Ocean waves are mainly caused by disturbances that transfer energy into the water, with wind generated waves being the most common type at the ocean surface. When wind blows across the sea, friction and pressure variations disturb the surface and create small ripples. If the wind continues, these disturbances can grow as the atmosphere transfers additional energy to the water.
Larger waves develop when the wind is sufficiently strong, persists for a sufficiently long time, and acts over a sufficiently large distance called fetch. Ocean waves therefore represent the propagation of energy through the water, while individual water particles generally move in orbital or oscillatory paths rather than travelling forward with the wave over the entire distance. Other mechanisms can generate waves, including earthquakes, volcanic eruptions, landslides, atmospheric pressure disturbances, tides, and gravitational interactions.
Wind waves begin through complex interactions between air and water. Very small surface irregularities alter the airflow immediately above the sea surface, producing pressure differences that can transfer energy into the water. As waves grow, the interaction between wind stress and the sloping wave surface becomes increasingly important. Wave growth depends on wind speed, the length of time the wind blows, the distance over which it acts, and the existing sea state.
A young sea with locally generated waves is often irregular and contains waves with different wavelengths and directions. As waves travel away from the generating region, longer and faster components can move ahead of shorter components, producing swell. Swell can travel across entire ocean basins and may arrive at a coastline even when local winds are weak or blowing in a different direction.
Wave motion is governed by gravity, surface tension, water depth, wavelength, wave height, and interactions among waves. Gravity is the dominant restoring force for most ordinary ocean waves, pulling displaced water back toward an equilibrium surface. Surface tension is more important for very short capillary waves. In deep water, the behaviour of waves depends strongly on wavelength, and longer waves generally travel faster than shorter waves.
In shallow water, the influence of the seafloor becomes increasingly important because orbital water motion extends downward far enough to interact with the bottom. As waves enter shallow water, they slow down, their wavelength decreases, and their height may increase. If the wave becomes sufficiently steep, it can break, converting organized wave energy into turbulence, currents, heat, and other forms of motion.
Waves can be classified according to their generating mechanisms and physical characteristics. Wind waves and swell are the most familiar surface gravity waves. Tsunamis are long wavelength waves generated mainly by large scale displacement of the ocean, commonly through undersea earthquakes, although submarine landslides and volcanic processes can also produce them. Tidal waves are associated with the astronomical tides, although the term tidal wave is often avoided in scientific communication because tides are not ordinary wind generated waves.
Internal waves occur within the ocean rather than primarily at the surface and can develop along density boundaries such as the pycnocline. Seiches are standing oscillations within enclosed or semi enclosed bodies of water and can be triggered by atmospheric pressure changes, winds, earthquakes, or other disturbances. Each type has distinct mechanisms, wavelengths, propagation characteristics, and environmental effects.
As waves approach a coastline, they undergo shoaling, refraction, reflection, diffraction, and breaking. Shoaling occurs because decreasing water depth alters wave speed and wavelength, causing wave energy to become concentrated into a smaller vertical and horizontal region. Refraction occurs when different parts of a wave move at different speeds because they encounter different depths, causing the wave crest to bend. Reflection occurs when wave energy is redirected from a coastline, cliff, seawall, harbour structure, or other boundary.
Diffraction involves the spreading of wave energy around obstacles or through openings. When a wave becomes too steep for the available water depth, it breaks. Breaking waves are major agents of coastal erosion and sediment transport and contribute to the development of beaches, sandbars, spits, and other coastal landforms.
Waves transport energy over considerable distances and are therefore important to marine and coastal systems. Although water particles in a progressive wave can undergo approximately circular or elliptical orbital motion, the wave itself carries energy forward. In deep water, the particle motion decreases rapidly with depth, while in shallow water the motion extends toward the seafloor and can mobilise sediments. Wave energy can therefore influence seabed habitats, sediment redistribution, coastal morphology, and pollutant transport.
Wave conditions are also important for navigation, offshore structures, marine engineering, and renewable energy systems. Wave power technologies attempt to capture part of the energy carried by ocean waves, although practical deployment requires consideration of extreme storms, corrosion, biological growth, maintenance, grid connection, and environmental effects.
The size and character of ocean waves depend on more than wind speed. A strong wind acting over a short distance may generate smaller waves than a moderate wind acting across a vast ocean region for a long period. The initial sea state also matters because existing waves alter the way new wind energy is transferred. When waves from different sources overlap, they can interfere constructively or destructively, producing complicated patterns of wave height.
Rogue waves, more precisely called abnormal or freak waves in some contexts, are unusually large individual waves that can arise through several possible mechanisms, including wave focusing, nonlinear interactions, currents, and the superposition of multiple wave systems. Their formation is an active research topic. Numerical wave models use atmospheric wind data, bathymetry, observations, and physical equations to forecast wave height, direction, period, and other properties.
A common misconception is that ocean waves are simply large quantities of water moving continuously toward a beach. In most ordinary surface waves, the dominant movement is the transfer of energy rather than the wholesale transport of water from the open ocean to the shore. There can nevertheless be net water transport associated with wave induced currents, breaking waves, Stokes drift, and other processes.
Ocean waves are therefore best understood as dynamic disturbances whose physical behaviour changes with depth, wavelength, wave period, wind forcing, and seafloor geometry. Their causes range from everyday wind action to rare geological events, and their effects extend from microscopic turbulence to major coastal erosion and tsunami hazards.
