Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 371

Depending on the classification, three to four different types of spray can be mentioned,
all of them due to wave breaking. (Note that in the case of Shats et al. , (2010), this is the
breaking of capillary waves.) Thus, in this regard the breaking takes on a different role in
the general system of air-sea interactions.
The spume's contribution to the dynamics of the wave boundary layer is most significant
at wind speeds in excess of 20m

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s this leads (perhaps in
conjunction with some other, e.g. aerodynamic effects), to the reduction of the sea drag.
As far as the spume in such conditions is concerned, models of its impacts have been
developing intensively, and at the present stage appear quite feasible in describing the
dynamics of the droplet-filled WBL. The persisting problem here is calibration of such
models and spray impacts. They rely on spray-production terms which are diverse and not
consistent in their estimates and predictions. Those would need experimental guidance, but
field data are largely unavailable, and the laboratory measurements do not seem to be able
to fill the gap.

s. At winds greater than 30m

9.1.3 Boundary layer at extreme breaking

The term 'extreme conditions' which we have loosely used in this chapter, requires defi-
nition. Obviously, what is extreme from the point of view of one set of applications can
be treated as moderate or even ordinary in other regards. This topic is dedicated to wave
breaking, and from this perspective we will call the circumstances extreme when the U 10
wind exceeds 20m s.

Indeed, as discussed in Section 9.1.2 , it is at this wind speed that spume starts being torn
from the top of wave crests to fill the lower boundary layer. This reduces the wave height
and takes energy away. Thus, by definition, this is the wave breaking, but this is a new kind
of breaking not seen at wind speeds below, when the breaking behaved differently and
mostly affected the ocean-surface features and dynamics of the upper ocean rather than
the atmospheric boundary layer. Even though the breaking in non-extreme winds does
influence the wind input and therefore the sea drag ( Section 8.3 ), at low breaking rates the
overall effect is small.
Even visually, this threshold signifies a transition to different surface coverage by break-
ing and its consequences. Sporadic whitecaps at strong wind forcing in Figure 1.1 are
replaced by a surface where the foam extends from crest to crest and the interface becomes
hazy in Figures 1.5 and 1.9 .
New roles of the breaking in such extreme conditions are yet to be fully understood.
Their traditional roles, those parameterised as dependences for frequency of breaking
occurrence, for breaking strength, for whitecapping production and coverage, also need
to be re-evaluated and perhaps re-quantified.
As a result of the breaking and its consequences, as well as other physical processes
close to the air-sea interface, which are either altered or only appear in extreme con-
ditions, the sea drag saturates at wind speeds in excess of 30m

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s and potentially even
goes down at even higher winds. This effect was long anticipated based on larger-scale

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