Frédéric Cyr

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.

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.

Observations of small-scale secondary instabilities during the shoaling of internal bores on a deep-ocean slope

AbstractThe Rockall Bank area, located in the northeast Atlantic Ocean, is a region dominated by topographically trapped diurnal tides. These tides generate up- and downslope displacements that can be locally described as swashing motions on the bank. Using high spatial and time resolution of moored temperature sensors, the transition toward the upslope flow (cooling phase) is described as a rapid upslope-propagating bore, likely generated by breaking trapped internal waves. Buoyant anomalies are found during the bore propagation, likely resulting from small-scale instabilities. The imbalance between the rate of disappearance of available potential energy and the dissipation rate of turbulent kinetic energy suggests that these instabilities are growing (i.e., young) and have high mixing potential.

On the influence of cold-water coral mound size on flow hydrodynamics, and vice versa

Using a combination of in situ observations and idealistic 2-D nonhydrostatic numericalsimulations, the relation between cold-water coral (CWC) mound size and hydrodynamics is explored forthe Rockall Bank area in the North Atlantic Ocean. It is shown that currents generated by topographicallytrapped tidal waves in this area cause large isopycnal depressions resulting from an internal hydrauliccontrol above CWC mounds. The oxygen concentration distribution is used as a tracer to visualize the flowbehavior and the turbulent mixing above the mounds. By comparing two CWC mounds of different sizesand located close to each other, it is shown that the resulting mixing is highly dependent on the size of themound. The effects of the hydraulic control for mixing, nutrient availability, and ecosystem functioning arealso discussed.

Thermal Fronts Atlas of Canadian Coastal Waters

Abstract Oceanic fronts are often associated with enhanced biological activity. Depending on their generation mechanism, they are often linked to specific geographical areas. Here we use 25 years of high-resolution satellite sea surface temperature (SST) daily images to generate maps of SST fronts over Canadian coastal waters. Results show that fronts are ubiquitous features, but some fronts are more persistent than others. We confirmed the location of previously known major fronts, but some new persistent frontal areas were also detected as a result of the use of high-resolution (1.1 km) data and a methodology adapted to detect smaller-scale frontal features. Results also show that some of the frontal areas are associated with enhanced phytoplankton biomass or higher trophic level organisms (whales and birds) confirming the ecological importance of this physical process.

Behavior and mixing of a cold intermediate layer near a sloping boundary

As in many other subarctic basins, a cold intermediate layer (CIL) is found during ice-free months in the Lower St. Lawrence Estuary (LSLE), Canada. This study examines the behavior of the CIL above the sloping bottom using a high-resolution mooring deployed on the northern side of the estuary. Observations show successive swashes/backwashes of the CIL on the slope at a semi-diurnal frequency. It is shown that these upslope and downslope motions are likely caused by internal tides generated at the nearby channel head sill. Quantification of mixing from 322 turbulence casts reveals that in the bottom 10 m of the water column, the time-average dissipation rate of turbulent kinetic energy is 𝜖10 m = 1.6×10−7Wkg−1, an order of magnitude greater than found in the interior of the basin, far from boundaries. Near-bottom dissipation during the flood phase of the M2 tide cycle (upslope flow) is about four times greater than during the ebb phase (downslope flow). Bottom shear stress, shear instabilities, and internal wave scattering are considered as potential boundary mixing mechanisms near the seabed. In the interior of the water column, far from the bottom, increasing dissipation rates are observed with both increasing stratification and shear, which suggests some control of the dissipation by the internal wave field. However, poor fits with a parametrization for large-scale wave-wave interactions suggests that the mixing is partly driven by more complex non-linear and/or smaller scale waves.

Turbulent nitrate fluxes in the Lower St. Lawrence Estuary, Canada

Turbulent vertical nitrate fluxes were calculated using new turbulent microstructure observations in the Lower St. Lawrence Estuary (LSLE), Canada. Two stations were compared: the head of the Laurentian Channel (HLC), where intense mixing occurs on the shallow sill that marks the upstream limit of the LSLE, and another station located about 100 km downstream (St. 23), more representative of the LSLE mean mixing conditions. Mean turbulent diffusivities and nitrate fluxes at the base of the surface layer for both stations were, respectively (with 95% confidence intervals): K¯HLC = 8.6(3.2,19) × 10−3 m2 s−1,K¯23 = 4.4(2.3,7.6) × 10−5 m2 s−1,F¯HLC = 95(18,300) mmol m−2 d−1, and F¯23 = 0.21(0.12,0.33) mmol m−2 d−1. Observations suggest that the interplay between large isopleth heaving near the sill and strong turbulence is the key mechanism to sustain such high turbulent nitrate fluxes at the HLC (two to three orders of magnitude higher than those at Station 23). Calculations also suggest that nitrate fluxes at the HLC alone can sustain primary production rates of 3.4(0.6,11) g C m−2  mo−1 over the whole LSLE, approximately enough to account for a large part of the phytoplankton bloom and for most of the postbloom production. Surfacing nitrates are also believed to be consumed within the LSLE, not leaving much to be exported to the rest of the Gulf of St. Lawrence.

Hypoxia in the St. Lawrence Estuary: How a Coding Error Led to the Belief that “Physics Controls Spatial Patterns”

Two fundamental sign errors were found in a computer code used for studying the oxygen minimum zone (OMZ) and hypoxia in the Estuary and Gulf of St. Lawrence. These errors invalidate the conclusions drawn from the model, and call into question a proposed mechanism for generating OMZ that challenges classical understanding. The study in question is being cited frequently, leading the discipline in the wrong direction.