Sensitivity of simulated global climate to perturbations in low cloud microphysical properties. Part II: Spatially localized perturbations

The sensitivity of the global climate to spatially localized (20°-70°N) perturbations in the microphysical properties of low clouds is investigated using a general circulation model coupled to a mixed layer ocean with fixed cloud distributions. By comparing with earlier experiments involving globally uniform perturbations, in-sights are obtained into the climate responses to spatially inhomogeneous radiative forcings, such as that due to the contrast in the effective drop radius of land and ocean clouds and the anthropogenic sulfate aerosol-induced alteration of cloud albedo. The main findings of this study are as follows: 1) The model's climate sensitivity (ratio of global-mean surface temperature response to the global-mean radiative forcing) is virtually independent of the distribution and magnitude of forcing. 2) Although the total feedback is very similar in the different experiments, the strengths of the individual feedback mechanisms (water vapor, albedo, lapse rate) are dissimilar. 3) For the localized perturbations, the climate response is essentially confined to the hemisphere in which the forcing occurs, owing to a poor interhemispheric energy exchange. 4) In spite of no forcing in the Southern Hemisphere in the localized experiments, there is a weak "remote" temperature response there. 5) For both global and localized perturbations, the temperature response in the tropical upper troposphere is larger than in the lower troposphere due to moist convective processes; in the localized experiments, while there is a strong vertical gradient in the temperature change at the Northern Hemisphere mid and high latitudes, the temperature change throughout the lower and midtroposphere of the Southern Hemisphere is uniform. 6) The localized experiments induce notable changes in the mean meridional circulation and precipitation near the equator, which are not obtained for the global perturbation cases. 7) The pattern of temperature response of the land and ocean areas in the Northern Hemisphere midlatitudes depends on whether the forcing occurs over both types of surfaces or over land only; the results suggest that the well-known contrast in drop radii between continental and maritime clouds exerts a significant influence on the surface temperature distribution within the zone and on the manner in which the surface energy balance is maintained.