Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 384

The other turbulent-production wave-related mechanism is that due to shear stresses
imposed by the orbital wave motion and the vortex instability of potential surface waves
( Benilov et al. , 1993 ; Babanin , 2006 ). In Chalikov & Belevich ( 1993 ), this is E cT , which,
for some reason is shown as turbulent energy flux from the mean layer currents rather
than from wind-waves in their scheme reproduced here in Figure 9.8 . This turbulence was
considered in Section 7.5 in the context of wave-energy dissipation and will be further
discussed in the next section, 9.2.2 , specifically with respect to the upper-ocean mixing.
Thus, in this section, we have discussed general schemes of transfer of the energy and
momentum from the wind to the ocean. These include descriptions of fluxes all the way
from the free atmosphere down to the ocean bottom. In such an overall pattern, the role of
the waves and their breaking may not be most determining, but it is essential, and it has
been highlighted and its relative place is shown in this general context. The next section
will specifically consider generation of ocean turbulence by the waves and due to wave
breaking.
9.2.2 Generation of turbulence
The most comprehensive and recent update on the effects of surface wave breaking in
the upper ocean is given by Benilov & Ly ( 2002 ), and we refer the reader to this paper
for details and numerous further references. Here, we will briefly summarise this wave-
breaking role. It should be pointed out from the very beginning that although, as mentioned
in Section 9.2.1 , the breaking is unlikely to directly facilitate the mixing through the ther-
mocline unless it is very shallow, its influence is felt by the turbulence production profiled
all the way through the mixed layer.
Benilov & Ly ( 2002 ) start from emphasising that, in contrast to the wall-layer turbu-
lent flows or geophysical flows over the land, turbulence in the upper ocean is governed
not just by the mean shear, but also by surface waves. (To an extent, it is also true for
the atmospheric flow over the ocean (see Chalikov & Belevich , 1993 , and discussions in
Section 9.2.1 ). Thus, the transport of momentum, energy, heat, moisture and gases across
the ocean interface are all affected or even controlled by the waves (see also Section 9.1.2
on the role of spray and Section 9.2.3 on the role of bubbles).
If approached analytically, turbulence in the ocean is a very interesting phenomenon of
fluid mechanics rather than just geophysics: turbulent motion in a liquid whose free sur-
face is subject to wind forcing which makes the surface vertically unstable and as a result
covered by growing and propagating oscillations (waves). These waves receive a consider-
able amount of momentum and energy from the wind, which they do not accumulate, but
pass over to currents and ocean turbulence. When doing this, however, they redistribute
this momentum and energy in time and space by means of breaking.
Benilov & Ly ( 2002 ) write:
“Breaking waves create a highly turbulent environment within the top few meters of the ocean. Wave
breaking provides a mechanism for injection of both momentum and turbulent kinetic energy from