Shear instability in the St. Lawrence Estuary, Canada: A comparison of fine-scale observations and estuarine circulation model results

A three-dimensional numerical model was used to predict the timing and the location of shear instabilities in the St. Lawrence Estuary. This model sug gests that significant mixing occur during flood tides in the upper estuary . This mixing is associated with a strong bottom density current made of th e cold Gulf of St, Lawrence intermediate waters flowing under the St. Lawre nce mixed surface waters. Guided by these results, a field experiment was u ndertaken in summer 1997 to verify this and to document the conditions that favor the development of instabilities. The instabilities were found as pr edicted and documented from acoustic imaging, current profiler, and density measurements. The instabilities first develop in the form of wavelike dist urbances before they break, like Kelvin-Helmholtz instabilities. The unstab le waves have wavelength of approximate to 140 - 150 m and extend verticall y between 10 and 25 m. The fine-scale observations of the semidiurnal evolu tion of the vertical structure of currents and density at the experimental site are compared with the numerical results. The model reproduces accurate ly the tidal variability of the currents hut underestimates by a factor of 2 the amplitude of the density fluctuations. The general patterns of the sh ear squared S-2 and the buoyancy frequency squared N-2 are reasonably well reproduced by the model, but their intensities are approximate to 2 times s maller than the observations, This difference is attributed to the limited vertical resolution of the model at the pycnocline. However, the modeled Ri chardson numbers, Ri equivalent to (NS-2)-S-2, are reasonably well reproduc ed and appeared to be useful for the prediction of instabilities in such a complex environment.