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
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
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 -
RC3: 'Comment on egusphere-2025-955', Anonymous Referee #3, 30 Apr 2025
"The Impact of the Stratospheric Quasi-Biennial Oscillation on Arctic Polar Stratospheric Cloud Occurrence"
Peer review
The manuscript investigates the influence of the quasi-biennial oscillation (QBO) on the occurrence of Arctic polar stratospheric clouds (PSCs) using CALIPSO satellite observations (2006-2021) and SLIMCAT model simulations (1979-2022). The study shows that PSC coverage is significantly larger during the westerly QBO (WQBO) phase compared to the easterly QBO (EQBO) phase, with a zonally asymmetric anomaly pattern. The authors analyze the mechanisms driving these differences, attributing them primarily to QBO-induced temperature changes, with secondary contributions from water vapor and nitric acid variations. Sensitivity tests further emphasize the dominant role of temperature.
The topic is highly relevant for understanding polar stratospheric chemistry and ozone depletion processes under future climate scenarios. The combined use of long-term satellite observations and chemical transport modeling is a strong methodological approach. The manuscript is generally well-structured, clearly written, and supported by comprehensive references. The sensitivity analysis provides valuable insight into the relative contributions of temperature, H2O, and HNO3.
I really enjoyed reading this work, and I believe it definitely deserves to be published. The manuscript is scientifically sound, well-presented, and makes a valuable contribution to the understanding of stratospheric processes. However, he authors might consider expanding it along the lines of the suggestions listed below.
General comments:
The CALIPSO dataset (16 years) is relatively short for robust statistical analysis, as noted by the authors. While SLIMCAT compensates with a longer timeframe, the observational validation remains limited. The authors may discuss potential biases or uncertainties arising from the short observational record and how SLIMCAT’s longer simulations mitigate this. Moreover, statistically significant differences related to the QBO phase appear over regions hosting important ground-based lidar stations with long-term data records. Have the authors tried to verify their findings by also making use of these datasets and/or referring to published results?
The SLIMCAT model uses simplified PSC schemes (e.g., fixed number densities for NAT/ice particles). How might this affect the representation of denitrification/dehydration processes? The authors may speculate on how more sophisticated microphysics (e.g., size-resolved NAT sedimentation) would alter the conclusions.
The authors dismiss BD circulation as the driver of H₂O anomalies but do not fully explore alternative mechanisms. As instance, the may clarify whether the H₂O accumulation is purely due to vortex isolation or if other processes (e.g., local tropopause temperature and permeability changes) may contribute.
While the paper mentions that PSC changes may affect ozone, the connection is not quantified. How much could QBO-driven PSC variability contribute to interannual ozone loss differences? The author may add a speculative estimate based on some proxy, as the volume of PSCs below a certain temperature threshold and subsequent ozone loss during spring.
The conclusion notes QBO disruptions under climate change but does not explore how projected QBO changes (e.g., weaker amplitude) might alter PSC trends, this may be briefly discuss this in the "Discussion and Conclusions" section.
Specific comments:
- Figure 1: It could be beneficial to add to the data points a colour coding the ENSO phase, to visually show what is the possible impact on PSC area. Moreover, it would help to quantify the slopes and R2 values of the regression lines for CALIPSO and SLIMCAT.
- Table 2: The description of "W_less HNO₃" and "E_more HNO₃" could be clearer. Specify that adding HNO₃ during WQBO reduces PSCs (due to less denitrification).
- Line 20-22: It would be beneficial to clarify early that H2O anomalies have a small direct but possibly significant indirect impact via radiative cooling. See Forster and Shine (2002) for quantitative estimates.
- Section 2.2: A brief summary of previous validations of SLIMCAT for PSC representation would strengthen confidence. Relevant references may include Feng et al. (2021) and Li et al. (2024).
- Figure 4 vs. Figure 5: Please explicitly state that the ENSO exclusion does not alter the primary conclusions, but does reduce significance areas due to reduced sample size.
- Page 20 (Sensitivity analyses): Consider emphasizing that temperature effects dominate mainly because the Arctic stratospheric temperatures are often near PSC thresholds, making them highly sensitive (Pitts et al., 2018).
- Minor: Typos like "SLICMAT" instead of "SLIMCAT" (Page 4) should be corrected.Citation: https://doi.org/10.5194/egusphere-2025-955-RC3
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