Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 396

To finalise this section, we will say that the importance of wave-induced turbulence, both
due to the breaking and due to the mean orbital motion, i.e. unrelated to the breaking, is
briefly reviewed. The presence of this turbulence poses a principal difference to the wall-
layer turbulence schemes, which are often employed to describe the upper-ocean mixing.
Near the ocean surface, production of turbulence by breaking dominates over the mean-
shear effects, and the wall-layer approach is inapplicable. Immediately below, there is a
transitional layer, followed further down by the layer where the shear stresses are dominant
and the wall-law logarithmic profile is observed. Characteristics of this profile, however,
still 'know' about the wave-breaking turbulence above and to some extent are defined by
the breaking at the surface.
A substantial part of this subsection is dedicated to a discussion of the non-breaking
wave-induced turbulence. We believe this discussion is relevant here since most of the
present-day turbulence-production and ocean-mixing schemes miss this source of turbu-
lence generation and turbulence diffusion, and without it the picture of the upper-ocean
dynamics is not complete. Out of the wave-turbulence interaction mechanisms on the water
side of the interface, the non-breaking turbulence is weak by comparison with the breaking
turbulence only near the surface. While the breaking is the surface-concentrated source
of the turbulence, the non-breaking wave-induced turbulence production is distributed
through the water column and potentially can be responsible even for the mixing through
the thermocline (see also Section 7.5 ). Needless to say that in the absence of breaking (i.e.
swell case), this is the main source of turbulence production by the waves.
In this section, the theoretical background for such non-breakingwave-induced turbulence
is outlined, direct numerical simulations of this turbulence are presented, and laboratory
experiments and field observations of the mixing due to such turbulence are described.
The wave-turbulence diffusion coefficient and production term for Reynolds averaged
Navier-Stokes schemes are discussed in detail, and improvements due to their inclusion
intowater-mixingmodels, ranging through applications from laboratorywave tanks through
sediment transport in a sea to global ocean-mixing applications, are highlighted.
9.2.3 Injecting the bubbles; gas exchange across the surface
The topic of the bubbles on and under the water interface and of their role in gas exchange
across the ocean surface is so vast that there are topics dedicated specifically to this issue
(e.g. Bortkovskii , 1987a ; Brennen , 1995 ) and there are series of international conferences
on this particular subject (e.g. International Symposiums on Gas Transfer at Water Surfaces,
the 6th of which took place in Kyoto, Japan in May, 2010). Since in the ocean the bubble
production and injections are primarily due to wave breaking, which is the topic of this topic,
we should outline such a connection in this subsection. Note that other natural sources of
bubbles also exist, e.g. Langmuir vortices, but at a much lesser extent than those due to
the breaking. A review of bubble physics, however, and its relevance for the wave-energy
dissipation, ocean dynamics and air-sea exchanges cannot be attempted here. Rather, we
will only indicate the problem and point the interested reader to some relevant references.