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Warming induced changes in Arctic vegetation have to date been studied through observational and experimental field studies, leaving significant uncertainty about the representativeness of selected field sites as well as how these field scale findings scale up to the entire pan-Arctic. The purposes of this thesis were therefore to 1) analyze remotely-sensed/modeled temperature, Normalized Difference Vegeta- tion Indices (NDVI) and plant Net Primary Productivity (NPP) to assess coarse- scale changes (1982–2006) in vegetation; and 2) compare field, remote sensing and model outputs to estimate limitations, challenges and disagreements between data formats. The following data sources were used: • Advanced Very High Resolution Radiometer Polar Pathfinder Extended (APP- x, temperature & albedo) • Moderate Resolution Imaging Spectroradiometer (MODIS, Normalized Dif- ference Vegetation Index (NDVI) & Enhanced Vegetation Index (EVI) ) • Landsat Enhanced Thematic Mapper (Landsat ETM, NDVI) • Global Inventory Modeling and Mapping Studies (GIMMS, NDVI) • Global Productivity Efficiency Model (GloPEM, Net Primary Productivity (NPP)) Over the pan-Arctic (1982-2007), increases in temperature, total annual NPP and maximum annual NDVI were observed. Increases in NDVI and NPP were found to be closely related to increases in temperature according to non-parametric Sen’ slope and Mann Kendall tau tests. Variations in phenology were largely non- significant but related to increases in growing season temperature. Snow melt onset and spring onset correspond closely. MODIS, Landsat and GIMMS NDVI data sets agree well, and MODIS EVI and NDVI are very similar for spring and summer at Fosheim Peninsula. GloPEM NPP and field estimates of NPP are poorly correlated, whereas GIMMS NDVI and GloPEM NPP are well correlated, indicating a need for better calibration of model NPP to field data. In summary, increases in pan-Arctic biological productivity indicators were ob- served, and were found to be closely related to recent circumpolar warming. How- ever, these changes appear to be focused in regions from which recent field studies have found significant ecological changes (Alaska), and coarse resolution remote sensing estimates of ecological changes have been less marked in other regions. Dis- crepancies between results from model, field data and remote sensing, as well as central questions remaining about the impact of increases in productivity on soil- vegetation-atmosphere feedbacks, indicate a clear need for continued research into warming induced changes in pan-Arctic vegetation.

Warming induced changes in Arctic vegetation have to date been studied through observational and experimental field studies, leaving significant uncertainty about the representativeness of selected field sites as well as how these field scale findings scale up to the entire pan-Arctic. The purposes of this thesis were therefore to 1) analyze remotely-sensed/modeled temperature, Normalized Difference Vegeta- tion Indices (NDVI) and plant Net Primary Productivity (NPP) to assess coarse- scale changes (1982–2006) in vegetation; and 2) compare field, remote sensing and model outputs to estimate limitations, challenges and disagreements between data formats. The following data sources were used: • Advanced Very High Resolution Radiometer Polar Pathfinder Extended (APP- x, temperature & albedo) • Moderate Resolution Imaging Spectroradiometer (MODIS, Normalized Dif- ference Vegetation Index (NDVI) & Enhanced Vegetation Index (EVI) ) • Landsat Enhanced Thematic Mapper (Landsat ETM, NDVI) • Global Inventory Modeling and Mapping Studies (GIMMS, NDVI) • Global Productivity Efficiency Model (GloPEM, Net Primary Productivity (NPP)) Over the pan-Arctic (1982-2007), increases in temperature, total annual NPP and maximum annual NDVI were observed. Increases in NDVI and NPP were found to be closely related to increases in temperature according to non-parametric Sen’ slope and Mann Kendall tau tests. Variations in phenology were largely non- significant but related to increases in growing season temperature. Snow melt onset and spring onset correspond closely. MODIS, Landsat and GIMMS NDVI data sets agree well, and MODIS EVI and NDVI are very similar for spring and summer at Fosheim Peninsula. GloPEM NPP and field estimates of NPP are poorly correlated, whereas GIMMS NDVI and GloPEM NPP are well correlated, indicating a need for better calibration of model NPP to field data. In summary, increases in pan-Arctic biological productivity indicators were ob- served, and were found to be closely related to recent circumpolar warming. How- ever, these changes appear to be focused in regions from which recent field studies have found significant ecological changes (Alaska), and coarse resolution remote sensing estimates of ecological changes have been less marked in other regions. Dis- crepancies between results from model, field data and remote sensing, as well as central questions remaining about the impact of increases in productivity on soil- vegetation-atmosphere feedbacks, indicate a clear need for continued research into warming induced changes in pan-Arctic vegetation.

Arctic warming is assumed to be four times the global warming. A published study by Ivanov et al. (2019) shows that the annual average temperature of Franz Joseph Land (the world’s northernmost island region, a Russian territory) has increased by 5.2 °C from 2000-2017. This result supported the idea of determining whether Svalbard (Norwegian territory) is experiencing similar warming. Svalbard has historically been an attractive research center for examining climate change in the Arctic. Due to easier accessibility, the vast majority of weather stations have been located on the western part of the main island, Spitsbergen, which does not provide a representative picture of the entire archipelago. Therefore, this project has focused on eastern Spitsbergen. Data from six stations have been processed to analyze the temperature changes based on linear regression (the same method as at Franz Joseph Land). As eastern Spitsbergen has never been a priority, only short datasets are available, with the longest one dating from 2009. Because of this, no statistically significant result could be elucidated. Instead, data from Longyearbyen, which is located southwest were implemented, allowing analysis over the same period as Franz Joseph Land (2000-2017). This result suggested a temperature increase of 5.6 °C for the same period, with a statistical significance of P = 0.13, as well as that the winters are extra vulnerable to warming. The stations from eastern Spitsbergen’s local variability were also examined, which showed that the local climate varies although the stations are relatively close. Among others, Pyramiden seemed to be most affected by the lapse rate feedback, meaning a significant strong warming at the surface.

Identifying the drivers of Arctic climate variability is essential for understanding the recent rapid changes in local climate and determining the mechanisms that cause them. Remote tropical sea surface temperatures (SST) have been identified in previous studies as having contributed to the recent positive trends in surface temperature and geopotential height at 200 hPa over north Canada and Greenland (1979-2012) through poleward propagating Rossby waves. However, the source and direction of wave propagation on to north Canada and Greenland (NCG) differs across climate datasets indicating that there are still uncertainties surrounding the mechanisms for how the tropics influence the NCG climate. This thesis aims to further investigate the robustness of the trends over NCG and understand how circulation in this region responds to imposed tropical SST perturbations. The eddy 200 hPa geopotential height (Z200) trends over NCG are assessed in a number of different datasets and compared to the response of eddy Z200 over NCG to imposed tropical SST perturbations in a number of sensitivity studies using the HadGEM3 atmosphere-only model. These model experiments are forced with observed differences in SSTs from the beginning and end of the satellite record (1979-1988 and 2003-2012), with spatial perturbations for [i] the entire tropics, [ii] global SSTs, [iii] the tropical Pacific only, [iv] the tropical Atlantic SST only, [v] the tropical Indian Ocean only. The positive spatial trends of eddy Z200 over NCG from ERA-Interim reanalysis is largely captured in ensemble means of two available climate datasets, UPSCALE and AMIP, indicating that this is a robust climate pattern, however, these trends appear to be stronger in the latter part of the record specifically over the UPSCALE period (1985 to 2011). The model sensitivity studies show that a negative eddy Z200 anomaly over NCG was found in response to all imposed tropical SST perturbations (2003-2012) relative to a background state (1979-1988). This was due a stationary trough over the region that was able to intensify in response to a lack of a strong anomalous wave forcing from changes in mid-tropospheric temperature and zonal winds. The forcing from the tropical Atlantic, relative to the other tropical ocean basins, resulted in the largest eddy Z200 response over NCG, indicating its dominance in forcing the large scale tropical signal. The influence of extratropical SST perturbations relative to tropical SST perturbations were also investigated and it was demonstrated that this negative anomaly is largely driven by the change in tropical sea surface temperatures.

L'étude de la variabilité des cyclones extra-tropicaux (ETC) est non seulement un sujet d'intérêt pour la communauté scientifique mais aussi d'une grande importance en raison de ses impacts socio-économiques. Toutefois, l'étude continué de la variabilité des ETC et de leurs impacts est encore rare, en particulier a l'échelle de temps du Xeme siècle. Cette thèse vise a étudier la variabilité des trajectoires de tempêtes et de leurs dégâts associés du début du Xxeme siècle a 2010. Pour ce faire, le travail est divisé en deux sections principales, l'une dédiée a la climatologie des ETCs au cours du siècle dernier a partir de données de réanalyse, et la seconde centrée sur le calcul d'indices de pertes et l'évaluation des risques induits par les tempêtes. On s'intéresse en premier lieu a l'étude de la variabilité des ETCs par l'application d'un algorithme de suivi de cyclone, sur la réanalyse de long terme du Centre Européen (ECMWF) ERA-20C. Le nombre annuel d'ETC modérées a intenses fait ressortir trois périodes historiques distinctes. Deux périodes, l'une au début et la seconde à la fin du Xeme siècle (1900-1935 et 1980-2010) ne présentent aucune tendance tandis qu'au milieu du siècle (1930-1980) une tendance significative à l'augmentation apparait. Cette dernière peut toutefois être interrogée en raison de l'inhomogénéité temporelle des réanalyses de long terme. Pour cette raison, un ensemble de paramètres physiques sont analysés en vue d'interpréter physiquement les trois périodes. Durant la période 1930-1980, un refroidissement général de l'atmosphère est observé, en particulier aux hautes latitudes, qui augmente le gradient méridien de température et en conséquence la baroclinicité et la conversion barocline. Par ailleurs, cette augmentation de la fréquence d'ETC est observée spécifiquement sur le Pacifique (Atlantique) au cours de la première (seconde) moitié de la période en lien avec une inversion de l'indice Oscillation Décennale du Pacifique (Oscillation Multidecennale Atlantique). La seconde partie de la thèse s'intéresse à l'analyse des tempête scausant les plus forts dégâts du Xeme siècle. Tout d'abord, on calcule un champ d'indices de dégâts de vents forts pour plus de vingt pays. On développe ensuite une Méthode de Suivi de Tempêtes de Forts Dégâts et les résultats de l'algorithme de suivi sont combinés avec les indices de dégâts de vents forts pour chaque pays. [...] / Extratropical cyclones (ETCs) variability is not only a subject that raises interest among the scientific community, but also extremely important in terms of social-economical impacts. Nevertheless, the study of both the extratropical cyclones variability and windstorms impacts is still scarce, particularly at time-scales that cover the twentieth century. This thesis aims to study, both storms track variability and associated losses from the beginning of the 20th century until 2010. In order to do so, the work was separated in two main parts, one witch focus on ETCs climatology during the last century using reanalysis data and another focused on loss indexes calculations and risk assessment of windstorms. The first part of this PhD concerns the study of ETCs variability after applying a tracking algorithm on the long-term ECMWF reanalysis ERA-20C. The number of ETCs per year shows three distinct periods for the moderate and deep cyclones. Two periods, one at the beginning and another at end of the century (1900-1935 and 1980-2010) for which no significant e trends are observed and a middle-century period between 1935-1980 which presents a significant positive trend. This last trend, however, a deeper analysis on this period should be done due to time-inhomogeneity of long-term reanalysis datasets. For this reason, a set of physical parameters are analysed and a physical interpretation made for each one of the periods. During the middle period, a general cooling of the atmosphere is observed, particularly at high-latitudes, which increases the meridional gradients of temperature and consequently baroclinicity and baroclinic conversion. Besides that, this increase is also observed more specifically in the Pacific (Atlantic) in the first (second) half of this period and linked with a Pacific Decadal Oscillation (Atlantic Multidecadal Oscillation) change in signs. On the opposite, the first and third periods are related with warmer polar temperatures that are more intense in the third period but never reach the upper levels of the troposphere. This creates differential changes in baroclinicity. On the one hand, baroclinicity decreases at lower levels and, on the other hand increases at upper levels. The second part of this thesis is focused on the analysis of the most damaging windstorms of the century. First, Loss and Meteorological indexes Pinto et al 2012 are computed for more than twenty countries. Then, a High-Loss Tracking Method is developed and the tracking algorithm trajectories are matched with the LI and MI information for each country. [...]