the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Nov 2024
Exploring the interplay between observed warming, atmospheric circulation, and soil-atmosphere feedbacks on heatwaves in a temperate mountain region
Abstract. This study investigates the exceptional heatwaves of 2022 in the Pyrenees, focusing on their physical drivers and environmental influences. The June heatwave was advective in nature, with stronger mountain-induced circulations resulting in heterogeneous temperature anomalies, while the July event had subsiding and weaker atmospheric flow, leading to more uniform temperatures. The interplay of the synoptic circulation with the complex topography, or the pre-existing soil moisture deficits, played an important role in driving the spatial variability of temperature anomalies in the heatwaves and contributed significantly to their regional amplification. In addition, human-induced climate change has exacerbated these extreme weather phenomena, with more intense heatwaves in the recent period (1986–2021) compared with the past (1950–1985). This research contributes to a more realistic assessment of the impact of climate change on heatwaves in mountain regions.
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RC1: 'Comment on nhess-2024-192', Anonymous Referee #1, 02 Dec 2024
Review of “Exploring the interplay between observed warming, atmospheric circulation, and soil-atmosphere feedbacks on heatwaves in a temperate mountain region” by Lemus-Canovas et al.
This study investigates the physical drivers of two heatwaves that occurred in a temperate mountain region in 2022. The authors systematically quantified the contributions of adiabatic, diabatic, and advective processes to temperature trends. Their analysis revealed that the June heatwave was primarily driven by advection, while the July heatwave was largely caused by subsidence. By employing flow analogues, the study further demonstrated that heatwaves during 1986–2021 were more intense compared to those during 1950–1985. Additionally, the authors highlighted how interactions between synoptic circulation, topography, and soil moisture contribute to the spatial variability of temperature. This research provides valuable insights into the mechanisms behind heatwaves and underscores the role of land-atmosphere interactions in amplifying such events. While the manuscript is generally well-written, a few points require further clarification.
Major comments
- Lines 104-106. What is the magnitude of the diabatic term compared to other terms in Equation (1)?
- Lines 127-158. Could you make a schematic of how the flow analogue experiments are conducted, clearly showing all the steps? For example, how do you reconstruct the expected mean Tx?
- Figure 1. The temperature is lower than 14 °C, which is quite low compared to heatwaves in general, although the temperatures in the identified periods are heatwaves statistically. Could you show that such a temperature is really a concern?
- Lines 256-263. The temperature difference between the two periods from Figure 5b is at least half of that from Figure 5a. You cannot say that the temperature difference between the analogs of the two climate periods is mainly linked to a long-term trend, unless the difference from Figure 5b is much smaller than that from Figure 5a.
- Lines 271-274. Could you provide more explanations on the spatial structure of synoptic situations? Why RMSD could clearly represent the spatial structure?
Minor comments
Line 58. Tx is maximum temperature but is not explicitly defined.
Lines 110-120. The same should be used everywhere.
Line 263. Inconsistent font size
Citation: https://doi.org/10.5194/nhess-2024-192-RC1 -
RC2: 'Comment on nhess-2024-192', Anonymous Referee #2, 24 Jan 2025
In the manuscript “Exploring the interplay between observed warming, atmospheric circulation, and soil-atmosphere feedbacks on heatwaves in a temperate mountain region," Lemus-Canovas et al. investigated the thermodynamic processes that contributed to the two most intense heatwaves in the Pyrenees during 2022. The authors then evaluated temperature anomalies during analogous synoptic events in reanalysis data to draw conclusions on the relative roles of anthropogenic warming vs. land-atmosphere feedbacks on heatwave intensity. Ultimately, the authors conclude that global mean warming due to anthropogenic greenhouse gas emissions underlies the trend in maximum surface air temperature anomalies in the Pyrenees and surrounding areas. Still, land-atmosphere feedbacks related to reduced soil moisture conditions in the days preceding heatwave events, particularly in the low lying areas south of the Pyrenees, can amplify warming during an event.
The study is well-motivated and addresses important questions regarding the role of topography in shaping the spatial distribution of surface temperature anomalies during extreme heat events. The results seem highly relevant for evaluating local and regional vulnerability to future heatwaves in the greater Pyrenean area. The analyses are logical and support the text well. The results presented would be a great addition to the literature and I do feel the study is fit for publication in Natural Hazards and Earth System Sciences following revisions.
General Comments:
I think you need to justify that the thermodynamic processes operating at 700 hPa are significantly correlated to the surface environment. It is reasonable to perform your analysis at this pressure level to avoid intersecting with the topography as long as you can demonstrate that the surface conditions are directly responding to changes in the 700 hPa conditions.
Even though the authors use daily minimum temperatures (Tn) to evaluate heatwave occurrence and intensity, these results never make it into the paper. Given the importance of Tn in regard to human impacts, the authors might want to consider adding in any analyses and/or discussion on the drivers of anomalously warm Tn and if this varied between the two heatwaves at all?
I found it a little bit difficult to follow the results section describing the flow analogs, which could just be due to my unfamiliarity with this method. If I’m reading your plots correctly, it seems that the RMSD of the flow analogs is ~25-50% of the magnitude of the geopotential height anomaly associated with the 2022 events relative to climatology. Could you elaborate on if the size of the flow analog RMSD relative to the size of the 2022 geopotential height anomalies impacts their utility in any way? Would it be worthwhile to include a supplemental figure showing a composite of the Z500 field for the 30 flow analogs for the two events in order to get a sense of where the circulation patterns agree or disagree? For example, it could be useful to see if the flow analogs are well matched over the subtropical ridge area but disagree on the strength of the low over the Atlantic. Also, do you expect that the flow analogs are more relevant for characterizing the July heatwave in which you suggest that weaker wind patterns meant that the impact of the anomalously high geopotential height and subsidence was greater during this event than during the June heatwave?
Based on Fig 5, the Tx anomalies associated with the 2022 heatwaves is beyond the 99th percentile of the historical data, so an extreme outlier. Was the soil moisture anomaly during this event also a significant outlier? Given that you found only small impacts of non-linear interactions in the reanalysis data, I’m wondering if this is just because of the limitations of the data in being able to capture the rarity of the 2022 event? I think you try to explain this in the conclusions by proposing a negative feedback between low cloud formation and Tx warming during hot and dry conditions that mitigates extreme temperatures in the higher topography regions. Is there any evidence to support this in your data, did you look at the low level cloud fields or cloud radiative effects?
Specific Comments:
Lines 44—45: “However, a more local approach is lacking for mountain areas and their surrounding regions.” Can you be more specific about what a “more local approach” is in relation to the studies cited in the previous statement.
Lines 46—49: this sentence is very long, considering breaking it up to improve clarity.
Line 53: for (2), I think you want to specify that you’re examining the role of anthropogenic greenhouse gas forcing on the intensity of the heatwaves, “climate change” feels a bit too vague
Lines 51—55: It’s not clear to me how objective (3) is different than (1)
Line 58: I think these are listed backwards – typically Tx is associated with maximum temperature and Tn with minimum temperature (and this seems to be the convention used in the rest of the manuscript).
Line 61: Here you begin to shorten heatwave to HW, however, you use the term heatwave several times in the introduction already. You should be consistent about calling HW after the first instance of using heatwave. See also Section 2.2.1, where you switch between using heatwave and HW, creating more inconsistency.
Line 74: unclear if the text inside of the parentheses is the long name related to CLIMPY or if it is just explaining what CLIMPY is?
Section 2.2.2 and Section 2.2.3: both are labelled “Thermodynamic Equation”
Lines 128—129: The first sentence of this section needs to be re-written in order to make it clear what the purpose of the analogue experiments are. As the sentence “We use the analogue approach, which …” is currently written, it does not explain what you use the analogue approach to do.
Figure 1: The first sentence of the caption seems incomplete “(a) Intensity and duration of the summer 2022.” I believe the dots refer to the intensity/duration of heat above some threshold during JJA 2022 but the sentence reads like they correspond to differences in the intensity of the summer itself. The figure caption needs to be checked for consistency overall – the authors I think accidentally use “y” instead of “and” at one point and the soil moisture units aren’t appropriately superscripted. Also, the labels of the 3 cities on panel a are small and a bit pixelated, making them difficult to read.
Line 162: I believe the ‘a’ in this sentence is a typo
Lines 193—194: “However, the July HW exhibited higher 500 hPa geopotential height anomalies than the June HW, with up to 40 meters of geopotential more” sounds quite awkward. More appropriate phrasing would be something like “500 hPa geopotential height anomalies were up to 40 m greater during the July HW than the June HW” or “Compared to the June HW, geopotential height anomalies during the July HW were up to 40 m greater.”
Lines 195—197: Reference Figure 2a.
Figure 2: add a row of panels showing the differences between the two heatwaves?
Line 206: I don’t believe that you definitively demonstrated that both heatwaves were caused by the subtropical ridging. It would be more accurate to say that both heatwaves occurred during periods of stronger ridging.
Lines 214—219: It would be helpful if you could provide a sense for the days associated with each phase. For example, the “preconditioning” phase corresponds to days -4 to -2 preceding the maximum Tx of the heatwave. Or better yet, reference the color shading in Figure 3 (e.g. preconditioning (Fig. 3, purple shading).
Figure 3: If this is a timeseries of days from the beginning of each heatwave episode, why does the x-axis not start at 0 and why are all of the days negative? I would expect that either the peak temperature day would correspond to day 0 and the preceding days negative and subsequent days positive, or that the onset of the change in temperature (beginning of purple phase) would be 0 and all subsequent days positive?
Lines 248—250: Can you provide a metric that describes how close the 30 flow analogues actually are to the 2022 events? Perhaps standard deviation or RMSE between the 30 analogue composite and the event? Okay I see that you do get to this on Lines 271—274 (referencing panels 5c,f), but I wonder if it should be mentioned earlier in the section?
Line 263: erroneous change in font size?
Lines 277—278: This sentence reads awkwardly, re-phrase to improve clarity.
Lines 397—398: The boundary layer, by definition, extends to the surface so this statement is a bit confusing. Perhaps it is the mixing between the warmer boundary layer air and cooler mid-tropospheric air masses?
Citation: https://doi.org/10.5194/nhess-2024-192-RC2
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