the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jul 2025
Characterization of Past Marine Heatwaves around South Pacific Island Countries: What really matters?
Abstract. Marine heatwaves (MHWs) can have devastating and lasting impacts on marine ecosystems. We investigated past MHW characteristics around 12 southwestern Pacific Island countries and territories (PICTs) using two observed sea surface temperature products and an ocean reanalysis product. PICTs are highly dependent on their marine resources for their livelihoods: a better understanding of MHW characteristics is needed for planning and adaptation to risks associated with MHWs. Our research builds on previous studies where MHWs have been detected and described using a point-based definition. We first revisit past MHW characteristics based on their spatial extent, vertical extent and seasonality. We show that filtering MHWs by size (spatial extent) and seasonality can greatly affect their characterisation and help trace their physical drivers. We then characterise past events inside each EEZ (Economic Exclusive Zone) and at the coast with MHW indices tailored to benefit Pacific Island stakeholders. We consider two types of events: large-scale events, covering a large part of the EEZ, likely to affect pelagic fisheries, and events affecting coastal zones and ecosystems. We distinguish between events occurring in the hot season (November to April), and in the cold season (May to October). We show that all 12 PICTs experienced MHWs in the past 30 years that are getting more frequent with greater spatial extents, longer durations, but with less intensity. New Caledonia, Vanuatu, Fiji and Tonga appear to be more exposed to MHWs with longer duration, higher maximum intensity, and deeper extent compared to other countries.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3281', Anonymous Referee #1, 27 Sep 2025
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RC2: 'Comment on egusphere-2025-3281', Anonymous Referee #2, 02 Oct 2025
The authors present a study on the characteristics of marine heatwaves (MHWs) in the South Pacific, with a focus on their spatial distribution, vertical extent, seasonality, and long-term trends. Using multiple observational and reanalysis products, the paper contributes to our understanding of MHWs. The study is innovative in its filtering of MHWs by size and seasonality, and in quantifying their vertical structure. The methodology is generally well described, and the figures are informative, although some improvements in clarity and consistency would strengthen the presentation.
In my view, the manuscript could be improved by clarifying the criteria for regional subdivision and considering whether detrending would provide additional insights into the drivers of long-term changes. Overall, the paper addresses an important and timely topic, advancing our understanding of MHWs in the South Pacific. In general, I suggest a minor revision, and more detailed comments are coming as follows:
Section 2.2: Why is the data not detrended? The long-term trend in water temperature affects both the detection and duration of MHWs, and it also influences ecological processes and fisheries. The author should either explain why they don't detrend the data or provide some comparisons between with/without detrend.
Figure 1: Please fill the land areas with color (e.g., using gray patches) to better illustrate the land of each country. The current light gray outlines make the contrast between land and ocean unclear, especially since the EEZ boundaries are also marked with light gray curves.
Section 3, line 245: The phrase “eddy-rich region” is unclear for readers unfamiliar with the South Pacific. I suggest including an additional figure or a new layer in Figure 3 to illustrate this region, which seems to correspond to the areas highlighted in magenta.
Line 262 and Figure 4: It is unclear what metric was used to quantitatively divide the research region. It appears that the authors used MHW days as a criterion, but the boundaries look unnaturally straight. It seems that the five subregions were drawn by eye based on Figure 4a, rather than from a quantitative method. Please clarify.
Lines 265–266: Please keep the figure references consistent (e.g., Figure 5 → Fig. 5; Figure 6 → Fig. 6).
Section 3.3, Long-term trend and Figure 7: From my perspective, there is a clear geophysical distribution of increasing MHW days across regions 1, 2, and 5, which is consistent with the increasing duration (as indicated by the black arrow in the attached supplement). This raises questions about whether there are underlying physical processes driving this pattern, and whether the criteria for subregion division (Section 3.2) are appropriate for analyzing long-term trends.
Figure 8 shows results without detrending. It would be informative to also include a detrended version of Figure 7. Is climate warming the primary driver of increased MHW states over the past 40 years? What other drivers might also play a role?
Page 25, Lines 508–509 and Figure 14b: Most coastal regions are shown in blue to green colors (10–15 days), except for a single pixel at [19°S, 180°] showing 20 days. This location is close to the neighboring coast, so why is there such a large difference? Could this be a numerical artifact from interpolation, or is there a physical explanation?
Section 4.3, Figure 15a: The results suggest that Vanuatu has a large seasonal difference in MHW days, while neighboring Fiji does not. The authors attribute this to high-intensity events (Line 553), but this explanation is unconvincing without supporting evidence. Additional justification is needed. Maybe pointing out the specific event observed in the data set.
Lines 666–676: The characterization of MHWs along Pacific island coasts is interesting, particularly because the indices and impacts may differ from those observed along continental coasts. The authors could expand their discussion on this point.
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General Comments
The authors have studied the characteristics of marine heatwaves (MHWs) across twelve Pacific Island countries and territories (PICTs) using multiple sea surface temperature products and an ocean reanalysis. The paper addresses an important topic, given the reliance of these island nations on marine resources and the growing concern over climate-change-related impacts on coastal and pelagic ecosystems.
By considering spatial and vertical extent, seasonality, and differentiating between large-scale and coastal events, the study provides insights that are both scientifically relevant and of practical value to stakeholders in the region. The methods are sufficiently outlined and the conclusions are clear. The study builds upon existing knowledge on MHWs in the study area, while taking a step forward by providing information that is usable for stakeholders.
In my view, the manuscript could be further strengthened by briefly expanding on i) how results could be practically used by stakeholders (e.g., coral reef management, early warning systems) and ii) the uncertainties associated with differences between products, since these may influence stakeholder confidence in the results. The latter will also increase the paper’s contribution to the ongoing discussion within the MHW community on the dependence of MHW characteristics on different products and methods.
I have no major concerns with the manuscript and I kindly encourage the authors to take into account the aforementioned suggestions and the specific comments provided below.
Specific comments:
Abstract:
Line: less intensity -> lower intensity
Sect. 3.2
Line 282: Keep the term max intensity throughout the text, as the manuscript currently switches between intensity and max intensity
Line 283: Is this a hypothesis or is it derived from an analysis not included? I suggest supporting it better, or, if it is an assumption based on literature, rephrasing accordingly.
Sect. 3.3
Line 357-8: as above, keep the term max intensity, or state at the beginning that intensity refers to maximum intensity in the context of this work (same for lines 437, 441, 549).
Sect. 4.1
Line 429: are typically short duration -> are typically of short duration
Line 437-440: There is some repetition in the first two sentences, I suggest rephrasing for clarity
Fig. 11: I like the visualization approach for combining season/severity/extent. However, using more distinct colors (and line styles in the legend) would improve readability.
Sect. 4.2
Line. 461: Consider mentioning (possibly in Methods) why these specific cases are analyzed (same for the previous section)
Line. 465: MHW -> MHWs
Line 472: Any idea why in GLORYS12 we observe more MHW days, longer durations and higher max intensities in both seasons compared to OSTIA? It would be interesting advancing this discussion even through hypothesis.
Line 518: Clarify what exactly corresponds to the 95th percentile
Line 525: What do you mean by "coastal MHW days" ? Please, rephrase.
Fig. 12: The caption reads a bit confusing to me, an alternative could be: Total number of MHW days, mean Duration and mean Max Intensity in coastal New Caledonia: a,b,c in OSTIA and d,e,f in GLORYS12, hot season; g,h,i in OSTIA and j,k,l in GLOSYS12, cold season.
Fig. 14: “The data in the maps refer to average changes in these metrics per decade over the 29 years”. I am not sure if this sentence adds information here. Also, since the mapped trends are only the significant ones, this should better be noted in the caption.
Sect. 5.1
Line 605: no need for acronym here
Sect. 5.3
Line 700: I suggest adding in Methods a brief explanation on the choice for fixed baseline
Line 701: I suggest replacing “because we think that…” with “in order to capture total heat exposure, encompassing both temporary...”
Sect. 5.4
Line 724: You refer to global results from previous studies? Not clear. If yes, add references here