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This thesis investigates physical processes in Lake Erie, a large, shallow mid-latitude lake, from two perspectives: climate change impacts on the thermal structure and dissolved oxygen concentration and small-scale eddy dynamics generated by internal Kelvin wave propagation. A three-dimensional hydrodynamic and aquatic ecological coupled model ELCOM-CAEDYM, validated by the field data collected in 2008, is first used to investigate the responses of the thermal structure and dissolved oxygen concentration in Lake Erie to potential changes in air temperature and wind speed. A new method is presented to define spatially and temporally varying regions for the epilimnion, thermocline, and hypolimnion. Four metrics are selected to quantify the characteristics of the thermal structure: mean epilimnion temperature, mean hypolimnion temperature, onset and breakdown of stratification, and thermocline depth. Exploiting the power of the three dimensional model to provide a more authentic characterization of thermal structure in such large lakes, it is shown that patterns inferred from simple isotherm dynamics, as typically done with one dimensional models, are not always accurate. In the dissolved oxygen studies similar analyses are presented. Three factors related to lake hydrodynamics have strong influences on hypolimnetic hypoxia: water temperature, stratification duration, and hypolimnion thickness. The present results show the potential for complicated and interactive effects of climate forcing on important biogeochemical processes in Lake Erie as well as other large mid-latitude lakes. Taking advantage of high performance computing, the generation of eddies when a baroclinic Kelvin wave propagates past a peninsula is studied using the MITgcm. The grid resolution can be refined from 2 km to 200 m in the parallel computing environment. With the finer resolution small-scale processes which cannot be resolved in the coarse resolution applied previously are able to be explored. The eddy dynamics are studied in detail in both an idealized lake and in Lake Erie. This work presents a first attempt at simulating small-scale hydrodynamic processes in large lakes and contributes to our understanding of how energy is moved from large scales (the scale of the basins in Lake Erie) to smaller scales (the scale of the peninsula or Point Pelee).

This thesis investigates physical processes in Lake Erie, a large, shallow mid-latitude lake, from two perspectives: climate change impacts on the thermal structure and dissolved oxygen concentration and small-scale eddy dynamics generated by internal Kelvin wave propagation. A three-dimensional hydrodynamic and aquatic ecological coupled model ELCOM-CAEDYM, validated by the field data collected in 2008, is first used to investigate the responses of the thermal structure and dissolved oxygen concentration in Lake Erie to potential changes in air temperature and wind speed. A new method is presented to define spatially and temporally varying regions for the epilimnion, thermocline, and hypolimnion. Four metrics are selected to quantify the characteristics of the thermal structure: mean epilimnion temperature, mean hypolimnion temperature, onset and breakdown of stratification, and thermocline depth. Exploiting the power of the three dimensional model to provide a more authentic characterization of thermal structure in such large lakes, it is shown that patterns inferred from simple isotherm dynamics, as typically done with one dimensional models, are not always accurate. In the dissolved oxygen studies similar analyses are presented. Three factors related to lake hydrodynamics have strong influences on hypolimnetic hypoxia: water temperature, stratification duration, and hypolimnion thickness. The present results show the potential for complicated and interactive effects of climate forcing on important biogeochemical processes in Lake Erie as well as other large mid-latitude lakes. Taking advantage of high performance computing, the generation of eddies when a baroclinic Kelvin wave propagates past a peninsula is studied using the MITgcm. The grid resolution can be refined from 2 km to 200 m in the parallel computing environment. With the finer resolution small-scale processes which cannot be resolved in the coarse resolution applied previously are able to be explored. The eddy dynamics are studied in detail in both an idealized lake and in Lake Erie. This work presents a first attempt at simulating small-scale hydrodynamic processes in large lakes and contributes to our understanding of how energy is moved from large scales (the scale of the basins in Lake Erie) to smaller scales (the scale of the peninsula or Point Pelee).

The transport of low-salinity waters through the Canadian Arctic Archipelago links the North Pacific, Arctic, and North Atlantic Oceans. This transport is influenced by many related small-scale processes including mixing, internal hydraulics, and internal tide generation. In this thesis, I quantify and elucidate the physics of such processes with aims of addressing discrepancies between observed and simulated fluxes through the Archipelago and advancing the skill of numerical models by identifying shortcomings and informing where and how progress can be achieved. To address the dearth of mixing rates across the network of channels, I first use a large-scale model to obtain baseline estimates of the spatial and seasonal variability of the vertical buoyancy flux. Much of the mixing occurs in the eastern half of the Archipelago and is attributed to the abundance of sills and narrow channels. Indeed, the so-called 'central sills area' is shown to be a mixing hot spot. I investigate this region further using high-spatial-resolution observational transects to examine the role of tides, which are excluded from the large-scale model. The many shallow channels here accelerate tidal currents and thereby induce strong bottom boundary layer dissipation. This is the largest energy sink within an observationally constrained energy budget. The generation of internal tides is another primary sink of barotropic tidal energy. Because the study site lies poleward of the critical latitudes of the dominant tidal constituents, internal tides propagate as internal Kelvin waves. Idealized, process-oriented modelling demonstrates that the amplitudes of such waves, or similarly the energy extracted from the barotropic tide, is sensitive to channel width because waves generated at each side of the channel interfere. Given the multiple connecting channels of the Archipelago, it is difficult to make a priori estimates of internal tide generation for a given channel. Nevertheless, the phenomenology I describe will be detectable in, and a requisite to understanding, pan-Arctic or global three-dimensional tidal models, which are becoming more prevalent. / Graduate

Le phénomène El Niño est le mode dominant de la variabilité du climat aux échelles de temps interannuelles dans le Pacifique tropical. Il modifie considérablement le climat régional dans les pays voisins, dont le Pérou pour lequel les impacts socio-économiques peuvent être dramatiques. Comprendre et prévoir El Niño reste un enjeu prioritaire pour la communauté climatique. Des progrès significatifs dans notre compréhension du phénomène El Niño et dans notre capacité à le prédire ont été réalisés dans les années 80, en particulier grâce à la mise en place du système d'observation dans le Pacifique tropical (programme de TOGA, en particulier, ainsi que l'émergence de l'ère des satellites). À la fin du XXe siècle, alors que de nouvelles théories scientifiques ont été proposées et testées, les progrès réalisés dans le domaine de la modélisation numérique et de l'assimilation de données ont conduit à l'idée que le phénomène El Niño pourrait être prévu avec au moins deux ou trois saisons à l'avance. Or, depuis le début du 21ième siècle, les manifestations du phénomène El Niño ont réduit cette expectative: un nouveau type d'El Niño est a été découvert - identifié par des anomalies de température moins intenses et localisées dans le centre du Pacifique équatorial. Ce phénomène, connu sous le nom CP El Niño pour El Niño Pacifique Central ou El Niño Modoki a placé la communauté scientifique devant un nouveau défi. Cette thèse est une contribution à l'effort international actuel pour comprendre la dynamique de ce nouveau type d'El Niño, dans le but de proposer des mécanismes expliquant sa présence accrue au cours des dernières décennies. Plus précisément, l'objectif de cette thèse est d'étudier le rôle des ondes longues équatoriales dans le Pacifique tropical sur la dynamique océanique et la thermodynamique associées au phénomène El Niño de type Pacifique Central. Cette thèse s'intéresse tout d'abord au premier CP El Niño du 21ième siècle, le phénomène El Niño 2002/03, à partir des sorties d'un modèle de circulation océanique général. Ensuite, nous documentons les caractéristiques des ondes équatoriales de Kelvin aux fréquences Intra Saisonnières (ISKw) sur la période 1990-2011, fournissant une statistique de l'activité des ondes ISKw durant l'évolution des événements El Niño de type Central Pacifique. Nos résultats montrent que l'onde ISKw subit une forte dissipation dans le Pacifique Est, qui est interprétée comme provenant de la dispersion des ondes lorsqu'elles rencontrent le front zonal de la stratification dans l'Est du Pacifique (i.e. la pente de la thermocline d'Ouest en Est). Une réflexion partielle de l'onde ISKw en onde de Rossby équatoriale de près de 120°W est également identifiée, ce qui peut expliquer le confinement dans le Pacifique central des anomalies de température de surface associées aux événements El Niño de type Central Pacifique. Nous suggérons que la fréquence accrue au cours des dernières années des événements CP El Niño peut être associée à l'état froid - de type La Niña - observé dans le Pacifique Equatorial depuis les années 90 et les changements dans la variabilité saisonnière de la profondeur de la thermocline depuis les années 2000. / The El Niño phenomenon is the dominant mode of climate variability at interannual timescales in the tropical Pacific. It modifies drastically the regional climate in surrounding countries, including Peru for which the socio-economical impacts can be dramatic. Understanding and predicting El Niño remains a top-priority issue for the climatic community. Large progress in our understanding of El Niño and in our ability to predict it has been made since the 80s thanks to the improvement of the observing system of the tropical Pacific (TOGA program and emergence of the satellite era). At the end of the Twentieth century, whereas new theories were proposed and tested, progress in numerical modeling and data assimilation led to the idea that El Niño could be predicted with at least 2 or 3 seasons in advance. The observations since the beginning of the 21st century have wiped out such expectation: A new type of El Niño, known as the Central Pacific El Niño (CP El Niño) or Modoki El Niño has put the community in front of a new challenge. This thesis is a contribution to the current international effort to understand the dynamics of this new type of El Niño in order to propose mechanisms explaining its increased occurrence in recent decades. More specifically, the objective of the thesis is to study the role of the oceanic equatorial waves in the dynamic and thermodynamic along the equatorial Pacific Ocean, focusing on the CP El Niño. This thesis first takes a close look at the first CP El Niño of the 21st century of this type, i.e. the 2002/03 El Niño, based on an Oceanic General Circulation Model. Then it documents the characteristics of the IntraSeasonal Kelvin waves (ISKws) over the period 1990-2011, providing a statistics on the ISKws activity during the evolution of CP El Niño events. We find that the ISKw experiences a sharp dissipation in the eastern Pacific that is interpreted as resulting from the scattering of energy associated to the zonal contrast in stratification (i.e. sloping thermocline from west to east). Partial reflection of the ISKw as Rossby waves near 120°W is also identified, which may explain the confinement of CP El Niño warming in the central Pacific. We suggest that the increased occurrence of CP El Niño in recent years may be associated to the La Niña-like state since the 90s and changes in the seasonality of the thermocline since the 2000s.

A 48-layer version of the COMMA-LIM (Cologne Model of the Middle Atmosphere – Leipzig Institute for Meteorology) three-dimensional global mechanistic model of the Earth\'s atmosphere from 0 km to 135 km with logarithmic pressure height coordinates was developed. The model is capable of reproducing the global structures and propagation of different planetary waves in the middle atmosphere. The contribution of gravity waves, tides, Rossby and Kelvin waves into the zonally averaged momentum budget of the mesosphere / lower thermosphere region has been investigated. / Eine neue Version des COMMA-LIM (Cologne Model of the Middle Atmosphere – Leipzig Institute for Meteorology) wurde im Zusammenhang mit der Erhöhung der vertikalen Schichtauflösung entwickelt. Das COMMA ist ein dreidimensionales globales mechanistisches Modell der Erdatmosphäre mit einer Ausdehnung von ca. 0 – 135 km in logarithmischen Druckkoordinaten. Damit können globale Eigenschaften der mittleren Atmosphäre sowie die Ausbreitung verschiedener planetarer Wellen nachvollzogen werden. Die Beiträge der Schwerewellen, thermischer Gezeiten, Rossby und Kelvin Wellen zur zonal gemitteltem Impulsbalance der Mesosphäre und unteren Thermosphäre wurden untersucht.

A 48-layer version of the COMMA-LIM (Cologne Model of the Middle Atmosphere – Leipzig Institute for Meteorology) three-dimensional global mechanistic model of the Earth\''s atmosphere from 0 km to 135 km with logarithmic pressure height coordinates was developed. The model is capable of reproducing the global structures and propagation of different planetary waves in the middle atmosphere. The contribution of gravity waves, tides, Rossby and Kelvin waves into the zonally averaged momentum budget of the mesosphere / lower thermosphere region has been investigated. / Eine neue Version des COMMA-LIM (Cologne Model of the Middle Atmosphere – Leipzig Institute for Meteorology) wurde im Zusammenhang mit der Erhöhung der vertikalen Schichtauflösung entwickelt. Das COMMA ist ein dreidimensionales globales mechanistisches Modell der Erdatmosphäre mit einer Ausdehnung von ca. 0 – 135 km in logarithmischen Druckkoordinaten. Damit können globale Eigenschaften der mittleren Atmosphäre sowie die Ausbreitung verschiedener planetarer Wellen nachvollzogen werden. Die Beiträge der Schwerewellen, thermischer Gezeiten, Rossby und Kelvin Wellen zur zonal gemitteltem Impulsbalance der Mesosphäre und unteren Thermosphäre wurden untersucht.

Dans cette thèse, nous avons abordé plusieurs thématiques liées aux modes de variabilité climatique dans l'Atlantique tropical à l'échelle interannuelle. Les analyses statistiques nous ont permis dans un premier temps de mettre en évidence les deux principaux modes dominants de cette variabilité interannuelle: un mode équatorial et un mode méridien. Le mode équatorial est responsable d'anomalies de température de surface de la mer (SST) principalement dans le Golfe de Guinée et est identifié par des variations de la pente du niveau de la mer dans la bande équatoriale. Il est dû à des rétroactions dynamiques entre le vent, le niveau de la mer et la SST. Quant au mode méridien, il se manifeste par des fluctuations inter-hémisphériques de SST et est contrôlé par des rétroactions dynamiques et thermodynamiques entre le vent, l'évaporation et la SST. L'évaluation du couplage de ces variables clés du mode méridien nous a permis de proposer un modèle conceptuel pour expliquer les principaux mécanismes responsables des oscillations du mode méridien. Le modèle a montré que le mode méridien résulte de la superposition d'un mécanisme auto-entretenu basé sur les rétroactions positives et négatives générant des oscillations régulières de haute fréquence (2-3 ans) et d'un autre mécanisme d'oscillation basse fréquence (4-9 ans) lié à l'influence d'ENSO du Pacifique Est. Comme l'évolution de ces deux modes est fortement liée au déplacement méridien de la zone de convergence intertropicale (ITCZ) qui transporte les pluies, nous avons ensuite identifié la signature de ces modes sur la salinité de la surface de la mer à l'aide observations in situ et d'une simulation numérique régionale. Les processus océaniques et/ou atmosphériques responsables de la signature de chaque mode ont été également identifiés grâce à un bilan de sel dans la couche de mélange du modèle validé. Le bilan de sel a révélé que le forçage atmosphérique, lié à la migration de l'ITCZ, contrôle la région équatoriale tandis que l'advection, due à la modulation des courants, du gradient vertical et le mélange à la base de la couche de mélange, explique les variations de SSS dans les régions sous l'influence des panaches. [...] / In this thesis, we investigate several topics related to the interannual climatic modes in the tropical Atlantic. Statistical analyses allows us to extract the two main dominant modes of interannual variability: an equatorial mode and a meridional mode. The equatorial mode is responsible for Sea Surface Temperature (SST) anomalies mainly found in the Gulf of Guinea and is linked to variations of the sea-level slope in the equatorial band. It is due to dynamic feedbacks between zonal wind, sea level and SST. The meridional mode is characterised by inter-hemispheric SST fluctuations and is controlled by dynamic and thermodynamic feedbacks between the wind, evaporation and SST. After quantifying the coupling between key variables involved in the meridional mode, we develop a conceptual model to explain the main mechanisms responsible for meridional mode oscillations. The model shows that the meridional mode results from the superposition of a self-sustaining mechanism based on positive and negative feedbacks generating regular oscillations of high frequency (2-3 years) and another low frequency oscillation mechanism (4-9 years) related to the influence of ENSO. As the evolution of these two modes is strongly linked to the meridional shift of the Intertropical Convergence Zone (ITCZ) and associated rainfall maximum, we identify the signature of these modes on Sea Surface Salinity (SSS) using in situ observations and a regional numerical simulation. Oceanic and/or atmospheric processes responsible for the signature of each mode are also identified through a mixed-layer salt budget in the validated model. The salt balance reveals that the atmospheric forcing, related to the ITCZ migration, controls the equatorial region while the advection, due to the modulation of current dynamics, the vertical gradient and mixing at the base of the mixed layer, explains SSS variations in regions under the influence of plumes. Finally, we study the Equatorial Kelvin wave characteristics and influences on the density that are involved in the meridional and equatorial mode connection processes, using a very simplified model of gravity wave propagation along the equator. After a brief description of this model, which was initially constructed to study dynamics in the equatorial Pacific, we apply it to the specific case of the equatorial Atlantic by validating its analytical and numerical solutions under adiabatic conditions. [...]