Global mean lower stratosphere temperatures rose abruptly in January 2020 reaching values not experienced since the early 1990s. Anomalously high lower stratospheric temperatures were recorded for 4 months at highly statistically significant levels. Here, we use a combination of satellite and surface-based remote sensing observations to derive a time-series of stratospheric biomass burning aerosol optical depths originating from intense SouthEastern Australian wildfires and use these aerosol optical depths in a state-of-the-art climate model. We show that the S.E. Australian wildfires are the cause of this lower stratospheric warming. We also investigate the radiatively-driven dynamical response to the observed stratospheric ozone perturbation and find a significant strengthening of the springtime Antarctic polar vortex suggesting that biomass burning aerosols play a significant role in the observed anomalous longevity of the ozone hole in 2020.

Abstract. In June 2019 the Raikoke volcano located in the Kuril Islands, northeast of Japan, erupted explosively and emitted approximately 1.5 Tg ± 0.2 Tg of SO2 and 0.4–1.8 Tg of ash into the upper troposphere and lower stratosphere. Volcanic ash is usually neglected in modelling stratospheric climate changes since larger particles have generally been considered to be short-lived in terms of their stratospheric lifetime. However, recent studies have shown that the coagulation of mixed particles with ash and sulfate is necessary to model the evolution of aerosol size distribution more accurately. We perform simulations using a nudged version of the UK Earth System Model (UKESM1) that includes a detailed 2-moment aerosol microphysical scheme for modelling the oxidation of sulfur dioxide (SO2) to sulfate aerosol and the detailed evolution of aerosol microphysics in the stratosphere. We compare the model with a wide range of observational data. The current observational network including satellites and surface based lidars and high-altitude sun-photometers means that smaller-scale eruptions such as Raikoke provide unprecedented detail of the evolution of volcanic plumes and processes, but there are significant differences in the evolution of the plume detected using the various satellite retrievals. These differences stem from fundamental differences in detection methods between e.g. lidar and limb-sounding measurement techniques and the associated differences in detection limits and the geographical areas where robust retrievals are possible. This study highlights that, despite the problems in developing robust and consistent observational constraints, the balance of evidence suggests that including ash in the model emission scheme provides a more accurate simulation of the evolution of the volcanic plume within UKESM1.

Abstract. In June 2019 the Raikoke volcano, located in the Kuril Islands northeast of the Japanese archipelago, erupted explosively and emitted approximately 1.5 Tg ± 0.2 Tg of SO2 and 0.4–1.8 Tg of ash into the upper troposphere and lower stratosphere. Volcanic ash is usually neglected in modelling stratospheric climate changes since larger particles have generally been considered to be short-lived particles in terms of their stratospheric lifetime. However, recent studies have shown that the coagulation of mixed particles with ash and sulfate is necessary to model the evolution of aerosol size distribution more accurately. We perform simulations using a nudged version of the UK Earth System Model (UKESM1) that includes a detailed two-moment aerosol microphysical scheme for modelling the oxidation of sulfur dioxide (SO2) to sulfate aerosol and the detailed evolution of aerosol microphysics in the stratosphere. We compare the model with a wide range of observational data. The current observational network, including satellites, surface-based lidars, and high-altitude sun photometers means that smaller-scale eruptions such as Raikoke provide unprecedented detail of the evolution of volcanic plumes and processes, but there are significant differences in the evolution of the plume detected using the various satellite retrievals. These differences stem from fundamental differences in detection methods between, e.g. lidar and limb-sounding measurement techniques and the associated differences in detection limits and the geographical areas where robust retrievals are possible. This study highlights that, despite the problems in developing robust and consistent observational constraints, the balance of evidence suggests that including ash in the model emission scheme provides a more accurate simulation of the evolution of the volcanic plume within UKESM1.

Global mean lower stratosphere temperatures rose abruptly in January 2020 reaching values not experienced since the early 1990s. Anomalous lower stratospheric temperatures were recorded for 4 months at highly statistically significant levels (p-values of 0.0004 to 0.02). While the warming event of 1991-1993 has been definitively attributed to absorption of sunlight by stratospheric sulfate from the eruption of Pinatubo, no candidate volcanic eruption for explaining the 2020 stratospheric heating exists. Here, we use a combination of satellite and surface-based remote sensing observations to derive a time-series of stratospheric biomass burning aerosol optical depths originating from intense S.E. Australian wildfires and apply these aerosol optical depths to a state-of-the-art climate model. We show beyond doubt that the S.E. Australian wildfires are the cause of this lower stratospheric warming with implications for stratospheric dynamics and stratospheric ozone should this type of event become more frequent in the future.

<p>Global mean lower stratosphere temperatures rose abruptly in January 2020 reaching values not experienced since the early 1990s. Anomalous lower stratospheric temperatures were recorded for 4 months at highly statistically significant levels (p-values of 0.0004 to 0.02). While the warming event of 1991-1993 has been definitively attributed to absorption of sunlight by stratospheric sulfate from the eruption of Pinatubo, no candidate volcanic eruption for explaining the 2020 stratospheric heating exists. Here, we use a combination of satellite and surface-based remote sensing observations to derive a time-series of stratospheric biomass burning aerosol optical depths originating from the intense 2019/20 S.E. Australian wildfires and apply these to a state-of-the-art climate model. We show beyond doubt that the biomass burning aerosols emitted by the S.E. Australian wildfires are the cause of this lower stratospheric warming, with implications for stratospheric dynamics and stratospheric ozone should this type of event become more frequent in the future.</p>