Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 383

On the left, turbulent mixing mechanisms are indicated. Wave-breaking turbulence is
located near the water surface, Langmuir circulation penetrates deeper. This circulation is
a consequence of an instability resulting from the waves interacting with both the Stokes
and wind drifts (e.g. Langmuir , 1938 ; Craik & Leibovich , 1976 ; Phillips , 1998 ). Thus, this
is still a wave-related mechanism.
Below lies what is labelled as 'normal' eddy diffusion, that is turbulent diffusion unre-
lated to the waves. This is turbulence produced by the mean shear flow. Bottom boundary-
layer wave and current turbulence concludes the scheme. It should be mentioned in passing
that internal waves at the thermocline and throughout the stratified ocean, their break-
ing and interactions with other circulation and turbulent processes, are another source of
momentum fluxes in the ocean, which will not be, however, considered here.
In this context, we will also point out that the overall pattern of the upper-ocean mixing,
even in the purely wind-sea case, is much more complex than the general schemes outlined
here. For example, Ardhuin et al. ( 2009b ) further subdivided the Stokes drift and quasi-
Eulerian currents, likely related to the wave breaking and/or Langmuir circulation, and
concluded that such decomposition is important for the estimation of energy fluxes to the
upper ocean.
We will be discussing the concept of the different turbulence layers and turbulence
mechanisms in more detail in Section 9.2.2 below. In this section, dedicated to the gen-
eral scheme of momentum and energy fluxes across the interface and in the upper ocean,
we would like to additionally mention two mechanisms of turbulence generation, that is
Langmuir turbulence and non-breaking wave-induced turbulence. In the schemes presented
above they are indicated, but one needs to be particularly perceptive to notice that.
Langmuir turbulence is essentially the same physical mechanism as Langmuir circula-
tion (e.g. McWilliams et al. , 1997 ; Sullivan & McWilliams , 2010 ). It deserves a specific
highlight, however, both in phenomenological terms and because of its potentially special
role in the upper-ocean mixing.
Langmuir circulation is typically pictured as rolls rotating perpendicularly to the main
wave direction and displaying themselves as streaks on the ocean surface, parallel to the
wave propagation (e.g. Langmuir , 1938 ; Smith , 1992 ; Phillips , 2001 , 2002 , 2005 ). Their
surface spacing can grow from millimetres to kilometres ( Phillips , 2001 ; Thorpe , 2004 ),
and their depth is more or less restricted by the depth of the mixed layer. Their rate of rota-
tion is relatively slow (of the order of cm/s); nevertheless, Langmuir circulations provide
a means to advect turbulence kinetic energy vertically (see Babanin et al. , 2009b ,fora
discussion).
What McWilliams et al. ( 1997 ) called Langmuir turbulence is a continuous spectrum
of Langmuir circulations, generated by the waves interacting with background drift cur-
rents through the water column. They would provide superposition of different spatial and
velocity rotational scales and not necessarily extend and exhibit themselves at the water
surface. Effectively, this is a turbulence distributed in the upper ocean where the instabili-
ties of wave-drift interactions can persist. As the turbulence is distributed vertically, it can
play an obvious role in the upper-ocean mixing and the schemes mentioned in this section.