An interactive model study of the influence of the Mount Pinatubo aerosol on stratospheric methane and water trends

We have used the Goddard Space Flight Center interactive two-dimensional mo del to evaluate the effects of the Mount Pinatubo volcanic aerosol cloud on subsequent trends in upper stratospheric CH4 and H2O. The effects of the a erosol cloud were included in the model photolysis, heating rate, and heter ogeneous chemistry parameterizations, resulting in thermal, chemical, and d ynamical responses to the aerosol forcing. The simulation used observed ext inctions and surface area densities from the Stratospheric Gas and Aerosol Experiment II instrument between 1990 and 1997. Model upper stratospheric C H4 concentrations respond to a volcanically enhanced tropical vertical upwe lling of up to 24%, which increases CH4 by similar to 10% in early 1992. Th e decay of this CH4 pulse between 1992 and 1997 produces negative upper str atospheric trends which are in better agreement with observations than a no nvolcanic simulation but are still substantially weaker than the similar to 30-40% CH4 decreases observed in the upper stratosphere by the Halogen Occ ultation Experiment between 1992 and 1997. Model temperature perturbations following the eruption of similar to2-3 K between 20 and 50 hPa agree well with observations in both magnitude, timing, and duration. The temperature of the model tropopause increases by similar to0.5 K and results in a subst antial transfer of H2O into the stratosphere across the model tropopause. T he H2O signal takes several years to propagate into the upper stratosphere, resulting in peak increases of similar to6% at the start of 1995. Middle t o upper stratospheric model H2O trends between 1992 and 1997 agree better w ith observations than a nonvolcanic simulation, but atmospheric observation s lack the strong signature of a pulse propagating from the tropical lower stratosphere as seen in the model simulation. These comparisons suggest tha t the aerosol from the Mount Pinatubo eruption could have contributed to th e observed changes in CH4 and H2O following the eruption but was probably n ot the sole driver of the observed changes.