Most ocean waves are formed when wind transfers energy into the surface of the water. Small disturbances grow into waves as the wind continues to blow, with their development controlled mainly by wind speed, how long the wind blows and the distance it blows across the water, known as fetch. Once formed, waves can carry that energy far beyond the weather system that created them.
What causes ocean waves?
Wind is responsible for most of the surface waves we see in the ocean. As moving air interacts with the sea surface, some of its energy is transferred into the water. Small irregularities on the surface begin to grow as the wind continues to act on them.
The result is not simply water being pushed from a storm all the way toward the coast. A wave is primarily a transfer of energy through water. The wave can travel over a large distance while individual water particles move much less distance overall.
How does wind create waves?
Wave formation begins with an exchange of energy between the atmosphere and the ocean. Wind moving across the surface creates pressure and frictional forces that disturb the water. Once small waves exist, the wind can continue transferring energy into them and make them grow.
A light wind acting over a small area for a short time produces relatively small waves. Strong wind blowing consistently across a large area of ocean for a long period can transfer much more energy into the sea and create a substantially larger wave field.
What determines how large wind waves become?
Three of the most important controls on wind-wave development are wind speed, wind duration and fetch. Large waves generally require all three to be sufficient.
- Wind speed: stronger wind can transfer more energy into the water.
- Wind duration: the longer suitable wind continues to blow, the more time waves have to develop.
- Fetch: the longer the uninterrupted distance over water across which the wind blows, the greater the opportunity for waves to grow.
Strong wind alone does not guarantee large waves. If it only blows briefly, there may not be enough time for a large wave field to develop. Likewise, strong persistent wind over a very short fetch is limited by the relatively small area over which it can transfer energy into the ocean.
What is fetch?
Fetch is the distance over open water that wind blows in a broadly consistent direction. It is one of the fundamental controls on the development of wind-generated waves.
A storm over a large ocean can have an enormous fetch, allowing waves to receive energy across hundreds or even thousands of kilometres. A similar wind over a small enclosed body of water has far less distance available for wave growth.
Fetch therefore helps explain why the open ocean can produce wave systems that cannot develop in the same way on smaller bodies of water.
Do waves move water across the ocean?
Ocean waves primarily transmit energy rather than transporting the same mass of water across the ocean. In deep water, water particles affected by a passing surface wave move in approximately circular orbital paths and remain relatively close to where they started after the wave has passed.
The orbital motion becomes smaller with increasing depth. This is why surface waves can travel across an ocean even though the individual water particles are not travelling the same distance with them.
Near the coast, this behaviour changes as the wave begins interacting with the seabed and eventually breaks.
What are wind waves?
Waves that are still being actively generated by local or regional wind are commonly referred to as wind waves or wind sea. Because multiple wave components are being generated at the same time, the sea surface can appear irregular and disorganised.
The size and character of that wind sea depend on the generating wind and the amount of time and space available for wave development. The distinction becomes particularly useful when comparing groundswell and windswell, which can produce very different conditions at the coast.
How does wind become swell?
Once waves move away from the winds that generated them, or the generating wind weakens, the wave energy can continue travelling across the ocean as swell. Swell is generally more organised than the actively generated wind sea and can travel for very large distances.
Different wave components do not all travel across deep water at the same speed. Longer-period components travel faster than shorter-period components. Understanding wave period helps explain how a complex storm-generated sea becomes more organised as it moves away from its source.
This is why waves arriving at a coastline can originate from weather systems far beyond the local horizon. The wind at the beach does not necessarily tell you where the incoming swell was created.
Can waves travel without wind?
Yes. Wind is needed to generate ordinary wind-driven waves, but the wind does not need to remain with them as they travel. Once energy has entered the wave system, swell can continue propagating through the ocean after leaving the region where it was generated.
This explains why a coastline can receive organised swell on a day when local winds are light. The energy may have originated in a distant storm many hours or days earlier.
Why do waves change as they approach the coast?
Deep-water waves can travel long distances with little interaction with the seabed. As they enter shallower water, that changes. The orbital motion associated with the wave begins interacting with the bottom, altering the wave's speed and shape.
The wave slows, its wavelength decreases and it can become progressively steeper. Eventually, if it becomes unstable enough, the crest breaks and much of the wave's energy is dissipated in the surf zone.
The exact transformation depends on more than the incoming wave alone. Water depth, seabed shape and coastal geography all influence what the wave becomes before it reaches shore.
Are all ocean waves caused by wind?
No. Wind generates the ordinary surface waves and swell most people encounter at the coast, but ocean waves can have other sources.
Underwater earthquakes, landslides and volcanic activity can rapidly displace large volumes of water and generate tsunamis. Tides are also wave phenomena, driven primarily by the gravitational interactions between Earth, the Moon and the Sun rather than by wind.
These processes operate on very different scales and should not be treated as interchangeable with ordinary wind-generated waves.
From ocean storm to breaking wave
A useful way to understand ordinary ocean waves is as a chain of energy transfer. Wind first transfers energy into the ocean. Wind speed, duration and fetch determine how much the wave field can develop. Waves that leave their generating weather system can organise into swell and travel across deep water. When that energy reaches shallow coastal water, interaction with the seabed transforms the waves until they eventually break.
That journey also explains why local conditions only tell part of the story. The waves at a beach can carry information from weather systems thousands of kilometres away.
To translate that information into actual coastal conditions, the next step is understanding how to read swell and surf forecasts and then how to interpret waves at the coast.