bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the Magnetospheric ULF Wave Counterpart of Substorm Onset
Authors: Smith A. W. , Rae I. J. , Forsyth C. , Watt C. E. J. , Murphy K. R. 
Published: April 2020

AbstractOne near‐ubiquitous signature of substorms observed on the ground is the azimuthal structuring of the onset auroral arc in the minutes prior to onset. Termed auroral beads, these optical signatures correspond to concurrent exponential increases in ground ultralow frequency (ULF) wave power and are likely the result of a plasma instability in the magnetosphere. Here, we present a case study showing the development of auroral beads from a Time History of Events and Macroscale Interactions during Substorms (THEMIS) all‐sky camera with near simultaneous exponential increases in auroral brightness, ionospheric and conjugate magnetotail ULF wave power, evidencing their intrinsic link. We further present a survey of magnetic field fluctuations in the magnetotail around substorm onset. We find remarkably similar superposed epoch analyses of ULF power around substorm onset from space and conjugate ionospheric observations. Examining periods of exponential wave growth, we find the ground‐ and space‐based observations to be consistent, with average growth rates of ∼0.01 s−1, lasting for ∼4 min. Cross‐correlation suggests that the space‐based observations lead those on the ground by approximately 1–1.5 min. Meanwhile, spacecraft located premidnight and ∼10 RE downtail are more likely to observe enhanced wave power. These combined observations lead us to conclude that there is a magnetospheric counterpart of auroral beads and exponentially increasing ground ULF wave power. This is likely the result of the linear phase of a magnetospheric instability, active in the magnetotail for several minutes prior to auroral breakup.
A Subtidal Box Model Based on the Longitudinal Anomaly of Potential Energy for Narrow Estuaries. An Application to the Guadalquivir River Estuary (SW Spain)
Authors: Cobos M. , Baquerizo A. , Díez‐Minguito M. , Losada M. A. 
Published: April 2020

AbstractThe objective of the present study is to demonstrate the informative capacity of the longitudinal anomaly of potential energy (LAPE) in the analysis of the magnitude and spatiotemporal variability of estuarine processes. For this purpose, a LAPE balance equation is formulated. The LAPE integrates and varies with the vertical and longitudinal density distribution. The formulation is applied on a subtidal scale to each box or stretch of the Guadalquivir River estuary, a narrow, highly turbid, weakly stratified, and strongly anthropized estuary. Data recorded by a large network of monitoring stations in 2008 and 2009 are used to quantify advective transports as well as the transports associated with longitudinal dispersion and vertical turbulent mixing in different hydraulic regimes. In low‐river flow conditions, (river flows Q<40 m3s−1), the magnitude of LAPE transports decreases upstream and varies locally, depending on neap‐spring tidal cycles. The direction of the net LAPE transport creates convergence zones that are particularly consistent with maximum levels of estuarine turbidity. During high‐river flows (Q>400 m3s−1), this convergence disappears and the maximum longitudinal density gradient moves towards the mouth. More specifically, tidal pumping ‐induced LAPE governs during these conditions and manages to compensate the sum of the mean nontidal and dispersive and differential advective LAPE transports. However, during the post‐riverflood period, the mechanisms controlling recovery downstream from the mouth are the longitudinal dispersive and differential advective LAPE transports. Furthermore, the convergence zone reappears with a longitudinal gradient of the net LAPE transport that is even greater than in low‐river flow conditions.
The Influence of Diapycnal Nutrient Fluxes on Phytoplankton Size Distribution in an Area of Intense Mesoscale and Submesoscale Activity off Concepción, Chile
Authors: Corredor‐Acosta A. , Morales C. E. , Rodríguez‐Santana A. , Anabalón V. , Valencia L. P. , Hormazabal S. 
Published: April 2020

AbstractMesoscale and submesoscale processes that contribute to localized increases in nutrients in the sunlit layer can stimulate phytoplankton growth and community changes, but the mechanisms involved remain sparsely documented with in situ data in the case of Eastern Boundary Upwelling Systems (EBUSs) and of most ocean regions. The role of diapycnal mixing in providing nutrients to the upper layer and in influencing phytoplankton size structure was analyzed in an area of intense mesoscale and submesoscale activity during the coastal upwelling season off Concepción (~36–37°S), the Humboldt Current EBUS. Diapycnal nutrient fluxes based on conductivity, temperature, and depth vertical eddy diffusivity (Kz) values (the Thorpe scale method) and on nutrient gradients were assessed in association with size‐fractionated chlorophyll‐a and microdiatom abundance derived from in situ sampling in an area including a mesoscale intrathermocline eddy (ITE) adjacent to a coastal upwelling front (CUF). The indirect estimates of Kz values spanned between 0.01 and 4 × 10−4 m2 s−1, and maxima in diapycnal nitrate flux per station ranged between 0.08 and 19.1 mmol m−2 day−1. Maxima in the upward fluxes were detected at the subsurface (15–40 m depth) in the CUF and ITE areas, coinciding with maxima in the micro‐ and nano‐chlorophyll‐a fractions and in microdiatom abundance. These results suggest that ITE and CUF features, as well as their interaction, can generate intense diapycnal mixing and, thereby, contribute to increasing nutrient availability below the mixed layer. In turn, these processes enhance the contribution of larger phytoplankton cells in the coastal transition zone of EBUSs.
Surface Methane Concentrations Along the Mid‐Atlantic Bight Driven by Aerobic Subsurface Production Rather Than Seafloor Gas Seeps
Authors: Leonte Mihai , Ruppel Carolyn D. , Ruiz‐Angulo Angel , Kessler John D. 
Published: April 2020

AbstractRelatively minor amounts of methane, a potent greenhouse gas, are currently emitted from the oceans to the atmosphere, but such methane emissions have been hypothesized to increase as oceans warm. Here, we investigate the source, distribution, and fate of methane released from the upper continental slope of the U.S. Mid‐Atlantic Bight, where hundreds of gas seeps have been discovered between the shelf break and ~1,600 m water depth. Using physical, chemical, and isotopic analyses, we identify two main sources of methane in the water column: seafloor gas seeps and in situ aerobic methanogenesis which primarily occurs at 100–200 m depth in the water column. Stable isotopic analyses reveal that water samples collected at all depths were significantly impacted by aerobic methane oxidation, the dominant methane sink in this region, with the average fraction of methane oxidized being 50%. Due to methane oxidation in the deeper water column, below 200 m depth, surface concentrations of methane are influenced more by methane sources found near the surface (0–10 m depth) and in the subsurface (10–200 m depth), rather than seafloor emissions at greater depths.
A Physically Based Method for Real‐Time Monitoring of Tidal River Discharges From Water Level Observations, With an Application to the St. Lawrence River
Authors: Bourgault Daniel , Matte Pascal 
Published: April 2020

AbstractA fast, physically based, fully nonlinear, and nonsteady reach‐averaged river model is developed to provide instantaneous freshwater discharge rates in the tidally influenced stretch of the St. Lawrence River (Canada) from water level measurements alone at two tide gauge stations. The model does not require any a priori knowledge of the river geometry (width and depth) or hydraulic conditions (hydraulic slope and friction coefficient) as those parameters are inferred by inverse modeling. The model is fast in that several decades of hourly discharges can be reproduced in few seconds on any modern desktop computer. The method is accurate in that it reproduces to within 3% the observed tidal discharge variability at 8 different cross‐sections distributed along the tidally influenced portion of the St. Lawrence River. Although much simpler, the new method performs as well as, or better than, much more sophisticated models for reproducing daily and monthly discharge averages. Being physically based, it is also shown that the model can perform well outside the limited range of measurements that are generally available for calibration. The method is simple and easy to manage in that the main function only contains a few lines of code such that it could be readily implemented as a tool for real‐time discharge monitoring of the St. Lawrence River near Québec City. Being physically based, the model developed here could likely be applied more generally to highly unsteady tidal rivers with flow reversals.
Radiation Controls the Interannual Variability of Evaporation of a Subtropical Lake
Authors: Xiao Wei , Zhang Zhen , Wang Wei , Zhang Mi , Liu Qiang , Hu Yongbo , Huang Wenjing , Liu Shoudong , Lee Xuhui 
Published: April 2020

AbstractDoes lake evaporation increase or decrease under the scenario of climate warming? This paper aims to answer this question by investigating the controlling mechanism of interannual variations in lake evaporation at a subtropical lake. The research methodology is based on continuous eddy covariance measurement over >7 years and a diagnostic analysis using the surface energy balance principle. The results indicate that lake evaporation was enhanced mainly by increasing energy inputs including solar radiation and incoming longwave radiation and was weakened by surface feedback through outgoing longwave radiation. The incoming longwave radiation was positively correlated with cloud cover. Bowen ratio and surface albedo had slight effect on the change of lake evaporation. The annual lake evaporation can be predicted by the Priestly‐Taylor model using a larger coefficient of 1.39 than the original value of 1.26, suggesting that advection or entrainment in the atmospheric boundary layer may play a role in lake evaporation.
Enhanced New Particle Formation Above the Marine Boundary Layer Over the Yellow Sea: Potential Impacts on Cloud Condensation Nuclei
Authors: Takegawa N. , Seto T. , Moteki N. , Koike M. , Oshima N. , Adachi K. , Kita K. , Takami A. , Kondo Y. 
Published: April 2020

AbstractAircraft measurements of aerosols were conducted over the Yellow Sea and the East China Sea from February 14 to March 10, 2013, during the Aerosol Radiative Forcing in East Asia 2013 Winter campaign. Ground‐based measurements of aerosols were also conducted on Fukue Island (32.8°N, 128.7°E), located near the boundary of the Yellow Sea and the East China Sea. The aircraft measurements revealed large increases in particle number concentrations in the nucleation mode (2.5–10 nm and up to ~105 cm−3) at altitudes of ~0.5–3 km over the Yellow Sea. This was attributed to enhanced new particle formation (NPF) above the marine boundary layer over the Yellow Sea. The number ratios of Aitken‐mode (10–90 nm) or accumulation‐mode (>90 nm) particles to black carbon particles (tracer for primary emissions) were used to quantify the effects of NPF on particle number concentrations. We estimated that NPF increased median Aitken‐mode particle number concentrations by an order of magnitude at altitudes of ~1–3 km over the Yellow Sea. Synchronized aircraft and ground‐based data suggest that vertical transport of Aitken‐mode particles formed over the Yellow Sea led to a large variability in the particle growth events observed on Fukue Island. A shift of sea surface temperature near Fukue Island and the resultant promotion of vertical mixing are probably the key mechanisms responsible for this complex feature. Evolution of accumulation‐mode particles was observed farther downwind over the East China Sea, suggesting significant impacts of NPF on the regional distributions of cloud condensation nuclei.
Convectively Generated Gravity Waves During Solstice and Equinox Conditions
Authors: Heale C. J. , Lund T. S. , Fritts D. C. 
Published: April 2020

AbstractPrevious studies suggest that mesospheric concentric wave patterns are more observable in the equinox months than solstice months, despite concentric and semiconcentric waves being readily observed in summer months in the stratosphere. This study uses a 3‐D nonlinear model to simulate the propagation and dissipation of convectively generated gravity waves under averaged equinox and solstice conditions to investigate the influences of wave breaking upon these concentric patterns. It is found that the relatively weak atmospheric winds and shears during the equinox months means that waves can propagate up to the lower thermosphere before breaking. In contrast, strong zonal winds and shears in the summer solstice months lead to in situ breaking in the mesosphere, which causes disruption to the concentric pattern. While the western propagating portion of the concentric pattern is filtered by the stratospheric winds, the eastern propagating portion rapidly transitions to turbulence in the mesosphere making it less likely to observe concentric patterns in the solstice months. The concentric pattern is more complete in the equinox months because the waves are only weakly filtered by winds and relatively little in situ breaking occurs in the mesosphere. However, in situ breaking does not affect the pattern in the stratosphere, where the morphology is primarily dictated by wind filtering. This also helps explain why concentric and semiconcentric patterns are observed in the summer stratosphere but not regularly in the mesosphere.
Mechanisms Responsible for Stratosphere‐to‐Troposphere Transport Around a Mesoscale Convective System Anvil
Authors: Phoenix Daniel B. , Homeyer Cameron R. , Barth Mary C. , Trier Stanley B. 
Published: April 2020

AbstractRecent observational studies have shown that stratospheric air rich in ozone (O3) is capable of being transported into the upper troposphere in association with tropopause‐penetrating convection (anvil wrapping). This finding challenges the current understanding of upper tropospheric sources of O3, which is traditionally thought to come from thunderstorm outflows where lightning‐generated nitrogen oxides facilitate O3 formation. Since tropospheric O3 is an important greenhouse gas and the frequency and strength of tropopause‐penetrating storms may change in a changing climate, it is important to understand the mechanisms driving this transport process so that it can be better represented in chemistry‐climate models. Simulations of a mesoscale convective system (MCS) around which this transport process was observed are performed using the Weather Research and Forecasting model coupled with Chemistry. The Weather Research and Forecasting model coupled with Chemistry model adequately simulates anvil wrapping of ozone‐rich air. Possible mechanisms that influence the transport, including small‐scale static and dynamic instabilities and MCS‐induced mesoscale circulations, are evaluated. Model results suggest that anvil wrapping is a two‐step transport process (1) compensating subsidence surrounding the MCS, which is driven by mass conservation as the MCS transports tropospheric air into the upper troposphere and lower stratosphere, followed by (2) differential advection beneath the core of the MCS upper‐tropospheric outflow jet which wraps high O3 air around and under the MCS cloud anvil. Static and dynamic instabilities are not a leading contributor to this transport process. Continued fine‐scale modeling of these events is needed to fully represent the stratosphere‐to‐troposphere transport process.
Troposphere‐Stratosphere Coupling in Subseasonal‐to‐Seasonal Models and Its Importance for a Realistic Extratropical Response to the Madden‐Julian Oscillation
Authors: Schwartz Chen , Garfinkel Chaim I. 
Published: April 2020

AbstractThe representation of upward and downward stratosphere‐troposphere coupling and its influence on the teleconnections of the Madden‐Julian oscillation (MJO) to the European sector is examined in five subseasonal‐to‐seasonal models. We show that while the models simulate a realistic stratospheric response to transient anomalies in troposphere, they overestimate the downward coupling. The models with a better stratospheric resolution capture a more realistic stratospheric response to the MJO, particularly after the first week of the integration. However, in all models examined here the connection between the MJO and vortex variability is weaker than that observed. Finally, we focus on the MJO‐SSW (sudden stratospheric warming) teleconnection and specifically initializations during the MJO phase with enhanced convection in the west/central pacific (i.e., 6 and 7) that preceded observed SSW. The integrations that simulated a SSW (as observed) can be distinguished from those that failed to simulate a SSW by the realism of the Pacific response to MJO 6/7, with only the simulations that successfully simulate a SSW capturing the North Pacific low. Furthermore, only the simulations that capture the SSW subsequently simulate a realistic surface response over the North Atlantic and Europe.
Decreased Soil Organic Matter in a Long‐Term Soil Warming Experiment Lowers Soil Water Holding Capacity and Affects Soil Thermal and Hydrological Buffering
Authors: Werner W. J. , Sanderman J. , Melillo J. M. 
Published: April 2020

AbstractLong‐term soil warming can decrease soil organic matter (SOM), resulting in self‐reinforcing feedback to the global climate system. We investigated additional consequences of SOM reduction for soil water holding capacity (WHC) and soil thermal and hydrological buffering. At a long‐term soil warming experiment in a temperate forest in the northeastern United States, we suspended the warming treatment for 104 days during the summer of 2017. The formerly heated plot remained warmer (+0.39 °C) and drier (−0.024 cm3 H2O cm−3 soil) than the control plot throughout the suspension. We measured decreased SOM content (−0.184 g SOM g−1 for O horizon soil, −0.010 g SOM g−1 for A horizon soil) and WHC (−0.82 g H2O g−1 for O horizon soil, −0.18 g H2O g−1 for A horizon soil) in the formerly heated plot relative to the control plot. Reduced SOM content accounted for 62% of the WHC reduction in the O horizon and 22% in the A horizon. We investigated differences in SOM composition as a possible explanation for the remaining reductions with Fourier transform infrared (FTIR) spectra. We found FTIR spectra that correlated more strongly with WHC than SOM, but those particular spectra did not differ between the heated and control plots, suggesting that SOM composition affects WHC but does not explain treatment differences in this study. We conclude that SOM reductions due to soil warming can reduce WHC and hydrological and thermal buffering, further warming soil and decreasing SOM. This feedback may operate in parallel, and perhaps synergistically, with carbon cycle feedbacks to climate change.
Riparian Lowlands in Clay Till Landscapes Part II: Nitrogen Reduction and Release Along Variable Flow Paths
Authors: Petersen R. J. , Prinds C. , Jessen S. , Iversen B. V. , Kjaergaard C. 
Published: April 2020

AbstractRiparian lowlands are known to control catchment nitrogen (N) balances. This study examined the role of agricultural tile drainage systems, often present in clay till landscapes, on the transport, transformation, and mass balance of N species in four riparian peat lowland transects receiving agricultural tile drainage water. Monitoring of N speciation of drain, stream, and groundwater, combined with a previously established water balance, enabled the determination of N mass balances for different flow paths including groundwater, subsurface drain water, and overland flow for each piezometer transect. The type of overland flow largely affected nitrate‐N (NO3‐N) removal efficiency, as determined by the total N output from a transect relative to the NO3‐N loading (%). Infiltration and subsurface flow followed by exfiltration (short return flow) allowed an efficient removal of NO3‐N (71–94%), while direct overland flow strongly lowered NO3‐N removal (25%) in one transect. The hydraulic loading rate versus the lowland infiltration capacity determined the transport pathways and thus the resulting NO3‐N removal efficiency. For all transects there was a net export of organic N and/or ammonium, associated with in situ N release from peat decomposition, through overland flow and groundwater discharge. These exports partly counterbalanced NO3‐N removal and significantly reduced the overall total N removal for the riparian lowlands. However, the N removal efficiencies remained positive (1–56%). The study indicates that N budgets for riparian lowlands need to account for overland flow as a transport pathway for N.
Nondispersive Drainage Direction Simulation Based on Flexible Triangular Facets
Authors: Wu Pengfei , Liu Jintao , Han Xiaole , Liang Zhongmin , Liu Yangyang , Fei Junyuan 
Published: April 2020

AbstractA new eight‐direction algorithm (iFAD8) for the simulation of drainage directions on grid digital elevation models (DEMs) is presented. In this algorithm, a flexible triangular facet construction technique (ND∞) is developed to provide local drainage directions ranging continuously from 0° to 360°. Subsequently, a flow aggregation technique using global deviations of local drainage directions is proposed to simplify the flow paths into a nondispersive format. Another algorithm (FAD8) accompanying the iFAD8 is also presented, which uses the D∞ directions as the local drainage directions. Then FAD8, iFAD8, and three existing algorithms are compared on 10 abstract terrains including nine basic terrains and a Himmelblau terrain. The algorithms are tested to reproduce exact slope lines and specific catchment areas derived from the terrain functions. The results show that iFAD8 has better performance than FAD8. Both iFAD8 and FAD8 outperform existing algorithms in most cases. The flow aggregation technique is shown to be an excellent choice for nondispersive drainage direction simulation based on infinite directions. The ND∞ direction based on flexible triangular facets is also an improvement to the local infinite direction of D∞. We conclude that the iFAD8 algorithm can provide better definitions of drainage directions.
Heat Flow on the U.S. Beaufort Margin, Arctic Ocean: Implications for Ocean Warming, Methane Hydrate Stability, and Regional Tectonics
Authors: Hornbach Matthew J. , Harris Robert N. , Phrampus Benjamin J. 
Published: April 2020

AbstractResults from the first focused heat flow study on the U.S. Beaufort Margin provide insight into decadal‐scale Arctic Ocean temperature change and raise new questions regarding Beaufort Margin evolution. This study measured heat flow using a 3.5‐m Lister probe at 103 sites oriented along four north‐south transects perpendicular to the ~700‐km long U.S. Beaufort Margin. The new heat flow measurements, corrected both for seasonal ocean temperature fluctuations and bathymetric effects, reveal low average heat flow values (~35 mW/m2) at seafloor depths of 300–900 m below sea level (mbsl) and anomalously high (~80 mW/m2) values at seafloor depths of >1,000 mbsl, near the predicted continent‐ocean transition. Anomalously low heat flow values measured on the upper margin are consistent with previous studies suggesting decadal‐scale ocean temperature warming to ~500 mbsl. Our results, however, indicate this ocean warming likely extends to depths as great at 900 mbsl—400 m deeper than previous studies suggest—implying widespread, ongoing, methane hydrate destabilization across much of the U.S. Beaufort Margin. The cause of the anomalously high heat flow values observed at seafloor depths >1,000 at the continent‐ocean transition is unclear. We suggest three candidate processes: (1) higher heat production and lower thermal conductivity on the margin edge due to the thickest sedimentary cover at the ocean‐continent transition, (2) seaward migrating subsurface advection, and (3) possible fault‐reactivation at the northern boundary of the Alaskan Microplate.