| 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.
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.