Large-scale circulation and precipitation decline in the Mediterranean

Though Global Climate Models have long projected a robust precipitation decrease in the Mediterranean region, mechanistic explanations for this decline are still unclear. Two mechanisms have been put forward to account for the projections: a “dry-get-drier” scenario, based on the amplification of current moisture divergence, and a poleward shift of the Hadley circulation and mid-latitude storm tracks. Here we discuss these mechanisms, and take a regional approach to provide a mechanistic understating of precipitation change across the region, emphasizing local changes in simulated atmospheric circulation that act on the strong, existing gradients of temperature and humidity. Initial results based on the CMIP5 archive data suggest that, while the Eastern Mediterranean precipitation decline may be remotely forced through a substantial subsidence signal, the drying over the western end of the basin could be a consequence of local changes in the circulation that advect hot and dry air from the Sahara to Northwest Africa.