the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Mar 2025
The Impact of the Stratospheric Quasi-Biennial Oscillation on Arctic Polar Stratospheric Cloud Occurrence
Abstract. Polar stratospheric clouds (PSCs) play a critical role in stratospheric ozone depletion. Previous studies have shown that the quasi-biennial oscillation (QBO) influences the Arctic stratospheric polar vortex and ozone, yet no studies have deeply analyzed the impact of the QBO on Arctic PSC occurrence. This study analyzes this impact using CALIPSO observations from 2006 to 2021 and SLIMCAT simulations from 1979 to 2022. The results show that the winter PSC coverage area is significantly larger during the westerly QBO (WQBO) phase than during the easterly QBO (EQBO) phase, with a zonal asymmetry in PSC occurrence frequency anomalies. The QBO influences the temperature, water vapour (H2O), and nitric acid (HNO3) in the Arctic stratosphere, which are key factors affecting PSC formation. During the WQBO phase, Arctic stratospheric temperatures show negative anomalies, with the centre of this anomaly biased towards North America. In addition, H2O shows positive anomalies in the Arctic lower stratosphere, mainly due to the stronger polar vortex preventing the transport of high-moisture air at high latitudes to mid-latitudes, causing H2O to accumulate inside the polar vortex. HNO3 shows negative anomalies, primarily caused by denitrification through nitric acid trihydrate (NAT) sedimentation. Sensitivity analyses further indicate that the QBO-induced temperature anomalies are the main factor influencing PSC area, while the direct effect of H2O anomalies on PSCs is relatively small. The reduction of HNO3 mainly affects PSCs in late winter and early spring. This work implies that future changes in the QBO may influence ozone through its impact on PSCs.
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RC1: 'Comment on egusphere-2025-955', Anonymous Referee #1, 14 Apr 2025
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This manuscript investigates the impact of the stratospheric quasi-biennial oscillation (QBO) on the occurrence of Arctic polar stratospheric clouds (PSCs). This is an interesting topic, as PSCs play a critical role in ozone depletion. The QBO is a major mode of variability in the tropical stratosphere and its effects on the polar vortex and ozone have been studied, but its specific impact on PSCs has not been explored in depth. Therefore, this study fills a gap in the existing literature and is innovative. In this study, using the CALIPSO satellite observations and the SLIMCAT chemical transport model, the authors found that QBO can have a significant effect on the Arctic PSC, characterized by a clear zonal asymmetry. Moreover, the authors also found that QBO affects Arctic H2O and HNO3 in two different ways. These conclusions are based on observations and simulations and appear reasonable. Overall, this paper is well written. However, part of the analysis needs to be clarified and improved. I encourage the authors to revise it before publication.
General comments:
- You mentioned that during the EQBO phase, the distribution of the PSC area is skewed, with a peak near zero. This is understandable, as the polar vortex is generally weaker and the temperatures inside the vortex are higher during the EQBO phase, which is unfavorable for PSC formation. However, during the WQBO phase, the polar vortex is stronger and the temperatures inside the vortex are lower, which favors PSC formation. So why is the distribution of the PSC area during the WQBO phase not a skewed distribution with a higher peak, but rather a uniform distribution?
- I suggest dividing Section 3 into several subsections to improve the structural clarity and logical coherence of the manuscript, such as (1) The impact of QBO on PSCs; and (2) The key factors responsible for QBO’s impact.
- The current analysis relies on a single observational dataset (CALIPSO), which has a relatively limited temporal coverage. Are there other PSC observations covering different time periods that could be incorporated? I suggest the authors consider using additional observational datasets to further validate the key conclusion of the manuscript, that PSC occurrence is more frequent during WQBO phases compared to EQBO phases. This would help enhance the robustness and credibility of the findings.
Specific comments:
P1, L12: analyzes -> examines
P1, L13: there is -> there exists
P1, L27: I would suggest an explanation of Clx.
P1, L29: Delete the “.” after sunlight.
P2, L47: Add a comma after “HNO3”.
P2, L49: atmospheric-> atmosphere
P3, L69-L70: SSW->SSWs; which have -> which has
P4, L22: spans -> span
P5, L133: vertical range spanning from 316 to 0.00215 hPa -> vertical range of 316 hPa to 0.00215 hPa; Delete “an”.
P6, L163: surface density-> surface area density
P6, L187: Are the PSC coverage areas of SLIMCAT and CALIPSO daily? You need clarify.
P8, L215: on 500 K -> on the 500 K
P8, L221: How do you perform the composite analyses, was it WQBO-EQBO?
P8; L221-222: are removed -> were removed
P8, L226: level -> levels
P8, L227: differences -> differences in PSC area
P8, L228: Why are the differences in the SLIMCAT PSC area larger than those observed by CALIPSO? Does SLIMCAT reproduce the observed PSCs well?
P10, L254: zonal asymmetry of -> zonal asymmetry in
P10, L255: changes -> shifts
P10, L266: that in -> those in
P12, L291: during 1979–2022 -> for the period 1979–2022
P13, L314: Arctic -> the Arctic
P18, L409: strength-> the strength
Citation: https://doi.org/10.5194/egusphere-2025-955-RC1 -
RC2: 'Comment on egusphere-2025-955', Anonymous Referee #2, 27 Apr 2025
reply
General comments:
This manuscript, "The impact of the Stratospheric Quasi-Biennial Oscillation on Arctic Polar Stratospheric Occurrence" , by Li et al. investigates how the QBO modulates Arctic PSC formation and, by extension, ozone depletion processes. The paper addresses a relatively underexplored but important linkage between QBO phase and PSC variability using satellite observations and SLMICAT simulations, and separates the roles of temperature, H2O and HNO3, via sensitivity experiments. It is topical given the implications for Arctic ozone recovery.
Specific comments:
1. My major concern lies with the sensitivity experiments. While the use of composites to assess QBO-related anomalies is methodologically reasonable, it remains unclear whether the observed changes in temperature (T), water vapor (H₂O), and nitric acid (HNO₃) are causally induced by the QBO alone.
In reality, the anomalies during WQBO/EQBO phases are influenced by multiple factors, and not solely by the QBO. Thus, attributing the entire composite difference to QBO forcing may overestimate its impact. I recommend that the authors address how they isolate QBO-specific variability from other confounding influences (e.g., ENSO, solar variability, volcanic aerosols).
If a full isolation is not feasible with the current dataset, the authors should clearly acknowledge this limitation and discuss the potential implications on the interpretation of their sensitivity results.2. ENSO is a major interannual variability of the NH polar vortex. The authors show that the significance of the PSC occurrence anomalies during W/E-QBO is substantially reduced after excluding strong ENSO years (Fig. 5). However, it remains unclear how much of the observed differences are independently attributable to the QBO versus ENSO. Would the results change after regressing out the ENSO variability? I recommend that the authors clarify the influence of ENSO years quantitatively.
3. It is clear that the chemical transport model (CTM) used in this study does not include interactive radiative-dynamical feedbacks.
This is an important limitation, as feedbacks between radiation, temperature, and circulation could alter the stratospheric response to QBO forcing. I recommend the authors explicitly discuss how the absence of radiative-dynamical coupling may affect their results, particularly the sensitivity experiments and the interpretation of temperature-driven PSC variability.Technical corrections:
1. In Figures 3 and 4 (PSC occurrence anomalies), the color scales could be optimized to better show the large positive anomalies. Currently, they are all just dark red.
2. Abstract, Line 13: change "no studies have deeply analyzed" to "few studies have thoroughly analyzed" for a better scientific tone.
3. Line 157: "shown accurately simulate" missing "to".
4. Line 205: Repeating "samples"
5. Line 240: "variation overtime on the left panel". Typo: should be "over time"
6. Line 409: "as strength of the polar vortex increases" --> "as the strength of the polar vortex increases"
7. Line 320: "Figure. 7" --> "Figure 7".
Citation: https://doi.org/10.5194/egusphere-2025-955-RC2
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