Enhanced dissipation of kinetic energy beneath surface waves

TRANSFER of momentum from wind to the surface layer of lakes and oceans plays a central part in driving horizontal and vertical circulation of water masses. Much work has been devoted to understanding the role of waves in momentum transfer across the air–sea interface, but less is known about the energetics of the near-surface turbulence responsible for the mixing of momentum and mass into the underlying water column. In particular, it has remained unclear whether the structure of the turbulence in the surface layer can be described by analogy to wall-bounded shear flows or whether waves, either through breaking or wave–current interaction, introduce new length- and timescales which must be modelled explicitly. Here we report observations of turbulence in Lake Ontario, taken under conditions of strong wave breaking, which reveal a greatly enhanced dissipation rate of kinetic energy close to the air–water interface, relative to the predictions of wall-layer theory. Because wave breaking is intermittent, short-term measurements of the kinetic energy dissipation in the near-surface layer may therefore result in considerable underestimates, and any general treatment of upper mixed layer dynamics will have to take wave breaking explicitly into account.