Summary

This paper investigates how extratropical cyclone occurrences influence extreme seasonal conditions using 1050 years of CESM-LE (large-ensemble) climate simulations and ERA5 reanalyses in present-day climate (1990-1999). It identifies patterns in extratropical cyclone frequency, intensity, and stationarity associated with extreme seasons across the Northern and Southern Hemispheres. Key findings include the link between increased extratropical cyclone frequency and intensity with wet and windy winters, while dry and calm seasons are marked by fewer or weaker cyclones. Regional variability is large, with the study highlighting shifts in storm tracks and distinct characteristics across different regions. The results highlight the importance of extratropical cyclones in shaping extreme seasonal weather and provide insights for future climate projections, although the authors note that the model’s coarse resolution limits the capture of finer-scale features.

General Comments

I very much welcome this paper for publication given that it makes use of Large Ensemble climate simulations, which are critical to address uncertainty in historical and (for a potential future study) future projections. I also very much welcome the focus of the paper on extratropical cyclone associated extremes seasonality. Finally, I was very impressed by the scientific rigour and the level of detail of the scientific analysis.

The only major point I feel to flag is that I am not entirely convinced by how just looking at two (different for the type of extreme) seasons from ERA5 data can prove consistency with CESM-LE simulations, also considering the 2013/14 season is outside the CESM-LE sample years 1990-1999. I'd encourage the authors to better explain or perform a little further analysis in demonstrating how this link may be robust, for instance looking at more seasons from ERA5 data. I hypothesize this may be straightforward to perform, e.g. just re-use diagnostics tools already developed on easily downloadable era5 data, thus I suggest publication with minor revisions. 

Minor revisions

• Abstract: I had a hard time following this long abstract. I think the paper would benefit greatly if the abstract could be made more concise and more tailored on key results
• Intro: If possible, please make a stronger link between how having a large ensemble of simulations helps/or has helped in previous studies to understand the extratropical cyclone extremes seasonality
• Methods: Please explain better the cyclone detection algorithm and adaptation of extreme season detection.
• Results: The inclusion of the 2013/14 and 1988/89 seasons as real-world examples has the potential to strengthen the study. However, 2013/14 is outside the present-day CESM-LE sample, and they are just two. To better connect the statistical findings from the CESM analysis I suggest the authors to look at more ERA5 seasons, so as to create also a more coherent narrative, from which the paper would benefit.
• Summary:
  • I find this sentence a bit vague at line 453-455 “The results also indicate that changes in the number, geographical distribution and properties of extratropical cyclones with global warming would significantly affect extreme seasons around the globe. An essential next step is to evaluate such changes and their impact on extreme seasons in future climate simulations.” More in details, I think it would be very valuable if the authors could add some insights on how the methodology contained in this paper could be used for warmer climate simulations, or even just analysing two different chunks of historical data, e.g. 1950-60 and 2010-2020, the latter already experiencing a warmer climate. 
  • When discussing the limitations related to the relatively coarse resolution of CESM-LE at line 456, the authors should highlight the trade-off between resolution and computational cost. Running CESM-LE with finer spatial resolution (e.g., 0.25° or less) would dramatically increase computational demands, potentially limiting the length or ensemble size of the simulations. Noting this trade-off would better contextualize the decision to use the current resolution and emphasize the value of leveraging large ensemble simulations for statistical robustness.