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1. Introduction

The products of a major explosive volcanic eruption produce significant changes in the radiation fields in the stratosphere, thereby affecting the stratospheric heating and photolysis rates, and the stratospheric temperature and circulation patterns. Additionally, the sulfate aerosol produced in situ from the oxidation of sulfate-containing gases provides the medium for relatively fast heterogeneous chemical reactions that transfer active nitrogen oxides to the more passive nitrous acid form, which leads to changes in the ozone concentration. The ozone changes, in turn, can affect the radiation, temperature and dynamics in the middle and lower stratosphere. Therefore, a theoretical and observational study of the post-volcanic changes can substantially improve our understanding of the different physical and chemical processes in the atmosphere and their relationships. Studies of the consequences of volcanic eruptions can facilitate understanding of how the climate system responds to the stratospheric perturbation and enable estimation of the magnitude of the possible influence of stratospheric processes on the tropospheric climate/weather system. Modelling efforts and comparison of the simulated changes in post-volcanic periods will also allow determination of how successfully the actual processes in the atmosphere can be simulated by a model, and what part of a model should be improved to obtain better performance.

Among the major volcanic eruptions (e.g., McCormic et al. [1995]; Rampino and Self[1984]; Robock and Mao [1995]), Pinatubo is an exception owing to the numerous observations collected during and after its eruption in June, 1991. The formation and evolution of the aerosol cloud, optical properties of the aerosol, radiative fluxes, chemical composition, temperature and the dynamical state of the atmosphere have been observed by a variety of satellite, balloon, lidar, airborne and ground-based instruments.

The availability of different satellite measurements has allowed development of a dataset of aerosol optical properties (Stenchikov et al. [1998]; Andronova et al. [1999]) which can be used in the simulation of the radiative and climatic effects of the Pinatubo aerosol. However, from analyses of the observations alone it is difficult to understand which processes are responsible for the observed changes of ozone and temperature following the Pinatubo eruption. Ozone, temperature and dynamics are closely linked in the real atmosphere. Therefore, only model simulations of the consequences of the Mount Pinatubo eruption with state-of-the-art models, sensitivity studies therewith, and comparison of their results with the observations can answer the question: Which processes are mainly responsible for the observed ozone and temperature changes?

During the last 7 years a number of modeling studies of the effects of the Pinatubo eruption have been performed, mainly with 2-D zonally averaged stratospheric models. These modeling efforts mainly addressed the causes of the ozone changes and the potential role of heterogeneous processes in the observed ozone depletion. The results obtained by Brasseur and Granier [1992], Pitari and Rizi [1993], Kinnison et al. [1994], Tie et al. [1994], Jackman et al. [1996] and Solomon et al. [1996] proved that heterogeneous chemistry is the most important factor for the simulated ozone depletion in the tropics and polar areas. However, in the paper by Rosenfield et al. [1997] it was found that the main process responsible for 60% of the ozone depletion in the tropics is the increased upward motion resulting from the enhanced heating rates. In this model, heterogeneous chemistry determines only about 20% of the total ozone decrease in the tropics. This was explained as being due to the use in this simulation of a new, highly interactive 2-D model, with a parameterized description of planetary waves and a direct calculation of the eddy mixing. This shows how significant it is to include in the model, interactively, all the relevant processes. However, some inherent limitations of 2-D models hamper their ability to simulate interactively the many chemical, hydro-thermodynamical, radiative and hydrological processes that are involved in determining the influence of the Pinatubo aerosol on the atmosphere. A more appropriate tool for such studies is a General Circulation Model (GCM).

Currently, GCMs are focusing on the simulation of the temperature and circulation changes caused by prescribed ozone and volcanic aerosol properties. Hansen et al. [1992] applied the GISS GCM, which has a coarse horizontal resolution of 8° latitude by10° longitude and only 1-2 layers in the stratosphere, to simulate the Post-Pinatubo atmosphere. Therefore, the dynamical interaction between the stratosphere and troposphere in that model was very simplified. Graf et al. [1993] investigated the relation between the northern hemisphere circulation and the surface air anomalies after the Pinatubo eruption by using the results of a perpetual January simulation with the ECHAM-2 GCM forced by an artificial decrease of solar radiation at the top of the model, and calculated the radiative heating "off-line". They concluded that the observed winter warming over the northern hemisphere continents is associated with the enhancement of the polar night jet (PNJ) due to anomalous radiative heating in the tropical lower stratosphere by the volcanic aerosol. A set of ensemble simulations has recently been performed by Kirchner et al. [1999] to study the climate response of the atmosphere to the Pinatubo eruption. They used the ECHAM-4 GCM and realistic aerosol optical properties to perform three sets of 2-year-long simulations with and without volcanic-aerosol forcing and with different sets of sea surface temperature (SST). It was concluded that the model simulates reasonably well the observed general cooling in the troposphere and winter warming near the surface over northern hemisphere continents. However, about a 4K warming in the lower tropical stratosphere was simulated which exceeds the observed value by up to 2K. It was pointed out that this overestimation of the warming can be explained by the absence of the QBO in the model and by the cooling of the lower stratosphere due to volcanically induced ozone depletion. Yang [1999] and also studied the temperature and circulation changes observed following the Pinatubo eruption using singular-value decomposition and the UIUC stratosphere-troposphere GCM, with emphases on identification and separation of the temperature changes induced by the Pinatubo aerosol and the overlapping ENSO events. However, it is difficult to predict what the model response would be if interactive ozone chemistry was introduced into the model because of the strong non-linearity of the coupled model and the very complicated system of feedbacks in the fully interactive model. It should be noted that several GCMs with interactive chemistry (Zhao et al., [1997]; Knight et al. [1998]) have already been developed and used to estimate the changes in the stratosphere caused by the Pinatubo eruption. However, they mainly addressed the problem of ozone depletion and the partition of the reactive nitrogen and chlorine. They have not provided or discussed any information about the temperature and circulation changes in the stratosphere. Moreover, the above-mentioned stratospheric GCMs are not able to simulate any tropospheric processes , hence they cannot be used to analyze stratosphere-troposphere interactions. To fill this gap we have performed the first-ever Post-Pinatubo simulations with a Stratosphere-Troposphere GCM that has interactive chemistry (ST-GCM/PC).