How the extreme 2019–2020 Australian wildfires affected global circulation and adjustments
Abstract. Wildfires are a significant source of absorbing aerosols in the atmosphere. Especially extreme fires, such as those during the 2019–2020 Australian wildfire season (Black Summer fires), can have considerable large-scale effects. In this context, the climate impact of extreme wildfires not only unfolds because of the emitted carbon dioxide, but also due to smoke aerosol released as high as the stratosphere. The overall aerosol effects depend on a variety of factors, such as the amount emitted, the injection height, and the composition of the burned material, and is therefore subject to considerable uncertainty. In the present study, we address the global impact caused by the exceptionally strong and high-reaching smoke emissions from the Australian wildfires using simulations with a global aerosol-climate model. We show that the absorption of solar radiation by the black carbon contained in the emitted smoke led to a shortwave radiative forcing of more than +5 W m−2 in the southern mid-latitudes of the lower stratosphere. Subsequent adjustment processes in the stratosphere slowed down the diabatically driven meridional circulation, thus redistributing the heating perturbation on a global scale. As a result of these stratospheric adjustments, a positive temperature perturbation developed in both hemispheres leading to additional longwave radiation emitted back to space. According to the model results, this adjustment occurred in the stratosphere within the first two months after the event. At the top of atmosphere (TOA), the net effective radiative forcing (ERF) in the southern hemisphere was initially dominated by the instantaneous positive radiative forcing of about +0.5 W m−2, for which the positive sign resulted mainly from the presence of clouds above the Southern Ocean. The longwave adjustments led to a compensation of the initially net positive TOA ERF, which is seen in the southern hemisphere, the tropics and the northern mid-latitudes. The changes in the lower stratosphere also affected the upper troposphere through a thermodynamic downward coupling mechanism in the model. Subsequently, increased temperatures were also obtained in the upper troposphere, causing a decrease in relative humidity, cirrus amount, and the ice water path. As a result, surface precipitation also decreased, which was accompanied by a weakening of the tropospheric circulation due to the given energetic constraints. In general, it appears that the radiative effects of smoke from single extreme wildfire events can lead to global impacts that affect the interplay of tropospheric and stratospheric cycles in complex ways. This emphasizes that future changes in extreme wildfires need to be included in projections of aerosol radiative forcing.
Fabian Senf et al.
Status: open (until 24 Mar 2023)
- RC1: 'Review of Senf et al.', Anonymous Referee #1, 08 Mar 2023 reply
Fabian Senf et al.
Dataset associated with Senf et al. (2023): "How the extreme 2019-2020 Australian wildfire affected global circulation and adjustments" https://doi.org/10.5281/zenodo.7568466
Jupyter Notebooks for Plotting and Analysis of the "Circulation Responses for WiFi-AUS" study, Submission Release https://doi.org/10.5281/zenodo.7575809
Fabian Senf et al.
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In this study, the authors use a climate model to simulate the large Australian wildfires of December 2019 and January 2020. They build on the study of Heinold et al. (2022), which looked at injection height and aerosol properties, to focus instead on rapid adjustments in the stratosphere and troposphere. They find that these adjustments are substantial and modulated by underlying clouds. Dynamical adjustments in the stratosphere are driven by different responses in the south and north branch of stratospheric circulation. In the troposphere, adjustments impact the water budget and may ultimately impact precipitation.
The paper is well written, and the description of the mechanisms of the response is generally convincing. Some aspects of the simulations and the discussion could however be clarified, as commented below. The revisions to address those comments should not require additional analyses, so should be minor revisions.
Line 41: The first instance of “radiative forcing” is ambiguous. Do you mean effective radiative forcing, or the radiative effect of the adjustments? From context I would say the latter, but that is not clear.
Line 53: Is pyrocumulonimbus formation always happening when wildfire aerosols are injected high in the atmosphere?
Line 76: I do not understand the use of “for” in this sentence. Isn’t it the other way around, that radiative coupling between troposphere and stratosphere allows the chain of effect to happen?
Lines 135-136 and 241-242: I am not sure why the FIRE experiments need to be rescaled and averaged. Aren’t the ensemble simulations enough to deal with statistical significance? And then check from the scaled FIRE experiments whether perturbations are indeed linear functions of the aerosol injection amounts?
Lines 157-160: This is an unusual ensemble initialization technique. Does that risk making the ensemble too narrow by applying only a small perturbation?
Lines 269-270: I understand how the mere presence of clouds modulate the SW forcing, but how does that work in terms of LW adjustments?
Lines 275-276: Just to confirm, that heating is directly due to absorption by the injected aerosols?
Lines 279-281: And the heating decay is driven by the decrease in aerosol mass (or absorption)?
Line 340: What range of pressures do you call the “upper troposphere” here?
Lines 357-358: I do not understand what that sentence is saying, and what it implies for the results that were just presented.
Line 371: I would be good to specify which variability modes are referred to here.
Line 51: in several kilometers -> several kilometers
Line 59: smoke -> of smoke
Line 129: impact -> the impact on
Line 132: analysis -> analyse