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Falina A.,RAS Shirshov Institute of Oceanology | Sarafanov A.,RAS Shirshov Institute of Oceanology | Mercier H.,French National Center for Scientific Research | Lherminier P.,French Research Institute for Exploitation of the Sea | And 2 more authors.
Journal of Physical Oceanography | Year: 2012

Hydrographic data collected in the Irminger Sea in the 1990s-2000s indicate that dense shelf waters carried by the East Greenland Current south of the Denmark Strait intermittently descend (cascade) down the continental slope and merge with the deep waters originating from the Nordic Seas overflows. Repeat measurements on the East Greenland shelf at ;200 km south of the Denmark Strait (658-668N) reveal that East Greenland shelf waters in the Irminger Sea are occasionally as dense (s0 . 27.80) as the overflow-derived deep waters carried by the DeepWestern Boundary Current (DWBC). Clear hydrographic traces of upstream cascading of dense shelf watersare found over the continental slope at 64.38N, where the densest plumes (s0 . 27.80) originating from the shelf are identified as distinct low-salinity anomalies in the DWBC. Downstream observations suggest that dense fresh waters descending from the shelf in thenorthern Irminger Sea can be distinguished in theDWBCup to the latitude ofCape Farewell (;608N) and that thesewatersmake a significant contribution to the DWBC transport. © 2012 American Meteorological Society. Source

Shteinbuch-Fridman B.,Bar - Ilan University | Makarov V.,National Polytechnic Institute of Mexico | Carton X.,Laboratoire Of Physique Des Oceans | Kizner Z.,Bar - Ilan University
Physics of Fluids | Year: 2015

The so-called carousel tripoles are constructed and characterized in the framework of two-layer quasi-geostrophic contour dynamics, and their stability is examined. Such a tripole is a steadily rotating doubly symmetric ensemble of three collinear vortices, or more specifically, uniform-potential-vorticity patches, with the central, core vortex, located in the upper layer, and the two remaining, satellite vortices, in the lower layer, or vice versa. The carousel tripole solutions are obtained with the use of a numerical iterative procedure. A tripole with zero total potential vorticity can be generally identified by a point in the plane spanned by two parameters, namely, the typical size of the patches relative to the Rossby deformation radius, and some shape parameter. We consider two kinds of the parameter plane by taking as the second parameter either the distance d between the centroids of the core and one of the satellites (termed also separation) or, alternatively, the minimal distance h between the core centroid and the satellite contour, measured along the symmetry axis that passes through the centroids of the core and satellites. Accordingly, to capture the stationary tripoles, we use two alternative numerical procedures, which are based on fixing the first or the second pair of parameters. This is done because the areas of convergence of the two procedures differ somewhat from each other. The areas of convergence are plotted in the parameter planes, and in each of the planes, two branches of solutions are found bifurcating from some segments of the lines bounding the convergence areas. Stability is tested in numerical simulations with the numerical noise taken as a perturbation factor. Stability/instability of a tripole is determined by examining the oscillations in the perimeter of one of the vortex satellites. For each tripole size, both stable and unstable solutions exist. The stability bounds coincide with the bifurcation lines, so that one branch of the solutions is stable while the other is not. As a whole, tripoles with considerable separation behave stably. © 2015 AIP Publishing LLC. Source

De Verdiere A.C.,Laboratoire Of Physique Des Oceans
Journal of Physical Oceanography | Year: 2010

Although the instability of the thermohaline circulation has been widely observed in numerical ocean models, theoretical advances have been hindered by the nonlinearity of heat and salt transports, a circulation governed by lateral temperature, and salinity gradients. Because the instability occurs initially in polar waters through the formation of haloclines and the halt of convection, any explanatory model must have at least a surface and a deep layer. The model proposed here (two surface boxes above a deep one) reduces to a 2 degrees-of-freedom dynamical system when convection is active and 3 degrees when it is interrupted. The instability that is induced by a negative freshwater perturbation in polar waters has three stages. The first stage is a rapid 5-yr adjustment to a transient thermal attractor that results from an approximate balance between heat advection and air-sea heat fluxes. The second stage is a slow evolution that self-organizes near this attractor, which preconditions the instability, as it can be shown that the circulation becomes more sensitive to changes in salinity gradients than in temperature gradients. The slow O(100 yr) growth of salinity in the subtropics is the critical precursor of the instability while at the same time the subpolar salinity rises against the initial perturbation to stabilize the system by increasing the overturning and restoring convection. When the overturning becomes smaller than the value at the unstable fixed point, the third stage occurs, which is when the subpolar salinity decreases at last on a fast O(10 yr) time scale, precipitating the fall of the overturning. During the last two stages of the instability, the horizontal thermal gradient increases, but its stabilizing effect is just barely unable to prevent the outcome. The return to stability occurs frequently through a regime of multidecadal oscillations with intermittent convection. The hypothesis of mixed boundary conditions has been relaxed by coupling the ocean box model to an atmospheric energy balance model to show that the coupling increases the stability of the oceanic circulation; however, the precursors of the instability are unchanged. © 2010 American Meteorological Society. Source

Sokolovskiy M.A.,Russian Academy of Sciences | Koshel K.V.,RAS Ilichev Pacific Oceanological Institute | Carton X.,Laboratoire Of Physique Des Oceans
Geophysical and Astrophysical Fluid Dynamics | Year: 2011

In a two-layer quasi-geostrophic model, the evolution of a symmetric baroclinic multipole, composed of a central vortex with strength μκ in the upper layer, and A satellites with strength κ in the lower layer, is studied. This multipole is imbedded in a center-symmetric shear/strain field, either steady or time-periodic. Special attention is given to the case of the tripole (A=2). The stability of this tripole is assessed and its oscillations in the external field are analyzed. Conditions for resonance of these oscillations are derived and transition to chaos is described. © 2011 Taylor & Francis. Source

Sokolovskiy M.A.,Southern Federal University | Filyushkin B.N.,RAS Shirshov Institute of Oceanology | Carton X.J.,Laboratoire Of Physique Des Oceans
Ocean Dynamics | Year: 2013

The interaction of meddies with a complex distribution of seamounts is studied in a three-layer quasi-geostrophic model on the f-plane. This study aims at understanding if and how this seamount chain can represent a barrier to the propagation of these eddies and how it can be involved in their decay. The eddies are idealized as vortex patches in the middle layer, interacting with a regional cyclonic current and with ten idealized seamounts. The numerical code is based on the contour surgery technique. The initial position, radius, shape, number and polarity of the eddies are varied. The main results are the following: (1) Though they do not describe the unsteady flow, the streamlines of the regional and topographic flow provide a useful estimate of the vortex trajectories, in particular towards the major seamounts, where stronger velocity shears are expected. (2) The tallest and widest seamounts which have the largest vorticity reservoir are able to considerably erode the vortices, but also to draw anticyclones towards the seamount top. The ability of narrower seamounts to erode vortices is related to their multiplicity. (3) Only 1/3 of the anticyclones with about 30-km radius reach the southern boundary of the seamount chain, and their erosion is larger than 50 %. The other anticyclones are either completely eroded or trapped over a wide seamount top. Cyclones are less affected by seamounts because they oppose the topographic draft towards the seamount top and they drift along the side of the seamount. (4) Large vortices resist topographic erosion more efficiently. The rate of erosion grows from a few percent to about 35-50 % as the vortex radius decreases from about 60 to 30 km. Small cyclones are not eroded, contrary to small anticyclones (which completely decay), in relation with the different trajectories of these eddies in the vicinity of the seamounts. (5) The detailed vortex shape does not appear critical for their evolution, if they are close enough to the seamount chain initially. The interaction between a group of vortices initially north of the seamount chain can modify their trajectory to such an extent that they finally avoid collision with seamounts. (6) Finally, meddy trajectories across the Horseshoe Seamounts (data from the AMUSE experiment) show qualitative similarity with the vortex paths in the model. Several events of vortex decay also occur at comparable locations (in particular over the wide and tall seamounts) in the model and observations. © 2013 Springer-Verlag Berlin Heidelberg. Source

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