Le cadre général de ce travail est l'étude des problématiques de couplage de modèles dans le contexte applicatif de l'hydraulique et de l'océanographie. Ce sont des domaines dans lesquels les besoins sont nombreux, utilisant une vaste gamme de modèles tels que les équations de Navier-Stokes, de Saint-Venant ou des modèles intermédiaires tels que les équations primitives ou de Navier-Stokes hydrostatiques. Pour aborder ces questions de couplage, on a choisi d'utiliser les méthodes de Schwarz, pour leur caractère non-intrusif vis-à-vis des codes de calcul. On a travaillé tout d'abord sur les équations de Navier-Stokes, en envisageant deux discrétisations : une méthode de projection où la vitesse et la pression sont calculées dans deux étapes différentes, et un schéma couplé où toutes les inconnues sont calculées lors de la même étape. À chaque pas de temps, on est ramené à une équation de Stokes, pour laquelle différents opérateurs d'interface ont été considérés (Dirichlet, absorbants exacts et approchés...). La convergence de la méthode de Schwarz a été étudiée et testée numériquement dans la configuration classique de la cavité entraînée. Ces conditions ont ensuite été étendues, sans être testées numériquement, à des équations proches des équations de Navier-Stokes, à savoir dans un premier temps aux équations d'Oseen (qui sont une version linéarisée des équations de Navier-Stokes), et dans un deuxième temps en vue d'un couplage entre les équations de Navier-Stokes et les équations de Navier-Stokes hydrostatiques.

Les épisodes de fortes précipitations, caractéristiques du climat méditerranéen, souffrent parfois d'importantes erreurs de prévision. Ces erreurs proviennent essentiellement d'incertitudes sur les conditions initiales du modèle ou la représentation des processus physiques. L'impact des différentes sources d'incertitudes est classiquement étudié à partir de prévisions d'ensemble. Au cours de ce travail, nous avons développé une méthodologie d'ensemble visant à étudier la sensibilité des prévisions à différentes configurations initiales des anomalies dynamiques d'altitude. Ces anomalies sont des zones à fort gradient, susceptibles de constituer des zones d'erreurs de prévision privilégiées. Elles sont par ailleurs un précurseur connu des perturbations météorologiques. La méthodologie proposée repose sur l'advection de la variable conservative associée aux anomalies d'altitude : le tourbillon potentiel (PV). Le modèle utilisé, MIMOSA, advecte le PV sur un niveau isentrope, sur une grille de haute résolution, permettant de mieux représenter les petites échelles et les zones de gradient tout en contrôlant les grandes échelles grâce à une relaxation vers un PV de contrôle. Le PV MIMOSA a été introduit dans un modèle en équations primitives à l'aide d'un outil d'inversion du PV. Le choix retenu pour l'inversion est un algorithme qui fournit des solutions équilibrées au sens des équations du modèle. Différentes configurations d'advection du PV ont fourni un ensemble de conditions initiales au modèle de prévision ARPEGE. Cet ensemble dénommé ARPEGE-MIMOSA, a été appliqué à 5 cas réels de fortes précipitations : le premier épisode, ou "catastrophe de Draguignan" sur le département du Var le 15 juin 2010, est associé à une prévisibilité particulièrement faible ; les 4 autres cas ont été observés en 2012 durant la campagne HyMeX et ont été utilisés comme un premier échantillon pour évaluer l'apport statistique du système de prévision d'ensemble ARPEGE-MIMOSA. Le cas du 15 juin 2010 a fait l'objet d'une étude détaillée, qui a notamment confronté l'impact de perturbations d'altitude à celui des perturbations de basse-couche. Pour cette situation, nous concluons que les incertitudes des champs d'altitude ont joué un rôle mineur durant la première phase de l'épisode en raison d'une absence de couplage entre les anomalies de surface et d'altitude. Le rôle des incertitudes de surface a été examiné au moyen de plusieurs expériences perturbées: des modifications manuelles de la pression au niveau de la mer et du champ d'humidité se sont avérées toutes deux capables d'améliorer la première phase de l'épisode. Cette amélioration a été attribuée au déclenchement d'un système fortement précipitant le long d'une zone frontale. Pour la seconde phase de l'épisode, nous avons mis en évidence le rôle de l'humidité à travers une simulation qui permet de se rapprocher du scénario observé sans toutefois atteindre le niveau de précipitation observé. Une explication alternative a été alors proposée, mettant en jeu l'insuffisance du modèle à reproduire la progression d'une onde d'altitude dans un contexte fortement diabatique. Enfin, une évaluation objective du système de prévision d'ensemble ARPEGE-MIMOSA a été réalisée à partir des 4 cas HyMeX. Les résultats obtenus avec l'ensemble ARPEGE-MIMOSA ont été comparés aux résultats issus de deux systèmes de prévision d'ensemble opérationnels. Cette comparaison a permis de mieux quantifier l'erreur de prévision attribuable à la seule dynamique d'altitude et de mettre en évidence la valeur ajoutée du système ARPEGE-MIMOSA pour les seuils de fortes précipitations.

Understanding the interactions between ocean and atmosphere in regions of oceanographic fronts is of vital importance for the improvement of numerical models for weather and climate forecasting. In the South Atlantic Ocean (SAO) the meeting between the warm waters of the Brazil Current (BC) and the cold waters of the Malvinas (Falkland) Current (MC) in the region known as the Brazil-Malvinas Confluence (BMC), results in intense mesoscale oceanic activity and, for this reason, this region is considered one of the most energetic of the Global Ocean. The interactions resulting from the thermal contrast in regions oceanographic fronts of the OAS are investigated in this work through estimates of heat fluxes based on data collected in situ and by satellite. The results of this study show that the response to the thermal contrasts found in the ocean is in the form of heat fluxes and these fluxes are critical in modulating the atmospheric boundary layer (ABL). Estimation based on data collected in situ show that in the warm side (north) of the oceanographic front the fluxes are more intense (latent heat: 62 W/m² and sensible heat: 0.6 W/m²) than in the cold side (south) (latent heat: 5.8 W/m² and sensible heat: -13.8 W/m²). In the South Atlantic Current (SAC) along the 30° S parallel, heat fluxes are directly related to the meandering characteristic of the current. The data collected in situ, in addition to allow heat flux estimates at a better spatial resolution, were used to develop a new method for estimating the heat energy exchanged between the atmosphere and the ocean caused by the presence of mesoscale oceanic structures. This methodology consists in the comparison of a radiosonde profile taken over waters of the structure of interest and another taken over waters which do not belong to this structure. The methodology was used to estimate the heat energy transfer between the atmosphere and the ocean over the top of three structures sampled in the OAS. The estimation of the heat energy transferred by a warm eddy detached from the BC points to an energy in the latent (sensible) form of 1.6 1017 J (-2.8 1016 J) which corresponds to approximately 0.011 % of the total heat energy of the eddy transferred to the atmosphere during the field experiment and 0.78 % transferred during the supposed lifetime of the eddy (3 months). Along the CSA two oceanic structures were studied: (i) a cold meander that receives from the atmosphere energy in the latent (sensible) form of 1.4 106 J/m2 (5.4 105 J/m2), and (ii) warmer waters associated with a detached eddy from the Agulhas Current (AC) that transfer to the atmosphe heat energy of approximately 4 106 J/m2 an 5.7 106 J/m2 in the latent and sensible forms, respectively. The estimation of heat energy transfer on top of mesoscale oceanic structures clearly demonstrate the importance of these structures for the heat exchanges between the ocean and the atmosphere and must be taken into account in future works about this subject in the SAO. / A compreensão das interações entre oceano e atmosfera em regiões de frentes oceanográficas é de vital importância para o melhoramento de modelos numéricos de previsão do tempo e clima. No Oceano Atlântico Sul (OAS) o encontro entre as águas quentes da Corrente do Brasil (CB) com as águas frias da Corrente das Malvinas (CM), na região denominada Confluência Brasil-Malvinas (CBM), resulta em intensa atividade oceânica de mesoescala e, por esse motivo, essa região é considerada uma das mais energéticas do Oceano Global. As interações resultantes do contraste termal ao longo de regiões de frentes oceanográficas no OAS são investigadas neste trabalho através de estimativas de fluxos de calor baseadas em dados de satélite e dados coletados in situ. Os resultados do trabalho demonstram que a resposta aos contrastes termais encontrados no oceano se dá na forma de fluxos de calor e que esses fluxos são fundamentais na modulação da Camada Limite Atmosférica (CLA). As estimativas com base em dados coletados in situ demonstram que no lado quente (norte) da frente oceanográfica os fluxos são mais intensos (calor latente: 62 W/m² e calor sensível: 0,6 W/m²) que nos lado frio (sul) (calor latente: 5,8 W/m² e calor sensível: -13,8 W/m²). Na Corrente Sul Atlântica (CSA), ao longo do paralelo de 30° S, os fluxos de calor estão diretamente relacionados a característica meandrante da corrente. Os dados coletados in situ, além de possibilitarem estimativas de fluxo de calor com uma melhor resolução espacial, foram usados no desenvolvimento de uma nova metodologia para estimativa da energia calorífica trocada entre oceano e atmosfera em virtude da presença de estruturas oceânicas de mesoescala. Essa metodologia consiste na comparação entre um perfil de radiossonda tomado sobre águas da estrutura de interesse e outro tomado sobre águas que não pertencem a essa estrutura. A metodologia desenvolvida foi utilizada para determinar a transferência de energia calorífica entre oceano e atmosfera em três estruturas amostradas no OAS. A estimativa da energia calorífica transferida por um vórtice quente desprendido da CB aponta para uma energia na forma latente (sensível) de 1,6 1017 J (-2,8 1016 J) que corresponde a aproximadamente 0,011 % da energia calorífica total do vórtice transferida durante o experimento de campo e de 0,78 % da energia do vórtice transferidos durante o tempo suposto de vida do vórtice (3 meses). Ao longo da CSA, duas estruturas oceânicas foram estudadas: (i) um meandro frio que recebe da atmosfera uma energia na forma latente (sensível) de 1,4 106 J/m2 (5,4 105 J/m2) e (ii) águas mais quentes associadas a um vórtice desprendido da Corrente das Agulhas (CA) que transferem para a atmosfera uma energia calorífica de aproximadamente 4 106 J/m2 e 5,7 106 J/m2 nas formas latente e sensível, respectivamente. As estimativas da transferência de energia calorífica sobre estruturas oceânicas de mesoescala demonstram claramente a importância destas nas trocas de calor entre o oceano e a atmosfera e devem ser levadas em consideração em trabalhos futuros sobre o tema no OAS.

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2016-03-18T13:04:49Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_Gbekpo_Aubains_2015.pdf: 10869276 bytes, checksum: 0b22521ef971c75f3112dcbc74fcbb7a (MD5) / Made available in DSpace on 2016-03-18T13:04:50Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_Gbekpo_Aubains_2015.pdf: 10869276 bytes, checksum: 0b22521ef971c75f3112dcbc74fcbb7a (MD5) Previous issue date: 2015-04-08 / As interações do sistema oceano-atmosfera no Atlântico tropical e suas contribuições à grande variabilidade da precipitação ao longo da costa do nordeste do Brasil (NEB) foram investigadas para os anos de 1974-2008. Os núcleos das estações chuvosas de Março-Abril e de Junho-Julho foram identificados para a parte norte do Nordeste do Brasil (NNEB) e a parte leste do Nordeste do Brasil (ENEB), respectivamente. As regressões lineares defasadas entre as anomalias da Temperatura da Superfície do Mar (TSM), da Pseudo tensão de cisalhamento de vento (PWS), do Fluxo de calor latente (LHF), da Umidade especifica do ar, e as anomalias (positivas e negativas) de precipitação forte no NNEB e no ENEB mostram que a variabilidade da precipitação dessas regiões é diferentemente influenciada pela dinâmica do Atlântico tropical. Quando a zona de convergência intertropical (ZCIT) é anormalmente deslocada para o sul alguns meses antes da estação chuvosa do NNEB, a fase negativa do Modo Meridional do Atlântico (AMM) (fortalecimento dos ventos alísios do nordeste, relaxamento dos ventos alísios do sudeste, maior evaporação no hemisfério norte, menor evaporação no hemisfério sul, TSM mais fria no hemisfério norte, e TSM mais quente no hemisfério Sul), aumenta a precipitação durante a estação chuvosa. O efeito oposto ocorre durante a fase positiva do AMM. Além disso, o estudo mostra a grande influência e um efeito preditivo da região Noroeste do Atlântico Equatorial noroeste (NWEA) sobre a precipitação do NNEB. Com relação ao estado subsuperficial do oceano, os resultados indicam que uma camada de barreira mais fina na NWEA de Novembro-Dezembro até Março-Abril é associada ao resfriamento progressivo da TSM, ao reforço do componente meridional do vento nordeste e precipitações intensas sobre o NNEB. Já a influência da dinâmica do Atlântico tropical sobre a variabilidade da precipitação no ENEB em Junho-Julho indica uma propagação para noroeste de uma área de forte correlação positiva de TSM e de Umidade específica do ar, deslocando-se da parte sudeste do Atlântico tropical (de Fevereiro-Março) para a região da Piscina Quente do Atlântico Sudoeste (SAWP), situada ao largo do Brasil (Junho-Julho). A área de propagação das anomalias, observada segue globalmente o caminho do ramo sul da Corrente Sul Equatorial (sSEC), que é responsável pelo transporte de calor oceânico de leste para oeste no Atlântico tropical sul. O deslocamento da fase mais quente da advecção horizontal de calor oceânico, na camada de mistura, de leste da bacia (entre 5º-15ºS) para a costa do Brasil em Junho-Agosto corrobora a influência da sSEC sobre o núcleo da chuva do ENEB. Uma aceleração dos ventos alísios de sudeste, associada a uma convergência da anomalia do vento sobre a SAWP, produz excesso de umidade do ar sobre a região e provoca mais precipitação sobre ENEB. O efeito oposto ocorre para os episódios menos chuvosos. De acordo com o estudo, a SAWP se mostra como uma área de potencial para o estabelecimento de um índice de previsão de chuvas no ENEB. / Tropical Atlantic Ocean-atmosphere interactions and their contributions to strong variability of rainfall along the Northeast Brazilian coast (NEB) were investigated for the years 1974-2008. The core rainy seasons of March-April and June-July were identified for northern Northeast Brazil (NNEB) and eastern Northeast Brazil (ENEB), respectively. Lagged linear regressions between sea surface temperature (SST), pseudo wind stress (PWS), latent heat flux (LHF) and air specific humidity anomalies over the entire tropical Atlantic and strong rainfall anomalies in NNEB and ENEB show that the rainfall variability of these regions is differentially influenced by the dynamics of the tropical Atlantic. When the intertropical convergence zone (ITCZ) is abnormally displaced southward a few months prior to the NNEB rainy season, the associated meridional mode (strengthening of the northeast trade winds, relaxation of the southeast trade winds, strong evaporation in the north, weak evaporation in the south, colder SST in the North, and warmer SST in the South) increases precipitation during the rainy season. The opposite effect occurs during the positive phase of the dipole. Additionally, this study shows strong influence and predictive effect of the Northwestern Equatorial Atlantic (NWEA) on the NNEB rainfall. Thinner barrier layer in the NWEA from November-December to March-April is associated with progressive cooling of SST, strengthening of meridional component of the northeasterly wind and intense precipitations over the NNEB. The dynamical influence of the tropical Atlantic on the June-July ENEB rainfall variability shows a northwestward-propagating area of strong, positively correlated SST and air specific humidity from the southeastern tropical Atlantic (February-March) to the Southwestern Atlantic Warm Pool (SAWP) offshore of Brazil (June-July). The northwestward-propagating area, observed from February-March to June-July, follows the same pathway of the southern branch of south equatorial current (sSEC), which is responsible of the oceanic heat transport from east to west in the southern tropical Atlantic. The displacement of the warmest phase of horizontal advection of the oceanic mixed layer heat from the eastern part (between 5-15ºS) to the Brazilian coast in June-August confirms this influence of the sSEC on core rainy season in the ENEB. Furthermore, according to our study, the SAWP could be used as index of rainfall prediction in ENEB. An early acceleration of the southeasterly trade winds, associated with a strong convergence of the wind anomaly over the SAWP, produces excessive humidity over the region and causes more precipitation over ENEB. The opposite effect occurs for less rainy episodes.

No presente trabalho utiliza-se um m´etodo multi-escala para estudar de forma te´orica as intera¸coes nao lineares entre o oceano e a atmosfera atrav´es de ressonancia onda-onda. Desenvolve-se uma hierarquia de modelos acoplados oceano-atmosfera nao lineares que foram escalonados convenientemente para representar as principais escalas de variabilidade clim´atica (i.e., intrasazonal, interanual, e decenal). A enfase dos modelos desenvolvidos foi dado para a regiao tropical. As fontes de nao linearidade inclu´das no modelo sao de dois tipos: I) nao linearidade intr´nsica (advectiva) e II) nao linearidade relacionada com os termos da f´sica e ambas sao abordadas neste trabalho. Para obter as equa¸coes que regem a dinamica de intera¸coes ressonantes a partir da hierarquia de modelos acoplados, aplicou-se um m´etodo perturbativo multi-escala. As solu¸coes sao escritas em termos de solu¸coes de ordem dominante e solu¸coes seculares. Para as solu¸coes de ordem dominante e seculares utilizam-se as fun¸coes base do problema linear, em uma aproxima¸cao do tipo Galerkin. As propriedades das fun¸coes base permitem calcular de forma anal´tica os coeficientes de intera¸cao associados com os termos nao lineares, assim como tamb´em permitem projetar estes termos nos modos de oscila¸cao natural do sistema (ressonancia). Com este m´etodo obt´em-se modelos reduzidos que permitem determinar as contribui¸coes de diversos processos para a evolu¸cao em escala lenta de um determinado modo de variabilidade natural. Para aplicar estes conceitos ao problema de acoplamento oceano-atmosfera utiliza-se como Ansatz (hip´otese inicial para a solu¸cao do problema) um tripleto composto por duas ondas atmosf´ericas e uma onda oceanica, sendo uma onda de Kelvin e de Rossby na atmosfera e uma onda Kelvin no oceano. O tripleto escolhido representa uma aproxima¸cao de v´arios fenomenos encontrados na regiao tropical, e.g. o desenvolvimento do El Nino, a intera¸cao da oscila¸cao de Madden-Julian com o oceano, a intera¸cao entre el Nino e variabilidade intrasazonal. No presente trabalho ´e mostrado que existe a ressonancia envolvendo ondas atmosf´ericas e oceanicas e que a modula¸cao em baixa frequencia produto desta ressonancia pode afetar desde escalas r´apidas sin´oticas equatoriais, intrasazonais, interanuais e at´e variabilidade da ordem de dezenas de anos. Palavras chave: Dinamica Equatorial nao linear, Intera¸coes Ressonantes, Modelos Acoplados Oceano-Atmosfera, El Nino, Oscila¸cao de Madden Julian, Oscila¸coes Decenais (Decadal) / In the present work a multiscale method is used to study resonant nonlinear wave-wave interactions between the ocean and the atmosphere. A hierarchy of coupled atmosphere-ocean models is developed using typical scalings found in the tropical region with the aim to represent some of the dominant modes of climate variability (intraseasonal, interannual and decadal). The sources of nonlinearity included into model are of two types: I) intrinsic nonlinearity (advective form) and II) nonlinearity related to physical terms. A multi-scale perturbation method is applied to obtain equations governing dynamics of ressonant interactions. The solutions are described in terms of dominant and secular solutions. For the dominant modes basis functions of the linear problem are used in a approximation of the Galerkin type. The properties of the basis functions allows the analytical computation of the interaction coefficients associated with non-linear terms and the projection into the natural oscillation modes of the system (resonance). Using this method it is possible to obtain reduced models to determine the contributions of several processes to the slow time evolution of a specific mode of natural variability. To apply these concepts to the problem of atmosphere-ocean coupling an Ansatz composed of a three waves (two atmospheric Rossby and Kelvin waves and an ocean Kelvin wave) is used. The triad chosen represents a aproximation of several phenomena found in the tropical region, e.g. desenvolving of El Nino, interaction of the Madden-Julian oscillation with the ocean, interaction between El Nino and intra-seasonal variability, etc. It is shown that system allows a resonance involving atmospheric and oceanic waves and that the low-frequency modulation resulting from these ressonance can affect the system from fast equatorial synoptic scales to decadal timescales, including the intermediate scales i.e., intraseasonal and interannual.

Thesis (Ph.D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences and the Woods Hole Oceanographic Institution), 2000. / Includes bibliographical references (p. 284-290). / by Brian Kenneth Arbic. / Ph.D.

Thesis (Ph.D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences and the Woods Hole Oceanographic Institution), 2000. / Includes bibliographical references (p. 217-222). / Estimation of the upper ocean heat budget from one year of observations at a moored array in the north central Arabian Sea shows a rough balance between the horizontal advection and time change in heat when the one-dimensional balance between the surface heat flux and oceanic heat content breaks down. The two major episodes of horizontal advection, during the early northeast (NE) and late southwest (SW) monsoon seasons, are both associated with the propagation of mesoscale eddies. During the NE monsoon, the heat fluxes within the mixed layer are not significantly different from zero, and the large heat flux comes from advected changes in the thermocline depth. During the SW monsoon a coastal filament exports recently upwelled water from the Omani coast to the site of the array, 600 km offshore. Altimetry shows mildly elevated levels of surface eddy kinetic energy along the Arabian coast during the SW monsoon, suggesting that such offshore transport may be an important component of the Arabian Sea heat budget. The sea surface temperature (SST) and mixed layer depth are observed to respond to high frequency (HF, diurnal to atmospheric synoptic time scales) variability in the surface heat flux and wind stress. The rectified effect of this HF forcing is investigated in a three-dimensional reduced gravity thermodynamic model of the Arabian Sea and Indian Ocean. Both the HF heat and wind forcing act locally to increase vertical mixing in the model, reducing the SST. Interactions between the local response to the surface forcing, Ekman divergences, and remotely propagated signals in the model can reverse this, generating greater SSTs under HF forcing, particularly at low latitudes. The annual mean SST, however, is lowered under HF forcing, changing the balance between the net surface heat flux (which is dependent on the SST) and the meridional heat flux in the model. A suite of experiments with one-dimensional upper ocean models with different representations of vertical mixing processes suggests that the rectified effect of the diurnal heating cycle is dependent on the model, and overstated in the formulation used in the three-dimensional model. / by Albert Sok Fischer. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (p. 221-236). / This thesis focuses on ocean circulation and atmospheric forcing in the Atlantic Ocean at the Last Glacial Maximum (LGM, 18-21 thousand years before present). Relative to the pre-industrial climate, LGM atmospheric CO₂ concentrations were about 90 ppm lower, ice sheets were much more extensive, and many regions experienced significantly colder temperatures. In this thesis a novel approach to dynamical reconstruction is applied to make estimates of LGM Atlantic Ocean state that are consistent with these proxy records and with known ocean dynamics. Ocean dynamics are described with the MIT General Circulation Model in an Atlantic configuration extending from 35°S to 75°N at 1° resolution. Six LGM proxy types are used to constrain the model: four compilations of near sea surface temperatures from the MARGO project, as well as benthic isotope records of [delta]¹⁸O and [delta]¹³C compiled by Marchal and Curry; 629 individual proxy records are used. To improve the fit of the model to the data, a least-squares fit is computed using an algorithm based on the model adjoint (the Lagrange multiplier methodology). The adjoint is used to compute improvements to uncertain initial and boundary conditions (the control variables). As compared to previous model-data syntheses of LGM ocean state, this thesis uses a significantly more realistic model of oceanic physics, and is the first to incorporate such a large number and diversity of proxy records. A major finding is that it is possible to find an ocean state that is consistent with all six LGM proxy compilations and with known ocean dynamics, given reasonable uncertainty estimates. Only relatively modest shifts from modern atmospheric forcing are required to fit the LGM data. The estimates presented herein succesfully reproduce regional shifts in conditions at the LGM that have been inferred from proxy records, but which have not been captured in the best available LGM coupled model simulations. In addition, LGM benthic [delta]¹⁸O and [delta]¹³C records are shown to be consistent with a shallow but robust Atlantic meridional overturning cell, although other circulations cannot be excluded. / by Holly Janine Dail. / Ph.D.

Cette thèse présente différents cas d'acclimatation d'une espèce de phytoplancton (Isochrysis sp.) à des modifications de son environnement physico-chimique avec une attention particulière portée sur la composante lipidique de la réponse aux changements. Dans ce contexte nous avons étudié les lipides à la fois comme des produits (triacylglycerol) et des acteurs (glycolipides) de la photosynthèse. Tout d'abord, nous avons étudié l'influence de plusieurs types de limitation par l'azote sur la teneur en lipides neutres des algues. Nous avons montré que la relation entre la disponibilité de l'azote et la teneur en lipide n'était pas monotone et qu'elle présentait un caractère hystérétique. Les cellules d'Isochrysis sp. n'accumulent des lipides neutres que transitoirement lors du passage d'une croissance forte vers une croissance faible. Nous nous sommes ensuite intéressé aux relations qui existent entre le cycle nycthéméral et l'accumulation de réserves énergétiques sous forme de lipides et de sucres. Les réserves sont accumulées pendant le jour en disproportion du reste du carbone et consommées pendant la nuit pour alimenter la division cellulaire et la croissance nocturne. La carence en azote dans ces conditions nycthémérales ne provoque pas d'accumulations significatives de lipides. Les variations diurnes de la teneur en lipides neutres sont plus importantes que les variations obtenues en carence. A travers ces différentes expériences nous avons pu mettre en évidences les relations qui existaient entre l'environnement, la croissance, et l'accumulation de réserves. La compréhension de ces mécanismes a permis de déterminer les conditions environnementales qui favorisent la production d'huile en vue d'une utilisation biocarburant. Nous avons aussi étudié l'influence des conditions de croissance sur la composition des glycolipides des membranes des chloroplastes. Les résultats montrent que les changements de la teneur en MGDG, DGDG et SQDG et de l'insaturation des acides gras de ces lipides accompagnent les modifications d'activité photosynthétique des cellules en limitation azotée et en cycle Jour/Nuit.

Les tourbillons et les écoulements méso-échelles océaniques contribuent de façon importante au transport de masses d'eau et de chaleur dans l'océan. La compréhension de leurs dynamiques, et de leurs interactions avec la topographie, est primordiale, pour caractériser les échanges thermohalins, le déplacements d'espèces biologiques ou de polluants. En particulier, les courants et tourbillons de surface, situés dans les cent ou deux cents premiers mètres de profondeur, jouent un rôle capital dans les échanges entre la zone côtière et la zone hauturière. Cette thèse porte sur la modélisation expérimentale idéalisée de deux phénomènes pouvant participer activement aux échanges horizontaux et verticaux dans la couche supérieure océanique. Les sillages d'îles peuvent être le lieu d'instabilités fortes dans les zones anticycloniques de l'écoulement. Des expériences ont été réalisées sur la Plateforme Coriolis (LEGI) à Grenoble, pour identifier les régimes dynamiques de ces instabilités. Ce dispositif (13 mètres de diamètre) a permis d'atteindre de faibles rapports d'aspects géométriques, tout en travaillant avec un dispositif couche-mince. Des visualisations avec colorants et des mesures du champ de vitesse horizontal par intercorrélation de particules ont été réalisées, afin d'étudier la sensibilité des instabilités dans les anticyclones pour différentes gammes de nombres de Reynolds et de Rossby. Nous montrons que ces instabilités sont présentes dès la couche limite anticyclonique, autour de l'île. L'autre partie de ce travail concerne la dynamique d'un tourbillon de surface, en interaction avec la côte ou le talus océanique. Des mesures récentes (campagne EGYPT-1) nous ont permis de mesurer la dérive particulière d'un anticyclone de surface le long de la côte libyenne. Cette dérive vers l'est ne peut s'expliquer par la simple prise en compte de l'effet beta ou de l'effet miroir le long de la côte. Des expériences idéalisées sont menées sur la table tournante de l'ENSTA, afin de modéliser de façon simple l'interaction d'une côte ou d'un talus sur la dynamique d'un anticyclone, à la surface d'un dispositif bi-couche. La vitesse de ce tourbillon de surface voit notamment sa dérive doubler lorsqu'il se trouve initialement à la base de la pente, montrant ainsi l'existence d'une rétroaction possible de la couche du fond sur la dynamique de la couche de surface.