Climate model simulations of winter northern hemisphere atmospheric blocking : statistical assessment, dynamical perspective, regional impacts and future change

Characterized by its persistence and quasi-stationary features, atmospheric blocking is often responsible for extreme weather events, which in turn can have enormous impacts on human life, economy and environment. Despite considerable effort made in the comprehension and numerical simulation of this phenomenon, some dynamical aspects of blocking are still not well understood and its poor representation in climate models is a long-standing problem, which has important implications for climate change projections. The basis for this thesis lies in improving understanding of mid-latitude winter-time atmospheric blocking over the two preferred regions of blocking development of the Northern Hemisphere (the Euro-Atlantic and North Pacific sectors), through a multi-faceted study, which includes research on phenomena such as stratospheric sudden warmings and extreme temperature and precipitation events. First, the ability of six European coupled Atmosphere-Ocean General Circulation Models to simulate atmospheric blocking is evaluated, considering various sources of errors and uncertainties. Although model performance can be affected by the choice of the blocking detection method, errors in representing the frequency, duration and geographical location of blocks are mostly inherent to the models. These errors are primarily linked with the time-mean atmospheric circulation bias (too strong average westerlies and zonality of the flow) and, to a lesser extent, with the high-frequency activity of the transient eddies in the areas affected by blocking. The blocking responses to errors in the low-frequency variability are substantial, but differ according to the region and model considered. The second aspect of this thesis investigates potential dynamical mechanisms linking blocking lifetime and variability with stratospheric sudden warmings in the IPSL-LMDZ fifth-generation multicentury climate model simulation. The results show a robust connection between the occurrence of stratospheric sudden warmings and blocking in both the Euro-Atlantic and Pacific regions, up to 40 days before and after the onset date of the warmings. The results are explained through the evolution of the largeand small-scale planetary wave disturbances during the weakening and breakdown of the stratospheric polar vortex. Finally, the influence of European blocking on extreme weather precipitation and temperature events over Europe is examined from the observation/re-analysis dataset and in the present-day and future climates of the IPSL-LMDZ4 variable-grid Atmosphere-only General Circulation Model. The observations reveal that warm and wet extreme weather conditions are four times more frequent (cold and dry five times less frequent) over northern Europe, while over central and southern Europe cold and dry extremes are twice as frequent (warm and wet five times less frequent) during European blocking than during average conditions. The model is able to reproduce these overall relationships reasonable well, except that it overestimates the blocking impact by up to twice the observed values. In the future climate, blocking episodes bring less extreme conditions over northern Europe, while over central Europe blocking-driven weather events are projected to shift toward warmer and wetter extremes. The robustness of the relationships between blocking and extreme events to changes in horizontal model resolution in the core of the blocking region (where the wind is the weakest) implies that the mechanisms responsible for the occurrence of extremes under the action of the blocking high are weakly sensitive to the horizontal resolution of the IPSL-LMDZ4 model.