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Supertidal Frequency Internal Waves on the Continental Shelf South of New England

Derek M. BurrageCollege of Marine Studies,. University of Delaware, Newark DE 19716

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Richard W. GarvineCollege of Marine Studies,. University of Delaware, Newark DE 19716

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Abstract

Large-amplitude internal waves of supertidal frequency are commonly observed on continental shelves during the stratified mason. We present a series of intensive observations of such waves 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. Our data analysis technique permitted accurate estimation of isopycnal displacement time series despite the large amplitudes (11 m) present. We found 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−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. We also found evidence of local mixing in the pycnocline following passage of the larger amplitude waves. Such mixing could have important consequences for the subtidal mean balances of mass, momentum, salt, and heat for the shelf circulation during the stratified season.

Abstract

Large-amplitude internal waves of supertidal frequency are commonly observed on continental shelves during the stratified mason. We present a series of intensive observations of such waves 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. Our data analysis technique permitted accurate estimation of isopycnal displacement time series despite the large amplitudes (11 m) present. We found 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−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. We also found evidence of local mixing in the pycnocline following passage of the larger amplitude waves. Such mixing could have important consequences for the subtidal mean balances of mass, momentum, salt, and heat for the shelf circulation during the stratified season.

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