Impact, drivers and pathways of two Arctic atmospheric rivers in April 2020

Avilés-Podgurski, Luisa E.; Martineau, Patrick; Lu, Hua; Yamamoto, Ayako; Maycock, Amanda C.; Orr, Andrew; Phillips, Tony; Bracegirdle, Thomas J.; Hogg, Anna E.; Muszynski, Grzegorz; Fleming, Andrew

doi:10.5194/egusphere-2025-6285

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https://doi.org/10.5194/egusphere-2025-6285

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05 Jan 2026

| 05 Jan 2026

Status: this preprint is open for discussion and under review for Weather and Climate Dynamics (WCD).

Impact, drivers and pathways of two Arctic atmospheric rivers in April 2020

Luisa E. Avilés-Podgurski, Patrick Martineau, Hua Lu, Ayako Yamamoto, Amanda C. Maycock, Andrew Orr, Tony Phillips, Thomas J. Bracegirdle, Anna E. Hogg, Grzegorz Muszynski, and Andrew Fleming

Abstract. Atmospheric rivers (ARs) play a major role in transporting heat and moisture into the Arctic, yet their thermodynamic structure and regional impacts remain poorly understood. Here, we adopt a combined Eulerian-Lagrangian framework to investigate two intense ARs that penetrated into the central Arctic within one week in April 2020, providing a comprehensive view of their large-scale dynamics, moisture sources, and thermodynamic evolution.

The first AR entered the Arctic via the Siberian sector, driven by a highly anomalous quasi-stationary anticyclone over north-central Siberia. The second followed an Atlantic pathway and was associated with an unusually deep and persistent cyclone over Baffin Bay. Despite their distinct origins and pathways, both events produced extreme surface impacts, including widespread warming across Eurasia exceeding 9 °C over a 7-day period and intense precipitation along the Greenland coast and in the central Arctic. The events coincided with a notable decline in sea ice extent in the Barents-Kara Sea and along eastern Greenland, that is highly correlated with the AR-induced warming and rainfall.

Backward trajectory analysis of air parcels associated with extreme Arctic precipitation reveals distinct pathways and thermodynamic evolution. One group of parcels associated with overall cooling and increases in potential temperature exhibits classic AR characteristics: warm, moist, low-pressure airmasses that ascended upon arrival and released intense precipitation. Moisture sources however differed by pathway: the Atlantic AR drew from the warm Gulf Stream region, while the Siberian AR was fed by continental Eurasia. These findings highlight the diverse origins and mechanisms of ARs and their capacity to drive rapid Arctic climate and cryospheric changes.

Received: 16 Dec 2025 – Discussion started: 05 Jan 2026

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Luisa E. Avilés-Podgurski, Patrick Martineau, Hua Lu, Ayako Yamamoto, Amanda C. Maycock, Andrew Orr, Tony Phillips, Thomas J. Bracegirdle, Anna E. Hogg, Grzegorz Muszynski, and Andrew Fleming

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RC1: 'Comment on egusphere-2025-6285', Anonymous Referee #1, 02 Feb 2026 reply

In this study, the authors explore two atmospheric river systems that impacted the Arctic basin on two separate occasions in April 2020. These two events were identified as notable due to their cooccurrence with two unusual pressure systems and alignment with in-situ data associated with the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC). Beyond observational data from MOSAiC, the study also utilized atmospheric reanalysis data from ERA5 and sea ice data from the EUMETSAT Ocean and Sea Ice Satellite Application Facility. Results from the study showed that these two atmospheric river systems led to the increased rate of sea ice loss and regional warming within the basin. Additionally, through synoptic-scale and backward parcel trajectory analysis, the study demonstrated that the unusual pressure systems present during each event were the primary drivers of heat and moisture into the central Arctic basin. I would recommend this paper be accepted following minor revisions.

Major Comments:

Lines 99 – 100: The abstract does a wonderful job detailing why the low- and high-pressure systems associated with the Arctic ARs were considered to be unusual. However, the features that define them as unusual are not defined in the main body of the text. It may be useful to include a description of what makes these pressure systems unusual at or before this point. A more complete definition is provided between lines 190 – 195, but it would benefit the reader to include a broader definition earlier in the paper.

Line 146: In the AR detection section, tARget v4 has Eulerian and Lagrangian tracking capabilities. In the Lagrangian parcel tracking section, it might be helpful to clarify why LAGRANTO v2.0 is being used and why the Lagrangian tracking features of tARget v4 aren’t capable of achieving the goals that LAGRANTO v2.0 is being used for.

Minor Comments:

Figure 1: It is slightly hard to tell which AR is of Atlantic or Siberian origin. Perhaps different contour colors can be used, with one color denoting the Atlantic AR and another color denoting the Siberian AR.

Figure 3: The y-axis label on subplot (d) is a bit awkward to read. Is it possible to add more white space between the two columns of plots (e.g., between the subplots (a) and (c) column and the subplots (b) and (d) column)?

Lines 29 – 31: It is mentioned that ARs enter the Arctic via the Pacific and North Atlantic sectors. Following that, it is mentioned that this is due to cyclones forming and deepening near Greenland. In this case, are Greenland cyclones responsible for both Pacific and North Atlantic AR intrusion? Further clarification might be needed for the mechanism that drives Pacific AR intrusion.

Line 93: It is mentioned that precipitation and rainfall data are retrieved from ERA5. Are these variables the same, or is the precipitation variable measuring both rainfall and snowfall?

Lines 126 – 131: More of a clarification question, since it is not mentioned prior to this point, do both ARs of interest move directly over the ‘Met City’ observation site? If not, how close does each system get?

Line 414: Please add a space in “align,indicating”, such that there is a space between the comma and the word “indicating”.

Point to consider:

When studying ARs entering the central Arctic basin, the use of additional ARDTs can provide further insights into AR characteristics. Historically, the tARget suite of algorithms tends to do a better job at capturing ARs closer to the midlatitude storm track, with tARget V4 receiving significant modifications for improved polar AR detection. With that said, other algorithms have been developed (e.g., Wille vIVT [Wille et al. 2021: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JD033788]) to capture meridionally propagating ARs that might reach the central Antarctic and Arctic regions and that deviate from the mid-latitude storm track. For this study, did you explore how the results change through the use of AR detection algorithms that differ in their general method of AR detection? Adding more than one ARDT to this study is not needed; this is just a curiosity I had.

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Citation: https://doi.org/10.5194/egusphere-2025-6285-RC1
RC2: 'Comment on egusphere-2025-6285', Anonymous Referee #2, 05 Feb 2026 reply

Using reanalysis data and MOSAiC in situ observations, the authors present a detailed and comprehensive analysis of two Arctic atmospheric rivers (ARs) that occurred within one week in April 2020. They show that both events were associated with a cyclone–anticyclone couplet and had substantial impacts on the Arctic environment, including intense warming, sea-ice loss, and extreme precipitation. Using a Lagrangian back-trajectory approach, the manuscript further characterizes the thermodynamic evolution of these two ARs and identifies the principal moisture source regions contributing to AR-induced precipitation. Overall, the manuscript is well written and logically organized. The combined Eulerian–Lagrangian framework provides valuable mechanistic insight into Arctic AR processes. I therefore believe this manuscript is suitable for publication after minor revisions, which I outline below.
Specific comments:
Line 26: The increases in AR frequency over the Arctic are not spatially uniform, with faster increases over the Atlantic sector (Ma et al., 2024a).
Line 211: ‘Coinciding’ is not very accurate here. It seems that Q2m begins to decline sharply only after precipitation returns to ~0. Please explain why the decline in Q2m lags behind changes in precipitation.
Fig. 2a: The results shown here are consistent with the findings of Ma et al. (2024b) (see their Fig. 10). Arctic ARs are usually driven by a cyclone–anticyclone couplet. This couplet can be further divided into a high-pressure-dominant regime (associated with the Siberian AR), with a relatively weak low-pressure system to the left, and a low-pressure-dominant regime (associated with the Atlantic AR), with a relatively weak high-pressure system to the right. I think the authors should provide additional discussion in the Introduction on the flow regimes that drive Arctic AR formation. This would give readers more context for interpreting the results presented in this study.
Fig. 5b: Sea-ice concentration decreases of comparable magnitude can also be observed in the marginal sea-ice regions of the Pacific sector. However, ARs were not observed in those regions. Is it possible that the sea-ice reduction shown in Fig. 5b is simply driven by the seasonality of sea-ice melt? Please also examine the sea-ice concentration differences between 12 April and 22 April in years when no AR event influenced these regions.
Fig. 6c: What drives the precipitation peaks prior to the occurrence of ARs? Are they also driven by extreme moisture-transport events that are not captured by the Guan and Waliser (2024) AR detection algorithm? I recommend that the authors also plot IVT evolution in Fig. 1 using shaded contours and use line contours to indicate SLP anomalies. This would allow readers to assess whether these precipitation peaks are also associated with enhanced moisture transport.
Lines 267-268: Based on Fig. 6d, precipitation starts to increase and peaks after the passage of the AR.
Fig. 9 caption: Should be “moisture uptake minus the absolute magnitude of moisture loss”?
Line 346: I can see the strong moisture loss in the figure. However, I don’t see how strong upward motion is reflected in the figures. Additional explanations might be helpful here.
Fig. 5: I suggest that the authors also plot the spatial distribution of precipitation intensity. This would help readers better understand the magnitude of extreme precipitation associated with Arctic ARs.
Fig. 7: Please explain how you calculated this spatial density distribution for parcel trajectories. This can help readers better interpret the results.
Fig. 10: Since all these parcels are released in central Arctic, shouldn’t central Arctic have the highest density distribution?
Fig. 12: Please explain why there is a lack of moisture loss over the central Arctic where extreme AR precipitation occurs, and parcels are released.
Lines 364-365: If that is the case, why not track the parcels backward in time for more than 7 days?
Line 366: Should be decreasing pressure.
Lines 398–400: This is consistent with the Arctic AR trajectory-clustering results shown in Fig. 7d of Ma et al. (2025). They show that ARs that travel along the U.S. East Coast or the western Atlantic before entering the Arctic through the Atlantic pathway tend to be associated with enhanced moisture uptake from the warm waters of the Gulf Stream.
References:
Ma, W., Wang, H., Chen, G. et al. The role of interdecadal climate oscillations in driving Arctic atmospheric river trends. Nat Commun 15, 2135 (2024). https://doi.org/10.1038/s41467-024-45159-5
Ma, W., Wang, H., Chen, G., Qian, Y., Baxter, I., Huo, Y., and Seefeldt, M. W.: Wintertime extreme warming events in the high Arctic: characteristics, drivers, trends, and the role of atmospheric rivers, Atmos. Chem. Phys., 24, 4451–4472, https://doi.org/10.5194/acp-24-4451-2024, 2024.
Ma, W., Wang, H., Zhang, S., Singh, B., Qian, Y., Huo, Y., et al. (2025). Quantifying moisture sources of Arctic atmospheric rivers during the recent historical period. Journal of Geophysical Research: Atmospheres, 130, e2025JD043918. https://doi.org/10.1029/2025JD043918

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Citation: https://doi.org/10.5194/egusphere-2025-6285-RC2
RC3:
'Comment on egusphere-2025-6285', Anonymous Referee #3, 06 Feb 2026 reply
General remarks:
In their manuscript title “Impact, drivers and pathways of two Arctic atmospheric rivers in April 2020”, the authors, in details, introduced two ARs penetrating into the Arctic Circle within several days in April 2020. Evidenced by reliable observations by MOSAiC and supported by extremeness analyses using ERA5 data, these two ARs were found to bear great impacts on the Arctic surface, including near surface temperature, precipitation, and sea ice. Although these two intense ARs have been investigated before, the authors complemented further in this study with revealing their thermodynamic evolution by means of backward trajectory analysis, the findings of which are novel and interesting. The manuscript is clearly written, and all the figures are well polished, but there are some of my concerns below that I would like the authors to elaborate on.
For the drivers of the two ARs: the authors emphasized the steering impacts of a cyclone-anticyclone couplet. Although the two high/low MSLP centers in between in Fig. 2a have also been analyzed, but it seems that their roles are secondary compared to those two highlighted in Fig. 2b-c. I was wondering whether the cyclone-anticyclone couplet is a must for intense ARs to penetrate into the Arctic. Also, why the anomalous cyclone is important for the Atlantic AR and the anomalous anticyclone for the Siberian AR? Are they random case-dependent or more like common features?

According to the distinct pathways, the Atlantic AR have strong impacts on Greenland and central Arctic, whereas the Siberian AR on Eurasia and also central Arctic. These are evident results that have been revealed by previous studies. I would like to see some further discussion about comparing the strength of Siberian and Atlantic AR impacts. For example, which of them could exert greater surface impacts on the central Arctic (e.g., the Atlantic AR seems to bring about more precipitation over the central Arctic in Fig. 3b)? In this case, it is difficult to isolate the impacts of the two ARs because they ended up merged together. But the simultaneous occurrence of the Atlantic and Siberian ARs would not always take place, very likely I supposed, thus the difference between them is worth being discussed.

I suggest, if there is any, the authors could also provide some simple characteristics of similar/comparable Atlantic or Siberian ARs in the global AR database, which could help support the findings. This is also a potential way to scope with the first two concerns.

Specific remarks:
L31-32: I don’t understand why the “observed increase …” is in line with “projections”? Do the projections here mean future predictions or historical simulations?
L54: Since the strong near-surface winds have been mentioned here, I suggest the effect of the surface wind should also be discussed, besides the results in Fig. 6.
L87: The Atlantic AR has not shown up on 15 April within the Arctic Circle.
L99-105: Why the calculations of the reference distribution are different for different variables (MSLP, T2m, and precipitation)? It sounds a bit complicated and subjective (how about changing the length of days to include more or fewer days?). It would be better if some of them could be unified.
Figure 4b: It would be more meaningful also providing the PDF of absolute T2m, like that in Fig. 3a. Or just provide readers the climatology mean of all 7-day mean T2m, helping contextualize the anomaly magnitude.
L280: Differences “relative to their trajectory endpoints” more sound like the initial status minus the endpoint, but the authors actually mean the reverse.
L302: The orographic precipitation augmentation by the steep topography over the southeastern Greenland is a key factor for the in-situ extreme precipitation in Fig. 5a. I suggest this should be mentioned more and in advance.

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Citation: https://doi.org/10.5194/egusphere-2025-6285-RC3
RC4:
'Comment on egusphere-2025-6285', Anonymous Referee #4, 06 Feb 2026 reply
Review of “Impact, drivers and pathways of two Arctic atmospheric rivers in April 2020”
This manuscript examines two intense and well-documented Arctic atmospheric river (AR) events that occurred during the MOSAiC campaign in April 2020, with a focus on their thermodynamic characteristics and regional surface impacts using ERA5 and MOSAiC observations. While these AR events have been widely studied, the authors apply a Lagrangian parcel tracking approach to further investigate their moisture sources and thermodynamic evolution along the transport pathways. Overall, the paper is well written and provides an interesting and valuable perspective on diagnosing Arctic AR events and their surface impacts. I have only a few major/minor comments and suggestions that may help improve clarity and strengthen the manuscript.
Major/clarifying comments
The AR identification is based on Guan and Waliser (2024), who also employ a Lagrangian tracking framework. According to their tracking methodology, the Siberian AR event and the Atlantic AR evet appear to be merged into a single AR event (if my understanding is correct). However, throughout the manuscript the two events are described as separate ARs. It would be helpful to include a brief clarification explicitly stating that, under the Guan–Waliser AR algorithm, these two ARs are identified as a single merged event, while they are analyzed separately in this study.

Regarding Figure 3: In April, decreases in net shortwave radiation associated with ARs can also contribute to the net surface energy budget (SEB), in addition to enhanced net longwave radiation (Zhang et al., 2025). It would be helpful to discuss the contribution of net shortwave radiation to net SEB, as net shortwave radiative appears to decrease during the two AR events shown in Figure 3e, which may help explain the slight decrease on net SEB on April 16.

In addition, prior to the arrival of the Siberian AR, net longwave radiation, 2-m temperature (T2m), precipitation, 2-m specific humidity (Q2m), and net SEB already show increases, while during the Siberian AR itself the net longwave radiation remains relatively steady. Could the authors please elaborate on the atmospheric conditions preceding the arrival of the Siberian AR that may explain these features?
I also suggest computing T2m, precipitation, Q2m, and surface energy budget components (including longwave, shortwave, sensible, and latent heat fluxes) from ERA5 and comparing them with the MOSAiC in situ observations shown in Figure 3. This comparison could help readers to appreciate a broader spatial context and allow for an assessment of the consistency between the reanalysis and the MOSAiC observations.
Minor comments:
Lines 19–22: I recommend citing the canonical AR definition paper by Ralph et al. (2018).

Figure 1: Please consider using two distinct colors to clearly differentiate the Siberian AR from the Atlantic AR.

Figure 3 (title): Please specify that the in situ observations refer to measurements fromRV Polarstern.

Line 145 (Section 2.5): Please provide additional details on the Lagrangian Analysis Tool (LAGRANTO v2.0) used in the parcel tracking analysis.

Lines 343–345: Figure 12 shows

References:
Zhang, C., Cassano, J. J., Seefeldt, M. W., Wang, H., Ma, W., and Tung, W.: Quantifying the impacts of atmospheric rivers on the surface energy budget of the Arctic based on reanalysis, The Cryosphere, 19, 4671–4699, https://doi.org/10.5194/tc-19-4671-2025, 2025.
Guan, B. and Waliser, D. E.: A regionally refined quarter-degree global atmospheric rivers database based on ERA5, Scientific Data, 11, https://doi.org/10.1038/s41597-024-03258-4, 2024.
Ralph, F. M., Dettinger, M. C. L. D., Cairns, M. M., Galarneau, T. J., and Eylander, J.: Defining “Atmospheric river”: How the glossary of meteorology helped resolve a debate, B. Am. Meteorol. Soc., 99, 837–839, https://doi.org/10.1175/BAMS-D-17-0157.1, 2018.

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Citation: https://doi.org/10.5194/egusphere-2025-6285-RC4
- RC5:
  'Addendum to RC4', Anonymous Referee #4, 07 Feb 2026 reply
  I apologize for the omission in my previous comment (RC4). I would like to add the following clarification:
  Lines 343–345: Figure 12 shows nTnθ, but the text refers to nTpθ. Please correct this inconsistency.
  
  Reply
  
  Citation: https://doi.org/10.5194/egusphere-2025-6285-RC5

Luisa E. Avilés-Podgurski, Patrick Martineau, Hua Lu, Ayako Yamamoto, Amanda C. Maycock, Andrew Orr, Tony Phillips, Thomas J. Bracegirdle, Anna E. Hogg, Grzegorz Muszynski, and Andrew Fleming

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Short summary

Atmospheric rivers (ARs) are narrow filaments transporting vast amounts of water vapour poleward. Rarely, they reach the Arctic, driving strong warming and melt. In April 2020, two ARs reached the central Arctic within one week, raising near-surface temperatures by up to 30 °C and leading to extreme precipitation. Their distinct paths and thermodynamic evolution reveal diverse AR impacts on Arctic sea ice and precipitation extremes.


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