Evaluation of CMIP6 DECK Experiments With CNRM‐CM6‐1

TLDR

CNRM‑CM6‑1 is a fully coupled atmosphere‑ocean general circulation model of the sixth generation, jointly developed by CNRM and Cerfacs for CMIP6. The paper details each component of CNRM‑CM6‑1, its coupling method, new online output software, and major improvements in atmospheric, land processes, and mass‑energy conservation. The model’s climate is evaluated through CMIP6 historical and DECK experiments and compared with equivalent CMIP5 CNRM‑CM5.1 runs. Results show surface biases of similar magnitude but different spatial patterns, reduced deep‑ocean biases, thinner Arctic sea ice, consistent climate variability, and an increased equilibrium climate sensitivity of 4.9 K, near the upper CMIP5 bound.

Abstract

Abstract This paper describes the main characteristics of CNRM‐CM6‐1, the fully coupled atmosphere‐ocean general circulation model of sixth generation jointly developed by Centre National de Recherches Météorologiques (CNRM) and Cerfacs for the sixth phase of the Coupled Model Intercomparison Project 6 (CMIP6). The paper provides a description of each component of CNRM‐CM6‐1, including the coupling method and the new online output software. We emphasize where model's components have been updated with respect to the former model version, CNRM‐CM5.1. In particular, we highlight major improvements in the representation of atmospheric and land processes. A particular attention has also been devoted to mass and energy conservation in the simulated climate system to limit long‐term drifts. The climate simulated by CNRM‐CM6‐1 is then evaluated using CMIP6 historical and Diagnostic, Evaluation and Characterization of Klima (DECK) experiments in comparison with CMIP5 CNRM‐CM5.1 equivalent experiments. Overall, the mean surface biases are of similar magnitude but with different spatial patterns. Deep ocean biases are generally reduced, whereas sea ice is too thin in the Arctic. Although the simulated climate variability remains roughly consistent with CNRM‐CM5.1, its sensitivity to rising CO 2 has increased: the equilibrium climate sensitivity is 4.9 K, which is now close to the upper bound of the range estimated from CMIP5 models.