Polar lows dans l'hémisphère Nord : influence de l'environnement de grande échelle, de sa variabilité et de ses modifications avec le changement climatique

Abstract : Polar lows (PLs) are intense meso-cyclones forming in winter at hight latitudes over open water. Despite their low spatial and temporal extension, they are particularly dangerous, being associated with strong winds and heavy snowfall, causing very low visibility and high waves. The purpose of this thesis is to study the impact of climate variability on their outbreak, and in particular to make an assessment of their evolution with climate change. Firsly, large-scale environment leading to development of Pls is determined using all available homogeneous PLs observations. Geopotential height at 500 hPa, the temperature difference between the surface of the ocean and 500 hPa, wind and temperature near the surface, and the potential vorticity (PV) at 300 hPa present significant anomaly patterns over large areas centrerd over PL genesis zones. The focus is done on the Nordic Seas for which there is a wide and homogeneous climatology of Pls, although other regions of the Northern Hemisphere are also studied (Labrador Sea, Gulf of Alaska ...). Pls develop after a certain build-up, the outbreak day been marked by strong wind and PV intensification. Significant disparities between the different formation areas indicate some heterogeneity in the relative influence of formation mechanisms. A second part is devoted to study the influence of climate variability on Pls. Two approaches to this variability are then considered: the teleconnection patterns over the North Pacific and weather regimes over the North Atlantic and Europe. Teleconnection patterns based on the idea that the atmospheric variability can be described by variations in the intensity of specific large-scale, persistent and recurrent patterns. For each North Pacific area, the study of the relationship between these patterns and key variables for Pls formation highlight respective influences of each teleconnection patterns on Pls development. The idea behind the concept of weather regimes is that the atmospheric circulation variability can be described as an alternation between different preferential quasi-stationary circulation states. Regimes have a typical duration of 8-10 days, similar to the existence of a typical environment associated with the development of Pls. They appear particularly discriminating for PL outbreaks: over the Norwegian and Barents seas, 75% Pls observed form in Altantic Ridge or in the negative phase of the North Atlantic Oscillation (NAO), while in the Labrador Sea most observed Pls develop during the positive phase of the NAO. The last part is an exploratory study of PL behavior evolution with climate change. Climate change may affect the distribution and frequency of occurrence of Pls by changing the conditions of large-scale monitoring their genesis. The study focuses primarily on the role of sea ice and its evolution on Pls. Finally, the evolution of key variables in PL formations be studied from the ECHAM5/MPI-OM climate simulation outputs. The results of this study indicate an overall decrease in the activity of Pls with strong regional disparities, as well as changes in the seasons. The results of this thesis reinforce knowledge of PLs and provide new elements to improve their forecasting in the short and longer term.
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Paul-Etienne Mallet. Polar lows dans l'hémisphère Nord : influence de l'environnement de grande échelle, de sa variabilité et de ses modifications avec le changement climatique. Climatologie. Ecole Polytechnique X, 2013. Français. ⟨pastel-00932363⟩

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