Internal Solitary Waves on the Southern Flank of Georges Bank

James D. Irish, Peter H. Wiebe, Bob Beardsley and William J. Williams

Internal solitary waves have been observed on the Northeast continental shelf from the Scotian south into the New York Bight. During the Summer and Fall of 1996, internal solitary waves were observed by a rapidly-sampling SeaCat at 20 meters depth on the Southern Flank mooring on Georges Bank. Also, in June 1997, internal waves were observed from acoustic backscattering records taken by the Greene Bomber on the southern flank of Georges Bank. These data indicate that these solitary waves are not isolated occurrences, but the waves are to be found in most stratified shelf regions with strong semi-diurnal tides.

These internal waves are generated at the shelf break by the off-bank tidal flow which depresses the density structure creating potential energy in the density field. When the off- bank tidal velocity is stronger than the group velocity (about 60 cm/sec), the energy in the disturbance can not propagate up onto the bank against the current, but builds up into a larger disturbance. When the tidal current reverses, the disturbance can propagate up onto the bank, and forms a train of rank ordered downward depressions. The are waves of depression because the surface layer (down to the pycnocline) is thinner than the bottom layer (pycnocline to bottom). Since the waves are generated by the tides, the groups of several pulses reoccur at the semi- diurnal tidal frequency (every 12.4 hours). In 1996 these waves were observed to occur from May through September - that is whenever a density stratification exists. The temperature and density time series show what appear to be noise spikes at first glance. However, the spikes occur at semi-diurnal intervals (with occasional gaps) showing the strong coupling with the tides. When these spikes are expanded they reveal the typical rank ordered solitary wave form.

The acoustic record shows stronger backscattering returns, indicating that there is some process (such as stronger turbulence) at the interface associated with the presence of the solitons. Thus, the acoustics backscattered signal is able to trace out the "profile" of the waves as the ship moves across the shelf. The presence of this turbulence could be a mechanism for increased predator prey encounter and for increased mixing on the Bank. The internal waves disappear as they reach the tidally well mixed region which surrounds the top of the bank (there is no density stratification to support the internal waves there). Also, since the waves only propagate up onto the Bank, they provide a mechanism which may aid in transporting mass (and plankton and larvae) up onto the bank and aid in the retention of organisms on the Bank. Further study of this potential for mixing and retention is suggested.