Swells

Swells are long-wavelength surface gravity waves that originate from distant weather systems and travel across the oceans after the winds that generated them have shifted or ceased. They are distinct from locally generated wind waves, which are irregular and short-period; swells form a coherent train of waves with more uniform height, direction, and period.

Generation and propagation: Swells are produced by winds over large fetches, often associated with storms, extratropical cyclones, or hurricanes. The energy radiates outward from the storm, forming a wave field that can travel thousands of kilometers from its source. Because longer-period waves travel faster than shorter-period waves, swells that originate from a distant storm may arrive with a different waveform than the winds that created them. As they move into shallower water, dispersion and refraction alter their height and direction, often focusing energy on distinct coastlines and creating surfable conditions.

Characteristics: Swell periods typically range from about 6 to 20 seconds (sometimes longer), with wavelengths of tens to hundreds of meters. Wave heights vary with distance from the source, wind strength, and bathymetry, but swells generally appear as a more organized and less choppy sea state than wind seas.

Measurement and forecasting: Swell parameters are monitored by ocean buoys, satellites, and coastal wave radars, and are forecast by numerical models such as SWAN and WAVEWATCH III. Forecasts include swell period, height, direction, and arrival time, useful for surfers, mariners, and coastal managers.

Impact: Swells influence coastal processes, beach morphology, and surf conditions. They can generate longshore currents and rip currents and may cause elevated wave run-up on shorelines. In remote areas they provide information for navigation and offshore operations.