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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
A submesoscale modelling approach to understanding the past, present, and future carbonate chemistry balance of the Salish Sea (2023)
Over the past 250 years, atmospheric levels of carbon dioxide (CO₂) have increased at an unprecedented rate. The ocean has absorbed a significant proportion of this anthro- pogenic carbon, resulting in changes to marine chemistry that negatively impact a wide variety of ocean organisms. Disproportionately productive coastal systems may be par- ticularly vulnerable to these changes; however, the detection and impact of secular carbon trends in these systems is complicated by heightened natural variability as compared to open-ocean regimes and is comparatively poorly understood. This dissertation inves- tigates the changing carbonate chemistry of the Salish Sea, a representative Northeast Pacific coastal system.Both physical-oceanographic factors and primary productivity exert a control on the inorganic carbon balance. I first investigate their interaction in the Salish Sea by apply- ing a clustering method to four factors relating to stratification and to depth-integrated phytoplankton biomass extracted from an existing biophysical model, finding coherent biophysical provinces. I then develop and evaluate a carbonate chemistry model for the Salish Sea and use it to evaluate biogeochemical changes between the pre-industrial and modern periods. I find that to date, the increase in inorganic carbon in the Salish Sea has been 29-39 mmol m−3, a modest amount relative to other parts of the global ocean. However, because of the naturally high inorganic carbon content of Pacific waters, this increased carbon drives the estuary towards domain-wide undersaturation of aragonite year-round, negatively impacting shell-forming organisms. I then use a global database of coastal carbonate chemistry observations to show that estuaries throughout the Pacific Rim have likely already undergone a similar saturation state regime shift. I then quan- tify the relative components of the Salish Sea carbonate chemistry budget in the context of the identified biophysical provinces. I note the dominant role and large variability of lateral boundary fluxes in setting the inorganic carbonate chemistry of the system. Fi- nally, I estimate that future anthropogenic carbon increase in the system by year 2050, under a conservative emissions scenario, will be approximately 80% of the increase from pre-industrial to present, and that this increase will have further significant effects on aragonite saturation states.
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Tracing ocean pathways : modelling manganese and lead in the Canadian Arctic (2023)
Human-induced climate change is rapidly altering the Arctic sea ice regime, ocean dynamics, and freshwater cycle. These changes impact biogeochemical cycling, including the cycling of trace elements, but the exact manifestations remain unclear. Over the past decade, the international GEOTRACES program has greatly expanded the coverage of trace element observations in the Arctic Ocean, capturing the current state of the system. These observations, in conjunction with improvements in model representations, allow the development of trace element models to investigate drivers of the spatial distribution and seasonal variability of trace element concentrations, and to estimate sensitivity of trace element cycling to climate change.This dissertation describes some of the first three-dimensional models of the dissolved micronutrient manganese (Mn) and pollutant lead (Pb) in the Canadian Arctic Ocean, including the Canada Basin, the Canadian Arctic Archipelago (CAA), and Baffin Bay. I highlight sources that control the distributions of Mn and Pb in this region, and use Mn and Pb to trace sea ice sediments and river runoff, and Atlantic Water. The Mn model highlights the significance of the long-range transport of sediments by sea ice for micronutrients such as Mn in the Canada Basin. The disruption of the transpolar sea ice drift could reduce Mn supply to the Canada Basin and downstream. Sensitivity experiments varying the Mn content in runoff identify distinct continental and glacial runoff fingerprints of influence in the southwestern and northern CAA, respectively. Glacial runoff carries micronutrients southward from Nares Strait in the late summer and may help support longer phytoplankton blooms in the Pikialasorsuaq Polynya. The Pb model illustrates the continued impact of anthropogenic pollution on Pb concentrations in the Arctic through aerosol deposition, boundary transport, and, likely, river runoff and sediment resuspension. The Labrador Sea is a net source of Pb to Baffin Bay via the West Greenland Current and Pb highlights pathways of Atlantic Water in Baffin Bay.The model results presented in this thesis highlight trace element concentrations and supply mechanisms in the Arctic Ocean and their sensitivity to climatic changes, and illustrates the use of trace element models in extending knowledge gained from observations.
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Wind-driven upwelling and nutrient supply in a productive estuarine sea (2022)
Marine ecosystems are under increasing pressure due to climate change. Wind-driven circulation in the upper ocean is one of the primary ways that climate determines ecosystem behavior. One example is wind-driven upwelling of nutrients to the euphotic zone. In small enclosed seas where stratification is often strong and wind sporadic, the significance of nutrient upwelling is not well-established. These small systems can be important habitats for juvenile migrating fish such as Pacific salmon. In this thesis, wind-driven upwelling and the effect on surface nitrate availability were investigated in the Strait of Georgia on the Canadian Pacific coast using a high-resolution, coupled biophysical ocean model. This investigation was conducted in three parts. First, the model skill was evaluated against observations from local monitoring programs, and the sensitivity of tuning the surface wave breaking parameterization using wave model results from the region was tested. Second, principal component analysis (PCA) was performed on five years of modelled hourly surface nitrate and temperature fields to identify the wind-driven upwelling modes and determine their significance relative to other processes. Spectral analysis of the principal component (PC) loadings and correlations between the PC loadings and the surface wind stress were used to attribute the dominant PCA modes to physical phenomena. Third, depth of upwelling estimates from cross-shore density transects during a comprehensive set of wind events across the five-year simulation were compared to a theoretical cross-shore upwelling model depending only on wind stress, stratification and cross-shore bottom slope to identify the physical parameters that control upwelling. Upwelling accounted for approximately one-third of summer surface nitrate variance based on the PCA results. Modelled upwelling depth generally agreed with the theoretical prediction and was thus interpreted to depend primarily on wind stress and stratification. The deepest upwelling and strongest nitrate anomalies occurred in the northern Strait of Georgia, which is consistent with the presence of an along-axis stratification gradient due to the Fraser River. These results establish a direct link between climate forcing and the factors that determine upwelling in the Strait of Georgia, with strong implications for summer productivity in the region.
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Transport through submarine canyons (2020)
Exchanges of water, nutrients, and oxygen between coastal and open ocean are key componentsof on-shelf nutrient budgets and biogeochemical cycles. Submarine canyons are underwater topographic features that incise the continental shelf and enhance physical processes such as cross-shelf mass exchanges and mixing. There is a good understanding of the flow around upwelling submarine canyons; however, the flux of biologically relevant tracers and the collective impact of canyons is less well understood. This dissertation investigates the collective impact of submarine canyons on cross-shelf exchange of tracers and water and their distribution on the shelf, taking into account the impact of locally-enhanced mixing within the canyon and the initial geometry of the tracer profile. I performed numerical experiments, using the Massachusetts Institute of Technology general circulation model (MITGCM), simulating an upwelling event near an idealized canyon. To investigate the role of mixing I added a passive tracer with an initially linear profile, varying the spatial distribution of vertical eddy diffusivity and its magnitude. To investigate the impact of the initial tracer profile I added 10 passive tracers with initial profiles representing nutrients, carbon and dissolved gasses. I found that locally enhanced vertical diffusivity has a positive effect on the tracer that is advected by the upwelling flow and can significantly increase canyon-upwelled tracer flux; tracer flux also depends on the initial vertical tracer gradient within the canyon, the depth of upwelling and the upwelling flux. I identified a pool of low oxygen and high nutrient, methane, dissolved inorganic carbon and total alkalinity concentrations on the shelf bottom, downstream of the canyon. The horizontal extent of the pool depends on the canyon-induced advective fluxes feeding the pool and the initial background distribution of tracers on the shelf. The interaction between two identical canyons during an upwelling event was investigated using a laboratory model on a rotating tank. I found that canyons are primarily independent for the parameter regime explored but may interact through the arrival of the upstream canyon’s pool to the downstream canyon’s head.
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Changes to the near surface waters of the Canada Basin, Arctic Ocean from 1993-2009 : an examination of the consequences of warming and freshening to the water mass structure and optical environment (2011)
The near-surface water mass structure in the Canada Basin of the Arctic Ocean was examined from 1993 through 2009. This was a period of rapid change due to warming air and ocean temperatures and subsequent sea ice melt. During this time, the Southern Canada Basin transitioned from a perennially ice-covered to an almost seasonally ice-free ocean. It was found that the freshwater from sea ice melt increased the near-surface stratification. Solar radiation was stored below the surface mixed layer as a near-surface temperature maximum (NSTM). From 1993-2009, the NSTM warmed by up to 1.5◦C, freshened by up to 4 salinity units, expanded northwards, and formed at successively shallower depths. Below the NSTM is a temperature minimum identified as the remnant of the previous winter’s surface mixed layer (rML). Similar to the NSTM, the rML warmed by up to 0.5◦C and freshened by up to 2 salinity units from 1993-2009. Using a 1-D model of heat diffusion, it was found that heat from both the NSTM and Pacific Summer Water (PSW) is diffused to the rML. In warmer years, more heat was diffused to the rML from the NSTM. The freshening of both the NSTM and rML was greatest at stations that were located inside the anticyclonic Beaufort Gyre and this is likely because downwelling caused freshwater from sea ice melt to accumulate inside the gyre.An examination of light attenuation to estimate suspended particle concentrations identified six common attenuation features. These features were at the surface, within the summer halocline, within water that has high fluorescence, within cold water that had the salinity range 32.9 - 33.1, within Atlantic water, and at the bottom. It was found that there was no evidence of increased particle concentrations in the basin from 2003-2008. However, the chlorophyll maximum inside the basin deepened from an average of 45 m in 2003 to 61 m in 2008 and it is likely that this is because the nutricline also descended. The deepening of the chlorophyll maximum is one example of how changes to the near-surface water mass structure from climate change can impact the Arctic Ocean ecosystem.
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Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Pacific sources of biologically significant constituents in the Salish Sea using Lagrangian particle tracking (2023)
The Salish Sea is a semi-enclosed coastal sea between Vancouver Island and the coast of British Columbia and Washington State, invaluable from both an economic and ecologic perspective. Pacific inflow to the Sea is the main contributor of many biologically important constituents. The contribution of Pacific water masses to the flow through Juan de Fuca Strait (JdF), the Salish Sea’s primary connection to the Pacific Ocean, is explored. Quantitative Lagrangian particle tracking within Ariane, an offline Lagrangian tool capable of volume transport calculations, was applied to two numerical ocean models to track the paths and properties of water parcels before entering JdF and within the Salish Sea. The Coastal Ice Ocean Prediction System (CIOPS) for the west coast was used to track water parcels from JdF backwards in time to analyse their paths on the shelf and offshore before entering the Salish Sea, while SalishSeaCast was used to track water parcels forwards in time to assess the success of the water masses identified in the CIOPS analysis at reaching the Sea’s inner basins as opposed to being advected back out to the shelf region. During summer upwelling, intermediate flow from the north shelf and offshore dominate inflow, while during winter downwelling, intermediate flow from the south shelf and surface flow from the Columbia River plume are the dominant sources. A weaker and less consistent estuarine flow regime in the winter leads to less Pacific inflow overall and a smaller percentage of said inflow reaching the Salish Sea's inner basins than in the summer. Nevertheless, it was found that winter dynamics are the main driver of interannual variability, in part due to the strongly anti-correlated behaviour and distinct properties of the two dominant winter sources. This analysis extends the knowledge on the dynamics of Pacific inflow to the Salish Sea and highlights the importance of winter inflow to the interannual variability in biogeochemical conditions in the region.
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Characterizing the lowest oxygen waters on the southern continental shelf off Vancouver Island (2020)
A shortage of dissolved oxygen in seawater can adversely impact marine life and ecosystems. Coastal waters deeper than 100 m do not gain oxygen directly from the surface, and thus transient, seasonal, or permanent oxygen deficit conditions can occur in such deeper coastal waters. A low oxygen, dense pool of water is formed every summer over the mid-shelf off southwest Vancouver Island in the Juan de Fuca Eddy region, known for its high primary productivity. In this thesis, the waters of the dense pool are traced back to their source using Lagrangian Particle tracking and a regional numerical ocean model, and upwelling hotspots of deep water leading to the pool are discovered. The model accuracy in simulating the local circulation is evaluated based on a statistical skill score, root mean square error and bias upon comparing the model variables to the observations. The numerical model simulates the local circulation well, except it under-predicts the variation along isopycnals. Due to this lack of variation in the model, only the particles which agree well between the model results and the observations are selected using a K-Means clustering algorithm. Tracking these particles backwards in time showed that the dense pool under the Juan de Fuca Eddy is primarily composed of water from the California Undercurrent, water from Washington State shelf and offshore water. Signatures from mixing the source waters in proportion closely approximate the final signatures of water inside the dense pool. The investigation of upwelling hotspots revealed that 1) the dense pool water primarily upwells through Spur Canyon and the convoluted Juan de Fuca Canyon bathymetry near Swiftsure Bank, and 2) The southern side of Nitinat Canyon acts as a dominant upwelling site for water ending on the south-outer shelf of South Vancouver Island. This study helps in climate change predictions, as the pathways identified for different source water origins help to determine the change in source composition over time and understand the potential low dissolved oxygen implications over the Vancouver Island shelf, in Juan de Fuca Strait and in the Strait of Georgia.
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Circulation and upwelling in Mackenzie Canyon, a dynamically wide submarine canyon in the Beaufort Sea (2019)
Mackenzie Canyon, in the southeastern Beaufort Sea, is a site for strong upwelling compared to the adjacent continental shelf and slope and potentially supplies the shelf with significant levels of nitrate. Research regarding the circulation and upwelling mechanisms in submarine canyons has previously been limited to dynamically narrow canyons, and most studies have used numerical models with idealized bathymetry. The main goal of the study presented in this thesis is to describe the circulation and upwelling in Mackenzie Canyon, which is classified as a dynamically wide canyon. This study also identifies key flow features that act as significant modifiers of upwelling, examines differences between idealized and realistic model simulations, and estimates the canyon-induced upwelling of nitrate.To address these goals, the circulation and upwelling associated with an upwelling event induced by an impulsive wind forcing in Mackenzie Canyon was simulated using a nested-grid modelling system configuration based on the Nucleus for European Modelling of the Ocean framework. Numerical simulations were conducted using realistic and idealized bathymetry and three cases of wind stress forcing. The model performance was evaluated using observational data from Mackenzie Canyon during an upwelling event. This study finds that near-geostrophic flows are topographically steered around the Mackenzie Canyon walls. Strong cyclonic vorticity is generated on the upstream corner of the canyon mouth and evolves into a closed, cyclonic eddy, which becomes a site for strong upwelling. A coastal trapped wave (CTW) is induced on the downstream side of the canyon and propagates upstream. It is characterized as a shelf wave using a model that searches for the free wave solutions of CTWs along straight coastlines. An upwelling signal in the canyon exits the canyon and propagates along the slope with the CTW. Unlike narrow canyons, upwelling in Mackenzie Canyon is stronger on the upstream side than on the downstream side, likely as a consequence of the upstream propagation of the CTW. The nitrate flux across the nitracline depth supplied by upwelling in Mackenzie Canyon during the initial 36 hours of an upwelling event is estimated to be twice the seasonal draw-down in the Beaufort Sea.
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A comprehensive simulation study of dissolved Barium and Oxygen isotope ratio in the Arctic Ocean (2017)
The Arctic Ocean freshwater plays important roles in regional and global climate. Dissolved Barium and the Oxygen isotope ratio are two tracers that provide key information on the river runoff and the sea-ice melt water as two Arctic Ocean freshwater components. In this research, an offline tracer model was developed with dissolved Barium and Oxygen isotope ratio modules and appropriate boundary conditions were applied to the Arctic Ocean to simulate the spatial and temporal variations of the two tracers. The tracer model was run from 2002 to 2013 after a 24-year spin-up. The simulation results show reasonable tracer climatology and seasonal cycles, agree well with field observations and the Arctic freshwater cycle. The tracer model was applied to investigate the atmospheric driven freshwater variabilities in the upper 130m through linear trend and Empirical Orthogonal Function (EOF) analysis. The linear trend result shows the increase in the transport of Eurasian runoff from the Makarov Basin to the Beaufort Sea and concurrent with the increase in the winter-spring Arctic Oscillation (AO). The three EOF modes show the role of the dipole anomaly, the interannual impact of the North Atlantic Oscillation (NAO) and the Beaufort Sea anticyclonic anomalous wind, respectively on changing the pathway of the high Barium concentration North American runoff and the impact of the Eurasian runoff along the continental shelves and in the central Arctic. A case study of the Beaufort Gyre freshwater in 2007-2008 revealed the change of Eurasian runoff pathways in three stages with the dipole anomaly and the transport of Eurasian runoff in the developing stage, the strong anti-cyclonic wind in the Beaufort Sea in the mature stage and the weakening of the Beaufort Gyre in the final stage. A linear mixing model result confirms the increase of the Eurasian runoff in the Beaufort Gyre in the winter of 2007.
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Evaluation of a NEMO model of the Strait of Georgia and insights into mixing and transport of the Fraser River plume (2017)
The goals of this modeling study of the Fraser River plume, located in the Strait of Georgia,British Columbia, are twofold. Firstly, it aims at improving the Fraser River plume propertiesby evaluating the model results with various available observations. Secondly, mixing and transport processes within the plume, driven by different forcing factors, are investigated with theimproved configuration to understand the plume dynamics in the model. The problems foundby comparing with ferry-based salinity data, drifter data and CTD data in the modeled FraserRiver properties are: (1) too weak cross-strait velocities; (2) too strong along-strait flows; (3)too salty surface water. To fix the problems, a longer and deeper river channel was createdand added into the model. The results show promising improvements with stronger cross-straitmotions. Background vertical eddy viscosity was reduced from 1 x10-⁴to 1 x 10-⁵ m²s-¹, which tends to reduce the along-strait velocities. In addition, background vertical eddy diffusivitywas reduced to 1 x 10-⁶ m²s-¹ which reduced the surface salinity. Furthermore, effects of riverdischarge, tides, winds and the Coriolis force are explored on plume mixing and transport. Asexpected, plume size increases with increasing river outflow. Tides are important in mixing atthe river mouth and inside the river channel during low and moderate river flow periods withwind magnitude smaller than 5 m s-¹, whereas winds become the dominant factor in mixingover almost the entire plume domain when wind speed is greater than 5 m s-¹ . The Coriolisforce strengths the northward flux across a transect north of the river mouth when winds arenot strong, resulting in a fresher plume in English Bay, north of the City of Vancouver. Thisthesis provides both a guide to accurately modeling the Fraser River plume and insight into plume dynamics.
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Modeling 230Th (and 231Pa): as an approach to study the intermediate and deep water circulation in the Arctic Ocean (2017)
Recently observed ²³⁰Th concentrations in 2007 and 2009 documented very high ²³⁰Th values within the Atlantic layer in the Canada Basin of the Arctic Ocean. Similar levels of high ²³⁰Th had only been previously observed in the Alpha Ridge region, implying that the Alpha Ridge is the potential source of the high ²³⁰Th waters. As the Alpha Ridge is downstream in the classic cyclonic circulation, that circulation is believed to have changed. Motivated by this, a three-dimensional Arctic ²³⁰Th model is configured for the first time to study such change.To simulate the tracer, I coupled a scavenging model, which describes the exchange of ²³⁰Th (and ²³¹Pa) between the dissolved and particulate phases, to an offline NEMO model (the Nucleus for European Modelling of the Ocean) that provides the advection and mixing processes that redistribute the tracers within the ocean. As the scavenging rates of such tracer elements are strongly affected by oceanic particle concentrations, the scavenging rates are parameterized as a function of ice concentration, which, to a great extent, influences the biological processes in the water. Model output produced an increase of ²³⁰Th concentration in the south Canada Basin. Sensitivity experiments confirm such change is not caused by a change in the particle field but a change in the intermediate circulation from cyclonic to anticyclonic throughout the Amerasian Basin. This shift in circulation is the reason for a subsequent transport of high ²³⁰Th concentration from the Alpha Ridge to the south Canada Basin. The model circulation and density fields suggest that the change in the flow is caused by increased dense water flux into the Arctic Ocean, primarily through the Barents Sea route. This increase of dense water inflow alters the density distribution in the Arctic and results in a quick adjustment in the Atlantic layer (~1 year) through propagation of boundary trapped internal Kelvin waves.
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The Inorganic Carbonate Chemistry of the Southern Strait of Georgia (2015)
A one-dimensional, biophysical, mixing layer model was modified to hindcast pH and aragonite saturation state (OmegaA) in the southern Strait of Georgia. The model skill in predicting spring phytoplankton bloom timing in previous studies was a key factor in its selection. Dissolved inorganic carbon (DIC) and total alkalinity (TA) were added as state variables, coupled to the existing nitrogen-based biological equations. Additional processes determined to be important to the system such as air-sea gas exchange and nutrient-limited excess carbon uptake were also implemented. pH and OmegaA could then be calculated from DIC and TA. Modeled DIC, TA, pH, and OmegaA were evaluated against data collected between 2003 and 2012. Modeled and observed quantities agreed except in some summers, with surface disagreement driven primarily by plume variability and subsurface disagreement driven primarily by model overproductivity. Model outputs demonstrated a near-surface seasonal cycle characterized by low pH and OmegaA in winter and high pH and OmegaA in summer. In order to evaluate the sensitivity of model pH and OmegaA to local forcing quantities, the model was run in one year increments over the period from 2001 through 2012. For each year, each of three forcing records (wind speed, freshwater flux, cloud fraction) was shifted across all possible years during the same period for a total of 432 experimental runs. When regressed against spring wind speed, model surface pH demonstrated a clear, negative correlation. Model spring OmegaA demonstrated a negative correlation to cloud fraction. Summer pH and OmegaA were most sensitive to freshwater flux, both showing negative correlations. Model pH and OmegaA sensitivity to freshwater TA and pH were also evaluated over the same period using a set of realisitic freshwater chemistry scenarios determined from observations in the Fraser River. Model pH and OmegaA demonstrated opposite correlations to freshwater TA with sensitivities at opposite extremes of freshwater pH. The sensitivity results identify important links between local processes and the carbonate chemistry in the southern Strait of Georgia, and perhaps provide some simple forecasting tools to be tested in the future.
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Flow dynamics oaround downwelling submarine canyons (2014)
Flow dynamics around a downwelling submarine canyon were analyzed with the Massachusetts Institute of Technology general circulation model. Blanes Canyon (Northwest Mediterranean) was used for topographic and initial forcing conditions. Fourteen scenarios were modelled with varying forcing conditions. Rossby number and Burger number were used to determine the significance of Coriolis acceleration and stratification (respectively) and their impacts on flow dynamics. A new non-dimensional parameter (χ) was introduced to determine the significance of vertical variations in stratification. Downwelling (downwards advection of density) occurs under all forcing conditions and is enhanced within the canyon. High Burger numbers lead to negative vorticity and a trapped anticyclonic eddy within the canyon, as well as an increased density anomaly. Low Burger numbers lead to positive vorticity, cyclonic circulation and weaker density anomalies. Vertical variations in stratification affect zonal jet placement. Under the same forcing conditions, the zonal jet is pushed offshore in more uniformly stratified domains. Offshore jet location generates upwards density advection away from the canyon, while onshore jets generate downwards density advection everywhere within the model domain. Increasing Rossby values across the canyon axis, as well as decreasing Burger values, increase negative vertical flux at shelf break depth (150 m). Increasing Rossby numbers lead to stronger downwards advection of a passive tracer (nitrate). Comparisons were made to previous studies to explain how variations in initial forcing conditions impact regional flow dynamics.
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Modeling herring and hake larval dispersal in the Salish Sea (2013)
The Salish Sea includes Juan De Fuca Strait, Puget Sound, and the Strait of Georgia(SoG), which separates Vancouver Island from mainland British Columbia. Hake and herring arecommercially important fish and both species use SoG as larval rearing grounds. Drift tracks oflarvae for these species were simulated using a regional circulation model and a particle-trackingmodel, for up to six weeks after they hatch. Larvae with different behaviors (such as surfacedrifters or performing diel vertical migration) are traced in the springs of each of the years 2007,2008, and 2009. Since herring larvae stay in the top 12m, their distribution is heavily influencedby the wind storms. Strong winds to the north during the hatching period wash herring larvae outof SoG and lead to poor recruitment later. Alternatively, wind storms blowing to the south helpretain herring larvae in the Salish Sea. Northern and southern parts of SoG are weakly connectedfor herring larvae. Hake larvae reside deeper in the water column (50-200m) and the distributionof the hake larvae released in the central SoG is shaped by a deep gyre with cross-strait currents.Behavior changes distribution for both types of larvae but there is no single pattern. Behaviormay enhance retention in SoG for the northern herring larvae. This study helps to identifyimportant herring larvae habitat in the Strait of Georgia.
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Physical modelling of tidal resonance in a submarine canyon (2013)
The Gully, Nova Scotia (44 degrees N) is unique amongst studied submarine canyons poleward of 30 degrees due to the dominance of the diurnal (K₁) tidal frequency, which is subinertial at these latitudes. Length scales suggest the diurnal frequency may be resonant in the Gully. A physical model of the Gully was constructed in a tank and tidal currents were observed using a rotating table. Resonance curves were fit to measurements in the laboratory canyon for a range of stratifications, background rotation rates and forcing amplitudes. Resonant frequency increased with increasing stratification and was not affected by changing background rotation rates, as expected. Dense water was observed upwelling onto the continental shelf on either side of the laboratory canyon and travelled at least one canyon width along the shelf. Most of this upwelled water was pulled back into the canyon on the second half of the tidal cycle. Friction values measured in the laboratory were much higher than expected, possibly due to upwelled water surging onto the shelf on each tidal cycle, similar to a tidal bore. By scaling observations from the laboratory to the ocean and assuming friction in the ocean is also affected by water travelling onto the shelf, a resonance curve for the Gully was created. Resonance curves explain why the diurnal frequency dominates over the semi-diurnal (M₂) frequency throughout the year at the Gully, even if stratification at the shelf break varies.
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Impact of wind and river flow on the timing of the Rivers Inlet spring phytoplankton bloom (2010)
The primary objective of this masters study is to develop an understanding of the physical processes driving the timing of the spring phytoplankton bloom in Rivers Inlet. The spring bloom is initiated as light limitation is lifted causing an increase in growth which overcomes loses due to grazing and advection. The bloom is terminated by nitrate exhaustion. The physical system can impact the spring bloom through variations of winds, cloud coverage, and river input. Strong winds showed two effects. First, strong winds increased the mixing layer depth which decreased the amount of light available for phytoplankton, thus delaying the timing of the spring bloom. Second, large outflow winds caused flushing events to occur resulting in rapid horizontal advection removing the plankton population from the area. River discharge has two opposite effects on the timing of the spring bloom. High river discharge causes the water column to stratify, reducing the mixing layer depth which provides more light available for growth and results in an earlier bloom. High discharge will also result in higher upwelling advection leading to a larger advective loss term for phytoplankton, delaying the bloom. Changes in cloud coverage will directly affect the incoming solar radiation and the light available for photosynthesis.A coupled bio-physical model is used to explore the driving forces involved in the timing of the spring phytoplankton bloom in Rivers Inlet, British Columbia, Canada.The primary control on the timing of the spring bloom in Rivers Inlet is wind speed and direction. Secondary control on the timing is due to freshwater flow; high river discharge delays the bloom in Rivers Inlet. Single outflow wind events can result in a 7 day delay in the bloom timing. The shift in bloom timing resulting from multiple outflow wind events is greater than the sum of the individual wind events. Implications of flushing events in fjords along the British Columbia coastline are also discussed.
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