Séminaire climatologie

[GD]

mais pas ouvert à tous...

SEMINAIRE DE GWENDAL RIVIERE (CNRS/GAME) le 12 AVRIL 2006 à 11 H EN SALLE DE CONFERENCES DU CERFACS

Titre:

"Caractéristiques des dépressions synoptiques au dessus de l'Atlantique

et leur role dans le mécanisme de l'Oscillation Nord Atlantique (NAO)"

Résumé:

The seminar will focus on feedbacks of the high-frequency eddy activity

onto the quasi-stationary circulation, particularly with regard to the

North Atlantic Oscillation (NAO). The methodology consists in analyzing

observations from NCEP reanalysis data and sensitivity runs from a high

resolution non hydrostatic regional model developed at Geophysical Fluid

Dynamics Laboratory (GFDL).

Consistently with recent studies, results show that the jet displacement

characteristic of the NAO phenomenon depends strongly on the dynamics

of synoptic-scale waves and the way they break. Positive and negative

phases of the NAO are closely related to anticyclonic and cyclonic wave

breaking respectively. The aim of our study is thus to undertand what

makes the waves break cyclonically and anticyclonically in order to

determine the factors that promote negative and positive phases of the

NAO. The original aspect of our work is to show that wave breaking over

the Atlantic is strongly sensitive to the spatial and temporal

characteristics of the upper-level waves coming from the Eastern Pacific

and North America. Another crucial factor is related to surface effects;

depending on the strength of the surface moisture fluxes, upper-level

waves will tend to break more cyclonically or anticyclonically. The

presentation will also emphasize the role played by some extratropical

storms, in particular those occurring in the Eastern coast of US, and

their ability to change suddenly the NAO index and to affect its sign

during an entire month. Finally, I will present some arguments to show

how these results could be useful to interpret the influence of other

teleconnections on the NAO or to better understand the decadal trend of

the NAO.

[GD]

un autre....

*VENDREDI 14 AVRIL 2006 ý 14 H*

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SÈminaire spÈcial CNRM

Salle de ConfÈrences du CNRM

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ThËme : **´MathÈmatiques et MÈtÈorologieª** **

´ ModÈlisation probabiliste des Ècoulements atmosphÈriques turbulents afin** d'en filtrer la mesure par approche particulaire** ª** **par Christophe Baehr GMEI*

Bien que le travail sous-jacent soit assez thÈorique, nous allons effectuer une prÈsentation sans le symbolisme mathÈmatique et adaptÈe au public le plus large. ProblÈmatique : La mesure de la turbulence nÈcessite une approche intÈgrÈe pour la conception des instruments, l'acquisition ý haute cadence des mesures et leur traitement. Pour le traitement, compte tenu de la nature alÈatoire du fluide observÈ, nous sommes face ý un problËme d'estimation non-linÈaire d'un phÈnomËne chaotique. Nous avons alors besoin conjointement d'un modËle de l'Ècoulement turbulent atmosphÈrique et d'un modËle de notre dispositif de mesure. Les deux sont fortement non linÈaires et affectÈs de distributions non gaussiennes. Ces deux caractÈristiques justifient pleinement l'utilisation de l'approche particulaire (discrÈtisations alÈatoires de l'espace d'Ètat et estimations des lois de probabilitÈs conditionnelles en ces points). Plan de la prÈsentation:- Nous commenÁons par exposer la technique d'estimation stochastique par particule dans le cas gÈnÈral.- Nous verrons un exemple simple o˜ l'estimateur de Kalman est comparÈ ý l'estimation particulaire.- Nous exposerons comment l'estimation de la dynamique d'un fluide avec une observation bruitÈe peut s'insÈrer dans le cadre du filtrage stochastique.- Nous aborderons ensuite le cas prÈcis de la turbulence et de l'algorithme de traitement que nous avons dÈveloppÈ pour le vent.- Nous examinerons les rÈsultats de notre filtre sur des mesures rÈelles au sol rÈalisÈes par un thermo-anÈmomËtre sonique avec divers bruits artificiels.- Nous terminerons par l'exposÈ des dÈveloppements envisagÈs.

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[GD]

et encore un!

*Jeudi 13 AVRIL 2006 ý 14 h***

SÈminaire CNRM

Salle de ConfÈrences du CNRM

ìResponse of the Japan Sea to synoptic atmospheric forcingî

par Daisuke Inazu (UniversitÈ du Japon)

Synoptic atmospheric forcing drive the variations of flow and sea level of the semi-enclosed Japan Sea. The sea level response of the Japan Sea to synoptic pressure forcing lags behind inverted barometer because of the large drag in and around the shallow straits. Our analysis of sea level data at tidal stations around the Japan Sea reveals that the barometric response lags proportionally to the distances from the Tsushima or Tsugaru Straits. A numerical simulation shows that the barometric response has zonally asymmetric patterns of amplitude/phase in the Japan Sea. The response amplitude is larger (smaller) than 1 cm/hPa in the eastern (western) part of the basin. The phase lag takes the maximum near the central region and gradually becomes small in the eastern/western part of the basin. The simulated response is in good agreement with the analyzed response at each tidal station. It is found that the zonal asymmetry arises from the eastward traveling pressure forcing with the wavelength larger but not much larger than the spatial scale of the Japan Sea. Our numerical experiments also show that the growing low pressure systems generate the large phase lag in the western region of the Japan Sea.

On the other hand, the wind forcing associated with synoptic system drives the water budgets through the straits and changes the mean sea level of the basin. The numerical simulation suggests that the water exchange through the Tsushima Strait contributes the main variation of the mean sea level due to the synoptic wind forcing. It is also suggested that the wind-driven transports through the Tsugaru and Soya Straits are mainly caused by the sea level difference between the Japan Sea and the Pacific or the Sea of Okhotsk. The transports due to the sea level difference seem to have larger variability than those driven by local wind stress. A simple estimation based on Toulany and Garrett (1984) or Wright (1987) well accounts for the lags of the transports through the Tsugaru and Soya Straits behind the mean sea level in the Japan Sea.