the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Jan 2024
Climatology and Trends in Concurrent Temperature Extremes in the Global Extratropics
Abstract. Simultaneous occurrences of multiple heatwaves or cold spells in remote geographical regions have drawn considerable attention in the literature, due to their potentially far-reaching impacts. We introduce a flexible toolbox to study such concurrent temperature extremes, with adjustable parameters that different users can tailor to their specific needs. We then use the toolbox to present a climatological analysis of spatially compounding heatwaves and cold spells in the global midlatitudes. Specific geographical areas, such as Western Russia, Central Europe, Southwestern Eurasia and Western North America, emerge as hotspots for concurrent temperature extremes. Concurrent heatwaves are becoming more frequent, longer-lasting and more extended in the Northern Hemisphere, while the opposite holds for concurrent cold spells. Concurrent heatwaves in the Southern Hemisphere are comparatively rare, but have been increasing in both number and extent. Notably, these trends in concurrent temperature extremes are significantly stronger than the corresponding trends in all temperature extremes.
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RC1: 'Comment on esd-2023-45', Anonymous Referee #1, 17 Mar 2024
This paper delves into the concurrent surface temperature extremes using a novel toolbox tailored for understanding such occurrences. The toolbox is well introduced, with clear delineation of the necessary parameters and steps. The authors identify regions with concurrent temperature extremes, characterized by relatively higher frequencies, durations, or temperature anomalies of concurrent extremes. They meticulously analyze the trend of concurrent heatwaves and cold spells across both hemispheres in summer and winter, concluding that the trend of concurrent extremes surpasses that of individual temperature extremes.
Given the characterization of this manuscript, the presentation of temperature extreme trends is comprehensive, logically structured, and well-organized. However, one aspect warrants attention: the focus of the letter. It's imperative to clarify whether the primary emphasis lies in analyzing trends of concurrent temperature extremes or in introducing and propagating the toolbox. The statement in lines 107-108 regarding not attempting a full parameter sweep but rather selecting values for comparison with previous literature suggests an intent to showcase the toolbox's effectiveness in reproducing prior results. If so, revisiting the title and abstract might be beneficial. Alternatively, if the letter aims to highlight trends in concurrent temperature extremes, it would be advisable to incorporate parameter sensitivity tests and potentially explore additional content, such as dynamical feature trends in the upper-troposphere (if feasible within the toolbox). Such enhancements would underscore the toolbox's efficacy for this type of research.
Citation: https://doi.org/10.5194/esd-2023-45-RC1 -
RC2: 'Comment on esd-2023-45', Anonymous Referee #2, 19 Mar 2024
The manuscript introduces a toolbox for analysing concurrent temperature extremes with adjustable parameters. Using the toolbox, the authors present a brief climatological analysis of spatially compounding (concurrent) heatwaves (HW) and cold spells (CS).
My biggest concern regarding the present state of the manuscript is an inadequately explained definition of concurrent HW (CS) and its potential conflict with toolbox’s clustering algorithm. If I understand correctly from the Figure 2 caption, a HW (CS) is regarded as concurrent if another HW (CS) is present within the selected domain (northern/southern mid-latitudes). It seems that a resulting frequency of concurrent HW (CS) would differ considerably under different clustering and minimum extend settings (Figure 1c). For example, if the clustering distance and minimum area are small, almost every HW (CS) would be accompanied by other events, resulting in too many concurrent extremes. By contrast, if the clustering distance and minimum area are set as large, the algorithm would merge HW (CS) thorough the domain, resulting in too little concurrent extremes.
In my opinion, the usage of the toolbox would require quite a lot of expert judging to capture concurrent extremes properly. Before publishing the toolbox, authors should consider adding default parameters or, alternatively, their recommended range for individual extreme events. Additionally, authors may consider evaluating other variables then temperature (Line 50), to enhance interdisciplinarity which is required by the ESD journal. My other comments are as follows:
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Regarding the toolbox
- The authors plan to publish the toolbox’s python code after acceptance (Line 271). I am not an expert in programming, so I am wondering if it is possible to add a more user-friendly graphical interface to make the toolbox available for a broader community. Please comment.
- The toolbox is planned to be published via authors’ GitHub (Line 272). Please make sure that it will receive a permanent DOI identifier  Â
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Regarding the climatological analysis
- Line 112: ‘roughly 40% of single-gridbox heatwaves at those locations are part of a set of multiple concurrent, large-scale heatwaves across the NH’ – if I understand the definition correctly, only another single heat wave (related or not) is sufficient to fulfil the criterion for a concurrent event. Therefore, in my opinion, ‘across the NH’ is exaggerated.
- Line 130–132: The explanation regarding a duration of compound (concurrent?) extreme is hard to follow. Please rewrite for better clarity.
- Line 153: ‘Concurrent cold spells show mirror trends’ – Not everywhere. For example, negative HW and CS trends (blue colour) can be found in parts of the US, while positive HW and CS trends are in parts of Siberia and Scandinavia. Please modify the wording.
- Figure 4: I suppose these are linear trends (?) Please clarify in the caption.
- Line 168: It should be mentioned that the large HW trend in Western North America is probably related mainly to the extraordinary 2021 heat wave
- I believe that the southern domain should be cropped to exclude Antarctica’s coast and its shelf ice
Citation: https://doi.org/10.5194/esd-2023-45-RC2
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