[1] Stratospheric sulfate aerosol particles from strong volcanic eruptions produce significant transient cooling of the troposphere and warming of the lower stratosphere. The radiative impact of volcanic aerosols also produces a response that generally includes an anomalously positive phase of the Arctic Oscillation (AO) that is most pronounced in the boreal winter. The main atmospheric thermal and dynamical effects of eruptions typical of the past century persist for about two years after each eruption. In this paper we evaluate the volcanic responses in simulations produced by seven of the climate models included in the model intercomparison conducted as part of the preparation of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). We consider global effects as well as the regional circulation effects in the extratropical Northern Hemisphere focusing on the AO responses forced by volcanic eruptions. Specifically we analyze results from the IPCC historical runs that simulate the evolution of the circulation over the last part of the 19th century and the entire 20th century using a realistic time series of atmospheric composition (greenhouse gases and aerosols). In particular, composite anomalies over the two boreal winters following each of the nine largest low-latitude eruptions during the period 1860-1999 are computed for various tropospheric and stratospheric fields. These are compared when possible with observational data. The seven IPCC models we analyzed use similar assumptions about the amount of volcanic aerosols formed in the lower stratosphere following the volcanic eruptions that have occurred since 1860. All models produce tropospheric cooling and stratospheric warming as in observations. However, they display a considerable range of dynamic responses to volcanic aerosols. Nevertheless, some general conclusions can be drawn. The IPCC models tend to simulate a positive phase of the Arctic Oscillation in response to volcanic forcing similar to that typically observed. However, the associated dynamic perturbations and winter surface warming over Northern Europe and Asia in the post-volcano winters is much weaker in the models than in observations. The AR4 models also underestimate the variability and long-term trend of the AO. This deficiency affects high-latitude model predictions and may have a similar origin. This analysis allows us to better evaluate volcanic impacts in up-to-date climate models and to better quantify the model Arctic Oscillation sensitivity to external forcing. This potentially could lead to improving model climate predictions in the extratropical latitudes of the Northern Hemisphere.

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[1] Explosive eruptions can inject large amounts of volcanic gases into the stratosphere. These gases may be scavenged by hydrometeors within the eruption column, and high uncertainties remain regarding the proportion of volcanic gases, which eventually reach the stratosphere. These are caused by the difficulties of directly sampling explosive volcanic eruption columns and by the lack of laboratory studies in the extreme parameter regime characterizing them. Using the nonhydrostatic nonsteady state plume model Active Tracer High Resolution Atmospheric Model (ATHAM), we simulated an explosive volcanic eruption. We examined the scavenging efficiency for the climatically relevant gases within the eruption column. The low concentration of water in the plume results in the formation of relatively dry aggregates. More than 99% of these are frozen because of their fast ascent to low-temperature regions. Consideration of the salinity effect increases the amount of liquid water by one order of magnitude, but the ice phase is still highly dominant. Consequently, the scavenging efficiency for HCl is very low, and only 1% is dissolved in liquid water. However, scavenging by ice particles via direct gas incorporation during diffusional growth is a significant process. The salinity effect increases the total scavenging efficiency for HCl from about 50% to about 90%. The sulfur-containing gases SO 2 and H 2 S are only slightly soluble in liquid water; however, these gases are incorporated into ice particles with an efficiency of 10 to 30%. Despite scavenging, more than 25% of the HCl and 80% of the sulfur gases reach the stratosphere because most of the particles containing these species are lifted there. Sedimentation of the particles would remove the volcanic gases from the stratosphere. Hence the final quantity of volcanic gases injected in a particular eruption depends on the fate of the particles containing them, which is in turn dependent on the volcanic and environmental conditions.