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sur le développement des zones arides et semi-arides

Titre : Mechanisms of summertime Mediterranean drying with warming in CMIP6 models

Auteur : Lanteri, Paolo

Université de soutenance : Universität Innsbruck

Grade  : Master of Science in Environmental Meteorology 2022

Résumé
Projected climate changes threaten to modify patterns of precipitation and fresh water availability worldwide, affecting human societies possibly even more than changes in temperature, with the most dramatic effects on the most vulnerable societies. Therefore, being able to predict such changes is one of the major challenges of climate science. It is expected that climate change will alter not only the frequency and intensity of extreme events of precipitation and droughts in many regions of the world, but also the spatial distribution of cumulative seasonal precipitation. A warmer atmosphere can retain more water vapour, according to the Clausius Clapeyron scaling, at a rate of ∼ +7% for each degree of warming. This simple scaling has large effects on thermodynamic and dynamic mechanisms that govern precipitations. This thesis aims to study mechanisms of regional precipitation changes in the Mediterranean and European region, using the ensemble projection of 20 CMIP6 climate models. Applying a decomposition of the predicted mean water vapour fluxes, we attribute changes in precipitation to changes in temperature, relative humidity and circulation. The SSP5-8.5 CMIP6 ensemble predicts a strong summer drying in the region, with widespread reduction of precipitation (−24% on average over land) and net precipitation. We find that models that predict the largest degree of warming are also the ones that predicts the most severe drying. The thermodynamic strengthening of water vapour fluxes turns out to have a major role in net precipitation decline, but a strong contribution also comes from the predicted poleward shift of the circulation patterns. Poleward shift of mean meridional circulation over the Atlantic sector confirms theoretical expectations of the poleward expansion of the storm track. An analysis on European summer precipitation using ERA5 reanalysis identifies a strong relation between the AMO phases, the influence of the Azores high over Western Europe and precipitation patterns. The competing roles of natural variability and increasing greenhouse gases (GHGs) in driving last-decade changes in precipitation remains a challenging question.

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