Major comments.

This paper examines 2m temperature, surface winds, and precipitation extremes associated with merged Atlantic-African jet during northern winter. It is well written with an easy-to-follow structure. The introduction is particularly well-written. However, I do have one major concern about how the statistical significance is performed.

My worry is that the merged-jet regime is treated as an independent dynamic regime from the NAO and ENSO. It could well be that those winters involve both extratropical and tropical forcing, with the latter further involving latent heat realise in additional to tropical SSTs.  More rigorous statistical significance tests are thus required to be sure that the signals are associated with the merged jets rather than the NAO, EI Nino or their combined effect.  

• Quite a lot of the samples of the merged jets are associated with the negative NAO phases (-NAO, 9 out of 14) and with EI Nino (7 out of 14). With only 5 or 7 independent samples, the statistics of composite or linear regression analysis become limited and may lead to unreliable estimates. Furthermore, the merged-jet subgroup contains individual months for a given winter, e.g. 12/2009, 01/2010, 02/2010 and the signal has a strong imprint of tropical SST anomalies, which may persist over a winter. Thus, whether those months can be viewed as independent samples remains a question. To make sure that the results are truly different from -NAO and EI Nino, the authors need to explain exactly how the Monte Carlo simulation is performed. They may consider using other statistical techniques that are more robust with small sample sizes, such as Partial Least Squares Regression (PLS) or Ridge Regression that adds a penalty to the regression coefficients. Have the authors cross-validation the stability and reliability of the results using bootstrapping?
• Statistical significance tests are required for the composite differences shown in Figures 4, 6, 8, and 9.
• Have you examined the composite anomalies based on the eight winter months during which -NAO and EI Nino occurred at the same time and compare with the anomalies associated with the -ZJI regime?
• The discussion about the difference of daily average surface temperatures (figures 2 and 3), surface winds (figure 5), and total precipitation using box plot for the selected regions is somehow confusing. Because no confidence intervals are shown for the median nor for the mean, these box plots do not prove those measures are either larger or smaller than the climatology statistically.

Specific comments:

• Title: during winter -> during northern winter
• Line 62-64. This sentence has little to do with the 2009/2010 winter or add anyting to the paragraph. Consider removing.
• There are quite a few “Northern Hemisphere” in the paper. Consider abbreviating it to “NH”.
• Line 67: make it clearer that the warm extremes over North Africa and Arab are linked to positive NAO and during summer, or delete this part of the sentence because your focus is on the winter season.
• Line 73: insert “subtropical” before “eastern US coast”.
• Line 125: do you mean that the eddy kinetic energy shown in figure 9 is estimated based on 10-day highpass filter?
• Line 126-127: I am not should that mean flow corresponds to the data filtered with a 10-day low pass filter. Quasi-stationary waves will stay in the low-passed part but they should not be regarded as the mean state of the flow, also in which figure that the low-pass covariances between the high-pass fields are assessed?
• Lines 142-150. This is a very long sentence. Try to break it into two so that the readers can understand exactly what is involved.
• Line 150: how “significantly”. Consider reserve the word “significant” to statistically tested quantity.
• Line 174: NAO -> -NAO and other similar expression.
• Lines 190-215: reads like discussion rather than results. Move to last section and make the section title to “conclusions and discussion”?
• Lines 202 and 204: I do not think that you have studied the warming trend. These are anomalies associated with either the -NAO, EI Nino or the -ZJI.
• Line 239: can you be a little specific about “the sources of momentum”? Is the merged jet eddy driven or thermal driven? If it is too elaborated to go into the details, I would suggest removing these texts. Mixing lengthy discussion of previous work with new results can be confusing sometimes.
• Line 249: This explains -> This is consistent with.
• Lines 251-254: This may indicate an anomalous …”. This is too speculative especially figure 6b does not involve any statistical test, the rather small sample size, and the small magnitude of the wind anomalies (< 1 m s^-1).
• Line 272: “Additionally, there are notable extreme daily precipitation from the climatology that fall within the ZJI winter days”. I wonder how can you make a statement like this just based on the box plot. Each box plot is associated with different subgroup of winter days with no information about the climatology. For a given day that is shared by climatology, -NAO or -MJI group, it can be shown outside of the whiskers in one boxplot, e.g. climatology, but included in other subgroups.
• Line 314: “A significant proportion”. This is not supported by the results because no statistical significance is shown in figure 9.
• Lines 247-350: again speculative sentence without any concrete results backing.
• Line 318: Conclusions -> Conclusions and Discussion. Also, I would think that the conclusion can be much more concise than what has been written.
• Line 335: no trend has been studied here.
• Line 337: A connection between -ZJI and Arctic Amplification may be related to the paper https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GL082714.
• Line 362: remove due to merged jet formation.

The authors have submitted an manuscript analyzing the characteristics of the large-scale atmospheric circulation over the North Atlantic and Europe when a merged polar and sub-tropical jet occur, and specifically look into the resulting impacts in terms of cold temperatures, strong winds and precipitation. While the paper deals with an interesting topic and includes promising results, it also features several shortcomings in terms of the methodology and physical interpretation, that in my opinion hampers its potential scientific value. Thus, I suggest a major review and would be willing to review the manuscript again if required

Major points

i) the manuscript does not include the composites of the 5 groups (line 95ff), particularly in terms of zonal wind at 250 hPa, which makes the understanding of the rest of the paper quite difficult. Please include the composite figures and describe the possible differences

ii) Following on the above, and specifically: from the 14 "merged jet" periods, 9 are NAO-. I would really like to know what special in the 5 periods with „merged-jet but not NAO-“. For me it is difficult to conceive a „merged-jet” situation with a neutral or positive NAO, maybe this is only a question of thresholds??

iii) I wonder in how far it is possible to obtain statistical significant information from the small samples (e.g. only 5 samples), as I imagine that the case-to-case variability will be quite high.

iv) I also wonder about the physical interpretation of the relationship to ENSO, particularly considering the small sample, the indirect influence of ENSO in the European climate and the multitude of modifiers that affect this relationship. How much is the actual explained variance???

v) Given all of the above, and even though I have refrained to provide detailed comments on the figures / text - as these will probably changed in a revised version - I would like to suggest the authors to downtone the statements regarding the current results, particularly given on iii) above.

I am looking for to re-evaluate a revised version of the manuscript.