Improved Climatological Hadley Circulation Reduces Model Bias in Cloud–Circulation Coupling and Model Uncertainty in Climate Sensitivity

Recent analyses of global climate models have shown that uncertainty in the coupling between mid-latitude clouds and the atmospheric circulation contributes to uncertainty in climate sensitivity. However, the reasons behind model differences in the cloud-circulation coupling have remained unclear, and have been suspected to arise from small-scale cloud microphysics. Here, we use a global climate model in idealized aquaplanet setup to show instead that the climatological circulation, which in many models is biased equatorward, controls much of the model differences in the cloud-circulationcoupling. For the same poleward shift of the Hadley circulation (HC) edge, models with narrower climatological HCs exhibit stronger mid-latitude cloud-induced shortwave warming than models with wider climatological HCs. This cloud-induced warming results predominantly from a subsidence warming of the planetary boundary layer, which decreases low-level cloud fraction and is stronger for narrower HCs because of a larger meridional gradient in the vertical velocity. A comparison of our aquaplanet results with global climate models suggests that about half of the model uncertainty in the mid-latitude cloud-circulation coupling stems from this impact of the circulation on the large-scale temperature structure of the boundary layer, and thus could be removed by improving the climatological circulation in models. This illustrates how understanding of large-scale dynamics can help reduce uncertainty in clouds and their response to climate change.