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: open (until 30 Oct 2025)
- RC1: 'Comment on egusphere-2025-4269', Anonymous Referee #1, 05 Sep 2025 reply
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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.