Summary

This study investigates the linkages between mid-latitude heatwave frequency and atmospheric dynamics. They use an idealized dry dynamic model with thermal perturbations to get different storm track climatologies. They find a heatwave frequency minimum polewards of the mid-latitude storm track; and show that shifts in the position of the storm track are a good predictor for heatwave characteristics. They compare these idealized results with trends in ERA5 reanalysis in the Southern Hemisphere, and infer that the observed poleward shift in the jet may have reduced the persistence of heatwaves.

Overall, the analysis and visuals are compelling, and make some interesting arguments for linkages of heatwaves and dynamics. However, I feel a major revision is necessary to address some key issues I have, particularly in relation to the SH reanalysis component of the paper.

Major Comments

• A comparison with the observed SH storm track trends is attempted, but even though the main contributor to these trends is polar stratospheric cooling via ozone depletion, no polar stratospheric cooling experiment is performed. Though it’s true that the tropical warming experiment also results in a poleward shifted jet, the dynamics involved are somewhat different (e.g., see Butler et al. 2010), and there is little discussion of this in the text. In addition, I find the ERA5 SH analysis confusing because as shown in, e.g., Banerjee et al. 2020, since 2000 there has been some indication of ozone recovery in the atmospheric circulation, so that if the early period is subtracted from the later as is done in Figure 6 and 7b, it should reflect not the ozone depletion period but rather the ozone recovery signal (e.g., a weakening and equatorward shift of the jet, see Banejee et al. 2020 Fig 2c,f,i). Though this signal has weakened with more recent years added on (e.g. Shaw et al. 2024), it still is not clear why these two periods were selected, rather than just the ozone depletion period (e.g., year 2000 minus 1979) or perhaps the first period minus the second (opposite of what is shown). Visually at least, it also appears in Figure 7a that the noted shift in SH heatwave frequency minima was larger between 1979 and 2000 compared to 2000-2020, which would line up with this slowdown in the SH poleward jet shift. But then interpreting Fig 6b and 7b is not quite clear since it’s a difference between these two periods.
• The background information/motivation in lines 25-48 and 303-305 could be improved/reorganized. It seems odd, for example, to delve into Arctic amplification and details of waviness metrics, and not mention the counteracting effect of upper tropical warming and polar stratospheric cooling until the second paragraph, which feels like an after-thought. Also, the phrasing of lines 28-30 is not exactly true in terms of the “weakened westerly jet stream” part; if only Arctic amplification were occurring, this may be valid, but the fact of the matter is, there are other effects and as such what we’ve actually seen is weak poleward shifts of the jet (Woollings et al. 2023). There is also lack of discussion of seasonality of these effects, which seems important to mention; for example, while there seems to be more solid evidence of a sea ice effect on the circulation in the NH summer, this effect does not seem detectable in the winter. The ozone depletion signal is also going to be primarily important in austral spring/summer. Overall, I suggest trying to rewrite so that the contributing “forcings” are all discussed first, and then explaining how these are expected to (or already have) driven trends in the NH and SH separately.
• While the authors acknowledge this issue, it is difficult to disentangle the thermodynamic effects (and also, oceanic effects) in the ERA5 reanalysis. The authors do not try to detrend the ERA5 temperature data, but I do wonder if there would be some way to try to isolate the dynamic changes from the thermodynamic changes better. Perhaps by removing the global-mean from each year and grid box? I also wonder whether the minimum in heatwave frequency shows up so nicely in the SH in 7a because this is where the Southern Ocean takes up much of the heat, and its colocation with the storm track is thus coincidental.

Minor Comments

Line 31: This is the first mention of “waviness” which may confuse unfamiliar readers. Could an explanation of waviness be linked somewhere in line 29 to the weakened jet stream? (as a general note, maybe the in-depth description of waviness metrics lines 31-36 should instead be moved to the Methods section?)

Line 34-35: I don’t understand what is meant by “render thermodynamic feedbacks more effective”; could more explanation be provided?

Line 42-43: “supported historically”- it’s unclear what this is supposed to mean. Do you mean, the trends due to ozone depletion are in the same direction as those due to greenhouse gases, and so they are additive? Or do you mean, the modelled poleward trends due to tropical upper tropospheric warming are in the same direction as observed trends due to ozone depletion?

Line 45: See Major comment, but “more than twenty years” is a strange period to discuss here (especially when the references themselves are from 20+ years ago- but surely those references were looking at SAM trends over the decades before they were published?). It should be mentioned that these trends have weakened in the last twenty years due to ozone recovery, and the Banerjee et al. 2020 paper should be cited.

Line 46-48: This jumps back to the NH; see Major comment about reordering/reorganizing these paragraphs.

Line 47: Another reference here could be Lee et al. 2019 (https://www.nature.com/articles/s41586-019-1465-z)

Line 49: Can you explain why (e.g., because the signal will start to exceed the noise)?

Line 56-57 (also lines 257-258): are these results in idealized models, or realistic? For both hemispheres, or just one or the other?

Line 72: Can you comment about whether things like the SAM and associated jet shift in response to a forcing can be captured well by this configuration (e.g., Gerber et al. 2008, Chan and Plumb 2009)? Are there implications for your results?

Line 81: It would be nice to either add to this table or have a separate table that summarizes/quantifies some of the key changes in jet shift, phase speed, etc across all the experiments.

Line 97 (also, lines 306, 317): Is EKE really the best metric for “waviness”, per se? I think of waviness as contemporaneous regions of high and low pressure around a longitude circle, but EKE is highest in the storm tracks. In Geen et al. (2023), this is not even mentioned as a waviness metric. Alternatively, maybe “waviness” isn’t exactly the right word for what you’re evaluating here?

Line 134: This sensitivity to altitude is shown in Butler et al. (2010) so could refer to that here

Line 139-141: could the fact that the Tropical heating exp (poleward shifted jet) has a larger magnitude strengthening than the Arctic amplification (equatorward shifted jet) weakening magnitude be due in part to the stronger magnitude imposed heating in the Tropical heating case compared to the Arctic amplification case?

Line 143-145: It’s confusing whether this line is a conclusion of the line before it, or a new thought to be demonstrated in the analysis below.

Line 164-165: zonal wavenumber on the other hand only strongly changes in the mid-latitudes in the sensitivity experiments (Fig 2e)

Line 178: perhaps should be mentioned that the opposite is true from ~25-40N

Figure 2: would be useful to mention in caption or on figure that “positive” values are “eastward”

Lines 223-224: So, are the results here truly dynamically relevant, or do they arise in part or as a side effect of the threshold definition chosen here? Also, I think a few more lines could be added re: the question posed at the top of this page, which is, is there a limit on hot day persistence due to certain characteristics of the atmospheric circulation, and why? I think lines 210-225 are trying to get at that, but it would be nice to re-summarize what this means. For example, lines 226 says “In the previous section, we explored a mechanism…” but it was not exactly clear to me what the mechanism was.

Line 239-240: This quantification is useful, but one thing that is unclear is if it’s location specific (e.g., is it only true at the heatwave minimum latitude?). Or does it more generally mean, a poleward shifted jet means less frequent mid-latitude heatwaves overall?

Line 241-242: parts of this sentence are seemingly contradictory (“in the absence of thermodynamic feedbacks” and “in response to anthropogenic climate change”- which is going to be dominated by thermodynamics). Also, don’t you really mean in response to latitudinal shifts in the jet (which could be caused by climate change or ozone depletion or even natural forcings)?

Line 246: where is this shown? Fig 2d?

Line 260: describe based on idealized results in previous section what you expect to see in terms of heatwave frequency and hot day persistence

Line 282: by “does not meet the expectation from our model”- what would the expectation be? Could this be better quantified, as it was for the model results?

Line 300-301: where is it noted above? Fig 7c? At the heatwave density minimum or generally? This statement also seems to contradict lines 66-67, which argues that there is a role. Overall, could the conclusions in lines 298-301 be better explained? I’m not sure I follow how the Hovmoller plot means that the dynamics reduce heatwave frequency.

Line 329-332: It should again be mentioned the main reason for this poleward shift, which is only partly increasing greenhouse gases and mostly ozone depletion- which was not an experiment tested here and which might have different effects on the jet than tropical warming (since the dynamics involved are different).

Technical Edits

Line 8: “hot temperature extremes” is repetitive, can just say “hot extremes” (same in line 35 and elsewhere)

Line 16, 17: instead of “general”  “global”

Line 22: “block formation” – not clear what is meant here, though I think you mean atmospheric blocking. Could change to “atmospheric high-pressure ‘blocking’ formation”

Line 29: “at height” – specify at which height

Line 37: specify “NH” in front of “near-surface temperature gradient”

Line 39: here, specify “circulation response to tropical upper tropospheric warming”

Line 47: here, specify that you mean “on the atmospheric jet streams” or some equivalent in terms of the dominance of tropical warming over Arctic amplification

Line 59: move comma outside the parenthesis

Line 112: put a space between “forcing” and parenthesis

Line 127: change “modify” to “modified”

Line 132: is this exp 8?

Line 133: change to “indicating, to a large extent, a linear circulation…”

Line 138: here 5N is mentioned but in line 82 is what said the shift was 4N

Line 204: “lower” -> “slower”

Line 208: remove second “for”

Line 242: use of “waves” here is a bit confusing since it could be atmospheric waves or heat waves- suggest using “eddies” (to also match section title)

Line 272-273: is this the delta latitude, or the actual latitude? I think the former but it’s not clear from the wording

Line 296: “Fig 8d”  should be “Fig 8c”

Line 319: specify “mid-latitude” heatwave frequency reduction?

Summary:

The manuscript evaluates the link between the midlatitude heatwave (HW) frequency minimum and the position of the storm track using both idealized model experiments and ERA5 reanalysis data. By imposing idealized forcing that represents the Arctic amplification and the expansion of the tropics in the dry dynamical core model, the authors identified that the midlatitude HW frequency minimum is located about 6^o poleward of the extratropical storm track. The model results suggest that tropical warming leads to a coherent poleward shift of both the storm track and the HW frequency minimum, while Arctic warming causes both to shift equatorward together. Further analysis shows that the increase in zonal phase speed, associated with the poleward shift of the storm track, contributes to a reduction in heatwave frequency. The authors also argue that the response of HW frequency and duration are modulated by the storm track’s latitude instead of the magnitude of eddy kinetic energy (EKE). They further extended the analysis from idealized model experiments to ERA5 reanalysis in the Southern Hemisphere (SH) midlatitude. The authors found an increase in phase speed of upper-tropospheric meridional wind variance that resembles the tropical expansion in the idealized experiments. They further infer that the poleward shift of storm track related to the Southern Annular Mode (SAM) could reduce the hot day persistence.

This research contributes to distinguishing the dynamical impacts of Arctic warming and tropical expansion on midlatitude heat extremes. The authors did an excellent job interpreting the idealized model results, and I am convinced that, in an idealized setting, the zonal phase speed of synoptic waves modulate the HW duration and frequency. However, the argument regarding how observed poleward shift of the Southern Hemisphere storm track in ERA5 influences heatwaves appeared weak. Specifically, I am uncertain whether the positive trend in SAM is definitively connected to observed changes in heatwaves in SH midlatitudes. Therefore, I recommend that a substantial portion of the manuscript be revised before it can be considered for publication.

Major Concerns:

• It is unclear whether the shift in the position of SH storm tracks between the first and second halves of the ERA5 reanalysis is driven by SAM, tropical expansion, or a combination of both. The response of the power spectral density in Figure 6b resembles the result from the idealized tropical warming experiment (Figure 2b), which also produces a poleward shift in storm tracks. However, the underlying causes of tropical expansion and the positive phase of SAM are different. I agree with Referee #1 that without polar stratospheric ozone depletion experiments (or alternative experiments with stratospheric cooling) [e.g., 1], it is hard to establish a causal link between observed changes in heatwave frequencies and SAM. In addition, given that the jet shift associated with SAM is most pronounced in austral summer (i.e., December to February), I wonder why the authors analyze the entire year rather than focusing exclusively on that season.
• The observed responses of near-surface temperature and SH storm tracks are also closely linked to ocean dynamics [e.g., 2, 3]. For example, the stronger and poleward-shift westerlies due to ozone depletion before 2000 can increase northward Ekman drift and upwelling south of the ACC. The northward Ekman drift leads to northward expansion of the sea ice and a colder SST around 60^o As enhanced Ekman drift also pumps up warm waters from below the mixed layer, a slow warming trend is expected to reverse the initial cooling near 60^oS over decades [3]. The above mechanisms might be able to explain why there is a more notable poleward shift in SH heatwave frequency before 2000 compared to 2000 to 2020 period.

Specific Comments:

L5-7: Consider clearly mentioning in the Abstract that the idealized forcing experiments represent Arctic warming and tropical expansion.

L8: I am not sure if there is strong evidence supporting the impact of SH circulation on midlatitude hot extremes. In addition, the term “austral” is only mentioned here, but the ERA5 analysis appears to cover the entire year.

L9-10: This statement is oversimplified, given that the future response in the NH hot extremes could also be influenced by topography, land-sea contrast, atmosphere-ocean coupling and interactions with sea ice, in addition to Arctic amplification and tropical expansion.  

L31-34: I am not sure why the authors mention “waviness” here.  The rest of the manuscript is discussing the latitude and the strength of the eddy-driven jet instead of jet waviness.

L45: SAM also has a strong seasonality, but the authors did not mention it when analyzing the ERA5 data.

L45-47: "Does 'the slowly emerging trends' refer to the shift in extratropical storm tracks, or to something else?"

L49: The authors could elaborate more why the statistical significance of the circulation trends will increase.

L59: (Sec. 3,) -> (Sec. 3),

L60: (see Section 2.1,) -> (Section 2.1)

L81: Suggest providing a few sentences to clarify the reasoning for selectin exp4 and exp9.

L86-87: Why are different horizontal resolutions used for winds and near-surface temperature in the ERA5 analysis.

L90: Since the “heatwave frequency minimum” is quite important for the later analysis, suggest providing a clear definition earlier in the Methods section.

L93-94: This sentence “In ERA5 data…rolling window” is very confusing. Just to clarify, do you mean that for each calendar day, you first consider a 31-day window centered on that day and gather temperature data from multiple years to compute the 90^th percentile? In that case, the 90^th percentile temperature would change with time.

L96: Could you clarify why the ERA5 temperature data were used without detrending?

L97: I am not sure if “waviness” is the correct word to refer to the vertically integrated EKE. To measure “waviness”, you should consider metrics like finite local wave activity [4] or meridional circulation index [5].

L112: forcing(Held and Suarez, 1994) -> forcing (Held and Suarez, 1994)

L124: The response to Arctic forcing… -> The response of eddy heat flux to Arctic forcing…

L139-141: I don’t know whether the differences in jet strength are related to the different forcing magnitudes and profiles used in exp4 and exp9.

L141-143: I am not sure I fully understand this sentence.

L151: statistically highly significant -> statistically significant

L172: according to the definition employed here -> according to the definition employed in Section 2.2

L184: A question beyond the main discussion: why does heatwave duration show significant differences in low latitudes but not in high latitudes in Figure 3b?

Figure 3c and 3d: What is the pink dotted line? The figure caption for 3c and 3d is a bit confusing. By saying “Changes in zonal-mean hot day persistence with respect to the reference run…”, audience are not expecting the probability density difference plots here.

L193-195: Suggest clarifying in main text which latitude is used for the composite-mean temperature anomalies.

L201: In reanalysis data, the midlatitude heatwaves are also influenced by local land-atmospheric feedback, soil moisture, convection and other factors. Actually, the first paragraph of Introduction has already acknowledged that many other processes besides Rossby wave amplification are responsible for heatwaves in the real world.

L221-224: I am uncertain why “3 consecutive days” is chosen as threshold here. Some other heatwave metrics, such as the Heat Wave Duration Index (HWDI), use five consecutive days as the duration to classify a heatwave. Therefore, I suggest adding some justifications why this threshold is chosen.

L230: I don’t think there is a linear relationship between heatwave frequency minimum (or position of heatwave frequency minimum) and storm track strength.

L234: On average, the heatwave frequency minimum… -> On average, the latitude of the heatwave frequency minimum…

L241-242: I am not sure what the “absence of thermodynamic feedback” refers to. The authors could simply say “in an idealized model with Held-Suarez configuration”.

L246: Suggest clarifying which regions are expected to experience an increase in phase speed.

L253: Are circulation data in ERA5 detrended?

L254: Suggest deleting “however”.

L260: I am confused by the word “trends”. It seems that Figure 7a is not a trend plot.

Figure 7b: What is the pink dotted line?

L269-270: I am not sure whether attributing the poleward shift of observed heatwave frequency minimum to the trend of the SAM is correct. Please see major concern (1).  

L271: What is the “expected signal from the idealized model”?

L275-277: Could the authors elaborate more how ocean dynamics affect the observed SH midlatitude heatwaves? Please see major concern (2) for detailed comments.

Figure 8c: the orange line should be 1979-2000 and the blue line should be 2001-2022.

L295-296: Should both figures referred to be Figure 8c?

L208-300: I am not sure I fully understand this sentence.

L300-301: Where is the increase in heatwave frequency noted? Figure 7a?

L306: As mentioned earlier, “waviness” shouldn’t be used here.

L317: What does “over the broad range of different levels in waviness” mean?

L335-336: The argument of how Southern Ocean SST influences the hot day frequency is weak and non-convincing.

References:

[1] Dong, Y., Polvani, L.M., Hwang, YT. et al. Stratospheric ozone depletion has contributed to the recent tropical La Niña-like cooling pattern. npj Clim Atmos Sci 8, 150 (2025). https://doi.org/10.1038/s41612-025-01020-0

[2] Chemke, R. The future poleward shift of Southern Hemisphere summer mid-latitude storm tracks stems from ocean coupling. Nat Commun 13, 1730 (2022). https://doi.org/10.1038/s41467-022-29392-4

[3] Ferreira, D., J. Marshall, C. M. Bitz, S. Solomon, and A. Plumb, 2015: Antarctic Ocean and Sea Ice Response to Ozone Depletion: A Two-Time-Scale Problem. J. Climate, 28, 1206–1226, https://doi.org/10.1175/JCLI-D-14-00313.1.

[4] Chen, G., Lu, J., Burrows, D. A., & Leung, L. R. (2015). Local finite-amplitude wave activity as an objective diagnostic of midlatitude extreme weather. Geophysical Research Letters, 42(24), 10–952. https://doi.org/10.1002/2015gl066959

[5] Francis, J. A., & Vavrus, S. J. (2012). Evidence linking Arctic amplification to extreme weather in mid-latitudes. Geophysical Research Letters, 39(6), L06801. https://doi.org/10.1029/2012gl051000