Spread of model climate sensitivity linked to double-Intertropical Convergence Zone bias

摘要

Despite decades of climate research and model development, two outstanding problems still plague the latest global climate models (GCMs): the double-Intertropical Convergence Zone (ITCZ) bias and the 2-5 degrees C spread of equilibrium climate sensitivity (ECS). Here we show that the double-ITCZ bias and ECS in 44 GCMs from Coupled Model Intercomparison Project Phases 3/5 are negatively correlated. The models with weak (strong) double-ITCZ biases have high (low)-ECS values of similar to 4.1(2.2)degrees C. This indicates that the double-ITCZ bias is a new emergent constraint for ECS based on which ECS might be in the higher end of its range (similar to 4.0 degrees C) and most models might have underestimated ECS. In addition, we argue that the double-ITCZ bias can physically affect both cloud and water vapor feedbacks (thus ECS) and is a more easily measured emergent constraint for ECS than previous ones. It can be used as a performance metric for evaluating and comparing different GCMs.