Waves in the Ocean are propagating disturbances that transfer energy through seawater and occur in many forms produced by different mechanisms. Wind generated surface waves are among the most familiar, but ocean waves also include swell, capillary waves, internal waves, seismic sea waves and other long wavelength disturbances.
In a simple surface gravity wave, water particles generally move in approximately orbital paths while the wave energy propagates forward, meaning that the water itself does not necessarily travel with the wave over long distances at the same rate as the wave form. Wave behaviour depends on wavelength, period, height, water depth, gravity, wind conditions and interactions with the seabed and coast. Understanding ocean waves requires concepts from fluid dynamics, mechanics and coastal science and is essential for navigation, engineering, weather forecasting, sediment transport and coastal hazard assessment.
Wind generated waves develop when atmospheric winds transfer energy and momentum to the ocean surface. Small disturbances initially form through surface tension and gravity effects, after which larger waves can grow as wind continues to act over the surface. Wave growth depends on wind speed, the duration over which the wind blows and the fetch, which is the distance over which the wind acts without major interruption.
A fully developed sea state is approached when these factors provide sufficient energy for a statistical equilibrium between wave growth and energy loss. Waves can continue propagating after leaving their generating region, becoming swell with relatively long wavelengths and regular appearance. Because different wave components travel at different group speeds, a swell field can evolve as it moves across an ocean basin.
Wave characteristics include height, wavelength, period, frequency, direction and phase speed. Wave height is the vertical distance between crest and trough, while wavelength is the horizontal distance between successive crests or equivalent points. Period is the time between successive wave crests passing a fixed point, and frequency is the inverse of period.
Deep water occurs where water depth is sufficiently large relative to wavelength that the seabed has little influence on wave motion, while shallow water occurs when depth is small enough to affect wave speed and particle motion. Intermediate conditions occur between these regimes. The distinction matters because waves change as they move into shallower water. Their speed decreases, wavelength shortens and wave height may increase until breaking occurs under suitable conditions.
Wave transformation near coastlines involves refraction, diffraction, reflection, shoaling and breaking. Refraction occurs because different parts of a wave encounter different water depths and therefore travel at different speeds, causing wave crests to bend. This can concentrate wave energy around headlands and redistribute it along irregular coastlines. Diffraction allows wave energy to spread into regions behind barriers or through openings, while reflection occurs when waves encounter sufficiently steep or rigid boundaries.
Shoaling describes the increase in wave height associated with changes in depth as waves approach shore. Breaking occurs when a wave becomes unstable, commonly because its height becomes too large relative to local depth or because steepness becomes excessive. Breaking waves dissipate energy and generate currents and turbulence that strongly influence beaches and coastal sediments.
Long ocean waves can be produced by disturbances other than wind. Tsunamis are generated primarily by rapid displacement of large volumes of water, commonly associated with undersea earthquakes, submarine landslides or volcanic processes. In deep water, a tsunami can have a relatively small wave height but an extremely long wavelength and can travel rapidly across ocean basins. As it approaches shallow water, its speed decreases and wave height can increase substantially, potentially producing destructive coastal inundation.
Internal waves occur within the ocean where density changes with depth create stratification, allowing disturbances to propagate along internal density interfaces. Internal waves can transport energy and influence mixing, nutrient distributions and biological processes. These examples demonstrate that the term wave encompasses several physical phenomena with different generation mechanisms and properties.
Wave measurements use instruments including wave buoys, radar, satellites, pressure sensors and coastal gauges. Buoys can record surface elevation over time and derive statistical properties such as significant wave height and dominant period. The significant wave height is approximately the average height of the highest third of waves in a measured wave record and is widely used in oceanographic and engineering applications. Spectral analysis separates a complex sea state into wave energy distributed across frequencies and directions.
Numerical wave models use atmospheric wind fields, bathymetry and physical parameterisations to forecast wave conditions. Satellite radar altimetry and synthetic aperture radar can provide large scale information about wave height, wavelength and direction. Accurate observations are important because real seas contain mixtures of waves generated by different storms and distant sources rather than a single regular wave train.
Ocean waves strongly influence coastal morphology and sediment transport. Breaking waves generate turbulence and currents that move sand along beaches and offshore. Longshore currents can transport sediment parallel to coastlines, while wave driven cross shore transport can move sediment between beaches and nearshore bars. Repeated wave action contributes to coastal erosion, cliff retreat and the reshaping of beaches, although sediment supply, tides, storms, sea level and geology also influence these processes.
Extreme waves generated during storms can produce rapid changes in coastal morphology and damage infrastructure. Wave energy is also a major consideration in offshore engineering because structures must withstand repeated cyclic loading from waves. Ships, offshore platforms, wind turbines, breakwaters and coastal defences are designed using wave statistics appropriate to their locations and expected service conditions.
Waves differ fundamentally from currents because a wave is primarily a propagating disturbance and energy transfer, whereas a current represents directed transport of water. They also differ from tides because ordinary wind waves have much shorter periods and are generated principally by atmospheric forcing rather than astronomical gravitational cycles. Wave behaviour is complicated by nonlinear interactions, wave breaking, turbulence, variable bathymetry and interactions among different wave trains.
A common misconception is that individual water parcels simply travel from the generating storm to the beach with the wave; in many surface wave conditions the dominant effect is oscillatory particle motion with net transport produced through more complex processes. Ocean wave science is consequently important not only for understanding visible sea states but also for explaining energy transfer, coastal change, marine hazards, sediment movement and the engineering behaviour of structures exposed to the ocean.
