Stratopause trends observed by satellite limb instruments

Dubé, Kimberlee; Bourassa, Adam; Tegtmeier, Susann; Zawada, Daniel; Degenstein, Douglas

doi:10.5194/egusphere-2025-5470

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https://doi.org/10.5194/egusphere-2025-5470

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18 Nov 2025

| 18 Nov 2025

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Stratopause trends observed by satellite limb instruments

Kimberlee Dubé, Adam Bourassa, Susann Tegtmeier, Daniel Zawada, and Douglas Degenstein

Abstract. The stratopause, the boundary between the stratosphere and the mesosphere, is projected to cool and drop in response to anthropogenic greenhouse gas (GHG) emissions. A lack of long-term observations with high vertical resolution at the stratopause has made it difficult to quantify trends in this region. We use observations from the Optical Spectrograph and InfraRed Imager System (OSIRIS) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument to assess the annual and inter-annual variability and to quantify trends in the stratopause temperature and height. The SABER and OSIRIS observations at the stratopause are highly correlated, and both show that the stratopause cooled by ~0.5–1 K per decade during 2005–2021. The observations also suggest that the tropical stratopause moved lower during this time period by 300–475 m per decade. The observational stratopause trends are consistent with trends from chemistry climate models simulations.

Received: 04 Nov 2025 – Discussion started: 18 Nov 2025

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Kimberlee Dubé, Adam Bourassa, Susann Tegtmeier, Daniel Zawada, and Douglas Degenstein

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RC1: 'Comment on egusphere-2025-5470', Anonymous Referee #1, 02 Jan 2026 reply

Review of "Stratopause trends observed by satellite limb instruments" by Dubé et al.

This manuscript presents trends in stratopause height and temperature for 2005-2021 from the OSIRIS instrument and compares them with results from SABER. Overall, the manuscript is well written, and the results are sound and supported by the data and methods. The results presented are relevant to the global context of the study of the impacts of climate change in the middle atmosphere, where data scarcity is often a problem. Therefore, I recommend the publication of the manuscript in ACP after some revisions.

My main concern is:
- In subsection 2.5, the authors explain their method to compute the stratopause location based on the SPARC/IO3/GAW report. They acknowledge that this is based on the properties of ozone in the middle atmosphere. However, they do not clarify why this method is necessary, or whether it is better or worse than the simple determination based on temperature, which is the method proposed by the WMO. If the authors prefer, for whatever reason, the method based on the MLR equation presented, the differences in both methods should be discussed in the context of their needs and benefits for the computation of stratopause trends presented (not related to ozone), and a comparison of the results with both methods should be presented.

Other issues to be addressed:
- In line 5 of the Introduction, the authors state that "little attention has been given to the stratosphere". Well, it can be true compared to the study of surface climate, but considerable research on the stratosphere has been developed over the last decades, with substantial efforts such as all the work developed in the framework of the APARC, something that some of the authors are very aware of. The statement is made after referring to the changes in the limiting layers, the troposphere, and the mesosphere, and can translate to the wrong view that nothing has been done on the stratosphere, which is not true. I think that for a balanced discussion, it is necessary to make a specific mention here to the work on stratospheric contraction by Pisoft et al. (2021), which the authors cite later in the text.

- The caption of Fig. 1 should better explain the contents presented in the plots. For example, what color corresponds to altitude, and which one to temperature? Also, I recommend keeping the vertical axis at a fixed range for altitude across all plots. In this way, the plots would directly translate at a glance if the stratopause is higher or lower.

- The first paragraph on page 8, after Fig. 4, is badly explained. The authors could try to rewrite it to better explain that they are comparing SABER data across different periods, the rationale for doing so, and why they are comparing them to OSIRIS.

- Could you elaborate on the reason for the positive trend of altitude observed in Fig. 4 at 50ºN? Is it instrumental? Real climate variability?

- Page 9, the last paragraph in section 3: It would be good to state how the obtained trend values compare to previous ones in the literature.

- In the conclusions section, the authors claim to present "the first comprehensive analysis of stratopause height trends between 60ºS and 60ºN". I think this is an unnecessary (it adds nothing scientifically relevant) and bold statement, and it should be removed. First, I do not think that an analysis limited to the tropics and extratropics, and covering only a 15-year period, can be considered "comprehensive". Also, in the manuscript, the authors refer to previous works that include results on trends in the stratopause.

- The Code and Data Availability section includes a link to GitHub for the LOTUS regression code. GitHub is not a frozen repository; it is not reliable for long-term availability, and it is recommended that its users create Zenodo repositories to store code used for scientific purposes and publication. Therefore, although ACP does not currently enforce a strict policy on it, I recommend that the authors deposit the LOTUS code in a Zenodo repository, which, among other benefits, will provide a DOI and a proper way to cite it.

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Citation: https://doi.org/10.5194/egusphere-2025-5470-RC1
RC2:
'Comment on egusphere-2025-5470', Anonymous Referee #2, 19 Jan 2026 reply
The manuscript Stratopause trends observed by satellite limb instruments by Dubé et al. examines trends in temperature and stratopause height over the 2005-2021 period using OSIRIS observations. The results are compared with SABER measurements and chemistry-climate model simulations. The authors apply a newly developed OSIRIS temperature retrieval, allowing a latitude-resolved observational assessment of stratopause height variability and trends. Comparison of these trends with chemistry-climate models provides useful context for model representations of the middle atmosphere. Together, these aspects make the study a valuable contribution to middle atmosphere research. I recommend publication in Atmospheric Chemistry and Physics after the comments below are addressed.
Page 1, line 15: The statement “little attention has been given to the stratosphere” is bit misleading, as it is immediately followed by a discussion of the importance of stratospheric temperature trends, which have been investigated in several earlier studies. The authors should clarify that the lack of attention refers specifically to observational studies of stratospheric height rather than to stratospheric trends more generally.
Page 4, line 101: The stratopause is identified as the local maximum using cubic interpolation of monthly zonal means. Although cubic interpolation is reasonable it can introduce small oscillations, especially near boundaries or when gradients are weak. Have the authors tested whether the diagnosed stratopause height and trends are sensitive to the choice of interpolation method? Brief discussion or comparison with different method would help demonstrate robustness.
Page 4, Section 2.5:
Please state in Section 2.5 that LOTUS model was used (and version), and list the source of proxies implemented.

It would be helpful to state explicitly that the MLR is applied to the deseasonalized stratopause height and temperature anomalies described in Section 3.1.

The MLR equation includes QBO and ENSO as regression terms with constant coefficients which means they are treated as season-independent. Since the impacts of QBO and ENSO are seasonally modulated could the authors clarify this choice and comment on how sensitive the stratopause height and temperature trends are to this assumption?

Page 8, section 3.3: The model results would benefit from a short comparison with the stratopause height trends reported in previous modelling studies cited in the manuscript, i.e., Pisoft et al. 2021.
Page 7&8, Figure 3&4: The authors note that the OSIRIS sampling pattern affects the SABER stratopause height trends, particularly near 50°. As differences at other latitudes are relatively small, could the authors provide additional insight into why this latitude appears to be especially sensitive to the sampling choice?
Page 9, Figure 5: In section 2.3 authots states that 11 CCMI-2022 models are used, but Figure 5 shows distributions based on only seven models. Please clarify the discrepancy.
Page 9, line 186: Figure 5 presents a multimodel distribution of stratopause trends but each model contributes only a single free-running ensemble member. As a result, the spread reflects both structural model differences and internal variability based on single realization. One sample only is not representative, especially with the large variability in the tropics.This represents a limitation that should be mentioned by the authors.
Page 9, line 197: The authors describe this study as the first comprehensive observational analysis of stratopause height trends, while also noting in the Introduction that some observational studies have previously examined stratopause heights. These statements needs some modification (e.g., emphasizing the latitude-resolved nature of the analysis or the use of the new OSIRIS temperature retrieval) to avoid overstating complete lack of work in this area.

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Citation: https://doi.org/10.5194/egusphere-2025-5470-RC2

Kimberlee Dubé, Adam Bourassa, Susann Tegtmeier, Daniel Zawada, and Douglas Degenstein

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Short summary

The stratopause, the boundary between the stratosphere and the mesosphere, is projected to cool and drop in response to anthropogenic greenhouse gas emissions. We use observations from two satellite instruments to assess the annual and inter-annual variability and to quantify trends in the stratopause temperature and height. We find that the stratopause cooled by ~0.5–1 K per decade and that the tropical stratopause moved lower by 300–475 m per decade during 2005–2021.


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