the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Nov 2024
The impacts of climate change on tropical-to-extratropical transitions in the North-Atlantic basin
Abstract. As tropical cyclones migrate towards mid-latitudes, they can transform into extratropical cyclones, a process known as extratropical transition. In the North Atlantic basin, nearly half of the hurricanes undergo this transition. After transitioning, these storms can reintensify, posing significant threats to populations and infrastructure along the eastern coast of North America. While the impacts of climate change on hurricanes have been extensively studied, there remain uncertainties about its effects on extratropical transitions. This study aims to assess how climate change affects the frequency, location, intensity, and duration of these transitions. To achieve this, high-resolution regional simulations from an atmospheric regional climate model, based on the RCP 8.5 emissions scenario, were used to compare two 30-year periods: the present (1990–2019) and the end of the century (2071–2100). The results indicate a projected decrease in the number of tropical hurricanes, with no significant change in extratropical transition rates. September and October continue to be the primary months for extratropical transitions. However, the season’s peak appears to have shifted from September to October, suggesting that large-scale environmental conditions may become more favorable for extratropical transitions in October in the future. Although a poleward shift in the maximum intensity of tropical hurricanes is detected, the average latitude of the transitions does not change. Our findings suggest that transitioning storms will be more intense in the future, despite a less baroclinic atmosphere due to a stronger contribution from latent heat transfer. However, the risk of reintensification after transition is not expected to increase.
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RC1: 'Comment on egusphere-2024-3435', Anonymous Referee #1, 10 Dec 2024
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Garin et al. (2024) uses a regional climate model over two 30-year periods to examine the effects of climate change (under RCP8.5) on ET events in the North Atlantic. The authors find no significant change in the frequency of ET events in the future but a shift in their location (increase off the northeast coast) and increase in potential destructiveness.
Given the limited number of studies on ET and climate change, I appreciate this addition to the literature. The model simulations used in this study are high enough resolution to adequately capture TCs and ET events and storm tracking methods are in line with previous studies. I would, therefore, rate the scientific significance of this manuscript as “excellent-to-good”.
The overall presentation quality is also “excellent-to-good” in the sense that the manuscript is concise and easy to follow. The scientific quality, however, is “good-to-fair” as substantial discussion of how the presented results compare with previous studies is omitted and should be included before publication. Additionally, I noted several omitted references for the authors to include in their introduction and/or to help put their findings into context.
General Comments
- Here are some additional references that should be included throughout the introduction and results. The Bieli et al. references are of particular interest to the current study:
- Arnott et al. (2004): https://doi.org/10.1175/MWR2836.1
- Baatsen et al. (2015): https://doi.org/10.1007/s00382-014-2329-8
- Bieli et al. (2019): https://doi.org/10.1175/JCLI-D-17-0518.1
- Bieli et al. (2020): https://doi.org/10.1029/2019MS001878
- Haarsma et al. (2013): https://doi.org/10.1002/grl.50360
- Kitabatake (2011): https://doi.org/10.2151/jmsj.2011-402
- Kofron et al. (2010): https://doi.org/10.1175/2010MWR3180.1
- Wood and Ritchie (2014): https://doi.org/10.1175/JCLI-D-13-00645.1
- Of biggest concern is the lack of comparisons to previous studies throughout the results section. While there is some comparison on the Discussion and Conclusions section, the manuscript could benefit from additional comparisons and related discussion throughout. For each presented result, consider:
- How do these results compare to previous studies?
- What could account for the differences (e.g., methodologies, model environments, etc.)?
- A couple small changes to the figures would be helpful to increase readability:
- All box plot figures: Could be helpful for the reader to add grid lines and/or explicitly state the mean/median values either on the plots themselves or in the text.
- Figure 1: Add legend on plot as in other figures.
- Figure 3: Could be helpful to indicate which intensity ranges are significantly different in the future.
- Figure 6: The information provided in this figure could be better suited for a table instead.
Specific Comments
L93: The “ET” acronym was already defined in L28.
L106: In addition to precipitation validation, what data set was used to evaluate model TC tracks? I see some evaluation of the ET ratio in section 2.8 compared to IBTrACS and ERA5, but what about for the TC and ET tracks themselves? In particular, I would be curious to see how CRCM5/GEM 4.8 handles TCs in the eastern North Atlantic main development region.
L108: Is the precipitation comparison shown anywhere in the manuscript? What does a reasonable precipitation comparison mean for the model’s ability to represent the TC/ET climatology?
L108: Is it possible to evaluate over the full 30-year simulation period? If not, please clarify and state this limitation.
L222: Remove extra space between “to” and “cold-core”.
L261: As noted in General Comment #2 above, it could be helpful to compare this model’s simulated ET percentage to that from other modeling/observational studies.
L274: Are 14.3 and 18 the annual averages? Please clarify.
L280: Remove extra parenthesis after “studies”.
L299–301: Reference?
L304–305: Reference?
Citation: https://doi.org/10.5194/egusphere-2024-3435-RC1 - Here are some additional references that should be included throughout the introduction and results. The Bieli et al. references are of particular interest to the current study:
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RC2: 'Comment on egusphere-2024-3435', Anonymous Referee #2, 16 Dec 2024
reply
The authors evaluate how extratropical transition (ET) in the North Atlantic will change with climate change using high-resolution regional climate simulations. They find a decrease in the number of ET events associated with a decrease in the number of tropical cyclones, with no change in the ratio of cyclone that undergo ET. They show that there is a compensation between more intensification from surface latent heat fluxes but a weakening baroclinic instability. They show a shift in the region of ET but no significant change in the average latitude. This is an excellent and well written paper about an interesting area of research. An excellent paper to have in this journal. I have some minor corrections and clarifications below.
L106 - Is there a missing reference here? Otherwise, you need to show this evaluation in a supplement.L186 - Is the weight of each layer the mass?
L203 - What do you mean by "i.e. upper" in this paragraph
L218 - What do you do with cyclones that are diagnosed as having an onset of ET but not completing ET? The paper by Sarro and Evans (2022) (https://doi.org/10.1175/MWR-D-22-0088.1) would be good to reference here. The "instant warm seclusion" they describe, where the cyclones undergo ET but are always warm core, could be relevant.
L261 - I'm actually surprised at how close this is. I would have thought that IBTrACS underestimates ET due to reporting biases. Could you comment on this?
L292 - Is the latitude of minimum pressure dependent on ET? Do you count the minimum post ET or only prior?
Figs 2,3,4 - It would be good to also include IBTrACS on these figures as you did with figure 1, to give some idea of how close the model is to these observations (accepting that they can be biased)
Fig 5 - The Eady growth rate is shown at 200hPa, but earlier you only describe the calculation of Eady growth rate at 500hPa. Also, it is confusing that you say you use data at 400 and 500hPa to get the Eady growth rate at 500hPa. Would this not be better described as the Eady growth rate over that layer or at 450hPa assuming you are using 1st order differences.
L333 - The description of this weighting is slightly confusing. The monthly TC number is already in the ET ratio, so cancels out in the weighting and you would be left with the number of ET events in that month divided by the total number of TCs in the year. Is that correct?
L352 - "Indeed, TCs that are most likely to undergo ET need to sustain a minimum energy level at middle latitudes". This sounds reasonable, but I was wondering if anyone has actually shown this. Can you add a reference?
Fig 10 - Can you add some indication of statistical significance to the pattern shown, either to the figure or the discussionL383 - low transitioning -> slow transitioning?
L298 and L408 - You have used the term "available potential energy" interchangeably with "eady growth rate". While they are related, they are not the same. It would be better just to say Eady growth rate in the text as that is what is shown in the figures.
L457 - Decrease -> weakening
L480 - I think this paragraph could be split into multiple paragraphs. It's quite long and does discuss different things.
The code/data availability needs improvement. You could just write something about where the model data is stored, but I think the tracks you have generated should be made openly available which should be easy enough to do with zenodo. Similarly the code for the data analysis and figures could be uploaded to a zenodo repository.
Citation: https://doi.org/10.5194/egusphere-2024-3435-RC2
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