Hemispheric Contrast in Aerosol-Cloud Interactions: An Attempt for Detection and Attribution

The changes in the concentration, distribution, and composition of anthropogenic aerosols impact cloud properties and cloud radiative effects. A distinct feature of the anthropogenic perturbation of aerosols is the hemispheric contrast, with much larger perturbations in the Northern Hemisphere. Observations of clouds in the two hemispheres, particularly alongside model simulations, may help constrain the magnitude of the effective radiative forcing due to aerosol-cloud interactions. This study investigates the impact of flipping the distribution of anthropogenic aerosol emissions between hemispheres on aerosol-cloud interactions using the aerosol-climate model ICON1.3.0-A-HAM2.3. It is shown, globally, that a clear, detectable, and attributable impact in aerosol optical depth results from anthropogenic emissions changes based on the counterpart hemisphere, an increase in the Southern Hemisphere, and a decrease in the Northern Hemisphere. The response to changes in anthropogenic aerosols in cloud droplet number concentration and liquid water path is particularly strong over land and is attributable using satellite data as a reference. Changes in enhanced/reduced aerosol emissions, through their interactions with radiation and clouds, produce a positive effective radiative forcing of 2.85 W m^–2 in the Northern Hemisphere and a negative effective radiative forcing in the Southern Hemisphere of –2.63 W m^–2.