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Abstract An intermediate-complexity general circulation model is used to disentangle changes in the large-scale zonally asymmetric circulation in response to rising greenhouse gases. Particular focus is on the anomalous ridge that develops over the Mediterranean in future climate projections, directly associated with reduced winter precipitation over the region. Specifically, we examine changes in stationary waves forced by land–sea contrast, horizontal oceanic heat fluxes, and orography, following a quadrupling of CO₂. The stationary waves associated with these three drivers depend strongly on the climatological state, precluding a linear decomposition of their responses to warming. However, our modeling framework still allows a process-oriented approach to quantify the key drivers and mechanisms of the response. A combination of three similarly important mechanisms is found responsible for the rain-suppressing ridge. The first is part of a global response to warming: elongation of intermediate-scale stationary waves in response to strengthened subtropical winds aloft, previously found to account for hydroclimatic changes in southwestern North America. The second is regional: a downstream response to the North Atlantic warming hole and enhanced warming of the Eurasian landmass relative to the Atlantic Ocean. A third contribution to the Mediterranean Ridge is a phase shift of planetary wave 3, primarily associated with an altered circulation response to orographic forcing. Reduced land–sea contrast in the Mediterranean basin, previously thought to contribute substantially to Mediterranean drying, has a negligible effect in our integrations. This work offers a mechanistic analysis of the large-scale processes governing projected Mediterranean drying, lending increased understanding and credibility to climate model projections. 

Abstract While a poleward shift of the near-surface jet and storm track in response to increased greenhouse gases appears to be robust, the magnitude of this change is uncertain and differs across models, and the mechanisms for this change are poorly constrained. An intermediate complexity GCM is used in this study to explore the factors governing the magnitude of the poleward shift and the mechanisms involved. The degree to which parameterized subgrid-scale convection is inhibited has a leading-order effect on the poleward shift, with a simulation with more convection (and less large-scale precipitation) simulating a significantly weaker shift, and eventually no shift at all if convection is strongly preferred over large-scale precipitation. Many of the physical processes proposed to drive the poleward shift are equally active in all simulations (even those with no poleward shift). Hence, we can conclude that these mechanisms are not of leading-order significance for the poleward shift in any of the simulations. The thermodynamic budget, however, provides useful insight into differences in the jet and storm track response among the simulations. It helps identify midlatitude moisture and latent heat release as a crucial differentiator. These results have implications for intermodel spread in the jet, hydrological cycle, and storm track response to increased greenhouse gases in intermodel comparison projects.