Supertidal frequency internal waves on the continental shelf south of New England.

Abstract

Presents a series of observations on large amplitude internal waves (of supertidal frequency) at an anchor station south of New England using two CTD probes, one as a profiler and one at a fixed level in midpycnocline. Two week records of the current at two depths on a mooring there were also obtained with a sampling rate sufficient to resolve the waves. The data analysis technique permitted accurate estimation of isopycnal displacement time series despite the large amplitudes (11 m) present. Concluded that over one diurnal cycle the energy content in the internal tidal and supertidal wavebands were comparable. Two periods of higher supertidal energy were separated by a time corresponding to the semidiurnal tidal period, consistent with postulated generation of these waves at the shelfbreak by tidal currents. Measured currents at 20 m depth showed supertidal wave groups often separated by a period of a day with onshore surges of up to 46 cm s SUP - SUP 1. A power spectrum for the entire current record at 20 m showed greater total variance in the supertidal band than in the subtidal band. Thus, these waves present severe aliasing potential for routine hydrographic and current observations. Presents evidence of local mixing in the pycnocline following passage of the larger amplitude waves. Such mixing could have important consepuences for the subtidal mean balances of mass, momentum, salt, and heat for the shelf circulation during the stratified season. (from authors' abstract)