Surface temperature dependence of stratospheric sulfate aerosol clear-sky forcing and feedback

Abstract. Stratospheric sulfate aerosol originating from explosive volcanic eruptions can perturb the radiative budget for several years. However, the understanding of the state dependence of aerosol forcing and its effect on the radiative feedback is still incomplete. Using a one-dimensional radiative–convective equilibrium model of the tropical atmosphere, 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 aerosol forcing has a stronger surface temperature dependence than CO2 forcing. At surface temperatures from 280 to 300 K, the aerosol forcing becomes less negative (weaker) with increasing surface temperature because its longwave component becomes more positive. Additionally, the radiative feedback is less negative in the presence of the aerosol. The dependence of the feedback parameter on the aerosol concentration and of the forcing magnitude on temperature arises from the same process: aerosol absorbs in the spectral range in which the atmosphere is optically thin and thus spectrally masks the temperature-dependent surface emissions. 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.