Equilibrium Response of an Atmosphere–Mixed Layer Ocean Model to Different Radiative Forcing Agents: Global and Zonal Mean Response

Abstract

Abstract The equilibrium response to various forcing agents, including CO2, solar irradiance, tropospheric ozone, black carbon, organic carbon, sulfate, and volcanic aerosols, is investigated using an atmospheric general circulation model coupled to a mixed layer ocean model. The experiments are carried out by altering each forcing agent separately. Realistic spatial patterns of forcing constituents are applied but the magnitude of the forcing is adjusted so that each forcing constituent yields approximately the same strength of radiative forcing. It is demonstrated that the global mean temperature response depends on the types of forcing agents and the efficacy with respect to CO2 forcing ranges from 58% to 100%. The smallest efficacy is seen in one of the black carbon experiments and is associated with negative cloud feedback. The sign of the cloud feedback is shown to be sensitive to the vertical distribution of black carbon. The feedback analysis suggests that the small efficacy in tropospheric ozone is due to a large negative lapse rate feedback. Global mean precipitation increases when the earth warms except for the case of black carbon in which precipitation decreases. In all experiments, the global mean convective mass flux decreases when the earth’s surface warms. When the applied radiative forcing resulting from a particular forcing agent is stronger in one hemisphere, anomalous heat exchange between the hemispheres results in conjunction with changes in the Hadley circulation. The magnitude of interhemispheric heat transport is little sensitive to the details of the forcing, but is determined primarily by the interhemispheric contrast in forcing. The change in the Hadley circulation strongly impacts the precipitation changes in low latitudes.