the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Sep 2025
Characterising Marine Heatwaves in the Svalbard Archipelago and Surrounding Seas
Abstract. In the Arctic Ocean, satellite-based sea surface temperature data shows that marine heatwave (MHW) intensity, frequency, duration, and coverage have increased significantly in recent decades, raising concern for Arctic ecosystems. A high frequency (more than three events per year) of MHWs has been shown around the Svalbard Archipelago. Based on this, we investigate MHW trends around Svalbard at the surface and subsurface, using a regional reanalysis from TOPAZ (1991–2022). We find an increase in the frequency and duration of MHW events around the Svalbard Archipelago over the last decade. Furthermore, we observe an increase in MHW frequency and duration west of Svalbard, associated with a long-term rise in sea surface temperature in the region. Analysis of eight individual summer (June–September) MHW events lasting longer than 10 days in Svalbard West, indicated the presence of four shallow (≤ 50 m) and four deep (> 50 m) MHWs after 2010, with a mean duration of 29 days. Some events extended from Svalbard West into the Barents Sea. Heat budget analysis demonstrated a greater contribution of ocean heat transport compared to air-sea heat fluxes in driving the MHW events. Deep and shallow events were associated with ocean heat transport anomalies of up to 9 TW. This new understanding of MHW characteristics including their horizontal and vertical distribution is key to assessing ecological impacts.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4269', Anonymous Referee #1, 05 Sep 2025
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RC2: 'Comment on egusphere-2025-4269', Anonymous Referee #2, 28 Oct 2025
This study provides an overview of marine heatwave (MHW) characteristics around the Svalbard Archipelago in the Eurasian Arctic Ocean, using the regional ocean reanalysis TOPAZ over 1991-2022. This work covers MHW seasonality, long-term trends, and horizontal and vertical extent, and investigates the relative roles of surface heat fluxes and ocean heat transport in driving the MHW events. The article presents two very valuable aspects. First, MHW studies in the Arctic region still remain limited, and the present work constitutes a nice addition. Second, the authors characterise the vertical structure of MHWs, which is still rarely done–and should be more often, in my view. Therefore, I find the focus of this paper important and timely. The approach appears generally robust.
However, the manuscript currently presents some shortcomings, mainly a lack of detail in the methodology employed and insufficiently supported interpretations. Still, I believe these points can be addressed and that after revisions, this study will constitute a very valuable contribution to the journal and the MHW research field. My suggestions are detailed below, listing first my major/general comments, and more minor typesetting points thereafter.
Main comments:
1) The introduction is lacking several key MHW references, both from literature covering global MHW properties and trends, and from the limited literature on Arctic MHWs. This particularly applies to paragraphs lines 14-21 and 33-41. Below are examples of very relevant papers for the authors to include (either all or a selection of them):
- Capotondi et al. (2024), A global overview of marine heatwaves in a changing climate. 10.1038/s43247-024-01806-9
- Malan et al. (2025), Lifting the lid on marine heatwaves. 10.1016/j.pocean.2025.103539
- Hu et al. (2020), Marine heatwaves in the Arctic region: Variation in different ice covers. 1029/2020GL089329
- Golubeva, et al. (2021), Marine heatwaves in Siberian Arctic seas and adjacent region. 10.3390/rs13214436
- Mohamed et al. (2022), Marine Heatwaves Characteristics in the Barents Sea Based on High Resolution Satellite Data (1982–2020). 10.3389/fmars.2022.821646
- He et al. (2024), Arctic Amplification of marine heatwaves under global warming. 10.1038/s41467-024-52760-1
- Léon-FonFay et al. (2024), Sensitivity of Arctic marine heatwaves to half-a-degree increase in global warming: 10-fold frequency increase and 15-fold extreme intensity likelihood. 10.1088/1748-9326/ada029
- Gou et al. (2025), The changing nature of future Arctic marine heatwaves and its potential impacts on the eco-system.10.1038/s41558-024-02224-7
2) Some more details on the methodology could be needed. For example:
- Could you provide the reason for choosing the period “cut-off” in 2010-2011 as soon as it is introduced? Otherwise, this leaves the readers to wonder until rather late in the manuscript.
- What is the reasoning to pick a different split for the seasons than meteorological? I would find it OK, but it is good to provide a rational to readers and/or to explain how routine this is in the field.
- How are horizontal bounds defined for each event? I.e. how do you detect coherent events/patterns of MHW?
3) Part of the interpretations are not sufficiently or not clearly enough supported by analyses (or past literature, if relevant). It would be good to support interpretations of causal relationships with quantitative analyses & to directly illustrate them with corresponding figures. Importantly, the authors conclude that shallow and deep MHWs were associated with the transfer of ocean heat from different sources, with more heat from the NwAFC during shallow MHWs, and more from the NwASC for deep MHWs (l. 345-353 and l. 450-459). However, I don’t see a particularly clear separation between shallow and deep on Fig. 10. Given your small event sample size, is this separation robust? Could you provide more quantitative estimates for this?
4) How do the present findings of a dominant role of OHT around Svalbard fit with previous studies finding that surface MHWs in the Arctic are primarily driven by SHF? These are not necessarily mutually exclusive, but could you include some discussion on this point?
5) While the manuscript is generally clear enough to convey its message, I would still suggest added attention to improve the writing flow, and double check grammar if possible. There are some redundancies, occurrences of colloquial phrasing, and typesetting issues (unnecessary spaces, hyphens, or commas, and occasionally text/information that has been inserted twice in a row). I also know that paraphrasing findings or interpretations from previous studies can sometimes feel a little artificial. However, I noted a few cases of very direct usage of the complete phrasing appearing in past studies, and that, not just in the introduction but also when discussing results. Examples can be found e.g., l. 404-409 and l. 415-419 for the respective cited papers. I would encourage the authors to carefully review their phrasing, to avoid such re-uses and rather foster their own original formulations, wherever possible.
Minor comments & edits:
General: There are quite a lot of appendix figures. Are all panels absolutely essential to supporting the analyses? Would it be possible to reduce that appendix figure number by merging some together and/or by including key information in the main figures? This may lighten the overall manuscript a little, but this is not absolutely essential.
L. 52-54: Can you include a citation to support this sentence?
Fig. 1: Could we have some indication of the depth(s) of these currents? Rather surface / subsurface? I know the AW is in some places at the surface, in some subsurface. It could be good to indicate what is where somehow (e.g. dashed or thicker lines for subsurface). Also, please capitalise “mooring” consistently here.
L. 86: “no more than two days below the threshold ” could you justify why adding this somehow more relaxed condition?
L. 99: This is interesting. So, this may oversee events that do not have a surface expression, right? That is OK, just needs to be stated for clarity (and discussed in the discussion)
L. 106: How are anomalies computed (for each data product)? Against which reference period?
L. 115. What do you mean by “unique identified”?
L. 123. Rho should be rho_0?
L. 129: “z(λ) is the depth at each section”, meaning, down to the ocean floor?
L. 204: Here and throughout the manuscript, please remove parentheses after figure number/subpanels, when quoting in text
L. 204: “…r = 0.8, p <0.05); Fig. A1a), and a slightly weaker correlation for monthly anomalies (r = 0.7, p <0.05),…” Please round values consistently between text & figure (here the figures say 0.75 and 0.72)
L. 234: “…the highest MHW intensity is located at water mass fronts, for example south-east of Svalbard at∼74°N” Could you tell readers to which water mass front this corresponds? Same for subsequent examples.
L. 236-239: This is interesting. However, this should not be in a section titled “trends”. Please restructure or retitle accordingly. e.g., Seasonal variations and trends (with, in order, seasonal variations, annual trends, seasonal trends).
L. 239: To match the colourscale on the figure, might be best to say 1-to-3 events in summer, vs. 1-to-2 events in other seasons
L. 242: “the region shown in Fig. 3” So, the whole map shown? or the black box?
L. 243-244: This is a strange formulation. Why not give the intensity amplitude directly for all, as shown in the figure?
L. 251: “The largest change in intensity is observed in spring“ Interestingly, I see rather that large parts of the WSC experience a decrease. Intensity changes in summer and spring vary also quite a bit spatially, so it would be good to provide a bit more understanding of why, or at least propose reasons. And is this averaged on the box? or on the whole figure? This would need to be a bit more precisely described here.
Figure 2: this is a nice visual, but it is hard to see whether there is indeed an abrupt change in absolute intensity or whether this is a visual artifact from the colourbar and/or the fact that we are looking at anomalies here (so, bound to switch from blue negatives to red positives once the transient warming exceeds the 30-year climatological reference period).
L. 254-255: the same information appears twice in this sentence.
L. 256-257: Again, this is redundant with the first sentence, and the third time this information appears in this paragraph.
L. 265-269: “a 5–52% decrease in duration and 17–44% decrease in intensity” Is this a trend, a reduction of the trend, or a reduction in mean value? Where are the ranges coming from? What is the reason behind testing a shorter reference period for anomaly computation? A lot more information is needed here to understand how these results are obtained and their physical meaning.
L. 270: This section should probably be merged into one subsection “horizontal and vertical extents”
L. 299-309: In this section, could you provide more quantitative information? E.g. line 305, “SHF values considerably exceed the ±1 standard deviation range”: How many std variation? for how long/how frequent? Otherwise I would suggest shortening this part.
Figure 8: which kind of STD is this? (spatial, across events…)
Figure A2: Define your OHT “s, n, w” explicitly (I assumed it is through each bound of the box)
L. 313-315 and Figure A11: There is no residual term in your heat content tendency equation, so can you please explain how this quantity was calculated in your figure A11?
L. 325-331: which figure(s) supports this analysis here? Is it still based on Table A2? If yes, I do not see how the 2016 event has a larger northward transport compared to 2015, 2017 and 2019. The proposed causal relationships could be supported more directly with quantitative analyses & directly illustrated by figures.
L. 325: “implying an accumulation of heat”, could you elaborate on what is meant by this? By which processes would heat accumulate there? Could it also imply a possible enhanced heat loss from the ocean to the atmosphere, or a larger heat flux to the Svalbard shelf/coast?
L. 339: is a single-day snapshot really representative here? Would it be possible to average over a coherent time-period of days to weeks to give more robustness to your results?
L. 339-344: so, in other words, shallow events constrained to the NwAFC, and deep events relatively ubiquitous, right? If yes, that might be a more direct, clearer phrasing.
L. 347-351: The phrasing here is a little too vague here. It would be great to have some quantification, if possible.
L. 350: “showing relatively higher OHC compared to years with shallow events in the BSO. Meanwhile, in the NwAFC region, except for 2011, the OHC is largely higher during shallow events compared to deep events.” Is this robust? As mentioned in my main comments, on the figure, given your small event sample size, I don’t see a clear separation between shallow and deep.
L. 356-359: This sentence starts by introducing your own results, but ends with a citation. Please state more clearly what is a new finding, and what part has been discussed in other papers.
L. 361-362: “Our results also suggest that deep events received heat from the Norwegian Atlantic Slope Current (NwASC),” I am confused by this. Which part of the results showed this causal relationship?
L. 365-380: Have you considered detrending SST instead to compute SSTA? It does not necessarily need to be computed, but given the strong Arctic trends, it immediately comes to mind when discussing baselines and MHW definition approaches (and could be worth discussing, in my view).
L. 382: observe -> find (as it is not observed here, or not directly)
L. 384: “described in Mohamed et al. (2022); Barton et al. (2018)” I know the authors mean that the Polar Front is described in those papers, but it is not exactly clear here. Same as above, it would be very beneficial to discriminate what is your results vs. what is previous work. I would suggest moving these citations & the description of the polar front to the introduction.
L. 388: to my understanding, “momentum mixing” is an atmospheric process. It could be worth explaining what it is and why it is relevant here.
L. 391: There is some literature citation missing on the Atlantification in the Eurasian Basin, such as:
- Polyakov et al. (2017), Greater role for Atlantic inflows on sea-ice loss in the Eurasian Basin of the Arctic Ocean. 1126/science.aai8204
L. 415-417: I like this justification. However, I think it could be useful to have it earlier, so the reader is not left wondering why a 10-day duration was picked in the first place.
L. 421-430: Malan et al. (2025) recently proposed a new, detailed classification of MHW vertical structure. If possible, it would be great to see this discussed here (See full reference provided at the beginning of this review).
L. 454-455: Still cannot find where this is established (and I do not see this clearly in Figure 10)
Citation: https://doi.org/10.5194/egusphere-2025-4269-RC2
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- 1
The authors presented a comprehensive MHW study in the Svalbard Archipelago region using TOPAZ analysis, which is validated by various moorings and OISST analysis. They presented MHWs changes in timescales of decade and season, vertical and horizontal extent, provided heat budget analysis for each MHW events, and concluded that the most MHWs are driven by the ocean heat transport. The manuscript is well written and can be published in EGUsphere after revision. My major concern is what drive the deep MHW, if the surface heat flux, how?
Here are my detailed comments (L=line):
L54: Use consistent time unit in L54 °C year-1, L57 %y-1, L62 °C per decade
L110, 90th percentile. I am not sure whether the region is ice free during the summer from 1991-2022. If not, how the MHWs are defined in the ice covered region, since water temperature changes a lot when ice is melted. E.g. the threshold, which is calculated using the temperature with ice in the early period, may be difficult to applied to the time when ice in melted in the later period. Can you test how much MHW features are changed if the threshold is set to 95th percentile?
L128, equation (2), why is Tref is needed?
Figures A1B, A2B and other figures with p-value: 1.43e-24, 1.27e-17, check and revise.
L211-219, Figs. A5 (low correlation) and A6 (bifurcated correlation), these figures may indicate the biases of TOPAZ model in the coastal regions.
L234, note that the intensity decreases in many regions although frequency and duration increase in most of the regions. Therefore it might be helpful to use the cumulative intensity by integrating SSTA and time in units of degree-day (e.g. Huang et al. 2025, DOI 10.1175/BAMS-D-24-0337.1), which will enable us to see how MHWs intensify with time.
L240-246, it might be helpful to add implications or causes of those features, e.g. warming is strong in winter than summer etc.
L248-252, The definition of MHW differences is not straightforward: there are many regions without MHWs in left panels marked as “missing”, which results the difference in right panel are marked as “missing” or blank. Can the “missing” in the left panels be marked as “zero”? This should make the difference more reasonable. One alternative way is to assess the differences is to integrate MHWs in space and then compare their time evolution.
L279, “Note that MHWs are not analysed north of the sea ice edge (sea ice concentration ≥ 15%).” This might be noted much earlier in definition in section 2.1.
Figures 5, 6, “peak date of each MHW”. How is this defined? MHW evolution may not be synchronized in different regions, and therefore it is not straightforward to define “one” MHW within a large region (more than one grid point). What the black dots represent?
L315, 332, “With the exception of events in 2016 and 2017 (deep events),” Does this imply that the deep MHWs are driven by the surface heat flux, which is hard to imagine. If not, what drive the deep MHWs? “With the exception of events in 2016 and 2017 (deep events),” why?
Figure 10, suggest exchange the dotted with solid lines, which will highlight the MHWs.
Section 4, Discussion, The discussion is lengthy and should be shortened.