Assessment of Large-Scale Indices of Surface Temperature During the Historical Period in the CMIP6 Ensemble


We develop a statistical method to assess CMIP6 simulations of large-scale surface temperature change during the historical period (1850-2014), considering all timescales, allowing for the different unforced variability of each model and the observations, observational uncertainty, and applicable to ensembles of any size. The generality of this method, and the fact that it incorporates information about the unforced variability, makes it a useful model assessment tool. We apply this method to the historical simulations of the CMIP6 multi-model ensemble. We use three indices which measure different aspects of large-scale surface-air temperature change: global-mean, hemispheric gradient, and a recently-developed index that captures the sea-surface temperature (SST) pattern in the tropics (SST#; Fueglistaler and Silvers, 2021). We use the following observations: HadCRUT5 for the first two indices, and AMIPII and ERSSTv5 for SST#. In each case, we test the hypothesis that the model's forced response is compatible with the observations, accounting for unforced variability in both models and observations as well as measurement uncertainty. This hypothesis is accepted more often (75% of the models) for the hemispheric gradient than for the global mean, for which half of the models fail the test. The tropical SST pattern is poorly simulated in all models. Given that the tropical SST pattern can strongly modulate the relationship between energy imbalance and global-mean surface temperature anomalies on annual to decadal time scales (short-term feedback parameter), we suggest this should be a focus area for future improvements due to its potential implications for the global-mean temperature evolution in decadal time scales.