The Role of Southern Ocean Mean State Bias in Climate Sensitivity

Climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulate a considerable range of mean-state climates differing from the observations. In the meantime, there is an inter-model spread of the historical (1925-2014) global warming rate, leading to the large uncertainties of climate sensitivity based on model estimates. In this study, we analyze large-ensemble CMIP6 models to detect and investigate the inter-model connection between the simulated mean state and warming rate. A robust negative correlation has been identified between the Southern Ocean mean-state sea surface temperature (SST) and the global mean SST warming during the historical period. In other words, a model with a warmer Southern Ocean climatological SST tends to simulate a weaker global SST warming. Our preliminary analysis suggests that this negative correlation can potentially be explained by cloud radiative feedbacks and atmospheric feedbacks linked to surface turbulent heat flux. Our results imply an emergent constraint of Southern Ocean mean state on the climate sensitivity.