bluuebunny/crossref_metadata_2025 huggingface dataset:
Middle‐ and low‐latitude ionosphere response to 2015 St. Patrick's Day geomagnetic storm
Authors: Nava B. , Rodríguez‐Zuluaga J. , Alazo‐Cuartas K. , Kashcheyev A. , Migoya‐Orué Y. , Radicella S.M. , Amory‐Mazaudier C. , Fleury R.
Published: March 2016

This paper presents a study of the St Patrick's Day storm of 2015, with its ionospheric response at middle and low latitudes. The effects of the storm in each longitudinal sector (Asian, African, American, and Pacific) are characterized using global and regional electron content. At the beginning of the storm, one or two ionospheric positive storm effects are observed depending on the longitudinal zones. After the main phase of the storm, a strong decrease in ionization is observed at all longitudes, lasting several days. The American region exhibits the most remarkable increase in vertical total electron content (vTEC), while in the Asian sector, the largest decrease in vTEC is observed. At low latitudes, using spectral analysis, we were able to separate the effects of the prompt penetration of the magnetospheric convection electric field (PPEF) and of the disturbance dynamo electric field (DDEF) on the basis of ground magnetic data. Concerning the PPEF, Earth's magnetic field oscillations occur simultaneously in the Asian, African, and American sectors, during southward magnetization of the Bz component of the interplanetary magnetic field. Concerning the DDEF, diurnal magnetic oscillations in the horizontal component H of the Earth's magnetic field exhibit a behavior that is opposed to the regular one. These diurnal oscillations are recognized to last several days in all longitudinal sectors. The observational data obtained by all sensors used in the present paper can be interpreted on the basis of existing theoretical models.
Statistics of ionospheric disturbances over the Antarctic Peninsula as derived from TEC measurements
Authors: Galushko V. G. , Paznukhov V. V. , Sopin A. A. , Yampolski Y. M.
Published: March 2016

Results of ionospheric disturbances observations using multisite total electron content (TEC) measurements over the Antarctic Peninsula for the period from 2009 to 2012 are presented. Diurnal dependencies of the disturbance occurrence frequency, velocity, and direction of motion are established. Parameters of the disturbances are calculated using dynamic approach to the problem of ionospheric disturbances diagnostics which has been extended to allow for an arbitrary waveform of TEC perturbations. Specific data quality constraints are introduced to determine the presence of wavelike disturbances in the TEC records and to satisfy the required accuracy of parameter reconstruction. Statistical treatment of the results shows that during the daytime disturbances propagate predominantly in the northern and northeastern directions. In the evening and nocturnal hours the northwestern direction is prevailing, with the characteristic disturbance velocities ranging from 10 to 250 m/s. The most probable velocities are tens of meters per second, while the mean values are equal to about 100–130 m/s. In the daytime the velocities of the ionospheric disturbances are even higher. During the Antarctic winter time disturbances are usually observed at local noon time, in sharp contrast with the summer time, when they are present during the nights and mornings.
The effect of fracture density and stress state on the static and dynamic bulk moduli of Westerly granite
Authors: Blake O. O. , Faulkner D. R.
Published: March 2016

Elastic properties are key parameters during the deformation of rocks. They can be measured statically or dynamically, but the two measurements are often different. In this study, the static and dynamic bulk moduli (Kstatic and Kdynamic) were measured at varying effective stress for dry and fluid‐saturated Westerly granite with controlled fracture densities under isotropic and differential stress states. Isotropic fracturing of different densities was induced in samples by thermal treatment to 250, 450, 650, and 850°C. Results show that fluid saturation does not greatly affect static moduli but increases dynamic moduli. Under isotropic loading, high fracture density and/or low effective pressure results in a low Kstatic/Kdynamic ratio. For dry conditions Kstatic/Kdynamic approaches 1 at low fracture densities when the effective pressure is high, consistent with previous studies. Stress‐induced anisotropy exists under differential stress state that greatly affects Kstatic compared to Kdynamic. As a result, the Kstatic/Kdynamic ratio is higher than that for the isotropic stress state and approaches 1 with increasing axial loading. The effect of stress‐induced anisotropy increases with increasing fracture density. A key omission in previous studies comparing static and dynamic properties is that anisotropy has not been considered. The standard methods for measuring static elastic properties, such as Poisson's ratio, Young's and shear modulus, involve subjecting the sample to a differential stress state that promotes anisotropy. Our results show that stress‐induced anisotropy resulting from differential stress state is a major contributor to the difference between static and dynamic elasticity and is dominant with high fracture density.
An enhanced singular spectrum analysis method for constructing nonsecular model of GPS site movement
Authors: Wang Xiaoming , Cheng Yingyan , Wu Suqin , Zhang Kefei
Published: March 2016

Many GPS time series contain offsets, sometimes nonsecular trends, and seasonal signals with time‐varying amplitudes due to several different types of geophysical phenomena. Therefore, the use of nonsecular models to depict the real geophysical movement of GPS sites is better than a linear model. In this study, an enhanced singular spectrum analysis (SSA) method for fitting GPS time series and predicting its coordinates is proposed. Simulation results show that the root‐mean‐square (RMS) of differences between the reconstructed and simulated signals is 1.7 mm; the RMS of the differences between the predicted coordinates and simulated signal is about 3 mm for the first half 1.5 years of testing period and decreases to 10 mm for the last half 1.5 years. Fitting results for three GPS time series are obtained using maximum likelihood estimation (MLE), which is used to fit the time series with a piecewise linear trend plus an annual/semiannual components, SSA, and state space model (SSM). Both SSA and SSM perform similarly and better than the MLE in extracting the nonsecular trend and annual/semiannual components from the GPS time series. The prediction results from SSA have higher coefficients with raw time series and lower power of annual/semiannual in their residuals than that from MLE for two case studies. The differences between the linear trend estimated by Plate Boundary Observatory and SSA nonsecular model for 16 GPS time series are all larger than 2 mm in up direction, which is not negligible for a high‐accuracy terrestrial reference frame construction.
Lithospheric flexure under the Hawaiian volcanic load: Internal stresses and a broken plate revealed by earthquakes
Authors: Klein Fred W.
Published: March 2016

Several lines of earthquake evidence indicate that the lithospheric plate is broken under the load of the island of Hawai`i, where the geometry of the lithosphere is circular with a central depression. The plate bends concave downward surrounding a stress‐free hole, rather than bending concave upward as with past assumptions. Earthquake focal mechanisms show that the center of load stress and the weak hole is between the summits of Mauna Loa and Mauna Kea where the load is greatest. The earthquake gap at 21 km depth coincides with the predicted neutral plane of flexure where horizontal stress changes sign. Focal mechanism P axes below the neutral plane display a striking radial pattern pointing to the stress center. Earthquakes above the neutral plane in the north part of the island have opposite stress patterns; T axes tend to be radial. The M6.2 Honomu and M6.7 Kiholo main shocks (both at 39 km depth) are below the neutral plane and show radial compression, and the M6.0 Kiholo aftershock above the neutral plane has tangential compression. Earthquakes deeper than 20 km define a donut of seismicity around the stress center where flexural bending is a maximum. The hole is interpreted as the soft center where the lithospheric plate is broken. Kilauea's deep conduit is seismically active because it is in the ring of maximum bending. A simplified two‐dimensional stress model for a bending slab with a load at one end yields stress orientations that agree with earthquake stress axes and radial P axes below the neutral plane. A previous inversion of deep Hawaiian focal mechanisms found a circular solution around the stress center that agrees with the model. For horizontal faults, the shear stress within the bending slab matches the slip in the deep Kilauea seismic zone and enhances outward slip of active flanks.
Do the Amazon and Orinoco freshwater plumes really matter for hurricane‐induced ocean surface cooling?
Authors: Hernandez O. , Jouanno J. , Durand F.
Published: March 2016

Recent studies suggested that the plume of low‐saline waters formed by the discharge of the Amazon and Orinoco rivers could favor Atlantic Tropical Cyclone (TC) intensification by weakening the cool wake and its impact on the hurricane growth potential. The main objective of this study is to quantify the effects of the Amazon‐Orinoco river discharges in modulating the amplitude of TC‐induced cooling in the western Tropical Atlantic. Our approach is based on the analysis of TC cool wake statistics obtained from an ocean regional numerical simulation with ¼º horizontal resolution over the 1998–2012 period, forced with realistic TC winds. In both model and observations, the amplitude of TC‐induced cooling in plume waters (0.3–0.4ºC) is reduced significantly by around 50–60% compared to the cooling in open ocean waters out of the plume (0.6–0.7ºC). A twin simulation without river runoff shows that TC‐induced cooling over the plume region (defined from the reference experiment) is almost unchanged (∼0.03ºC) despite strong differences in salinity stratification and the absence of barrier layers. This argues for a weaker than thought cooling inhibition effect of salinity stratification and barrier layers in this region. Indeed, results suggest that haline stratification and barrier layers caused by the river runoff may explain only ∼10% of the cooling difference between plume waters and open ocean waters. Instead, the analysis of the background oceanic conditions suggests that the regional distribution of the thermal stratification is the main factor controlling the amplitude of cooling in the plume region.
Turbulent production in an internal wave bottom boundary layer maintained by a vertically propagating seiche
Authors: Henderson Stephen M.
Published: March 2016

Internal seiches, which supply the energy responsible for mixing many lakes, are often modeled as vertically standing waves. However, recent observations of vertical seiche propagation in a small lake are inconsistent with the standard, vertically standing model. To examine the processes responsible for such propagation, drag and turbulent production in the bottom boundary layer of a small lake are related to the energy supplied by a propagating seiche (period 10–24 h). Despite complex and fluctuating stratification, which often inhibited mixing within 0.4 m of the bed, bottom stress was well represented by a simple drag coefficient model (drag coefficient 1.5 × 10−3). The net supply of seiche energy to the boundary layer was estimated by fitting a model for internal wave vertical propagation to velocity profiles measured above the boundary layer (1–4.5 m above lakebed). Fitted reflection coefficients ranged from 0.3 at 1 cycle/d frequency to 0.7 at 2.4 cycles/d (cf. near‐unity coefficients of classical seiche theories). The net supply of seiche energy approximately balanced boundary layer turbulent production. Three of four peaks in production and energy flux occurred 0.8–2.2 days after strong oscillating winds, a delay comparable to the time required for seiche energy to propagate to the lakebed. A model based on the estimated drag coefficient predicted the observed frequency dependence of the seiche reflection coefficient. For flat‐bed regions in narrow lakes, the model predicts that reflection is controlled by the ratio of water velocity to vertical wave propagation speed, with sufficiently large ratios leading to weak reflection, and clear vertical seiche propagation.
Simulation of oil bioremediation in a tidally influenced beach: Spatiotemporal evolution of nutrient and dissolved oxygen
Authors: Geng Xiaolong , Pan Zhong , Boufadel Michel C. , Ozgokmen Tamay , Lee Kenneth , Zhao Lin
Published: March 2016

Numerical experiments of oil bioremediation of tidally influenced beach were simulated using the model BIOMARUN. Nutrient and dissolved oxygen were assumed present in a solution applied on the exposed beach face, and the concentration of these amendments was tracked throughout the beach for up to 6 months. It was found that, in comparison to natural attenuation, bioremediation increased the removal efficiency by 76% and 65% for alkanes and aromatics, respectively. Increasing the nutrient concentration in the applied solution did not always enhance biodegradation as oxygen became limiting even when the beach was originally oxygen‐rich. Therefore, replenishment of oxygen to oil‐contaminated zone was also essential. Stimulation of oil biodegradation was more evident in the upper and midintertidal zone of the beach, and less in the lower intertidal zone. This was due to reduced nutrient and oxygen replenishment, as very little of the amendment solution reached that zone. It was found that under continual application, most of the oil biodegraded within 2 months, while it persisted for 6 months under natural conditions. While the difference in duration suggests minimal long‐term effects, there are situations where the beach would need to be cleaned for major ecological functions, such as temporary nesting or feeding for migratory birds. Biochemical retention time map (BRTM) showed that the duration of solution application was dependent upon the stimulated oil biodegradation rate. By contrast, the application rate of the amendment solution was dependent upon the subsurface extent of the oil‐contaminated zone. Delivery of nutrient and oxygen into coastal beach involved complex interaction among amendment solution, groundwater, and seawater. Therefore, approaches that ignore the hydrodynamics due to tide are unlikely to provide the optimal solutions for shoreline bioremediation.
Fram Strait and Greenland Sea transports, water masses, and water mass transformations 1999–2010 (and beyond)
Authors: Marnela Marika , Rudels Bert , Goszczko Ilona , Beszczynska‐Möller Agnieszka , Schauer Ursula
Published: March 2016

The exchanges between the Nordic Seas and the Arctic Ocean are important for the ocean circulation and climate. Transports are here estimated using summer hydrographic data from the Greenland Sea and the Fram Strait. Geostrophic transports are computed from hydrographic sections at 75°N in the Greenland Sea and at about 79°N in the Fram Strait. Geostrophic velocities are adjusted with summer velocities derived from Argo floats, and four conservation constraints are applied to a box closed by the two sections. The estimated net volume transports are 0.8 ± 1.5 Sv southward. Net freshwater transports through the Greenland Sea section are estimated at 54 ± 20 mSv and through the Fram Strait section at 66 ± 9 mSv. Heat loss in the area between the two sections is estimated at 9 ± 12 TW. Convection depths in the Greenland Sea are estimated from observations and vary between about 200 and 2000 dbar showing no trend. Water mass properties in the Greenland Sea are affected both by convection and lateral mixing. Vertical mixing is estimated from hydrography and based on it about 1 Sv of diluted Arctic Ocean waters are estimated to enter the Greenland Sea. The properties of Atlantic, intermediate, and deep waters are studied. Deep water properties are defined using water mass triangles and are subject to decadal changes.
Anatomy of the Antarctic Circumpolar Current volume transports through Drake Passage
Authors: Koenig Zoé , Provost Christine , Park Young‐Hyang , Ferrari Ramiro , Sennéchael Nathalie
Published: March 2016

The 20 year (October 1992 to August 2013) observation‐based volume transport time series of the Antarctic Circumpolar Current (ACC) through Drake Passage (DP) across the Jason altimeter track #104 is analyzed to better understand the ACC transport variability and its potential causes. The time series of three transport components (total (TT), barotropic (BT), and baroclinic (BC)) referenced to 3000 m present energetic intraseasonal fluctuations, with a salient spectral peak at 50 and 36 days, with the largest (least) variance being associated with the BT (BC) component. Low‐frequency variations are much less energetic with a significant variance limited to the annual and biannual timescales and show a nonstationary intermittent link with the Southern Annular Mode and the Nino 3.4 index for interannual timescales. The region around 57°S in the Yaghan Basin appears to be a strategic point for a practical monitoring of the ACC transport, as the whole‐track TT is significantly correlated with the local TT (r = 0.53) and BT (r = 0.69) around 57°S. These local BT (and TT) variations are associated with a well‐defined tripole pattern in altimetric sea level anomaly (SLA). There is evidence that the tripole pattern associated with BT is locally generated when the BC‐associated mesoscale SLAs, which have propagated eastward from an upstream area of DP, cross the Shackleton Fracture Zone to penetrate into the Yaghan Basin. Barotropic basin modes excited within the Yaghan Basin are discussed as a plausible mechanism for the observed energy‐containing intraseasonal spectral peaks found in the transport variability.
Observations of bubbles in natural seep flares at MC 118 and GC 600 using in situ quantitative imaging
Authors: Wang Binbin , Socolofsky Scott A. , Breier John A. , Seewald Jeffrey S.
Published: March 2016

This paper reports the results of quantitative imaging using a stereoscopic, high‐speed camera system at two natural gas seep sites in the northern Gulf of Mexico during the Gulf Integrated Spill Research G07 cruise in July 2014. The cruise was conducted on the E/V Nautilus using the ROV Hercules for in situ observation of the seeps as surrogates for the behavior of hydrocarbon bubbles in subsea blowouts. The seeps originated between 890 and 1190 m depth in Mississippi Canyon block 118 and Green Canyon block 600. The imaging system provided qualitative assessment of bubble behavior (e.g., breakup and coalescence) and verified the formation of clathrate hydrate skins on all bubbles above 1.3 m altitude. Quantitative image analysis yielded the bubble size distributions, rise velocity, total gas flux, and void fraction, with most measurements conducted from the seafloor to an altitude of 200 m. Bubble size distributions fit well to lognormal distributions, with median bubble sizes between 3 and 4.5 mm. Measurements of rise velocity fluctuated between two ranges: fast‐rising bubbles following helical‐type trajectories and bubbles rising about 40% slower following a zig‐zag pattern. Rise speed was uncorrelated with hydrate formation, and bubbles following both speeds were observed at both sites. Ship‐mounted multibeam sonar provided the flare rise heights, which corresponded closely with the boundary of the hydrate stability zone for the measured gas compositions. The evolution of bubble size with height agreed well with mass transfer rates predicted by equations for dirty bubbles.
Wildfires in a warmer climate: Emission fluxes, emission heights, and black carbon concentrations in 2090–2099
Authors: Veira A. , Lasslop G. , Kloster S.
Published: March 2016

Global warming is expected to considerably impact wildfire activity and aerosol emission release in the future. Due to their complexity, the future interactions between climate change, wildfire activity, emission release, and atmospheric aerosol processes are still uncertain. Here we use the process‐based fire model SPITFIRE within the global vegetation model JSBACH to simulate wildfire activity for present‐day climate conditions and future Representative Concentration Pathways (RCPs). The modeled fire emission fluxes and fire radiative power serve as input for the aerosol‐climate model ECHAM6‐HAM2, which has been extended by a semiempirical plume height parametrization. Our results indicate a general increase in extratropical and a decrease in tropical wildfire activity at the end of the 21st century. Changes in emission fluxes are most pronounced for the strongest warming scenario RCP8.5 (+49% in the extratropics, −37% in the tropics). Tropospheric black carbon (BC) concentrations are similarly affected by changes in emission fluxes and changes in climate conditions with regional variations of up to −50% to +100%. In the Northern Hemispheric extratropics, we attribute a mean increase in aerosol optical thickness of +0.031±0.002 to changes in wildfire emissions. Due to the compensating effects of fire intensification and more stable atmospheric conditions, global mean emission heights change by at most 0.3 km with only minor influence on BC long‐range transport. The changes in wildfire emission fluxes for the RCP8.5 scenario, however, may largely compensate the projected reduction in anthropogenic BC emissions by the end of the 21st century.
How accurate are volcanic ash simulations of the 2010 Eyjafjallajökull eruption?
Authors: Dacre H. F. , Harvey N. J. , Webley P. W. , Morton D.
Published: March 2016

In the event of a volcanic eruption the decision to close airspace is based on forecast ash maps, produced using volcanic ash transport and dispersion models. In this paper we quantitatively evaluate the spatial skill of volcanic ash simulations using satellite retrievals of ash from the Eyjafjallajökull eruption during the period from 7 to 16 May 2010. We find that at the start of this period, 7–10 May, the model (FLEXible PARTicle) has excellent skill and can predict the spatial distribution of the satellite‐retrieved ash to within 0.5° × 0.5° latitude/longitude. However, on 10 May there is a decrease in the spatial accuracy of the model to 2.5°× 2.5° latitude/longitude, and between 11 and 12 May the simulated ash location errors grow rapidly. On 11 May ash is located close to a bifurcation point in the atmosphere, resulting in a rapid divergence in the modeled and satellite ash locations. In general, the model skill reduces as the residence time of ash increases. However, the error growth is not always steady. Rapid increases in error growth are linked to key points in the ash trajectories. Ensemble modeling using perturbed meteorological data would help to represent this uncertainty, and assimilation of satellite ash data would help to reduce uncertainty in volcanic ash forecasts.
The temporal evolution of a long‐lived contrail cirrus cluster: Simulations with a global climate model
Authors: Bock Lisa , Burkhardt Ulrike
Published: March 2016

The representation of contrail cirrus in climate models has advanced in the last years tremendously. Nevertheless, uncertainties in particular regarding the representation of contrail microphysics still remain. Properties of young contrail cirrus differ from those of natural cirrus due to the large ice crystal number concentration common in contrails. Consequently, microphysical process rates in contrail cirrus, which control its lifetime, can be very different to those in natural cirrus. We extend a contrail cirrus scheme within a climate model by implementing a microphysical two‐moment scheme and study the life cycle of a contrail cirrus cluster. In an idealized experiment we study the properties and microphysical process rates of a contrail cirrus cluster in a large and long‐lived ice supersaturated region. We find that at flight level contrail cirrus display their typical high ice crystal number concentration (of about 10–100 cm−3) for a few hours with far lower densities in lower levels caused by sedimentation. After about 7 h contrail cirrus have spread considerably so that even at flight level associated ice crystal number concentrations have dropped to values that prohibit fast relaxation of ice supersaturation. The reduced ice crystal number and the resulting limited water uptake in the contrail cirrus limit the lifetime of the contrail cirrus cluster to about 10 h even though surrounding conditions would be still favorable for contrail cirrus persistence. In our case studies, contrail cirrus resembles natural cirrus regarding their ice crystal number concentration and size after 5–7 h.
Using satellite data to estimate partial pressure of CO2 in the Baltic Sea
Authors: Parard Gaëlle , Charantonis Anastase Alexandre , Rutgersson Anna
Published: March 2016

In this study we focused on estimating the pressure partial of CO2 (pCO2) in the entire Baltic Sea which can be considered a coastal area in its entirety. We used the self‐organizing multiple linear output (SOMLO) method to estimate the ocean surface pCO2 in the Baltic Sea from the remotely sensed sea surface temperature, chlorophyll a, colored dissolved organic matter, net primary production, and mixed‐layer depth. Uncertainties in the estimates include sparsity of in situ data used to train the algorithms, in particular, for some sectors and seasons. For this application we divided the Baltic Sea in four basins. When comparing the results obtained with this division to those obtained in previous studies, we notice a decrease in the root‐mean‐square error (<40 μatm) between the reconstruction of the pCO2 and their corresponding in situ measurements, as well as an increase of the correlation coefficient (> 0.96) between them. The outputs of this research have a horizontal resolution of 4 km and cover the 1998–2011 period. For the first time, a climatological mean distribution of surface water pCO2 over the Baltic Sea was calculated based on the SOMLO method with a mean pCO2 of 368.3 ± 100 μatm, and a range of 234–514 μatm. The seasonal variability is similar throughout the Baltic Sea, being high in winter and low in summer.
Riparian zones attenuate nitrogen loss following bark beetle‐induced lodgepole pine mortality
Authors: Biederman Joel A. , Meixner Thomas , Harpold Adrian A. , Reed David E. , Gutmann Ethan D. , Gaun Janelle A. , Brooks Paul D.
Published: March 2016

A North American bark beetle infestation has killed billions of trees, increasing soil nitrogen and raising concern for N loss impacts on downstream ecosystems and water resources. There is surprisingly little evidence of stream N response in large basins, which may result from surviving vegetation uptake, gaseous loss, or dilution by streamflow from unimpacted stands. Observations are lacking along hydrologic flow paths connecting soils with streams, challenging our ability to determine where and how attenuation occurs. Here we quantified biogeochemical concentrations and fluxes at a lodgepole pine‐dominated site where bark beetle infestation killed 50–60% of trees. We used nested observations along hydrologic flow paths connecting hillslope soils to streams of up to third order. We found soil water NO3 concentrations increased 100‐fold compared to prior research at this and nearby southeast Wyoming sites. Nitrogen was lost below the major rooting zone to hillslope groundwater, where dissolved organic nitrogen (DON) increased by 3–10 times (mean 1.65 mg L−1) and NO3‐N increased more than 100‐fold (3.68 mg L−1) compared to preinfestation concentrations. Most of this N was removed as hillslope groundwater drained through riparian soils, and NO3 remained low in streams. DON entering the stream decreased 50% within 5 km downstream, to concentrations typical of unimpacted subalpine streams (~0.3 mg L−1). Although beetle outbreak caused hillslope N losses similar to other disturbances, up to 5.5 kg ha−1y−1, riparian and in‐stream removal limited headwater catchment export to <1 kg ha−1y−1. These observations suggest riparian removal was the dominant mechanism preventing hillslope N loss from impacting streams.
Origin and temporal variability of unusually low δ13C‐DOC values in two High Arctic catchments
Authors: Hindshaw R. S. , Lang S. Q. , Bernasconi S. M. , Heaton T. H. E. , Lindsay M. R. , Boyd E. S.
Published: March 2016

The stable carbon isotopic composition of dissolved organic matter (δ13C‐DOC) reveals information about its source and extent of biological processing. Here we report the lowest δ13C‐DOC values (−43.8‰) measured to date in surface waters. The streams were located in the High Arctic, a region currently experiencing rapid changes in climate and carbon cycling. Based on the widespread occurrence of methane cycling in permafrost regions and the detection of the pmoA gene, a proxy for aerobic methanotrophs, we conclude that the low δ13C‐DOC values are due to organic matter partially derived from methanotrophs consuming biologically produced, 13C‐depleted methane. These findings demonstrate the significant impact that biological activity has on the stream water chemistry exported from permafrost and glaciated environments in the Arctic. Given that the catchments studied here are representative of larger areas of the Arctic, occurrences of low δ13C‐DOC values may be more widespread than previously recognized, with implications for understanding C cycling in these environments.
Antenna selection in a SIMO architecture for HF radio links
Authors: Erhel Yvon , Lemur Dominique , Oger Martial , Le Masson Jérôme
Published: March 2016

This work takes place in the global design of a SIMO architecture (single input multiple output) for transhorizon radio links, aiming at a significant increase in the data rate when compared with standard modems based in general on a SISO scheme (single input single output). The project is subject to available space constraints at the receive end, involving mobile stations or onboard implementation. We consider solutions that appear as extensions of the compact and heterogeneous antenna array that we proposed previously: collocated antennas of different types are set up with the same phase center and present diversity in their polarization sensitivities to make array processing effective. Given the number NC of receive channels, we address the problem of selecting the most effective antennas in a set of NA possible candidates including monopoles, dipoles, loop antennas with various geometries, and orientations. The criterion to be maximized is the SIMO outage capacity, a quantity based on the statistical distribution of the SIMO Shannon capacity estimated for a large number of ionospheric channel realizations, each of them being quantified by its channel impulse response including the receive antenna directional responses. Results are presented in the context of a 1 × 2 SIMO structure: the identification of the two most effective antennas in a set of NA = 15 sensors indicates that the optimal structures involve two orthogonal horizontal dipoles or two vertical orthogonal loop antennas. In these conditions, the outage capacity reaches up to 2.23 bps/Hz, a value that significantly exceeds the performances of standard modems.
Seismotectonics of northeastern Sicily and southern Calabria (Italy): New constraints on the tectonic structures featuring in a crucial sector for the central Mediterranean geodynamics
Authors: Scarfì L. , Barberi G. , Musumeci C. , Patanè D.
Published: March 2016

The purpose of this study is to gain a better understanding on the tectonic structures featuring in a crucial sector of central Mediterranean area, including the Aeolian Islands, southern Calabria, and northeastern Sicily, where the convergence between Eurasian and African Plates has given rise to a complicated collisional/subduction complex. A high‐quality data set of about 3000 earthquakes has been exploited for local earthquake tomography and focal mechanisms computation together with available source mechanisms from published catalogues. The results depict new details of a network of faults which enables the concurrent existence of adjacent compressional and extensional domains. In particular, tomographic images, seismic events distribution, and focal mechanisms pinpoint the geometry and activity of a lithospheric‐scale tear faults system which, with a NW‐SE trend through Sicily and the Tyrrhenian and Ionian Seas, represents the southern edge of the Ionian subduction trench zone. At crustal depth, this tearing is well highlighted by a rotation of the maximum horizontal stress, moving across the area from west toward east. In addition, the shallow normal fault regime, characterizing the southern Calabria and northeastern Sicily mainland, south of the NW‐SE lineament, changes in the deeper part of the crust. Indeed, a NE‐SW earthquake distribution, gently dipping NW, and inverse fault solutions indicate a still active contractional deformation in eastern Sicily, caused by the Africa‐Eurasia convergence and well framed with the current compressive regime along the southern Tyrrhenian zone and at the front of the Sicilian Chain‐Foreland.
Timing of deformation in the Sarandí del Yí Shear Zone, Uruguay: Implications for the amalgamation of western Gondwana during the Neoproterozoic Brasiliano‐Pan‐African Orogeny
Authors: Oriolo Sebastián , Oyhantçabal Pedro , Wemmer Klaus , Basei Miguel A. S. , Benowitz Jeffrey , Pfänder Jörg , Hannich Felix , Siegesmund Siegfried
Published: March 2016

U‐Pb and Hf zircon (sensitive high‐resolution ion microprobe ‐SHRIMP‐ and laser ablation‐inductively coupled plasma‐mass spectrometry ‐LA‐ICP‐MS‐), Ar/Ar hornblende and muscovite, and Rb‐Sr whole rock‐muscovite isochron data from the mylonites of the Sarandí del Yí Shear Zone, Uruguay, were obtained in order to assess the tectonothermal evolution of this crustal‐scale structure. Integration of these results with available kinematic, structural, and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630–625 Ma during the collision of the Nico Pérez Terrane and the Río de la Plata Craton. The shear zone underwent dextral shearing up to 596 Ma under upper to middle amphibolite facies conditions, which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 Ma. After emplacement of the Cerro Caperuza granite at 570 Ma, the shear zone underwent only cataclastic deformation between the late Ediacaran and the Cambrian. The Sarandí del Yí Shear Zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the Río de la Plata Craton and the Nico Pérez Terrane. Furthermore, the obtained data reveal that strain partitioning and localization with time, magmatism emplacement, and fluid circulation are key processes affecting the isotopic systems in mylonitic belts, revealing the complexity in assessing the age of deformation of long‐lived shear zones.
Late Neogene tectonically driven crustal exhumation of the Sikkim Himalaya: Insights from inversion of multithermochronologic data
Authors: Landry Kyle R. , Coutand Isabelle , Whipp David M. , Grujic Djordje , Hourigan Jeremy K.
Published: March 2016

Apatite fission track and zircon (U‐Th)/He data are reported for 34 bedrock samples distributed between the foothills and the topographic crest of the Darjeeling‐Sikkim Himalaya. The pattern of observed cooling ages does not correlate with topography, rainfall distribution, and the deeply incised high‐relief Tista window, indicating that tectonic processes are mainly responsible for their spatial distribution. Inversion of this thermochronometric data set using 3‐D thermokinematic modeling constrained by independent geological and geophysical observations was performed to evaluate the contribution of slip partitioning, duplex development, and relief growth on the evolution of the thermal structure of the Himalaya during the last 12 Ma. Models involving significant relief growth do not show a substantial influence of topography evolution on the cooling age distribution, while models involving duplex growth demonstrate that tectonic processes exert a dominant influence on their distribution. In concert with equivalent studies in Bhutan, central Nepal, and NW India, our results attest that the lateral variation of the geometry and kinematics of the Himalayan basal décollement locally associated with duplex formation exert a leading influence on lateral variations of middle to upper crustal long‐term exhumation rates documented along the strike of the Himalaya.
Ice mass loss in Greenland, the Gulf of Alaska, and the Canadian Archipelago: Seasonal cycles and decadal trends
Authors: Harig Christopher , Simons Frederik J.
Published: March 2016

Over the past several decades mountain glaciers and ice caps have been significant contributors to sea level rise. Here we estimate the ice mass changes in the Canadian Archipelago, the Gulf of Alaska, and Greenland since 2003 by analyzing time‐varying gravimetry data from the Gravity Recovery and Climate Experiment. Prior to 2013, interannual ice mass variability in the Gulf of Alaska and in regions around Greenland remains within the average estimated over the whole data span. Beginning in summer 2013, ice mass in regions around Greenland departs positively from its long‐term trend. Over Greenland this anomaly reached almost 500 Gt through the end of 2014. Overall, long‐term ice mass loss from Greenland and the Canadian Archipelago continues to accelerate, while losses around the Gulf of Alaska region continue but remain steady with no significant acceleration.
The atmospheric partial lifetime of carbon tetrachloride with respect to the global soil sink
Authors: Rhew Robert C. , Happell James D.
Published: March 2016

The magnitude of the terrestrial soil sink for atmospheric carbon tetrachloride (CCl4) remains poorly constrained, with the estimated uncertainty range of CCl4 partial lifetimes between ~110 and 910 years. Field observations are sparse, and there are uncertainties in extrapolating these results to the global scale. Here we add to the published CCl4 fluxes with additional field measurements, and we employ a land cover classification scheme based on Advanced Very High Resolution Radiometer measurements that align more closely with the measurement sites to reevaluate the global CCl4 soil sink. We calculate an updated partial lifetime of CCl4 with respect to the soil sink to be 375 (288–536) years, which is 50 to 90% longer than the most recently published best estimates of the soil sink partial lifetime (195 and 245 years). This translates into a longer overall atmospheric lifetime estimate, which is more consistent with the observed atmospheric concentration trend and interhemispheric gradient.
Comparing GOSAT observations of localized CO2 enhancements by large emitters with inventory‐based estimates
Authors: Janardanan Rajesh , Maksyutov Shamil , Oda Tomohiro , Saito Makoto , Kaiser Johannes W. , Ganshin Alexander , Stohl Andreas , Matsunaga Tsuneo , Yoshida Yukio , Yokota Tatsuya
Published: March 2016

We employed an atmospheric transport model to attribute column‐averaged CO2 mixing ratios (XCO2) observed by Greenhouse gases Observing SATellite (GOSAT) to emissions due to large sources such as megacities and power plants. XCO2 enhancements estimated from observations were compared to model simulations implemented at the spatial resolution of the satellite observation footprint (0.1° × 0.1°). We found that the simulated XCO2 enhancements agree with the observed over several continental regions across the globe, for example, for North America with an observation to simulation ratio of 1.05 ± 0.38 (p < 0.1), but with a larger ratio over East Asia (1.22 ± 0.32; p < 0.05). The obtained observation‐model discrepancy (22%) for East Asia is comparable to the uncertainties in Chinese emission inventories (~15%) suggested by recent reports. Our results suggest that by increasing the number of observations around emission sources, satellite instruments like GOSAT can provide a tool for detecting biases in reported emission inventories.
Mapping 3D fault geometry in earthquakes using high‐resolution topography: Examples from the 2010 El Mayor‐Cucapah (Mexico) and 2013 Balochistan (Pakistan) earthquakes
Authors: Zhou Yu , Walker Richard T. , Elliott John R. , Parsons Barry
Published: March 2016

Fault dips are usually measured from outcrops in the field or inferred through geodetic or seismological modeling. Here we apply the classic structural geology approach of calculating dip from a fault's 3‐D surface trace using recent, high‐resolution topography. A test study applied to the 2010 El Mayor‐Cucapah earthquake shows very good agreement between our results and those previously determined from field measurements. To obtain a reliable estimate, a fault segment ≥120 m long with a topographic variation ≥15 m is suggested. We then applied this method to the 2013 Balochistan earthquake, getting dips similar to previous estimates. Our dip estimates show a switch from north to south dipping at the southern end of the main trace, which appears to be a response to local extension within a stepover. We suggest that this previously unidentified geometrical complexity may act as the endpoint of earthquake ruptures for the southern end of the Hoshab fault.
The importance of ENSO phase during volcanic eruptions for detection and attribution
Authors: Lehner Flavio , Schurer Andrew P. , Hegerl Gabriele C. , Deser Clara , Frölicher Thomas L.
Published: March 2016

Comparisons of the observed global‐scale cooling following recent volcanic eruptions to that simulated by climate models from the Coupled Model Intercomparison Project 5 (CMIP5) indicate that the models overestimate the magnitude of the global temperature response to volcanic eruptions. Here we show that this overestimation can be explained as a sampling issue, arising because all large eruptions since 1951 coincided with El Niño events, which cause global‐scale warming that partially counteracts the volcanically induced cooling. By subsampling the CMIP5 models according to the observed El Niño–Southern Oscillation (ENSO) phase during each eruption, we find that the simulated global temperature response to volcanic forcing is consistent with observations. Volcanic eruptions pose a particular challenge for the detection and attribution methodology, as their surface impacts are short‐lived and hence can be confounded by ENSO. Our results imply that detection and attribution studies must carefully consider sampling biases due to internal climate variability.
Volcanic lightning and plume behavior reveal evolving hazards during the April 2015 eruption of Calbuco volcano, Chile
Authors: Van Eaton Alexa R. , Amigo Álvaro , Bertin Daniel , Mastin Larry G. , Giacosa Raúl E. , González Jerónimo , Valderrama Oscar , Fontijn Karen , Behnke Sonja A.
Published: March 2016

Soon after the onset of an eruption, model forecasts of ash dispersal are used to mitigate the hazards to aircraft, infrastructure, and communities downwind. However, it is a significant challenge to constrain the model inputs during an evolving eruption. Here we demonstrate that volcanic lightning may be used in tandem with satellite detection to recognize and quantify changes in eruption style and intensity. Using the eruption of Calbuco volcano in southern Chile on 22 and 23 April 2015, we investigate rates of umbrella cloud expansion from satellite observations, occurrence of lightning, and mapped characteristics of the fall deposits. Our remote sensing analysis gives a total erupted volume that is within uncertainty of the mapped volume (0.56 ± 0.28 km3 bulk). Observations and volcanic plume modeling further suggest that electrical activity was enhanced both by ice formation in the ash clouds >10 km above sea level and development of a low‐level charge layer from ground‐hugging currents.
Enhanced ionospheric plasma bubble generation in more active ITCZ
Authors: Li Guozhu , Otsuka Yuichi , Ning Baiqi , Abdu M. A. , Yamamoto M. , Wan Weixing , Liu Libo , Abadi Prayitno
Published: March 2016

A close link between the atmospheric Intertropical Convergence Zone (ITCZ) and ionospheric plasma bubble has been proposed since the last century. But this relationship has often appeared to be less than convincing due to the simultaneous roles played by several other factors in shaping the global distribution of ionospheric bubbles. From simultaneous collaborative radar multibeam steering measurements at Kototabang (0.2°S, 100.3°E) and Sanya (18.4°N, 109.6°E), conducted during September–October of 2012 and 2013, we find that the total numbers of nights with bubble (i.e., occurrence rates) at the two closely located longitudes (Kototabang and Sanya) are comparable. But interestingly, the total number of nights with locally generated bubble (i.e., generation rate) over Kototabang is clearly more than that over Sanya. Further analysis reveals that a more active ITCZ is situated around the longitude of Kototabang. We surmise that the enhanced ionospheric bubble generation at Kototabang longitude could be caused by a higher gravity wave activity associated with the more active ITCZ.
On the electron diffusion region in asymmetric reconnection with a guide magnetic field
Authors: Hesse Michael , Liu Yi‐Hsin , Chen Li‐Jen , Bessho Naoki , Kuznetsova Masha , Birn Joachim , Burch James L.
Published: March 2016

Particle‐in‐cell simulations in a 2.5‐D geometry and analytical theory are employed to study the electron diffusion region in asymmetric reconnection with a guide magnetic field. The analysis presented here demonstrates that similar to the case without guide field, in‐plane flow stagnation and null of the in‐plane magnetic field are well separated. In addition, it is shown that the electric field at the local magnetic X point is again dominated by inertial effects, whereas it remains dominated by nongyrotropic pressure effects at the in‐plane flow stagnation point. A comparison between local electron Larmor radii and the magnetic gradient scale lengths predicts that distribution should become nongyrotropic in a region enveloping both field reversal and flow stagnation points. This prediction is verified by an analysis of modeled electron distributions, which show clear evidence of mixing in the critical region.
Unsolved problems: Mesoscale polar cap flow channels' structure, propagation, and effects on space weather disturbances
Authors: Lyons L. R. , Nishimura Y. , Zou Y.
Published: March 2016

Recent evidence indicates that the magnetosphere‐ionosphere system is frequently driven by dynamic mesoscale flow structures that are first seen near the dayside cusps, move across the open field line regions of the polar caps, and then enter the plasma sheet where they lead to a large variety of space weather disturbances. It will be important to uncover the features of these flow enhancements, including their structure, how they propagate across the polar cap, and what controls their dynamic effects after reaching the nightside plasma sheet.