Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 375

and bubbles and tends to shrink the transition layer. Soloviev & Lukas ( 2010 ) call this
condition 'marginal stability'.
The condition of marginal stability determines the lower limit for the sea-drag coeffi-
cient. The upper limit is defined by the customary Charnock wave resistance. Soloviev &
Lukas ( 2010 ) verify these limits against available data obtained at extreme winds in the
field and laboratory and argue that all such data are found between the two limits identified
in their model.
Thus, in this section we have briefly discussed the consequences of wave breaking in
extreme conditions. Direct observations and accounts of such breaking are rare, but multi-
ple models point to a number of peculiarities of this kind of breaking which distinguish it
from the breaking in benign environmental situations. These peculiarities include full flow
separation, disruption of the surface due to microbreaking caused by Kelvin-Helmholtz
instability, and tearing the wave crests by the high winds.
Speaking of breaking in extreme conditions, it is worth reminding that at approximately
the same wind speed at which the saturation of the sea drag occurs (9.12) , surface-wave
asymmetry (1.3) also saturates, at least in the laboratory experiments of Leikin et al. ( 1995 )
(see detailed discussion of these observations in Section 3.3 ). This fact can be interpreted
in two ways. Firstly, this may mean that breaking probability saturates at such a wind
speed (see Section 7.3.3 and Eqs. (7.18) - (7.19) ). Alternatively (or additionally), this fact
can mean that the breaking regime is changed, that is the waves no longer break because of
the modulational instability, which leads to the negative average asymmetry, but due to, for
example, direct wind forcing. One way or another, the onset of the hurricane wind speeds
appears to signify essential changes for the wave-breaking process as well, which may in
fact have an impact on the sea drag also.
Tearing the crests leads to production of spume in the atmospheric boundary layer, and
this spume along with other types of spray are discussed in Section 9.1.2 . All kinds of spray
result from the breaking and its consequences, such as whitecapping, and play a variety
of roles in the dynamics and thermodynamics of the boundary layer, air-sea exchanges,
including genesis of tropical cyclones, in producing aerosols, transferring moisture, serving
as condensation matter in meteorological processes, among many others.
In Section 9.1.1 , we touched on the topic of sea drag and the influences of wave break-
ing in this regard. Thus, the current section, 9.1 , briefly outlines and discusses issues
of the physics and dynamics of the atmospheric boundary layer where knowledge and
understanding of the wave breaking and its consequences can be important or helpful.
9.2 Upper-ocean mixing
Surface waves in general and wave breaking in particular are phenomena which owe their
existence to the presence of an air-water interface. The wave oscillations, however, also
involve orbital motions of the water particles through the water depths at wavelength scale,
and at a similar scale wave-induced fluctuations of velocity and pressure are felt in the
atmospheric boundary layer.