Discriminating between robust and non-robust atmospheric circulation responses to global warming

The robustness of the atmospheric circulation response to global warming in a set of atmospheric general circulation models (AGCMs) is investigated. The global-warmed climate is forced by a global pattern of warmed ocean surface temperatures that is extracted from a multi-model ensemble of coupled ocean-atmosphere climate model greenhouse warming simulations. The robustness of the warming response is evaluated by a hierarchical set of comparisons. The warming response is compared first between two independently developed AGCMs, then as a function of horizontal resolution in one model, and finally as a function of a single tuning parameter, related to orographic gravity wave drag. Across these levels of comparison, the tropical and subtropical response is generally robust in zonal wind and temperature, but the extratropical response is non-robust. On regional scales, almost every aspect of the response is non-robust, even to the variation of a single tuning parameter. Some evidence is presented that the non-robustness of the simulated response to global warming might be predicted from the (non global-warmed) control simulation. Finally, we will show results from a comparison of the global warming responses in a low versus a high top AGCM.