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In this paper, an idealized framework based on a cloud-topped mixed layer model is developed for investigating feedbacks between subtropical stratocumulus (Sc) and global warming. The two principal control parameters are Sc-region sea surface temperature (SST) and intertropical convergence zone (ITCZ) SST (which controls the temperature and mean subsidence profiles above the Sc). The direct effect of CO2 doubling (leaving all other parameters fixed) is tested and found to somewhat reduce liquid water path; discussion of this effect on the SST-change simulations is included. The presence of a cold boundary layer is found to significantly affect the temperature and subsidence rate just above cloud top by enhancing lower-tropospheric diabatic cooling in this region. A simple representation of this effect (easily generalizable to a more realistic boundary layer model) is developed. Steady-state solutions are analyzed as a function of local and ITCZ SST. Two climate change scenarios are considered. The first scenario is an equal increase of local and ITCZ SSTs. In this case, predicted boundary layer depth and cloud thickness increase. This is found in a simplified context to result from subsidence and entrainment decreases due to increased static stability in a warmer climate. In the second case, local SST change is diagnosed from a surface energy balance under the assumption that ocean heat transport remains unchanged. In this case, predicted boundary layer depth decreases. Cloud continues to thicken with rising ITCZ SST, but at a rate much reduced in comparison to the equal-warming scenario. This cloud shading feedback keeps SST in the Sc region nearly constant as the ITCZ SST increases. Model sensitivity to aerosol indirect effects is also considered by varying the assumed droplet concentration. The resulting change in liquid water path is small, suggesting a weaker dependence on second indirect effect than found in previous studies. [ABSTRACT FROM AUTHOR]

The subtropical North Pacific and North Atlantic are controlled by basin-scale anticyclones in boreal summer. Based on a novel metric regarding the strengths of the rotational and the divergent circulation of anticyclones, we investigated the possible future responses in the intensity of these two subtropical anticyclones to global warming. While the North Atlantic subtropical anticyclone (NASA) is projected to strengthen, the North Pacific subtropical anticyclone (NPSA) is projected to weaken, in terms of both the rotational and the divergent circulation. The distinct responses of the NPSA and NASA are corroborated by the models participating in the fifth and sixth phases of the Coupled Model Intercomparison Project (CMIP), under both intermediate and high emission scenarios. We further investigated the possible mechanism for their distinct responses by decomposing the effect of greenhouse gas forcing into the direct effect of increased CO2 concentration and the indirect effect through sea surface temperature (SST). The intensified NASA results from the CO2 direct forcing while the weakened NPSA is dominated by the SST warming. The CO2 direct forcing enhances the NASA by increasing land–ocean thermal contrast anchored by the largest subtropical continental area, the Eurasian–African continent. Both the uniform SST warming and the change in SST pattern act to weaken the NPSA by increasing the latent heating over the subtropical North Pacific basin, as suggested by atmospheric component model simulations. The distinct responses of the NPSA and the NASA may lead to zonal asymmetry of the subtropical climate change. [ABSTRACT FROM AUTHOR]