Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 376

Breaking waves disrupt the continuous surface-wave trains, and these events do affect
locally the momentum and energy fluxes in the atmospheric boundary layer ( Section 8.3 ).
In terms of the influence of the breaking on the wave orbital motion, however, this effect
seems to be small and thus the disruption of the orbital velocity is confined to the near-
surface area ( Section 7.3.3 ,see Figure 7.5 and its discussion).
Similarly, the direct effect of turbulence generation due to wave breaking in the water
column appears to be limited to the depths of the order of wave height beneath the surface
as discussed in Section 9.2.2 . This subsection is preceded by Section 9.2.1 which provides
a necessary brief overview of the general topic of momentum and energy transfer from the
wind to the ocean, including the waves and breaking waves as a mediator.
In the context of upper-ocean influences of the wave breaking, Section 9.2.3 will discuss
injection of the bubbles under the ocean surface. Like production of the spray
( Section 9.1.2 ), generation of the bubbles is primarily due to the breaking and therefore
is a relevant issue to be mentioned. This is an extensive research topic in its own right, as
such bubbles play a number of important roles in the air-sea interactions, from dynamic
dissipative consequences through acoustic noise to the heat and gas exchanges across the
surface which will be briefly outlined and will conclude Section 9.2 .

9.2.1 Transfer of energy and momentum from the wind to the ocean

Upper-ocean mixing is an outcome of a complicated chain of momentum and energy trans-
formations which connect energy and momentum fluxes from the outer atmosphere through
the boundary layer including WBL to the waves, currents and subsurface turbulence. As a
result, the upper tens of metres of the ocean are typically well-mixed. Under further external
forcing, this layer deepens or stratifies depending on the wind forcing and buoyancy flux.
The rate of deepening depends on the production of turbulent kinetic energy (e.g.
Richman & Garrett , 1977 ). Under unstable stratification, that is in autumn and the win-
ter months when the air is colder than the water, deepening of the mixed layer is mostly
determined by the vertical convection, which is stronger than the wind-caused turbulence
production. Such convection is apparently not induced by the waves and their breaking and
will not be discussed here.
The wind, as a forcing source, does not produce the turbulence in the water directly.
It creates the mean currents and waves which do that on its behalf.
The shear instability of such mean flow generates turbulence through the water column
and potentially at the base of the mixed layer if MLD is not very deep and the vertical
velocity gradients are still large enough. Such a mean current can be either caused directly
by the wind surface drift through the tangential viscous stress

τ ν in (7.36) or indirectly

through the waves' Stokes drift.
The wave orbital velocities are typically at least an order of magnitude larger than those
of the mean current, and with the exponentially decaying velocity profile are able to pro-
duce substantial shear stresses and turbulence (see Sections 7.5 and 9.2.2 ). Waves can also
inject turbulence directly when breaking.