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  • 1
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    Mesoscale eddies and Trichodesmium spp. distributions in the southwestern North Atlantic (2015)
    Wiley
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    Publication Date: 2015-05-06
    Description: Correlations of Trichodesmium colony abundance with the eddy field emerged in two segments of Video Plankton Recorder observations made in the southwestern North Atlantic during fall 2010 and spring 2011. In fall 2010, local maxima in abundance were observed in cyclones. We hypothesized surface Ekman transport convergence as a mechanism for trapping buoyant colonies in cyclones. Idealized models supported the potential of this process to influence the distribution of buoyant colonies over timescales of several months. In spring 2011, the highest vertically integrated colony abundances were observed in anticyclones. These peaks in abundance correlated with anomalously fresh water, suggesting riverine input as a driver of the relationship. These contrasting results in cyclones and anticyclones highlight distinct mechanisms by which mesoscale eddies can influence the abundance and distribution of Trichodesmium populations of the southwestern North Atlantic. 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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    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.
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    Topics: Geosciences , Physics
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  • 3
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    Surface gravity wave transformation across a platform coral reef in the Red Sea (2015)
    Wiley
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    Publication Date: 2015-12-19
    Description: The transformation of surface gravity waves across a platform reef in the Red Sea is examined using eighteen months of observations and a wave transformation model developed for beaches. The platform reef is 200 m across, 700 m long and the water depth varies from 0.3 to 1.2 m. Assuming changes in wave energy flux are due to wave breaking and bottom drag dissipation, the wave transformation model with optimal parameters characterizing the wave breaking (γ μ =0.25) and bottom drag (hydrodynamic roughness z o =0.08 m) accounts for 75% - 90% of the observed wave-height variance at four sites. The observations and model indicate that wave breaking dominates the dissipation in a 20 – 30 m wide surf zone while bottom drag dominates the dissipation over the rest of the reef. Friction factors (drag coefficients) estimated from the observed wave energy balance range from f w =0.5 to f w =5 and increase as wave-orbital displacements decrease. The observed dependence on wave-orbital displacement is roughly consistent with extrapolation of an empirical relationship based on numerous laboratory studies of oscillatory flow. As a consequence of the dependence on wave-orbital displacement, wave friction factors vary temporally due to changes in water depth and incident wave heights, and spatially across the reef as the waves decay. This article is protected by copyright. All rights reserved.
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  • 4
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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.
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  • 5
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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
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  • 6
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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
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  • 7
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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.
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  • 8
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    The origins of the anomalous warming in the California coastal ocean and San Francisco Bay during 2014-2016 (2017)
    Wiley
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    Publication Date: 2017-08-26
    Description: During 2014 exceptionally warm water temperatures developed across a wide area off the California coast and within San Francisco Bay (SFB) and persisted into 2016. Observations and numerical model output are used to document this warming and determine its origins. The coastal warming was mostly confined to the upper 100 meters of the ocean and was manifested strongly in the two leading modes of upper ocean (0-100 m) temperature variability in the extra-tropical eastern Pacific. Observations suggest that the coastal warming in 2014 propagated into nearshore regions from the west while later indicating a warming influence that propagated from south to north into the region associated with the 2015-16 El Niño event. An analysis of the upper ocean (0-100 m) heat budget in a Regional Ocean Modeling System simulation confirmed this scenario. The results from a set of sensitivity runs with the model in which the lateral boundary conditions varied supported the conclusions drawn from the heat budget analysis. Concerning the warming in the SFB, an examination of the observations and the heat budget in an unstructured-grid numerical model simulation suggested that the warming during the second half of 2014 and early 2016 originated in the adjacent California coastal ocean and propagated through the Golden Gate into the Bay. The finding that the coastal and Bay warming are due to the relatively slow propagation of signals from remote sources raises the possibility that such warming events may be predictable many months or even several seasons in advance.
    Print ISSN: 0148-0227
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  • 9
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    Spatial and Temporal Patterns of Chlorophyll Concentration in the Southern California Bight (2017)
    Wiley
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    Publication Date: 2017-12-28
    Description: Distinguishing between local, anthropogenic nutrient inputs and large-scale climatic forcing as drivers of coastal phytoplankton biomass is critical to developing effective nutrient management strategies. Here we assess the relative importance of these two drivers by comparing trends in chlorophyll- a between shallow coastal (0.1–16.5 km) and deep offshore (17–700 km) areas, hypothesizing that coastal regions influenced by anthropogenic nutrient inputs may have different spatial and temporal patterns in chlorophyll- a concentration from offshore regions where coastal inputs are less influential. Quarterly conductivity-temperature-depth (CTD) fluorescence measurements collected from three southern California continental shelf regions since 1998 were compared to chlorophyll- a data from the more offshore California Cooperative Fisheries Investigations (CalCOFI) program. The trends in the coastal zone were similar to those offshore, with a gradual increase of chlorophyll-a biomass and shallowing of its maximum layer since the beginning of observations, followed by chlorophyll- a declining and deepening from 2010 to present. An exception was the northern coastal part of SCB, where chlorophyll- a continued increasing after 2010. The long-term increase in chlorophyll- a prior to 2010 was correlated with increased nitrate concentrations in deep waters, while the recent decline was associated with deepening of the upper mixed layer, both linked to the low-frequency climatic cycles of the Pacific Decadal Oscillation and North Pacific Gyre Oscillation. These large-scale factors affecting the physical structure of the water column may also influence the delivery of nutrients from deep ocean outfalls to the euphotic zone, making it difficult to distinguish the effects of anthropogenic inputs on chlorophyll along the coast.
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  • 10
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    Community production modulates coral reef pH and the sensitivity of ecosystem calcification to ocean acidification (2017)
    Wiley
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    Publication Date: 2017-01-11
    Description: Coral reefs are built of calcium carbonate (CaCO 3 ) produced biogenically by a diversity of calcifying plants, animals and microbes. As the ocean warms and acidifies, there is mounting concern that declining calcification rates could shift coral reef CaCO 3 budgets from net accretion to net dissolution. We quantified net ecosystem calcification (NEC) and production (NEP) on Dongsha Atoll, northern South China Sea, over a two-week period that included a transient bleaching event. Peak daytime pH on the wide, shallow reef flat during the non-bleaching period was ∼8.5, significantly elevated above that of the surrounding open ocean (∼8.0-8.1) as a consequence of daytime NEP (up to 112 mmol C m −2 hr −1 ). Diurnal-averaged NEC was 390 ± 90 mmol CaCO 3 m −2 day −1 , higher than any other coral reef studied to date despite comparable calcifier cover (25%) and relatively high fleshy algal cover (19%). Coral bleaching linked to elevated temperatures significantly reduced daytime NEP by 29 mmol C m −2 hr −1 . pH on the reef flat declined by 0.2 units, causing a 40% reduction in NEC in the absence of pH changes in the surrounding open ocean. Our findings highlight the interactive relationship between carbonate chemistry of coral reef ecosystems and ecosystem production and calcification rates, which are in turn impacted by ocean warming. As open-ocean waters bathing coral reefs warm and acidify over the 21 st century, the health and composition of reef benthic communities will play a major role in determining on-reef conditions that will in turn dictate the ecosystem response to climate change.
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