Swell (ocean)
A swell, also sometimes referred to as ground swell, in the context of an ocean, sea or lake, is a series of mechanical waves that propagate along the interface between water and air under the predominating influence of gravity, and thus are often referred to as surface gravity waves.
More generally, a swell consists of wind-generated waves that are not greatly affected by the local wind at that time.
Swell waves often have a relatively long wavelength, as short wavelength waves carry less energy and dissipate faster, but this varies due to the size, strength, and duration of the weather system responsible for the swell and the size of the water body, and varies from event to event, and from the same event, over time.
Swells have a narrower range of frequencies and directions than locally generated wind waves, because they have dispersed from their generation area and over time tend to sort by speed of propagation with the faster waves passing a distant point first.
Swells take on a more defined shape and direction and are less random than locally generated wind waves.
Large breakers observed on a shore may result from distant weather systems over the ocean.
A fully developed sea has the maximum wave size theoretically possible for a wind of a specific strength and fetch.
The generation of wind waves is initiated by the disturbances of the crosswind field on the surface of the water.
For initial conditions of a flat water surface (Beaufort Scale 0) and abrupt crosswind flows on the surface of the water, the generation of surface wind waves can be explained by two mechanisms, which are initiated by normal pressure fluctuations of turbulent winds and parallel wind shear flows.
Phillips in 1957, the water surface is initially at rest, and the generation of the wave is initiated by turbulent wind flows and then by fluctuations of the wind, normal pressure acting on the water surface.
Due to this pressure fluctuation arise normal and tangential stresses that generate wave behavior on the water surface.
He found that the energy transfer from wind to water surface as a wave speed,
This relation shows the wind flow transferring its kinetic energy to the water surface at their interface, and thence arises wave speed,
The process was first described by Klaus Hasselmann (2021 Nobel prize winner) after investigating the non-linear effects that are most pronounced near the peaks of the highest waves.
The equation that Hasselmann[8] developed to describe this process is now used in the sea state models (for example Wavewatch III[9]) used by all the major weather and climate forecasting centres.
This is because both the wind sea and the swell have significant effects on the transfer of heat from the ocean to atmosphere.
The sorting of sand grain sizes, often seen on a beach,[10][11] is a similar process (as is a lot of life).
The dissipation of waves with periods larger than 13 seconds is very weak but still significant at the scale of the Pacific Ocean.
The reason for this behavior is still unclear, but it is possible that this dissipation is due to the friction at the air-sea interface.
For example, swells generated in the Indian Ocean have been recorded in California after more than half a round-the-world trip.
[13] This distance allows the waves comprising the swells to be better sorted and free of chop as they travel toward the coast.
Waves generated by storm winds have the same speed and will group together and travel with each other,[citation needed] while others moving at even a fraction of a meter per second slower will lag behind, ultimately arriving many hours later due to the distance covered.
The time of propagation from the source t is proportional to the distance X divided by the wave period T. In deep water it is
Hence swells with longer periods can transfer more energy than shorter wind waves.
