Observations of nonlinear internal waves on the New England continental shelf during the summer shelfbreak PRIMER conducted in July of 1996 are presented. The principal internal wave observations from this experiment were collected from an upward looking ADCP and three vertical thermistor moorings. The observations show energetic internal tides with 10-20 cm/s horizontal velocities and 15 to 30 m vertical displacements which rapidly evolve into an internal tide bore with the formation of high frequency soliton-like internal waves. The timescale of this development is approximately 1/5 of the tidal period. The observations reveal a high degree of variability in nonlinear steepening with some tide cycles showing strong steepening and other showing none. There is evidence of correlations between internal tide steepening and the shelfbreak front jet with strongest bore development occuring when the jet is oppositely directed to the internal tide propagation, reducing the effective group velocity of the wave packet and allowing more time for non-linear steepening. Estimates of the energy transfer rate between the thermal wind shear of the shelfbreak front jet and the internal tide are insufficient to explain the variability. Average vertically integrated potential energy densities are roughly 0.7 kJ/m^2, and average vertically integrated kinetic energy densities are roughly 1.8 kJ/m^2, which are consistent with WKB scaling. For a group velocity of 0.7 m/s shoreward energy fluxes are roughly 1.75 kW/m. Statistics of the tidal bore wave packets show rms travel time fluctuations from 0.84 to 1.7 hr, and average tidal bore durations from 12 to 9 hr. The observed distribution function of temporal spacing between high frequency internal waves is spread between 4 and 20 minutes.
Woods Hole Oceanographic Institution, MS#11
Applied Ocean Physics and Engineering Department
Woods Hole, MA, 02543-1053