The differing dynamical and thermodynamical mechanisms explaining drying of the Mediterranean over past decades in observations and models

Much of the Mediterranean region has experienced declining precipitation over past decades which is broadly consistent with climate model projections. While California, another Mediterranean-climate region (MCR), has not experienced drying, all three of the southern hemisphere climate regions have also been drying. In the Mediterranean and the three southern MCRs changes in atmospheric circulation have driven the drying. In the southern hemisphere the drying trends are related to a year-round, poleward shift and intensification of the eddy-driven jet and storm track consistent with a response to increasing CO₂ and ozone depletion. On this, models and observations agree. However, in the Mediterranean, observed drying is focused on mid-winter and in the central to western Mediterranean and is circulation-driven by a trend towards a more positive Northern Annular Mode (NAM). However, CMIP6 models, as a radiatively-forced multimodel ensemble mean, do not reproduce the observed circulation trends even as they have drying in much of the Mediterranean. Mediterranean winter drying and associated circulation changes are also not well simulated in SST-forced models. The mechanisms for the drying in CMIP6 models are also dynamic but appear different from those observed. We will also examine if the radiatively-forced response varies across large ensembles. This will allow assessment of whether the rough agreement between CMIP6 modeled and observed winter reductions in precipitation in the central to western Mediterranean can be taken as evidence of human-driven drying or is more fortuitous. Alternatively, the trend in the NAM might indeed be forced but with climate models underestimating the magnitude of this climate system response.