Response of Hadley Circulation to Warming and Sensitivity to Physical Parameters

Changes in global mean precipitation, clouds, and circulation under warming are intimately coupled. In the Coupled Model Intercomparison Project phase 5 (CMIP5) experiment, there is a large intermodel spread in the magnitude of the Hadley ascent tightening projected by most models under warming, which contributes significantly to the intermodel spread in global precipitation changes. This tightening is highly correlated to decreasing high cloud fraction with warming. While it is widely accepted that changes in circulation with warming are intimately coupled to changes in clouds, the details of such relationships remain elusive. Additionally, it is unclear what physics contribute most to the intermodel spread in the projected response of Hadley ascent tightening. A set of atmosphere-only experiments, in which parameters in the deep convective and microphysics schemes are perturbed independently, is performed with the Community Earth System Model (CESM) to identify the model physics to which these changes in circulation, clouds, and precipitation are most sensitive. Results from the parameter perturbation experiments suggest that the intermodel spread in the changes of high cloud fraction and Hadley ascent width per degree warming in CMIP5 models is largely due to different deep convective physics, rather than microphysics. Results from the microphysics experiment reveal an opposite cloud and circulation relation, suggesting competing feedbacks. We examine the relation between clouds and circulation using a two-box view of mass balance in the ascending and descending regions of the Hadley cell. Steps toward evaluating parameter sensitivity and observationally constraining model physics will also be discussed.