the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Aug 2026
Arctic Polar Vortex Strengthening on Seasonal Scales After Extreme Solar Proton Events: Quantitative Evidence for an Ozone–Dynamics Feedback
Abstract. Energetic particle precipitation (EPP) has long been known to influence the polar middle atmosphere. Extreme solar proton events (SPEs), the most intense type of EPP, are expected to influence the Arctic polar vortex (APV); however, their impacts and underlying pathways remain poorly understood. Here, we examine APV variability and the associated stratospheric processes following the two most extreme late-October SPEs, using the ERA5 reanalysis data. Both events were followed by persistent ozone depletion, cooling of the polar stratosphere, and sustained strengthening of the APV from autumn into late winter and spring. The ozone, temperature, and APV anomalies evolve coherently over time, with the APV transitioning from a relatively weak background state to a strongly enhanced state with respect to the climatological distribution. Thermodynamic budget diagnostics reveal that the temperature anomalies were dominated by dynamical cooling rather than by direct radiative cooling from ozone depletion. This dynamical cooling was accompanied by a tendency for downward planetary wave refraction at high latitudes and associated changes in wave–mean flow interactions. Collectively, these results provide quantitative evidence that extreme SPEs can influence the seasonal APV variability through coupled ozone and dynamical processes.
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Status: open (until 17 Sep 2026)
- RC1: 'Comment on egusphere-2026-4234', Anonymous Referee #1, 29 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-4234', Anonymous Referee #2, 16 Sep 2026
reply
The manuscript uses ERA5 reanalysis and (to some extend) WACCM-D-SD model simulations to assess ozone-radiative forcing-dynamics feedbacks during extended time periods following two SPEs. The topic itself is very interesting but the study represents some fundamental problems that needs addressing and, overall, (consider the extent of published research in the field) I find little new presented here.
Major issues:
1. Data
The authors are investigating two time periods: Oct-May 1989-1990 and Oct-May 2003-2004. There are extensive observations of the chemical state of the atmosphere following the Halloween SPEs, including NOx, HOx and ozone. None of the results presented here were contrasted to literature for checking the ERA5 ozone to observations. In this case though, it seems likely some of those observations were assimilated into ERA5. This leads to the bigger problem with the earlier SPE and ERA5. What is the ozone data quality for polar winter in ERA5 for 1989-1990? The Halloween SPEs were famously the first time we had polar night ozone observations during and following a major SPE. There are model simulations of the 1989 SPE impacts, but we do not have extensive ozone observations.
I also found many of the figures far too small to be able to see details in the manuscript pdf.
2. SPEs vs EPP
- The SPEs are not taking place in isolation in the real world. The enhanced levels of energetic electron precipitation after the Halloween events are well documented. Indeed, the enhanced levels of NOx and the following ozone depletion in early 2004 has been extensive attributed to the elevated stratopause event that followed the reformation of the polar vortex after the SSW and the enhanced levels of EEP.
- The authors attribute the missing NOy in their WACCM-D-SD simulations to missing high energy protons, but the main reason is more likely the missing EEP component.
- When looking at such an extended time period after the SPEs, it is not possible to attribute the ERA5 signals to the SPEs, the total EPP (=SPE + EEP) needs to be taken into account. This would need to be addressed, but I do not see how this is possible within the current scope.
- The fundamental importance is that the chemical-dynamical coupling mechanisms are not different for the two processes. We can see this clearly in Figure 5, which is large a reproduction of Figure 2 of Szelag et al. Nature Comms., 2022. The present manuscript adds planetary waves to the picture, but this is not new and has been demonstrated in the number of publications referenced in the manuscript.
- This really is the biggest problem: The manuscript is looking at EPP impacts, but with data limited to only two cases where we have strong SPE present. The ozone-dynamics feedback is not limited to SPEs and with the EPP viewpoint, the work seems to reproduce what is already known. It won't be possible to separate SPEs and EPP with reanalysis. This would need to be done with a model-only study (and not specified dynamics).
3. Literature and previous work
- The literature on SPE chemical impacts is extensive, particularly for the two most famous SPEs. Seems necessary to cite these in this particular context. The citations should go beyond the Nesse et al. 2026 review paper.
- Considering above point 2: There has been extensive work using both re-analysis data and model simulations to look at the heating effects from EPP-related ozone changes. Some important works include Meaner and Schmidt, ACP, 2018, who calculate radiative heating rates, separating between heating and cooling, and Lu et al. JGR, 2008; 2013, who already pointed to evidence of the dominance of dynamical heating over chemical heating in the springtime signals.
References:
Lu, H., M. A. Clilverd, A. Seppälä, and L. L. Hood (2008), Geomagnetic perturbations on stratospheric circulation in late winter and spring, J. Geophys. Res., 113, D16106, doi:10.1029/2007JD008915.
Lu, H., C. Franzke, O. Martius, M. J. Jarvis, and T. Phillips (2013), Solar wind dynamic pressure effect on planetary wave propagation and synoptic-scale Rossby wave breaking, J. Geophys. Res. Atmos., 118, 4476–4493, doi:10.1002/jgrd.50374.
Meraner, K. and Schmidt, H. (2018), Climate impact of idealized winter polar mesospheric and stratospheric ozone losses as caused by energetic particle precipitation, Atmos. Chem. Phys., 18, 1079–1089, doi:10.5194/acp-18-1079-2018.
Citation: https://doi.org/10.5194/egusphere-2026-4234-RC2
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- 1
The paper utilizes ERA5 reanalysis data, employs multiple diagnostic tools, and combines radiative transfer calculations with the Transformed Eulerian Mean (TEM) framework to systematically analyze the evolution of atmospheric responses from autumn to spring following two extreme SPEs in 1989 and 2003. To some extent, this fills the gap in understanding the impact of extreme SPEs on Arctic stratospheric dynamics. The proposed conceptual model of ozone–dynamics positive feedback has certain theoretical value. However, there are still some issues that need to be considered and improved.
Major Concerns:
Sample Size Issue
This study is based on only two extreme events, which is the most fundamental limitation. Although the authors acknowledge this limitation, both the title and conclusions use relatively definitive wording (e.g., “Quantitative Evidence”), which is inconsistent with the statistical basis of only two cases. I suggested weakening the causal language in the title, for example, use “Case Studies” or “Preliminary Evidence.” The authors should also discuss the uncertainty of the results more explicitly in the conclusions. Consider adding a brief analysis of additional moderate-intensity SPE events to assess the dose–response relationship.
Causality versus Correlation.
The core argument of the paper is that extreme SPEs influence the APV through the ozone–dynamics feedback pathway, but the analysis is essentially based on the temporal evolution of atmospheric anomalies following two events. Due to the lack of control experiments, it is impossible to rule out the possibility that internal atmospheric variability (such as natural variability in ocean-generated planetary wave activity) could produce similar anomalous impacts. The authors are advised to discuss these alternative explanations more systematically and assess their potential impact on the conclusions.
Physical Mechanisms Need Further Clarification
Although the authors discuss that SPEs affect stratospheric temperature not only through radiative processes but also through influencing planetary waves and thereby modulating the Brewer–Dobson circulation, which in turn affects temperature changes through adiabatic processes, it remains unclear why planetary waves would change. Is it possible that SPEs affect ozone, which then alters longwave radiative processes, thereby influencing the temperature and buoyancy frequency near the stratopause? Temperature changes would then affect the background wind field through thermal wind balance, thereby modulating vertical wind shear. Both buoyancy frequency and vertical wind shear would then affect the wave refractive index, consequently altering planetary waves.
Specific Comments:
L156: The unit “ppmm” should be “ppmv.”
L166: It is generally enhanced planetary waves that cause the westerlies to reverse to easterlies during SSWs. This sentence appears to suggest that the background wind field causes the enhancement of planetary waves; please pay attention to the expression of causality.
Figure 1: Over which altitude range in the polar region are the planetary waves shown in panels i–j calculated? Are they from the troposphere or the stratosphere? This is not clearly stated in the figure caption. Moreover, it is puzzling that the SSW in late December of the 2003/2004 case was clearly stronger than the SSW in February of the 1989/1990 case (as the authors also state in the main text), but why is the upward-propagating planetary wave activity in the second case weaker than in the first?
Figure 2a,b: Why does ozone depletion near the stratopause (around 1 hPa) lead to enhanced radiative heating?
Figure 3: The panels E3 and E4 appear somewhat odd. Figures 3k and 3l both show significant wave flux convergence from the mesosphere to the stratosphere. According to wave–mean flow interaction, this should correspond to significant westerly deceleration. Indeed, Figures 3o and 3p also show significant dynamical warming at these altitudes, but why are the temperatures in E3 and E4 (particularly stratospheric temperatures) negative anomalies? E4 is in the spring phase, when shortwave radiative cooling due to ozone loss is enhanced. Although the authors emphasize that dynamically induced temperature changes are consistently stronger than ozone-induced radiative processes, this does not appear to be the case from this figure. Generally, Arctic spring stratospheric temperature changes are jointly dominated by ozone-induced radiative processes and the resulting dynamical feedback processes.