the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Sep 2026
Parameterized orographic gravity wave forcing in the valve layer affects boreal stratospheric polar vortex in CMIP6
Abstract. The stratospheric polar vortex critically influences wintertime stratospheric circulation, as well as troposphere-stratosphere coupling and trace gas distribution. Reproducing correct statistics of stability and strength of the polar vortex is an important challenge for Earth system models as its variability affects regional climate predictability. The latest generation of the models are showing pronounced biases and intermodel spread in this regard. Here, we show how parameterized orographic gravity wave drag (OGWD) in climate models modulates planetary wave propagation, impacting SPV stability and morphology. We analyze simulations from 14 models produced within the Coupled Model Intercomparison Project Phase 6 (CMIP6). The orographic gravity wave drag varies between the models, depending on a parameterization scheme and the tuning of free parameters. We investigate how the intermodel differences in the parameterized gravity wave drag in the upper troposphere lower stratosphere relate to resolved wave activity and SPV characteristics. We find that stronger OGWD suppresses upward planetary wave propagation, leading to reduced wave breaking at the SPV edge and a more stable, broader vortex. By documenting the role of the orographic gravity wave drag for the boreal stratospheric polar vortex in the models, we highlight the potential of improvements or careful tuning of this parameterization for alleviating the climate model biases related to polar vortices.
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Status: open (until 24 Oct 2026)
- RC1: 'Comment on egusphere-2026-5319', Anonymous Referee #1, 06 Oct 2026 reply
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RC2: 'Comment on egusphere-2026-5319', Anonymous Referee #2, 13 Oct 2026
reply
Review of Parameterized orographic gravity wave forcing in the valve layer affects boreal stratospheric polar vortex in CMIP6, authored by Dominika Hájková, Aleš Kuchař, and Petr Šácha.
This manuscript analyzes the relation between intermodel differences in orographic gravity wave drag in the subtropical lower stratosphere and resolved wave activity and polar vortex variability, using CMIP6-AMIP simulations. It is shown that OGWD in the “valve” layer (30º-45ºN and 100-30 hPa, largely above the subtropical jet) has a positive correlation with the winds over the same region. Invoking the compensation mechanism between parametrized and resolved forcing (Cohen et al. 2014) and the mechanism by which the winds in that region control the meridional propagation of resolve waves (Sigmond and Scinocca 2010), it is shown that models with stronger OGWD have weaker resolved forcing in the extratropical stratosphere, and a generally stronger and wider vortex. The results also show a statistical connection between the intermodel spread in the climatological OGWD in the valve region and the frequency of days with weak vortex values measured by the vortex centroid and aspect ratio.
The paper is clearly written and easy to read. I have two major concerns regarding the novelty of the results and the direction of causality of the linear correlations. A list of comments is included below:
1) The novelty of the results needs to be emphasized. The connection between OGWD in the “valve” layer and the extratropical EP fluxes was published in Hájková and Šácha (2024), although with fewer models than those analyzed in this present study.
On the other hand, the results regarding the connection between the frequency of weak vortex days (in terms of vortex moments) and climatological OGWD could be better put into context. The given explanation involves the effects of the winds in the valve or neck region of the propagation of resolved wave activity, which appears similar to the results in Wu and Reichler (2020) and Hall et al. (2021). Also, in the Conclusions section the authors say that they could not find a connection between SSW frequency and OGWD. So how should Fig. 8 be interpreted then?
2) The direction of causality is not clear. Linear correlations throughout the manuscript are interpreted as the intermodel spread in OGWD being the cause of the spread in extratropical wave forcing and, as a consequence, in the variability of the vortex. However, this has not been demonstrated. There can be numerous factor affecting the strength of the subtropical jet and the winds in the neck region, which may affect the GW stress deposited in that region regardless of the tuning parameters of the GW schemes. Would it be possible to analyze differences in some of the tuning parameters and the associated impact on OGWD? If not, correlations should not be interpreted as having direction of causality in this study.
Other comments:
- Lines 59-60. Models differ in many other important details, such as the dynamical core and I guess most of the physics. Since potential impacts of horizontal resolution or the size of the model ensemble in the simulation of the vortex are not analyzed, I suggest not to mention them here.
- Lines 61-62. I understand that the standard deviation is computed across ensemble members. If so, only three or five members are used for each model, based on Table 1, and the standard deviatino is not a useful metric with such a small sample size. I would suggest to simply show the range of values across members as error bars.
- Line 66. I do not understand this sentence (i.e. what holds true?), please rephrase.
- Table 1. What is the purpose of showing information about the QBO in these models?
- Lines 117-118. What criterion has been followed to define the “valve” layer as the area enclosed by 30º-45ºN and 100-30 hPa? Is this based on any previous study? If the goal is to capture the region of strong OGWD in the upper flank of the subtropical jet, Fig. A1 suggests that by defining the bottom of the layer at 100 hPa a good part of the drag is not included. Also, the definition of this layer could be placed in the methodology section.
- Line 119. The direction of causality is not clear without firther analysis. I would change “indicating that OGWD in this region directly decelerates the westerly winds.” by “indicating that models with stronger OGWD in this region have weaker westerly winds”.
- Line 120. What is the meaning of an “absolute” correlation?
-Figure 3-5. I suggest to show all correlation contours to have a better view of the spatial pattern of the correlation.
- Lines 130-131. I would be useful to give more details on Hájková and Šácha (2024) results here, and compare or discuss in light of the present results.
- Lines 137. Please include the reference(s) for the mentioned previous findings.
Lines 179-180. This apparent agreement with Albers and Birner (2014) needs to be further explained. Albers and Birner showed OGWD composite around SSWs, and the anomalies started at 5 hPa and extented to upper levels. What is the agreement with the results of the present analysis?
- Line 191-192. The results reveal a connection between OGWD in the valve layer and EP flux divergence and variability of the vortex, but I do not believe the direction of causality has been resolved.
- Line 198. “OGWD-controlled index of refraction”. That the OGWD controls the index of refraction in this region has not been shown in this study.
- Line 201. Please include the reference(s) for the previous results.
- Lines 212-220. This paragraph is not clear. The text seems to be discussing on results that have not been shown. I would suggest to include some of those apparent negative results. For example, Fig. 8 seems to suggest that there could be a statistical connection between OGWD and SSW frequency, but this is not the case? How should Fig. 8 be interpreted?
- Lines 221-226. I do not fully understand this paragraph. If the QBO is believed to modulate the OGWD connection between the valve layer and the vortex, there is room in the manuscript to include the analysis. On the other hand, the QBO also affects the strength and position of the subtropical jet (e.g., Garfinkel and Hartmann, 2010), so one should be careful to interpret potential differences in the connection of OGWD and the vortex as being caused by the parametrized drag.
References:
Garfinkel, C. I. and Hartmann, D. L.: Influence of the quasi-biennial oscillation on the North Pacific and El Niño teleconnections, Journal of Geophysical Research: Atmospheres, 115, https://doi.org/10.1029/2010JD014181, 2010.
Hall, R. J., Mitchell, D. M., Seviour, W. J. M., and Wright, C. J.: Persistent Model Biases in the CMIP6 Representation of Stratospheric Polar Vortex Variability, Journal of Geophysical Research: Atmospheres, 126, e2021JD034759, https://doi.org/10.1029/2021JD034759, 2021.
Wu, Z. and Reichler, T.: Variations in the Frequency of Stratospheric Sudden Warmings in CMIP5 and CMIP6 and Possible Causes, Journal of Climate, 33, 10305–10320, https://doi.org/10.1175/JCLI-D-20-0104.1, 2020.
Citation: https://doi.org/10.5194/egusphere-2026-5319-RC2
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- 1
The manuscript ”Parameterized orographic gravity wave forcing in the valve layer affects boreal stratospheric polar vortex in CMIP6” by Hàjkovà et al. analyses the relationships between parameterized OGWD and stratospheric dynamics in 14 CMIP6 models. The authors demonstrate that a larger value of OGWD in the subtropical lower stratosphere (the valve layer) are associated with reduced upward and equatorward propagation of the planetary waves, increased wind speed at the equatorward flank of the polar night jet and reduced frequency of disturbed polar vortices, both displaced and elongated. The authors explain it by the compensation mechanism – changes in the parameterized OGWD typically lead to opposing changes in resolved wave drag.
The paper is written clearly; however, I am not sure whether the results are particularly novel. The effect of the compensation mechanism has previously been demonstrated for CMIP6 models for example by Hàjkovà and Šàcha (2024). Also, I am concerned that the causality of the relationship between OGWD and stratospheric winds is not sufficiently demonstrated. Therefore, I cannot recommend the manuscript for publication in its present form. Below are my specific comments.
Major comments:
1. It is not clear to me what is the novelty and significance of this manuscript. There is a discussion of how OGWD is important for SSW but the stated goal of the manuscript is to investigate “whether the intermodel differences in the boreal stratospheric SPV climatology and stability in CMIP6 can also be traced back to the differences in climatological parameterized OGWD.” I am not convinced it is much different from Hàjkovà and Šàcha (2024) who “has shown that the climatological intermodel differences of stratospheric dynamics in CMIP6 models are strongly correlated with the differences in the climatological parameterized OGWD in the lower stratosphere, especially in the valve layer.” I suggest the authors make it clearer what the results of this manuscript add to the existing knowledge and why it is important.
2. I find it somewhat disappointing that the authors do not try to connect their results to the properties of the parameterisation schemes used in the models. Ignoring this makes it difficult to establish causality - how certain is that the OGWD in the valve layers affects the properties of the zonal winds and resolved waves, and not the other way round? Gravity waves break in the lower stratosphere because of the strong wind gradients there. Could it be so that factors other than OGWD control wind gradients in the valve layer, and these wind gradients in turn affect gravity wave breaking and OGWD? I feel that you need to show that the properties of the GW parameterization schemes control the amount of OGWD in the valve regions. Without this the results are inconclusive. I suggest the authors link their results to the type of the parameterisation scheme used, and if possible to their properties, such as amount of gravity wave activity produced at the surface, criteria for wave breaking etc.
Other comments:
Figure 1: Figure 1 shows SSW statistics across the models but it is not discussed in the rest of the manuscript – how is SSW statistics related to OGWD? I think Figure 1 can be removed since its value for the manuscript is not clear.
L29: “the morphology of the edge of the SPV and the structure of potential vorticity can be used to describe the stability of the SPV”
I would say that the gradient of PV across the edge of the polar vortex can be used to quantify vortex stability. The current text does not sound clear to me, perhaps you need to rewrite it.
L31: I would add Charlton and Polvani (2007) to the reference list: Charlton, A., and L. Polvani, 2007: A new look at stratospheric sudden warmings. Part I: Climatology and modeling benchmarks. J. Climate, 20, 449–469.
L46: “the climatological intermodel differences of stratospheric dynamics in CMIP6 models are strongly correlated with the differences in the climatological parameterized OGWD in the lower stratosphere”
I think this needs to be explained better – what is correlated with what?
L56: What is the benefit of using AMIP experiments? Don’t you introduce unphysical biases due to the lack of atmosphere-ocean coupling?
L61: I do not think the sentence should start with “However,…”
L61 “the ensemble models” – do you mean “across ensemble members”?
L70: I suggest referring to DynVarMIP (Gerber and Manzini 2016), which made the dynamical outputs from CMIP6 possible:
Gerber, E. P. and Manzini, E.: The Dynamics and Variability Model Intercomparison Project (DynVarMIP) for CMIP6: assessing the stratosphere–troposphere system, Geosci. Model Dev., 9, 3413–3425, https://doi.org/10.5194/gmd-9-3413-2016, 2016.
L86-87: Do you calculate the vertical and meridional components of the EP-flux using data on pressure levels? How accurate these estimates are? You calculate the vertical gradient of potential temperature, which is probably not very accurate given the sparse vertical resolution of pressure levels. Are not the EP-flux components available from the CMIP6 database as a part of the DynVarMIP output?
L100: I am missing details of PV calculations. Do you calculate relative vorticity on isentropic levels? Are winds available on isentropic levels or do you interpolate winds from pressure levels onto isentropic levels? Do you calculate vertical gradient of potential temperature between pressure levels or between model levels? If it is between pressure levels then how accurate these vertical gradient estimates are? Is model data interpolated on the same horizontal grid?
L117: I suggest replacing “relationship” with “correlation” for clarity, because this is what is shown in Figure 3.
L117: To motivate the selection of the “valve layer” I suggest a latitude-pressure figure with multimodel mean OGWD. I assume the “valve layer” would show an enhanced values of OGWD.
L120: What do you mean by “absolute correlation” in this context?
L130: “In the CMIP6 models that relationship was documented by Hajkova and Šacha (2024) to govern the intermodel differences in the boreal stratospheric dynamics.”
This sentence should be reworded. It is not clear.
Figure 5a: The vertical component of the EP-flux in pressure coordinates, as given by Eq. 1, should be negative, because the potential temperature gradient with respect to pressure is negative. Is it negative in Figure 5a?
L212: “However, it has to be pointed out that the results presented here do not provide direct conclusions regarding SSWs and the intermodel differences in them. Naturally, we have tried to extend the analysis also towards this direction, but the results were not as clear as the ones presented in this manuscript.”
This text is not very informative. Can you explain what exactly is not clear?