Observations of nonlinear internal waves on the outer New England continental shelf during the summer Shelfbreak Primer study

Abstract

Observations are presented of nonlinear internal waves on the outer New England continental shelf during the summer Shelfbreak Primer study conducted between July 26 and August 5, 1996. Current and temperature measurements were made with an upward looking acoustic Doppler current profiler (ADCP) located on the 147 m isobath near the shelfbreak and three vertical thermistor moorings located upshelf. Data from the ADCP and two nearby thermistor chains show energetic internal tides propagating at roughly 0.9 m s−1 to the north‐northwest, nearly perpendicular to the local topography with 10–15 cm s−1 horizontal currents and 15–30 m vertical displacements. These waves evolve rapidly within a 5.8 km range into an undular internal tidal bore. Cross‐isobath barotropic tidal currents, responsible for generating the internal tides are in the 5–12 cm s−1 range. The bore formation is highly variable. There is evidence of a correlation between internal tide steepening and a shelfbreak front jet orientation that is oppositely directed to the internal tide propagation. There is no correlation between steepening and the jet's vertical shear. Statistics of the undular bores show rms travel time fluctuations from 0.8 to 1.7 hours and average tidal bore durations from 12 to 9 hours. The average undular bore speed is 0.9 m s−1, with an rms fluctuation of 0.4 m s−1. The number of high‐frequency waves in the bore varies from 0 to 8 near the shelfbreak and increases to 30 waves 26.7 km upshelf. The observed distribution function of temporal spacing between high‐frequency internal waves is spread between 4 and 20 min.