Tracking changes in climate sensitivity in CNRM climate models

The equilibrium climate sensitivity (ECS) in the latest version of CNRM climate model, CNRM-CM6-1, and in its high-resolution counterpart, CNRM-CM6-1-HR, is significantly larger than in the previous version (CNRM-CM5.1). The traceability of this climate sensitivity change is investigated using coupled ocean-atmosphere model climate change simulations. These simulations show that the increase in ECS is the result of changes in the atmospheric component. A particular attention is paid to the method used to decompose the equilibrium temperature response difference, by using a linearized decomposition of the individual radiative agents diagnosed by a radiative kernel technique. The climate sensitivity increase is primarily due to the cloud radiative responses, with a predominant contribution of the tropical longwave response (including both feedback and forcing adjustment) and a significant contribution of the extratropical and tropical shortwave feedback changes. A series of stand-alone atmosphere experiments is carried out to quantify the contributions of each atmospheric development to this difference between CNRM-CM5.1 and CNRM-CM6-1. The change of the convection scheme appears to play an important role in driving the cloud changes, with a large effect on the tropical longwave cloud feedback change.