Cloud-Radiative Impacts On Tropical Circulation Change in GFDL AM4.1

Abstract

<pre>We use the Geophysical Fluid Dynamics Laboratory (GFDL) state-of-the-art AM4.1 atmospheric model to assess the impact of clouds on the change in tropical circulation. Slab-ocean experiments where cloud microphysical properties are locked to either the pre-industrial or 4xCO<sub>2</sub> 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 CO<sub>2</sub> changes. The CO<sub>2</sub>-induced SST changes are shown to dominate the response in the boundary layer, 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 CO<sub>2</sub> 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 CO<sub>2</sub> and CREs make to the changes in dry-static and gross moist stability. Also, concerning the <span>meridional</span> circulation, we show that the response in the strength of the southern branch of the Hadley cell is largely due to CREs, while they have a much smaller impact in the north.</pre>