Disentangling projected stationary wave changes: implications for future drying of the Mediterranean region

An intermediate-complexity moist general circulation model is used to disentangle changes in the large-scale zonally asymmetric circulation due to rising GHGs. We run multiple idealized experiments in order to isolate, and subsequently synthesize, the physical processes driving these changes. In particular, we examine stationary wave changes forced by land–sea contrast, horizontal heat fluxes in the ocean, and orography, in response to a quadrupling of CO2 concentrations. A particular focus is on the anomalous ridge in the Mediterranean region associated with the decline in precipitation in this heavily populated region. 

 Our results suggest a combination of two mechanisms is responsible for future Mediterranean drying. The first is a global phenomena, a lengthening of intermediate-scale stationary waves due to strengthening of subtropical upper-tropospheric zonal mean zonal winds, shown previously to account for hydroclimatic changes in the western US. We find this mechanism to be dominated by change in waves forced by ocean horizontal heat fluxes. The second mechanism is a regional one, a strengthening of large-scale stationary wave modes over Europe and the north Atlantic, dominated by changes in stationary waves forced by land-sea contrast. This second mechanism is strongly tied to an altered temperature gradient between the North Atlantic and Europe, in response to rising GHGs. Our work demonstrates how large-scale upper-tropospheric circulation changes are directly tied to regional hydroclimate.