OBSERVATIONS OF INTERACTION BETWEEN THE INTERNAL WAVEFIELD AND LOW FREQUENCY -FLOWS IN THE NORTH ATLANTIC by BARRY RAYMOND RUDDICK

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 -1 at the less energetic moorings, (u < 10 cm s ), indicating that the magnitude of the eddy viscosity was under 200 cm 2/sec, with the sign of the energy transfer uncertain. This is considerably below the 0(4500 cm 2/sec) predicted by MUller (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 Muller, 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 ~. Stresses indicative of this effect were not observed in the data.