the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 May 2025
Influence of Major Sudden Stratospheric Warming With Elevated Stratopause on the Hydroxyl in the Polar Middle Atmosphere
Abstract. Based on the specified dynamics simulation of Whole Atmosphere Community Climate Model with ionosphere/thermosphere extension (SD-WACCM-X), the composite response of polar Hydroxyl (OH) layer in the mesosphere and lower thermosphere (MLT) to the Arctic major sudden stratospheric warming (SSW) events with elevated stratopause (ES) during 2004–2023 is investigated. A total of ten ES-SSW events are systematically analyzed. Before the onset of ES-SSW events, the OH concentration climatologically peaks at 7.4×10-9 mol/mol near ~82.4 km. During the stratospheric warming phase, relative to the climatology, the peak height of OH layer undergoes a distinct upward displacement reaching ~85.9 km accompanied by a reduction in the OH concentration to 2.9×10-9 mol/mol. This shift is closely linked to an ~11 % and ~90.8 % reduction in mesospheric temperature and atomic oxygen, respectively, due to enhanced upward residual circulation. During the elevated stratopause phase, the peak height of OH layer experiences a pronounced downward shift to ~80.6 km with a maximum in OH concentration to 6.8×10-9 mol/mol. This phase is characterized by ~3.7 % and ~137.3 % enhancements in mesospheric temperature and atomic oxygen concentrations, respectively, which is driven by intensified downward residual circulation. Further analysis suggests that OH concentration variations are positively correlated to mesospheric temperature anomalies and atomic oxygen redistribution induced by vertical transport, which is attributed to the significant influence of ES-SSW on gravity wave drag (GWs) in the mesosphere.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-2016', Anonymous Referee #1, 17 Jul 2025
- AC1: 'Reply on RC1', Hu Jin, 16 Sep 2025
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RC2: 'Comment on egusphere-2025-2016', Anonymous Referee #2, 07 Aug 2025
Comments on "Influence of Major Sudden Stratospheric Warming With Elevated Stratopause on the Hydroxyl in the Polar Middle Atmosphere" by Hu etal.
In the current manuscript, Hu etal have investigated the impact of major sudden stratospheric warming events in the northern hemisphere high latitude on the OH changes in the
mesosphere and lower thermosphere. Their work is mainly focusing on the model results (a specific-dynamic version of WACCM-X, SD-WACCM-X) with detailed middle-atmosphere chemistry and D-region chemistry (MAD).
They used the MERRA2 reanalysis data to constrain the modelled atmospheric temperature/winds below 50 km from 2004-2023, which can be well reproduce the major SSW events over this period.
The authors have made clear introduction and why this work is so important for the atmospheric research community.
First, they choose SSW event in January 2009 to start with and confirmed the elevated stratopause (ES_ and associated temperature/winds/OH changes (which has been studied/revealed by other studies before)
in SD-WACCM-X results.Then they found The sharp OH changes are well correlated with meriodinal temperature gradient for the ten SSW events over the past 20 years (2004-2023).
Figure 3 shows the climatological OH seasonal variations in the polar regions, which is nothing new here (this seems to be consistent with WACCM and other global atmospheric chemistry models, except the authors found some large discrepancies between SD-WACCM-X and other models).
Then the authors have made composite analysis of temperature, zonal mean wind, OH and its anomaly during the ES-SSW events (before 15 days and after 2 months)
to see the general pattern (and they thought it does not matter if SSW occurred in December, January or February, their Table1). This Figure 4 may cause some cautions
because we can still see there are still some large daily variation in their Figures 1-3.
Then they did similar for the atomic oxygen and temperature (Figure 5) to link the OH changes through the chemistry since OH can be depleted by reacting with O in the MLT region.
Figures 6-9 shows the OH, O, T and residual circulation for different ES-SSW stages. Finally the authors investigated the causes by the gravity waves.
Overall, the manuscript has very clear message and it reads well.
The results and discussions are useful. Personally I enjoy reading it. However, I am not sure if the conclusion would be changed if their current method changes (including the comments mentioned above).
I have noticed some differences/inconsistent of major SSW occurring date with published data at:https://csl.noaa.gov/groups/csl8/sswcompendium/majorevents.html
although the definition of SSW is slightly different to Charlton and Polvani (2007)
(https://journals.ametsoc.org/view/journals/clim/20/3/jcli3996.1.xml).For example, I did not see SSW events on 25 December 2018 (in Table 1) from the table in the above NOAA webpage (which showed it should be 2 January 2019 from MERRA2).
I expect ES-SSW onset date is later than SSW occurring date since the breaking of planetary waves propagate upward.
However, the Table 1 has earlier onset ES-SSW date than SSW.
Not sure if this caused by SD-WACCM-X simulation or the authors made some mistakes.
If the basic onset day in Table 1 is wrong, obviously, all other figures (except Figure 3), results and discussion need to be changed.
For the residual circulation, why SD-WACCM looks different to WACCM (for example,
Anne Smith's Figure at JGR(2011):please see the link:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2011JD016083
Maybe this is from WACCM6 physics and it is quite different to WACCM4 using in Smith et al. (2011).
Not sure why the authors choose the most expensive SD-WACCM6-X (MAD) model (FXSD) since their work is mainly focusing in the MLT below 95 km?
It seems to me this is the first long term SD-WACCM6-X simulation, it is unclear if this is from cesm2_1_3 or other version.
So additional model validation is required.
Even if this is from cesm2_1_3, what the inputdata (emissions etc) are used beyond January 2015?
It requires some clarification and make major changes before moving forward.Citation: https://doi.org/10.5194/egusphere-2025-2016-RC2 - AC2: 'Reply on RC2', Hu Jin, 16 Sep 2025
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I have reviewed the manuscript ‘Influence of Major Sudden Stratospheric Warming With Elevated Stratopause on the Hydroxyl in the Polar Middle Atmosphere’ by Hu et al.. The authors present the composite response of the polar OH layer in the MLT to 10 ES-SSW events during 2004-2023 using SD-WACCM-X. During the stratospheric warming phase, the peak height of OH layer undergoes a distinct upward displacement, which is closely synchronized with changes in mesospheric temperature, atomic oxygen concentrations, and the vertical component of the residual circulation in the MLT region. GWs play a pivotal role, as the enhanced downward (upward) motion driven by GWs leads to mesospheric warming (cooling) and a corresponding increase (decrease) in atomic oxygen, which in turn facilitates an increase (decrease) in OH concentration. The manuscript is well-written, and the methodology is solid. I have a few comments. My only concern is that this study is mainly based on model results. Please see my major comment for details.
Major comment:
WACCM model has deficiencies in its treatment of GW forcing, which could lead to an underestimation of OH variation during ES-SSWs. I suggest that the authors validate the model results using SABER OH observations—perhaps for at least one SSW event.
Minor Comments:
1) Lines 190-192: ‘In Figure 1a, the meridional temperature gradient (T[80−90]−T[60-70] K) is denoted by the pink solid line, and the height of the ES is indicated by the green dashed line.’ How is the height of the ES defined? A sentence in the manuscript would be helpful.
2) Figures 7-9 show latitude-altitude cross-sections of composite relative variation. How is the relative variation defined?