Is climate sensitivity related to dynamical sensitivity?

Abstract

AbstractThe atmospheric response to increasing CO2 concentrations is often described in terms of the equilibrium climate sensitivity (ECS). Yet the response to CO2 forcing in global climate models is not limited to an increase in global‐mean surface temperature: for example, the midlatitude jets shift poleward, the Hadley circulation expands, and the subtropical dry zones are altered. These changes, which are referred to here as “dynamical sensitivity,” may be more important in practice than the global‐mean surface temperature. This study examines to what degree the intermodel spread in the dynamical sensitivity of 23 Coupled Model Intercomparison Project phase 5 (CMIP5) models is captured by ECS. In the Southern Hemisphere, intermodel differences in the value of ECS explain ~60% of the intermodel variance in the annual‐mean Hadley cell expansion but just ~20% of the variance in the annual‐mean midlatitude jet response. In the Northern Hemisphere, models with larger values of ECS significantly expand the Hadley circulation more during winter months but contract the Hadley circulation more during summer months. Intermodel differences in ECS provide little significant information about the behavior of the Northern Hemisphere subtropical dry zones or midlatitude jets. The components of dynamical sensitivity correlated with ECS appear to be driven largely by increasing sea surface temperatures, whereas the components of dynamical sensitivity independent of ECS are related in part to changes in surface temperature gradients. These results suggest that efforts to narrow the spread in dynamical sensitivity across global climate models must also consider factors that are independent of global‐mean surface temperature.