A simulation of the possible consequences of a Volcanic Eruption on the General Circulation of the Atmosphere

Abstract

In view of the possible climatic importance of volcanic eruptions an initial attempt has been made to simulate the particulate impact of a large eruption on the general circulation via its interaction with the incoming solar radiation. A principal aim of the experiment was to separate out the direct radiative effects of the debris on the general circulation from the indirect effects associated with the large-scale transport mechanisms, in the framework of an evolving volcanic debris distribution. An 18-level, hemispheric, general circulation model constrained to annual mean conditions was used. This model in a prior run had reproduced many features of the observed general circulation. An amount of volcanic debris similar to that released by Krakatoa in IM was inserted as an initial zonal mean distribution in the model tropical stratosphere. The debris was subsequently permitted to be freely advected by the large-scale motions generated by the model for a total of 150 days. The debris's influence on the general circulation was restricted to backscattering the incoming solar radiation only, thus omitting the potentially important effect of the debris on the outgoing infrared radiation. Results presented show that the debris was diffused in a meaningful manner, and that the early debris distribution obtained was affected by whether or not coupling with the solar radiation was permitted. The debris produced perturbations in the wind fields of such a nature that, in general, the mean zonal wind was slightly reduced, while some changes in the synoptic wind distributions were also apparent. More marked differences were observed in the temperature distributions, although these were presumably overestimated owing to the omission of the thermal inertia of the oceans from the model. In particular, the hemispheric mean surface temperature was reduced by about 0.3 K, with cooling of approximately 0.7 K being noted in the tropics. At higher latitudes the perturbations caused by the volcanic debris tended to be obscured by ?local weather variations.? This limited latitudinal response was a consequence of the short time-scale of the experiment and of having a ?non-global? perturbation in the model, in contrast to most other model experiments that have involved global perturbations which produced maximum response at high latitudes. Outside of the tropics the temperature changes produced were a result of variations in the latitudinal transport of energy, especially latent heat, rather than an effect of the direct influence of the local debris distribution. The experiment suggests that individual large volcanic eruptions should have a transient, but not necessarily insignificant, impact on the climate.