| 13 Nov 2025
Sensitivity of marine heatwaves metrics to SST products, focusing on the Tropical Pacific
Abstract. Marine heatwaves (MHWs) are increasingly studied in climate sciences for their ecological impacts, for which accurate real-time bulletins and forecasts are essential. Yet, methodological choices in their detection affect metric estimates, underlining the need to better assess these sensitivities. This study provides a thorough assessment of the impact of Sea Surface Temperature (SST) product choice on MHW statistics, focusing on the tropical Pacific. MHW detection was performed on six daily gridded SST datasets: four widely used blended satellite observational products, one ocean reanalysis, and a multi-dataset ensemble mean computed from the four observational products. Sensitivity to SST products was evaluated for six MHW metrics (MHW days per year, number of events per year, duration, maximum intensity, cumulative intensity and onset rate) and for the Degree Heating Weeks (DHW), a widely used proxy for coral bleaching. Inter-product comparisons revealed a significant dispersion among MHW metric estimates, with the reanalysis GLORYS12v1 detecting fewer, longer and less intense MHWs while OISST detected more MHWs of shorter duration and higher intensity, likely related to their weak and strong high-frequency SST variability (periods shorter than 2 weeks) respectively. The sensitivity analysis showed that the onset rate was the most sensitive metric to SST product choice and the maximum intensity the most robust. Metrics uncertainties were quantified inside seven regions of the basin and were largest in the western Pacific Warm Pool. Co-occurrence analyses of MHWs revealed that, over the basin, 10 to 80 % of MHW days were detected simultaneously by all products, with the western Pacific Warm Pool showing the lowest agreement (10–40 %). Filtering MHWs by size also revealed that the detection of large-scale MHWs (> 5°x5°) was more consistent across products than smaller ones. Finally, over the studied period, inter-product differences tend to decrease with time. The DHW also revealed to be sensitive to SST products, with inter-product differences on DHW annual maximum reaching more than 1°C.weeks-1 and percentages of bleaching alert days (DHW ≥ 4°C.weeks-1) in common across products reaching 70 % at most across much of the basin. These findings contribute to a better understanding of how methodological choices affect the characterization of MHWs and DHW, and their associated uncertainties.
MHW detection and characterization (metrics) still have some open issues in their settings and criteria, such as the use or not of detrended SST data, baseline climatology, spatio-temporal constraints… In this work, the authors run an extensive analysis of the impact of the SST database selection on MHW detection and metrics in the Tropical Pacific Area.
The authors analyse a complete set of MHW metrics over different areas of the Pacific Ocean and different satellite and reanalysis SST datasets. They find differences between calculated metrics characteristics, regional differences and metrics dispersion depending on the selected dataset. They also analyse the temporal evolution of regional averaged MHW metrics.
The results in the manuscript show that for different metrics, the best results are observed with different databases. In the same direction, different MHW sizes yield different results are obtained depending on the SST used. They also observe that the variability of results between different databases has been decreasing in recent years. No clear distinction is obtained between one database and the others, nor is there one that obtains a better result in most metrics.
Work shown in the manuscript is methodologically consistent and provides interesting results on the impact of SST databases in MHW analysis. My recommendation is to publish the manuscript with minor revisions.
Main comments
My main concern comes from the methodology choice in detecting all pixels constituting an MHW in the 2.2.3- Filtering MHWs by size section. This is an important issue, as the authors separate MHWs in micro and macro scales, which needs to be better explained. Please, provide more details and the rationale on how “joint pixels” are detected. Which is the impact of the methodology (based on Bonino) on MHW detection? Have you tried any other methodology? Please, see the references below (global and Mediterranean scales) and discuss why you chose the methodology in Bonino, used in the Mediterranean where scales are much smaller than in the Pacific.
Sun, D., Jing, Z., Li, F. & Wu, L. Characterizing global marine heatwaves under a spatio-temporal framework. Prog. Oceanogr. 211, 102947 (2023).
Pastor, F., Paredes-Fortuny, L. & Khodayar, S. Mediterranean marine heatwaves intensify in the presence of concurrent atmospheric heatwaves. Communications Earth & Environment 5, 797 (2024).
Although you mention a possible impact of regridding in the MHW analysis. Have you checked the impact of regridding in the dataset characteristics? Some simple statistics, correlations… of this impact would be interesting to be included in the manuscript, maybe as supplementary material.
Is the climatology period 1993-2021 the same whole period studied? I understand that the full study period is the period analysed but it has to be clearly stated in the manuscript.
The authors separate MHW events in micro and macro scales, greater or smaller than 5x5 degree. How is this size threshold determined? Have you checked and compared results for other thresholds? An MHW of 4x4º occupies an extensive area, especially in the case of marginal seas. I would like to see some figures about mean size of micro-events, dispersion, percentiles that justify the 5x5 is a good choice. Some micro events can be almost as big as some macro events.
Maybe your threshold is appropriated for the open ocean, but this election needs to be better justified. Check methodology in Pastor (2023) to identify MHW area.
Pastor, F. & Khodayar, S. Marine heat waves: Characterizing a major climate impact in the Mediterranean. Science of the Total Environment 861, (2023).
Minor comments
Line 294 “for the maximum intensity (total MHW days) (Fig. 4b,f)”. Correct if necessary.
2.3.2 Temporal evolution
“The year attribution of a MHW was based on its time start”. Why do not use central date? Have you checked how many MHWs start and end on different years? And how many days of this event correspond to the end year?