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  • 1
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    Vertical distribution of buoyant Microcystis blooms in a Lagrangian particle tracking model for short-term forecasts in Lake Erie (2016)
    Wiley
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    Publication Date: 2016-06-28
    Description: Cyanobacterial harmful algal blooms (CHABs) are a problem in western Lake Erie, and in eutrophic fresh waters worldwide. Western Lake Erie is a large (3000 km 2 ), shallow (8 m mean depth), freshwater system. CHABs occur from July to October, when stratification is intermittent in response to wind and surface heating or cooling (polymictic). Existing forecast models give the present location and extent of CHABs from satellite imagery, then predict two-dimensional (surface) CHAB movement in response to meteorology. In this study, we simulated vertical distribution of buoyant Microcystis colonies, and 3D advection, using a Lagrangian particle model forced by currents and turbulent diffusivity from the Finite Volume Community Ocean Model (FVCOM). We estimated the frequency distribution of Microcystis colony buoyant velocity from measured size distributions and buoyant velocities. We evaluated several random-walk numerical schemes to efficiently minimize particle accumulation artifacts. We selected the Milstein scheme, with linear interpolation of the diffusivity profile in place of cubic splines, and varied the time step at each particle and step based on the curvature of the local diffusivity profile to ensure that the Visser time step criterion was satisfied. Inclusion of vertical mixing with buoyancy significantly improved model skill statistics compared to an advection-only model, and showed greater skill than a persistence forecast through simulation day 6, in a series of 26 hindcast simulations from 2011. The simulations and in-situ observations show the importance of subtle thermal structure, typical of a polymictic lake, along with buoyancy in determining vertical and horizontal distribution of Microcystis . This article is protected by copyright. All rights reserved.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    The characteristics and dynamics of wave-driven flow across a platform coral reef in the Red Sea (2016)
    Wiley
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    Publication Date: 2016-01-30
    Description: Current dynamics across a platform reef in the Red Sea near Jeddah, Saudi Arabia are examined using 18 months of current profile, pressure, surface wave, and wind observations. The platform reef is 700 m long, 200 m across with spatial and temporal variations in water depth over the reef ranging from 0.6 m to 1.6 m. Surface waves breaking at the seaward edge of the reef cause a 2 −10 cm setup of sea level that drives cross-reef currents of 5 – 20 cm s −1 . Bottom stress is a significant component of the wave setup balance in the surf zone. Over the reef flat, where waves are not breaking, the cross-reef pressure gradient associated with wave setup is balanced by bottom stress. The quadratic drag coefficient for the depth-average flow decreases with increasing water depth from C da =0.17 in 0.4 m of water to C da =0.03 in 1.2 m of water. The observed dependence of the drag coefficient on water depth is consistent with open channel flow theory and a hydrodynamic roughness of z o =0.06 m. A simple one-dimensional model driven by incident surface waves and wind stress accurately reproduces the observed depth-averaged cross-reef currents and a portion of the weaker along-reef currents over the focus reef and two other Red Sea platform reefs. The model indicates the cross-reef current is wave-forced and the along-reef current is partially wind-forced. This article is protected by copyright. All rights reserved.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Turbulence Observations beneath Larsen C Ice Shelf, Antarctica (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Increased ocean‐driven basal melting beneath Antarctic ice shelves causes grounded ice to flow into the ocean at an accelerated rate, with consequences for global sea level. The turbulent transfer of heat through the ice shelf‐ocean boundary layer is critical in setting the basal melt rate, yet the processes controlling this transfer are poorly understood and inadequately represented in global climate models. This creates large uncertainties in predictions of future sea‐level rise. Using a hot‐water drilled access hole, two turbulence instrument clusters (TICs) were deployed 2.5 and 13.5 meters beneath Larsen C Ice Shelf in December 2011. Both instruments returned a year‐long record of turbulent velocity fluctuations, providing a unique opportunity to explore the turbulent processes within the ice shelf‐ocean boundary layer. Although the scaling between the turbulent kinetic energy (TKE) dissipation rate and mean flow speed varies with distance from the ice shelf base, at both TICs the TKE dissipation rate is balanced entirely by the rate of shear production. The freshwater released by basal melting plays no role in the TKE balance. When the upper TIC is within the log‐layer, we derive an under‐ice drag coefficient of 0.0022 and a roughness length of 0.44 mm, indicating that the ice base is smooth. Finally, we demonstrate that although the canonical three‐equation melt rate parameterization can accurately predict the melt rate for this example of smooth ice underlain by a cold, tidally‐forced boundary layer, the law of the wall assumption employed by the parameterization does not hold at low flow speeds.
    Print ISSN: 2169-9275
    Electronic ISSN: 2169-9291
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Coral Reef Calcification and Production After the 2016 Bleaching Event at Lizard Island, Great Barrier Reef (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Severe coral bleaching events have affected the GBR causing massive losses of hard coral cover. Here we use flow respirometry approaches to assess coral reef net ecosystem calcification (NEC) and net ecosystem production following the 2015/2016 bleaching event at Lizard Island in the northern Great Barrier Reef, a heavily impacted area. Previous studies conducted in 2008 and 2009 (Silverman et al., 2014, http://10.1016/j.gca.2014.09.011) were used as preimpact data. Lagrangian and Eulerian approaches provided varied results. Estimated NEC (29.1 to 137.7 mmol m−2 day−1) and NEP (−876.7 to 50.5 mmol m−2 day−1) rates in 2016 were highly sensitive to assumptions about reef water residence times and oceanic end‐member concentrations. Replicating the methodology used for the 2008 and 2009 study resulted in postbleaching NEC in 2016 of 32 ± 10.8 mmol m−2 day−1, 40%–46% lower than prebleaching estimates in 2008 (61 ± 12 mmol m−2 day−1) and 2009 (54 ± 13 mmol m−2 day−1). The slopes of the total alkalinity versus dissolved inorganic carbon plot decreased from ~ 0.3 in 2008 and 2009 to 0.1 in 2016, indicating elevated organic production and a shift in community function. Changes in NEC relative to the previous study were not driven by changing Ωarag. Coral cover shifted from 8.3% and 7.1% in 2008 and 2009 to 3.0% in 2016. We demonstrate a clear decrease in coral reef NEC following bleaching and highlight that subtle assumptions/methodological differences may create bias in the interpretation of results. Therefore, comparing coral reef metabolism data sets and predicting long‐term coral reef calcification based on existing short‐term data sets needs to be done with care.
    Print ISSN: 2169-9275
    Electronic ISSN: 2169-9291
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Propagation and Vertical Structure of the Tidal Flow in Nares Strait (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract The southward freshwater flux through Nares Strait is an important component of the Arctic's freshwater budget. On short time scales, flow through the strait is dominated by the tides, and tidal dynamics may be important for the magnitude of the freshwater flux over longer periods. Here we build upon our existing knowledge of the tides in the region by exploring their propagation and vertical structure using data from four bottom‐mounted Acoustic Doppler Current Profilers deployed in Nares Strait between 2003 and 2006. We observe that propagating barotropic semidiurnal tidal waves interact to create a standing wave pattern, explaining the abnormally large tidal amplitudes that are observed in this region. In the along‐strait direction, semidiurnal tidal currents exhibit strong variations with depth. In contrast, the diurnal tides propagate northward through the strait as progressive waves, and the tidal currents are broadly depth invariant. Proximity of Nares Strait to the semidiurnal critical latitude and the topographical restriction imposed by the steep side wall of Ellesmere Island are primary drivers behind the observed vertical variability. In the upper part of the water column, baroclinic activity increases the tidal current amplitude by up to 25%. In the across‐strait direction, a two‐layer structure exists in both the diurnal and semidiurnal tidal flow, with a phase lag of approximately a quarter of a tidal cycle across the strait for the semidiurnal tide. Our results suggest that strong vertical motion exists against the side walls of Nares Strait, as the across‐strait flow interacts with the steeply sloping bathymetry.
    Print ISSN: 2169-9275
    Electronic ISSN: 2169-9291
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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