the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Mar 2026
A polar mesospheric precursor of sudden stratospheric warmings observed by the Aura Microwave Limb Sounder
Abstract. The observations of the Aura Microwave Limb Sounder (Aura/MLS) indicate the existence of a polar mesospheric precursor of major sudden stratospheric warmings (SSWs). The geopotential height data of Aura/MLS from 2004 to 2021 show that a burst of a 4 day-oscillation (zonally symmetric wave with zonal wavenumber zero) occurs at the stratopause and in the mesosphere at 80° N about 17 days before the central date of the SSW. The detection of this precursor in forecast analyses of weather and climate models has the potential to extend the subseasonal forecast of SSWs from three to five weeks.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-1111', Anonymous Referee #1, 07 Apr 2026
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RC2: 'Comment on egusphere-2026-1111', Anonymous Referee #2, 23 Apr 2026
This study uses temperature and geopotential height (GPH) products derived from the Aura/MLS instrument from 2004 to 2021, with the goal of identifying polar mesospheric precursory signals of sudden stratospheric warmings (SSW). In particular, the focus of the study is on oscillatory signals of the zonal-mean component of those fields. It is found that the amplitude of a 4-day period fluctuation of GPH is enhanced over the pole through the whole depth of the mesosphere around 20-15 days before the central date of SSWs (composite analysis of 10 SSWs). The signal is clearer in GPH than in temperature, although it is also present in the latter. It is concluded that this signal “has the potential to extend the subseasonal forecast of SSWs from three to five weeks”.
If confirmed, the identified signal is certainly relvant; however, a more thorough analysis and discussion are needed before the manuscript can be considered for publication.
Major comments:
1) The introduction falls short of adequately presenting the context and motivation for the study. What is currently known about tropospheric and stratospheric precursors of SSWs? What is known about zonal-mean fluctuations in the mesosphere and their statistical and physical connection to SSWs? What specific knowledge gap does this study aim to address?
2) The study relies on a composite of only 10 events. Without a significance test (e.g., Monte Carlo or bootstrapping), it is not possible to determine whether the 4-day fluctuation represents a robust signal. Additionally, I recommend that the author use data from (ideally multiple) reanalyses to assess whether the mesospheric signal is also present there. For example, the European (ERA5) and Japanesse (JRA-3Q) reanalyses would be particularly interesting to analyze since the ERA5 ingests MLS data (ozone and temperature, Hersbach et al. 2020), while JRA-3Q does not. Demonstrating the presence of this signal in reanalysis datasets, both during the MLS period and over the full record, would strengthen and complement the results. In this sense, there is an apparent contradiction between the results in this manuscript and those in Zhao et al. (2025). Figure 4 suggests that the 4-day fluctuation signal is present at levels above ~46 km, while Figure 1c of Zhao et al. (2025), using MERRA-2, does not display any 4-day signal at 47.3 km height.
3) The paper would enormously benefit from a hypothesis of the physical mechanism connecting the mesospheric signal and the development of SSWs. Without it, it is too soon to suggest that this signal is indeed a precursor of SSWs, and that its presence in weather forecasting models would imply a progress in subseasonal and seasonal forecasting (lines 4-5, 157-158). How do these fast zonal-mean fluctuations in the mesosphere affect the stratospheric wave driving that lead to SSWs? In order to assess the precursory role of this signal, it is also important to provide statistics of the 4-day fluctuation events: frequency of occurrence per winter, frequency of occurrence followed and not followed by SSWs, etc.
Other comments:
- Line 9: It is the zonal-mean zonal wind that reverses direction during SSWs, not the vortex.
- Line 12. Please be more specific regarding the tropospheric circulation changes after SSWs.
- Line 25-26. Please explain what “stratospheric preconditioning” refers to.
- Line 29-30. What is the physical justification for searching for SSW precursors in the polar mesosphere?
- Line 73. Please provide more details explaining why the measurement geometry of Aura/MLS is ideal for investigation of zonally symmetric waves in the middle atmosphere?
- Line 88-89. SSW central date has been already described in the previous paragraph.
- Line 95: Please define ∆ GPH: is it anomalies?
- Lines 96-97. Fig . 1 does not show pressure increases, but GPH.
- Line 102: “However, the interhemispheric effects of the SSWs seem to be too small in order to serve as a precursor of the SSW onset”. Please rephrase, I do not understand how the effects of SSW could serve as SSW precursory signals.
- Line 107-109: How many events present this peak? A statistical test is necessary.
- Figure 8 shows a stronger signal at periods of 10-12 days that at 4 days, in agreement with previous studies. However, it is stated that the former signal is not as well defined as the latter (lines 140-141). I would suggest to give more details on how this conclusion has been reached; it would seem necessary to a apply a statistical significance test in any case.
Citation: https://doi.org/10.5194/egusphere-2026-1111-RC2
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- 1
Using an analysis of Aura MLS data from 2004 to 2021, the paper identifies a mesospheric precursor for Sudden Stratospheric Warming events in the northern hemisphere. The study presents a valuable result which has potential for improving extended medium-range weather forecasts. The results are presented logically and the overall readability is good. However, I feel much more careful analysis is required to make the manuscript suitable for publication in Atmospheric Chemistry and Physics. Therefore, I recommend the manuscript to undergo major revision before it can be considered for publication.
In my opinion, the introduction section has scope for improvement. It would be appropriate to include a somewhat detailed discussion on the zonally symmetric oscillations especially during SSW events. It is also important since this aspect has not received much attention in contrast to stationary or traveling planetary wave enhancements during disturbed winter conditions.
Overall, the amount of new results presented in the manuscript is rather small. Several of the figures can be combined and further detailed analysis can be included. In my opinion, Figures 1-2, Figures 4-5 and Figures 6-7 could be combined.
Why are only major SSW events included in the analysis? Are minor SSW events accompanied by such precursors? If not, why are they absent? If so, what factors determine the occurrence of the precursor exclusive to major SSW events?
Another related question would be, are there S0 enhancements other than during SSW events? Has this been examined?
While the manuscript presents an interesting mesospheric precursor for SSW events, the interpretation is rather shallow and lacks detailed dynamical reasoning. By now, it is well established that zonally symmetric oscillations are primarily generated by non-linear wave-wave interactions. In the present study, there is no attempt made to identify the causative mechanism responsible for the early occurrence of 4-day oscillation in the mesosphere. I think it would be worthwhile to look at what wave modes contribute to the generation of the identified 4-day oscillation and, in particular, the relationship of the S0 precursor, if any, to SPW1 and SPW2 which are generally found to enhance during SSW events.
I would also urge the author to include a discussion on how the results presented in the manuscript are different from the ones presented already in previous literature, say, for instance in Pancheva et al (2007) and Zhao et al (2025).
Given the large variability that has been historically observed among SSW events, what makes the occurrence of the 4-day S0 approximately 20 days prior to major SSW events particularly robust?
I believe it would also be appropriate to have more details on the analysis techniques, such as for the bandpass filtering and the estimation of S0 amplitudes, in the manuscript text itself for improving the overall readability.
L 140: It is stated that the burst at longer periods is not well defined compared to the one at shorter periods. This has to be substantiated by estimating the confidence levels. It appears from Figure 8 that the oscillation at 10-12 days period is stronger than the one at 4-days period.
Technical comments:
L 107: change ‘controlled’ to something like ‘examined’.
Page 5, Paragraph 1: The figure number is incorrectly written. It should be Figure 5.
L 130: Insert ‘at’ after times.