The Southern Ocean Radiative Bias, Cloud Compensating Errors and
Equilibrium Climate Sensitivity in CMIP6 Models
Abstract
Analysis of Coupled Model Intercomparison Project Phase 6 (CMIP6) models
has focused on their higher equilibrium climate sensitivity (ECS)
relative to CMIP5 simulations, with higher ECS values attributed to
changing cloud feedbacks. We examine the simulation of the shortwave
cloud radiative effect (SW CRE) in CMIP6 and explore a potential link
between SW CRE errors and ECS. We derive mean and compensating errors in
model data relative to satellite observations using a cloud clustering
methodology. A statistically significant negative relationship between
the mean and compensating errors in SW CRE over the Southern Ocean is
identified. This relationship is observed elsewhere, but is only
significant over the Southern Ocean. This implies model tuning efforts
potentially hide biases in the representation of clouds in this region.
High ECS models tend to have lower mean and compensating errors over the
Southern Ocean, suggesting model biases are unlikely to explain the high
ECS values.