Climate Change in the Mediterranean Region: Changes in Atmospheric Circulations and the Impacts on the Mediterranean Hydroclimate

In this thesis, analyses of the moisture budget in the 5th ECMWF reanalysis (ERA5) and in phase 6 of the Coupled Model Intercomparison Project (CMIP6) historical simulations and future projections are leveraged to explore the maintenance and response to climate change of the hydroclimate in the Mediterranean region. Recent and future projected changes in the Mediterranean, a climate-change hot and dry spot. The transition toward a warmer and drier climate has substantial social and economic implications; hence, it is critical to invest in understanding the Mediterranean hydroclimate change over the coming decades. One of our goals is to complement previous work by further decomposing the mean flow into contributions by the zonal-mean flow, which is dominated by the mean meridional circulation, and by zonally anomalous circulations and/or moisture, namely the stationary eddies. In present-day climate, as depicted by ERA5, annual-mean net evaporation (negative P-E) over the ocean and net precipitation (positive P-E) over land are primarily due to submonthly transient eddies converging moisture originating from the sea into the surrounding land. Overall, total stationary eddies reinforce the transient tendency over the ocean but oppose it over land, with the zonal-mean meridional circulation exerting a minor drying tendency limited to the region's southernmost latitudes. These large-scale features are captured quite well in the ensemble-mean of ten CMIP6 models analyzed in this work. The same CMIP6 model subset is thus used to study the response of the Mediterranean hydroclimate at the end of the 21st century under the SSP5-8.5 scenario. According to the CMIP6 multi-model mean, the climatological annual mean P – E is projected to decrease drastically by the end of the 21st century, both over northern Mediterranean land regions as well as the sea. These changes are not due to the transient eddies, which exert an overall negative but weak tendency, but are driven by changes in the time-mean flow. Consistent with the mean climatological moisture budget, this drying arises from the zonally anomalous circulation term, that is, enhanced zonally anomalous descent and lower-level diverging wind patterns over the Mediterranean region. Our results highlight the importance of circulation changes within the Mediterranean region and their impacts on the hydrological cycle. However, the thermodynamic adjustment is never trivial in our region and is deserving of further investigation. In particular, we are exploring if and to what extent an extended scaling, which is based only on climatological quantities and changes in surface properties, performs better than the simple Clausius-Clapeyron scaling by including thermodynamic changes in advection. Our analyses indeed show that, both in the annual mean and through the seasonal cycle, the extended scaling better captures the full thermodynamic component, which, unlike the simple scaling, predicts a wettening over the ocean. While not fully accounting for the magnitude nor the extent of this wettening, the extended scaling outperforms the simple scaling. %Areas of better agreement also include the nearby land regions, including France and the Iberian Peninsula, where the extended scaling predicts a stronger drying. Throughout the target region, the differences between the two scalings primarily arise from the contribution of the terms involving the gradients of fractional changes in local relative humidity and near-surface temperature changes. Even if largely cancelling, these two terms give rise to a pattern grossly characterized by moistening over the ocean and drying over neighbouring regions. Overall, the results of this thesis highlight how changes in the hydrological cycle in the Mediterranean region result from a complex interplay between different mechanisms, arising from both thermodynamic and dynamical changes. In particular, our results emphasize how the overall drying tendency in the region is primarily due to zonally asymmetric circulation changes rather than by changes in the mean meridional circulation, and is augmented by changes in transient eddies and those arising through the simple thermodynamic wet-get-wetter mechanism, and is partly opposed over the ocean and reinforced over the land regions by thermodynamic changes in advection. By shedding light on all of the involved mechanisms, this work advances our understanding of the factors that make the Mediterranean region a climate-change hot and dry spot.