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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- 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.