Susan Elizabeth Allen

 
Prospective Graduate Students / Postdocs

This faculty member is currently not looking for graduate students or Postdoctoral Fellows. Please do not contact the faculty member with any such requests.

Professor

Research Classification

Oceans and Inland Waters
Prediction and Climatic Modeling

Research Interests

physical oceanography
coastal oceanography
forecast models
coupled bio-physics and chem-physics and all three models

Relevant Degree Programs

 

Research Methodology

numerical modelling
laboratory fluid modelling

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
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 (2010 - 2018)
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 around 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.Supplementary video materials available at: http://hdl.handle.net/2429/43966

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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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