Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 351

∂τ
∂

z =

0 (9.1)

where is the wind-stress vector (3.7) .

This defines the so-called constant-flux layer, that is the layer where the solution of (9.1)
produced flux

τ

τ

which does not depend on the distance to the surface. The height of this
layer is of the order of 10m (e.g. Komen et al. , 1994 ), and it may be less than that for light
winds or extend a few tens of metres high for stronger winds.
It is within this sublayer that the wind-wave interactions happen. The main difference
of the boundary layer over the ocean, compared to the atmospheric flow over the land and
fluid flow near the wall in general, is that the ocean surface is mobile and ever changing.
That is, the surface roughness is not constant, it evolves in response to the wind. In addi-
tion, the roughness elements are not stationary or even steady, they move and their motion
accelerates as the system evolves (see e.g. Kitaigorodskii , 1973 ).
Indeed, the wind pumps energy and passes momentum to the waves, which grow and
move faster as their spectrum peak downshifts. In turn, the waves can also alter the wind
(see, for example, Sections 7.4 and 7.5 ). Most importantly, the waves, and breaking waves
in particular, can alter the properties of this lower boundary layer itself. Thus, wind impacts
the waves, which then impact the wind and the boundary layer, the effects of which then
again impact the waves in this coupled small-scale air-sea system where such feedback
influences can hardly be unambiguously separated.
The constant-flux-layer dynamics can be further subdivided into subdynamics, and the
very constant flux itself consists of different contributions whose relative magnitude
depends on the wind forcing and actually depends on distance to the surface. In (7.36) ,
such contributions are subdivided into Reynolds turbulence stress

τ turb , wave-coherent-

turbulence stress τ ν . The former vanishes at the surface, but domi-

nates away from the surface, the latter dominates in light winds and becomes negligible in
strong winds (e.g. Kudryavtsev et al. , 2001 ).
The wave-induced turbulence represents 100% of the turbulent stress on the air-side
of the interface and rapidly decays upwards, becoming negligible at the height of the
order of

τ wave and viscous stress

2 (e.g. Donelan , 1999 ), which layer we will call the wave boundary layer
(WBL). Here,

λ/

τ wave can be further subdivided into the contributions of regular waves and
enhancement due to breaking (see Section 8.3 ). The breaking-induced stress constitutes up
to 50% of the wave stress at moderate-to-strong winds (e.g. Kudryavtsev et al. , 2001 ).
This fact highlights the role of the wave breaking as the most essential in the air-sea
interactions.
At extreme conditions, the breaking takes on yet another role in the boundary layer.
Makin ( 2005 ), for example, describes a situation when the sea drops saturate the near-
surface air to such an extent that one can talk about a thin (about 10% of significant wave
height) sublayer attached to the surface, with a yet different dynamics.
Subsections of this Section 9.1 outline these roles of the breaking in the dynamics and
description of WBL. Section 9.1.1 will highlight the potential of wave breaking to alter
the sea drag in parameterisations of the constant-stress layer. The generation of spray due