Satellites have opened a completely new chapter in the understanding of waves on the world's oceans in recent years. The latest measurements show that during powerful winter storms in the northern hemisphere, waves were recorded whose significant height averages approximately 20 meters – a height comparable to the Parisian Arc de Triomphe. But a far more important message lies in the way the energy of these waves spreads: a long oceanic swell travels thousands of kilometers from its source and, like an invisible messenger, brings destructive energy to shores that the storm has never touched. By comparing new satellite images with decades-old records of sea state, scientists have identified patterns that allow for early recognition of such events and more accurate risk assessments for coastal communities and infrastructure.

Waves, wind, and long periods: the physics that connects the storm and the coast

Waves are created by the action of wind on the sea surface. While a storm is ongoing, the sea is filled with a mixture of different wavelengths and directions – the so-called wind sea. When the system moves or weakens, the more orderly, longer waves that make up the swell remain in the foreground. It is precisely these long periods – for example, 18, 20 or more seconds between two crests – that are key to understanding the scale and power of the source storm. Long waves dissipate less, spread faster, and survive longer, so their "signals" reach far beyond the reach of the wind that created them. On the coast, this manifests as the sudden appearance of high, long waves and increased breaking in shallow areas, even when local winds are weak or variable.

The new satellite era: SWOT and the continuity of European altimetry missions

A breakthrough in swell detection came with the SWOT (Surface Water and Ocean Topography) mission, which combines classic radar altimetry with wide-swath imaging. Instead of measuring exclusively in a narrow strip below the satellite, SWOT maps the two-dimensional "topography" of the sea surface and thus captures the spatial patterns of long waves, their height, wavelength, and direction of propagation. This new geometry builds on multi-decade altimetry series collected by missions such as Jason-3, SARAL, CryoSat, Sentinel-3A/B, CFOSAT, and Sentinel-6, integrated into systems for climate and operational forecasting. At the European Space Agency, this continuous record of sea state is being developed as part of initiatives dedicated to "sea state" variables – a statistical description of the wave climate that includes wave heights, periods, and directions.

Record episode: storm "Eddie" and the journey of swell across two oceans

One of the most impressive displays of this new capability was recorded on December 21, 2024, when SWOT flew over the northern Pacific during the peak of a storm that researchers named "Eddie." The analysis showed significant wave heights of ~20 meters in the open ocean and enabled precise tracking of how the storm's energy, transformed into long swell, continued its journey through the Drake Passage all the way to the tropical Atlantic – during the period from December 21, 2024, to January 6, 2025. Such a transoceanic swell "beam" visually and quantitatively connected distant coasts with the source of the storm and confirmed in real time what models and earlier measurements had only indicated: that long waves act as messengers, long before local wind or air pressure forecasts signal danger.

Why this episode is scientifically important

Measuring extremely large waves from space has been a challenge for many years due to limited spatial resolution and rare flyovers. SWOT's wide "net" and synthesis with multi-year products from the European Space Agency (CCI Sea State) made it possible for the first time to systematically observe wave fields at the moments when energy was just "organizing" into long swell. This gave researchers direct, observed values for validating numerical wave models in extreme conditions, which is crucial for correcting energy calculations and risk assessments. Unlike point measurements from buoys or ships, wide-swath maps provide a coherent picture of how energy is transferred between wavelengths as waves travel across the oceans.

Correcting old understanding: the longest waves carry less energy than previously thought

Based on a comparison of a wide range of satellite missions and new SWOT images, scientists have found that traditional models systematically overestimated the energy in very long waves. Although long waves are extraordinarily efficient at transmitting information about a storm over great distances, it turned out that the truly largest part of the energy is concentrated in the dominant "peak" waves within the storm field. This nuanced distribution – less energy in the longest tail, more in the main peak of the spectrum – changes the expected mechanisms of coastal structure loading and the fine details in calculations of wave impacts, sea level rise due to waves (wave setup), and coastal erosion.

Lessons from history: the winter of 2013/2014 and storm Hercules

For comparison and risk calibration, the winter season of 2013/2014 is often cited, when a series of powerful cyclones hit the northeastern Atlantic. Among them, the episode known as "Hercules" stood out, to which waves over 20 meters high and widespread damage from Morocco to Ireland are attributed. At that time, floods hit parts of the Moroccan Atlantic coast, damage to tourist and port facilities was recorded, and Irish and French beaches suffered erosion and the displacement of huge amounts of sediment. Recent studies of that winter series show how the combination of longer periods, higher waves, and the Atlantic synoptic situation creates conditions for extreme events that last for days, not hours, so any additional precision in the forecast is of great practical value.

How satellites "see" a wave: from impulse to statistics

The basis of modern wave height products is radar altimetry. Instruments like Poseidon-4 on the Sentinel-6 Michael Freilich mission send microwave pulses toward the sea and measure the return time. The width and shape of the return signal reveal the roughness of the surface, from which the significant wave height (a statistical measure describing the height of the "upper third" of the waves) is calculated, and other parameters, including wind speed, are also estimated. SWOT introduces a key innovation: an interferometric radar with two antennas and a wide field of view, which enables two-dimensional mapping of the sea surface and the capture of longer wavelengths that earlier sensors often missed. This geometry creates a link between wave patterns and the storm source, which ultimately facilitates attribution and early warning.

When distant swell becomes a local problem

Even when a storm does not approach land, long waves can cause damage. On certain types of coasts, especially where the configuration of the seabed and coastline acts as an "amplifier," waves with periods of 16–22 seconds can produce unusually strong breaking, sudden flooding of low-lying areas, and damage to piers, breakwaters, and promenades. If they occur in coincidence with a high astronomical tide or a rise in the mean sea level, the effects are multiplied. Coastal protection systems are therefore increasingly using a combination of satellite maps, measurements from wave buoys, and local numerical models to issue targeted recommendations: temporary closures of exposed promenades, warnings to surfers and divers, redirection of ship traffic, or postponement of work on coastal structures.

What "significant wave height" means and why period changes the perception of risk

In everyday language, "five-meter waves" are often mentioned, but experts generally use significant wave height (Hs) and period (T). Two waves of the same height but different periods have a completely different mechanical effect on structures and the coast. A longer period means a greater orbital velocity of water particles and deeper penetration of wave energy toward the bottom, so breaking can be more dramatic, with greater horizontal reach of the water impact. This is precisely why modern services give great weight to predicting the period and direction of swell, and not just the height. In the context of maritime transport, this also means a different assessment of the comfort and safety of navigation: sailboats, tugboats, and large merchant ships react differently to combinations of height and period, so routes and speeds can be optimized according to the spectrum that is "coming," and not just according to the local wind forecast.

From science to service: how observations are translated into forecasts

Data from altimetry missions are fed into operational wave models that are used daily in maritime transport, fishing, and coastal management. Sentinel-6 provides a reference series for sea height and reliable estimates of significant wave height and wind speed in near real-time, while SWOT and other missions supplement the picture with spatial maps of swell. When this data is combined with measurements from buoys, coastal radars, and local models, a sufficiently precise forecast is obtained to, for example, prepare ports and shipping companies in advance, secure workplaces at sea, or restrict access to dangerous zones on beaches.

Example of a transoceanic sequence: from the North Pacific to the tropical Atlantic

The tracking of storm "Eddie" showed how the energetic core of the storm was transformed into a long swell that traveled about 24,000 kilometers: first to the southeast and south Pacific, then through the Drake Passage, and finally to the tropical Atlantic. On this journey, the waves are gradually "purified": longer wave components arrive first, shorter ones are late and dissipate faster. In practice, this means that authorities in regions thousands of kilometers away from the source can, from satellite maps and models, estimate the arrival time and intensity of the swell several days in advance and prepare coastal measures, including temporary bans on smaller vessels entering sensitive ports.

Climate context: trends, but also the limitations of statistics

The question of whether the frequency and intensity of large storms are changing with climate change is justified and common. Thanks to multi-decade records (from 1991 onwards), we can track changes in the wave climate, but proving trends in extremes requires very long series because the most severe episodes occur rarely, approximately once a decade. Researchers therefore combine longer satellite series with wind reanalyses and regional measurements to separate the influence of climate warming from natural variability. In addition, the geometry of the bottom and the local morphodynamics of the coast often decisively shape the wave situation at a microlocation, so a separate assessment of sensitivity and risk is necessary for each coast.

Lessons for coastal projects, wave energy, and maritime safety

For the design of protective walls, breakwaters, and piers, as well as for the planning of offshore wind farms and wave energy power plants, the distribution of energy in the spectrum is a key item. If more energy is concentrated in the dominant peak of the spectrum than in the very long tail, structures and operational procedures must be adapted to the most intense impact episodes, not to the "average." This implies different criteria for temporary port closures, different safety zones around workplaces at sea, and more cautious procedures for berthing during long-period episodes. For fishermen, tourist operators, and organizers of sports events on waves, a reliable signal about the period and direction of the swell is often more important than the height itself.

What follows until the end of 2025 and beyond

As the missions complement each other – with Sentinel-6B taking over the baton and routine SWOT flyovers – even more detailed mapping of swell is expected, including waves with a height of only a few centimeters with wavelengths up to about 1400 meters, which earlier sensors often missed. Continuous, multi-mission verification of models in extremes should bring more precise forecasts a day or two in advance, but also more reliable statistics for decades ahead: exactly what is needed for coastal communities and the maritime industry to adapt to new patterns of wave climate and the effects of rare but extremely powerful storms.