Sack Lunch Seminar (SLS)

Internal waves: mixing and scattering


The ocean is filled with fast, small-scale motions called internal waves, which
are too small to be resolved by numerical models, yet which are a very energetic
component of the ocean's dynamics. How do they affect the larger-scale
circulation? We look at two possible mechanisms. The first is horizontal
dispersion: even though a linear wave field is periodic, we show that a random
superposition of such waves can disperse particles in a diffusive manner because
of nonlinear corrections which are required to make the velocity field
dynamically consistent. The second mechanism is dissipation: we consider how an
internal wave is modified as it propagates over random topography. This is
intended to be a simplified model for the scattering and ensuing energy transfer
that a tidal mode undergoes as it propagates over small-amplitude undulating
topography, away from the large topographical features where it was generated.
We describe a peculiar focusing effect, and derive expressions for the decay
rate of the gravest mode. Our results appear to indicate that in some regions of
the ocean basin, scattering by bottom topography can act as a kind of friction,
and provide an effective decay mechanism for the internal tide.