Energy dissipation across the wave spectrum - Breaking and Dissipation of Ocean Surface Waves - page 319

compatible energy sink, like, for example, the swell friction against the air discussed above
in this section, should be present.
To conclude this section, we should stress again that, apart from the wave-breaking
dissipation, a number of other dissipation mechanisms certainly exist in the wave system.
In the presence of breaking, they may be small and negligible, but they are certainly most
essential when describing propagation of swell whose forecast is one of the most difficult
practical hydro-meteorological applications to date.
We should also remember that dissipation of waves is a source of energy for other mech-
anisms in the atmosphere-ocean system. Small it may be as a dissipation, but its role can
become significant and certainly not negligible as this source. Two such secondary-to-
breaking dissipation terms were identified as significant in this section: the wave-induced
turbulence and the wave-to-air fluxes.
In this context, for example, if the powerful dissipation breaking only injects turbulence
at the scale of wave height, the gentle dissipation mean-wave-orbit-induced shear stresses
generate turbulence directly through the water column at the scale of wave length. This
way, it is an essential mechanism of the upper-ocean mixing (e.g. Babanin et al. , 2009b ;
Pleskachevsky et al. , 2011 ).
The wave-against-air friction is essential in modifying the lower part of the atmospheric
boundary layer (e.g. Kudryavtsev , 2004 , see also Section 7.3.6 for discussion and further
references). Collard et al. ( 2009 ) demonstrated that it has a significant effect for wind
speeds in excess of 7m s(seealso Hanley et al. , 2010 ).

In this section, we mainly concentrated on the dissipation phenomena which could be
identified when observing swell propagation over large distances. There are, however, other
potential dissipation mechanisms whose role may be essential both in wind-generated wave
fields, i.e. small but not negligible by comparison with wave breaking, and in swells. One
of them is the loss of energy by large waves due to work spent on the modulation of short
waves (see e.g. and The WISE Group , 2007 ). In the case of swell, such process can be
active when swell propagates through wave fields generated by adverse wind.
This phenomenon is suggested as a combination of the maser mechanism ( Phillips ,
1963 ; Longuet-Higgins , 1969b ) and a theory of exchange of the potential energy between
long and short waves ( Hasselmann , 1971 ). Ardhuin & Jenkins ( 2005 ) demonstrated that
such dissipation is greater than the action of viscosity. Moreover, Hasselmann ( 1971 )
neglected associated modulations of the wind stress on the atmospheric side, which may
be significant and enhance the effect ( Garrett & Smith , 1976 ; Ardhuin & Jenkins , 2005 ,
2006 ).
Thus, the analytical theory of this particular dissipation phenomenon is well developed,
but experimentally its significance is still to be investigated and quantified. The consider-
able challenge in such experiments and observations of this and similar dissipation effects
is due to the fact that they are small in the presence of breaking and in the course of swell
propagation they are patchy. The interaction of swell with short waves, for instance, will
only occur if the swell encounters counter-propagating wind seas, which may or may not
have taken place in satellite observations similar to those of Ardhuin et al. ( 2009a ).

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