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Séminaires

by Webmaster Legos last modified Feb 07, 2012 11:59 AM

Jeudi 20 octobre - Empreinte basse-fréquence de la variabilité intrinsèque océanique

by SEMSOU last modified Sep 19, 2016 10:09 AM
When Oct 20, 2016
from 11:00 AM to 12:00 PM
Where Salle Lyot
Attendees Guillaume Serazin, post-doc LEGOS
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Guillaume Serazin,

LEGOS, Toulouse

 

Titre : Empreinte basse-fréquence de la variabilité intrinsèque océanique sur l’océan global à travers différents jeux de simulations turbulentes

 

Résumé :

Ce séminaire est l’occasion de présenter quelques résultats concernant la variabilité océanique intrinsèque générée spontanément sous forçage atmosphérique saisonnier dans des simulations océaniques à haute-résolution (1/4° et 1/12°), en présence de tourbillons de méso-échelle (~ 100 km). L'empreinte de la variabilité intrinsèque sur le niveau de la mer (SLA) est caractérisée dans plusieurs gammes d'échelles spatio-temporelles par des méthodes de filtrage. En particulier, il est montré que la variabilité petite-échelle inter-annuelle à décennale de SLA observée par les satellites altimétriques, a un caractère stochastique et est majoritairement intrinsèque. Aux échelles de temps multi-décennales, la variabilité intrinsèque de SLA dans les régions océaniques turbulentes est d'amplitude comparable à la variabilité interne simulée dans les modèles climatiques couplés (dénués de turbulence océanique), et pourrait constituer une source additionnelle d'incertitudes dans la détection de l'élévation régionale du niveau de la mer d'origine anthropique. Une analyse spectrale montre enfin que l'advection non-linéaire de vorticité relative transfère spontanément de l'énergie cinétique d'ondes frontales hautes-fréquences vers des tourbillons de méso-échelle plus lents, générant in fine de la variabilité intrinsèque basse-fréquence. Des résultats préliminaires concernant l'impact de la variabilité intrinsèque basse-fréquence sur le contenu de chaleur à différentes profondeurs seront également montrés, suite à l’analyse récente d’un ensemble novateur de 50 simulations au 1/4°. Ces travaux témoignent plus généralement du caractère chaotique de l'océan turbulent, dont l'interaction avec l'atmosphère est encore mal comprise.

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Jeudi 7 juillet - Crossroads of the Oceanic Circulation

by SEMSOU last modified Jun 27, 2016 10:23 AM
When Jul 07, 2016
from 11:00 AM to 12:00 PM
Where Salle Pyrénées
Attendees Alberto Baudena, PhD Student, LOCEAN Paris
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Alberto Baudena

LOCEAN Paris

Titre : Crossroads of the Oceanic Circulation

Résumé :

With the increasing pressure of human activities over the oceans, monitoring marine areas is assuming an even more important role. Here we examine a specific problem, namely the early detection of a harmful pollutant which is dispersed in the ocean upper layer. In particular, we ask the following question: assuming that a polluting is released into a determined area and advected by the currents, how can we optimally choose the position of monitoring sites in an attempt to retrieve the contaminants with the higher probability? In order to answer to this question, we borrow some concepts from network theory and develop a new Lagrangian diagnostic, that we call crossroadness (CR), which allows to locate the main sites (that we will call as stations) that represent "crossroads" of the surface flux and that therefore collect waters coming from larger ocean surfaces.\\
In order to verify the accuracy of the new diagnostic, we analysed the trajectories of 43 drifters released around the region of Kerguelen in November 2011. The stations computed with our method from regional and global altimetry products allow to intercept about the double of drifters caught with a regular grid. The extension of this diagnostic to scientific and societal applications will also be discussed.

Then I will discuss briefly about the importance of the features identified by this and other Lagrangian diangostic, such as the Lyapunov Exponents, as structures able to aggregate living organisms like fish. In order to do that we developed a 2D model, focusing on the Southern Ocean and in particular on the region around Kerguelen island.

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Mardi 5 juillet - Stochastic low-frequency variability in the turbulent ocean: insights from a global simulation ensemble

by SEMSOU last modified Apr 18, 2016 10:09 AM
When Jul 05, 2016
from 11:00 AM to 12:00 PM
Where Salle Lyot
Attendees Thierry Penduff, LGGE Grenoble
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Thierry Penduff,

Laboratoire de Glaciologie et Géophysique de l'Environnement, Grenoble

 

Title : Stochastic low-frequency variability in the turbulent ocean: insights from a global simulation ensemble.


Abstract : Idealized models have demonstrated the chaotic behavior of the ocean variability at high Reynolds number, up to multi-decadal timescales. Unlike laminar ocean models used in most current climate projections, eddying OGCMs that will be used in future climate projections spontaneously generate a substantial intrinsic variability from eddy scales to multi-decadal/basin scales, with a chaotic character, and a marked signature on SSH and SST where air-sea fluxes are maximum in Nature. Whether and how this ocean-driven low-frequency chaotic variability may ultimately impact biogeochemistry, the atmosphere and climate is an important but unsettled question.

Before addressing this question in fully-coupled simulations, it is necessary to explicitly simulate, characterize and quantify over a long period the stochastic character and scales of the low-frequency oceanic variability at high Reynolds number under full reanalyzed forcing, with a focus on climate-relevant indexes. In the framework of the OCCIPUT ANR/PRACE project, we have performed and are currently analyzing a 50-member ensemble of 1/4° global ocean/sea-ice NEMO-based 1/4° hindcasts driven by the same reanalyzed 1958-2014 atmospheric forcing. After a common spinup, the spread of the ensemble is seeded by applying stochastic perturbations within each member for one year; eddy interactions then take control of the subsequent growth of the ensemble spread and of its cascade toward long space and time scales. 

Along with reduced-size North Atlantic sensitivity experiments, this global ensemble simulation provides a probabilistic description of the global ocean/sea-ice evolution over the last 5 decades over a wide range of spatio-temporal scales, and direct estimates of the chaotic ocean variability (from the ensemble spread) and of the actual constraint exerted by the atmosphere (variability of the ensemble mean). We will present our strategy, describe the strong imprints of the atmospherically-modulated ocean stochastic variability on temperature, AMOC, SSH and water mass properties with a focus on interannual and longer time scales.

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