Revisiting the Historical Wintertime Drying of the Mediterranean in the LESFMIP Simulations

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. 
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 and hence cancelled out much of the GHG induced signal. The net effect when all forcings are present is a weak drying, and this weak drying and subtle ridging is consistent with recent observational evidence that the anthropogenically forced wintertime drying in the Mediterranean region has not yet robustly emerged. There is pronounced intermodel spread in both the sea level pressure and precipitation responses to both GHGs and aerosols, however, and the relation between this spread and the spread in 9 different climatic metrics is explored to help clarify dynamical mechanisms and the causes of intermodel spread. 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. Overall, the results of this work highlight that aerosols have influenced Mediterranean climate in the historical climate and helped mitigate the greenhouse gas induced drying.