The Role of Circulation Changes for the Future Mediterranean Precipitation Decline is Dependent on the Season

Future mean precipitation in the Mediterranean is projected to decrease year-round in response to global warming, threatening to aggravate water stress in the region, leading to social and economic difficulties. Yet, the origin of this drying remains debated and various potential causes ranging from circulation to thermodynamic changes have been proposed by different studies. We are aiming for clarification by disentangling possible causes of the Mediterranean drying in regional climate simulations. To test the influence of various large-scale drivers on the drying, we sequentially add them to multiple simulations. We show that the causes of the Mediterranean drying depend on the season. The summer drying results from the land-ocean warming contrast, and from lapse-rate and other thermodynamic changes, but only weakly depends on circulation changes. In contrast, the simulated Mediterranean winter drying occurs in response to changes in the circulation and atmospheric state. Among circulation changes, the role of mean circulation changes is more important than that of high-frequency circulation changes. Since land-ocean contrast, thermodynamic and lapse-rate changes are more robust in climate simulations than circulation changes, the uncertainty associated with the projected drying should be considered smaller in summer than in winter.