the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Apr 2026
Future projections of compound events around the Main Hawaiian Islands
Abstract. The consequences of overlapping environmental stressors — referred to as compound events — may be more harmful to marine ecosystems than as individual stressors. Using recently conducted submesoscale-permitting future projections for the Main Hawaiian Islands, we present the first assessment of future compound events for Hawaiian waters. Our analysis focuses on surface and sub-surface heat-stress, ocean acidification, and low-oxygen events and is based on three different greenhouse gas emission scenarios. We show that a large fraction of ocean around Hawai‘i will be subject to compound events in the near future. However, the projected event characteristics such as duration and intensity vary substantially across the region suggesting that potential ecosystem impacts may differ over short distances. Our results reveal that these spatial differences are mainly driven by considerably different magnitudes of natural variability in ocean physics and chemistry across the domain driven by mesoscale processes, while anthropogenic trends exhibit only minor spatial differences. Our analysis demonstrates that small-scale tidal variability can significantly mitigate compound events in near-shore regions including some designated Marine Protected Areas. Overall, our findings highlight the need for high-resolution numerical models as well as for an extended observation network for robust future projections of local extreme events.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-1573', Anonymous Referee #1, 08 May 2026
- AC1: 'Response to Reviewers', Tobias Friedrich, 01 Aug 2026
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RC2: 'Comment on egusphere-2026-1573', Anonymous Referee #2, 13 Jul 2026
The manuscript by Friedrich et al uses a downscaled regional model for the Hawaii region to study the frequency and intensity of compound extreme events over the course of the century. They use multiple runs with different SSP-RCP scenarios and evaluate the natural and emerging variability of hydrogen ion, dissolved oxygen and temperature variability at surface and subsurface, using different calculations to account for the effect of mesoscale and tidal influence in different regions around Hawaii. They find that compound event intensity is mostly dominated by acidity, that deeper waters experience more legacy effects from increased CO2, and that accounting for tidal variability delays the onset of extreme events in nearshore regions.
The study is well designed and well thought out, considering the limitations imposed by computational costs of conducting multiple long-term simulations at 4-km horizontal resolution and evaluating outputs at hourly/daily temporal resolution, and the methods are well explained. However, there are a few points that could be better discussed or explained. First, the manuscript would benefit from some review of water masses and mixing, and of oxygen and H+ distribution. While the text offers some information on which sources have more/less concentration of different variables, it would help to have more information on those values and a better sense of the spatial distribution of the water masses. Some information on mixed layer depth distribution (including seasonal) and rate of change would also be helpful even if it is highly variable and influenced by eddies. Was MLD distribution a factor in deciding the depth bins? If typical MLD hovers around 50m, for example, wouldn’t potential long-term trends need to be discussed in the results for the 0-50 and 50-200m bins? In general, more thorough justification for the choice of depths used would be welcome. While I understand that generalizations need to be done when dealing with this amount of data, it would be good to have a better understanding of how the oceanographic features in the region guided the choices.
The interpretation of figures at points seems a bit broad, and the figures themselves need some improvement to help the reader have a more immediate understanding of what it is they are looking at. Better placement of subplot letters, more informative titles, and labeling on axes should be reviewed. More focused suggestions and comments on these items are included below. Overall, it is a valuable contribution that expands the interpretation of compound events at scales that are of high importance for coastal environments, and it should be ready for publication with some reviews.
L41-43: to build this argument in the introduction it would be good to add information from observations as well. Are there references for the impact on biogeochemistry?
L71-75: While the model validation is published elsewhere, it would still be helpful to add some information on potential biases and differences between the model and observations
L90: You might want to explain why you are using [H+] instead of pH
L96-99: Perhaps this is a good place to introduce and discuss the period used for the calculations?
L107-108: Presentable and feasible is good, but also good if it’s based on oceanographic properties of the region. A discussion of general circulation, MLD distribution and variability would be helpful for the reader to understand the choices made.
L120-121: So for one year of simulation?
L127: “individual mean intensities” – do you mean per cell?
L141-142: confusing sentence, and is there a reference for that?
L 157: spelling of mesoscale
L195-200: Need references here, and which water masses are those?
L215: would be good to have some general characterization/description of seasonal cycle
L217: Need to be a bit more specific on how the figure shows that
Figure 4: I understand changing the colorbar to see the signal, but keeping the colorbar consistent would make it easier to compare depths
L247: might be good to at least point to the section where intensity/severity are defined
L269: how could potential changes in MLD affect the distinction between 0-50 and 50-200?
L287: spelling of patterns
Figure 10: letters on panels hard to find, y-axis on e/f need labeling
L351: At the surface, are you referring to the propagation between 2086-2089? It is easier to see it at depth indeed
L354-355: I’m not sure how to interpret that from the figure given the lags and the faster response of deeper layers
L357-359: The information from the velocity checks and interpretation seems reasonable, but it’s hard to see information on eddy activity from meridionally averaged plots
L360: perhaps change lower depths to deeper waters?
Citation: https://doi.org/10.5194/egusphere-2026-1573-RC2 - AC2: 'Response to Reviewers', Tobias Friedrich, 01 Aug 2026
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This manuscript presents the first regional compound-event assessment for Hawaiian waters, combining a submesoscale-permitting physical-biogeochemical model with three SSP scenarios. The explicit separation of tidal and mesoscale contributions to natural variability thresholds, and the finding that spatial CE patterns are driven primarily by natural variability rather than anthropogenic trends, represent substantial new insights beyond existing global-scale CE studies. The conclusions are well-matched to the results and end with a practical call to action. Minor revisions are recommended: quantifying the permanence of acidity threshold exceedance spatially, sharpening the epistemic framing of ecosystem impact claims, and adding specificity to the observation network recommendation. Subject to these revisions, the paper is suitable for publication.
Specific comments:
1. The conclusions acknowledge that the acidity threshold exceedance becomes permanent around 2040 in all three SSP scenarios, this is a striking result. However, neither the conclusions nor the main text quantify what fraction of the domain reaches this state by decade, nor how "combined intensity" is interpreted once one variable's threshold is permanently exceeded. A figure or supplemental table showing the spatial progression of permanence is recommended.
2. The conclusion states it is "reasonable to assume" substantial ecosystem changes under unabated emissions, which is a fair inference, but this framing is stronger than the evidence in Section 3.6 strictly supports, which relies primarily on single-stressor studies. The conclusion should more explicitly distinguish between (a) documented single-stressor effects, (b) limited multi-stressor evidence, and (c) the inference to compound events, making the epistemic steps transparent.
3. The authors acknowledge that depth-averaging within bins is debatable given the strong vertical gradients present in the domain, but offer no justification for the specific bin boundaries chosen nor any test of sensitivity to this choice. This is a non-trivial concern, particularly for the 50–200 m bin, which spans ecologically distinct zones including the base of the euphotic zone and the upper oxygen minimum layer. Averaging [H⁺] and [O2] across this range may suppress within-bin variability and systematically bias threshold amplitudes in ways that differ between the leeward and windward sub-domains, where eddy-driven vertical structure is known to differ. The authors should either provide a physical justification for why depth-averaging is appropriate within each bin, for instance, by showing that the variables of interest are relatively homogeneous within bins at representative locations, or demonstrate that the key spatial contrasts reported are not sensitive to bin choice. A vertical profile comparison of threshold amplitudes at one representative leeward and one windward location, computed at full vertical resolution versus depth-averaged, would be a concise and informative way to address this concern.
4. The final paragraph of the conclusions calls for an "extended observation network", which is a valuable recommendation, but does not specify what variables, depths, or temporal resolution would be needed. Even one sentence of specificity (e.g., sub-daily biogeochemical sensors in the 50–200 m range near designated MPAs) would make this actionable.
Minor comments:
Line 74, Section 2.1: 'and' and the semicolon are redundant. Replace with either a comma before 'and', or a semicolon alone without 'and'.
Line 158, Section 2.3 : "mescoscale" should be mesoscale
Line 254, Section 3.3: "combined intensities in the 0–50 m depth bin virtually follow a linear relationship", here I suggest changing "virtually" to "near-linear" or "approximately linear" as is more precise.
Line 394, Section 3.6: "oligotrphic" should be oligotrophic
Line 395, Section 3.6: "Prochlorochoccus" should be "Prochlorococcus" and should also be in italics
Line 395-398, Section 3.6: Inconsistent italics/capitalization for species names. Make sure Prochlorococcus, Synechococcus, and Trichodesmium are italicizes throughout manuscript, since standard scientific convention requires italic formatting for genus and species names.
Line 401, Section 3.6: "import" should be important.
Throughout the manuscript, use Hawaiʻi consistently (with ʻokina) , there are some instances where Hawaii without the ʻokina is used (e.g., Line 202, 459).
Line 463, Conclusions: "timely" here is used to mean "important now" but reads ambiguously; "urgent" or "now essential" is clearer in scientific writing.