Deconvolving the atmospheric impacts of the 1991 eruptions of Mount Pinatubo and Cerro Hudson

The eruption of Mount Pinatubo in the Philippines in June 1991 was the largest of the satellite era. The volcanically enhanced stratospheric aerosol layer in the years following the eruption was climatically important: attenuated solar radiation led to cooler surface temperatures, and absorbed longwave radiation led to warmer temperatures in the stratosphere. Further, while most volcanic eruptions observed by satellite have affected only the hemisphere in which they erupted, the Mount Pinatubo eruption had chemical and radiative effects in both hemispheres. Complicating the assessment of Pinatubo's impacts on the climate system was the eruption of Cerro Hudson in southern Chile in August 1991. Cerro Hudson only injected about 15% of sulfur dioxide as Pinatubo, and so has often been neglected in analyses of the post-Pinatubo atmosphere, but it had significant impact on the radiation and chemistry in the southern hemisphere in the months following its eruption. In particular, the rapid transport of the Cerro Hudson plume toward the South Pole led to ozone depletion during the Austral Spring of 1991. In this study we attempt to deconvolve the relative impacts of Pinatubo and Cerro Hudson on the radiation and chemistry in the atmosphere in a series of simulations performed using the Goddard Earth Observing System (GEOS-5) Earth system model with coupled chemistry and aerosol microphysics. We use the sectional Community Aerosol and Radiation Model for Atmospheres (CARMA) to simulate the evolving size distribution and radiative and chemical impacts of the background and volcanically perturbed sulfate aerosol. We perform a series of simulations of the years 1991-1994 investigating the impacts of these volcanic eruptions separately and combined and compare them with satellite and balloon borne in-situ measurements. Our simulations suggest that the Southern Hemisphere response often attributed to Pinatubo was strongly impacted by Cerro Hudson. We also find that the microphysical and radiative properties of the Cerro Hudson aerosols are sensitive to the dynamical changes caused by Pinatubo and assumptions about the still uncertain injection parameters associated with Pinatubo.