Geoscience Reference
In-Depth Information
1
Introduction
Wind-generated waves are the most prominent feature of the ocean surface. As much as
the oceans cover a major part of our planet, the waves cover all of the oceans. If there is
any object in oceanography that does not need too much of a general introduction, it is the
surface waves generated by the wind.
Being such a conspicuous entity, these waves, however, represent one of the most com-
plex physical phenomena of nature. Three major processes are responsible for wave evolu-
tion in general, with many more whose significance varies depending on conditions (such
as wave-bottom interaction which is only noticeable in shallow areas). The first process
is energy and momentum input from the wind. The waves are generated by turbulent
wind, and the turbulence is most important both for their initial creation and for subse-
quent growth (e.g.
Miles'
,
1957
;
Miles
,
1959
,
1960
;
Phillips'
,
1957
;
Janssen
,
1994
,
2004
;
Belcher & Hunt
,
1993
;
Belcher & Hunt
,
1998
;
Kudryavtsev
et al.
,
2001
, among many oth-
ers). There is, however, no fixed theory of turbulence to begin with. Experimentalists have
to deal with tiny turbulent fluctuations of air which are of the order of 10
−
5
-10
−
6
of the
mean atmospheric pressure and which must be measured very close to the water surface,
typically below the wave crests (e.g.
Donelan
et al.
,
2005
). The wind input process is quite
slow and it takes hours of wind forcing (thousands of wave periods) and tens and hundreds
of kilometres of fetch for waves to grow to a considerable height.
The second process is weak, resonant, nonlinear interactions within the wave system
which can only be neglected for infinitesimal waves. For most of its existence, the wind-
wave can be regarded as almost sinusoidal (i.e. linear), but its very weak mean nonlinearity
(i.e. deviation of its shape from the sinusoid) is generally believed to define the wave's
evolution. This is due to such waves, unlike linear sinusoids, exchanging energy when they
cross-path. They cross-path because waves of different scales (i.e. different frequencies
and wavelengths) propagate with different velocities, and also because waves tend to prop-
agate at a range of angles with respect to the mean wind direction. Such weak interactions
appear to be of principal importance. The longer (and higher) the waves are, the faster they
move, and therefore the visibly dominant waves move with speeds close to the wind speed.
This means that they virtually move in the still air, there is almost no wind for them. If they
are still obviously wind generated, how does the wind produce such waves? The answer
that is most commonly accepted now, is that the wind pumps energy mostly into shorter
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