General comments

This paper investigates the long-term impact of climate warming on the occurrence of marine heatwaves in the North Sea—a topic well within the scope of Ocean Science. The manuscript is well-written and clearly structured. The authors address the main research question appropriately and present interesting results that contribute to the scientific community. Overall, I believe the study meets the journal’s standards and could be published after moderate revision. Detailed comments and suggestions for improvement are provided below.

Specific comments

P2_L50:

The authors state that “the variability of MHW in the southern North Sea has been attributed to changes in stratification…”. I believe this is a misinterpretation of Chen et al. (2022). That study does not attribute MHW variability to stratification; rather, it argues the opposite—that the presence and persistence of stratification in the southern North Sea are attributed to the occurrence of MHWs.

P2_L71:

Throughout the Introduction, there is no mention of chlorophyll-a, yet the final sentence abruptly introduces it as a focus question regarding its response to MHWs. Even earlier in the paragraph, the stated goal of the study is to quantify the role of climate change, specifically increasing SST, in MHWs. This sudden shift lacks coherence. I recommend that the authors either omit this focus (along with Section 3.5) and reserve it for a future study (which may already form a complete narrative), or revise the introduction to systematically incorporate this aspect.

P5_L155–160:

I would not describe 2012/13 as indicating a “second regime shift.” A regime shift is not instantaneous; it marks a transition that may unfold over months or years. Figures 1a and 2 clearly illustrate such a transition. The regime shift appears to begin around 2000 and conclude in 2012. Rather than defining 2012/13 as a second regime shift, this study might more accurately be described as the first to delineate the full span of regime shift—from 2000 to 2012. Clarifying this timeframe provides greater scientific value than introducing an arguably redundant second shift.

P5_L173–175:

According to the domain-averaged SSTA (the thick black line in Figure 2), 2000 seems to be the transition point between the cold and transitional periods, as it is when the averaged SSTA reaches 0°C. Moreover, Figure 1 shows that, based on the Pettitt test, 70% of the North Sea experienced the transition after 2000, while only 30% (mainly the southern North Sea) transitioned between 1996 and 1998. If 1997 is used as the transition year, the affected area would be less than 10%.

P6_L187–189:

The authors state that “SST in the North Sea experienced two significant regime shifts in the late 1990s and after 2013.” In my view, the North Sea underwent a single regime shift between 2000 and 2012, transitioning from MCS dominance to MHW dominance.

P7:

Following my previous comment, I suggest that the authors indicate the regime shift period (2000–2012) in Figure 2.

P9_L251–252:

The authors state: “The increase in internal variability of SST leads to a broadening of the PDF of temperature, making the occurrence of MHW more likely.” However, Figure 4A–B shows a decrease in variance from the pre-2013 to post-2013 period. Does this imply that MHWs became less likely after 2013? This needs clarification.

P10_L266–268:

The authors write: “The frequency of MHW occurrence is higher in all months post-2013 than pre-2013, except for February and March...” In my view, a more outstanding difference is post-2013, the mean MHW frequency is considerably higher (almost doubled) than pre-2013 from June to December. This implies that climate warming mainly affects the appearance of MHW in the second half of the year. While this is mentioned in lines 272–273, the earlier description (lines 266–271) does not clearly highlight it.

P11_L285–287:

The statement “To date, no study has evaluated the relative role of the long-term trend and internal variability on the MHW in the North Sea” is not true. Chen and Staneva (2024) have addressed this very question, using similar data (1982–2022) and methodology (Hobday et al., 2016; MATLAB toolbox by Zhao & Marin, 2019). They also identified different MHW patterns over the last 30 years (1993–2002, 2003–2012, 2013–2022). To my knowledge, their study is the first of its kind in the North Sea. The authors should revise this claim and properly credit prior research, especially work so closely aligned with theirs.

Chen, W., & Staneva, J. (2024): Characteristics and trends of marine heatwaves in the northwest European Shelf and the impacts on density stratification: In: von Schuckmann, K., Moreira, L., Grégoire, M., Marcos, M., Staneva, J., Brasseur, P., Garric, G., Lionello, P., Karstensen, J., and Neukermans, G. (eds.): 8th edition of the Copernicus Ocean State Report (OSR8). Copernicus Publications, State Planet, 4-osr8, 7, doi:10.5194/sp-4-osr8-7-2024 

P14_L356:

Why were specific years selected rather than showing long-term trends? While Figure 8 provides spatial maps of annual mean MHW days, selecting individual years only highlights temporal variability within the same region (i.e., the North Sea). For instance, although 2022 and 2023 had similar total MHW days, the southern North Sea experienced different MHW durations. Presenting trends instead, like in Figure 2e–h of Chen and Staneva (2024), would better illustrate spatial variability.

P14_L365–367:

The statement that “SST variability and thus MHW in the North Sea are largely influenced by atmospheric rather than oceanic forcing, which is consistent with Tinker and Howes (2020)” is misleading. Tinker and Howes (2020) found that marine air temperature is the main driver of SST rise—not necessarily the dominant influence on MHWs.

P15_L379–381:

Why focus only on frequency and intensity? I would expect a discussion of trends in MHW duration and total days, or at least an exploration of the drivers behind MHW characteristics.

P15_L387:

What exactly is meant by “internal variability”? Do the authors refer to hydrodynamics?

P18_L440:

It's not only reduced wind mixing; stable stratification also suppresses vertical water mass exchange, thereby limiting heat transfer to deeper layers. As a result, heat accumulates in the surface layers.

P19_Section 3.5:

As previously mentioned, I do not see a clear connection between this section and the overall focus of the paper (nor is it reflected in the title). Either develop the introduction and methodology to properly integrate this topic, or consider removing it and addressing it in a future study.

P22_Conclusions:

The conclusion section is overly long and verbose. Please revise to make it more concise and focused.

Technical Corrections

P4_L125: Add a comma after the equation.

P4_L126: Change “Where” to lowercase: “where”.

Review of “Amplified Warming and Marine Heatwaves in the North Sea Under a Warming Climate” by Bayoumy Mohamed et al

Overview

The manuscript uses a long-term dataset of sea surface temperature (SST) observations in the North Sea to study the impact of the warming climate on SSTs and marine heatwaves (MHWs). The authors use established statistical techniques to confirm a regime shift in the 1990s, identify a further change in 2013, and attribute much of the recent increase in MHW occurrence and severity to increases in SST. MHWs strongly impact the marine environment, particularly the ecosystem and this study, showing ocean warming to be a major driver, is important for understanding how MHWs may develop in the future.

In addition to the SST/MHW analysis, a case study of a strong MHW event in spring 2024 and the impact of hot and cold events on chlorophyll-a concentrations are also studied.

Main comments

The analysis of the spring 2024 event and chl-a concentrations are rather disconnected from the SST/MHW analysis (sections 3.1 – 3.3) and not related to the manuscript title. These sections should be better integrated in the manuscript. For instance, the 2024 event is attributed to anomalous atmospheric conditions; would the event still have happened without SST warming? Also, the description of changes in the timing of the chl-a cycle pre- and post-2009 is interesting but difficult to relate to the SST or MHW changes which are shown for different time periods (pre- and post-2013).

One of the metrics used is MHW frequency, which does not account for the duration of events. An increase in heatwave frequency does not necessarily mean more time under MHW conditions compared to longer, less frequent events. Number of heatwave days per year would be a better metric. 

Please include a description of what constitutes and impacts on “internal variability” which is the other main driver of MHW change along with SST increases. 

Detailed comments

Lines 15-16 there is no analysis shown relating SST to the AMO and EAP (only to MHW intensity PC1 and AMO/EAP) so the “crucial role” statement is not supported here.

Line 16: the doubling of the SST trend is an important result, consider adding the rates in the abstract.

Lines 25-27: please clarify the sentence on chl-a results, However the results were mostly not significant so probably not appropriate for the abstract.

Line 47: since the AMO and EAP are used in the current analysis please include brief overviews of their major features and how they impact on SST/MHWs.

Line 57-58: “Therefore, climate change …” does not follow from the previous sentence on the North Sea being a productive fishery. Please rephrase.

Lines 64-66: results should not be included in the introduction.

Line 71: 2004 should be 2024.

Line 75: Dataset -> Datasets.

Line 83: some of these products are not used in the analysis – remove those that aren’t used from the list (eg latent heat etc).

Line 110-112: sentence ending “to detect MHWs” does not make sense. Please clarify.

Lines134-140: definition of PR is not clear: how does the threshold for P1 change each year? Please clarify.

Line 139 and line 142: what is the MHW “change” defined relative to?

Line 151: should be “top left”.

Line 157: should be “top right”.

Line 160: please define the abbreviation SSTA and state how it is calculated.

Line 160: in addition to the SST increase (0.8 deg C) please add the pre- and post-2013 mean temperatures.

Not sure that figure 1C is needed – figure 2 shows a clearer timeseries of SSTA and better supports the discussion (lines 172-174).

Line 185: why use the LOWESS method and how are the weights calculated?

Figure 3B: green line is not defined in the caption.

Line 211: How do the SST increases in this study compare with estimates from literature?

Line 227: please clarify what are the cumulative trends and what do they signify.

Lines 229-230: p>0.05 means that the correlations are not significant.

Line 232: brackets not needed (from 1.6 to 9.6).

Line 260: increase SSTA does not necessarily imply “an excessive trend in MHW”: MHWs are sustained increases (longer than 5 days) in temperature, please be careful of that point.

Line 312: use “maximum positive variability” instead of “opposite maximum variability”, which doesn’t make sense.

Line 314: there is no figure 6D.

Line 315: please clarify the statement “which corroborates a negative trend in variability intensity”.

Line 351: typo in 0.1.98.

Line 367: please elaborate on how the results suggest that “SST variability and thus MHW in the North Sea are largely influenced by atmospheric rather than oceanic forcing”.

Line 398 and elsewhere: end date of the spring 2024 event is given as July when it should be June.

Line 417: please elaborate or delete this statement on “atmospheric overheating”.

Figure 11: Is the caption correct “all anomalies were calculated by subtracting the daily climatological SST”? The text on the subfigures is too small and blurry, labels A-C are difficult to read. The figure needs more explanation, e.g. how are the anomalies calculated – is the seasonal signal removed? The spring 2024 event was obviously severe, it is possible to say why it was so much more severe than others? 

Line 466: “smaller trend” denotes a comparison, but compared to what?

Line 472: there is no blue line in figure 12.

Lines 473-475: figure 4C does not support this statement – it shows differences before and after 2013, not 2009.

Figure 12: yellow text is difficult to read and should be replaced. Please specify what the dots signify in B and C.

Line 501: how is the chl-a anomaly calculated?

Figure 13D: white contour is not visible.

Lines 550-551: the response of chl-a to MHWs and MCSs was not so clear-cut as a north-south split, please clarify.