Conclusions. What else do we need to know about wave breaking? - Breaking and Dissipation of Ocean Surface Waves - page 415

change is much faster compared to the downshifting brought about by the weak nonlinear
resonance. This topic is outlined in Section 8.1 where a model of such downshifting due to
breaking is also presented.
Next Section 8.2 is dedicated to the role of breaking in maintaining the level of the high-
frequency wind-wave spectrum tail. Mention of this level, and in particular of the transition
from

ω − 5 behaviour, are scattered around the topic, and the section concentrates
on this topic and highlights part of the breaking in this regard. It is argued that the

ω − 4

to

ω − 5
subinterval is always present, and it is the breaking of the short waves which is responsible
for this. For all but mature stages of wave development, the level of the

ω − 5 interval is
constant on average, and must be supported by a combined reaction of the severity and
probability of the small breakers in response to changing wind forcing.
An important outcome of Section 8.2 is the dependence of the transitional frequency
ω − 4 to ω − 5 as a function of a combination of the wind speed and peak frequency, or, in a

broader analytical sense, of the energy flux from the wind to the waves. This dependence
is a result of discussion in this section of two approaches to describing the physics which
forms the spectrum tail, those due to the wind action and due to nonlinear fluxes within the
wave system. It is argued that these approaches do not contradict each other, as they have
routinely been indicated to, but in fact are complementary and both explain the same spec-
trum shape of

ω − 4 , that is one through the energy input and the other through redistribution
of this input. The wave breaking competes with this mechanism for control over the spec-
trum shape, and where it starts to dominate the transition to ω − 5 occurs. It is feasible that

this transition also separates bands of predominantly-inherent and predominantly-induced
breakings in the wave spectrum.
The last section of Chapter 8 discusses the breaking-caused wind-input enhancement. It
approximately doubles over the breaking waves and can prove a significant increase if the
breaking rates are high. Therefore, while taking the energy and momentum from the wave
system and passing them on to the ocean turbulence and mean currents, the breaking at
the same time promotes the wind-energy/momentum intakes and in this way facilitates the
atmosphere-ocean exchanges.
Thus, Section 8.3 is further linked to a wider topic of the role of wave breaking in
the small-scale air-sea interactions in general which is the subject of the last chapter,
Chapter 9 . These interactions affect the physics of the atmospheric bottom-boundary layer
as discussed in Section 9.1 and of the upper ocean in Section 9.2 .
It has been shown, for example, that the wave-breaking characteristics alter the sea-
drag dependences in this layer. While such dependences bear a profound importance in
the large-scale air-sea interaction modelling, because the sea-drag coefficient is often the
parameter which solely defines the momentum flux between the atmosphere and the ocean
in such models, the influences of the breaking on the variations of this coefficient have
received very scant attention so far. This is the topic of Section 9.1.1 .
Another contribution of breaking to the air-sea interactions is generation of spray, which
makes the air close to the interface a two-phase fluid, as discussed in Section 9.1.2 . Such
a fluid has a different density since the water droplets are much heavier compared to the