Robust and non-robust atmospheric circulation responses to global warming (Formerly Paper JP3.3)

The robustness of the atmospheric circulation response to global warming in several different atmospheric GCMs (AGCMs) is investigated. Each AGCM is forced with a prescribed sea-surface temperature (SST) perturbation that is independent of the AGCM. This SST perturbation represents the ensemble-mean response of several different coupled atmosphere-ocean GCMs to transient greenhouse-gas increases, at the time of CO2 doubling. The robustness of the AGCM response to this perturbation as horizontal resolution is changed is also investigated.

The sensitivity to horizontal resolution, the sensitivity to time-sampling and the spread among models are quantified. Regarding the sensitivity to resolution, it is found that in regions where a quantity is non-robust (i.e., highly dependent on resolution) in the control simulation, the response to climate change also tends to be non-robust. For example, in the NH high latitude stratosphere in one AGCM, the boreal winter zonal wind is a non-monotonic function of resolution in the control simulation. In this region, the zonal wind response to climate change is also non-robust: the response switches sign as the resolution is increased. This analysis does not exclude other cases of non-robustness, for example, where a feature is insensitive to resolution in the control simulation but is sensitive to resolution in the warm simulation. But the rule of thumb appears to be that non-robustness in the control simulation is a good a priori predictor for non-robustness in the response to climate change.