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  • Articles  (20)
  • Wiley  (20)
  • Cell Press
  • Frontiers Media
  • National Academy of Sciences
  • 2015-2019  (20)
  • 1995-1999
  • 1980-1984
  • 1955-1959
  • Journal of Geophysical Research JGR - Oceans  (9)
  • Journal of Geophysical Research JGR - Atmospheres  (4)
  • Journal of Geophysical Research: Atmospheres  (4)
  • 7528
  • 7529
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  • Articles  (20)
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  • 2015-2019  (20)
  • 1995-1999
  • 1980-1984
  • 1955-1959
  • 2010-2014  (20)
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  • 1
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    Regional Widening of Tropical Overturning: Forced Change, Natural Variability, and Recent Trends (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract The width of the tropical Hadley circulation (HC) has garnered intense interest in recent decades, owing to the emerging evidence for its expansion in observations and models and to the anticipated impacts on surface climate in its descending branches. To better clarify the causes and impacts of tropical widening, this work generalizes the zonal mean HC to the regional level by defining meridional overturning cells (RC) using the horizontally divergent wind. The edges of the RC are more closely connected to surface hydroclimate than more traditional metrics of regional tropical width (such as the sea level pressure ridge) or even than the zonal mean HC. Simulations reveal a robust weakening of the RC in response to greenhouse gas increases, along with a widening of the RC in some regions. For example, simulated widening of the zonal mean HC in the Southern Hemisphere appears to arise in large part from regional overturning anomalies over the Eastern Pacific, where there is no clear RC. Unforced interannual variability in the position of the zonal mean HC edge is associated with a more general regional widening. These distinct regional signatures suggest that the RCs may be well suited for the attribution of observed circulation trends. The spatial pattern of regional meridional overturning trends in reanalyses corresponds more closely to the pattern associated with unforced interannual variability than to the pattern associated with CO2 forcing, suggesting a large contribution of natural variability to the recent observed tropical widening trends.
    Print ISSN: 2169-897X
    Electronic ISSN: 2169-8996
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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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
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  • 3
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    High resolution atmospheric inversion of urban CO2 emissions during the dormant season of the Indianapolis Flux Experiment (INFLUX) (2016)
    Wiley
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    Publication Date: 2016-04-08
    Description: Based on a uniquely dense network of surface towers measuring continuously the atmospheric concentrations of Greenhouse Gases (GHG's), we developed the first comprehensive monitoring systems of CO 2 emissions at high resolution over the city of Indianapolis. The urban inversion evaluated over the 2012-2013 dormant season, showed a statistically significant increase of about 20% (from 4.5 to 5.7 MtC ±0.23 MtC) compared to the Hestia CO 2 emission estimate, a state-of-the-art building-level emission product. Spatial structures in prior emission errors, mostly undetermined, appeared to affect the spatial pattern in the inverse solution and the total carbon budget over the entire area by up to 15%, while the inverse solution remains fairly insensitive to the CO 2 boundary inflow and to the different prior emissions ( i.e. ODIAC). Preceding the surface emission optimization, we improved the atmospheric simulations using a meteorological data assimilation system also informing our Bayesian inversion system through updated observations error variances. Finally, we estimated the uncertainties associated with undetermined parameters using an ensemble of inversions. The total CO 2 emissions based on the ensemble mean and quartiles (5.26 - 5.91 MtC) were statistically different compared to the prior total emissions (4.1 to 4.5 MtC). Considering the relatively small sensitivity to the different parameters, we conclude that atmospheric inversions are potentially able to constrain the carbon budget of the city, assuming sufficient data to measure the inflow of GHG over the city, but additional information on prior emission error structures are required to determine the spatial structures of urban emissions at high resolution.
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    Topics: Geosciences , Physics
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  • 4
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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
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  • 5
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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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  • 6
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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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  • 7
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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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  • 8
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    Lightning location, NOx production, and transport by anomalous and normal polarity thunderstorms (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Production and transport of NOx by convection is critical as it serves as a precursor to tropospheric ozone, an important greenhouse gas. Lightning serves as the largest source of nitrogen oxides (NOx = NO + NO2) to the upper troposphere (UT) and is one of the largest natural sources of NOx. Interest is placed on the vertical advection of NOx because its lifetime increases to several days in the UT compared to roughly three hours in the lower troposphere and boundary layer. Thus, lightning can play an important role in ozone production within the UT. However, the amount of NOx produced per flash and NOx advection in storms remain uncertain. This study investigates lightning NOx (LNOx) production and transport processes in anomalous (mid‐level positive charge) and normal polarity (mid‐level negative charge) thunderstorms by advecting parcels containing LNOx from the flash channels of over 5600 lightning flashes observed during the Deep Convective Clouds and Chemistry (DC3) field campaign. Results reveal most flash channels occur near 6‐8 km in the normal polarity thunderstorms and 5‐6 km within anomalous polarity thunderstorms. Larger flash rates and stronger updrafts in anomalous storms result in considerably larger LNOx mixing ratios (peaks of 0.75‐1.75 ppb) in the UT compared to normal polarity storms (peaks 〈 0.5 ppb). A slightly lower mean flash LNOx production was also found among all five storms in this study (storm mean values of 72‐158 moles per flash) compared to previous estimates, which generally parameterize LNOx by flash rate rather than flash rate.
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  • 9
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    Evaluation of regional CO2 mole fractions in the ECMWF CAMS real‐time atmospheric analysis and NOAA CarbonTracker Near‐Real Time reanalysis with airborne observations from ACT‐America field campaigns (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract This study systematically examines the regional uncertainties and biases in carbon dioxide (CO2) mole fractions from two of the state‐of‐the‐art global CO2 analysis products, namely the Copernicus Atmosphere Monitoring Service (CAMS) real‐time atmospheric analysis from the European Centre for Medium‐Range Weather Forecasts and the CarbonTracker Near‐Real Time (CT‐NRT) reanalysis from the National Oceanic and Atmospheric Administration, by evaluation against hundreds of hours of airborne in situ measurements from the summer 2016 and winter 2017 Atmospheric Carbon and Transport (ACT) ‐‐ America field campaigns. Both the CAMS and CT‐NRT analyses agree reasonably well with the independent ACT‐America airborne CO2 measurements in the free troposphere, with root‐mean‐square deviations (RMSDs) between analyses and observations generally between 1‐‐2 ppm, but show considerably larger uncertainties in the atmospheric boundary layer where the RMSDs exceed 8 ppm in the lowermost 1 km of the troposphere in summer. There are strong variations in accuracy and bias between seasons, and across three different subregions in the United States (Mid‐Atlantic, Midwest and South), with the largest uncertainties in the Mid‐Atlantic region in summer. Overall, the RMSDs of the CAMS and CT‐NRT analyses against airborne data are comparable to each other, and largely consistent with the differences between the two analyses. The current study provides uncertainty estimates for both analysis products over North America and suggests that these two independent estimates can be used to approximate regional CO2 analysis uncertainties. Both statistics are important in future studies in quantifying the uncertainties in regional CO2 mole fraction and flux estimates.
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  • 10
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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.
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