Internal solitary waves as a global ocean hazard observed continuously by automated Sentinel-1 classification

Beneath the surface of the open ocean, invisible to the naked eye, massive waves move through the water column. These are not the familiar swells that crash on a beach, but internal solitary waves. They form when tides push layers of water of different densities over underwater hills and ridges, creating a disturbance that travels for hundreds of kilometers. While the surface above might look calm, these hidden waves can displace water by more than one hundred meters and generate powerful currents that shift rapidly. For decades, scientists have known these waves exist, but they have been difficult to track. They are a hidden hazard to offshore oil rigs, underwater pipelines, and submarines, capable of exerting crushing forces on structures or disrupting the path of a vessel. Because they are so hard to find without specialized equipment, our understanding of where they occur has been limited to scattered reports from specific locations where researchers happened to look.

A new study changes this picture by turning a vast archive of satellite images into a continuous, global monitoring system. Researchers have developed an automated system that scans thousands of radar images taken over the open ocean, identifying the subtle surface patterns left by these hidden waves. By applying this method to more than four million images collected over several years, the team has created the first systematic record of where these waves appear across the globe. The result is a map that covers more than eighty-three percent of the open ocean, revealing that these dangerous waves are far more widespread than previously thought. This system does not just look for waves where scientists expect them to be; it scans everything, recording both where waves are found and where they are absent. This shift from searching in specific spots to watching the entire ocean at once transforms our ability to understand and predict this oceanic hazard.

For a long time, the story of internal solitary waves was written in fragments. Scientists would focus on a particular region, such as the South China Sea or the Andaman Sea, where they suspected these waves were active. They would gather data, find the waves, and publish their findings. However, this approach left large gaps in our knowledge. If researchers looked in an area and found nothing, that result was rarely published. Consequently, the scientific record became a collection of successful sightings rather than a true map of the phenomenon. It was like trying to understand the weather by only keeping a diary of days when it rained in a few specific towns, while ignoring the vast stretches of land where the sky remained clear. This patchwork record meant that we knew very little about how often these waves occurred in the deep ocean or how they moved across different regions. The lack of a complete picture made it difficult to assess the full risk they posed to the infrastructure and vessels that traverse the world's seas.

The breakthrough in this research comes from using artificial intelligence to analyze data that already exists. The Sentinel-1 satellite, which has been circling the Earth since 2014, continuously takes small radar pictures of the ocean surface. These images are valuable because radar can see through clouds and darkness, capturing the texture of the sea regardless of the weather or time of day. When an internal solitary wave passes beneath the surface, it creates a unique pattern of bright and dark bands on the water's surface, caused by the wave's currents smoothing or roughening the tiny ripples. For years, this massive archive of images sat largely unexamined because there were too many to look at by hand. The new study introduces a system that automatically scans these images, flagging the ones that show the tell-tale signs of a wave. This automated process allows researchers to process millions of images quickly, turning a static archive into a live, operating monitoring service.

The team applied this system to over four million radar snapshots harvested from the open ocean between late 2021 and September 2026. After filtering out images taken over land or in marginal seas, they analyzed more than 4 million ocean scenes. The automated classifier identified potential wave signatures, which were then reviewed by human experts to confirm their presence. This rigorous process resulted in nearly 19,000 confirmed detections. The study reveals that these waves are not confined to a few well-known hotspots but are distributed across the globe in a distinct pattern. They are most frequent in the tropics, drop off in the subtropical regions, and rise again in the mid-latitudes, particularly in the northern hemisphere. The researchers identified nineteen distinct centers of activity, with the six largest holding nearly three-quarters of all confirmed detections. Eleven of these active centers were in regions that had never been documented in scientific literature before, proving that the automated survey could find what human-led campaigns had missed.

One of the most significant findings is the sheer scale of the ocean now under observation. While previous studies had only examined about twelve percent of the open ocean, this new system brings more than eighty-three percent within reach. This is not just a matter of seeing more area; it represents a fundamental change in how we observe the ocean. The system records every image it takes, whether it finds a wave or not. This means scientists can now calculate the actual rate at which these waves occur, rather than just listing where they have been seen. The data shows that these waves are a global phenomenon, traveling vast distances from their generation sites. For instance, waves generated in deep water can travel hundreds of kilometers before reaching coastlines, where they can cause damage to harbors and coastal infrastructure. By detecting these waves while they are still far out at sea, the system offers a potential early warning capability, giving operators of offshore platforms and ships time to prepare for the arrival of strong currents.

The study also clarifies the limitations of what can be seen. The radar images used in this survey are sensitive to wind speed. If the wind is too light, the surface is too smooth for the radar to detect the wave's signature; if the wind is too strong, the surface becomes too rough, masking the wave. This means the system might miss waves during very calm or very stormy conditions, particularly in the mid-latitude storm tracks. However, the researchers are confident that the data provides a reliable lower bound on where these waves occur. The system is designed to improve over time, with the artificial intelligence model being retrained and the entire archive being re-analyzed as new data arrives. This ensures that the record becomes more complete and accurate as the years go on.

The implications of this work extend beyond just counting waves. By providing a continuous, global record, the study offers a new tool for understanding how the ocean mixes and moves heat and nutrients. Internal waves play a crucial role in stirring the ocean, which affects climate patterns and marine ecosystems. They can also transport larvae and sediments, shaping the environment for coral reefs and other coastal life. The ability to monitor these waves in near-real-time opens the door to new services, such as alerting submarine operators to dangerous currents or helping scientists understand how climate change might alter the frequency and intensity of these events. The researchers emphasize that this is the first operational monitoring service for internal solitary waves, moving the field from a collection of isolated case studies to a systematic, global science.

This new perspective shifts the focus from where we have looked to where the waves actually are. The study confirms that the open ocean is far more active with these hidden waves than previously imagined. It establishes a baseline for future research, allowing scientists to test theories about how these waves are generated and how they interact with other ocean features. As the satellite constellation grows and the archive expands, the system will continue to refine our understanding of this energetic and often dangerous aspect of the marine environment. The work demonstrates that by combining existing technology with advanced analysis, we can uncover patterns in the natural world that were previously invisible, turning a patchwork of observations into a coherent global picture.