Yue Dong, Kyle C. Armour, Mark D Zelinka, Cristian Proistosescu, David S. Battisti, Chen Zhou, Timothy Andrews

Radiative feedbacks depend on the spatial patterns of sea-surface temperature (SST) and thus can change over time as SST patterns evolve – the so-called ‘pattern effect’. This study investigates inter-model differences in the magnitude of the pattern effect and how these differences contribute to the spread in the effective equilibrium climate sensitivity (ECS) within CMIP5 and CMIP6 models. Effective ECS from abrupt4xCO2 simulations is on average 10% higher than that inferred from historical energy budget in CMIP5, this difference is reduced to 7% on average in CMIP6, but still with a wide range across models. The (negative) net radiative feedback weakens over the course of the abrupt4xCO2 simulations in the vast majority of CMIP5 and CMIP6 models, but this weakening is less dramatic on average in CMIP6. For both ensembles, the total variance in the effective ECS is found to be dominated by the spread in radiative response on fast timescales, rather than the spread in feedback evolution over time. Using Green’s functions derived from two AGCMs shows that the spread in feedbacks on fast timescales may be primarily determined by atmospheric model physics, whereas the spread in feedback evolution towards longer timescales is primarily governed by evolving SST patterns. Inter-model spread in feedback evolution is well explained by differences in the relative warming in the West Pacific warm-pool regions for the CMIP5 models, but this relation fails to explain differences across the CMIP6 models, suggesting that stronger sensitivity of extratropical clouds to surface warming may also contribute to feedback changes in CMIP6 models.