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.

Automated summary

The study investigates the increase in equilibrium climate sensitivity (ECS) in CNRM-CM6-1 and CNRM-CM6-1-HR compared to CNRM-CM5.1 using coupled ocean-atmosphere model climate change simulations. It finds that the change is primarily due to changes in the atmospheric component, particularly the cloud radiative responses with significant contributions from tropical longwave response and extratropical and tropical shortwave feedback changes.