Joël Chassé

Tidal circulation and buoyancy effects in the St. Lawrence Estuary

Abstract The action of tides on density‐driven circulation, internal gravity waves, and mixing was investigated in the St. Lawrence Estuary between Rimouski and Québec City. Time‐varying fields of water level, currents and density were computed under typical summer conditions using a three‐dimensional hydrostatic coastal ocean model that incorporates a second order turbulence closure submodel. These results are compared with current meter records and other observations. The model and the observations reveal buoyancy effects produced by tidal forcing. The semi‐diurnal tide raises the isopycnals over the sills at the head of the Laurentian Trough and English Bank, producing internal tides radiating seaward. Relatively dense intermediate waters rise from below 75‐m depth to the near surface over the sills, setting up gravity currents on the inner slopes. Internal hydraulic controls develop over the outer sills; during flood, surface flow separation occurs at the entrances of the Saguenay Fjord and the upper estuary west of Ilet Rouge Bank. Early during ebb flow (restratification), the surface layer deepens to encompass the tops of the sills. As the ebb current intensifies, the model predicts the formation of seaward internal jumps over the outer sills, which were confirmed from acoustic reflection observations. As the internal Froude number increases further, flow separation migrates up to sill height. As a result of these transitions, internal bores emanate from the head region one to two hours before low water. We find that the mixing of oceanic and surface waters near the sills is driven by the vertical shear produced during ebb in the channel south of Ilet Rouge, the shear produced in the bottom gravity flood currents, and, to a lesser extent, the processes over the sills.

Impacts of Climate Change in the Gulf of St. Lawrence

ABSTRACT To explore modifications in water temperature and salinity under warmer climate change conditions, we performed simulations from 1970 to 2069 with the CANadian Océan PArallélisé (CANOPA) model for the Gulf of St. Lawrence and the Scotian Shelf. The surface fields to drive CANOPA were provided by the Canadian Regional Climate Model (CRCM), driven by the outputs from the third-generation Canadian Global Climate Model (CGCM3) following the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1B climate change scenario. The sea-ice concentration and volume simulated by CANOPA are shown to have patterns consistent with those seen in observations; CANOPA is also shown to simulate sea surface temperature (SST) well. Although CANOPA can simulate the observed vertical structure of water temperature and salinity, it tends to underestimate the cold intermediate layer and overestimate water salinity in the central Gulf of St. Lawrence (GSL). In terms of the possible future climate, CANOPA simulations suggest that the GSL will be largely ice free in January, with ice volume in March steadily decreasing from about 80 km3 in the 1980s to near zero by the late 2060s. On average, the GSL water will become warmer and fresher over this time period. In January, maximum SST increases occur near eastern Cabot Strait, with amplitudes of about 1.5°–2.5°C, corresponding to reduced sea ice in that area, and there is no notable change along the western and northern coasts of the GSL. In July, maximum SST increases occur over the western GSL corresponding to the largest increases in surface air temperature in the region. The maximum decreases in surface salinity also occur near western coastal areas and the Scotian Shelf, whereas reductions in the eastern GSL are relatively weak. Finally, compared with the present climate, the cold intermediate layer is significantly weaker in 2040–2069 than in 1980–2009.

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.

Simulation of Circulation and Ice over the Newfoundland and Labrador Shelves: The Mean and Seasonal Cycle

Abstract A three dimensional ice–ocean coupled model with a 7 km horizontal resolution has been developed to examine spatial and seasonal variability of hydrography and circulation over the Newfoundland and Labrador Shelves. Daily atmospheric forcing is applied and monthly open boundary forcing is prescribed based on a global ocean assimilation model. Monthly mean results averaged over a simulation period from 1979 to 2010 are evaluated using a variety of temperature, salinity, current, and ice observations. In comparison with observations and previous model results, the present model shows good skill in simulating the inshore and shelf-edge Labrador Current. The model temperature and salinity agree well with observations. Model sea-ice extent compares well with observations. The model mean transport is approximately 7.5 and 0.7 Sv (Sv = 106 m3 s−1) for the shelf-edge and inshore branches of the Labrador Current, respectively, consistent with observational estimates. The modelled total transport from the coast to the central Labrador Sea is 27.5 Sv from June to September, in good agreement with the observational estimate. The seasonal range for the shelf-edge and inshore branches is 2.0 and 0.6 Sv, respectively, strong in winter and fall and weak in spring and summer. The model mean freshwater transport at the Seal Island and Flemish Cap transects is 0.12 and 0.14 Sv, respectively, consistent with observational estimates, and the range of the seasonal freshwater transport is 0.09 Sv and 0.04 Sv for each transect, respectively, which is approximately in phase with the volume transport.

Projected Changes in Surface Air Temperature and Surface Wind in the Gulf of St. Lawrence

Abstract The impacts of climate change on surface air temperature (SAT) and winds in the Gulf of St. Lawrence (GSL) are investigated by performing simulations from 1970 to 2099 with the Canadian Regional Climate Model (CRCM), driven by a five-member ensemble. Three members are from Canadian Global Climate Model (CGCM3) simulations following scenario A1B from the Intergovernmental Panel on Climate Change (IPCC); one member is from the Community Climate System Model, version 3 (CCSM3) simulation, also following the A1B scenario; and one member is from the CCSM4 (version 4) simulation following the Representative Concentration Pathway (RCP8.5) scenario. Compared with North America Regional Reanalysis (NARR) data, it is shown that CRCM can reproduce the observed SAT spatial patterns; for example, both CRCM simulations and NARR data show a warm SAT tongue along the eastern Gulf; CRCM simulations also capture the dominant northwesterly winds in January and the southwesterly winds in July. In terms of future climate scenarios, the spatial patterns of SAT show plausible seasonal variations. In January, the warming is 3°–3.5°C in the northern Gulf and 2.5°–3°C near Cabot Strait during 2040–2069, whereas the warming is more uniform during 2070–2099, with SAT increases of 4°–5°C. In summer, the warming gradually decreases from the western side of the GSL to the eastern side because of the different heat capacities between land and water. Moreover, the January winds increase by 0.2–0.4 m s−1 during 2040–2069, related to weakening stability in the atmospheric planetary boundary layer. However, during 2070–2099, the winds decrease by 0.2–0.4 m s−1 over the western Gulf, reflecting the northeastward shift in northwest Atlantic storm tracks. In July, enhanced baroclinicity along the east coast of North America dominates the wind changes, with increases of 0.2–0.4 m s−1. On average, the variance for the SAT changes is about 10% of the SAT increase, and the variance for projected wind changes is the same magnitude as the projected changes, suggesting uncertainty in the latter.

Stochastic dispersal increases the rate of upstream spread: A case study with green crabs on the northwest Atlantic coast

Dispersal heterogeneity is an important process that can compensate for downstream advection, enabling aquatic organisms to persist or spread upstream. Our main focus was the effect of year-to-year variation in larval dispersal on invasion spread rate. We used the green crab, Carcinus maenas, as a case study. This species was first introduced over 200 years ago to the east coast of North America, and once established has maintained a relatively consistent spread rate against the dominant current. We used a stage-structured, integro-difference equation model that couples a demographic matrix for population growth and dispersal kernels for spread of individuals within a season. The kernel describing larval dispersal, the main dispersive stage, was mechanistically modeled to include both drift and settlement rate components. It was parameterized using a 3-dimensional hydrodynamic model of the Gulf of St Lawrence, which enabled us to incorporate larval behavior, namely vertical swimming. Dispersal heterogeneity was modeled at two temporal scales: within the larval period (months) and over the adult lifespan (years). The kernel models variation within the larval period. To model the variation among years, we allowed the kernel parameters to vary by year. Results indicated that when dispersal parameters vary with time, knowledge of the time-averaged dispersal process is insufficient for determining the upstream spread rate of the population. Rather upstream spread is possible over a number of years when incorporating the yearly variation, even when there are only a few “good years” featured by some upstream dispersal among many “bad years” featured by only downstream dispersal. Accounting for annual variations in dispersal in population models is important to enhance understanding of spatial dynamics and population spread rates. Our developed model also provides a good platform to link the modeling of larval behavior and demography to large-scale hydrodynamic models.

Past, Present, and Future: Performance of Two Bivalve Species Under Changing Environmental Conditions

Globally, the production of marine bivalves has been steadily increasing over the past several decades. As the effects of human population growth are magnified, bivalves help provide food security as a source of inexpensive protein. However, as climate change alters sea surface temperatures (SST), the physiology, and thus the survival, growth, and distribution of bivalves are being altered. Challenges with managing bivalves may become more pronounced, as the uncertainty associated with climate change makes it difficult to predict future production levels. Modelling techniques, applied to both climate change and bivalve bioenergetics, can be used to predict and explore the impacts of changing ocean temperatures on bivalve physiology, and concomitantly on aquaculture production. This study coupled a previously established high resolution climate model and two dynamic energy budget models to explore the future growth and distribution of two economically and ecologically important species, the eastern oyster (Crassotrea virginica), and the blue mussel (Mytilus edulis) along the Atlantic coast of Canada. SST was extracted from the climate model and used as a forcing variable in the bioenergetic models. This approach was applied across three discreet time periods: the past (1986-1990), the present (2016-2020), and the future (2046-2050), thus permitting a comparison of bivalve performance under different temporal scenarios. Results show that the future growth is variable both spatially and interspecifically. Modelling outcomes suggest that warming ocean temperatures will cause an increase in growth rates of both species as a result of their ectothermic nature. However, as the thermal tolerance of C. virginica is higher than M. edulis, oysters will generally outperform mussels. The predicted effects of temperature on bivalve physiology also provided insight into vulnerabilities (e.g. mortality) under future SST scenarios. Such information is useful for adapting future management strategies for both farmed and wild shellfish. Although this study focused on a geographically specific area, the approach of coupling bioenergetic and climate models is valid for species and environments across the globe.

Climate Change on Newfoundland and Labrador Shelves: Results From a Regional Downscaled Ocean and Sea-Ice Model Under an A1B Forcing Scenario 2011–2069

ABSTRACT Climate change may affect ocean and ice conditions in coastal oceans and thus have significant impacts on coastal infrastructure, marine navigation, and marine ecosystems. In this study a three-dimensional ice–ocean model is developed to examine likely changes of ocean and ice conditions over the Newfoundland and Labrador Shelves in response to climate change. The model is configured with a horizontal grid of approximately 7 km and a vertical grid of 46 levels and is run from 1979 to 2069. The projection period is 2011 to 2069 under a median emission scenario A1B used by the Intergovernmental Panel on Climate Change. For the projection period, the surface atmospheric forcing fields used are from the Canadian Regional Climate Model over the North Atlantic. The open boundary conditions come from the Canadian Global Climate Model, Version 3 (CGCM3), adjusted for the 1981–2010 mean of the Simple Ocean Data Assimilation model output. The simulated fields over the 1981–2010 period have patterns consistent with observations. Over the Newfoundland and Labrador Shelves during the projection period, the model shows general trends of warming, freshening, and decreasing ice. From 2011 to 2069, the model projects that under A1B sea surface temperature will increase by 1.4°C; bottom temperature will increase by 1.6°C; sea surface salinity will decrease by 0.7; bottom salinity will decrease by 0.3; and sea-ice extent will decrease by 70%. The sea level will rise by 0.11 m at the St. Johns tide-gauge station because of oceanographic change, and the freshwater transport of the Labrador Current will double as a result of freshening. The regional ice–ocean model reproduces more realistic present climate conditions and projects considerably different future climate conditions than CGCM3.