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Correlation Between Cloud Adjustments and Cloud Feedbacks Responsible for Larger Range of Climate Sensitivities in CMIP6
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  • Nicholas Lutsko,
  • Matt Luongo,
  • Casey James Wall,
  • Timothy A Myers
Nicholas Lutsko
Scripps Institution of Oceanography.

Corresponding Author:[email protected]

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Matt Luongo
University of California, San Diego
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Casey James Wall
Scripps Institution of Oceanography
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Timothy A Myers
University of Colorado Boulder
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While the higher mean Equilibrium Climate Sensitivity (ECS) in CMIP6 has been attributed to more positive cloud feedbacks, it is unclear what causes the greater range of ECS values across CMIP6 models compared to CMIP5. Here we investigate the relationship between radiative forcing and cloud feedbacks across the two model generations to explain the very high ECS values in some CMIP6 models. The relationship is sensitive to the definition of the forcing, particularly in CMIP6, but fixed-SST simulations suggest the shortwave cloud feedback ($\lambda_{SW, cl}$) is anti-correlated with the forcing in CMIP5 and weakly positively correlated with the forcing in CMIP6. These relationships reflect the cloud adjustment to the forcing, which is anti-correlated with $\lambda_{SW, cl}$ in CMIP5 and positively correlated in CMIP6. Although we are unable to identify a systematic change across the model generations, we do show that modifications to the land components of climate models are not responsible for the change in the relationship between cloud adjustments and cloud feedbacks, and that cloud adjustments are generally driven by low and, especially mid-level clouds. Moreover, models derived from the MOHC and NCAR modeling centers seem to be responsible for much of the trend between CMIP5 and CMIP6. Our analysis is severely limited by the available simulations, highlighting the need for targeted simulations to probe the sources of intermodel differences in cloud adjustments.