Revisiting the volcanic forcing of high-latitude Northern Hemisphere eruptions

Abstract

High-latitude explosive volcanic eruptions can cause substantial cooling on a hemispheric scale. However, our current understanding of these events remains limited. In this study, we employ a whole atmosphere chemistry-climate model with prognostic stratospheric aerosols to simulate Northern Hemisphere (NH) high-latitude explosive volcanic eruptions of similar magnitude to the 1991 eruption of Mt. Pinatubo. Our simulations reveal that the lifetime of sulphur dioxide and the growth of volcanic sulphate aerosols are strongly influenced by the initial state of the NH polar vortex. The stability of the polar vortex during the first weeks after an eruption determines the latitudinal dispersion of the injected gases within the stratosphere. Consequently, interannual variability of the polar vortex stability introduces a variability in the modelled cumulative radiative forcing of more than 20 %. We also test the aerosol evolution’s sensitivity to co-injection of sulphur and halogens, injection season, and injection altitude, and show how processes related to aerosol formation, lifetime, and growth impact the resulting radiative forcing. Notably, several of the sensitivities are of similar magnitude to the variability stemming from initial conditions, highlighting the significant influence of atmospheric variability. Finally, we compare the modelled volcanic sulphate deposition over the Greenland ice sheet with the relationship assumed in reconstructions of past NH extratropical eruptions. Our analysis yields an estimated Greenland transfer function for NH extratropical eruptions of 0.44 × 10⁹ km² . If applied to ice core data, this value would produce volcanic stratospheric sulphur injections from NH extratopical eruptions 23 % smaller than in currently used volcanic forcing reconstructions. Furthermore, the uncertainty attached to the Greenland transfer function for NH extratropical eruptions, which propagates into the estimate of volcanic stratospheric sulphur injection, needs to be at least doubled to account for both atmospheric variability and unknown eruption source parameters. Our results offer insights into the complex dynamics shaping the impacts of both historical and future NH high-latitude eruptions. Furthermore, they underscore the need for more accurate representation of these events in existing volcanic forcing reconstructions, with the aim of improving our understanding of their climatic impacts.

Initial conditions control transport of volcanic volatiles, forcing and impacts

“…Notably, the order of the ensemble members differs between tropical and NH extratropical eruptions, underlining different factors controlling the transport of volcanic volatiles following tropical and NH extratropical eruptions. However, in both this study and Fuglestvedt et al (2023), the ensembles exhibit less northern poleward transport of SO 2 in the first to fourth members, as compared to the fifth and sixth members. The former four members (H1 to H4) correspond to El Niño or neutral ENSO initial states, while the latter two members (H5 and H6) correspond to La Niña states (Table 1).…”

“…The control of initial conditions on the transport of volcanic volatiles is also evident after NH extratropical eruptions, in this case primarily controlled by the initial polar vortex state. The northward transport and lifetime of SO 2 increases from members 1 to 6 (Fuglestvedt et al, 2023) with increasing initial polar vortex stability. Notably, the order of the ensemble members differs between tropical and NH extratropical eruptions, underlining different factors controlling the transport of volcanic volatiles following tropical and NH extratropical eruptions.…”

“…The control of initial conditions on aerosol and halogen transport and subsequent volcanic forcing is not only evident for tropical eruptions but also for NH extratropical eruptions, although with a different dominating factor. For NH extratropical eruptions at 64°N in January, the initial polar vortex conditions play a dominant role in the transport and evolution of SO 2 as further investigated in our related paper by Fuglestvedt et al (2023).…”

“…We have ordered the baseline experiments H1 to H6 according to less to more northward transport of volcanic volatiles, respectively. Note that this labeling differs from Fuglestvedt et al (2023), who ordered their ensemble members based on the stability of the polar vortex. During this boreal winter season, wave breaking is stronger in the winter hemisphere leading to strong mixing and distribution toward the NH high latitudes, which is evident especially in H6.…”

“…We also conduct sensitivity tests with sulfur injections only and in eruption season of July. In a related study by Fuglestvedt et al (2023), we analyze the effects of initial polar vortex conditions on the aerosol evolution, volcanic forcing and deposition of NH extratropical eruption at 64°N. Here in this study, we aim to understand the following questions: What impacts do initial conditions have on transport of volcanic volatiles and volcanic forcing after tropical eruptions co-injecting sulfur and halogens?…”