Sensitivity of projected storm track and jet latitude changes to the parameterization of convection: implications for mechanisms of the future poleward shift

While a poleward shift of the jet stream 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 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 mechanisms that have been proposed to lead to the poleward shift are present in all simulations (even those with no poleward shift), and hence we can conclude that these mechanisms are not of leading-order significance for the poleward shift in any of the simulations. In contrast, the thermodynamic budget is able to diagnose the reason the jet and storm track shift differs among the simulations, and helps identify midlatitude latent heat release as the 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.