Stratospheric Water Vapor Feedbacks in a simple GCM

Recent observational and modeling studies have highlighted the impact of stratospheric water vapor on surface climate across many time scales, from interannual variations to the equilibrium response to greenhouse gas forcing. Much of the theoretical work on stratosphere-troposphere coupling, however, has relied on idealized “dry” models of the atmosphere. We develop a model of an idealized moist atmosphere appropriate for the study of the coupled stratosphere-troposphere system. Our work builds on the idealized moist atmospheric model developed by Frierson et al. (2006), but we replace the gray radiation scheme with full radiation, comparable to that in comprehensive models. Radiative transfer is computed with specified carbon dioxide and ozone, and includes prognostic water vapor. We otherwise retain the simplified diffusive boundary layer and mixed layer aquaplanet surface boundary conditions, and the hydrological cycle can be computed with either large scale condensation only or with a highly simplified convection scheme. A key idealization is the absence of any cloud water or ice, so that the model atmosphere is cloud free. The surface albedo is tuned to compensate for the climatological radiative impact of clouds, producing a reasonable base climatology. We probe the impact of water vapor on the stratospheric circulation by comparing fully interactive integrations of the model to companion integrations where the coupling between the circulation and water vapor is disconnected by feeding the radiation scheme only the climatological mean water vapor concentrations. We focus on the impact of water vapor on the Brewer-Dobson Circulation and its response to increased greenhouse gas forcing.