Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 403

although the exact mechanisms are perhaps not that well understood and described. At
lower wind speeds, turbulent and molecular diffusion of gases is essential. For higher wind
speeds in excess of 10-12m

s, conclusions have been made that the bubbles may dominate
the air-sea gas exchange ( Thorpe , 1982 ; Merlivat & Memery , 1983 ). In any scenario, the
diffusive gas exchange weakens as the gas-concentration difference across the interface
decreases at strong winds (e.g. Merlivat & Memery , 1983 ; Oost et al. , 1995 ), but the wave
breaking, which significantly enhances the gas transfer through the bubble injection (e.g.
Jahne , 1990 ), is on the rise.
In the bubbles, the pressure is higher than in the surrounding waters and therefore the
gas transfer into the water continues even if it is already saturated and even over-saturated
under the given pressure (e.g. Bortkovskii , 2003 ). Therefore, bubbles enhance the gas flux
from the atmosphere to the ocean. A model capable of describing such fluxes, which agrees
with observations of bubble production at strong winds up to 20m

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/ s, was suggested by

Bortkovskii ( 2003 ).
McNeil & D'Asaro ( 2007 ) tried to explicitly parameterise the air-sea gas fluxes at winds
even stronger than that, those fluxes measured during Hurricane Frances in 2004 at U 10
winds up to 55m s, by means of air-deployed floats. In particular they intended to answer

questions regarding whether there are existing parameterisations not suitable at such winds,
what functional dependences could possibly describe such fluxes across the entire range of
wind speeds and, most importantly, what are the actual physical processes responsible for
the air-sea gas exchange in hurricanes?
The models to be excluded are those which predict cubic dependence on the wind speed
( Wannikhoff & McGillis , 1999 ). According to McNeil & D'Asaro ( 2007 ), at hurricane
wind speeds the fluxes still grow rapidly, but in relative terms at a much lower rate by
comparison with the moderately strong winds.
A change of regime of the air-sea gas transfer is observed at

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U 10 > 35m / s . (9.50)

This is remarkably close to the wind speed which characterises saturation of the sea drag
coefficient (9.12) and of the surface-wave asymmetry (3.26) . The drag coefficient C D
describes the momentum flux from the atmosphere to the ocean, and the asymmetry sig-
nifies the breaking probability and/or the breaking mechanism on the surface itself, and
such coincidence of the regime change for dynamic and gas-transfer properties of air-sea
interactions can hardly be random. Some fundamental physics should be responsible for
the alteration of all exchanges at the ocean interface at the same time, even though sea drag
and bubble-injection rates seem to be weakly related phenomena. As usual, such physics
is necessarily complex and hardly limited to a single phenomenon, but the wave breaking
may be one of the dynamics involved as it relates to both the surface roughness and bubble
production.
In this high-wind regime of gas transfer, bubbles of 1mm radius and smaller are trans-
ported down by vertical currents with speeds of 20-30 cm

s and mostly dissolve due to
hydrostatic compression before they reach depths of some 20m. This additional mechanism

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