Northern Hemisphere continental winter warming following the 1991 Mt. Pinatubo eruption: reconciling models and observations

Abstract

Abstract. 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.

Volcanic Eruption Signatures in the Isotope‐Enabled Last Millennium Ensemble

“…This has been previously attributed to a strengthening of the stratospheric polar vortex and associated positive phase of the North Atlantic Oscillation, bringing warmer air from the North Atlantic (Perlwitz & Graf, 1995;Robock, 2000;Robock & Mao, 1992;Zambri & Robock, 2016). This mechanism has recently been called into question owing to the importance of internal variability (Polvani et al, 2019); we have not isolated the mechanism for this warming in CESM, but note that it is a statistically robust feature of CESM climate. This may be due to the larger magnitude of Last Millennium eruptions relative to Pinatubo; the response appears notably stronger than that noted in Polvani et al (2019) for the larger Tropical eruptions.…”

“…This mechanism has recently been called into question owing to the importance of internal variability (Polvani et al, 2019); we have not isolated the mechanism for this warming in CESM, but note that it is a statistically robust feature of CESM climate. This may be due to the larger magnitude of Last Millennium eruptions relative to Pinatubo; the response appears notably stronger than that noted in Polvani et al (2019) for the larger Tropical eruptions.…”

Scant evidence for a volcanically forced winter warming over Eurasia following the Krakatau eruption of August 1883

“…This begs the question: how could the later, better models fail to simulate the causal connection reported with the earlier, simpler models? A resolution to this conundrum was recently proposed by Polvani et al (2019), hereafter PBS19, who showed that much of the earlier literature had failed to properly account for the large internal variability associated with the stratospheric polar vortex and with the NAO. Focusing primarily on the 199 Pinatubo eruption, PBS19 analyzed three "large ensembles" of model runs, and showed how tiny initial perturbations of the same model forced with the same volcanic aerosols result vastly different winter temperature anomalies over Eurasia.…”

Uncertainty and the basis for confidence in solar geoengineering research

“…Impact of stratospheric heating on tropospheric and surface climate High for certain aerosol choices, such as sulfate; stratospheric-heating effects on tropospheric and surface climate are known but uncertainties regarding magnitude of responses and robustness across models 6,47,168 High; stratospheric heating may be responsible for many of the surface-climate side effects of SAG 47…”