Conclusions. What else do we need to know about wave breaking? - Breaking and Dissipation of Ocean Surface Waves - page 417

disruption of the surface due to microbreaking caused by Kelvin-Helmholtz instability,
and tearing the wave crests by high winds. Most importantly, there appears to be a change
of regime for the wave breaking at approximately the same wind speed as the saturation of
the sea drag, as concluded in Section 9.1.3 . This is either due to saturation of the breaking
probability, or because the direct wind forcing replaces hydrodynamics mechanisms as a
cause of wave breaking, or both.
More byproducts of the wave-breaking studies can be found in the adjacent field of
upper-ocean mixing ( Section 9.2 ). Such mixing, at a seasonal scale, for example, regu-
lates the dynamic balance between the air and the water bodies, and ultimately negotiates
the weather and climate conditions. In this regard, the breaking is responsible for a large
proportion of the transfer of the momentum from the wind to the ocean, for generation of
turbulence, and for injecting the bubbles and thus facilitating the gas exchange across the
interface.
While being the dominant source as far as bubble generation is concerned, the role
of the breaking in mixing the upper layer is important, but not pivotal. The breaking-
injected turbulence is a surface energy source (that is occurs at depths comparable with
the wave height), and it does provide the boundary condition which then imparts on the
turbulence profiles throughout the layer. Schemes of the momentum exchange through the
ocean surface and the place of breaking in these schemes are described in Section 9.2.1 .
More specifically, the generation of turbulence by waves, including the breaking waves,
is discussed in Section 9.2.2 . Here, however, the non-breaking mean orbital motion can
serve as a distributed source of turbulence, that is this motion can produce such turbulence
directly at the depths scaled with the wave length. This turbulence source has been gen-
erally overlooked, and in this section it is discussed in some detail in the context of the
upper-ocean mixing schemes and turbulence generation, in order to provide a complete
picture of such mixing.
Section 9.2.3 of the topic discusses the physics and dynamics of the bubbles. The major-
ity of the bubbles at the ocean interface and in the upper ocean are due to wave breaking, so
the connection of the topic theme to this issue is apparent. The topic, however, is very large
and could only have been outlined here. Three main issues are discussed: the structure of
the bubble clouds, the dynamics of the bubble submersion and surfacing, and the role of
the bubbles in the gas transfer from the air to the upper ocean.
As there is a change in the dynamic regime of the atmospheric boundary layer men-
tioned above at wind in excess of 30m s (9.12) , and of the surface-wave breaking at

approximately the same wind speed (3.26) , so there is a change in the gas-transport regime
at similar threshold winds (9.50) . This poses an interesting question of whether such a
simultaneous change in the physics of the low-atmosphere, ocean-surface and upper-ocean
properties can be incidental. The answer is most likely negative, and the question certainly
needs further attention in the light of its apparent significance across the range of research
and applied topics and even fields. In any case, as far as the bubbles are concerned, at such
winds and perhaps even at the lighter wind speeds, the role of the bubbles and therefore of
the breaking is dominant in the air-sea gas exchange.

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