Swell (ocean)
Encyclopedia
A swell, in the context of an ocean, sea or lake, is a series surface gravity waves that is not generated by the local wind. Swell waves often have a long wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 but this varies with the size of the water body, e.g. rarely more than 150 m in the Mediterranean, and from event to event, with swells occasionally longer than 700 m away from the most severe storms. Swells have a narrower range of frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 and directions than the wind sea, because swell waves have dispersed from their generation area and have been dissipated.

Swell dissipation

The dissipation of swell energy is much stronger for short waves, which is why swells from distant storms are only long waves. The dissipation of waves with periods larger than 13 s is very weak but still significant at the scale of the Pacific Ocean. These long swells lose half of their energy over a distance that varies from over 20000 km (half the distance round the globe) to just over 2000 km.
This variation was found to be a systematic function of the swell steepness: the ratio of the swell height to the wavelength. The reason for this behaviour is still unclear but it is possible that this dissipation is due to the friction at the air-sea interface.

Swell dispersion and wave groups

Swells are often created by storms thousands of nautical mile
Nautical mile
The nautical mile is a unit of length that is about one minute of arc of latitude along any meridian, but is approximately one minute of arc of longitude only at the equator...

s away from the beach where they break, and the propagation of the longest swells is only limited by shorelines. For example swells generated in the Indian Ocean have been recorded in California after more than half a round-the-world trip. This distance allows the waves comprising the swells to be better sorted and free of wikt: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, while others moving at even a fraction of a metre 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 where g is the acceleration of gravity. For a storm located 10000 km away, swells with a period T=15 s will arrive 10 days after the storm, followed by 14 s swells another 17 hours later, and so forth.

This dispersive arrivals of swells, long periods first with a reduction in the peak wave period over time, can be used to tell the distance at which swells were generated.

Whereas the sea state in the storm has a frequency spectrum
Frequency spectrum
The frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...

 with more or less always the same shape (i.e. a well defined peak with dominant frequencies within plus or minus 7% of the peak), the swell spectra are more and more narrow, sometimes as 2% or less, as waves disperse further and further away. The result is that wave groups (called sets by surfers) can have a large number of waves. From about seven waves per group in the storm, this rises to 20 and more in swells from very distant storms.

Swell and surfing

Information on swell size and period is useful for surfers
Surfing
Surfing' is a surface water sport in which the surfer rides a surfboard on the crest and face of a wave which is carrying the surfer towards the shore...

, as swells are generally more desirable to surf on than normal, locally-generated waves and chop. Swell size is typically the average height of the largest 33% of waves in a set, measured from the highest point of a wave (crest
Crest (physics)
A crest is the point on a wave with the maximum value or upward displacement within a cycle. A trough is the opposite of a crest, so the minimum or lowest point in a cycle.-Interference:...

) to the lowest point (trough). Swell size is also known as the significant wave height
Significant wave height
In physical oceanography, the significant wave height is defined traditionally as the mean wave height of the highest third of the waves , but now usually defined as four times the standard deviation of the surface elevation...

. Period is the average length of time between each wave in a set. The significant period is the average period between the third largest waves in a wave record.

Swell and coastal impacts

Just like for all water waves the energy flux is proportional to the significant wave height squared times the group velocity
Group velocity
The group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....

. In deep water this group velocity is proportional to the wave period. Hence swells, with usually longer periods, can pack a lot more energy that shorter wind seas. Also, the amplitude of infragravity waves increases dramatically with the wave period (typically like the period squared), which results in
higher run-up
Run-up
Run-up, also runup, may be:*Run-up , type of movement made by a bowler*Run-up , aircraft verification procedure performed just prior to take-off*The maximum height on land reached by a tsunami as it encounters the shore...

.

As swell waves typically have long wavelengths (and thus a deeper wave base), they begin the refraction process (see water waves) at greater distances offshore (in deeper water) than locally generated waves.
Since swell-generated waves are mixed with normal sea waves, they can be difficult to detect with the naked eye (particularly away from the shore) if they are not significantly larger than the normal waves. From a signal analysis point of view, swells can be thought of as a fairly regular (though not continual) wave signal existing in the midst of strong noise (i.e., normal waves and chop).

Swell and navigation

Swells were used by Polynesia
Polynesia
Polynesia is a subregion of Oceania, made up of over 1,000 islands scattered over the central and southern Pacific Ocean. The indigenous people who inhabit the islands of Polynesia are termed Polynesians and they share many similar traits including language, culture and beliefs...

n navigators to maintain course when no other clues were available, such as on foggy nights.

External links

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