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Resonant Modulation of a Tidal Jet
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Lunenburg Bay, on the south eastern coast of Nova Scotia, is the site of an interdisciplinary coastal observatory with permanent moorings recording hydrodynamic, atmospheric and biological data. During the ebb phase of the tide a strong jet exits two coves along a narrow channel and enters the main bay with maximum velocities reaching 1m/s. Eddies and whorls form at the jet edge and it serves as an important mechanism for mixing throughout the bay. We report data from the observatory and an additional instrument array deployed during the summer of 2005. Pressure and velocity spectra reveal an oscillation with a period of approximately one hour. The oscillation appears to be a resonant amplification of the broad-banded high-frequency (on the timescales of hours) energy on the Scotian Shelf. The oscillation has larger amplitudes in winter, likely associated with increased storminess. During the field experiment, the oscillation sea surface displacement at the head of the bay was small ( similar to 0.05m) with little effect on the tidal currents in the main bay. However, there is a significant modulation of velocities within the jet (ca. 0.4 m/s). The observations are compared to numerical results using the hydrodynamic model Delft3d. A simple two-dimensional case reproduces the seiche motions well. An analysis of the phases of the oscillation throughout the bay indicates that the signal is consistent with wave propagation along the channel, which forces a pumping mode within the coves. The oscillation interacts nonlinearly with the M2 tide, causing a broadening of the oscillation spectral peak.
Mullarney, J., A. Hay, and T. Bowen. "Resonant Modulation of a Tidal Jet." Canadian Meteorological and Oceanographic Society, McDonald Building 150 Louis Pasteur, Ste 112 Ottawa, ON, K1N 6N5 Canada. [np]. CMOS Congress 2007 - Air, Ocean, Earth and Ice on the Rock, St. Johns, Newfoundland and Labrador, May 28 - June 1, 2007.