the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Aug 2025
First 2D record of a tsunami by SWOT satellite: observation data and preliminary numerical simulation of the 19 May 2023 tsunami near the Loyalty Islands
Abstract. The NASA-CNES altimetry mission SWOT (Surface Water and Ocean Topography) deployed in December 2022 embarks a Ka-band Radar Interferometer (KaRIn), providing a 120 km-wide swath sea level measurement. On 19 May 2023, SWOT was able to record a 2D signature of the tsunami generated by the Mw 7.7 earthquake southeast of the Loyalty Islands (southwest Pacific Ocean), about 1 hour after the earthquake, on a straight SSW-NNE path. Comparison between numerical models and real measurements was performed to assess SWOT’s ability to monitor tsunami waves. A uniform coseismic slip rupture model allows to satisfactorily fit the regional observations. Testing models against a dynamic representation of the tsunami wavefield (instead of static) show a good phase agreement, but simulated amplitudes and energy spectra are lower than the measurements. However, this SWOT unprecedented 2D observation critically inform on tsunami propagation and modeling, and offer a breakthrough perspective for better predictions.
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- RC1: 'Comment on egusphere-2025-3926', Anonymous Referee #1, 10 Oct 2025
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RC2: 'Comment on egusphere-2025-3926', Anonymous Referee #2, 15 Oct 2025
This paper presents a very interesting study on the use of SWOT satellite data for detecting a tsunami event generated by the Mw 7.7 earthquake southeast of the Loyalty Islands (Southwest Pacific Ocean). The authors used the Level-3 Low Rate Sea Surface Height SWOT product, which is easily accessible to researchers, and compared it with numerical model simulations. Their analysis shows a strong consistency between the SWOT observations and the model results.
Tsunami is a type of event for which observations by satellites are rare and depends on “luck”, but when captured, they provide unique opportunities to study the event and its dynamics in ways that models alone might not allow (especially with a satellite like SWOT with high spatial resolution and advanced capabilities). This article therefore opens the door for more in-depth analyses and demonstrates the potential of SWOT studying such oceanic phenomena and events.
Although I am not a tsunami expert, I found that the literature review about tsunami detection from satellite altimeters is well presented in the introduction. The presentation of the SWOT altimeter and its capabilities is also concise and very clear.
After checking the comments of the first reviewer, I find myself in agreement with their comments and I don’t have further technical corrections to provide. I therefore propose a minor revision, and two additional comment/suggestions that could further improve the manuscript:
- To better quantify the time lag between the simulated main peak and the SWOT peak, I suggest providing a comparison spatially restricted between −19° and −18° latitude and using a simulation output closer in time to the SWOT overpass above this restricted area. This would make the comparison more precise.
- I also propose that the authors change the title of section 4 to something more generic like “Comparisons and discussion”, and change the title of section 4.1 to something like “Comparison between observations and model output” to better reflect the content and structure of the section.
Citation: https://doi.org/10.5194/egusphere-2025-3926-RC2
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The work entitled “First 2D record of a tsunami by SWOT satellite: observation data and preliminary numerical simulation of the 19 May 2023 tsunami near the Loyalty Islands” focuses on satellite radar observation of a tsunami event on 19 May 2023. It is the first dynamic observation of a two-dimensional tsunami wave field. The satellite’s orbit allowed the observation of the wavefront propagating in both directions from the epicenter, about 1 h after the earthquake. I wonder why the authors call it 2D observation, when in fact it is a 3D observation because it includes the sea surface elevations (Sea Level Anomaly) within a two-dimensional domain. These observations are compared with numerical simulations performed with COMCOT model, showing generally good agreement with observations, although some differences reveal weak points of the simulations and indicate that there is room for future model improvements. Simulations show a small delay in the main peak of the tsunami wavefront and underestimated amplitudes, especially for the secondary waves in their propagation to the north. The results presented are relevant and highlight the usefulness of SWOT data to improve tsunami predictions. Some paragraphs and figures show inconsistencies that must be corrected (see attached file). The manuscript presents new data, and I recommend publication once the minor changes have been addressed. I hope my comments are useful to the authors and help improve the quality of the paper.