Abstract:
A total of four moorings from POLYMODE Array I and II were analyzed
in an investigation of internal wavefield-mean flow interactions. In
particular, evidence for wave-mean flow interaction was sought by
searching for time correlations between the wavefield vertically-acting
Reynolds stress (estimated using the temperature and velocity records),
and the mean shear. No significant stress-shear correlations were found
at the less energetic moorings, indicating that the
magnitude of the eddy viscosity was under 200 cm2/sec, with the sign of
the energy transfer uncertain. This is considerably below the
0(4500 cm2/sec) predicted by Müller (1976). An extensive error analysis
indicates that the large wave stress predicted by the theory should have
been clearly observable under the conditions of measurement.
Theoretical computations indicate that the wavefield "basic state"
may not be independent of the mean flow as assumed by Müller, but can
actually be modified by large-scale vertical shear and still remain in
equilibrium. In that case, the wavefield does not exchange momentum with
a large-scale vertical shear flow, and, excepting critical layer effects,
a small vertical eddy viscosity is to be expected. Using the Garrett-Munk
(1975) model internal wave spectrum, estimates were made of the maximum
momentum flux (stress) expected to be lost to critical layer absorption.
Stress was found to increase almost linearly with the velocity difference
across the shear zone, corresponding to a vertical eddy viscosity of
-100 cm2 s -1. Stresses indicative of this effect were not observed in
the data.
The only significantly non-zero stress correlations were found at
the more energetic moorings. Associated with the 600 m
mean velocity and the shear at the thermocline were a positively correlated
stress at 600 m, and a negatively correlated stress at 1000 m.
These stress correlations were most clearly observable in the frequency
range corresponding to 1 to 8 hour wave periods. The internal wavefield
kinetic and potential energy were modulated by the mean flow at both
levels, increasing by a factor of two with a factor of ten in the mean
flow. The observed stress correlations and energy level changes were
found to be inconsistent with ideas of a strictly local eddy viscosity,
in which the spectrum of waves is only slightly modified by the shear.
When Doppler effects in the temperature equation used to estimate vertical
velocity were considered, the observations of stress and energy changes
were found to be consistent with generation of short (0.4 to 3 km) internal
waves at the level of maximum shear, about 800 m. The intensity
of the generated waves increases with the shear, resulting in an effective
vertical eddy viscosity (based on the main thermocline shear) of
about +100 cm2 s-1 The stresses were not observable at the 1500 m level,
indicating that the waves were absorbed within 500 m of vertical travel.
The tendency for internal wave currents to be horizontally anisotropic
in the presence of a mean current was investigated. Using the Garrett-
Munk (1975) model internal wave spectrum, it was found that critical
layer absorption cannot induce anisotropies as large as observed. A
mechanical noise problem was found to be the cause of large anisotropies
measured with Geodyne 850 current meters. It could not be decided, however,
whether or not the A.M.F. Vector Averaging Current Meter is able to
satisfactorily remove the noise with its averaging scheme.