How a New Satellite Is Sizing Up Monster Storm Waves

Decoding the Ocean’s Giants

The world’s oceans are a realm of immense power, capable of generating waves of breathtaking size. For centuries, tales of “rogue waves” were the stuff of maritime legend, often dismissed as exaggeration. Today, we know that extreme waves are a real and dangerous phenomenon. These colossal walls of water, born in the heart of violent storms, pose a significant threat to ships, offshore platforms, and coastal communities. Understanding them isn’t just an academic exercise; it’s essential for safety, engineering, and predicting the impacts of a changing climate. Yet, the very storms that create these giants make them nearly impossible to study up close. Direct measurements from the most intense parts of a storm are incredibly rare, leaving a gap in our knowledge about the true upper limits of ocean wave height and power.

A groundbreaking approach is changing this. Scientists are now looking not into the eye of the storm, but at the messengers it sends across the globe: ocean swells. These long, rolling waves can travel thousands of kilometers from where they were formed, carrying with them a detailed fingerprint of the storm’s intensity. By using the advanced capabilities of the Surface Water and Ocean Topography (SWOT) satellite, a joint mission of NASA and France’s space agency CNES, researchers are learning to read these fingerprints. For the first time, they can measure the properties of swells with enough precision to reconstruct the characteristics of the extreme waves at the storm’s source, including their dominant wave period – the time between successive wave crests. This new method effectively allows scientists to size up the largest waves on Earth from a safe distance, revealing that the processes creating them are even more dynamic than previously believed. This work corrects long-held assumptions about how wave energy is distributed in storms and provides an unprecedented awareness of the true nature of the ocean’s giants.

The Challenge of Measuring a Monster

To appreciate the breakthrough of studying swells, one must first understand why measuring extreme waves directly is so difficult. The primary parameter used to describe the state of the sea is the significant wave height. It’s a statistical measure representing the average height of the highest one-third of the waves in a given area. In a storm with a significant wave height of 15 meters, some individual waves will be smaller, and others could be much larger, potentially reaching heights of 30 meters. These are the waves that can snap ships in two and toss massive boulders on shore.

The problem is that these conditions occur inside storms that are often hundreds of kilometers wide and located in the most remote and inhospitable parts of the ocean. Sending a research vessel into such a tempest is out of the question. Instrumented buoys can provide valuable data, but they are few and far between, and the chances of one being in the exact location of a storm’s peak intensity are vanishingly small.

For decades, our best tool for global wave monitoring has been the satellite altimeter. Instruments on satellites like Jason-3 and Sentinel-3 measure wave height by sending a radar pulse down to the ocean surface and timing how long it takes to reflect. By analyzing the shape of the returning signal, they can calculate the significant wave height along a narrow line directly beneath the satellite.

While altimeters have revolutionized oceanography, they have a fundamental limitation when it comes to capturing extreme events. A satellite travels at about seven kilometers per second, meaning it crosses a storm system in minutes. The storm’s core, where the waves are largest, might only be a few hundred kilometers across. The satellite’s path is a narrow, one-dimensional track, making it easy to miss the peak. It’s like trying to find the tallest tree in a forest by taking a single, random path through it. The odds are you’ll miss it.

This sampling issue is clear in the historical record. Despite decades of satellite altimetry, very few confirmed measurements of significant wave height have exceeded 16 meters. Yet, numerical wave models, which simulate the physics of the ocean, regularly predict wave heights well over 20 meters. This creates a dilemma: Are the models overestimating the waves, or are the satellites simply missing the biggest events? The evidence increasingly points to the latter. For example, during storm Bolaven in October 2023, a powerful extratropical cyclone in the Pacific, models calculated a peak significant wave height of over 20 meters. At the same time, the closest satellite altimeter pass measured only 5.4 meters. The satellite wasn’t wrong about what it saw, it just wasn’t in the right place at the right time. It missed the storm’s peak by several hundred kilometers. This illustrates the needle-in-a-haystack problem that has long plagued the study of extreme waves.

Swell: A Message From the Storm

When winds blow across the ocean surface, they transfer energy to the water, creating waves of all shapes and sizes. Inside a storm, the seascape is a chaotic mixture of waves moving in multiple directions, known as a “wind sea.” As these waves travel out of the storm system, they undergo a remarkable transformation. They begin to sort themselves out in a process called dispersion.

Dispersion happens because the speed of a deep-water wave depends on its period. Waves with a longer period (and thus a longer wavelength) travel faster than waves with a shorter period. Imagine a group of runners at the start of a race. When the race begins, the fastest runners quickly pull ahead of the pack, while the slower runners fall behind. Ocean waves do the same thing. The long, powerful waves generated by the storm’s fiercest winds race away from the source, while the shorter, slower waves lag.

After traveling hundreds or thousands of kilometers, this jumble of waves has organized itself into a clean, orderly pattern of long, rolling lines known as swell. This is the swell that surfers in California might ride from a storm that happened days earlier near Japan. These waves have retained precious information about their birthplace. By measuring the properties of these swells – their height, wavelength, and direction – scientists can trace them back across the basin to the storm that created them. The swell is, in effect, a message from the storm’s heart.

The very front of a swell field, known as the forerunner, consists of the longest-period waves that have outrun all the others. As envisioned by the famous oceanographer Walter Munk in the 1940s, these forerunners are the first hint of an approaching storm system. The new research takes this idea a step further. It shows that the entire structure of the dispersing swell field – how the wave height and wavelength change with distance from the storm – contains a detailed code. If this code can be cracked, it can reveal the properties of the waves back inside the storm, including those at the spectral peak, where most of the energy is concentrated. The key to cracking this code has arrived in the form of the SWOT satellite.

SWOT’s Revolutionary Two-Dimensional View

The Surface Water and Ocean Topography (SWOT) mission is a game-changer for oceanography. Unlike traditional altimeters that measure a single line, SWOT uses an advanced instrument called the Ka-band Radar Interferometer (KaRIn). It has two radar antennas mounted at either end of a 10-meter boom. By sending down radar signals from both antennas and analyzing the tiny differences in the return signals, SWOT can map the height of the sea surface over two broad swaths, each 50 kilometers wide, on either side of the satellite’s ground track.

This ability to see the ocean in two dimensions is what sets SWOT apart. It doesn’t just provide a single data point; it creates a high-resolution image of the ocean surface. For the first time, scientists can directly see the long, organized lines of swell waves. SWOT’s resolution is fine enough to resolve waves with wavelengths longer than about 500 meters, which corresponds to a period of 18 seconds – exactly the kind of long-period swells generated by major storms. It can also detect swell heights as small as just 3 centimeters.

This provides a massive advantage for studying extreme storms. While a traditional altimeter has to get lucky and fly directly over a storm’s peak, SWOT only needs to fly through the vast swell field that radiates outward from the storm. The target area is much larger, dramatically increasing the chances of gathering useful data. By observing a swell field, SWOT effectively observes the storm’s legacy from hundreds or even thousands of kilometers away. This is the technological leap that has enabled the new research. It provides the raw data – the detailed maps of swell height and wavelength – that are needed to decode the message sent by the storm.

Reading the Swell and Rewriting the Rules

Using SWOT’s unique data, researchers analyzed the swell fields from hundreds of storms. They discovered a remarkably consistent and previously unreported pattern. As they looked at swells farther and farther from the source storm, they saw that the wavelength predictably increased, just as linear wave theory would suggest due to dispersion. But they also found that the swell height dropped off with incredible speed, following a distinct mathematical relationship – it decreased with distance raised to a power of approximately nine.

This sharp decay in swell height is the key. It told the scientists something important about the “recipe” of waves back in the storm. This recipe is known as the wave spectrum. A wave spectrum isn’t a visual phenomenon; it’s a mathematical tool that describes how a storm’s total energy is distributed among waves of different frequencies (or periods). Most of the energy is typically concentrated around a “peak period,” but there is also energy in waves that are longer and shorter than the peak.

For decades, engineers and scientists have relied on standard models for this spectrum, like the famous JONSWAP spectrum, which was developed based on measurements from the North Sea in the 1970s. These models have worked well for average conditions but, as the SWOT data revealed, they are deeply flawed when it comes to the longest waves in the most powerful storms. Standard models predicted far more energy at very low frequencies (long periods) than what the swell observations implied. The SWOT data showed that the energy of waves with periods about 20% to 40% longer than the storm’s peak period was overestimated by a factor of 20.

This discrepancy points to the mechanism that builds monster waves: nonlinear four-wave interactions. This is a complex process where waves interact with each other, transferring energy among themselves. In a storm, shorter, steeper waves can feed their energy to longer waves, causing them to grow in height and period. This is how waves can develop that are so long they actually travel faster than the winds that create them. The SWOT observations strongly suggest that this process is the dominant source of energy for the long waves just below the spectral peak. The rapid fall-off of swell height is a direct consequence of the physics of these interactions.

Based on this, the researchers proposed an updated shape for the wave spectrum. Their new model incorporates the steep energy slope suggested by the SWOT data for frequencies below the peak. This revised spectrum not only aligns with the new observations but also provides a direct physical link between the measurable properties of the distant swell and the peak period of the waves back in the storm. By fitting their model to the observed decay of swell height with distance, they can now accurately estimate the storm peak period (SPP), a parameter that was previously hidden within the storm’s chaos. This is the heart of the breakthrough: using the subtle characteristics of tiny swells across the ocean to size up the most powerful waves at their source.

Putting the Method to the Test: The Stories of Bolaven and Eddie

The power of this new technique is best illustrated by looking at its application to specific, powerful storms.

Storm Bolaven

In October 2023, Tropical Cyclone Bolaven underwent a transformation in the North Pacific Ocean, becoming an intense extratropical storm. Numerical models – computer simulations of ocean physics – calculated that Bolaven generated a maximum significant wave height of 20.3 meters. This was the largest modeled value anywhere on the globe for the entire year. However, the region of these extreme waves was small, less than 300 kilometers across, and moved quickly eastward. As is often the case, no satellite altimeter passed close enough to the peak to verify this.

But Bolaven sent out a powerful swell field that spread across half of the Pacific Ocean. Days later and thousands of kilometers away, the SWOT satellite passed through this field. SWOT’s instruments mapped the long, rolling waves, measuring their height and wavelength. Applying their new method to the SWOT data, researchers analyzed how the swell height and wavelength changed along the satellite’s track. By fitting this data to their updated wave spectrum model, they calculated a storm peak period for Bolaven of 19.4 seconds. This value was very close to the 19.6-second peak period predicted by the wave model at the storm’s most intense moment. This agreement gave strong validation to both the new SWOT-based method and the wave model’s performance in extreme conditions. It showed that by observing faint swells, one could accurately infer the timing of the most powerful waves in a monster storm.

Storm Eddie

An even more compelling case occurred in December 2024 with a massive North Pacific storm that the researchers nicknamed “Eddie.” This storm generated an enormous swell that had a major impact across the Pacific basin. It produced large waves in Hawaii for the famous big wave surfing competition, The Eddie Aikau Big Wave Invitational, and caused extensive damage to coastal areas from Canada all the way to Peru.

SWOT observed the swell from Eddie at great distances. At 5,000 kilometers from the storm’s center, it measured swells with a mean wavelength exceeding 1,200 meters, corresponding to an incredibly long period of 28 seconds. Even farther out, at 5,600 kilometers, it could still detect swells with a wavelength of 1,360 meters (a 30-second period), though their height had diminished to just 6 centimeters, near the limit of SWOT’s detection capability.

What makes Eddie a landmark event is that, by a stroke of luck, a traditional satellite altimeter did get a direct look near the storm’s core. Just six hours before the storm reached its modeled peak, the altimeter on board the SWOT satellite itself (which also carries a conventional altimeter) measured a significant wave height of $19.7 pm 0.3$ meters. This is the largest significant wave height ever officially reported from a satellite altimeter.

This direct measurement provided a rare opportunity to check the consistency of the swell-based method. The analysis of Eddie’s vast swell field yielded a storm peak period of $20.2 pm 0.6$ seconds. According to established relationships between wave height and period, a wave field with that peak period should have a significant wave height of between 19 and 20.5 meters. The direct altimeter measurement of 19.7 meters falls squarely within this range. All the pieces of the puzzle fit together perfectly. The direct measurement confirmed the extreme height, and the swell analysis provided the corresponding wave period, demonstrating a powerful consistency between the two independent methods. It was a remarkable validation of the new science, confirming that swells are indeed reliable messengers from the heart of the storm.

A New Awareness of the Ocean

The ability to accurately estimate both the height and period of the most extreme waves has wide-ranging implications. For marine engineering and coastal engineering, the period of a wave is just as important as its height. Long-period waves carry more energy and can exert much larger forces on structures like offshore wind turbines, oil rigs, and coastal defenses. Designing these structures to withstand the true forces they will face requires accurate data on not just the highest waves, but also the longest ones. The updated wave spectrum provides a much more realistic foundation for these engineering calculations.

The research also has applications in coastal management and climate adaptation. As the climate changes, storm tracks and intensity may shift. Understanding the full character of the waves these storms produce is essential for predicting coastal erosion, flooding, and damage to infrastructure. The new method provides a tool to build a more robust global climatology of extreme wave events, which can help communities better prepare for future hazards.

Beyond engineering, these findings connect to other areas of Earth science. For decades, seismologists have detected a persistent, low-frequency hum in the Earth’s crust known as a microseism. This seismic noise is generated by the interaction of ocean waves. One particular signal, a mysterious “gliding tremor” with a period of around 26 seconds, has puzzled scientists. The observation of swells with periods as long as 30 seconds provides a compelling link, suggesting that these unexplained seismic signals could be directly related to the longest waves generated in the planet’s most intense storms. The SWOT data could help calibrate the relationship between microseism intensity and ocean wave height, potentially allowing scientists to use historical seismic records, which go back over a century, as a proxy to study past storm activity.

Summary

The largest ocean waves have long been a mystery, their immense power shrouded by the violent storms that create them. Direct measurements are rare and dangerous, leaving scientists to rely on models that have been difficult to validate. A new era of understanding has begun, thanks to the innovative use of data from the SWOT satellite. By looking at the faint, long-period swells that travel thousands of kilometers from a storm’s center, scientists can now reconstruct the properties of the extreme waves at their source.

This research has revealed that the standard models used to describe wave energy in storms were significantly overestimating the energy of the longest waves. The unique pattern of swell decay observed by SWOT points to the dominant role of nonlinear interactions in building monster waves, a process where shorter waves feed energy to longer ones. This insight has led to a revised, more accurate model of the storm wave spectrum.

The successful application of this method to major storms like Bolaven and Eddie – including the landmark case where a direct 19.7-meter wave height measurement confirmed the swell-based analysis – validates this groundbreaking approach. It gives us a new tool to fill in the gaps in our knowledge of the ocean’s most powerful events. This enhanced awareness of wave properties has immediate practical applications, from improving the design of offshore structures and refining coastal hazard assessments to helping solve long-standing mysteries in seismology. For the first time, we are truly beginning to size up the giants of the open ocean.

Reference: Sizing the largest ocean waves using the SWOT mission by Fabrice Ardhuin, Taina Postec, Mickael Accensi, Jean-François Piolle, Guillaume Dodet, Marcello Passaro, Marine De Carlo, Romain Husson, Gilles Guitton, and Fabrice Collard.