Historic and future extreme precipitation and wind events over southern Baffin Island, more specifically Iqaluit, Kimmirut, Pangnirtung and Cape Dorset are examined. Two sets of modeled re-analysis data, the Canadian Regional Climate Model (CRCM) forced with European Centre for Medium-Range Weather Forecasts Regional Analysis 40 (ERA40) and the other the North American Regional Re-analysis (NARR) dataset were used to characterize the atmosphere during historic events. Two sets of CRCM data forced with Canadian Global Climate Model (CGCM) data, one from 1961-1990 and the other from 2041-2070, are compared to assess the changes in extreme events in the future. Extreme events were defined by daily precipitation and sustained wind thresholds. Based on the CRCM future projection, events were inferred to increase in intensity for all communities and increase in frequency for 3 of the 4 communities. A shift in the Arctic storm season was also inferred in the future projection.

Historic and future extreme precipitation and wind events over southern Baffin Island, more specifically Iqaluit, Kimmirut, Pangnirtung and Cape Dorset are examined. Two sets of modeled re-analysis data, the Canadian Regional Climate Model (CRCM) forced with European Centre for Medium-Range Weather Forecasts Regional Analysis 40 (ERA40) and the other the North American Regional Re-analysis (NARR) dataset were used to characterize the atmosphere during historic events. Two sets of CRCM data forced with Canadian Global Climate Model (CGCM) data, one from 1961-1990 and the other from 2041-2070, are compared to assess the changes in extreme events in the future. Extreme events were defined by daily precipitation and sustained wind thresholds. Based on the CRCM future projection, events were inferred to increase in intensity for all communities and increase in frequency for 3 of the 4 communities. A shift in the Arctic storm season was also inferred in the future projection.

L'étude de la variabilité climatique dans les moyennes et hautes latitudes est très complexe, principalement en raison des nombreux mécanismes physiques mis en jeu. Cette variabilité climatique résulte de deux contributions majeures : la variabilité interne associée à des processus internes au système climatique et la variabilité forcée qui est liée aux forçages externes, qui peuvent être d'origine naturelle (comme le volcanisme, les aérosols naturels) ou anthropique (GES, aérosols anthropiques). Ces forçages externes jouent un rôle important sur le climat et sa variabilité. Le défi de la recherche climatique est de comprendre leurs effets sur le climat et leurs rôles face à la variabilité interne. Cette thèse a comme objectif une meilleure compréhension des rôles respectifs de la variabilité interne et des différents forçages externes sur la variabilité passée de la circulation atmosphérique dans les deux hémisphères et de l'activité synoptique associée, ainsi que de leur évolution pour le climat futur en utilisant des ré-analyses atmosphériques et des simulations issues du modèle de climat CNRM-CM5. Nous nous sommes intéressés dans un premier temps à l'étude des changements des modes annulaires dans les hémisphères nord et sud, appelés respectivement NAM (Northern Annular Mode) et SAM (Southern Annular Mode). Nous montrons que la tendance positive observée du SAM sur la période 1960-1990 en été austral est bien reproduite par le modèle. Cependant, celle-ci est reproduite lorsque la diminution d'ozone stratosphérique et l'augmentation des gaz à effet de serre sont toutes deux prescrites au modèle, alors que certaines études suggéraient qu'elle était principalement expliquée par la seule présence de l'ozone. Nous nous sommes ensuite focalisés sur les changements de circulation \linebreak atmosphérique dans l'hémisphère nord. Ceux-ci sont plus complexes que dans l'hémisphère sud. En effet, l'augmentation des GES provoque un réchauffement global différent suivant l'altitude : maximum dans la haute troposphère tropicale ainsi qu'aux hautes latitudes en surface. Ce dernier est en réalité expliqué par la fonte de la banquise Arctique. De nombreux processus physiques et de nombreuses rétroactions sont mis en jeu et rendent la compréhension des changements compliquée. Pour cela, nous avons mis en place un protocole expérimental avec le modèle couplé CNRM-CM5 pour séparer l'effet direct des GES (réchauffement de l'atmosphère) de l'effet indirect (fonte de la banquise Arctique et rétroaction sur l'atmosphère). Cette fonte est responsable de l'augmentation des flux de chaleur entre l'océan et l'atmosphère qui perturbe ensuite la circulation atmosphérique. Nous montrons que la diminution de glace de mer Arctique pourrait être à l'origine de la réponse barocline dans l'hémisphère nord. Enfin, la dernière partie de cette thèse a fait l'objet d'une étude plus spécifique, puisque l'on s'est intéressé aux évolutions passée et future des dépressions atmosphériques nord Atlantique. De nombreuses incertitudes persistent en raison de processus complexes mettant en jeu la vapeur d'eau, le gradient méridien de température et la stabilité statique. Nous retrouvons la réponse tripolaire dans le RCP8.5, mentionnée dans le 5\up{ème} rapport du GIEC, qui correspond à une diminution dans la partie sud du domaine Atlantique nord et la mer Méditerranée, une augmentation sur les îles Britanniques et la Scandinavie et une diminution à l'est du Groenland. Nous montrons que ce signal est peu robuste sur la période dite historique en raison de la forte variabilité interne associée aux systèmes dépressionnaires. / Climate variability in mid and high latitudes is very complex due to numerous physical mecanims implied. This climate variability can be decomposed into 2 components : the internal variability associated with internal processes and the forced variability linked to the external forcings which can be natutal (volcanism, natural aerosols) or anthropogenic (greenhouse gases, anthropogenic aerosols). These external forcings play a crucial role on the climate and its variability. The challenge in the climate research is to understand their effects on the climate and their roles relatively with the internal variability. The objective of this thesis is a better understanding of the respective roles of internal variability and forced variability on the past and future atmospheric circulation in both hemispheres characterized by the annular mode and the synoptic activity associated using atmospheric reanalysis and experiments performed with the coupled climate model CNRM-CM5. First, we focus on the annular mode changes in both hemispheres, named the NAM (Northern Annular Mode) and the SAM (Southern Annular Mode). We show that the observed positive trend of the SAM in the 1960s in austral summer is well reproduced by the climate model. However, contrarily to other studies which suggest that this positive trend can be explained by only stratospheric ozone depletion, it is reproduced in the CNRM-CM5 model when the ozone depletion and greenhouse gases (GHG) increase are both prescribed. Then, we investigate the changes in the Northern Hemisphere atmospheric circulation. These are more complex than in the Southern Hemisphere. Indeed, the increase of GHG in the atmosphere causes a general global warming maximum in the tropical high troposphere and over the pole at the surface which is mainly explained by Arctic sea ice loss. So the understanding of the changes is very complex due to several physical processes and retroactions. Thus, we have conducted a protocol with the coupled climate model CNRM-CM5 in order to assess the respective role of Arctic sea ice loss and GHG increase. Arctic sea ice loss is reponsible for an increase in the heat flux between the atmosphere and the ocean which modify the atmospheric circulation. We show that Arctic sea ice loss can cause the baroclinic response in the Northern Hemisphere. Finally, the last part of the thesis is the study of past and future changes in the North Altantic storm-tracks. There are still sereval uncertainties because of the complex processes involving the water vapour, the meridional temperature gradient and the static stability. We find the tripolar response, already found by other studies, consisting of a significant decrease in the south of the basin and over the Mediterranean sea, a small increase over the British Isles, and a decrease east of the Greenland. We show that the signal in the historical period is not robust, due to large chaotic variability associated with storms.

The research presented in this dissertation attempted to determine whether climate variability is critical to sea level changes in coastal BC. To that end, a number of statistical models were proposed to clarify the relationships between five climate variability indices representing large-scale atmospheric circulation regimes and sea levels, storm surges, extreme winds and storm track variability in coastal BC. The research findings demonstrate that decadal to inter decadal climatic variability is fundamental to explaining the changing frequency and intensity of extreme atmospheric and oceanic environmental variables in coastal BC. The trends revealed by these analyses suggest that coastal flooding risks are certain to increase in this region during the next few decades, especially if the global sea-levels continue to rise as predicted. The out come of this study emphasis the need to look beyond climatic means when completing climate impact assessments, by clearly showing that climate extremes are currently causing the majority of weather-related damage along coastal BC. The findings highlight the need to derive knowledge on climate variability and change effects relevant at regional to local scales to enable useful adaptation strategies. The major findings of this research resulted in five independent manuscripts: (i) Sea level responses to climatic variability and change in Northern BC. The Manuscript (MC) is published in the Journal of atmospheric and oceans (AO 46 (3), 277-296); (ii) Extreme sea-level recurrences in the south coast of BC with climate considerations. This MC is in review with the Asia Pacific Journal of Climate Change (APJCC); (iii) Extreme sea-surge responses to climate variability in coastal BC. This MC is currently in review in the Annals of the AAG (AN-2009-0098); (iv) Extreme wind regime responses to climate variability and change in the inner-south-coast of BC. This MC is published in the Journal of Atmosphere and Oceans (AO 47 (1), 41-62); (v) Sensitivity of winter storm track characteristics in North-eastern Pacific to climate variability. This manuscript is in review with the Journal of Atmosphere and Oceans (AO (1113)). The findings of this research program made key contributions to the following regional sea level rise impact assessment studies in BC: (i) An examination of the Factors Affecting Relative and Absolute Sea level in coastal BC (Thomson et al., 2008). (ii) Coastal vulnerability to climate change and sea level rise, Northeast Graham Island, Haida Gwaii (formally known as the Queen Charlotte Islands), BC (Walker et al., 2007). (iii) Storm Surge: Atmospheric Hazards, Canadian Atmospheric Hazards Network - Pacific and Yukon Region, C/O Bill Taylor.

Les objectifs de cette thèse sont d'analyser l'influence des processus humides sur les dépressions des moyennes latitudes en s'intéressant plus particulièrement aux déplacements des dépressions et à la formation des vents forts près de la surface. Ces recherches ont été effectuées à l'aide de simulations idéalisées du modèle de méso-échelle Méso-NH. La première étude s'est focalisée sur l'effet des processus humides sur le déplacement des dépressions dans la direction perpendiculaire à l'axe du courant-jet, c'est à dire principalement le déplacement vers les pôles. Le mécanisme expliquant ce déplacement est dû à l'advection non-linéaire de la dépression de surface par les anomalies cyclonique et anticyclonique d'altitude se trouvant de part et d'autre de la dépression de surface. En présence d'humidité, l'anomalie anticyclonique d'altitude se renforce du fait du dégagement de chaleur latente dans les zones d'ascendances, et ainsi le déplacement perpendiculairement à l'axe du jet et vers les pôles s'accélère. On montre aussi que le déplacement vers l'est des dépressions s'accélère en incluant les processus humides. La seconde étude porte sur la formation des vents forts près de la surface et notamment sur ce qu'on appelle les "sting jets". Les "sting jets" sont des jets dont les masses d'air associées descendent rapidement du milieu de la troposphère au niveau de la tête du nuage jusqu'au sommet de la couche limite et peuvent déclencher des vents dévastateurs en surface. Ceux-ci se forment lorsque le front chaud se déplace à l'arrière de la dépression et qu'une fracture frontale apparaît. Les simulations montrent que, lorsqu'une dépression traverse l'axe du courant-jet de son côté chaud à son côté froid, le retour en arrière du front chaud se produit tandis qu'une dépression naissant côté froid du courant-jet possède un front chaud beaucoup moins actif. Ainsi, la formation d'un " sting jet " n'apparaît que dans le premier cas , moyennant une résolution verticale élevée. C'est la présence d'un forçage géostrophique dans un environnement neutre vis-à-vis de l'instabilité symétrique qui facilite la descente des masses d'air et la formation du " sting jet ". / The objective of this thesis is to analyze the influence of moist processes on mid-latitude cyclones, and specifically the tracks of cyclones and the formation of strong winds near the surface. These researches have been made using idealized simulations of the mesoscale model Méso-NH. The first study focused on the effects of moist processes on the motion perpendicularly to the zonal jet axis, poleward. The main mechanism explaining this motion involves the nonlinear advection of the surface cyclone by the upper-level cyclonic and anticyclonic perturbations located on both sides of the surface cyclone. In the presence of humidity, the upper-level anticyclonic anomaly strengthens due to the latent heat release in the ascent zones, and the poleward motion is reinforced. The eastward motion of the cyclone is also reinforced with the inclusion of moist processes. The second study focuses on the formation of near-surface strong winds which are triggered by the so-called " sting-jets ". These jets correspond to descending air masses from the mid-troposphere near the cloud head down to the top of the boundary layer, and can trigger devastating surface winds. In the case where synoptic perturbations are initialized south of the baroclinic zonal jet, the bent-back warm front phase occurs, whereas when they are initialized on the zonal jet axis, the surface cyclone has a less active warm front. Thus, the formation of a sting-jet only occurs in the first case, and with a high vertical resolution. It is the geostrophic forcing in a globally neutral environment relative to conditional symmetric instability which facilitates the airstreams descent and the formation of the sting-jet.

Large-scale atmospheric disturbances play important roles in determining the general circulation of the atmosphere during the North Pacific boreal winter. A number of scientific questions have been raised due to these disturbances’ spatial and temporal complexity as well as the hydrological implication associated with them. In this dissertation, the principal goal is to further improve our understanding of the atmospheric high frequency (HF) and intermediate frequency (IF) disturbances active over the North Pacific. The study focuses on their energetics, intraseasonal and interannual variability, and the resulting hydrological impact over the eastern North Pacific and Western U.S. including extreme events. To delineate the characteristics of HF and IF disturbances in the troposphere, we first derive a new set of equations governing the local eddy kinetic energy (EKE), and assess the critical processes maintaining local budgets of the HF and IF EKE. The diagnosis assesses the 3-D patterns of energy flux convergence (EFC), barotropic conversion (BT), baroclinic conversion (BC), and cross-frequency eddy-eddy interaction (CFEI). The local EKE budget analysis is followed by an investigation of the modulation of HF and IF eddy activity by different modes of low frequency climate variability. On interannual timescales, the response of various local energetic processes to El Niño-Southern Oscillation (ENSO) determines the HF and IF EKE anomalies and the role of CFEI process is important in producing these anomalies. Also on interannual timescales, winter precipitation deficits associated with suppressed cyclonic activity, i.e., negative HF EKE anomalies, are linked to severe droughts over the U.S. Southern Great Plain (SGP) region. The suppressed cyclonic activity is, in turn, tied to phase changes in the West Pacific (WP) teleconnection pattern. On intraseasonal timescales, variations in HF disturbances (a.k.a. storm tracks) over the North Pacific are closely coupled with tropical convection anomalies induced by the Madden-Julian Oscillation (MJO), and partly drive larger scale intraseasonal flow anomalies in this region through eddy-eddy interactions. Anomalous HF eddy activity induces subseasonal transitions between “wet” and “dry” regimes over the west coast of North America. Also on intraseasonal timescales, the East Asian cold surge (EACS) is found to provide a remote forcing of the winter precipitation anomalies in the western U.S. This modulation is achieved through “atmospheric rivers” (ARs), which are narrow channels of concentrated moisture transport in the atmosphere and are responsible for over 70% of the extreme precipitation events in the western U.S.. EACS effectively modulates the IF disturbance activity over the North Pacific, and the anomalous IF disturbances lead to the formation of an AR over the eastern North Pacific that ultimately induces precipitation anomalies in the western U.S. Analyses of the simulations from the NCAR Community Climate System Model version 4 (CCSM4) demonstrate that the connections among the EACS, AR and western U.S. precipitation are better captured by a model with higher spatial resolutions. The improved simulation of these connections is achieved mainly through a better representation of the IF disturbances, and the associated scale-interaction processes in the higher resolution model.