Isolating the boreal winter response to the Pinatubo and Krakatoa eruptions using large-ensemble single-forcing simulations

Abstract. The Northern Hemisphere wintertime circulation response to the eruptions of Krakatoa and Pinatubo is revisited in large ensembles from eight modeling centers with only one time-varying external forcing: volcanic eruptions. All eight models show a warming of the tropical lower stratosphere. In six of the models, the meridional temperature gradient in the winter stratosphere is enhanced, leading to a strengthened stratospheric polar vortex, a positive phase of the North Atlantic Oscillation, a poleward shift in storm tracks, and warm surface temperatures during the winter over subpolar Eurasia. While this warming over subpolar Eurasia is statistically significant in the multi-model mean and in four of the individual models, at least 34 eruptions are needed before it can be robustly distinguished from the global mean cooling with 5 % confidence. An El Niño response is evident shortly after eruption in these models, which transitions from a Central Pacific morphology in the first winter to an East Pacific morphology in the second, and subsequently to an La Niña response in the third and fourth winters. While these ENSO responses require 48 or more eruptions to emerge from the noise, they nonetheless lead to surface impacts over North America. However, these surface impacts do not resemble those classically associated with ENSO in the first year after eruption, likely because the tropical precipitation response differs as well due to the large-scale reduction in tropical precipitation. There is substantial diversity in the magnitude of these responses across models, likely owing to differences in tropical stratospheric diabatic heating despite the models using the same forcing.