| 04 Sep 2025
Marine Heatwaves across the central South Pacific: characteristics, mechanisms, and modulation by the El Niño Southern Oscillation
Abstract. Marine heatwaves (MHWs) are intensifying with climate change, endangering ecosystems such as coral reefs. Yet their regional characteristics and drivers remain poorly understood in many parts of the Pacific. Here we provide a comprehensive assessment of MHWs in the central South Pacific and across the five archipelagos of French Polynesia (representing ~77 atolls, more than half of world’s atolls, and more than 5 million km2 of maritime area, a region as vast as Europe), using sea surface temperature observations and an ocean reanalysis to investigate underlying mechanisms. MHW exposure varies widely across the region: its northern and southern parts (the Marquesas and Austral archipelagos respectively) experience the highest number of MHW days and the strongest cumulative intensities, especially during the warm season (Nov–Apr). In contrast, its central part (the Society, Tuamotu, and Gambier Islands) exhibits more moderate MHW characteristics. Heat budget analyses highlight the seasonally and regionally diverse mechanisms shaping MHWs. In central FP during the warm season, most of MHWs are driven by air–sea heat fluxes, while in the northern part, those driven by oceanic horizontal advection dominate. During the cold season, more MHWs driven by horizontal advection are observed in the whole region since the thicker seasonal mixed layer reduce the proportion of MHWs driven by air-sea fluxes. El Niño– Southern Oscillation (ENSO) strongly modulates MHWs occurrence: El Niño favors MHWs occurrence in northeastern FP, while La Niña increases MHW occurrence in the southwest with different spatial extent across ENSO flavors (Central and Eastern Pacific ENSO events). This modulation arises from reduced wind-evaporation cooling with reduced wind speed, shoaled mixed layers, and enhanced horizontal heat advection. These results greatly improve our understanding of MHW characteristics, dynamics and variability in this ecologically-fragile region.
Review of OS manuscript 2025-4166 submitted to Ocean Science
The manuscript entitled “Marine heatwaves across the central South Pacific: characteristics, mechanisms, and modulation by the El Niño Southern Oscillation” by Bastien Pagli et al., submitted to Ocean Science, investigates the contribution and importance of El Niño Southern Oscillation (ENSO) to marine heatwave occurrences and evolution in the central South Pacific and across French Polynesia.
Overall, I find the manuscript to be a valuable contribution to our understanding of ENSO on marine heatwaves across this region that is sensitive to ENSO phase both temporally and spatially. I recommend the manuscript be potentially acceptable for publication once the comments below have been satisfactorily addressed by the authors.
Comments and Concerns
L2: In the title, I recommend deleting “the” as with “the El” it’s like a double-“the”
LL18-19: “MHW exposure varies widely across the region” – at this point, the reader is unaware what the authors mean by “exposure” here, which is a term most often used when referring to species and/or ecosystem exposure to a hazard
L23: “FP” acronym is undefined; delete “of”
L24: Is there really a “cold season” across French Polynesia?
L26: change “MHWs occurrence” to “MHW occurrences”, here and throughout the manuscript
L33: “known as prolonged periods of extreme ocean temperature” – MHWs are not periods of extreme temperature (which would be measured in time units), but rather they are temperature extremes that persist (measured in temperature units)
LL35-36: “and are projected to continue rising in the future (Oliver et al., 2018)” – this last part of the sentence is not covered by the cited reference which focuses on historical data, and not projections. The authors should cite another reference for this.
L36: “Developing skillful … their impacts.” – This is a leap in a single sentence. It needs to be explained further. Skillful forecasts do not necessarily translate to reduced ecological impacts.
LL37-38: “Such forecasts … variability.” – A relevant paper to cite here is Holbrook et al. (2020, NREE) which makes and builds the case for understanding MHW predictability and prediction.
L43: “increased MHW” – also include “and suppressed MHW”
L45: “(Gregory et al., (2024)” should be “Gregory et al. (2024)”
L47: acronym “SST” is undefined
L53: “MHWs” should be “MHWs’ “
L59 and L64 and throughout: I consider it more appropriate to replace “impacts” with “influences”, since the word “impacts” is more appropriate for socio-ecological impacts, while “influences” is between the physical components
L81: “OISST” – write in words first and then use the acronym
L82: delete “resolution” – the data are interpolated to a 0.25o grid, which does not necessarily reflect the true “resolution” of the original sampling
LL88-89: “Due to the known … ERA-Interim” – why not use ERA5 throughout?
LL138-140: “Whether or not … Amaya et al., 2023; Capotondi et al., 2024).” – This is not a sentence, please rewrite. Also, note that the Amaya et al. (2023) paper was challenged by Sen Gupta et al. (2023, Nature) and which helped lead to the paper by Smith et al. (2025, Prog Oceanogr). It would be good to reflect this in the manuscript.
LL140-141: “Both approaches are complementary … address.” – The key reference here is Smith et al. (2025, Prog Oceanogr).
LL141-144: Another approach would be to follow the guidance of Smith et al. (2025), as the terminology “total heat exposure” suggests that species are indeed exposed to the hazard. I’m not sure we can absolutely assume that?
L179: acronym “MLD” is undefined
LL184-186: the word “net” should be explicitly used in each term in the equation, e.g. “LHF the net latent heat flux” etc.
L191: In Equation (5), why is “dk” used rather than “dt”?
L199: “sea-level (SSH)” – should this be “sea surface height”? Sea level and sea surface height can be subtly different.
LL210-212: Following the Hobday et al. (2016) definition, why isn’t the average total number of MHW days/year close to 10% of the year (~36/year) everywhere?
L213: “During the cold season … compared to the warm season.” – Based on the 90th percentile seasonally varying threshold applied using Hobday et al. (2016), why are there less cold season MHWs than in the warm season?
L233: “cumulative impacts” should be “cumulative intensities”
L250: Figure 2 caption – “map”, I think should be “legend”
LL273-274: “However some differences … than in OISST.” – This is consistent with the analysis of Pilo et al. (2019) based on ACCESS-OM2 simulations across different model resolutions. I suggest this would be an appropriate reference to cite as context.
L394: Figure 6 choice of colorbar for panels a-c and e-g make it difficult to discern proportions <40%.
L542: Conclusions section usually follows the Discussion section.
L568: I think “types” would be better used to replace the terminology of “categories” in the context used here, as “categories” is typically reserved for measures of MHW intensity.
LL605-606: Another earlier study of MHWs in the region and relevant reference here is Holbrook et al. (2022, Glob Planet Change).
L632: “Gupta and Sil (2024)” should be “(Gupta and Sil, 2024)”
References
Holbrook NJ et al., 2022: Impacts of marine heatwaves on tropical western and central Pacific Island nations and their communities. Global and Planetary Change, 208, 103680, https://doi.org/10.1016/j.gloplacha.2021.103680.
Holbrook NJ et al., 2020: Keeping pace with marine heatwaves. Nature Reviews Earth and Environment, 1, 482-493, https://doi.org/10.1038/s43017-020-0068-4.
Sen Gupta A et al., 2023: Marine heatwaves: definition duel heats up. Nature, 617, 465, https://www.nature.com/articles/d41586-023-01619-4.
Smith KE et al., 2025: Baseline matters: Challenges and implications of different marine heatwave baselines. Progress in Oceanography, 231, 103404, https://doi.org/10.1016/j.pocean.2024.103404.
Pilo G S et al., 2019: Sensitivity of marine heatwave metrics to ocean model resolution. Geophysical Research Letters, 46, 14604-14612, doi:10.1029/2019GL084928.