the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Sep 2024
The Joint Effect of Mid-latitude Winds and the Westerly Quasi-Biennial Oscillation Phase on the Antarctic Stratospheric Polar Vortex and Ozone
Abstract. The quasi-biennial oscillation (QBO) dynamically interacts with the extratropical atmosphere. However, the relationship between the QBO in austral winter and the Antarctic stratospheric polar vortex in spring remains unclear. Here, we proposed a joint predictor involving the QBO for the Antarctic polar vortex and ozone in austral spring. During the westerly QBO phase (WQBO), positive anomalies in the zonal-mean zonal wind at 20° S−40° S in the upper stratosphere in July, named as the extratropical positive mode, can lead to a stronger Antarctic stratospheric polar vortex and lower ozone concentration in November, with correlations reaching 0.75 and 0.60. The mechanism is summarized as follows: the positive extratropical mode triggers a secondary circulation, which further alters the environmental condition for wave propagation in the stratosphere, pushing the positive anomalous zonal-mean zonal wind towards the pole. While during the easterly QBO phase (EQBO), the correlation of the extratropical mode and the strength of polar vortex is only 0.1. Due to stronger upward motions in the tropics, which opposes the secondary circulation caused by the extratropical mode, the EQBO cannot sustain the positive anomalous zonal-mean zonal wind until November. Our results highlight that the extratropical mode during WQBO could serve as a reliable predictor of the Antarctic stratospheric polar vortex and Antarctic ozone hole with a five-month time lag.
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RC1: 'Comment on egusphere-2024-2669', Anonymous Referee #1, 03 Oct 2024
This paper establishes a robust connection between the QBO signal in winter and the stratospheric polar vortex in spring with a time-lag of five months. Their results indicated that zonal-mean zonal winds in the mid-latitude upper stratosphere play a crucial role in facilitating the tropic-polar connection in Southern Hemisphere. Specifically, during WQBO, the positive zonal-mean zonal winds anomalies at 20°S−40°S in the upper stratosphere in July can lead to a stronger Antarctic stratospheric polar vortex and lower ozone concentrations in November. This finding on predicting the Antarctic polar vortex and ozone in spring could be of broad interest, and the authors have presented a comprehensive body of work on it. Overall, this paper presents an interesting and convincing and well-written analysis. I think this study would be of interest to the readership Atmospheric Chemistry and Physics and recommend its publication after addressing the comments listed below.
General comments:
- In the introduction, the authors mentioned that most researches focus on the QBO-polar connection in the Northern Hemisphere (NH), where the upward-propagating planetary waves are strong. A more detailed explanation of the underlying mechanisms, along with a discussion of whether this connection in the NH is robust, would strengthen the introduction. I think this would offer readers more useful information about why there is less attention on the QBO-polar connection in the Southern Hemisphere.
- The study defines the QBO phase using zonal-mean zonal wind at 20 hPa. However, I noticed that most researches define the QBO as being in its easterly (westerly) phase using the zonal mean zonal wind at 50 hPa. It would be very instructive to show why defining the QBO phase at 20 hPa is reasonable for establishing the QBO-polar connection in the SH.
- The correlation between the winter extratropical mode and the polar vortex reaches 0.75 during WQBO. It seems the winter extratropical mode could serve as a good predictor of the spring Antarctic stratospheric polar vortex. Additionally, in Figure 3c, a positive extratropical mode in July usually corresponds to a strong polar vortex. Would it be possible that the author uses these relationships to ‘predict’ the strength of the Antarctic polar vortex from 1950 to 1979 using the ERA5 reanalysis?
Specific comments:
Line 25 Please specify how QBO modify the upward-propagating planetary waves.
Line 51 The QBO is only considered as a predictor of the Arctic stratospheric polar vortex and the near-surface climate in the NH. Please clarify this point.
Line 74 Monthly to monthly
Line 77 what is the vertical range being considered?
Line 96 Why not use the traditional refraction index to diagnose the wave-propagation in the stratosphere?
Line 102 The word size of the equation (7) is too large. Please correct.
Line 136 I note that in the CESM, the stratospheric conditions are nudged to the JRA-55 reanalysis. Why is the model forced using different types of reanalysis data?
Figure 1b It seems no apparent connection between the QBO in July and the Antarctic stratospheric polar vortex in austral spring. However, in the introduction, how previous studies have shown that the QBO can modulate the Antarctic polar vortex during austral spring?
Figures 3a and 3b The first paired mode explains 98.2% of the total variance. How is it possible for the first mode to account for nearly all of the total variance?
Figure 4 ‘horizontal component unit: 107 kg s−2; vertical component unit: 105 kg s−2’. Please correct the units.
Line 249 W* to w*
Line 306 ‘25 ensembles’ to 20 ensembles
Line 381 positive anomalies in the zonal-mean zonal wind à positive zonal-mean zonal wind anomalies
Line 400 ‘EQBO has a greater influence on the stratospheric polar vortex than the WQBO’ Please explain.
Citation: https://doi.org/10.5194/egusphere-2024-2669-RC1 -
RC2: 'Comment on egusphere-2024-2669', Anonymous Referee #2, 17 Oct 2024
Comments on “The Joint Effect of Mid-latitude Winds and the Westerly Quasi-Biennial Oscillation Phase on the Antarctic Stratospheric Polar Vortex and Ozone” by Wang et al.
Summary
Using the reanalysis and model simulations, this study analyzes the possible impact of the subtropical stratospheric wind mode on the QBO-Southern Hemisphere stratospheric polar vortex. The authors find that the westerly winds in the subtropics can increase the correlation between the QBO and the polar vortex wind, while this relation is weak during the easterly winds in the subtropics. This finding is very interesting and important for seasonal forecast in the Arctic circulation and ozone. Therefore, I suggest to publish this paper with the following comments considered.
Specific comments
- This study finds that the subtropical westerlies can increase the relationship between the WQBO and the strong polar vortex. However, this relationship is asymmetric. I meant that if the subtropical easterlies can increase the relationship between the EQBO and the weak polar vortex.
- This study states that few studies focus on the relationship between QBO and the SH stratospheric polar vortex, and that the relation between them might be absent and non-existent. However, previous studies have reported the possible impact of QBO on the SH stratospheric polar vortex. The maximized response of the SH stratosphere to the QBO appears in boreal spring, not in winter (Rao et al. 2023a, b).
- This study used model simulation by nudging methods. However, the method does not describe the necessity of the experiment. All figure captions should also mention if the results are based on ERA5 or ERA5. If the stratosphere is nudging, what can we learn from the experiments with t this study still focusing on the stratospheric variability?
- L15: anomalous zonal-mean zonal wind => zonal-mean zonal wind anomalies
- L21: References are required for this sentence.
- L22: wind … descend => wind … descends
- L31: Arctic ozone => Arctic ozone and water vapor (https://doi.org/10.1016/j.wace.2023.100627)
- L33: It depends on the timescale concerned. On the interannual timescale, the chemical processes are weaker than transport.
- L36: less thermal contrast => weaker thermal contrast
- L38, L41-44: Two recent publications mentioned the impact of QBO on the Southern Hemisphere stratosphere.
- L48: It is too certain to say so.
- L50: The QBO of which period varies irregularly in the range from 17 to 38 months is considered as a reliable predictor => The QBO period varies irregularly in the range from 17 to 38 months, which is considered as a reliable predictor
- L70: Here it is worth mentioning that the QBO index at 20 hPa is used, different from studies for Northern Hemisphere.
- L74: Monthly => monthly
- L95: mean stream function => mean mass stream function
- L100: phi is geopotential or potential height? Please check.
- L107, 111: dm, what is the meaning of m?
- L120: ’’ denotes meridional average? I did not see ’’ in all equations.
- L125: 1.9*2.5 => 0.95*1.25
- L148: several months => Please tell the specific number.
- L151: influence => influences
- L158: This relation indeed exists, but the maximum response of the polar vortex to QBO is in austral spring.
- L166: measure => depict
- L168: and polar => and polar regions.
- L239: negative anomalous temperature => negative temperature anomalies
- L255: exhibit a … anomalies => remove a
- L276: as a results => as a result
- L356: moths => months?
- L379: It is unknow which figure are based on the sensitivity experiments.
- L386: anomalous … wind => … wind anomalies
Citation: https://doi.org/10.5194/egusphere-2024-2669-RC2 -
CC1: 'Comment on egusphere-2024-2669', Yousuke Yamashita, 20 Oct 2024
This paper describes the relationship between the SH mid-latitudes wind and the Antarctic polar vortex under the westerly QBO condition. The relationship is derived with the statistical method. However, there is a lack of proper discussion about the dynamical linkage between them partially, as listed below. In addition, there is a lack of proper discussion about the previous studies. Thus, I have a concern about the present manuscript.
p.2, L35-40: Baldwin and Dunkerton (1998) also mentioned “the zonal-mean zonal wind in the lower stratosphere decelerates more rapidly from September to October during the EQBO than during the WQBO”.
p.2, L40, 45: The relationship between the below two sentences are not clear.
“the extratropical response to the QBO in late-winter SH can be interpreted as a modulation of the final warming by the QBO”
“the timing of the Antarctic stratospheric polar vortex’s response to the QBO signal remains unclear”
p.2, L45: “Wang et al. (2022) examine the QBO signals in stratospheric ozone.” The implication of the Garcia and Solomon (1987) will be also mentioned in this context.
p.3 L70: There is no reasonable explanation for why 20 hPa was chosen as a single pressure level.
p.5, L130: In this nudging method, the zonal asymmetric component is relaxed to the observation as well as the zonal component. The wave propagation (zonal asymmetric component) is affected in this method.
p.6, L135: The MERRA-2 is used in this study (L60), while the atmospheric conditions of the model are nudged to the JRA-55. The equatorial 40-hPa data is lacked in JRA-55 (it is important to the QBO disruption in 2016.)
p.6, L145: “consistently reaching their maximum in November”: This result will be compared to that of Baldwin and Dunkerton (1998).
p.6, L155: The “direct impact” is not clear. The results only show that the correlation between the QBO index and the zonal wind at 60S, 70hPa.
p.6, L165: What is the “winter extratropical mode”? Is it formed through the secondary circulation of the QBO?
p.6, L180: “Thus, we can conclude that the winter extratropical mode, in conjunction with the WQBO, is closely linked to the spring Antarctic polar vortex and ozone.”: This probably responds of the upper stratospheric vortex shift to the upper in July, together with the poleward and downward movement from winter to summer (Yamashita et al. 2018, DOI:10.2151/jmsj.2018-057).
p.9, L210: “We further revealed that the positive anomalies in zonal-mean zonal wind can be reasonably explained by the E-P flux divergence anomalies and their poleward shift from July to November.”: The causality of the zonal-mean zonal wind is explained in this context, while the E-P flux divergence term is generally balanced with the fv* tern in the monthly timescales. Thus, the causality is difficult and there is only a consistency between the TEM terms.
p.11, L240: “favours heterogeneous chemistry”: Is the PSC area also increased in this time?
p.11, L260: “maintained by the downward motion”: The w* northward of 10 hPa, 30S is not clear and the relationship between the w* and temperature is not seen.
p.12, L265: “induced by the extratropical mode”: I suppose that the B-D circulation difference is induced by the difference in the E-P flux divergence in the TEM terms.
p.13, L275-280: “which dominates the wave refractive index and results in anomalous E-P flux divergence around 50S in the upper stratosphere in July”: Please indicate the contribution of the PV anomaly to the total wave refractive index.
Figure 7: The unit of the du/dt must be the same as that of E-P flux divergence. In addition, the fv* term will be also discussed in this context.
p.18, L385: “pushing the positive anomalous zonal-mean zonal wind towards the pole”: The causality is not derived from this analysis, as mentioned above.
p.18, L395: “we propose an upper stratospheric pathway for the QBO’s impact on the stratospheric polar vortex”: The comparison to the Yamashita et al. (2018)’s results will be needed in this context.
Other comments
Figure 2(c): “Sepetmber” -> “September”
Figure 3: The “solid blue line” is not found.
Citation: https://doi.org/10.5194/egusphere-2024-2669-CC1
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