Ocean swell originates from distant storms. When strong winds blow across open water, they transfer energy into the surface, creating chaotic, choppy waves. As those waves travel away from the storm, they organize themselves by speed and wavelength into the smooth, rolling waves that eventually reach coastlines hundreds or even thousands of miles away. That transformation from rough storm seas to clean, evenly spaced waves is what defines swell.
How Storms Build Waves
Three factors determine how much wave energy a storm produces: wind speed, wind duration, and fetch (the uninterrupted distance of open water the wind blows across). All three must be large for big waves to form. Strong wind blowing for only a few minutes produces nothing significant, even over unlimited open water. Likewise, sustained gale-force winds over a short stretch of ocean won’t build much either. The biggest swells come from intense low-pressure systems that maintain high winds for days over thousands of miles of open ocean.
Both tropical and extratropical storms generate swell. In the North Atlantic and North Pacific, powerful extratropical cyclones (large winter storms) are the primary swell factories, spinning up massive wave fields that radiate outward in every direction. Tropical cyclones, including hurricanes and typhoons, also produce significant swell that can affect coastlines far from the storm’s actual path. Climate patterns like El Niño and the North Atlantic Oscillation shift where these storms form and how intense they get, which directly changes swell patterns year to year.
What Separates Swell From Wind Waves
Directly under a storm, the ocean surface is a mess. Waves of all sizes crash into each other from different directions. These are called “seas” or wind waves. They’re short-crested, irregular, and have periods (the time between one crest and the next) typically ranging from 3 to 25 seconds.
Swell is what those waves become once they leave the storm area. Free from the wind that created them, the waves sort themselves out through a process called dispersion. Longer waves travel faster than shorter ones, so they pull ahead and separate from the pack. The result is a well-organized train of waves with smooth, defined crests and consistent spacing. Swell waves are more uniform, more predictable, and travel in cleaner lines than the chaotic seas they started as.
How Swell Travels Across Oceans
Once swell leaves a storm, it can cross entire ocean basins. In late 2024, satellites tracked a single swell event radiating 24,000 kilometers from the North Pacific, through the Drake Passage at the southern tip of South America, and into the tropical Atlantic. The journey took roughly two weeks. This kind of long-range propagation is not unusual. Swells generated by storms near Antarctica regularly reach beaches in California, and North Atlantic winter storms send energy to the coasts of West Africa and Brazil.
The speed of a swell group depends on its period. Longer-period swells (with more seconds between crests) move faster. A swell with a 20-second period travels roughly twice as fast as one with a 10-second period. This is why, after a distant storm, surfers and forecasters first see long-period swell arrive, followed hours or days later by shorter-period waves from the same event.
Swell loses surprisingly little energy in deep water. The two biggest reasons wave height drops over distance aren’t actually energy loss at all. They’re geometric: the wave field spreads out in a fan shape (angular spreading), and faster waves pull ahead of slower ones (frequency dispersion), stretching the energy over a larger area. The total energy stays roughly the same, but it gets distributed more thinly. Actual energy dissipation from friction and turbulence is minimal in deep water. Research published in the Journal of Geophysical Research estimated that small-amplitude swells can travel with almost no measurable energy loss, while steeper swells lose energy with a halving distance of at least 2,000 kilometers. That’s still an enormous range.
What Happens Near the Coast
Swell behaves very differently once it reaches shallower water. As the ocean floor rises, waves slow down and their energy compresses, causing wave height to increase. This is called shoaling, and it’s why a swell that was barely visible in the open ocean can produce powerful surf near shore.
The shape of the seafloor also bends the direction waves travel. This bending, called refraction, is the dominant factor that determines how wave energy is distributed along a coastline. Waves bend toward shallower areas. An underwater ridge or seamount will attract and concentrate wave energy, focusing it on the stretch of coast behind it. A submarine canyon does the opposite, allowing waves to pass through with less resistance and reducing surf on the adjacent beach. This is why two beaches a few miles apart can have dramatically different wave conditions on the same day, receiving the same swell but filtering it through different underwater terrain.
Where the Biggest Swells Originate
The most powerful and consistent swells on Earth come from a few key regions. The Southern Ocean, the band of open water encircling Antarctica, produces some of the longest-traveling and most energetic swells on the planet. With virtually no landmass to interrupt the wind, fetch is essentially unlimited. Storms there generate swells that fan northward into the Pacific, Atlantic, and Indian Oceans simultaneously.
The North Pacific is another prolific source, particularly in winter. Extratropical cyclones forming near Japan and the Aleutian Islands send long-period swell toward Hawaii, California, and Mexico. The North Atlantic produces similar swells from storms tracking between Newfoundland and Iceland, delivering waves to Western Europe, the Canary Islands, and the Caribbean. In the tropics, hurricane season creates intense but more localized swell events, often sending energy in all directions from a slow-moving or stationary storm.
The swell arriving at any given beach is the combined signature of storms that may have occurred days earlier and thousands of miles away. A single winter day in Southern California might receive overlapping swell from a North Pacific storm, a Southern Hemisphere groundswell, and residual energy from a tropical system near Central America, each arriving from a different direction with a different period and wave height.