The interaction between short internal gravity waves and a
larger-scale mean flow in the ocean is analysed in the Wkbj
approximation. The wave field determines the radiation-stress term
in the momentum equation of the mean flow and a similar term in the
buoyancy equation. The mean flow affects the propagation
characteristics of the wave field. This cross-coupling is treated as
a small perturbation. When relaxation effects within the wave field
are considered, the mean flow induces a modulation of the wave field
which is a linear functional of the spatial gradients of the mean
current velocity. The effect that this modulation itself has on the
mean flow can be reduced to the addition of diffusion terms to the
equations for the mass and momentum balance of the mean flow.
However, there is no vertical diffusion of mass and other passive
properties. The diffusion coefficients depend on the frequency
spectrum and the relaxation time of the internal-wave field and can
be evaluated analytically. The vertical viscosity coefficient is
found to be vv [ape ] 4 x 103cm2/s and exceeds
values typically used in models of the general circulation by at
least two orders of magnitude.