Simulation of the climate impact of Mt. Pinatubo eruption using ECHAM5 – Part 1: Sensitivity to the modes of atmospheric circulation and boundary conditions

Large volcanic eruptions and their subsequent climate responses are relatively short-lived perturbations to the climate system. They provide an excellent opportunity to understand the response of the climate system to a global radiative forcing and to assess the ability of our climate models to simulate such large perturbations. The eruption of 5 Mt. Pinatubo in Philippines in June 1991 was one of the strongest volcanic eruptions in the 20th century and this well observed eruption can serve as an important case study to understand the subsequent weather and climate changes. In this paper, the most comprehensive simulations to date of the climate impact of Mt. Pinatubo eruption are carried out with prescribed volcanic aerosols including observed SSTs, QBO and 10 volcanically induced ozone anomalies. This is also the first attempt to include all the known factors for the simulation of such an experiment. Here, the climate response is evaluated under different boundary conditions including one at a time, thereby, investigating the radiative and dynamical responses to individual and combined forcings by observed SSTs, QBO and volcanic effects. Two ensembles of ten members each, for 15 unperturbed and volcanically perturbed conditions were carried out using the middle atmosphere configuration of ECHAM5 model. Our results show that the pure aerosol response in lower stratospheric temperature is insensitive to the boundary conditions in the tropics and does not show some observed features which results from the boundary conditions. To simulate realistically the lower stratospheric temperature response, 20 one must include all the known factors. The pure QBO and ocean responses are simulated consistent with earlier studies. The dynamical response manifested as the winter warming pattern is not simulated in the ensemble mean of the experiments. Our analysis also shows that the response to El Niño conditions is very strong in the model and that it partially masks the effects due to volcanic forcing.