CoAuthors

Guillaume Dodet (Laboratoire d’Océanographie Physique et Spatiale (LOPS), Institut français de recherche pour l'exploitation de la mer (IFREMER), Brest, France); Fernando Méndez (Universidad de Cantabria, Spain); Ana Rueda (Universidad de Cantabria, Spain); Alba Cagigal (Universidad de Cantabria, Spain)

Long-period swells generated in the North and South Pacific frequently thread the low-lying Pacific islands and atolls. The accuracy of wave forecasting models is key to efficiently anticipating and reducing damage during these flooding episodes. In remote areas, spectral wave observations are sparse in space and time. However, spectral wave information acquired by satellite missions brings huge potential to map and monitor the ocean surface, being useful for the analysis of wave climate variability and coastal hazards applications. In this work, we present a satellite-driven swell forecast system that can be applied to any ocean location worldwide to predict the arrival of swells. The methodology relies on the dispersive behavior of ocean waves, assuming that the energy travels along great circle trajectories with a celerity that only depends on its frequency. Satellite data for this analysis corresponds to the L2 spectral product collected by the CFOSAT on board SWIM instrument (Hauser et al., 2017). The proposed workflow includes a) filtering of global-coverage data considering a temporal and geographical criterion (the spatial scale delimits the effective energy source that can reach the target location); b) comparison of collocated wave parameters from SWIM and numerical wave models to remove the 180º directional ambiguity; c) selection of the spectral energy sector that points towards the study site; d) backward wave energy propagation to locate the energy sources and incorporate dispersion related to the distance to the storm (Collard et al., 2009); e) analytical propagation of wave partitions to obtain the target spectral energy over time. As an example of application, results in Samoa Island are presented. The time evolution of wave systems approaching the site is evaluated against spectral energy from available in situ wave measurements and numerical model outputs. The results exhibit a promising ability to reproduce the swell fields.

The proposed method may be used to track swells across the ocean, control the arrival of the visible swells or locate remote storms. For the Small Island Developing States, the output of the methodology can undoubtedly be of great help for stakeholders and decision-makers to produce indexes and risk metrics and to create strategies that minimize the vulnerability of these small islands to coastal flooding at a very meager computational effort.

ACKNOWLEDGEMENTS
This work was supported by the ISblue project and co-funded by a grant from the French government under the program "Investissements d'Avenir". The authors also acknowledge the Spanish Ministry of Science and Innovation for the project Beach4cast PID2019-107053RB-I00. ARu acknowledges the funding from the Juan de la Cierva-Incorporación IJC2020-043907-I / MCIN/AEI / 10.13039/501100011033 and the European Union “NextGenerationEU”/PRTR.

REFERENCES
Collard, F.; Ardhuin, F.; Chapron, B. Monitoring and analysis of ocean swell fields from space: New methods for routine observations. J. Geophys. Res. 2009, 114, C07023.

Hauser, D., Tison, C., Amiot, T., Delaye, L., Corcoral, N., Castillan, P., 2017. SWIM: The First Spaceborne Wave Scatterometer. IEEE Transactions on Geoscience and Remote Sensing 55, 3000–3014. https://doi.org/10.1109/TGRS.2017.2658672