the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Feb 2025
Leading Dynamical Processes of Global Marine Heatwaves
Abstract. Marine heatwaves (MHWs) have emerged as a very active area of research due to the devastating impacts of these events on marine ecosystems across different trophic levels. Yet, a clear understanding of the local drivers of these extreme ocean conditions is still limited at a global scale. Observations of the terms needed to constrain ocean heat budgets are very sparse, ocean reanalysis products are generally non-conservative and inadequate to conduct accurate heat budget analyses, and the fidelity of climate models in simulating MHWs is still unclear. In this study, we make use of Argo floats observations, a satellite-based sea surface temperature product, and the Estimating the Circulation and Climate of the Ocean (ECCO) state estimate to assess MHW characteristics over the global ocean. ECCO is then used to evaluate local MHW drivers. ECCO assimilates observations using the adjoint methodology, which optimizes the system trajectory given the observational constraints in a conservative fashion, making it an ideal product for the estimation of heat budgets. The representation of MHWs in ECCO is overall consistent with observations, although ECCO tends to underestimate MHW frequency and intensity and overestimate duration, relative to the observational products. Atmospheric forcing emerges as the dominant contributor to MHW onset and decline across most regions, while ocean dynamics, including advective and diffusive convergence of heat, play crucial roles in the equatorial regions, specific extra-tropical zones (e.g., western boundary currents such as the Gulf Stream and Kuroshio), and the Southern Ocean. Regional analyses in the Northeast Pacific, Southwest Pacific, and Tasman Sea, show diversity in leading dynamical mechanisms for MHW onset and decline both across regions and across events in the same regions: while air-sea exchanges of heat may contribute most frequently to MHW onset and decline, other mechanisms can also often provide dominant contributions and at times be the main driver. A more complete understanding of MHWs and their drivers is crucial for predicting their initiation, duration, intensity and decline, to ultimately inform the development of mitigation and adaptation strategies for affected communities.
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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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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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Supplement
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-548', Anonymous Referee #1, 22 Apr 2025
- AC1: 'Reply on RC1', Jacopo Sala, 15 May 2025
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RC2: 'Comment on egusphere-2025-548', Anonymous Referee #2, 27 Jun 2025
The authors address the fundamental mechanisms driving surface marine heat waves in the global ocean using a combination of observational products and data assimilative model state estimates. The introduction does an excellent job of motivating the work. The methods and budget analyses are clearly articulated. They identify the key regions where atmospheric drivers and oceanic dynamics dominate the generation and decay of MHWs. They have already astutely responded to a bevy of comments from another referee and have indicated their responses to these criticisms, which are well supported in my opinion. Their conclusions are justified and substantiated by their analyses. The paper is eminently suitable for publication.
Citation: https://doi.org/10.5194/egusphere-2025-548-RC2 -
AC2: 'Reply on RC2', Jacopo Sala, 13 Jul 2025
We thank the reviewer for their positive and supportive comments, their feedback is much appreciated. We also appreciate their recognition of our efforts to address the earlier reviewer’s comments.
Citation: https://doi.org/10.5194/egusphere-2025-548-AC2
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AC2: 'Reply on RC2', Jacopo Sala, 13 Jul 2025
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RC3: 'Comment on egusphere-2025-548', Anonymous Referee #3, 07 Jul 2025
Main comments:
This study presents a comprehensive analysis of the leading dynamical processes of global MHWs during the onset and decline periods, based on ECCO and OISST datasets. While the paper is generally clear and the results could contribute to our understanding of MHW evolution, significant shortcomings preclude recommending it for publication in its current form.
Firstly, the authors used daily and monthly datasets to calculate MHWs after removing linear trends. The spatial patterns of the MHW metrics differ significantly between the OISST and ECCO datasets (see Figures 3 and 4). This suggests that ECCO does not capture MHW characteristics in the same way as observations. Therefore, I suggest that the authors use more ocean reanalysis datasets, such as GLORYS or BRAN.
Secondly, in the section discussing the mechanism of global MHWs, the authors conducted a heat budget analysis, but did not present the residual term. Does this mean that ECCO can perform a close heat budget analysis?
Finally, I agree with RC1 that the identification of leading terms is based on predefined thresholds (30%, 16%) that are not explained. The authors should demonstrate this more clearly.
Citation: https://doi.org/10.5194/egusphere-2025-548-RC3 - AC3: 'Reply on RC3', Jacopo Sala, 13 Jul 2025
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EC1: 'Comment on egusphere-2025-548', Meric Srokosz, 15 Jul 2025
In light of the reviewers' comments a revised manuscript may be suitable for publication.
Citation: https://doi.org/10.5194/egusphere-2025-548-EC1
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-548', Anonymous Referee #1, 22 Apr 2025
The manuscript presents an effort to characterize the physical drivers of marine heatwaves (MHWs) using the ECCO state estimate framework. The authors should be commended for undertaking the challenge of quantifying the upper ocean heat budget globally and attributing MHWs to specific dynamical processes. However, there are several important concerns that limit confidence in the presented results. First, there is a notable mismatch between the ECCO product and observations, raising questions about the representativeness and reliability of the diagnosed drivers. Second, there are questions regarding the heat budget closure and identification of dominant processes, complicating the interpretation of the contributions from different terms. These issues, along with several concerns related to the interpretation and presentation of results, warrant further consideration and clarification.
Major concerns:
- Comparisons between ECCO and OISST reveal clear mismatches in the number, duration, and intensity of MHWs globally (Figures 3–5). This raises the question: how representative are MHWs in ECCO of those in the real ocean? The title of the manuscript is ambitious, yet the results are evidently dependent on the fidelity of the ECCO product.
- The calculation of heat budget terms, which is key to the manuscript, is not clearly described. Offline calculations of the heat budget often fail to close due to the use of temporally averaged velocity and temperature fields, which do not account for nonlinear covariance terms like ∇∙(θ′T′). Depending on the region, these covariance terms can be significant. The authors do not report how well the budget closes or the magnitude of the residual term, which raises questions about the accuracy of the diagnosed contributions from different processes.
- The partitioning of contributions into advective and diffusive terms is inherently tied to the model’s resolution. Coarse-resolution products may misattribute unresolved advective processes to subgrid-scale diffusion. Without acknowledging this limitation, the attribution results risk being potentially misleading.
- The identification of leading terms is based on predefined thresholds (30%, 16%) that are not rationalized. The conclusions drawn in Sections 4.2 and 4.3 are thus sensitive to these subjective choices, which undermines their robustness.
Other comments:
Page 7, The claim that "ECCO provides an overall good representation..." is somewhat overstated. ECCO underestimates the magnitude of the trend in the Northwest Pacific, Northwest Atlantic, and Southwest Atlantic. The spatial patterns in MHW metrics also show clear mismatches in Figures 3–6.
Page 9, The statement that "intensity is slightly underestimated..." is problematic. ECCO upper ocean heat content and OISST surface temperatures are not directly comparable.
Page 10, “In these regions” should be “in other regions”?
Page 11, “Figure 8a d” should be “Figure 7a d”.
Page 11, “consistent with the onset phase being on average longer than the decline phase (Figure 4).” Why longer duration means overall pattern being similar to that of onset, especially the counter-argument is made later in the same paragraph? Besides, Figure 4 doesn’t show onset vs decline.
Page 14, “How often each budget terms is "the" leading term vs "a" leading term (excluding overlap with "the") across NEP, SWP, and TASMAN regions”. The definition of “the” and “a” leading term needs to be clearly explained.
Pages 15-18, Figures 11-13: 1. the percentage contribution should be based on the total temperature change, i.e., , where is the temperature change during onset or decline, so the contribution from one term can exceed 100%, if there is one negative term or more. 2. the numbers are only meaningful if the heat budget is closed, that is, equals . Otherwise, if there are residuals in the budget, these numbers are less meaningful.
Pages 15-18: all the numbers depend on how the percentage of the heat budget terms are calculated, and the threshold used for ranking contributors.
Citation: https://doi.org/10.5194/egusphere-2025-548-RC1 - AC1: 'Reply on RC1', Jacopo Sala, 15 May 2025
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RC2: 'Comment on egusphere-2025-548', Anonymous Referee #2, 27 Jun 2025
The authors address the fundamental mechanisms driving surface marine heat waves in the global ocean using a combination of observational products and data assimilative model state estimates. The introduction does an excellent job of motivating the work. The methods and budget analyses are clearly articulated. They identify the key regions where atmospheric drivers and oceanic dynamics dominate the generation and decay of MHWs. They have already astutely responded to a bevy of comments from another referee and have indicated their responses to these criticisms, which are well supported in my opinion. Their conclusions are justified and substantiated by their analyses. The paper is eminently suitable for publication.
Citation: https://doi.org/10.5194/egusphere-2025-548-RC2 -
AC2: 'Reply on RC2', Jacopo Sala, 13 Jul 2025
We thank the reviewer for their positive and supportive comments, their feedback is much appreciated. We also appreciate their recognition of our efforts to address the earlier reviewer’s comments.
Citation: https://doi.org/10.5194/egusphere-2025-548-AC2
-
AC2: 'Reply on RC2', Jacopo Sala, 13 Jul 2025
-
RC3: 'Comment on egusphere-2025-548', Anonymous Referee #3, 07 Jul 2025
Main comments:
This study presents a comprehensive analysis of the leading dynamical processes of global MHWs during the onset and decline periods, based on ECCO and OISST datasets. While the paper is generally clear and the results could contribute to our understanding of MHW evolution, significant shortcomings preclude recommending it for publication in its current form.
Firstly, the authors used daily and monthly datasets to calculate MHWs after removing linear trends. The spatial patterns of the MHW metrics differ significantly between the OISST and ECCO datasets (see Figures 3 and 4). This suggests that ECCO does not capture MHW characteristics in the same way as observations. Therefore, I suggest that the authors use more ocean reanalysis datasets, such as GLORYS or BRAN.
Secondly, in the section discussing the mechanism of global MHWs, the authors conducted a heat budget analysis, but did not present the residual term. Does this mean that ECCO can perform a close heat budget analysis?
Finally, I agree with RC1 that the identification of leading terms is based on predefined thresholds (30%, 16%) that are not explained. The authors should demonstrate this more clearly.
Citation: https://doi.org/10.5194/egusphere-2025-548-RC3 - AC3: 'Reply on RC3', Jacopo Sala, 13 Jul 2025
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EC1: 'Comment on egusphere-2025-548', Meric Srokosz, 15 Jul 2025
In light of the reviewers' comments a revised manuscript may be suitable for publication.
Citation: https://doi.org/10.5194/egusphere-2025-548-EC1
Peer review completion
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Donata Giglio
Antonietta Capotondi
Thea Sukianto
Mikael Kuusela
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(42316 KB) - Metadata XML
-
Supplement
(2388 KB) - BibTeX
- EndNote
- Final revised paper
The manuscript presents an effort to characterize the physical drivers of marine heatwaves (MHWs) using the ECCO state estimate framework. The authors should be commended for undertaking the challenge of quantifying the upper ocean heat budget globally and attributing MHWs to specific dynamical processes. However, there are several important concerns that limit confidence in the presented results. First, there is a notable mismatch between the ECCO product and observations, raising questions about the representativeness and reliability of the diagnosed drivers. Second, there are questions regarding the heat budget closure and identification of dominant processes, complicating the interpretation of the contributions from different terms. These issues, along with several concerns related to the interpretation and presentation of results, warrant further consideration and clarification.
Major concerns:
Other comments:
Page 7, The claim that "ECCO provides an overall good representation..." is somewhat overstated. ECCO underestimates the magnitude of the trend in the Northwest Pacific, Northwest Atlantic, and Southwest Atlantic. The spatial patterns in MHW metrics also show clear mismatches in Figures 3–6.
Page 9, The statement that "intensity is slightly underestimated..." is problematic. ECCO upper ocean heat content and OISST surface temperatures are not directly comparable.
Page 10, “In these regions” should be “in other regions”?
Page 11, “Figure 8a d” should be “Figure 7a d”.
Page 11, “consistent with the onset phase being on average longer than the decline phase (Figure 4).” Why longer duration means overall pattern being similar to that of onset, especially the counter-argument is made later in the same paragraph? Besides, Figure 4 doesn’t show onset vs decline.
Page 14, “How often each budget terms is "the" leading term vs "a" leading term (excluding overlap with "the") across NEP, SWP, and TASMAN regions”. The definition of “the” and “a” leading term needs to be clearly explained.
Pages 15-18, Figures 11-13: 1. the percentage contribution should be based on the total temperature change, i.e., , where is the temperature change during onset or decline, so the contribution from one term can exceed 100%, if there is one negative term or more. 2. the numbers are only meaningful if the heat budget is closed, that is, equals . Otherwise, if there are residuals in the budget, these numbers are less meaningful.
Pages 15-18: all the numbers depend on how the percentage of the heat budget terms are calculated, and the threshold used for ranking contributors.