the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Feb 2025
An Atlantic wide assessment of marine heatwaves beyond the surface in an eddy-rich ocean model
Abstract. Periods of prolonged anomalously high temperatures in the ocean, known as marine heatwaves (MHWs), can have devastating effects on ecosystems. While MHWs are extensively studied in the near-surface ocean, little is known about MHWs at depth. As continuous observations in space and time are very sparse away from the surface, basin wide studies on MHWs at depth have to rely on models. This introduces additional challenges due to the long adjustment timescale of the deep ocean, resulting in a long-term drift following the model’s initialisation. This unrealistic model drift dominates the MHW statistics below approximately 100 m when a fixed baseline is used. As a result, MHW studies at depth require a long model spin-up, or have to apply a linear baseline removing temperature trends. Based on a comparison of two model configurations with eddy-permitting and eddy-rich horizontal resolution, we show that the representation of mesoscale dynamics leads to pronounced differences in the characteristics of MHWs, in particular along the boundaries and along pathways of highly variable currents. Our results highlight the importance of horizontal and vertical heat transport within the ocean on sub-surface, but also on near-surface, MHWs. By investigating the vertical coherence of MHWs in an example region, here the Cape Verde archipelago, we show that MHWs are coherent over layers of a few 100 to 1000 m thickness, independent of the baseline used. These ranges are closely related to the vertical structure of the temperature field.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(8085 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(8085 KB) - Metadata XML
- BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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CC2: 'Comment on egusphere-2025-571', Lorenzo Berra, 04 Apr 2025
- AC3: 'Reply on CC2', Tobias Schulzki, 03 Jun 2025
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RC1: 'Comment on egusphere-2025-571', Neil Malan, 22 Apr 2025
This paper is a nice analysis of marine heatwaves with depth from an ocean model, which emphasises the role of model drift, baseline choice and resolution on marine heatwaves detection and statistics. Vertical coherence and drivers are also discussed as part of a case study.
The paper is interesting, and a good contribution to the field. However, I think that recommendations around baselines and spin up periods could be clarified, and believe that the heat budget analysis, while not incorrect, may not be fit for purpose.
Major Comments
Heat budget
I have concerns about the application of the heat budget. As used in the paper, it allows the diagnosis of the drivers of changes in heat content. However, MHWs are defined as discrete threshold exceedances relative to a local climatology, not by absolute temperature tendencies. Thus, areas of persistent heat convergence are not necessarily directly comparable to discrete MHW events. The manuscript currently suggests a causal link between sustained heat convergence and MHW occurrence or vertical coherence, but does not address this fundamental distinction. In order to prove this causal link, I believe that the heat budget would have to be performed on an event-by-event basis, and vertical coherence would have to be considered very carefully in terms of the boundary conditions for the budget for each event.
While this would be a very interesting analysis, I think it would be beyond the scope of the paper in its current form. In fact, I do not think that the heat budget analysis adds much to the outcomes of this work, and so my recommendation would be to restrict the analysis to that of vertical coherence, and remove, or at least strongly tone down and place the heat budget in the context of warming, and not of MHWs.
Recommendations
The paper makes recommendations around the use of sufficient model spin up periods, as well as the resolution at which MHW statistics should be calculated. I think that these recommendations will be very useful to researchers planning experiments for MHW use. However, I think the recommendations could be clarified in the text as they are sometimes not clearly laid out.
Is it possible to include more detail about the spin up process required? How much drift is removed in, for example the 4th cycle as opposed to the 6th? Would a repeat year forcing spin up be sufficient?
I find the finding about resolution very compelling, i.e. that while high resolution is needed to resolve mesoscale processes, MHW statistics can, in most cases be calculated on a coarser resolution grid. I note that this finding is not emphasised in either the abstract or conclusions of the paper and would suggest that it should be.
Minor Comments
I found the terminology and methology related baselines a little confusing at times. Smith et al (2025) recently published a detailed investigation of the effects of different baselines. For consistency with future literature, I would suggest citing this paper, and adopting their terminology (e.g. ‘detrended baseline’ instead of ‘linear baseline’)
I’m confused about the difference between model drift and real temperature trends – using a detrended baseline will remove both indiscriminately, while using a long enough spin up will remove model drift, but preserve natural temperature trends – is this correct? If so, it should be made clearer in the manuscript.
References
Smith, K. E., Gupta, A. S., Amaya, D., Benthuysen, J. A., Burrows, M. T., Capotondi, A., ... & Wernberg, T. (2025). Baseline matters: Challenges and implications of different marine heatwave baselines. Progress in Oceanography, 231, 103404.
Citation: https://doi.org/10.5194/egusphere-2025-571-RC1 - AC1: 'Reply on RC1', Tobias Schulzki, 03 Jun 2025
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RC2: 'Comment on egusphere-2025-571', Anonymous Referee #2, 07 May 2025
- AC2: 'Reply on RC2', Tobias Schulzki, 03 Jun 2025
Interactive discussion
Status: closed
-
CC2: 'Comment on egusphere-2025-571', Lorenzo Berra, 04 Apr 2025
Students of the course unit Physics of the Hydrosphere and the Cryosphere for the Laurea magistrale (MSc) in Geophysics are requested to select a paper or a preprint and prepare a report about the chosen work for the exam. I selected this manuscript and I attach the report that I prepared for the exam, as I had a few questions about the contents of the paper
- AC3: 'Reply on CC2', Tobias Schulzki, 03 Jun 2025
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RC1: 'Comment on egusphere-2025-571', Neil Malan, 22 Apr 2025
This paper is a nice analysis of marine heatwaves with depth from an ocean model, which emphasises the role of model drift, baseline choice and resolution on marine heatwaves detection and statistics. Vertical coherence and drivers are also discussed as part of a case study.
The paper is interesting, and a good contribution to the field. However, I think that recommendations around baselines and spin up periods could be clarified, and believe that the heat budget analysis, while not incorrect, may not be fit for purpose.
Major Comments
Heat budget
I have concerns about the application of the heat budget. As used in the paper, it allows the diagnosis of the drivers of changes in heat content. However, MHWs are defined as discrete threshold exceedances relative to a local climatology, not by absolute temperature tendencies. Thus, areas of persistent heat convergence are not necessarily directly comparable to discrete MHW events. The manuscript currently suggests a causal link between sustained heat convergence and MHW occurrence or vertical coherence, but does not address this fundamental distinction. In order to prove this causal link, I believe that the heat budget would have to be performed on an event-by-event basis, and vertical coherence would have to be considered very carefully in terms of the boundary conditions for the budget for each event.
While this would be a very interesting analysis, I think it would be beyond the scope of the paper in its current form. In fact, I do not think that the heat budget analysis adds much to the outcomes of this work, and so my recommendation would be to restrict the analysis to that of vertical coherence, and remove, or at least strongly tone down and place the heat budget in the context of warming, and not of MHWs.
Recommendations
The paper makes recommendations around the use of sufficient model spin up periods, as well as the resolution at which MHW statistics should be calculated. I think that these recommendations will be very useful to researchers planning experiments for MHW use. However, I think the recommendations could be clarified in the text as they are sometimes not clearly laid out.
Is it possible to include more detail about the spin up process required? How much drift is removed in, for example the 4th cycle as opposed to the 6th? Would a repeat year forcing spin up be sufficient?
I find the finding about resolution very compelling, i.e. that while high resolution is needed to resolve mesoscale processes, MHW statistics can, in most cases be calculated on a coarser resolution grid. I note that this finding is not emphasised in either the abstract or conclusions of the paper and would suggest that it should be.
Minor Comments
I found the terminology and methology related baselines a little confusing at times. Smith et al (2025) recently published a detailed investigation of the effects of different baselines. For consistency with future literature, I would suggest citing this paper, and adopting their terminology (e.g. ‘detrended baseline’ instead of ‘linear baseline’)
I’m confused about the difference between model drift and real temperature trends – using a detrended baseline will remove both indiscriminately, while using a long enough spin up will remove model drift, but preserve natural temperature trends – is this correct? If so, it should be made clearer in the manuscript.
References
Smith, K. E., Gupta, A. S., Amaya, D., Benthuysen, J. A., Burrows, M. T., Capotondi, A., ... & Wernberg, T. (2025). Baseline matters: Challenges and implications of different marine heatwave baselines. Progress in Oceanography, 231, 103404.
Citation: https://doi.org/10.5194/egusphere-2025-571-RC1 - AC1: 'Reply on RC1', Tobias Schulzki, 03 Jun 2025
-
RC2: 'Comment on egusphere-2025-571', Anonymous Referee #2, 07 May 2025
- AC2: 'Reply on RC2', Tobias Schulzki, 03 Jun 2025
Peer review completion
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Franziska U. Schwarzkopf
Arne Biastoch
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(8085 KB) - Metadata XML
Students of the course unit Physics of the Hydrosphere and the Cryosphere for the Laurea magistrale (MSc) in Geophysics are requested to select a paper or a preprint and prepare a report about the chosen work for the exam. I selected this manuscript and I attach the report that I prepared for the exam, as I had a few questions about the contents of the paper