Geoscience Reference
In-Depth Information
All but micro-breaking events of ocean waves produce whitecapping which is a bubble
cloud. The significance of whitecaps in general and individual bubbles which form the
clouds in particular, for remote sensing, for underwater acoustics, and for spray and aerosol
production has been discussed extensively throughout the topic (e.g.
Sections 3.1
,
3.5
,
3.6
and
9.1.2
).
Surface whitecap signatures of the breaking are important in remote sensing, for good or
bad reasons, i.e. they either provide a useful signal for measurements of, for example, wave
dissipation and waves as a source of spray, for estimates of surface winds, or alternatively
an undesirable optical noise when dealing with information based on the ocean colour.
In the latter case, knowledge and understanding of the whitecap-bubble generation is still
necessary in order to filter their contribution out.
Similarly, the acoustic underwater noise, produced by the breaking waves when the bub-
bles are formed and by the passive whitecaps when the bubbles coalesce or collapse, can
be a useful or unwanted signal. When dealing with the breaking, such noise allows to
identify the breaking events, study their statistics, the speed of propagation of breakers
and other relevant breaking properties. Acoustic signals emitted by individual bubble-
formation events can be used to classify the spectral distribution of the breaking probability
and even to measure the severity of the wave breaking and ultimately the wave-energy
dissipation due to breaking, which are the most difficult properties to estimate in field
observations of ocean waves. At the other end of the underwater acoustic applications,
the breaking-bubble-produced sound is a distraction if other sources of underwater bubble
production, whether natural or artificial, or other sources of underwater sound are sought.
Bubble bursting at the ocean surface is a primary mechanism of generation of small-
scale spray in moderate weather conditions. It plays a pivotal role in negotiating moisture
fluxes to the atmosphere and providing aerosols which are then carried up and around by
the atmospheric convection and winds, even across continents.
Most essential is the role of the bubbles in the wave-energy dissipation physics. Labora-
tory measurements estimate from 14% to 50% of the energy lost in a breaking event to be
spent on work against the buoyancy forces when injecting the air bubbles into the water in
the course of the breaking (see e.g.
Section 3.4
). If so, the bubble dynamics has to be given
a proper account when modelling the wave-breaking dissipation, along with the physics of
the turbulence generation by breaking waves.
Since all these features of the wave-breaking bubbles have already been discussed or at
least outlined in the sections mentioned above, in this section we will concentrate on the
process of injecting the bubbles as such, and on its consequences in terms of the bringing to
and dissolving of atmospheric gases in the ocean. As usual, there are a number of physical
mechanisms facilitating such exchanges, even if all of them are by means of the bubbles,
and there are differences in those mechanisms in benign, moderate and extreme weather
circumstances. We will try to indicate this variety too.
Before discussing the injection and exchange issues, however, it is helpful to outline
the bubble-field structure. This includes both the external and internal structure of the
whitecapping, such as the total whitecap coverage, geometry and spatial distribution of
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