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Optimal Perturbations of an Oceanic Vortex Lens ArchiMer
Meunier, Thomas; Menesguen, Claire; Carton, Xavier; Le Gentil, Sylvie; Schopp, Richard.
The stability properties of a vortex lens are studied in the quasi geostrophic (QG) framework using the generalized stability theory. Optimal perturbations are obtained using a tangent linear QG model and its adjoint. Their fine-scale spatial structures are studied in details. Growth rates of optimal perturbations are shown to be extremely sensitive to the time interval of optimization: The most unstable perturbations are found for time intervals of about 3 days, while the growth rates continuously decrease towards the most unstable normal mode, which is reached after about 170 days. The horizontal structure of the optimal perturbations consists of an intense counter-shear spiralling. It is also extremely sensitive to time interval: for short time...
Tipo: Text Palavras-chave: Vortex lenses; Intrathermocline eddies; Instability; Optimal perturbations.
Ano: 2018 URL: https://archimer.ifremer.fr/doc/00454/56605/58297.pdf
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Tracer stirring around a meddy: The formation of layering ArchiMer
Meunier, Thomas; Menesguen, Claire; Schopp, Richard; Le Gentil, Sylvie.
The dynamics of the formation of layering surrounding meddy-like vortex lenses is investigated using primitive equation (PE), quasigeostrophic (QG), and tracer advection models. Recent in situ data inside a meddy confirmed the formation of highly density-compensated layers in temperature and salinity at the periphery of the vortex core. Very high-resolution PE modeling of an idealized meddy showed the formation of realistic layers even in the absence of double-diffusive processes. The strong density compensation observed in the PE model, in good agreement with in situ data, suggests that stirring might be a leading process in the generation of layering. Passive tracer experiments confirmed that the vertical variance cascade in the periphery of the vortex...
Tipo: Text Palavras-chave: Circulation/ Dynamics; Advection; Dynamics; Atm/Ocean Structure/ Phenomena; Vortices; Models and modeling; Primitive equations model; Quasigeostrophic models; Tracers.
Ano: 2015 URL: http://archimer.ifremer.fr/doc/00255/36599/35160.pdf
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Oceanic vortex mergers are not isolated but influenced by the beta-effect and surrounding eddies ArchiMer
De Marez, Charly; Carton, Xavier; L'Hegaret, Pierre; Meunier, Thomas; Stegner, Alexandre; Le Vu, Briac; Morvan, Mathieu.
Oceanic vortices are ubiquitous in the ocean. They dominate the sub-inertial energy spectrum, and their dynamics is key for the evolution of the water column properties. The merger of two like-signed coherent vortices, which ultimately results in the formation of a larger vortex, provides an efficient mechanism for the lateral mixing of water masses in the ocean. Understanding the conditions of such interaction in the ocean is thus essential. Here, we use a merger detection algorithm to draw a global picture of this process in the ocean. We show that vortex mergers are not isolated, contrary to the hypothesis made in most earlier studies. Paradoxically, the merging distance is well reproduced by isolated vortex merger numerical simulations, but it is...
Tipo: Text
Ano: 2020 URL: https://archimer.ifremer.fr/doc/00657/76873/78117.pdf
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Meridional Overturning and Heat Transport From Argo Floats Displacements and the Planetary Geostrophic Method (PGM): Application to the subpolar North Atlantic ArchiMer
Colin De Verdiere, Alain; Meunier, Thomas; Ollitrault, Michel.
A direct method is presented to obtain the meridional overturning and heat transport in oceanic basins from observations under the sole assumptions of geostrophy and hydrostatics. The method is made possible because of the rising Argo float displacements database that can provide a reference level at 1000 dbar for the time mean circulation at 1 degrees x 1 degrees resolution. To achieve the overturning and heat transport objectives, the absolute geostrophic time mean circulation must have nondivergent barotropic transports and this requires the solutions of two Poisson equations with suitable boundary conditions, one for the geopotential at 1000 dbar and one for the barotropic stream function. Applied to the subpolar Atlantic for the period 2000-2009, an...
Tipo: Text Palavras-chave: Oceanic heat transport; Meridional overturning; Subpolar Atlantic.
Ano: 2019 URL: https://archimer.ifremer.fr/doc/00589/70139/68105.pdf
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Observations of Layering under a Warm-Core Ring in the Gulf of Mexico ArchiMer
Meunier, Thomas; Pallas Sanz, Enric; Tenreiro, Miguel; Ochoa, Jose; Angulo, Angel Ruiz; Buckingham, Christian.
Two glider transects in the Gulf of Mexico reveal fine-vertical-scale thermohaline structures within a Loop Current eddy (LCE). Partially compensating temperature and salinity anomalies are shown to organize as thin layers below the eddy and near its edges. The anomalies have vertical scales ranging from 2 to 60 m and extend laterally over distances up to 120 km. These structures are evident in synthetic acoustic reflectivity derived from the glider data and are reminiscent of the intense layering observed in seismic imagery around meddies, Agulhas rings, and warm-core Kuroshio rings. The observed layers are aligned with the geostrophic streamfunction rather than isopycnals and develop preferentially in zones of intense vertical shear. These observations...
Tipo: Text Palavras-chave: Eddies; Mesoscale processes; Small scale processes; In situ oceanic observations; Quasigeostrophic models; Tracers.
Ano: 2019 URL: https://archimer.ifremer.fr/doc/00600/71242/69612.pdf
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Intrathermocline Eddies Embedded within an Anticyclonic Vortex Ring ArchiMer
Meunier, Thomas; Tenreiro, M.; Pallas-sanz, Enric; Ochoa, Jose; Ruiz-angulo, Angel; Portela, Esther; Cusi, Simo; Damien, Pierre; Carton, Xavier.
High‐resolution hydrographic measurements reveal the presence of three Intra‐Thermocline Eddies (ITEs) embedded within a Loop Current Eddy (LCE). ITEs are lenticular bodies of nearly homogeneous water which contrasts with the well stratified surrounding water. Their radii and thickness ranged between [19‐32] km and [150‐250] m. Negative relative vorticity within their cores (down to ‐0.85 times the Coriolis frequency), along with a large negative stratification anomaly, result in low Ertel Potential Vorticity (PV) and intense negative Ertel Potential Vorticity Anomalies (PVA). Vortex stretching and relative vorticity have comparable contributions to PVA, resulting in Burger numbers of order unity. The similarity of thermohaline properties within the ITE's...
Tipo: Text Palavras-chave: Intrathermocline eddies; Submesoscale coherent vortices; Vortex lenses; Loop current eddy; Gulf of Mexico; Submesoscale.
Ano: 2018 URL: https://archimer.ifremer.fr/doc/00440/55122/56578.pdf
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Upwelling filaments off Cap Blanc: Interaction of the NW African upwelling current and the Cape Verde frontal zone eddy field? ArchiMer
Meunier, Thomas; Barton, E. D.; Barreiro, B.; Torres, R..
The hydrographical and dynamical properties of the upwelling filaments forming off Cap Blanc (Mauritania) are investigated using remotely sensed and in situ data collected in April/May 2009 during the strongest upwelling season. The area is situated at the southern edge of the NW African upwelling system, where the Cape Verde Frontal Zone (CVFZ) separates warmer, saltier North Atlantic Central Water (NACW) and cooler, fresher South Atlantic Central Water (SACW). Sea surface temperature images indicated the presence of an upwelling filament extending >280 km offshore, rooted over the Cap Blanc promontory and entrained around a warm-core anticyclonic eddy. After this filament started to decay, a new cold filament developed at the approximate same...
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Ano: 2012 URL: http://archimer.ifremer.fr/doc/00098/20920/18578.pdf
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Vortex stability in a multi-layer quasi-geostrophic model: application to Mediterranean Water eddies ArchiMer
Carton, Xavier; Sokolovskiy, Mikhail; Menesguen, Claire; Aguiar, Ana; Meunier, Thomas.
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...
Tipo: Text
Ano: 2014 URL: https://archimer.ifremer.fr/doc/00216/32711/31363.pdf
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Heat Content Anomaly and Decay of Warm‐core Rings: the Case of the Gulf of Mexico ArchiMer
Meunier, Thomas; Sheinbaum, Julio; Pallàs Sanz, Enric; Tenreiro, Miguel; Ochoa, José; Angulo, Angel Ruiz; Carton, Xavier; Marez, Charly.
In this study, we harness the 25‐year satellite‐altimeter record, in concert with a vast array of in‐situ measurements, to estimate the heat content anomaly of 32 warm‐core rings in the Gulf of Mexico (GoM). The decay rate of these mesoscale eddies is studied in detail, and it is shown that they release the majority of their heat as they drift in the central GoM (away from topographic obstacles). The surface heat fluxes from the eddies is shown to be small in comparison to the total rate of heat loss from the eddies, suggesting that heat is primarily released towards the surrounding watermasses. Integrating the total heat evolution equation over the warm‐core rings yields an estimate of their effective lateral diffusivity coefficient. The long term impact...
Tipo: Text
Ano: 2020 URL: https://archimer.ifremer.fr/doc/00606/71769/70249.pdf
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Explosive instability of geostrophic vortices. Part 1: baroclinic instability ArchiMer
Carton, Xavier; Flierl, Glenn R.; Perrot, Xavier; Meunier, Thomas; Sokolovskiy, Mikhail.
In a quasi-geostrophic model, we study the baroclinic instability of a two-layer vortex. The singular unstable modes for potential vorticity anomalies are compared with the classical normal modes. Short-time singular modes are explosively unstable and, at short times, depend only on the baroclinic component of the flow. As time progresses, they evolve towards the normal modes and their sensitivity to flow parameters is explored. Asymptotic solutions are provided.
Tipo: Text Palavras-chave: Quasi-geostrophic equations; Inviscid flows; Vortex stability; Normal or singular modes; Parametric resonance.
Ano: 2010 URL: http://archimer.ifremer.fr/doc/00002/11352/7925.pdf
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Explosive instability of geostrophic vortices. Part 2: parametric instability ArchiMer
Carton, Xavier; Meunier, Thomas; Flierl, Glenn R.; Perrot, Xavier; Sokolovskiy, Mikhail.
In a two-layer quasi-geostrophic model, a baroclinic vortex is submitted to a periodic forcing of its mean baroclinic azimuthal velocity. It is shown that parametric effects could stabilize a vortex which is baroclinically unstable in the absence of forcing. Conversely, parametric resonance can destabilize a baroclinically stable vortex, under conditions on the vortex parameters, on the ratio of layer thicknesses or on the forcing frequency.
Tipo: Text Palavras-chave: Quasi-geostrophic equations; Inviscid flows; Vortex stability; Parametric resonance.
Ano: 2010 URL: http://archimer.ifremer.fr/doc/00002/11353/7926.pdf
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Study of the stability of a large realistic cyclonic eddy ArchiMer
De Marez, Charly; Meunier, Thomas; Morvan, Mathieu; L’hégaret, Pierre; Carton, Xavier.
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...
Tipo: Text Palavras-chave: Mesoscale; Eddy; Instability; Submesoscale; Vortex.
Ano: 2020 URL: https://archimer.ifremer.fr/doc/00598/70967/69196.pdf
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