Ocean Waves and Internal Waves

Waves carry energy across the ocean without bulk transport of water — from wind-generated surface waves and far-traveling swell to the much larger, slower internal waves that undulate along density interfaces deep within the stratified ocean.

Definition

Ocean waves are oscillatory disturbances that propagate along the sea surface (surface waves) or along density interfaces within the stratified interior (internal waves), restored by gravity and, for the largest waves, modified by Earth's rotation.

Scope

This topic covers the generation, dispersion, and propagation of surface gravity waves (wind waves and swell), their growth, breaking, and shoaling; the physics of internal gravity waves on stratification, including the internal tide; and the role of these waves in transporting energy and driving mixing.

Core questions

• How do wind waves grow, disperse into swell, and travel across ocean basins?
• What is the dispersion relation that links a wave's period, wavelength, and speed in deep and shallow water?
• How do internal waves arise on density stratification, and why are they so much larger than surface waves?
• How do breaking surface and internal waves transfer energy into turbulence and mixing?

Key theories

Linear wave dispersion

Small-amplitude surface gravity waves obey a dispersion relation in which longer waves travel faster, so a storm-generated spectrum sorts itself into orderly swell as it propagates away from its source.

Internal waves on stratification

In a stably stratified ocean, buoyancy acts as a restoring force for internal gravity waves whose frequencies lie between the inertial and buoyancy frequencies, allowing them to propagate vertically and horizontally through the interior.

Mechanisms

Wind transfers energy and momentum to the sea surface, building a spectrum of waves that disperse with longer waves outrunning shorter ones to form swell; as waves enter shallow water they shoal, steepen, and break, releasing energy. Internal waves are generated where tides and currents flow over topography, propagating along stratification until they break and drive interior mixing.

Clinical relevance

Wave forecasting is critical to shipping, offshore engineering, and coastal hazard warning, while breaking internal waves supply much of the mixing that maintains the deep ocean's stratification and overturning circulation.

History

The mathematical theory of water waves was developed by Airy and Stokes in the nineteenth century; mid-twentieth-century work by Munk and others on wave forecasting and the discovery of swell propagation across whole oceans, together with Hasselmann's spectral wave theory, established modern wave science.

Key figures

• George Gabriel Stokes
• Walter Munk
• Klaus Hasselmann

Seminal works

• talley2011
• kundu2015

Frequently asked questions

What is the difference between sea and swell?

Sea refers to the short, choppy waves still being driven by local wind, whereas swell is the smoother, longer-period waves that have outrun their generating storm and travelled far across the ocean.

Why are internal waves often much larger than surface waves?

Internal waves propagate along the small density difference between water layers rather than the large air-water contrast, so the restoring force is weaker and the waves can reach tens of metres in height while staying hidden below the surface.