Universiẗat, Diss., 1999--Bremen.

Made available in DSpace on 2015-03-26T13:50:02Z (GMT). No. of bitstreams: 1 texto completo.pdf: 3607155 bytes, checksum: 0ba931da234b886d1f1b534578664639 (MD5) Previous issue date: 2009-07-28 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The global thermohaline circulation (THC) is the transport of ocean water masses caused by differences in the sea water density due to variations in temperature and salinity. Studies have shown that increased precipitation in high latitudes of the Northern Hemisphere and the sea ice melting of the Arctic region generates a freshwater flow of in the North Atlantic which is able to shutoff the formation of deep water and hence desintensifies the THC. Considering the above, this study investigates the anomalous pattern of the oceanic and atmospheric circulation by introducing an increase of 1 Sverdrup (Sv) (1 Sv = 106m3s-1) of freshwater into the North Atlantic, based on simulations performed with the LOVECLIM model. The results show that the weakening of the THC leads to strong cooling in the North Atlantic region whereas the extratropical Southern Hemisphere warms. The weakening of the THC also changes the patterns of atmospheric circulation, inducing a reduction in the subtropical jet speed due to the smooth thermal gradient between the equator and the southern hemisphere polar region. Furthermore, the intertropical convergence zone moves southwards and changes the precipitation regime of the north and the northeastern part of Brazil. The reduction of the THC also leads to changes in the baroclinic instability in the middle and high latitudes of the Southern Hemisphere. / A Circulação Termohalina Global (CTG) consiste no transporte de massas d água oceânicas associado a diferenças na densidade da água do mar devido a variações de temperatura e salinidade. Estudos têm demonstrado que o aumento da precipitação em altas latitudes do Hemisfério Norte e o derretimento do gelo da região do Ártico podem gerar um fluxo de água doce no Oceano Atlântico Norte, capaz de interromper a formação de água profunda e, conseqüentemente, reduzir a CTG. Diante do exposto, o objetivo do trabalho é investigar o comportamento anômalo das circulações oceânica e atmosférica devido a um aumento de 1 Sverdrup (Sv) (1 Sv = 106m3s-1) no transporte de água doce no Atlântico Norte, com base em simulações realizadas com um modelo climático acoplado (LOVECLIM). Os resultados demonstram que o enfraquecimento da CTG provoca um forte resfriamento no Atlântico Norte enquanto que a região extratropical do Hemisfério Sul aquece. A inibição da CTG também muda os padrões da circulação atmosférica, se observa uma redução na corrente de jato subtropical devido o menor gradiente térmico entre o equador e a região polar austral. Além disso, a zona de convergência intertropical desloca-se para sul alterando o regime de precipitação das regiões norte e nordeste do Brasil. Por outro lado existe um enfraquecimento da instabilidade baroclínica nas latitudes médias e altas do Hemisfério Sul.

La mer Ionienne est une zone à la géodynamique active en raison de la convergence entre les plaques Nubie et Eurasie. Elle correspond aux derniers stades de vie d’un océan, la Téthys. De ce fait, la tectonique et la sédimentation y sont très réduites ; et les faibles flux sédimentaires permettent ainsi d’enregistrer une multiplicité de processus sédimentaires.Néanmoins, c’est une zone encore peu étudiée d’un point de vue sédimentaire, notamment sur les échelles de temps de l’ordre de la centaine de milliers d’années. Ce travail est basé sur une étude sédimentologique d’archives marines complétée par des données acoustiques (bathymétrie et multi-faisceau) issue de campagnes océanographiques du SHOM. L’analyse détaillée des faciès et des séquences sédimentaire a permis, dans un premier temps, d’établir un calendrier des risques naturels (séismes, tsunamis, volcanisme), leurs sources et leurs processus de dépôt dans le bassin pour les derniers 330 000 ans. Puis, dans un second temps, de retracer l’évolution sur les derniers 400 000 ans de la circulation et de l’oxygénation des eaux de fond dans le bassin ionien et l’influence du détroit de Sicile, et notamment de la plate-forme de Malte, sur les échanges entre les bassins occidental et oriental. L’intégration des données sédimentologiques dans un modèle géoacoustique développé par le SHOM a finalement permis de déterminer l’impact des variations sédimentaires (distribution spatiale, lithologie, stratification) sur la propagation des ondes acoustique pour différentes gammes de fréquences (300 Hz - 3000 Hz) et d’angle d’incidence (0 -90°) et d’établir une cartographie de la réponse sédimentaire du le signal acoustique. / The Ionian Sea is an active geodynamic area because of the convergence between theNubia and the Eurasia plate. It corresponds to the last stage of the Tethys ocean life. Therefore,the tectonics and the sedimentation are much reduced; and the low sedimentary supply enables torecord a multiplicity of sedimentary processes. Nevertheless, this area still poorly studied from asedimentary point of view, in particular on timescales on the order of hundred thousand years.This work is based on a sedimentological study of marine archives supplemented by acoustic data(bathymetry and multibeam imagery) recovered during oceanographic campaign leaded by theSHOM. The detailed sedimentary analysis of facies and sequences allows, at first, to established acalendar of the natural hazard (earthquakes, tsunamis, volcanism), their origins and theirdepositional processes into the basin over the last 330 000 years. Then, secondly, to reconstructthe evolution over the last 400 000 years of the circulation and the oxygenation of bottom waterthrough the Ionian basin and the influence of the strait of Sicily, in particular of the Malta Plateau,on the exchanges between the western and the eastern basins. The integration of thesedimentological data in a geoacoustic modelling developed by the SHOM finally allowed todetermine the impact of the sedimentary variability (special distribution, lithology, stratification)on the acoustic waves propagation for various frequency bands (300 Hz - 3000 Hz) and incidentangle (0 - 90°) and to established a mapping of the sedimentary answer of the acoustic signal.

<p>Stochastic information, to be understood as &quot;information gained by the application of stochastic methods&quot;, is proposed as a tool in the assessment of changes in climate.</p> <p>This thesis aims at demonstrating that stochastic information can improve the consideration and reduction of uncertainty in the assessment of changes in climate. The thesis consists of three parts. In part one, an indicator is developed that allows the determination of the proximity to a critical threshold. In part two, the tolerable windows approach (TWA) is extended to a probabilistic TWA. In part three, an integrated assessment of changes in flooding probability due to climate change is conducted within the TWA.</p> <p>The thermohaline circulation (THC) is a circulation system in the North Atlantic, where the circulation may break down in a saddle-node bifurcation under the influence of climate change. Due to uncertainty in ocean models, it is currently very difficult to determine the distance of the THC to the bifurcation point. We propose a new indicator to determine the system's proximity to the bifurcation point by considering the THC as a stochastic system and using the information contained in the fluctuations of the circulation around the mean state. As the system is moved closer to the bifurcation point, the power spectrum of the overturning becomes &quot;redder&quot;, i.e. more energy is contained in the low frequencies. Since the spectral changes are a generic property of the saddle-node bifurcation, the method is not limited to the THC, but it could also be applicable to other systems, e.g. transitions in ecosystems. </p> <p>In part two, a probabilistic extension to the tolerable windows approach (TWA) is developed. In the TWA, the aim is to determine the complete set of emission strategies that are compatible with so-called guardrails. Guardrails are limits to impacts of climate change or to climate change itself. Therefore, the TWA determines the &quot;maneuvering space&quot; humanity has, if certain impacts of climate change are to be avoided. Due to uncertainty it is not possible to definitely exclude the impacts of climate change considered, but there will always be a certain probability of violating a guardrail. Therefore the TWA is extended to a probabilistic TWA that is able to consider &quot;probabilistic uncertainty&quot;, i.e. uncertainty that can be expressed as a probability distribution or uncertainty that arises through natural variability.</p> <p>As a first application, temperature guardrails are imposed, and the dependence of emission reduction strategies on probability distributions for climate sensitivities is investigated. The analysis suggests that it will be difficult to observe a temperature guardrail of 2&deg;C with high probabilities of actually meeting the target.</p> <p>In part three, an integrated assessment of changes in flooding probability due to climate change is conducted. A simple hydrological model is presented, as well as a downscaling scheme that allows the reconstruction of the spatio-temporal natural variability of temperature and precipitation. These are used to determine a probabilistic climate impact response function (CIRF), a function that allows the assessment of changes in probability of certain flood events under conditions of a changed climate. </p> <p>The assessment of changes in flooding probability is conducted in 83 major river basins. Not all floods can be considered: Events that either happen very fast, or affect only a very small area can not be considered, but large-scale flooding due to strong longer-lasting precipitation events can be considered. Finally, the probabilistic CIRFs obtained are used to determine emission corridors, where the guardrail is a limit to the fraction of world population that is affected by a predefined shift in probability of the 50-year flood event. This latter analysis has two main results. The uncertainty about regional changes in climate is still very high, and even small amounts of further climate change may lead to large changes in flooding probability in some river systems.</p> / <p>Stochastische Information, zu verstehen als &quot;Information, die durch die Anwendung stochastischer Methoden gewonnen wird&quot;, wird als Hilfsmittel in der Bewertung von Klima&auml;nderungen vorgeschlagen.</p> <p>Das Ziel dieser Doktorarbeit ist es, zu zeigen, dass stochastische Information die Ber&uuml;cksichtigung und Reduktion von Unsicherheit in der Bewertung des Klimawandels verbessern kann. Die Arbeit besteht aus drei Teilen. Im ersten Teil wird ein Indikator entwickelt, der die Bestimmung des Abstandes zu einem kritischen Grenzwert ermöglicht. Im zweiten Teil wird der &quot;tolerable windows approach&quot; (TWA) zu einem probabilistischen TWA erweitert. Im dritten Teil wird eine integrierte Absch&auml;tzung der Ver&auml;nderung von &Uuml;berflutungswahrscheinlichkeiten im Rahmen des TWA durchgeführt.</p> <p>Die thermohaline Zirkulation (THC) ist ein Zirkulationssystem im Nordatlantik, in dem die Zirkulation unter Einfluss des Klimawandels in einer Sattel-Knoten Bifurkation abreißen kann. Durch Unsicherheit in Ozeanmodellen ist es gegenw&auml;rtig kaum m&ouml;glich, den Abstand des Systems zum Bifurkationspunkt zu bestimmen. Wir schlagen einen neuen Indikator vor, der es erm&ouml;glicht, die N&auml;he des Systems zum Bifurkationspunkt zu bestimmen. Dabei wird die THC als stochastisches System angenommen, und die Informationen, die in den Fluktuationen der Zirkulation um den mittleren Zustand enthalten sind, ausgenutzt. Wenn das System auf den Bifurkationspunkt zubewegt wird, wird das Leistungsspektrum &quot;roter&quot;, d.h. die tiefen Frequenzen enthalten mehr Energie. Da diese spektralen Ver&auml;nderungen eine allgemeine Eigenschaft der Sattel-Knoten Bifurkation sind, ist die Methode nicht auf die THC beschränkt, sondern weitere Anwendungen k&ouml;nnten m&ouml;glich sein, beispielsweise zur Erkennung von &Uuml;berg&auml;ngen in &Ouml;kosystemen.</p> <p>Im zweiten Teil wird eine probabilistische Erweiterung des &quot;tolerable windows approach&quot; (TWA) entwickelt. Das Ziel des TWA ist die Bestimmung der Menge der Emissionsreduktionsstrategien, die mit sogenannten Leitplanken kompatibel sind. Diese Leitplanken sind Begrenzungen der Auswirkungen des Klimawandels, oder des Klimawandels selber. Der TWA bestimmt daher den Spielraum, den die Menschheit hat, wenn bestimmte Auswirkungen des Klimawandels vermieden werden sollen. Durch den Einfluss von Unsicherheit ist es aber nicht m&ouml;glich, die betrachteten Auswirkungen des Klimawandels mit Sicherheit auszuschlie&szlig;en, sondern es existiert eine gewisse Wahrscheinlichkeit, dass die Leitplanke verletzt wird. Der TWA wird daher zu einem probabilistischen TWA weiterentwickelt, der es erm&ouml;glicht, &quot;probabilistische Unsicherheit&quot;, also Unsicherheit, die durch eine Wahrscheinlichkeitsverteilung ausgedr&uuml;ckt werden kann, oder die durch den Einfluß von nat&uuml;rlicher Variabilit&auml;t entsteht, zu ber&uuml;cksichtigen.</p> <p>Als erste Anwendung werden Temperaturleitplanken betrachtet, und die Abh&auml;ngigkeit der Emissionsreduktionsstrategien von Wahrscheinlichkeitsverteilungen über die Klimasensitivit&auml;t wird bestimmt. Die Analyse ergibt, dass die Einhaltung einer Temperaturleitplanke von 2&deg;C sehr schwierig wird, wenn man hohe Wahrscheinlichkeiten des Einhaltens der Leitplanke fordert.</p> <p>Im dritten Teil wird eine integrierte Absch&auml;tzung der &Auml;nderungen von &Uuml;berflutungswahrscheinlichkeiten unter Einfluss des Klimawandels durchgef&uuml;hrt. Ein einfaches hydrologisches Modell wird vorgestellt, sowie ein Skalierungsansatz, der es erm&ouml;glicht, die raum-zeitliche nat&uuml;rliche Variabilit&auml;t von Temperatur und Niederschlag zu rekonstruieren. Diese werden zur Bestimmung einer probabilistischen Klimawirkungsfunktion genutzt, einer Funktion, die es erlaubt, die Ver&auml;nderungen der Wahrscheinlichkeit bestimmter &Uuml;berflutungsereignisse unter Einfluss von Klima&auml;nderungen abzusch&auml;tzen.</p> <p>Diese Untersuchung der Ver&auml;nderung von &Uuml;berflutungswahrscheinlichkeiten wird in 83 großen Flusseinzugsgebieten durchgef&uuml;hrt. Nicht alle Klassen von &Uuml;berflutungen k&ouml;nnen dabei ber&uuml;cksichtigt werden: Ereignisse, die entweder sehr schnell vonstatten gehen, oder die nur ein kleines Gebiet betreffen, k&ouml;nnen nicht ber&uuml;cksichtigt werden, aber großfl&auml;chige &Uuml;berflutungen, die durch starke, langanhaltende Regenf&auml;lle hervorgerufen werden, k&ouml;nnen ber&uuml;cksichtigt werden. Zuguterletzt werden die bestimmten Klimawirkungsfunktion dazu genutzt, Emissionskorridore zu bestimmen, bei denen die Leitplanken Begrenzungen des Bev&ouml;lkerungsanteils, der von einer bestimmten Ver&auml;nderung der Wahrscheinlichkeit eines 50-Jahres-Flutereignisses betroffen ist, sind. Letztere Untersuchung hat zwei Hauptergebnisse. Die Unsicherheit von regionalen Klima&auml;nderungen ist immer noch sehr hoch, und au&szlig;erdem k&ouml;nnen in einigen Flusssystemen schon kleine Klima&auml;nderungen zu gro&szlig;en &Auml;nderungen der &Uuml;berflutungswahrscheinlichkeit f&uuml;hren.</p>

Die Tiefenkonvektion ist ein wesentlicher Bestandteil der Zirkulation im Nordatlantik. Sie beeinflusst den nordwärtigen Wärmetransport der thermohalinen Zirkulation. Ein Verständnis ihrer Stabilität und Variabilität ist daher nötig, um Klimaveränderungen im Bereich des Nordatlantiks einschätzen zu können. <br /> <br /> Diese Arbeit hat zum Ziel, das konzeptionelle Verständnis der Stabilität und der Variabilität der Tiefenkonvektion zu verbessern. Beobachtungsdaten aus der Labradorsee zeigen Phasen mit und ohne Tiefenkonvektion. Ein einfaches Modell mit zwei Boxen wird an diese Daten angepasst. Das Ergebnis legt nahe, dass die Labradorsee zwei koexistierende stabile Zustände hat, einen mit regelmäßiger Tiefenkonvektion und einen ohne Tiefenkonvektion. Diese Bistabilität ergibt sich aus einer positiven Salzgehalts-Rückkopplung, deren Ursache ein Netto-Süßwassereintrag in die Deckschicht ist. Der konvektive Zustand kann schnell instabil werden, wenn der mittlere Antrieb sich hin zu wärmeren oder weniger salzhaltigen Bedingungen ändert. <br /> <br /> Die wetterbedingte Variabilität des externen Antriebs wird durch die Addition eines stochastischen Antriebsterms in das Modell eingebaut. Es zeigt sich, dass dann die Tiefenkonvektion häufig an- und wieder ausgeschaltet wird. Die mittlere Aufenthaltszeit in beiden Zuständen ist ein Maß ihrer stochastischen Stabilität. Die stochastische Stabilität hängt in glatter Weise von den Parametern des Antriebs ab, im Gegensatz zu der deterministischen (nichtstochastischen) Stabilität, die sich abrupt ändern kann. Sowohl das Mittel als auch die Varianz des stochastischen Antriebs beeinflussen die Häufigkeit von Tiefenkonvektion. Eine Abnahme der Konvektionshäufigkeit, als Reaktion auf eine Abnahme des Salzgehalts an der Oberfläche, kann zum Beispiel durch eine Zunahme der Variabilität in den Wärmeflüssen kompensiert werden. <br /> <br /> Mit einem weiter vereinfachten Box-Modell werden einige Eigenschaften der stochastischen Stabilität analytisch untersucht. Es wird ein neuer Effekt beschrieben, die wandernde Monostabilität: Auch wenn die Tiefenkonvektion aufgrund geänderter Parameter des Antriebs kein stabiler Zustand mehr ist, kann der stochastische Antrieb immer noch häufig Konvektionsereignisse auslösen. Die analytischen Gleichungen zeigen explizit, wie die wandernde Monostabilität sowie andere Effekte von den Modellparametern abhängen. Diese Abhängigkeit ist für die mittleren Aufenthaltszeiten immer exponentiell, für die Wahrscheinlichkeit langer nichtkonvektiver Phasen dagegen nur dann, wenn diese Wahrscheinlichkeit gering ist. Es ist zu erwarten, dass wandernde Monostabilität auch in anderen Teilen des Klimasystems eine Rolle spielt. <br /> <br /> Insgesamt zeigen die Ergebnisse, dass die Stabilität der Tiefenkonvektion in der Labradorsee sehr empfindlich auf den Antrieb reagiert. Die Rolle der Variabilität ist entscheidend für ein Verständnis dieser Empfindlichkeit. Kleine Änderungen im Antrieb können bereits die Häufigkeit von Tiefenkonvektionsereignissen deutlich mindern, was sich vermutlich stark auf das regionale Klima auswirkt. / Deep convection is an essential part of the circulation in the North Atlantic Ocean. It influences the northward heat transport achieved by the thermohaline circulation. Understanding its stability and variability is therefore necessary for assessing climatic changes in the area of the North Atlantic. <br /> <br /> This thesis aims at improving the conceptual understanding of the stability and variability of deep convection. Observational data from the Labrador Sea show phases with and without deep convection. A simple two-box model is fitted to these data. The results suggest that the Labrador Sea has two coexisting stable states, one with regular deep convection and one without deep convection. This bistability arises from a positive salinity feedback that is due to the net freshwater input into the surface layer. The convecting state can easily become unstable if the mean forcing shifts to warmer or less saline conditions. <br /> <br /> The weather-induced variability of the external forcing is included into the box model by adding a stochastic forcing term. It turns out that deep convection is then switched &quot;on&quot; and &quot;off&quot; frequently. The mean residence time in either state is a measure of its stochastic stability. The stochastic stability depends smoothly on the forcing parameters, in contrast to the deterministic (non-stochastic) stability which may change abruptly. The mean and the variance of the stochastic forcing both have an impact on the frequency of deep convection. For instance, a decline in convection frequency due to a surface freshening may be compensated for by an increased heat flux variability. <br /> <br /> With a further simplified box model some stochastic stability features are studied analytically. A new effect is described, called wandering monostability: even if deep convection is not a stable state due to changed forcing parameters, the stochastic forcing can still trigger convection events frequently. The analytical expressions explicitly show how wandering monostability and other effects depend on the model parameters. This dependence is always exponential for the mean residence times, but for the probability of long nonconvecting phases it is exponential only if this probability is small. It is to be expected that wandering monostability is relevant in other parts of the climate system as well. <br /> <br /> All in all, the results demonstrate that the stability of deep convection in the Labrador Sea reacts very sensitively to the forcing. The presence of variability is crucial for understanding this sensitivity. Small changes in the forcing can already significantly lower the frequency of deep convection events, which presumably strongly affects the regional climate. <br><br>----<br>Anmerkung:<br> Der Autor ist Träger des durch die Physikalische Gesellschaft zu Berlin vergebenen Carl-Ramsauer-Preises 2003 für die jeweils beste Dissertation der vier Universitäten Freie Universität Berlin, Humboldt-Universität zu Berlin, Technische Universität Berlin und Universität Potsdam.

Two long-lived halogen radionuclides (36Cl, T1/2 = 301 kyr, and 129I, T1/2 = 15.7 Myr) have been studied by means of Accelerator Mass Spectrometry (AMS) at the Uppsala Tandem Laboratory. The 36Cl measurements in natural samples using a medium-sized tandem accelerator (~1 MeV/amu) have been considered. A gas-filled magnetic spectrometer (GFM) was proposed for the separation of 36Cl from its isobar, 36S. Semi-empirical Monte-Carlo ion optical calculations were conducted to define optimal conditions for separating 36Cl and 36S. A 180° GFM was constructed and installed at the dedicated AMS beam line. 129I has been measured in waters from the Arctic and North Atlantic Oceans. Most of the 129I currently present in the Earth's surface environment can be traced back to liquid and gaseous releases from the nuclear reprocessing facilities at Sellafield (UK) and La Hague (France). The anthropogenic 129I inventory in the central Arctic Ocean was found to increase proportionally to the integrated 129I releases from these reprocessing facilities. The interaction and origin of water masses in the region have been clearly distinguished with the help of 129I labeling. Predictions based on a compartment model calculation showed that the Atlantic Ocean and deep Arctic Ocean are the major sinks for the reprocessed 129I. The variability in 129I concentration measured in seawater along a transect from the Baltic Sea to the North Atlantic suggests strong enrichment in the Skagerrak–Kattegat basin. The 129I inventory in the Baltic and Bothnian Seas is equal to ~0.3% of the total liquid releases from the reprocessing facilities. A lake sediment core sampled in northeastern Ireland was analyzed for 129I to study the history of the Sellafield releases, in particular the nuclear accident of 1957. High 129I concentration was observed corresponding to 1990 and later, while no indication of the accident was found. The results of this thesis research clearly demonstrate the uniqueness and future potential of 129I as a tracer of processes in both marine and continental archives.

Generalized linear stability theory is used to calculate the optimal initial conditions that result in transient amplification of the thermohaline circulation (THC) in a zonally-averaged single basin ocean model. The eigenmodes of the tangent linear model verify that the system is asymptotically stable but the nonnormality of the system permits the growth of perturbations for a finite period through the interference of nonorthogonal eigenmodes. It is found that the maximum amplification of the THC anomalies occurs after 6 years with both the thermally driven and salinity driven components playing major roles in the amplification process. The transient amplification of THC anomalies is due to the constructive and destructive interference of a large number of eigenmodes and the evolution over time is determined by how the interference pattern evolves. It is found that five of the most highly nonnormal eigenmodes are critical to the initial cancellation of the salinity and temperature contributions to the THC while 11 oscillating modes with decay timescales ranging from 2 to 6 years are the major contributors at the time of maximum amplification. This analysis demonstrates that the different dynamics of salinity and temperature anomalies allows the dramatic growth of perturbations to the THC on relatively short (interannual to decadal) timescales. In addition the ideas of generalized stability theory are used to calculate the stochastic optimals which are the spatial patterns of stochastic forcing that are most efficient at generating variance growth in the THC. It is found that the optimal stochastic forcing occurs at high latitudes and induces low-frequency THC variability by exciting the salinity-dominated modes of the THC. The first stochastic optimal is found to have its largest projection on the same five highly nonnormal eigenmodes found to be critical to the structure of the optimal initial conditions. The model’s response to stochastic forcing is not controlled by the least damped eigenmodes of the tangent linear model but rather by the linear interference of these highly nonnormal eigenmodes. The process of pseudoresonance suggests that the nonnormal eigenmodes are excited and sustained by stochastically induced perturbations which in turn lead to maximum THC variance. Finally, it was shown that the addition of wind stress did not have a large impact on the nonnormal dynamics of the linearised system. Adding wind allowed the value of the vertical diffusivity to be reduced to achieve the same maximum linearised THC amplitude as was used in the case with no wind stress.

Generalized linear stability theory is used to calculate the optimal initial conditions that result in transient amplification of the thermohaline circulation (THC) in a zonally-averaged single basin ocean model. The eigenmodes of the tangent linear model verify that the system is asymptotically stable but the nonnormality of the system permits the growth of perturbations for a finite period through the interference of nonorthogonal eigenmodes. It is found that the maximum amplification of the THC anomalies occurs after 6 years with both the thermally driven and salinity driven components playing major roles in the amplification process. The transient amplification of THC anomalies is due to the constructive and destructive interference of a large number of eigenmodes and the evolution over time is determined by how the interference pattern evolves. It is found that five of the most highly nonnormal eigenmodes are critical to the initial cancellation of the salinity and temperature contributions to the THC while 11 oscillating modes with decay timescales ranging from 2 to 6 years are the major contributors at the time of maximum amplification. This analysis demonstrates that the different dynamics of salinity and temperature anomalies allows the dramatic growth of perturbations to the THC on relatively short (interannual to decadal) timescales. In addition the ideas of generalized stability theory are used to calculate the stochastic optimals which are the spatial patterns of stochastic forcing that are most efficient at generating variance growth in the THC. It is found that the optimal stochastic forcing occurs at high latitudes and induces low-frequency THC variability by exciting the salinity-dominated modes of the THC. The first stochastic optimal is found to have its largest projection on the same five highly nonnormal eigenmodes found to be critical to the structure of the optimal initial conditions. The model’s response to stochastic forcing is not controlled by the least damped eigenmodes of the tangent linear model but rather by the linear interference of these highly nonnormal eigenmodes. The process of pseudoresonance suggests that the nonnormal eigenmodes are excited and sustained by stochastically induced perturbations which in turn lead to maximum THC variance. Finally, it was shown that the addition of wind stress did not have a large impact on the nonnormal dynamics of the linearised system. Adding wind allowed the value of the vertical diffusivity to be reduced to achieve the same maximum linearised THC amplitude as was used in the case with no wind stress.

Le but de cette thèse est d’analyser les impacts de la variabilité atmosphérique grande échelle sur la circulation océanique. Ceci a déjà fait l’objet de nombreuses publications, dans lesquelles la variabilité atmosphérique est analysée en termes de modes de variabilité, déterminés par analyse en composantes principales (EOF en anglais) des anomalies de pression de surface. Ces modes sont l’Oscillation Nord-Atlantic (NAO), le Pattern Est-Atlantique (EAP) et le Pattern Scandinave (SCAN). La décomposition en EOF implique que les modes sont orthogonaux et symétriques. Cette dernière hypothèse a été montrée comme étant invalide pour la NAO. Par conséquent, un nouveau concept est proposé dans cette étude pour estimer la variabilité atmosphérique, celui des régimes de temps. Ces derniers sont des structures spatiales de grande échelle, récurrents et quasi-Stationnaires qui permettent de capturer la variabilité des forçages atmosphériques. De plus, ils permettent de séparer les patterns spatiaux des deux phases de la NAO. Ces régimes de temps sont donc une alternative prometteuse pour l’analyse de la variabilité océanique forcée par l’atmosphère. A partir d’observation et de modèles numériques (réalistes ou idéalisés), nous avons montré que les régimes Atlantic Ridge (AR), NAO− et NAO+ induisent une réponse rapide (échelles mensuelles à interannuelles) des gyres subtropical et subpolaire (via un mécanisme de Sverdrup topographique) et de la cellule de retournement (MOC, ajustement aux anomalies de transport d’Ekman). Aux échelles décennales, le gyre subpolaire s’intensifie lors de conditions NAO+ et BLK persistantes via un ajustement barocline aux flux de flottabilité et s’affaiblit pour AR via un ajustement barocline aux anomalies de rotationnel de vent. Ce dernier mécanisme explique aussi l’augmentation du gyre subtropical pour une NAO+ persistante et son affaiblissement pour un AR persistant. La réponse des gyres pour des conditions de NAO− persistantes est un déplacement vers le sud des gyres (l’intergyre gyre). L’intensité de la MOC est augmentée pour des conditions de NAO+ et BLK persistantes, dû à l’augmentation de la formation d’eau dense en mer du Labrador, et inversement pour NAO− et AR. Finalement, des bilans de contenu de chaleur dans la gyre subpolaire et les mers nordiques ont été effectués dans quatre modèles océaniques globaux. Les moyennes d’hiver de convergence océanique de chaleur dans la partie ouest de la gyre subpolaire sont positivement corrélées aux occurrences d’hiver de NAO−, ce qui est dû à la présence de l’intergyre, tandis que cette convergence est négativement corrélée aux occurrences d’AR, ce qui est dû à la réduction des deux gyres qui lui est associée. Les flux de chaleur vers l’océan dans la gyre subpolaire sont négativement corrélés aux occurrences d’hiver de la NAO+ et inversement pour la NAO−. Dans les mers Nordiques, ils sont positivement corrélés aux occurrences de BLK et, dans une moindre mesure, aux occurrences de AR. De plus, nous suggérons que la variabilité du contenu de chaleur dans la partie ouest du gyre subpolaire est la réponse décalée (lag de 6 ans) à l’intégration temporelle du forçage lié au régime NAO+, due à la combinaison de la réponse en phase (0-Lag) des flux de chaleur et à la réponse décalée (lag de 3 ans) de la convergence de chaleur. / The aim of the PhD is to investigate the impacts of the large-Scale atmospheric variability on the North- Atlantic ocean circulation. This question has already been addressed in a large number of studies, in which the atmospheric variability is decomposed into modes of variability, determined by decomposing sea-Level pressure anomalies into Empirical Orthogonal Function (EOFs). These modes of variability are the North-Atlantic Oscillation (NAO), the East-Atlantic Pattern (EAP) and the Scandinavian Pattern (SCAN). EOF decomposition assumes that the modes are orthogonal and symmetric. The latter assumption, however, has been shown to be inadequate for the NAO. Hence, a different framework is used in this study to assess the atmospheric variability, the so-Called weather regimes. These are large-Scale, recurrent and quasi-Stationary atmospheric patterns that have been shown to capture well the interannual and decadal variability of atmospheric forcing to the ocean. Furthermore, they allow to separate the spatial patterns of the positive and negative NAO phases. Hence, these weather regimes are a promising alternative to modes of variability in the study of the ocean response to atmospheric variability. Using observations and numerical models (realistic or in idealised settings), we have shown that the Atlantic Ridge (AR), NAO− and NAO+ regimes drive a fast (monthly to interannual) wind-Driven response of the subtropical and subpolar gyres (topographic Sverdrup balance) and of the meridional overturning circulation (MOC, driven by Ekman transport anomalies). At decadal timescales, the subpolar gyre strengthens for persistent NAO+ and Scandinavian Blocking (BLK) conditions via baroclinic adjustment to buoyancy fluxes and slackens for persistent AR conditions via baroclinic adjustment to wind-Stress curl anomalies. The latter mechanism also accounts for the strengthening of the subtropical gyre for persistent NAO+ conditions and its weakening for persistent AR conditions. The gyres response to persistent NAO− conditions reflects the southward shift of the gyre system (the intergyre gyre). The MOC spins-Up for persistent NAO+ and BLK conditions via increased deep water formation in the Labrador Sea, and conversely for the NAO− and AR regimes. Last, heat budget calculations in the subpolar gyre and the Nordic Seas have been performed using four global ocean hindcasts. The winter averaged heat convergence in the western subpolar gyre is positively correlated with the NAO− winter occurrences, which is due to the intergyregyre circulation, while it is negatively correlated with AR winter occurrences, because of the wind-Driven reduction of both gyres. Downward surface heat flux anomalies are negatively correlated with NAO+ occurrences, and conversely for the NAO−. In the Nordic Seas, they are positively correlated with BLK and to a lesser extent AR occurrences. Furthermore, we suggest that the heat content variability in the western subpolar gyre is the signature of the delayed response (6-Year lag) to the time-Integrated NAO+ forcing, due to the combination of the immediate (0-Lag) response of surface heat flux and the lagged (3 year lag) response of ocean heat convergence.

Les flux air-mer, la convection profonde et la cyclogénèse sont étudiés en Méditerranée<br />grâce au développement d'un modèle régional couplé (AORCM). Il reproduit correctement<br />ces processus et permet de quantifier et d'étudier leur variabilité climatique. Le couplage<br />régional a un impact significatif sur le nombre de cyclogénèses intenses en hiver et sur<br />les flux et précipitations associés. Il simule une variabilité interannuelle plus faible qu'en<br />mode forcé pour les flux et la convection et permet de comprendre les rétroactions<br />qui la pilotent. L'impact régional d'un scénario climatique est analysé avec les modèles<br />non-couplés : le nombre de cyclogénèses diminue, les pluies associées augmentent au<br />printemps et en automne et diminuent en été. En outre, la Méditerranée se réchauffe,<br />se sale et sa circulation thermohaline s'affaiblit fortement. Cette thèse conclut de plus à<br />la nécessité des AORCMs pour étudier l'impact du changement climatique en Méditerranée.