Surface Temperature Dependence of Stratospheric Sulfate Aerosol Forcing and Feedback

Abstract. Stratospheric sulfate aerosol originating from explosive volcanic eruptions can perturb the radiative budget for several years following the eruption. However, the understanding of the state dependence of aerosol forcing and its effect on the radiative feedback is still incomplete. We quantify the contributions to clear-sky forcing and feedback from absorbing and re-emitting longwave radiation, stratospheric heating, and enhanced stratospheric water vapour. We show that, at surface temperatures from 280 K to 300 K, the aerosol forcing becomes less negative (weaker) with increasing surface temperature because its longwave component becomes more positive. Aerosol forcing has a stronger surface temperature dependence than CO₂ forcing. This stronger dependence arises because, unlike CO_2, the aerosol predominantly absorbs in the spectral range in which the atmosphere is optically thin and thus spectrally masks the surface-temperature dependence of emissions. Additionally, the radiative feedback to surface temperature change is less negative in the presence of the aerosol. This is mainly due to the fact that the temperature of the aerosol layer is largely independent of the surface temperature, leading to a masking of emission changes through the aerosol layer. The study highlights the critical role played by the spectral nature of aerosol longwave absorption in determining the surface temperature dependence of the forcing and in reducing the feedback in comparison to an atmosphere without stratospheric aerosol.