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the Creative Commons Attribution 4.0 License.
| 11 Aug 2026
MJO unlocks the stratospheric polar vortex influence on winter extreme precipitation over South China
Abstract. Winter extreme precipitation over South China (SC) exerts profound impacts, yet its stratospheric drivers remain limited understood, with a long-standing puzzle of the statistically weak linkage between the stratospheric polar vortex (SPV) and SC rainfall extremes. Here, using observational analyses and CMIP6 historical simulations, we demonstrate that a Eurasia-shifted SPV (ESSPV) can increase SC winter extreme precipitation, but this modulation is strictly phase-locked to the Madden–Julian Oscillation (MJO), occurring predominantly during MJO phases 2–5. Two synergistic pathways explain this phenomenon. First, the ESSPV excites a Rossby wave across Eurasia, inducing a low-level northeasterly anomaly north of SC that strengthens local moisture convergence. Second, the ESSPV couples with the westerly quasi-biennial oscillation (WQBO), which amplifies the MJO-associated anomalous anticyclone over the western North Pacific and enhances low-latitude moisture transport toward SC. CMIP6 multi-model analyses further confirm that the fidelity of MJO phase 2–5-associated western North Pacific anticyclone determines a model's ability to reproduce the observed linkage, firmly establishing the MJO as the indispensable gateway linking the SPV to SC extreme precipitation.
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Status: open (until 22 Sep 2026)
- RC1: 'Comment on egusphere-2026-4214', Anonymous Referee #1, 19 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-4214', Anonymous Referee #2, 07 Sep 2026
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Review of “MJO unlocks the stratospheric polar vortex influence on winter extreme precipitation over South China” by Zhou et al.
This paper argues that winter extreme precipitation over South China is influenced by the nonlinear interaction between two individual drivers: a Eurasian-shifted stratospheric polar vortex (EESPV) and the Madden–Julian Oscillation (MJO). The authors find that when the vortex is shifted toward Eurasia and the MJO is in phases 2–5, the probability of extreme precipitation over South China is enhanced. They then diagnose the processes associated with this relationship and suggest that it arises from changes in the local subtropical high and enhanced moisture convergence.
I find the results interesting and potentially valuable. Understanding how multiple large-scale drivers interact to influence regional weather and climate extremes is an important problem. However, I found the manuscript difficult to follow in places, and I think the authors frequently overstate what can be concluded from their analysis. My main concerns are outlined below.
General comments
1. Presentation and interpretation
1a. The writing style is unusually dense and, in my view, does not fit well with the style of this journal. The manuscript reads as though it was originally prepared for a high-impact journal and subsequently submitted here without substantial adaptation. This makes the paper unnecessarily difficult to follow.
For example, the paper contains only five figures, one of which is a schematic accompanied by a scatter plot that is not clearly explained or discussed. At the same time, the discussion of the proposed “mechanism” is very terse (e.g., lines 261–276 and 287–300), making it difficult to understand how the authors arrive at their interpretation. Adding intermediate figures that explicitly walk the reader through the different steps of the analysis would substantially improve the manuscript.
1b. The tone of the manuscript is also too strong in several places, with conclusions that are more definitive than the analysis appears to support. For example, the first sentence of the abstract describes a “long-standing puzzle of the statistically weak linkage between the stratospheric polar vortex (SPV) and SC rainfall extremes.” I am not convinced that this constitutes a “long-standing puzzle.” It is certainly a worthwhile research question, but this wording overstates the motivation. Similarly, the use of “firmly established” on line 231 is stronger than warranted by the results.
1c. More fundamentally, I do not think the analysis presented here constitutes a mechanistic explanation of the interaction between the MJO and SPV. Rather, it provides a diagnosis showing that the precipitation anomalies are dynamically and hydrologically consistent with changes in the subtropical circulation and moisture convergence. This is useful, but it is different from establishing a mechanism for how the MJO and SPV interact nonlinearly.
For example, the paper does not address several important physical questions: Why should an EESPV influence precipitation over South China in the first place? Why is the MJO-related wavetrain extending into the subtropics stronger during EESPV events? The authors do not necessarily need to answer these questions for the paper to be publishable, but the terminology should be adjusted accordingly. In particular, I would avoid referring to the diagnostics as establishing a “mechanism” (e.g., line 248) and instead describe them as evidence consistent with a particular dynamical pathway.
The authors also invoke a downward arching of QBO winds from the subtropics. However, this effect is barely evident during December and January in reanalysis data (Garfinkel et al., 2026). Is the proposed pathway also evident when the analysis is restricted to DJF, or is it primarily present in February?
As an aside, Ning et al. (2026) recently examined how shifts of the vortex toward or away from Eurasia can affect precipitation using mechanistic model experiments. This study appears highly relevant to the interpretation presented here and should be discussed.
There is also a concern regarding multiple testing. The authors effectively construct 16 different composites, and it is not surprising that some subsets show statistically significant differences. The authors should provide a stronger physical motivation for considering all of these combinations rather than presenting them primarily as an exploratory search. In addition, some of these composites must be based on relatively small sample sizes, given that reliable MJO observations are available only since approximately the 1970s. I think the authors should include an appropriate field-significance or multiple-testing correction that accounts for the number of hypotheses being tested.
2. Definition of EESPV and EWSPV
The authors classify entire winters as EESPV or EWSPV. However, the morphology of the polar vortex can change substantially within a winter, and it is entirely possible for one month to be characterized by an EESPV configuration while the following month is characterized by an EWSPV configuration. Would the results be more robust or physically meaningful if the SPV classification were performed at the monthly rather than seasonal timescale?
Given that the MJO is already defined using daily data, using monthly EESPV/EWSPV classifications should not require a major change to the methodology. At minimum, I think the authors should test whether their main results are sensitive to this choice.
3. The MJO–precipitation relationship needs to be examined more thoroughly
The authors first establish a relationship between SPV configuration and South China precipitation and then investigate how the MJO modifies this relationship. However, what is largely missing is an equivalent analysis of the MJO–South China extreme precipitation relationship independent of the stratosphere.
The top row of Figure 3 provides some analysis of this possibility, but the treatment is substantially less extensive than the analysis of the SPV–precipitation relationship, which occupies essentially all of Figure 1. As a result, the paper currently feels somewhat imbalanced. A more thorough characterization of the MJO–precipitation relationship independent of SPV state would help establish what is genuinely nonlinear about the combined response.
4. Model anomaly calculation
Line 108: Why are the model anomalies calculated relative to the observed/ERA5 climatology rather than the climatology of each individual model? The methodology as currently described seems likely to introduce or amplify the influence of model mean-state biases. Unless there is a specific physical reason for using the observational climatology, I think the authors should calculate anomalies relative to each model's own climatology and explain the implications of this choice.
5. Journal suitability
Overall, I think the paper is more naturally suited to Weather and Climate Dynamics than Atmospheric Chemistry and Physics. However, I will leave the final decision on journal suitability to the editor.
Specific comments
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Lines 165–167: Several papers have discussed the downward arching of QBO winds into the subtropics in considerable detail (e.g., Garfinkel and Hartmann, 2011; Gray et al., 2018). These studies should be cited here.
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Lines 261–264: I do not understand the dynamical argument being made here. In particular, why should two adjacent ridges induce upward motion? Please explain the dynamical reasoning more explicitly, ideally with an accompanying figure.
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Line 286: The word “validate” is inappropriate here. CMIP6 models cannot be used to “validate” an observed relationship. They can provide additional samples and therefore help address sampling uncertainty, but the models have their own biases, making it difficult to use them to independently validate a specific observed relationship.
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Lines 310–316: The MME1 and MME2 bars appear almost identical. Are the differences between them statistically significant and robust? Please provide quantitative evidence rather than relying on the visual differences in the figure.
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Lines 321–325: Is the difference in the subtropical high and moisture convergence between MME1 and MME2 essentially imposed by the way the two groups are defined? If so, the logic of this section appears somewhat circular. The authors should clarify what is being independently demonstrated here versus what follows directly from the definition of the two groups.
References mentioned:
Garfinkel, C. I., and Hartmann, D. L.: The influence of the quasi-biennial oscillation on the troposphere in winter in a hierarchy of models. Part I: Simplified dry GCMs, J. Atmos. Sci., 68, 1273–1289, https://doi.org/10.1175/2011JAS3665.1
Gray, L. J., Anstey, J. A., Kawatani, Y., Lu, H., Osprey, S. M., and Schenzinger, V.: Surface impacts of the Quasi Biennial Oscillation, Atmos. Chem. Phys., 18, 8227–8247, https://doi.org/10.5194/acp-18-8227-2018
Garfinkel, C. I., Avisar, D., Osprey, S. M., Smith, D., Rao, J., and Wright, J. S.: Revisiting the surface impacts of the QBO in the Large Ensemble Single Forcing MIP simulations: are teleconnections still too weak?, Weather Clim. Dynam., 7, 1133–1152, https://doi.org/10.5194/wcd-7-1133-2026.
Ning, W., Garfinkel, C. I., Cohen, J., White, I. P., and Rao, J.: The tropospheric response to zonally asymmetric momentum torques: implications for the downward response to wave reflection and SSW events, Weather Clim. Dynam., 7, 277–295, https://doi.org/10.5194/wcd-7-277-2026
Citation: https://doi.org/10.5194/egusphere-2026-4214-RC2 -
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Comments to Authors:
This study investigates the modulatory effect of the Eurasia-shifted stratospheric polar vortex (ESSPV) on winter extreme precipitation over South China (SC), with a particular focus on the phase-locking role of the Madden–Julian Oscillation (MJO). The authors identify that the statistically weak SPV–extreme precipitation linkage at the seasonal mean scale is strongly amplified during MJO phases 2–5, and propose two synergistic dynamical pathways (high-latitude Rossby wave train and low-latitude moisture transport amplified by the westerly quasi-biennial oscillation). The CMIP6 multi-model analysis further confirms that the fidelity of the MJO-associated western North Pacific anticyclone determines the model’s ability to reproduce the observed linkage.
Overall, this is a well-designed, logically coherent study with clear scientific novelty. It addresses the long-standing puzzle of why the SPV shows a weak and unstable statistical relationship with extreme precipitation in southern China, and provides a physically meaningful explanation from the perspective of intraseasonal–interannual scale interaction. The manuscript uses multiple datasets and model simulations to cross-validate the conclusions, and the findings have important implications for subseasonal-to-seasonal prediction of winter extreme precipitation in East Asia. I recommend publication in ACP after minor-to-moderate revisions to address the following comments.
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Minor Comments