number and form of the equilibria are determined as a function of two parameters: the external strain rate and the vertical separation between the vortices. A curve is determined analytically which separates the domain of existence of one saddle-point, and that of one neutral point and two saddle-points. Then, a Contour-Advective Semi-Lagrangian (hereafter referred to as CASL) numerical model of the coupled QG-SQG equations is used to simulate the time-evolution of a sphere of uniform potential vorticity, with radius $R$ at depth $−2H$ interacting with a disk of uniform density anomaly, with radius $R$, at the surface. In the absence of external strain, distant vortices co-rotate, while closer vortices align vertically, either completely or partially (depending on their initial distance). With strain, a fourth regime appears in which vortices are strongly elongated and drift away from their common center, irreversibly. An analysis of the vertical tilt and of the horizontal deformation of the internal vortex in the regimes of partial or complete alignment is used to quantify the three-dimensional deformation of the internal vortex in time. A similar analysis is performed to understand the deformation of the surface vortex.
Keywords: coupled QG-SQG model, point-vortex, CASL
Citation: Perrot X., Reinaud J. N., Carton X., Dritschel D. G.,  Homostrophic vortex interaction under external strain, in a coupled QG-SQG model, Regular and Chaotic Dynamics, 2010, vol. 15, no. 1, pp. 66-83
Maze G., Carton X., Lapeyre G.
Dynamics of a 2D vortex doublet under external deformation
2004, vol. 9, no. 4, pp.  477-497
The influence of an external strain (or shear) field on the evolution of two identical vortices is investigated in a two-dimensional incompressible fluid. Using point vortex modeling, two regimes of the vortex doublet (co-rotation and irreversible separation) are determined; the critical intensity of the large scale flow separating these two regimes for a given initial separation of vortices, is calculated. Finite-area effects are then considered for the vortices. The steady states of piecewise constant vortices are computed algebraically and numerically; positive strain (or shear) favors vortex deformation. This deformation has a dominant elliptical component. An elliptical model of two vortices confirms the point vortex model results for centroid trajectories, and the steady state model results concerning the influence of positive strain on vortex deformation. It also provides an estimate of critical merger distance in the presence of large scale flow. Finally, the finite-time, nonlinear evolution of the vortex doublet is simulated with a numerical code of the 2D vorticity equation. The various regimes (stationarity, merger, co-rotation, ejection) are classified in the plane of initial vortex separation and of external deformation. These regimes are analyzed, and the critical merger distance is evaluated for negative and positive external strain; the results are in agreement with the elliptical model prediction. Merger efficiency, defined as the ratio of final to initial vortex circulation, is computed; for the same initial distance, it is smaller for negative strain. It also depends in a more complex way of the initial vortex distance.
Citation: Maze G., Carton X., Lapeyre G.,  Dynamics of a 2D vortex doublet under external deformation, Regular and Chaotic Dynamics, 2004, vol. 9, no. 4, pp. 477-497
Pavec M., Carton X.
Parametric instability of a two-layer wall jet
2004, vol. 9, no. 4, pp.  499-507
The parametric instability of a wall jet, with time-varying potential vorticity or transport of the baroclinic mean flow, is studied in a two-layer quasi-geostrophic model. This wall jet is composed of two superimposed strips of uniform potential vorticity, and the layer thicknesses are equal. The steady flow is stable with respect to short waves and its domain of linear instability grows with stratification. The time-dependent flow evolution is governed by a Hill equation which allows parametric instability. This instability indeed appears in numerical flow calculations. It is favored near the marginal stability curve of the steady flow. Near that curve, the evolution equation of the flow is calculated with a multiple time-scale expansion. This equation shows that for zero baroclinic transport of the mean flow, subcritical steady flows can be destabilized by flow oscillation, and supercritical steady flows can be stabilized by medium frequency oscillations. For finite baroclinic transport, this parametric instability vanishes in the limit of short waves or of long waves and narrow potential vorticity strips. Consequences for coastal flows in the ocean are drawn.
Citation: Pavec M., Carton X.,  Parametric instability of a two-layer wall jet, Regular and Chaotic Dynamics, 2004, vol. 9, no. 4, pp. 499-507

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