Peter Galbraith

Relating Overturns to Mixing and Buoyancy Flux

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