Mechanisms of the Pattern Effect in CAM6

The pattern effect describes that global-mean radiative feedbacks depend strongly on the pattern of surface warming. The pattern effect can be studied in GCMs using SST patch experiments, ensembles of experiments in which SST patches are warmed (or cooled) in isolation one-by-one across the globe. Here we evaluate a new set of patch experiments run with CAM6 with a focus on mechanisms. The CAM6 patch experiments were prepared following the GFMIP protocol. The CAM6 patch experiments are unique in that they are run with a satellite simulator turned on. This allows for the influence of the SST pattern effect on cloud feedbacks to be decomposed into its six components that follow the WCRP assessment. We find that the pattern effect influences cloud feedbacks primarily through the tropical marine low-cloud feedback, with partial compensation from the high-cloud altitude and tropical anvil cloud area feedbacks. Further, we use this ensemble of patch experiments to evaluate proposed mechanisms of the pattern effect. Saturation MSE at 500hPa is highly correlated with the tropical marine low cloud feedback, suggesting that the “Circus Tent” mechanism exerts strong control over the pattern effect in the CAM6 patch experiments.