Revisiting the Historical Drying of the Mediterranean in the LESFMIP Simulations

Abstract. Simulations from the Large Ensemble Single Forcing Model Intercomparison Project are used to isolate the impact of greenhouse gases (GHGs) and anthropogenic aerosols for historical (1850–2014) wintertime drying in the Mediterranean region, and to clarify the importance of different dynamical mechanisms for the intermodel spread. Increasing GHGs have already led to a clear ridging signal across the Mediterranean and a precipitation reduction of up to 15 %. Anthropogenic aerosols, on the other hand, led to Mediterranean troughing in most models. There is pronounced intermodel and intramodel spread in both the sea level pressure and precipitation responses however, and the relation between this spread and the spread in 9 different climatic metrics is explored to help clarify dynamical mechanisms. A stronger tendency towards Mediterranean ridging is found in models and ensemble members with a more pronounced North Atlantic warming hole, a stronger stratospheric polar vortex, and to a lesser degree with a larger poleward shift of the eddy-driven jet. While these three sensitivities are as expected, others are not. Namely, a larger increase of global mean temperature is associated with troughing over the Mediterranean, opposite to naive expectations. Moreover, the single-forcing experiments indicate that a warmer land relative to the ocean (over the Mediterranean) is associated with troughing, rather than the previously proposed ridging. Other sensitivities are weak: the spread in the historical response cannot be explained by spread in shifts of the Hadley cell edge or the zonal-mean subtropical jet. Many of these sensitivities differ, however, when considering each model’s large ensemble as compared to treating all available members as a single large ensemble. This result highlights the importance of multi-model ensembles over single-model ensembles in revealing relations between dynamical and climate sensitivities. Overall, the results of this work highlight that aerosols had a detectable influence on Mediterranean climate in the historical climate, implying that removal of these aerosols will have an impact in future decades.