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Image of the month
2024
Oct. 2024: Swot & swell

Swot & swell

Image of the Month - October 2024

Sea level variations, in shades of blue, along Swot Swath ground track, measured by KaRIn, showing the swell in the Atlantic from storm Kirk (ex-hurricane). This swell reached the North-East Atlantic coasts on October 9, 2024. Sea level is filtered between 250 and 5 km wavelength to highlight the swell. Colors are saturated above a 10-cm amplitude, so amplitude is slightly higher than the amplitude shown on the image. The animation also includes backscatter coefficient, sea level anomalies, spectra, coherence phase and location of the measurements (Credit LOPS/Ifremer)

Hurricane season is currently at its peak in Atlantic Ocean. Some hurricanes have a catastrophic impact on the North American continent when their path goes that way. Others have a path which stays mostly over the ocean before landing on western European coasts as storms with heavy rains.
Kirk was such a hurricane. It intensified up to category 5 (major hurricane), but didn't come close to the North American continent. It induced heavy rains and flooding on October 9-10 in France, though.
While it was in the Atlantic Kirk generated waves, and those waves propagated radially as swell over the whole ocean basin. One of the unexpected outputs of Swot is that such storm-generated swell with wavelengths between 500 m and 1.3 km are clearly seen in KaRIn Sea Surface Height, even those with amplitude as small as 3 cm. They appear as series of nearly parallel lines, looking like fingerprints; criss-crossing of swell can also be seen, when there are several of them, generated by difference sources, or reflected by coasts and islands or even diffracted through straits. Detecting such swells, measuring their amplitudes, wavelengths and directions can be used to identify storms from these fingerprints and quantify storm energy and evolution.

References:

Ardhuin, F., Molero, B., Bohé, A., Nouguier, F., Collard, F., Houghton, I., et al. (2024). Phase-resolved swells across ocean basins in SWOT altimetry data: Revealing centimeter-scale wave heights including coastal reflection. Geophysical Research Letters, 51, e2024GL109658. https://doi.org/10.1029/2024GL109658
Ardhuin F., M. De Carlo, A. Ollivier, A. Nigou, Altimetry and the many scales of the ocean surface elevation: wave heights, wave groups, skewness, and sea level "noise”, "30 years of progress in radar altimetry Symposium, Montpellier, France, 2-7 September 2024

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