Daniel Bourgault

Wave-induced boundary mixing in a partially mixed estuary

We present observations that reveal the existence of horizontally propagating, tidally-driven, high-frequency internal wave (IW) packets in a channel of the partially mixed St. Lawrence Estuary. The packets propagate transversely to the channel axis and collide with the shoaling lateral boundaries. The structure and energy of IWs are diagnosed with a two-dimensional, nonlinear nonhydrostatic model, and the results are compared with weakly nonlinear Korteweg-de-Vries (KdV) theory. The behavior of IWs running into the shoaling lateral boundary is examined in terms of published laboratory and numerical experiments. Our analysis indicates that IWs break on the slope, during which 6% of their energy is converted into potential energy through vertical mixing. The corresponding buoyancy flux, when averaged over the surf zone and the time of the mixing event, is more than an order of magnitude larger than values predicted by a published non-IW-resolving three-dimensional (3D) baroclinic circulation model of the region. Even averaging across the full domain and tidal period yields mixing rates that are a significant proportion of those in the 3D circulation model. These indirect inferences suggest that wave-induced boundary mixing may be of general significance in partially mixed estuaries.

Real‐time monitoring of the freshwater discharge at the head of the St. Lawrence Estuary

Abstract The freshwater discharge at the head of the St. Lawrence Estuary near Quebec city in eastern Canada, was monitored at monthly timescales from 1955 to 1988 by the Quebec Department of Environment and Fauna (DEF). Since 1988, these estimates have been discontinued. Using the 1962–1988 data, two models are developed to estimate the freshwater discharge at the head of the Estuary. The first is a regression model which estimates the discharge at monthly timescales using sea level data available at Neuville near Quebec city. A second order polynomial fits the data with a correlation coefficient of R = 0.93. The second model is a one‐dimensional numerical model which estimates the hourly discharge using hourly sea level data available at Neuville and at Lauzon as upstream and downstream boundary conditions respectively. The numerical model is calibrated with current measurements for the tidal variability, and with the DEFs estimates for monthly means. The linear least‐squares fits between the monthly‐a...

On the Reflectance of Uniform Slopes for Normally Incident Interfacial Solitary Waves

Abstract The collision of interfacial solitary waves with sloping boundaries may provide an important energy source for mixing in coastal waters. Collision energetics have been studied in the laboratory for the idealized case of normal incidence upon uniform slopes. Before these results can be recast into an ocean parameterization, contradictory laboratory findings must be addressed, as must the possibility of a bias owing to laboratory sidewall effects. As a first step, the authors have revisited the laboratory results in the context of numerical simulations performed with a nonhydrostatic laterally averaged model. It is shown that the simulations and the laboratory measurements match closely, but only for simulations that incorporate sidewall friction. More laboratory measurements are called for, but in the meantime the numerical simulations done without sidewall friction suggest a tentative parameterization of the reflectance of interfacial solitary waves upon impact with uniform slopes.

Turbulent nitrate fluxes in the Amundsen Gulf during ice‐covered conditions

3(2, 5) × 10 −3 m 2 s −1 and decreased exponentially to a depth of ∼50 m, below which it was roughly constant at the background value Kb =3 (2, 5) ×1 0 −6 m 2 s −1 .T he nitracline, centered around 62 m depth, was subject to an eddy diffusivity close to the background value Kb and the mean diffusive nitrate flux across the nitracline was Fnit = 0.5(0.3, 0.8) mmol m −2 d −1 . These observations are compared with other regions and the role of vertical mixing on primary production in the Amundsen Gulf is discussed. Citation: Bourgault, D., C. Hamel, F. Cyr, J.‐E. Tremblay, P. S. Galbraith, D. Dumont, and Y. Gratton (2011), Turbulent nitrate fluxes in the Amundsen Gulf during ice‐covered conditions, Geophys. Res. Lett., 38, L15602, doi:10.1029/2011GL047936.

A Laterally Averaged Nonhydrostatic Ocean Model

A laterally averaged nonhydrostatic model for stratified flow in dynamically narrow domains is presented. Averaging laterally yields the computational efficiency of a two-dimensional model, while retaining some effects associated with variable domain width, such as flow acceleration through contracting channels. The model may be run in both hydrostatic and nonhydrostatic modes, and in the latter case it converges rapidly if the flow is approximately hydrostatic. The model’s strengths and weaknesses are illustrated with a series of test cases of increasing complexity. Side-by-side comparisons with laboratory observations show the ability of the model to simulate the structures of nonhydrostatic flows, including shear instabilities and overturning internal waves, with discrepancies becoming apparent mainly for transition to three-dimensional turbulence. Similar results are demonstrated in an application to the stratified sill flow in Knight Inlet, British Columbia. The model reproduces nonhydrostatic features thought to be dynamically important to this system, including the generation of largeamplitude lee waves and shear instabilities.

Observations of a Large-Amplitude Internal Wave Train and Its Reflection off a Steep Slope

Abstract Remote and in situ field observations documenting the reflection of a normally incident, short, and large-amplitude internal wave train off a steep slope are presented and interpreted with the help of the Dubreil–Jacotin–Long theory. Of the seven remotely observed waves that composed the incoming wave train, five were observed to reflect. It is estimated that the incoming wave train carried Ei = (24 ± 4) × 104 J m−1 to the boundary. The reflection coefficient, defined as the ratio of reflected to incoming wave train energies, is estimated to be R = 0.5 ± 0.2. This is about 0.4 lower than parameterizations in the literature, which are based on reflections of single solitary waves, would suggest. It is also shown that the characteristics of the wave-boundary situation observed in the field are outside the parameter space examined in previous laboratory and numerical experiments on internal solitary wave reflectance. This casts doubts on extrapolating current laboratory-based knowledge to fjord-like...

Interfacial solitary wave run-up in the St. Lawrence Estuary

Density variations show evidence of interfacial solitary waves (ISW) running up the sloping boundary of an island in the St. Lawrence Estuary, confirming inferences based remote sensing. Further detail is suggested by simulations created with a two-dimensional nonhydrostatic numerical model. The simulations confirm theoretical predictions of the location of wave breaking, something that is difficult to observe in the field. Two other results of the simulations match laboratory findings: the creation of turbulent boluses that propagate upslope of the breaking zone, and the creation of an intermediate layer that transports mixed water away from the mixing site. Although our sampling could not resolve the intermediate mixing layer, it did provide evidence of boluses. In addition to ISW breaking the bolus and intrusion effects may also be important in coastal regions.

Numerical simulations of the spread of floating passive tracer released at the Old Harry prospect

The Gulf of St Lawrence is under immediate pressure for oil and gas exploration, particularly at the Old Harry prospect. A synthesis of the regulatory process that has taken place over the last few years indicates that important societal decisions soon to be made by various ministries and environmental groups are going to be based on numerous disagreements between the private sector and government agencies. The review also shows that the regulatory process has taken place with a complete lack of independent oceanographic research. Yet, the Gulf of St Lawrence is a complex environment that has never been specifically studied for oil and gas exploitation. Motivated by this knowledge gap, preliminary numerical experiments are carried out where the spreading of a passive floating tracer released at Old Harry is examined. Results indicate that the tracer released at Old Harry may follow preferentially two main paths. The first path is northward along the French Shore of Newfoundland, and the second path is along the main axis of the Laurentian Channel. The most probable coastlines to be touched by water flowing through Old Harry are Cape Breton and the southern portion of the French Shore, especially Cape Anguille and the Port au Port Peninsula. The Magdalen Islands are less susceptible to being affected than those regions but the probability is not negligible. These preliminary results provide guidance for future more in-depth and complete multidisciplinary studies from which informed decision-making scenarios could eventually be made regarding the exploration and development of oil and gas at the Old Harry prospect in particular and, more generally, in the Gulf of St Lawrence.

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 suggests that significant mixing occur during flood tides in the upper estuary. This mixing is associated with a strong bottom density current made of the cold Gulf of St. Lawrence intermediate waters flowing under the St. Lawrence mixed surface waters. Guided by these results, a field experiment was undertaken in summer 1997 to verify this and to document the conditions that favor the development of instabilities. The instabilities were found as predicted and documented from acoustic imaging, current profiler, and density measurements. The instabilities first develop in the form of wavelike disturbances before they break, like Kelvin-Helmholtz instabilities. The unstable waves have wavelength of ≈140–150 m and extend vertically between 10 and 25 m. The fine-scale observations of the semidiurnal evolution of the vertical structure of currents and density at the experimental site are compared with the numerical results. The model reproduces accurately the tidal variability of the currents but underestimates by a factor of 2 the amplitude of the density fluctuations. The general patterns of the shear squared S2 and the buoyancy frequency squared N2 are reasonably well reproduced by the model, but their intensities are ≈2 times smaller than the observations. This difference is attributed to the limited vertical resolution of the model at the pycnocline. However, the modeled Richardson numbers, Ri ≡ N2S−2, are reasonably well reproduced and appeared to be useful for the prediction of instabilities in such a complex environment.

Water renewals in the Saguenay Fjord

Water renewals and renewal times of the Saguenay Fjord are investigated and classified according to their intrusion depth. Renewal dynamics are controlled by a shallow sill ( 20 m) at the fjord mouth, by large tides that are a distinguishing feature of the Saguenay Fjord and by large vertical mixing inside the inner basin (K 1024 m2 s21). A mooring was deployed in the inner basin of the fjord to provide a clearer quantitative understanding of the complexity and seasonality of water renewals in this seasonally ice-covered fjord. The mooring provided information on currents over nearly the entire water column, along with temperature-salinity at a few discrete depths. Hydrographic temperature and salinity transects spanning multiple seasons and years as well as turbulence profiles were also collected. The observations show that the fjord dynamics are more complex than previously hypothesized, with large changes in renewal event depths leading to three different renewal regimes. Part of this renewal depth variability may be explained by the seasonality of the St. Lawrence estuarine circulation. Because of the large turbulence within the inner basin bottom layer, the density decreases over time such that new deep renewals can occur every year. The mechanisms behind the large vertical mixing cannot yet be clearly identified but a statistically significant correlation (K / N21:3) suggests that internal wave breaking may be a significant contributor to deep turbulence mixing in the inner basin. The renewal time of the inner basin waters is estimated to be between 1 and 6 months.