Tracking Changes in Climate Sensitivity in CNRM Climate Models

Saint-Martin, David ; Geoffroy, Olivier ; Voldoire, Aurore ; Cattiaux, Julien ; Brient, Florent ; Chauvin, Fabrice ; Chevallier, Matthieu ; Colin, Jeanne ; Decharme, Bertrand ; Delire, Christine ; Douville, Hervé ; Guérémy, Jean-François ; Joetzjer, Emilie ; Ribes, Aurélien ; Roehrig, Romain ; Terray, L. ; Valcke, S.

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<p align=justify>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.</p>
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