Impacts of Cloud Feedbacks on Tropical Circulation Change in GFDL AM4.1

Abstract:

We use the Geophysical Fluid Dynamics Laboratory (GFDL) state-of-the-art AM4.1 atmospheric model to assess the impact of clouds on the changes in tropical circulation. Slab-ocean experiments where clouds are locked to either the pre-industrial or 4xCO2 conditions allow us to cleanly separate the circulation changes into a part caused by the cloud radiative effects (CREs), and to a part caused by the CO2 changes. The CO2-induced SST changes are shown to dominate the response in the lower troposphere, but are rivaled by the impacts of CREs in the mid to upper troposphere. The reduction in the east-to-west sea level pressure difference over the Pacific is solely caused by the increasing CO2 and SST, but they only account for about half of the change in the mid-tropospheric Walker circulation. The weakening of the free-tropospheric circulation is shown to be mostly caused by the near-equal contributions the CO2 and CREs make to the changes in dry-static and gross moist stability. We also analyze changes in the Hadley circulation, and show that the CREs play a larger role in the Southern Hemisphere.