Non-dissipative effects of breaking on the wave field - Breaking and Dissipation of Ocean Surface Waves - page 322

8
Non-dissipative effects of breaking on the wave field
One of the main roles of wave breaking in the ocean-wave fields is providing the dissipation
sink of wave energy (see Chapters 1 and 7 ). The waves do not grow unlimited in height, and
breaking is the main process which restricts this growth even in conditions of continuous
wind-energy input over unlimited wave fetches.
Being the loss from the wave system, the breaking dissipation provides a link for the
wind-generated waves with other phenomena in the lower atmosphere, upper ocean and on
the ocean interface itself. It is a source of energy and momentum for infra-gravity waves,
near-surface currents, upper-ocean mixing and the atmospheric boundary layer (e.g. and
The WISE Group , 2007 ).
Some features and physical processes in the air-sea system, related to the breaking,
however, are not necessarily of dissipative nature or can be described in terms of the wave-
energy dissipation as such, and these are the phenomena to which the current chapter will
be dedicated. Some of them have already been mentioned or discussed throughout the
topic. It was needed and essential to highlight them in order to understand the described
features of breaking and whitecapping dissipation.
Others, for example, the spectral-peak downshift which is the topic of the next
section, 8.1 , have been mentioned as those which contribute to spectral dissipation (see
Section 7.3.1 ). There is no contradiction here. In the context of the spectral distribution of
the dissipation, energy lost from any part of the wave spectrum is interpreted as dissipation
of waves of the respective scales. On the total-energy scale, however, this energy is not
necessarily lost to the wave system in general, and can be transferred to the shorter waves
(high-frequency tail of the spectrum) or to the longer waves (for instance, contribute to
the downshift of the spectral peak). On an even broader picture of ocean waves, it can be
converted into energy of infra-gravity waves at the scales of wave groups (e.g. Smith &
Mocke , 2002 ). These are still surface waves, even though outside the range customarily
attributed to wind-generated waves, and therefore the question of whether this energy was
dissipated from the wave field or not is a matter of definition.
Thus, in this chapter, Section 8.1 is dedicated to the downshift of the spectral wave power
and the next section, 8.2 to the roles wave breaking plays in maintaining the level of the
spectrum tail. As discussed in that section, these roles are few. Some of them are indeed
not due to the loss of wave energy as such. Others are definitely dissipative in nature, but
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