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A recent study has presented compelling new evidence suggesting that the observed Eurasian warming in the winter following the 1992 Pinatubo eruption was, in all likelihood, unrelated to the presence of volcanic aerosols in the stratosphere. Building on that study, we turn our attention to the only other low-latitude eruption in the instrumental period with a comparably large magnitude: the Krakatau eruption of August 1883. We study the temperature anomalies in the first winter following that eruption in detail, analyzing (1) observations, (2) reanalyses, and (3) models. Three findings emerge from our analysis. First, the observed post-Krakatau winter warming over Eurasia was unremarkable (only between 1σ and 2σ of the distribution from 1850 to present). Second, reanalyses based on assimilating surface pressure alone indicate the existence of very large uncertainties, so much so that a Eurasian cooling is not incompatible with those reanalyses. Third, models robustly show the complete absence of a volcanically forced Eurasian winter warming: here, we analyze both a 100-member initial-condition ensemble and 140 simulations from Phase 5 of the Coupled Model Intercomparison Project. This wealth of evidence strongly suggests that, as in the case of Pinatubo, the observed warming over Eurasia in the winter of 1883–84 was, in all likelihood, unrelated to the Krakatau eruption. This, taken together with a similar result for Pinatubo, leads us to conclude that if volcanically forced Eurasian winter warming exists at all, an eruption with a magnitude far exceeding these two events would be needed to produce a detectable surface warming.

Major mid-winter stratospheric sudden warmings (SSWs) are the largest instance of wintertime variability in the Arctic stratosphere. Because SSWs are able to cause significant surface weather anomalies on intra-seasonal timescales, several previous studies have focused on their potential future change, as might be induced by anthropogenic forcings. However, a wide range of results have been reported, from a future increase in the frequency of SSWs to an actual decrease. Several factors might explain these contradictory results, notably the use of different metrics for the identification of SSWs and the impact of large climatological biases in single-model studies. To bring some clarity, we here revisit the question of future SSW changes, using an identical set of metrics applied consistently across 12 different models participating in the Chemistry–Climate Model Initiative. Our analysis reveals that no statistically significant change in the frequency of SSWs will occur over the 21st century, irrespective of the metric used for the identification of the event. Changes in other SSW characteristics – such as their duration, deceleration of the polar night jet, and the tropospheric forcing – are also assessed: again, we find no evidence of future changes over the 21st century., Blanca Ayarzagüena was funded by the European Project 603557-STRATOCLIM under the FP7-ENV.2013.6.1-2 programme and “Ayudas para la contratación de personal postdoctoral en formación en docencia e investigación en departamentos de la Universidad Complutense de Madrid”. Blanca Ayarzagüena and Ulrike Langematz wish to acknowledge the Deutsche Forschungsgemeinschaft (DFG) within the research programme SHARP under the grant LA 1025/15-1. Lorenzo M. Polvani is grateful for the continued support of the US National Science Foundation. The work of Neal Butchart, Steven C. Hardiman, and Fiona M. O'Connor was supported by the Joint BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101). Neal Butchart and Steven C. Hardiman were supported by the European Community within the StratoClim project (grant 603557). Olaf Morgenstern and Guang Zeng acknowledge the UK Met Office for use of the Met Office Unified Model (MetUM). This research was supported by the New Zealand Government's Strategic Science Investment Fund (SSIF) through the NIWA programme CACV. Olaf Morgenstern acknowledges funding by the New Zealand Royal Society Marsden Fund (grant 12-NIW-006) and by the Deep South National Science Challenge (http://www.deepsouthchallenge.co.nz, last access: 21 March 2018). The authors wish to acknowledge the contribution of New Zealand eScience Infrastructure (NeSI) high-performance computing (HPC) facilities to the results of this research. New Zealand's national facilities are provided by NeSI and funded jointly by NeSI's collaborator institutions and through the Ministry of Business, Innovation & Employment's Research Infrastructure programme (https://www.nesi.org.nz, last access: 21 March 2018). The EMAC simulations were performed at the German Climate Computing Centre (DKRZ) through support from the Bundesministerium für Bildung und Forschung (BMBF). DKRZ and its Scientific Steering Committee are gratefully acknowledged for providing the HPC and data archiving resources for the consortial project ESCiMo (Earth System Chemistry integrated Modelling). CCSRNIES's research was supported by the Environment Research and Technology Development Funds of the Ministry of the Environment (2-1303) and Environment Restoration and Conservation Agency (2-1709), Japan, and computations were performed on NEC-SX9/A(ECO) and NEC SX-ACE computers at the Center for Global Environmental Research, NIES. The authors wish to thank two anonymous referees for their helpful comments.

It has been suggested, and is widely believed, that the anomalous surface warming observed over the Northern Hemisphere continents in the winter following the 1991 eruption of Mt. Pinatubo was, in fact, caused by that eruption, via a stratospheric pathway that involves a strengthening of the polar vortex. However, most studies that have examined multiple, state-of-the-art, coupled climate models report that, in the ensemble mean, the models do not show winter warming after the Mt. Pinatubo eruption. This lack of surface warming in the multi-model mean, concomitant with a frequent lack of strengthening of the polar vortex, is often interpreted as a failure of the models to reproduce the observations. In this paper we show that this interpretation is erroneous, as averaging many simulations from different models, or from the same model, is not expected to yield surface anomalies similar to the observed ones, even if the models were highly accurate, owing to the presence of strong internal variability. We here analyze three large ensembles of state-of-the-art, coupled climate model simulations and show that, in all three, many individual ensemble members are able to produce post-Pinatubo surface warming in winter that is comparable to the observed one. This establishes that current-generation climate models are perfectly capable of reproducing the observed surface post-eruption warming. We also confirm the bulk of previous studies, and show that the surface anomaly is not statistically different from zero when averaged across ensembles of simulations, which we interpret as the simple fact that the volcanic impact on continental winter temperatures is tiny compared to internal variability. We also carefully examine the stratospheric pathway in our models and, again confirming previous work, show that any strengthening of the polar vortex caused by the Mt. Pinatubo eruption is very small (of the order of a few meters per second at best). Such minuscule anomalies of the stratospheric circulation are completely overwhelmed by the tropospheric variability at midlatitudes, which is known to be very large: this explains the lack of surface winter warming in the ensemble means. In summary, our analysis and interpretation offer compelling new evidence that the observed warming of the Northern Hemisphere continents in the winter 1991–1992 was very likely unrelated to the 1991 Mt. Pinatubo eruption.

This paper examines the nature and robustness of internal stratospheric variability, namely the variability resulting from the internal dynamics of the stratosphere itself, as opposed to that forced by external sources such as the natural variability of the free troposphere. Internal stratospheric variability arises from the competing actions of radiative forcing, which under perpetual winter conditions strengthens the polar vortex, and planetary wave breaking, which weakens it. The results from a stratosphere-only model demonstrate that strong internal stratospheric variability, consisting of repeated sudden warming-type events, exists over a wide range of realistic radiative and wave forcing conditions, and is largely independent of other physical and numerical parameters. In particular, the coherent form of the variability persists as the number of degrees of freedom is increased, and is therefore not an artifact of severe model truncation. Various diagnostics, including three-dimensional representations of the potential vorticity, illustrate that the variability is determined by the vertical structure of the vortex and the extent to which upward wave propagation is favored or inhibited. In this paper, the variability arising from purely internal stratosphere dynamics is isolated by specifying thermal and wave forcings that are completely time independent. In a second paper, the authors investigate the relative importance of internal and external variability by considering time-dependent wave forcing as a simple representation of tropospheric variability.