Review for: CMIP6 Multi-model Assessment of Northeast Atlantic and German Bight Storm Activity, by Krieger and Weisse.

My recommendation is that this paper is rejected based on the scope of the journal as describe here: https://www.earth-system-dynamics.net/about/aims_and_scope.html

Specifically, it is stated that “Regional studies drawing conclusions of chiefly regional or local relevance are considered outside of the journal's scope.” Unfortunately, I fail to see how this manuscript does anything other than “draw conclusions of chiefly regional or local relevance”.

Having said that, I enjoyed reading the manuscript and strongly recommend the authors to consider submitting to a more relevant journal. I have provided my full review comments below with the intention that they may be useful to consider for this purpose.

Summary

The manuscript examines future projections of storminess in the Northeast Atlantic region based on simulations from CMIP6 and the MPI Grand Ensemble, each using a range of SSP scenarios. The storminess diagnostic employed has been developed by the lead author in previous studies using long-term direct station observations of surface pressure and as such provides a unique method for comparing the models to the real world. The results are broadly consistent with previous studies, though the diagnostic employed provides some new insights into the relationship between future changes in storminess and changes in wind direction. Overall I’d be very happy to see this work published, subject to the following comments  being addressed.

Main Comments

My main concern is that as presented, the study appears rather incremental. There are many studies examining model projections of North Atlantic storminess (as you summarise in your introduction), and your key conclusion of an overall reduction in storminess in future model projections but with an increase in the intensity of the most extreme storms, has been noted numerous times before. Please tweak the framing of your work to address this concern (particularly in the introduction) to better inform the reader exactly how this study aims to advance current understanding. Formulating one or two explicit research questions might help with this.

To my mind, one key advance is the comparison of storminess between the climate models and the long-term dataset of direct observations, because the vast majority of climate model studies just compare against reanalysis products. However, this comparison is not mentioned in the abstract, and even a basic description of the observational dataset is omitted from the manuscript. I’d urge you to make more of this aspect in the text, and to extend the observational comparison to all relevant figures (e.g. 3, 4, 6, 7, 8) if possible.

Your storminess diagnostics are annual in the sense that you don’t subset the data to a particular season. However, I imagine most of the >95%ile geostrophic wind events happen in autumn/winter and so your projected future changes represent most closely the changes in these seasons. Given projected future changes in storminess contain important seasonal variations, please add a discussion on this point to aid interpretation.

Other Comments

Abstract: The last two sentences appear contradictory because you state “the upper percentiles of winds speeds from these directions decrease” and then “the most extreme storms may become stronger or more likely”. I think the former is referring to the 95^th percentile of the wind speeds whereas the latter is referring to more extreme percentiles. Please clarify.

L25 and the following paragraphs: Please be explicit about the seasonality of the projected changes in storminess presented in these papers. Some I know explicitly refer to winter only, and others I am not sure about.

L56: “upper wind speed percentiles” is unclear (I thought it meant upper-tropospheric wind speeds initially). Please clarify, e.g. “upper percentiles of near-surface wind speeds”. Similar comment applies to L58.

L91: Is CMIP6 psl data daily means or instantaneous?

L97: Just to be clear, do you standardise the annual 95^th percentiles for each triangle separately, or average them together and then standardise?

L115: I presume that the gradients are computed using the distances between the model grid points (which differ for each model), rather than the original station locations? Please specify.

L133: The observed timeseries has not been introduced. Please add a description of it in section 2.

L146: You claim that “the full pool of ensemble members can represent the variability present in the observations”, but this is misleading and clearly must depend on the timescale examined. If I understand correctly, all the timeseries are independently standardised, so the interannual variability is by construction captured by the ensemble, at least during the period 1960-1990. What you show are ten year running means, so I assume your claim is something like “the full pool of ensemble members can represent the variability on decadal timescales”. Please clarify.

Section 4: Several recent papers have highlighted deficiencies in the ability of climate models to simulate multi-decadal variability in the North Atlantic, and have questioned the reliability of model projections in this region as a result (e.g. see here, and references therein: Smith et al., 2025, https://doi.org/10.1038/s41558-025-02277-2). Given their importance, I’d urge you to extend your discussion to include reference to them and relate to the findings of your study.

L290: This paper presents a statistical methodology for assessing future changes from multi-model ensembles of differing sizes, which is very relevant to your suggestion: Zappa et al. (2013) A multimodel assessment of future projections of North Atlantic and European extratropical cyclones in the CMIP5 climate models. Journal of Climate, 26(16), pp.5846-5862.

Fig 3 caption: Please state the periods over which trends are computed (I assume the full experiment periods, but best to be precise).

Fig 4 caption: Are daily geostrophic wind directions? Please clarify.

Fig 8: To what extent are the differences here statistically robust? Can you construct confidence intervals? (here and/or Fig 9)

Typos

Fig 7 caption: Repeated “the”

L225: “increase” -> “increase in frequency”

General comments:

The manuscript "CMIP6 Multi-model Assessment of Northeast Atlantic and German Bight Storm Activity" presents a study on extratropical storms for the named regions and analyses potential future changes due to anthropogenic climate change. The data basis for this study is a large multi-model ensemble of CMIP6 simulations as was as a single-model initial condition large ensamble, produced by means of MPI-ESM. This very comprehensive data basis from the newest generation of GCM simulations is a strong plus of the present study compared to many other papers on storms for the named region. This fact combined with a very clear and concise presentation of most elements of the study makes it a valuable contribution to scitific progress and potentially suitable for publication in ESD. However, there are some issues that should be addressed or at least discussed before publication:

A) A number of analyses in this study lack any assessment of robustness, e.g. by checking for statistical significance or provision of confidence intervals. In particular this is the case for results presented in Figs. 4, 6, 7, 8, 9, 10, and 11. I would be willing to accept this in case of Fig. 11 in order to ensure readability but for the other Figs. an analysis of statistical significance (nothing mentioned in the text either regarding these results) as well as including any indication in this respect in the Figs. seems necessary and possible. The authors themselves discuss the strong sensitivity of such studies depending on choice of metric, integration period, storm identification method, ... (lines 259-262). Especially in these cases, a thorough assessment of robustness and statistical significance is unavoidable for a proper scientific study.

B) The authors introduce very briefly, why the analysed parameters matter. Apart from lines 14-19 which sketch a few possible impacts of Northeast Atlantic storms, there is nothing. In particular, they provide no reasoning why wind direction is interesting besides wind speed/storm activity alone. A number of reasons are very clear to me but readers not familiar with the specific of potential storm impacts will have no clue why wind direction is relevant beyond academic exercise.

C) I am a little worried regarding the compilation of the multi-model ensemble. The authors themselves stress a problem related to their procedure of selecting one ensemble member from each model which leads to thos models with just one member always remaining part of the multi-model ensemble. This is a major shortcoming of this study. I am grateful that the authors have the courage to outline this shortcoming themselves. This is a great example of good scientific practice and they present an approach to asses the potential influence by compiling a second multi-model ensemble, bootstrapping only from thos models with 5 or more members. Anyway, I am worried for one more reason: If I understand correctly, the authors perform the bootstrapping for those models with more than one member separately for the historical experiment and the individual scenarios. When doing so, a chosen scenario run is probably unrelated to its historical counterpart (at least in most cases). This yields inhomogeneities masking physical meaningful climate signals. In the end, it probably does not matter so much, given that the averaging over the multii-model ensemble is done, smoothing out this inconsistency between the end of the historical period and three separate beginnings of future scenarios. However, I would argue that choosing scenario simulations belonging to the chosen historical parent simulation had been a better solution. If my understanding is correct, I would ask the authors to include this issue in the discussion, too.

D) lines 113-115: Choosing the nearest grid point from each model for a given observational site may lead to some distortion in areas of steep orographic gradients, namely Bodoe and Bergen. I understand that the authors use SLP, however, an extrapolation of modelled surface pressure into the ground of the one model (where the closest grid point is inland) may be somewhat different from SLP practically identical to modelled surface pressure of another model (where the closest grid point is flat terrain or even ocean). I do not insist a priori on including a discussion of this matter in the manuscript but I challenge the authors to think about this possible case (or perform some sensitivity test) and provide arguments here, why this should not be relevant for their findings.

E) I am pretty confused by the section title of Sec. 3.2. "Internal variability" refers to temporal fluctuations of various variables inherent to the natural earth system, observed or modelled. Single-model initial condition large ensembles (SMILE) are great tools to distinguish extrenally forced signals from internal variability. However, this is not what you are analysing in the large part of this section. Instead, the main focus of this section are heterogeneous climate change signals for different parts of the storm intensity spectrum as well as depending on wind direction. I would suggest choosing a more suitable title for Sec. 3.2 plus deleting a few sentences in the first paragraph of Sec. 3.2 (see my specific comment below).

Specific comments:

1. lines 26-28: Actually, the synthesis of Feser et al. is the other way around (increasing north of 60°N, decreasing south of 60°N) in line with a poleward shift of the NH storm track.

2. lines 38-39: I don't understand the line of argumentation here. The previous sentence is about low model agreement, hence large model-related uncertainty. This sentence here now seems to present a link to substantial changes in wind extremes when combined with changes in storm track locations. It seems something is missing here in between.

3. line 97: Did the authors analyse if discrepancies are introduced when using 3-hourly data for MPI-ESM compared to daily averages for the other models? An average of the eight 3-hourly values is usually in quite good agreement with a respective daily mean. So, it might even haven been an alternative to calculate such a proxy daily mean for MPI-ESM before analysing its simulations consistently with the other models.

4. Sec. 2.2: It is not entirely clear from this section if MPI-GE is also used as part of the CMIP6 multi-model ensemble and hence included in respective analyses or not.

5. Sec. 2.3: This section lacks the information that an averaging over all ten triangles is performed to yield results for the Northeast Atlantic. This fact is explicitely written only in lines 271-272, that is the discussion.

6. lines 115-117: Sory, I don't understand the procedure for "triangles" that basically fall onto a line. I am lost when you write about "the observation site that is most distant to the corresponding gridpoint". Which gridpoint? It is three observation sites with three associated grid points... Please rephrase (or correct?) your description here.

7. line 125 & 149: I would refrain from using the term "multidecadal oscillation" here. For me this term implies some type of natural variability associated with these fluctuations. But this is not possible given that you analyse the ensemble mean (and certain ensemble quantiles) here. These fluctuations must be either by chance (unlikely in these cases) or also result from external forcing common to all individual simulations. If you discuss these fluctuations, you have to provide possible drivers here.

8. Fig. 3: I would suggest swapping subfigures vertically in order to present the same order as in previous figures: first, the results from the bootstrapped MME, second, the results from all members.

9. line 178-179: "Internal variability" refers to fluctuations inherent to the natural earth system, observed or modelled. Single-model initial condition large ensembles (SMILE) are great tools to distinguish extrenally forced signals from internal variability. However, you cannot diagnose the internal variability from the SMILE's ensemble mean. It seems to me that you are doing that here. I would suggest eliminating everything from line 178 to line 196, replacing that with a better introduction for the direction- and intensity-dependent analyses, and then continue with line 197.

10. line 201: It looks to me as if the (positive) frequency changes are rotated clockwise (not counter-clockwise) compared to the historical frequencies.

11. lines 206-208 and ff: Why are the changes of upper percentiles of wind speeds only analysed from MPI-GE. This could have been done from the CMIP6-MME as well or not?

12. line 210: Here you write about "significantly weaker". Have you checked statistical significance? If not, refrain from using this term here but as said in my general comment A) you will have to assess statistical significance.

13. line 219-222: Why does only MPI-GE allow for the analysis of the extreme events? You could have done the same with the full CMIP6-MME, or not?

14. line 304-305: Rephrase this sentence about the internal variability. This is not what you are looking at.

15. I would suggest at least one or two outlook sentences... What are possible next steps that could be done based on your findings?

Technical corrections:

a) line 52: Insert "that is" before "most pronounced in the CMIP3...".

b) line 110: The reference to Krueger et al. (2019) makes no sense here. It is correctly placed in line 111, so please delete it here.

c) line 277: Replace "second part" by "final part". It's just this last element and not half of your study.