Conclusions. What else do we need to know about wave breaking? - Breaking and Dissipation of Ocean Surface Waves - page 406

dynamics and the resulting spectral-dissipation signature of the modulational-instability
breaking is different to that of the linear-focusing breaking. The former leads to loss of
energy by the primary waves, accompanied by the cumulative effect at higher frequencies,
whereas the latter mainly results in the dissipation of smaller-scale waves. There are indica-
tions that breakings brought about by other physical mechanisms, e.g. amplitude focusing,
are characterised by yet different spectral signatures.
Thus, the potential reasons for the waves reaching the breaking limiting steepness may
be different, but there are multiple indications that in moderate-wind deep-water conditions
modulational instability is the most likely cause of dominant breaking. The answer to the
question of whether this kind of instability is active in directional wave fields with typically
observed mean steepness, emerges as positive. This conclusion does not deny the break-
ing due to, for example, linear superposition, but the probability of the latter is low since
the limiting steepness would require a superposition of a large number of ocean waves
of the typical background steepness. Breaking of waves shorter than dominant scales is
induced by the underlying longer waves, particularly of those further away from the peak
in Fourier space.
Apart from dissipation of the wave energy, breaking plays other important and some-
times pivotal roles across many processes at the ocean interface, as well as above it and
below. Among those most essential in the system of surface waves are the downshifting of
the wave energy, defining the level of the wave spectrum, enhancement of the energy and
momentum input from the wind to the waves. The last feature illustrates the connection
of wave breaking with the dynamics of the atmospheric boundary layer where the break-
ing alters the sea drag and ejects spray, which effect is particularly significant in extreme
weather conditions. Below the ocean surface, the breaking is a major player in the upper-
ocean mixing schemes, in the production of turbulence and generation of bubbles which
facilitate the air-to-sea gas transfer.
In this Conclusion, we will avoid using references unless the relevant papers have not
been mentioned before - the references are very detailed in the chapters above. A wide
spectrum of achievements in the research on wave breaking, and research related to wave
breaking, have been attended to through the nine chapters. The first six chapters largely
follow the review of Babanin ( 2009 ). This review was dedicated to studies of the breaking
phenomenon as such, but respective chapters in this topic had to be updated or even essen-
tially rewritten. Progress in the field is rapid, and in two years since publishing the review
the advances have to be noted, and some pending questions have already been answered,
notably the question about modulational instability in directional wave fields.
Introductory Chapter 1 presented the topic and brought in the concept of wave break-
ing. This concept is initiated through its main physical consequence - dissipation of wave
energy.
Chapter 2 formulated definitions pertinent to wave breaking, to be used throughout the
topic. One of the advantages of presenting the wave-breaking phenomenon is the fact that,
unlike many other physical processes, including those oceanographic, this phenomenon
is apparent even to non-specialists. Everybody has a perception of wave breaking on the