Northern Greenland transect stacked ice cores as a proxy for winter extreme events in Europe

Gagliardi, Alessandro; Rimbu, Norel; Lohmann, Gerrit; Ionita, Monica

doi:10.5194/egusphere-2025-3071

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https://doi.org/10.5194/egusphere-2025-3071

Preprints

25 Jul 2025

| 25 Jul 2025

Northern Greenland transect stacked ice cores as a proxy for winter extreme events in Europe

Alessandro Gagliardi, Norel Rimbu, Gerrit Lohmann, and Monica Ionita

Abstract. High-resolution ice core records from the Greenland ice sheet provide critical insights into past climate variability across seasonal to multidecadal timescales. A key proxy in these reconstructions is the concentration of stable oxygen isotopes (δ¹⁸O), which reflects both regional climatic conditions, such as temperature, as well as atmospheric and oceanic circulation patterns. While recent studies have linked δ¹⁸O variability to synoptic-scale phenomena, particularly atmospheric blocking, its relationship to extreme hydroclimatic events in Europe remains underexplored. This study demonstrates that a stacked record of δ¹⁸O from the Northern Greenland Transect (NGT), spanning 1602 to 2011, serves as a proxy for hydroclimatic extremes in Europe. The connection between δ¹⁸O anomalies and European atmospheric circulation patterns is investigated across two periods: the observational era (1920–2011) and a longer historical context (1602–2003) using paleoclimate reanalysis data. Composite analysis reveals that years characterized by low δ¹⁸O values in the NGT record correspond to an increased frequency of atmospheric blocking over Europe. These blocking events are associated with distinct hydroclimatic extremes. Specifically, the analysis shows a consistent pattern of enhanced frequency of extreme precipitation along Norwegian coast and more frequent extreme drier conditions over southern Europe during such years. The persistence of this linkage in both modern observations and long-term reconstructions underscores the robustness and temporal stability of the relationship between Greenland δ¹⁸O variability and European hydroclimatic extremes driven by atmospheric blocking.

Received: 27 Jun 2025 – Discussion started: 25 Jul 2025

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Journal article(s) based on this preprint

06 May 2026

Northern Greenland transect stacked ice cores as a proxy for winter extreme events in Europe

Alessandro Gagliardi, Norel Rimbu, Gerrit Lohmann, and Monica Ionita

Clim. Past, 22, 935–955, https://doi.org/10.5194/cp-22-935-2026,https://doi.org/10.5194/cp-22-935-2026, 2026

Short summary

Alessandro Gagliardi, Norel Rimbu, Gerrit Lohmann, and Monica Ionita

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-3071', Mathieu Casado, 20 Aug 2025

See the attached file

Citation: https://doi.org/10.5194/egusphere-2025-3071-RC1
- AC2: 'Reply on RC1', Alessandro Gagliardi, 10 Oct 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3071/egusphere-2025-3071-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3071-AC2
RC2:
'Comment on egusphere-2025-3071', Pascal Yiou, 02 Sep 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3071/egusphere-2025-3071-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-3071-RC2
- AC3: 'Reply on RC2', Alessandro Gagliardi, 10 Oct 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3071/egusphere-2025-3071-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3071-AC3
CC1:
'Comment on egusphere-2025-3071', Dániel Erdélyi, 18 Sep 2025

I read this interesting and well-written study with great interest. I would like to use the opportunity of open community comment to ask for clarification of certain less clear points. Some of these questions are resonating with some points raised already by Mathieu Casado but proposed from a different perspective.
So, links between firn and ice core stable isotope time series from Greenland and weather/climate variability over Europe have been intensively studied during the past decades. Several excellent results were delivered just right from the team of the authors of the current paper, as well.
In this study a relatively new stacked δ18O record is tested as a proxy European hydroclimatic extremes driven by atmospheric blocking. The firn and ice cores used to compile the stacked δ18O record were actually distributed across north and central Greenland (Hörhold et al., 2023). The spatial extent of the ice core array scratches from ~71 to 80N and from ~30 to 50 W. However, different regions of the Greenland Ice Sheet experience distinct changes in the average moisture source locations, which is manifested in the stable isotopic signal of ice/firn (Sodemann et al., 2008 https://doi.org/10.1029/2007JD008503 ). Different regions of the Greenland Ice Sheet experience distinct changes in average moisture source locations, with the strongest variability observed in north and west Greenland. This strong sensitivity of Greenland winter precipitation to the NAO has a substantial impact on the stable isotope composition of Greenland precipitation, which is crucial to consider when interpreting ice-core δ¹⁸O records.
These isotope-hydrometeorological domains reflect the major regions of the Greenland delineated based on a synoptic survey of precipitation and possible effects of orography on moving cyclones (Chen et al., 1997 https://doi.org/10.1175/1520-0442(1997)010<0839:POGRBA>2.0.CO;2 ).
However, the employed stacked δ18O record amalgamates distinct regions considering the isotope-NAO relationship. My question is whether a subset of the firn/ice core δ18O records of Hörhold et al., 2023, rather than the NGT-2012 composite, might provide a better proxy for the European hydroclimatic extremes driven by atmospheric blocking?
Supporting the regionalization approach a previous study (Ortega et al., 2014, which is already cited in this manuscript) evaluated a set of shallow ice core derived δ18O series from Greenland also pointed out spatially variable depletion/enrichment in the isotopic composition of ice/firn depending on the dominant NAO mode. In addition, ice core d18O records across Greenland exhibited distinctive spatial arrangements regarding change points, all corresponded to changepoints in the NAO, indicative of a consistent atmospheric influence over the past millennium (Hatvani et al., 2022 https://doi.org/10.3390/atmos13010093 ).

Specific comments
The ”Results” section actually contains some discussion so section title could be changed. In addition, to improve the discussion the results, beside the previously mentioned studies, a very recent paper (Brönnimann, et al. (2025). https://doi.org/10.1038/s41561-025-01654-y ) largely overlapping with the current one could be compared.
The “main set of extremely consecutive positive years between 1927 and 1932” mentioned in line 115 corresponds well with the changepoint identified around 1933 in ice-core δ¹⁸O records from southern and central Greenland (Hatvani et al., 2022).
The last comment on this part is that the text in lines 182-188 that could fit better to Methods. In addition, region names and citations of Fig 6 panels might need careful checking in the subsequent paragraphs (e.g. “Scandinavia” should be replaced with “Baltic” in line 192 however then

Minor technical issues with the reference list are:
-Authors’ name for the first reference (line261) should be checked
-One of the duplicated items (lines 321-325; lines 349-352) should be removed from the reference list

Citation: https://doi.org/10.5194/egusphere-2025-3071-CC1
- AC1: 'Reply on CC1', Alessandro Gagliardi, 10 Oct 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3071/egusphere-2025-3071-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3071-AC1

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-3071', Mathieu Casado, 20 Aug 2025

See the attached file

Citation: https://doi.org/10.5194/egusphere-2025-3071-RC1
- AC2: 'Reply on RC1', Alessandro Gagliardi, 10 Oct 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3071/egusphere-2025-3071-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3071-AC2
RC2:
'Comment on egusphere-2025-3071', Pascal Yiou, 02 Sep 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3071/egusphere-2025-3071-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-3071-RC2
- AC3: 'Reply on RC2', Alessandro Gagliardi, 10 Oct 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3071/egusphere-2025-3071-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3071-AC3
CC1:
'Comment on egusphere-2025-3071', Dániel Erdélyi, 18 Sep 2025

I read this interesting and well-written study with great interest. I would like to use the opportunity of open community comment to ask for clarification of certain less clear points. Some of these questions are resonating with some points raised already by Mathieu Casado but proposed from a different perspective.
So, links between firn and ice core stable isotope time series from Greenland and weather/climate variability over Europe have been intensively studied during the past decades. Several excellent results were delivered just right from the team of the authors of the current paper, as well.
In this study a relatively new stacked δ18O record is tested as a proxy European hydroclimatic extremes driven by atmospheric blocking. The firn and ice cores used to compile the stacked δ18O record were actually distributed across north and central Greenland (Hörhold et al., 2023). The spatial extent of the ice core array scratches from ~71 to 80N and from ~30 to 50 W. However, different regions of the Greenland Ice Sheet experience distinct changes in the average moisture source locations, which is manifested in the stable isotopic signal of ice/firn (Sodemann et al., 2008 https://doi.org/10.1029/2007JD008503 ). Different regions of the Greenland Ice Sheet experience distinct changes in average moisture source locations, with the strongest variability observed in north and west Greenland. This strong sensitivity of Greenland winter precipitation to the NAO has a substantial impact on the stable isotope composition of Greenland precipitation, which is crucial to consider when interpreting ice-core δ¹⁸O records.
These isotope-hydrometeorological domains reflect the major regions of the Greenland delineated based on a synoptic survey of precipitation and possible effects of orography on moving cyclones (Chen et al., 1997 https://doi.org/10.1175/1520-0442(1997)010<0839:POGRBA>2.0.CO;2 ).
However, the employed stacked δ18O record amalgamates distinct regions considering the isotope-NAO relationship. My question is whether a subset of the firn/ice core δ18O records of Hörhold et al., 2023, rather than the NGT-2012 composite, might provide a better proxy for the European hydroclimatic extremes driven by atmospheric blocking?
Supporting the regionalization approach a previous study (Ortega et al., 2014, which is already cited in this manuscript) evaluated a set of shallow ice core derived δ18O series from Greenland also pointed out spatially variable depletion/enrichment in the isotopic composition of ice/firn depending on the dominant NAO mode. In addition, ice core d18O records across Greenland exhibited distinctive spatial arrangements regarding change points, all corresponded to changepoints in the NAO, indicative of a consistent atmospheric influence over the past millennium (Hatvani et al., 2022 https://doi.org/10.3390/atmos13010093 ).

Specific comments
The ”Results” section actually contains some discussion so section title could be changed. In addition, to improve the discussion the results, beside the previously mentioned studies, a very recent paper (Brönnimann, et al. (2025). https://doi.org/10.1038/s41561-025-01654-y ) largely overlapping with the current one could be compared.
The “main set of extremely consecutive positive years between 1927 and 1932” mentioned in line 115 corresponds well with the changepoint identified around 1933 in ice-core δ¹⁸O records from southern and central Greenland (Hatvani et al., 2022).
The last comment on this part is that the text in lines 182-188 that could fit better to Methods. In addition, region names and citations of Fig 6 panels might need careful checking in the subsequent paragraphs (e.g. “Scandinavia” should be replaced with “Baltic” in line 192 however then

Minor technical issues with the reference list are:
-Authors’ name for the first reference (line261) should be checked
-One of the duplicated items (lines 321-325; lines 349-352) should be removed from the reference list

Citation: https://doi.org/10.5194/egusphere-2025-3071-CC1
- AC1: 'Reply on CC1', Alessandro Gagliardi, 10 Oct 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3071/egusphere-2025-3071-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3071-AC1

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

ED: Reconsider after major revisions (14 Oct 2025) by Nancy Bertler

AR by Alessandro Gagliardi on behalf of the Authors (25 Nov 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (18 Dec 2025) by Nancy Bertler

RR by Anonymous Referee #3 (04 Jan 2026)

Suggestions for revision or reasons for rejection

It is the first time for me to review this work. The manuscript focuses on the association between the δ18O record of the Northern Greenland Transect (NGT) stacked ice cores and winter extreme hydroclimatic events in Europe. The research design spanning the observational period and long-term reconstruction period is rigorous, providing a novel approach for paleoclimate extreme event reconstruction. In my opinion, overall, the authors have effectively addressed core issues raised in previous reviews, including insufficient discussion on NAO connections, the impact of tree-ring biases, and ambiguous definitions of extremes. Key technical errors (e.g., misattribution of the 2D blocking index, missing figure citations) have been corrected. The manuscript’s completeness and rigor have been enhanced through expanded discussions, supplementary data explanations, and revised expressions.
The work aligns well with the scope of Climate of the Past and contributes meaningfully to the field. I have several minor suggestions that can be seamlessly integrated to further strengthen the manuscript.

1. The study centers on the causal chain “δ18O negative anomalies – atmospheric blocking – European extreme climate”, with a clear mechanistic explanation for the latter segment (atmospheric blocking to European extreme climate). However, the first link (δ18O negative anomalies to atmospheric blocking) remains superficially addressed, lacking in-depth elaboration of the underlying physical mechanisms. Strengthening this part would significantly enhance the rigor of the study.
2. L176, The sentence contains a redundant “the” (likely a typo). Please revise for conciseness.
3. L175-180, Consider clarifying that linear detrending does not alter the main patterns of positive/negative years (e.g., by referencing supplementary material if applicable).
4. L186, The phrase “results not shown” may weaken the argument. Consider including key results in the main text or supplementary materials to enhance credibility.
5. L195-240: The mechanistic explanation for the first link (“δ18O negative anomalies → atmospheric blocking”) is insufficiently developed. For instance, δ18O is also a well-recognized temperature proxy—clarifying whether it reflects local temperature changes in northern Greenland and how these changes are physically linked to European atmospheric blocking would significantly strengthen the argument. Elaborating this pathway is critical for validating the causal chain.
6. L218-219, the authors are discussing the negative years of δ18O in this paragraph, and Fig. 2b should be cited instead of Fig. 2a.
7. L249, what is “now dry air masses”
8. Fig. 4, Panel (c) shows the cool nights, should be the warm nights, according to the caption of Figure 4. Also, the captions of panel (b) and (c) should be re-arranged: (b) should be detrended integrated vapor transport (IVT) and (c) should be detrended warm nights index TN10p.
9. L277-278, should be Fig. 5b and c.
10. L327-328, The claim that Figure 5a indicates a northward jet stream shift during low δ18O periods requires clearer justification. Please specify the visual or analytical evidence supporting this interpretation.
11. L342-343, please rephrase this sentence.
12. Conclusions: When referencing “extreme weather events” in the conclusions, specific types should be explicitly described (e.g., extreme precipitation along the Norwegian coast, extreme drought, and low temperatures in Southern Europe) to avoid ambiguity and align with the study’s empirical findings.

Hide

RR by Pascal Yiou (09 Feb 2026)

ED: Publish subject to minor revisions (review by editor) (05 Mar 2026) by Nancy Bertler

Dear Authors,
Both reviewers—including the new reviewer—are satisfied with your revisions. I am grateful for their thoughtful assessments, and I want to thank you for the substantial effort you invested in revising the manuscript. Your work has significantly strengthened its clarity and the scientific discussion.
Please address the remaining minor comments outlined in Review Reports 1 and 2 below.
I look forward to reading your revised manuscript.
Kind regards,
Nancy Bertler

Report 1:
1. The study centers on the causal chain “δ18O negative anomalies – atmospheric blocking – European extreme climate”, with a clear mechanistic explanation for the latter segment (atmospheric blocking to European extreme climate). However, the first link (δ18O negative anomalies to atmospheric blocking) remains superficially addressed, lacking in-depth elaboration of the underlying physical mechanisms. Strengthening this part would significantly enhance the rigor of the study.
2. L176, The sentence contains a redundant “the” (likely a typo). Please revise for conciseness.
3. L175-180, Consider clarifying that linear detrending does not alter the main patterns of positive/negative years (e.g., by referencing supplementary material if applicable).
4. L186, The phrase “results not shown” may weaken the argument. Consider including key results in the main text or supplementary materials to enhance credibility.
5. L195-240: The mechanistic explanation for the first link (“δ18O negative anomalies → atmospheric blocking”) is insufficiently developed. For instance, δ18O is also a well-recognized temperature proxy—clarifying whether it reflects local temperature changes in northern Greenland and how these changes are physically linked to European atmospheric blocking would significantly strengthen the argument. Elaborating this pathway is critical for validating the causal chain.
6. L218-219, the authors are discussing the negative years of δ18O in this paragraph, and Fig. 2b should be cited instead of Fig. 2a.
7. L249, what is “now dry air masses”
8. Fig. 4, Panel (c) shows the cool nights, should be the warm nights, according to the caption of Figure 4. Also, the captions of panel (b) and (c) should be re-arranged: (b) should be detrended integrated vapor transport (IVT) and (c) should be detrended warm nights index TN10p.
9. L277-278, should be Fig. 5b and c.
10. L327-328, The claim that Figure 5a indicates a northward jet stream shift during low δ18O periods requires clearer justification. Please specify the visual or analytical evidence supporting this interpretation.
11. L342-343, please rephrase this sentence.
12. Conclusions: When referencing “extreme weather events” in the conclusions, specific types should be explicitly described (e.g., extreme precipitation along the Norwegian coast, extreme drought, and low temperatures in Southern Europe) to avoid ambiguity and align with the study’s empirical findings.

Report 2:
Please streamline the notations for latitude (capital or lowercase \phi?).

The authors state (l. 312): "Furthermore, the eruptions of El Chichón in 1982 and Pinatubo in 1991 are also associated with warming across northern Europe and very low values of δ18O in the NGT stacked series." This seems to be a controversial issue: Polvani et al. (https://doi.org/10.5194/acp-19-6351-2019) made a case that their "analysis and interpretation offer compelling new evidence that the observed warming of the Northern Hemisphere continents in the winter 1991–1992 was very likely unrelated to the 1991 Mt. Pinatubo eruption. Therefore, any causal (or association of) mechanisms should be discussed with caution.

L. 348: I do not see where the isotopic data is available.

Hide

AR by Alessandro Gagliardi on behalf of the Authors (12 Mar 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (30 Mar 2026) by Nancy Bertler

AR by Alessandro Gagliardi on behalf of the Authors (07 Apr 2026)

Journal article(s) based on this preprint

06 May 2026

Northern Greenland transect stacked ice cores as a proxy for winter extreme events in Europe

Alessandro Gagliardi, Norel Rimbu, Gerrit Lohmann, and Monica Ionita

Clim. Past, 22, 935–955, https://doi.org/10.5194/cp-22-935-2026,https://doi.org/10.5194/cp-22-935-2026, 2026

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Alessandro Gagliardi, Norel Rimbu, Gerrit Lohmann, and Monica Ionita

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This study shows that stable oxygen isotope ratios from Greenland ice cores can help identify extreme winter events in Europe. In years with a lack of the heavier oxygen isotope, we found changes in the atmospheric circulation over Europe. These changes bring warmer, wetter conditions to the Norwegian coast and colder, drier conditions to southern Europe. The pattern appears in both recent and past data, staying stable over the last 400 years.


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