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Short-term climatic impact of the 1991 volcanic eruption of Mt. Pinatubo and effects on atmospheric tracers

Large explosive volcanic eruptions are capable of injecting considerable amounts of particles and sulphur gases (mostly sulphur dioxide) above the tropopause, caus- ing increases in the stratospheric aerosol optical depth that may be even larger than one order of magnitude. The e- folding particle lifetime in the stratosphere is much longer than in the troposphere (one year versus a few days) so that climatic perturbations in a timeframe of a few years are pro- duced after major volcanic eruptions. A climate-chemistry coupled model is used here to study the dynamical effects of the radiative forcing due to stratospheric aerosols formed after the June, 1991 cataclysmic eruption of Mt. Pinatubo in the Philippines. It is shown that the dynamical perturba- tion is twofold: (a) the stratospheric mean meridional circu- lation is affected by local aerosol radiative heating (mostly located in the tropical lower stratosphere); (b) the planetary wave propagation in the mid- to high-latitude lower strato- sphere is altered as a consequence of decreasing atmospheric stability due to the climatic perturbation. Dynamical results of the climate model are compared with available observa- tions; a discussion is made regarding the similarities with the dynamical regime of the easterly phase of the equatorial quasi-biennial oscillation. Major findings of this study are: (a) radiatively forced changes in the stratospheric circulation during the first two years after the eruption may, to a large ex- tent, explain the observed trend decline of long-lived green- house gases (CH4 and N2O, in particular); (b) the dynamical perturbation helps explain why simple photochemical stud- ies of the ozone trends during 1991-1993 generally fail in reproducing the satellite observed feature consisting of a 2% additional global ozone depletion during 1993 with respect to 1992. In both cases we conclude that an increase in the mid- to high-latitude downward flux at the tropopause is the key factor for explaining the behaviour of these atmospheric tracers during 1991/92.