Evolution of isolated interior vortices in the ocean (original) (raw)
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Journal of Physical Oceanography, 2011
Large-scale vortices, that is, eddies whose characteristic length scale is larger than the local Rossby radius of deformation R d , are ubiquitous in the oceans, with anticyclonic vortices more prevalent than cyclonic ones. Stability or robustness properties of already formed shallow-water vortices have been investigated to explain this cyclone-anticyclone asymmetry. Here the focus is on possible asymmetries during the generation of vortices through barotropic instability of a parallel flow. The initial stage and the nonlinear stage of the instability are studied by means of linear stability analysis and direct numerical simulations of the one-layer rotating shallow-water equations, respectively. A wide variety of parallel flows are studied: isolated shears, the Bickley jet, and a family of wakes obtained by combining two shears of opposite signs.
Dynamics of intrathermocline vortices in a gyre flow over a seamount chain
Ocean Dynamics, 2013
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Hydrodynamical Modeling Of Oceanic Vortices
Surveys in Geophysics - SURV GEOPHYS, 2001
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, ß-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 en...
Dynamics of wind-forced coherent anticyclones in the open ocean
Journal of Geophysical Research, 2009
We study numerically the dynamics of coherent anticyclonic eddies in the ocean interior in hydrostatic, rotating, stably-stratified turbulence by using a high-resolution, primitiveequation model forced by small-scale winds in a idealized configuration. Many properties of the horizontal motions are found to be similar to those of two-dimensional turbulence.
Vortex stability in a multi-layer quasi-geostrophic model: application to Mediterranean Water eddies
Fluid Dynamics Research, 2014
ABSTRACT The stability of circular vortices to normal mode perturbations is studied in a multi-layer quasi-geostrophic model. The stratification is fitted on the Gulf of Cadiz where many Mediterranean Water (MW) eddies are generated. Observations of MW eddies are used to determine the parameters of the reference experiment; sensitivity tests are conducted around this basic case. The objective of the study is two-fold: (a) determine the growth rates and nonlinear evolutions of unstable perturbations for different three-dimensional (3D) velocity structures of the vortices, (b) check if the different structure of our idealized vortices, mimicking MW cyclones and anticyclones, can induce different stability properties in a model that conserves parity symmetry, and apply these results to observed MW eddies. The linear stability analysis reveals that, among many 3D distributions of velocity, the observed eddies are close to maximal stability, with instability time scales longer than 100 days (these time scales would be less than 10 days for vertically more sheared eddies). The elliptical deformation is most unstable for realistic eddies (the antisymmetric one dominates for small eddies and the triangular one for large eddies); the antisymmetric mode is stronger for cyclones than for anticyclones. Nonlinear evolutions of eddies with radii of about 30 km, and elliptically perturbed, lead to their re-organization into 3D tripoles; smaller eddies are stable and larger eddies break into 3D dipoles. Horizontally more sheared eddies are more unstable and sustain more asymmetric instabilities. In summary, few differences were found between cyclone and anticyclone stability, except for strong horizontal velocity shears.
Vortex Generation by Topography in Locally Unstable Baroclinic Flows*
Journal of Physical Oceanography, 2003
The dynamics of a quasigeostrophic flow confined in a two-layer channel over variable topography on the beta plane is numerically investigated. The topography slopes uniformly upward in the north-south direction (in the beta sense) and is a smooth function of the zonal coordinate. The bottom slope controls the local supercriticality and is configured to destabilize the flow only in a central interval of limited zonal extent. Linearized solutions indicate that, for a wide enough channel, unstable modes exist for an arbitrary short interval of instability, confirming previous analysis on disturbances with no meridional variation. For small local maximum supercriticality, the instability is maintained by a short bottom-trapped wave localized at the downstream edge of the unstable region and oscillating in phase with the upper-layer disturbance. When nonlinearity is retained in the problem, the equilibration of the bottom-trapped wave is associated with the formation of coherent vortices. Both cyclones and anticyclones are formed continuously at the northeastern edge of the unstable interval. Through vortex stretching mechanisms, dipoles inside the interval of instability can split upon reaching the northern wall: Anticyclones move downstream along the north wall and propagate into the downstream stable region, while cyclonic structures tend to remain trapped inside the interval of instability. The authors suggest the relevance of their results to the observed eddy field of the Labrador Sea.
The role of vorticity fluxes in the dynamics of the Zapiola Anticyclone
Journal of Geophysical Research, 2008
1] The Argentine Basin in the South Atlantic Ocean is one of the most energetic regions in the ocean with complicated dynamics, which plays an important role in the global climate. A number of observations have discovered an intense anticyclonic gyre of barotropic circulation around the Zapiola Rise in the center of the basin. Theoretical studies have shown that the Zapiola Anticyclone represents an eddy-driven flow controlled by bottom friction. Recent advances in high-resolution global-ocean data syntheses, performed using NASA supercomputing facilities, provide realistic simulations of the circulation and the variability in the Argentine Basin. Using these simulations and satellite altimeter observations, we analyzed the vorticity balance of the Zapiola Anticyclone. Our results suggest the dominance of vorticity fluxes and the advection of the potential vorticity over a nonuniform bottom topography in determining the variability of the gyre, while the impact of the local wind stress is small. The divergence of the relative vorticity anomaly advection by eddies is found to be the most important contributor to the relative vorticity flux divergence influencing the variability of the Zapiola Anticyclone. Our results demonstrate that the relative vorticity influencing the variability of the anticyclone is mainly advected from the south where the northern branch of the Antarctic Circumpolar Current at the Subpolar Front is located.
Influence of topography on the dynamics of baroclinic oceanic eddies
1994
In this work we study motion of a baroclinic upper-ocean eddy over a large-scale topography which simulates a continental slope. We use a quasigeostrophic f-plane approximation with continuous stratification. To study this problem we develop a new numerical technique which we call "semi-lagrangian contour dynamics". This technique resembles the traditional 2-D contour dynamics method but differs significantly from it in the numerical algorithm. In addition to "Lagrangian" moving contours it includes an underlying "Eulerian" regular grid to which vorticity or density fields are interpolated. To study topographic interactions in a continuously stratified model we use density contours at the bottom in a similar manner as vorticity contours are used in the standard contour dynamics. For the case of a localized upper-ocean vortex moving over a sloping bottom the problem becomes computationally 2-dimensional (we need to follow only bottom density contours and the position of the vortex itself) although the physical domain is still 3-dimensional.