# Peter Galbraith

### Thesis approved August 1992

Oceanic mixing occurs at molecular diffusion
and viscous scales, called the Batchelor and
Kolmogorov scales, although it has signatures at
larger scales. For example, the rate of creation of
temperature fluctuations by overturning against a
mean temperature gradient is balanced by the rate of
dissipation at the Batchelor scale. In potential
energy terms, buoyancy flux accumulates into a
standing crop of available potential energy of the
fluctuations (APEF), which in turn decreases due to
the potential energy dissipation term, raising the
mean potential energy of the water column. If a
steady-state exists, then both the buoyancy flux and
potential energy dissipation rate are equal to the
APEF divided by a suitable decay time.
This parameterisation of mixing is separated in
two turbulence cases: growing isotropic overturning
scales and steady-state overturning scales with
balanced inertial and buoyancy forces. The decay
time is shown to be inversely proportional to
overturn-scale shear and proportional to overturning
time; this becomes proportional to the buoyancy
period for turbulence in inertial-buoyancy balance,
whether it be isotropic or not. Buoyancy flux is
estimated from overturning scale quantities, which
are much easier to measure than mixing at the
smaller viscous and diffusive scales. Predictions of
buoyancy flux and mixing efficiency compare
favourably with laboratory turbulence data and to
lake and oceanic data, provided that salinity-
compensated intrusions can be excluded from the
analysis. Overturn scales are subsequently used in
the St. Lawrence estuary to estimate mixing rates;
data suggest that solitons create more mixing at the
head of the Laurentian channel than does the larger
scale internal tide.

*Peter is now working at Institute Maurice-Lamontagne in
Mont-Joli, Quebec. He can be
reached by e-mail at* Peter.Galbraith@dfo-mpo.gc.ca