Peter S. Galbraith

Identifying Overturns in CTD Profiles

Abstract The authors propose a scheme to test whether inversions in CTD density profiles are caused by overturning motions (from which mixing rates may be inferred) or by measurement noise. Following a common practice, possible overturning regions are found by comparing the observed profile ρ(z) and an imaginary profile ˆρ(z) constructed by reordering ρ(z) to make. it gravitationally stable. The resulting “reordering regions” are subjected to two tests. • The “Thorpe fluctuation” profile ρ′(z) = ρ(z) − ˆρ(z) is examined for “runs” of adjacent positive or negative values. The probability density function (PDF) of the run length is compared with the corresponding PDF of random noise. This yields a threshold value for rms run length within individual reordering regions that must be exceeded for adequate resolution of overturns, taking into account both CTD characteristics and local hydrographic properties. • Temperature and salinity covariations with respect to density are screened for systematic CTD errors ...

A summary of the formation and seasonal progression of the Northeast Water Polynya

Abstract A summary of the seasonal development of the Northeast Water Polynya ice cover characteristics is presented. This is based primarily on satellite remote sensing observations, with some in situ measurements, including both new and published data.

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.

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...

Marine Mammal Strandings and Environmental Changes: A 15-Year Study in the St. Lawrence Ecosystem

Understanding the effects of climatic variability on marine mammals is challenging due to the complexity of ecological interactions. We used general linear models to analyze a 15-year database documenting marine mammal strandings (1994–2008; n = 1,193) and nine environmental parameters known to affect marine mammal survival, from regional (sea ice) to continental scales (North Atlantic Oscillation, NAO). Stranding events were more frequent during summer and fall than other seasons, and have increased since 1994. Poor ice conditions observed during the same period may have affected marine mammals either directly, by modulating the availability of habitat for feeding and breeding activities, or indirectly, through changes in water conditions and marine productivity (krill abundance). For most species (75%, n = 6 species), a low volume of ice was correlated with increasing frequency of stranding events (e.g. R2 adj = 0.59, hooded seal, Cystophora cristata). This likely led to an increase in seal mortality during the breeding period, but also to increase habitat availability for seasonal migratory cetaceans using ice-free areas during winter. We also detected a high frequency of stranding events for mysticete species (minke whale, Balaenoptera acutorostrata) and resident species (beluga, Delphinapterus leucas), correlated with low krill abundance since 1994. Positive NAO indices were positively correlated with high frequencies of stranding events for resident and seasonal migratory cetaceans, as well as rare species (R2 adj = 0.53, 0.81 and 0.34, respectively). This contrasts with seal mass stranding numbers, which were negatively correlated with a positive NAO index. In addition, an unusual multiple species mortality event (n = 114, 62% of total annual mortality) in 2008 was caused by a harmful algal bloom. Our findings provide an empirical baseline in understanding marine mammal survival when faced with climatic variability. This is a promising step in integrating stranding records to monitor the consequences of environmental changes in marine ecosystems over long time scales.

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.

Factors affecting fast‐ice consolidation in Southeastern Hudson Bay, Canada

Abstract Analysis of satellite images of southeastern Hudson Bay taken over aperiod of 13 years led to the classification of ice distribution into three categories. The first category is for complete fast‐ice cover of the area, the second for fast ice covering only half the area and the third for the absence of fast ice extending away from the coast. Of the three factors considered‐ wind, water circulation and air temperature—the occurrence of strong southwesterly winds during the freezing period is probably the main factor regulating the extent of the fast‐ice cover for the first two categories. Through melting action, above‐freezing air temperatures appear to prevent the consolidation of ice into a solid cover giving rise to the rare third category of ice distribution.

Large-Scale Atmospheric and Oceanic Control on Krill Transport into the St. Lawrence Estuary Evidenced with Three-Dimensional Numerical Modelling

ABSTRACT A three-dimensional circulation model, coupled to a Lagrangian particle drift model, is used to understand the processes leading to krill transport from the northwest Gulf of St. Lawrence (nwGSL) towards the head of the Lower St. Lawrence Estuary (LSLE), a well-known site of krill accumulation. An analysis of the circulation at the scale of the Gulf of St. Lawrence (GSL) over five years (2006 to 2010) evidenced four major findings. (i) There are two main seasonal circulation patterns, one in winter–spring and one in summer–fall, driven by local wind forcing and transport at Cabot Strait and at the Strait of Belle Isle. (ii) The freshwater runoff variability does not control the observed inflow events at the mouth of the LSLE. (iii) Extratropical storms passing over the GSL are important for the transport of krill into the LSLE through the generation of inflow events at Pointe-des-Monts. (iv) The contribution of the transport in the surface layer (where krill are found at night) during these inflow events is also important in modulating the variability of the transport of krill into the LSLE. The inflow events, combined with the presence or absence of high krill densities in the nwGSL, partly control the interannual variability of the transport of krill into the LSLE.

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.

Generation of internal solitary waves by frontally forced intrusions in geophysical flows.

Internal solitary waves are hump-shaped, large-amplitude waves that are physically analogous to surface waves except that they propagate within the fluid, along density steps that typically characterize the layered vertical structure of lakes, oceans and the atmosphere. As do surface waves, internal solitary waves may overturn and break, and the process is thought to provide a globally significant source of turbulent mixing and energy dissipation. Although commonly observed in geophysical fluids, the origins of internal solitary waves remain unclear. Here we report a rarely observed natural case of the birth of internal solitary waves from a frontally forced interfacial gravity current intruding into a two-layer and vertically sheared background environment. The results of the analysis carried out suggest that fronts may represent additional and unexpected sources of internal solitary waves in regions of lakes, oceans and atmospheres that are dynamically similar to the situation examined here in the Saguenay Fjord, Canada.