Waves, turbulent motions and mixing - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 97

Figure 4.4 Images of the sea surface taken from the bridges of research ships in the Celtic Sea
(height above sea level approximately 25 metres) showing whitecaps for wind speeds of 25 m s 1
on the left and 15 m s 1 on the right. The whitecaps are growing or decaying areas of high
turbulence produced by wave breaking. The fraction of the ocean surface occupied by whitecaps
increases with wind speed, as does the energy transfer to turbulence. (Photos by J. Sharples.)
of the surface is disrupted and air is entrained into the sea surface layers as large numbers
of small bubbles are formed. Because of their small size, the bubbles rise rather slowly
and some are drawn downwards in the turbulent circulation which follows whitecapping.
The high pressure within the bubbles accelerates the transfer of gaseous components into
solution in the surface waters (see Thorpe, 2005 , Chapter 9 for a detailed account).
Surface waves are important, not only because they transmit mechanical energy
and stir the near surface layers, but also because they transfer momentum to the
surface layers. As we have just seen, when waves break their energy is dissipated, but
the wave momentum, associated with the Stokes drift (4.1.3), is transferred to the
surface current system and adds to the momentum input directly by the wind stress.
The Stokes drift of surface waves is also involved in the generation of an important
component of near surface flow, known as Langmuir circulation. This circulation is
responsible for the formation of parallel lines of foam and other buoyant material,
termed 'windrows', which are often apparent when the water surface in the ocean and
in lakes is subjected to wind stress. The windrows are aligned roughly in the wind
direction, as is evident in Fig. 4.5a where the surface wind-generated waves can been
seen with wave fronts orthogonal to the lines of foam. Windrows are the result of
surface convergence in the transverse circulation in the vertical plane normal to the
wind stress as shown in the schematic of Fig. 4.5b .
The mechanism responsible for the transverse motions appears to involve a subtle
interaction between surface waves and the shear flow. A small perturbation of an
initially uniform downwind Stokes drift flow is amplified by transverse forces in
an instability which leads to convergence and, hence, sinking of fluid where the flow
perturbation is growing. Conversely, divergence and upwelling occur where the down-
wind flow is reduced (see Fig. 4.5b ). The details of this mechanism have been formu-
lated theoretically (Leibovich, 1998) and studied in numerical simulations (see Thorpe,
2005 , chapter 9 ). In both observations and simulations, the cell structure is found to be