🌊 Record: here is the highest wave ever measured

The Franco-American satellite SWOT has just set a new record: 19.7 meters (64.6 feet), the greatest wave height ever measured in the ocean. Most importantly, its unique ability to finely map the ocean makes it possible to reconstruct wave properties at the very heart of storms, where no instrument has ever gone before. A major advance for understanding extreme phenomena and improving safety at sea.

Elusive extreme waves

The most spectacular ocean waves are born under very particular conditions: when the strongest winds of a storm move at exactly the same speed as the waves themselves. This synchronization allows the waves to grow in height and length for several hours, concentrating phenomenal energy in an area less than 300 km (186 miles) in diameter. Then this energy gradually disperses as swell across the entire ocean.

An international team led by researchers from CNRS Earth & Universe (see box) has cataloged and classified all global storms since 1991 based on their maximum waves. While weather forecast models predict heights reaching 23 meters (75 feet) offshore, the altimetry satellites that have succeeded one another since 1992 have never exceeded 18.5 meters (60.7 feet). The reason? These instruments only cover a very small portion of the ocean and systematically miss the most extreme areas of storms, which only last a few hours.

SWOT: seeing the invisible

The SWOT satellite (Surface Water and Ocean Topography), launched in December 2022, is a game-changer. Beyond the simple height measurement of conventional altimeters, SWOT maps sea level with an unprecedented resolution of 250 meters (820 feet), making it possible to "see" individual waves and simultaneously measure their height, length, and direction.

Even when it doesn't fly directly over the heart of a storm, SWOT captures the swell that escapes from it and propagates over thousands of kilometers. Researchers can then trace back to the source, like going up a river, and reconstruct the properties of the waves at the time of their formation in the storm. On December 21, 2024, SWOT passed exceptionally close to the heart of storm "Eddie" in the North Pacific: it recorded a new absolute record with a significant height of 19.7 meters (64.6 feet), the highest wave ever measured by a satellite since the beginning of space observations in 1991. This value also exceeds all records made by sea buoys.

The "energy cascade" revealed

The analysis of hundreds of swell trajectories reveals major discoveries. The swells generated by Eddie reached lengths of 400 to 1,600 meters (1,312 to 5,249 feet) and were observed from the North Pacific to the tropical Atlantic. At 5,600 km (3,480 miles) from the storm, SWOT still detected waves 1,360 m (4,462 ft) long (corresponding to a period of 30 seconds) and only 6 cm (2.4 inches) high, at the limit of its sensitivity.

The height of the waves decreases very rapidly with distance from the storm, following a spectacular power law: proportional to d⁻⁹ (where d is the distance). This behavior confirms a theory formulated in 1962 by German physicist Klaus Hasselmann¹: the most energetic waves transfer part of their energy to slightly longer waves through nonlinear four-wave interactions, creating a true "energy cascade" that allows waves to reach phenomenal heights. For the first time, this cascade is observed for waves as long as 1.5 times the dominant period of the storm.

Concrete impacts

These discoveries have led researchers to correct the empirical spectral shapes used since the 1960s in all wave models. The old formulation overestimated the energy of the longest waves (between 1.2 and 1.4 times the dominant period) by a factor of 20. This update will improve marine weather forecasts and refine our knowledge of extreme waves, crucial information for the design of offshore and coastal infrastructure.

And also a major safety issue: the swell generated by storm Eddie caused casualties and considerable damage from Canada to Peru, thousands of kilometers from its point of origin. These observations also open new perspectives for interpreting the seismic signals generated by ocean waves, recorded for more than a century, with periods that can reach 26 seconds.

Note

¹ Klaus Hasselmann received the Nobel Prize in Physics in 2021 for his contributions to the physical modeling of Earth's climate and the detection of human-caused climate change. His work on nonlinear interactions between ocean waves, formulated as early as 1962, also constitutes a major contribution to physical oceanography.