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  • Articles  (42,533)
  • Wiley  (42,533)
  • 2010-2014  (35,707)
  • 1985-1989  (5,756)
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  • 1945-1949  (318)
  • Physics  (42,533)
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  • Articles  (42,533)
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  • 1
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    Differentiating flavors of the Indian Ocean Dipole using dominant modes in tropical Indian Ocean rainfall (2014)
    Wiley
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    Publication Date: 2014-12-07
    Description: Phase-locked to austral winter and spring, canonical positive Indian Ocean Dipoles (pIODs) generally peak in spring. In recent decades, there has been an increase in unseasonable pIODs which, different from canonical pIODs, peak and decay by September. Distinguishing unseasonable pIODs from canonical pIODs is important, as conditions leading to more frequent unseasonable events are projected to persist in a warming climate. Here using superimposition of the first two seasonally evolving dominant modes of tropical Indian Ocean rainfall variability, we differentiate these types of pIODs. The first mode reflects characteristics of canonical pIODs, in which anomalies intensify with seasonal evolution. However, the second mode, with cool and dry anomalies extending from the eastern pole, reverses from winter to spring, signifying the demise of unseasonable pIODs. Processes embedded in the second mode reflect timing of propagation in equatorial Kelvin waves, and their relative importance to the first mode, in generating different pIODs.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 2
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    The Oslo temperature series 1837–2012: homogeneity testing and temperature analysis (2014)
    Wiley
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    Publication Date: 2014-12-07
    Description: ABSTRACT A 175 years long homogenized composite record of monthly mean temperatures is presented for Oslo, the capital of Norway. The early raw data have been digitised and quality controlled, and monthly means have been calculated. Some early original observations carried out in a Wild screen (1877–1936) were found to be spuriously high because of inappropriate sheltering from sunlight. These spurious temperatures were not used in the composite record, but alternative temperatures measured (1837–1933) by thermometers placed outside windows at the Astronomical Observatory were used instead. No inhomogeneity was detected in the latter series after adding an instrument correction of +0.3 °C, but the start year of the correction remains uncertain. The more recent part of the composite record used the long-term series (1937 to present) from Blindern in Oslo, the premises of The Norwegian Meteorological Institute. Two small inhomogeneities were detected in the Blindern series, possibly caused by a weak urban heat island effect or growing/cutting of trees. The study revealed that the annual mean temperature has increased by 1.5 °C in the period 1838–2012. The most pronounced increase in annual temperature occurred during the last 50 years, and in the early 20th century that ended with a local maximum in the 1930s. The temperature has increased significantly in all seasons; however, the temperature increase in summer was less than a half of that in winter and spring, which were the seasons with largest increase. In addition the monthly mean temperature of the coldest month in each year has increased two times faster than the warmest one. The most significant temperature variations were associated to ∼ 5-year time scales in its early part, but since 1930 and up to present, the dominant time scales were 10–20 years.
    Print ISSN: 0899-8418
    Electronic ISSN: 1097-0088
    Topics: Geosciences , Physics
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  • 3
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    Issue Information (2014)
    Wiley
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    Publication Date: 2014-12-13
    Description: No abstract is available for this article.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
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  • 4
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    Extremely Low Frequency Band Station for Natural Electromagnetic Noise Measurement (2014)
    Wiley
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    Publication Date: 2014-12-14
    Description: A new permanent ELF measurement station has been deployed in Sierra Nevada, Spain. It is composed of two magnetometers, oriented NS and EW, respectively. At 10 Hz, their sensitivity is 19 μ zV/pT and the Signal to Noise Ratio (SNR) is 28 dB for a time-varying signal of 1 pT, the expected field amplitude in Sierra Nevada. The station operates for frequencies below 24 Hz. The magnetometers, together with their corresponding electronics, have been specifically designed to achieve such an SNR for small signals. They are based on high-resolution search coils with ferromagnetic core and 10 6 turns, operating in limited geometry configuration. Different system noise sources are considered and a study of the SNR is also included. Finally, some initial Schumann resonance measurements are presented in order to validate the performance of the measurement station, including one hour length spectra, daily variations of resonance amplitudes and frequencies for the different seasons, and a three day spectrogram.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
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  • 5
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    Steered migration in hard rock environments (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: ABSTRACT Hard rock seismic exploration normally has to deal with rather complex geological environments. These types of environments are usually characterized by a large number of local heterogeneity (e.g., faults, fracture zones, and steeply dipping interfaces). The seismic data from such environments often have a poor signal-to-noise ratio because of the complexity of hard rock geology. To be able to obtain reliable images of subsurface structures in such geological conditions, processing algorithms that are capable of handling seismic data with a low signal-to-noise ratio are required for a reflection seismic exploration. In this paper, we describe a modification of the 3D Kirchhoff post-stack migration algorithm that utilizes coherency attributes obtained by the diffraction imaging algorithm in 3D to steer the main Kirchhoff summation. The application to a 3D synthetic model shows the stability of the presented steered migration to the presence of high level of the random noise. A test on the 3D seismic volume, acquired on a mine site located in Western Australia, reveals the capability of the approach to image steep and sharp objects such as fracture and fault zones and lateral heterogeneity.
    Print ISSN: 0016-8025
    Electronic ISSN: 1365-2478
    Topics: Geosciences , Physics
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  • 6
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    Issue Information (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: No abstract is available for this article.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    Simulating Diurnal Variations Over the Southeastern United States (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: The diurnal variations from a high-resolution regional climate model (Regional Spectral Model; RSM) are analyzed from 6 independent decade long integrations using lateral boundary forcing data separately from the National Centers for Environmental Prediction Reanalysis 2 (NCEPR2), and European Center for Medium-Range Weather Forecasts (ECMWF) 40-year Reanalysis (ERA40) and the 20 th Century Reanalysis (20CR). With each of these lateral boundary forcing data, the RSM is integrated separately using two convection schemes: the Relaxed Arakawa-Schubert (RAS) and Kain-Fritsch (KF) schemes. The results show that RSM integrations forced with 20CR have the least fidelity in depicting the seasonal cycle and diurnal variability of precipitation and surface temperature over the Southeastern United States (SEUS). The remaining four model simulations show comparable skills. The differences in the diurnal amplitude of rainfall during the summer months of the 20CR forced integration from the corresponding NCEPR2 forced integration, for example, is found to be largely from the transient component of the moisture flux convergence. The root mean square error (RMSE) of the seasonal cycle of precipitation and surface temperature of the other four simulations (not forced by 20CR) were comparable to each other and highest in the summer months. But the RMSE of the diurnal amplitude of precipitation and the timing of its diurnal zenith were largest during winter months and least during summer and fall months in the four model simulations (not forced by 20CR). The diurnal amplitude of surface temperature in comparison showed far less fidelity in all models. The phase of the diurnal maximum of surface temperature however showed significantly better validation with corresponding observations in all of the 6 model simulations
    Print ISSN: 0148-0227
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  • 8
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    Tracking major storms from microseismic and hydroacoustic observations on the seafloor (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Ocean wave activity excites seismic waves that propagate through the solid earth, known as microseismic noise. Here we use a network of 57 ocean-bottom seismometers (OBS) deployed around La Réunion Island in the southwest Indian Ocean to investigate the noise generated in the secondary microseismic band as a tropical cyclone moved over the network. Spectral and polarization analyses show that microseisms strongly increase in the 0.1 - 0.35 Hz frequency band as the cyclone approaches, and that this noise is composed of both compressional and surface waves, confirming theoretical predictions. We infer the location of maximum noise amplitude in space and time, and show that it coincides roughly with the location of maximum ocean-wave interactions. Although this analysis was performed retrospectively, microseisms recorded on the seafloor can be considered a novel source of information for future real-time tracking and monitoring of major storms, complementing atmospheric, oceanographic and satellite observations.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 9
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    Galápagos Coral Reef Persistence after ENSO Warming Across an Acidification Gradient (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Anthropogenic CO 2 is causing warming and ocean acidification. Coral reefs are being severely impacted, yet confusion lingers regarding how reefs will respond to these stressors over this century. Since the 1982-83 El Niño-Southern Oscillation warming event, the persistence of reefs around the Galápagos Islands has differed across an acidification gradient. Reefs disappeared where pH 〈 8.0 and aragonite saturation state (Ω arag ) ≤ 3 and have not recovered, whereas one reef has persisted where pH 〉 8.0 and Ω arag  〉 3. Where upwelling is greatest, calcification by massive Porites is higher than predicted by a published relationship with temperature despite high CO 2 , possibly due to elevated nutrients. However, skeletal P/Ca, a proxy for phosphate exposure, negatively correlates with density ( R  = -0.822, p  〈 0.0001). We propose that elevated nutrients have the potential to exacerbate acidification by depressing coral skeletal densities and further increasing bioerosion already accelerated by low pH.
    Print ISSN: 0094-8276
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  • 10
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    Changes to the temporal distribution of daily precipitation (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Changes to the temporal distribution of daily precipitation were investigated using a dataset of 12,513 land-based stations from the Global Historical Climatology Network (GHCN). The distribution of precipitation was measured using the Gini index (which describes how uniformly precipitation is distributed throughout a year) and the annual number of wet days. The Mann-Kendall test and a regression analysis were used to assess the direction and rate of change to both indices. Over the period of 1976-2000, East Asia, central America and Brazil exhibited a decrease in the number of both wet and light precipitation days, and eastern Europe exhibited a decrease in the number of both wet and moderate precipitation days. In contrast, the USA, southern South America, western Europe and Australia exhibited an increase in the number of both wet and light precipitation days. Trends in both directions were field significant at the global scale.
    Print ISSN: 0094-8276
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  • 11
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    Effects of the post-perovskite phase change on the observed geoid (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: In the lowermost mantle, seismic velocity variations beneath Pacific margins have been related to the perovskite (pv) to post-perovskite (pPv) phase transition. We investigate the influence of this phase transformation on the geoid using 3D spherical mantle circulation models based on a seismic tomography model and strong lateral viscosity variations in the lower mantle. We demonstrate that the geoid anomalies are strongly affected by the presence of pPv because of phase-dependent viscosity changes relative to the surrounding mantle. Whereas geoid heights above subduction zones are increased for high-viscosity pPv, the presence of weak pPv reduces them, thereby improving the fit to the observed geoid. An investigation using two different tomography models, different pPv density contrasts, and the presence or absence of a global thermal boundary layer and of lateral viscosity variations in the lower mantle demonstrate the various effects of weak pPv on the geoid.
    Print ISSN: 0094-8276
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  • 12
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    Interdecadal Pacific variability and eastern Australian mega-droughts over the last millennium (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: The Interdecadal Pacific Oscillation (IPO) influences multidecadal drought risk across the Pacific, but there are no millennial-length, high resolution IPO reconstructions for quantifying long-term drought risk. In Australia, drought risk increases in positive phases of the IPO, yet few suitable rainfall proxies and short (~100 y) instrumental records mean large uncertainties remain around drought frequency and duration. Likewise, it is unknown whether mega-droughts have occurred in Australia's past. In this study, an atmospheric teleconnection in the Indian Ocean mid-latitudes linking East Antarctica and Australia is exploited to produce the first accurate, annually dated millennial-length IPO reconstruction from the Law Dome (East Antarctica) ice core. Combined with an eastern Australian rainfall proxy from Law Dome, the first millennial-length Australian mega-drought (〉5 y duration) reconstruction is presented. Eight mega-droughts are identified including one 39 y drought (AD 1174–1212), which occurred during an unprecedented century of aridity (AD 1102–1212).
    Print ISSN: 0094-8276
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  • 13
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    The onset, retreat and the length of growing season in the north-eastern region of Sri Lanka (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: ABSTRACT The onset, retreat and the length of growing season in the north-eastern region of Sri Lanka were investigated using daily rainfall data for the period 1961 to 2000. Data from three weather stations situated in the coastal belt in the northern and eastern parts of Sri Lanka (Jaffna, Trincomalee and Batticaloa) that receive rainfall predominantly from the northeast monsoon were selected for this study. A method based on cumulative rainy days was utilized in the determination of the onset and retreat dates. It is shown that there is substantial interannual variability in onset and retreat dates. The mean onset and retreat dates fall on the standard week 38.3 ± 2.7 and 53.0 ± 2.9, respectively. The mean duration of the growing season is 14.7 ± 3.4 weeks. The retreat date and thus the length of growing season could be extended by 2 weeks if the probability of occurrence of rain during the onset is favourable for the retreat. The results indicate that there has been no significant trend in the onset and retreat dates during the last 40 years in the dry zone of Sri Lanka. The onset date and the length of growing season are weakly correlated with early onset dates leading to longer growing seasons. The study concludes that rainy days could be used successfully to determine the mean rainfall onset and retreat dates in the dry zone of Sri Lanka.
    Print ISSN: 0899-8418
    Electronic ISSN: 1097-0088
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  • 14
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    Revision and extension of the composite Carlisle rainfall record, northwest England: 1757–2012 (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: ABSTRACT A 256-year composite monthly rainfall record representative of the lowlands around Carlisle, northwest England is presented, providing the third longest instrumental record of rainfall available in the UK and expanding the sparse network of long homogenous rainfall series. This article describes the construction of the rainfall record for the period 1757–2012 and presents analyses of long-term annual and seasonal variability, with a particular focus on wet/dry extremes. Three primary rainfall records from Carlisle underpin the reconstruction, with others selected based on length of record and proximity to the primary recording stations. Linear regression and adjustment factors were applied to create a homogenous continuous rainfall record, assessed by cross-comparison with other long-term UK rainfall records. Double-mass curve and standard normal homogeneity tests using long records representative of Manchester and Appleby confirmed that the Carlisle record is homogenous, but includes a period of uncertainty during the period 1886–1911, which is within the recognized instrumental phase of recording. Analysis of the series shows long-term temporal rainfall variability, with seasonal rainfall totals showing a significant increase in winter rainfall. An increasing trend in annual rainfall was also identified but is not significant. Several previously documented notable extreme wet (e.g. winter 1834) and dry (e.g. summer 1995) seasons were identified, along with several additional seasons (e.g. winter 1764 and summer 1891) that can be considered as extreme and occurred during the more poorly understood 18th and early 19th centuries. These results allow recent extremes to be placed within the context of long-term natural variability. At the decadal timescale seasonal rainfall totals are highly variable. The wettest season fluctuates between autumn and summer until the late 20th century (1990s), when winter became wetter than any other season for the first time on record.
    Print ISSN: 0899-8418
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  • 15
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    A conceptual model for assessing rainfall and vegetation trends in sub-Saharan Africa from satellite data (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: ABSTRACT Policymakers, governments and aid agencies require operational environmental monitoring in support of evidence-based policy-making and resource deployment in crisis situations. For Africa, this is only feasible at sub-continental scale with a large network of automated meteorological stations, a large number of highly coordinated field observers or with satellite remote sensing. The challenge with satellite data lies in the derivation of meaningful environmental indicators. This article describes a conceptual framework for understanding satellite-derived indicators of rainfall and vegetation greenness trends over Africa. It attributes observed vegetation changes to climatic (i.e. rainfall linked) and non-climatic drivers. A decade of annual rainfall and vegetation data over sub-Saharan Africa was analysed using satellite-based rainfall estimates [Famine Early Warning System Rainfall Estimation 2.0 (FEWSNET RFE 2.0)] from National Oceanic and Atmospheric Administration's (NOAA's) Climate Prediction Centre and the Normalized Difference Vegetation Index (NDVI) obtained from the Satellite Pour l'Observation de la Terre Vegetation (SPOT-VGT) sensor. Rainfall and vegetation greenness trends were analysed for 759 administrative regions of sub-Saharan Africa to identify those regions that have experienced a negative, positive or stable rainfall/vegetation trend over the period 2001–2010. The character of the relationship between the annual rainfall and max NDVI trends were examined to identify areas where the changes in greenness could be attributed to climatic (rainfall) and non-climatic (human land use or ecological disturbance) changes. Regions where increasing rainfall was associated with vegetation greening were found in West Africa, Central African Republic, West Cameroon and northeastern part of South Africa, whereas areas with evidence of ‘climatic vegetation degradation’ were located in Southern Madagascar, Nigeria, Kenya and the Garden Route region of South Africa.
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  • 16
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    Field Observations of Wave-Driven Circulation over Spur and Groove Formations on a Coral Reef (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: Spur and groove (SAG) formations are found on the forereefs of many coral reefs worldwide. Modeling results have shown that SAG formations together with shoaling waves induce a nearshore Lagrangian circulation pattern of counter-rotating circulation cells, but these have never been observed in the field. We present results from two separate field studies of SAG formations on Palmyra Atoll which show their effect on waves to be small, but reveal a persistent order 1 cm/s depth-averaged Lagrangian offshore flow over the spur and onshore flow over the grooves. This circulation was stronger for larger, directly-incident waves and low alongshore flow conditions, consistent with predictions from modeling. Favorable forcing conditions must be maintained on the order of one hour to accelerate and develop the SAG circulation cells. The primary cross- and alongshore depth-averaged momentum balances were between the pressure gradient, radiation stress gradient and nonlinear convective terms, and the bottom drag was similar to values found on other reefs. The vertical structure of these circulation cells was previously unknown and the results show a complex horizontal offshore Lagrangian flow over the spurs near the surface driven by alongshore variability in radiation stress gradients. Vertical flow was downward over the spur and upward over the groove, likely driven by alongshore differences in bottom stress and not by vortex forcing. This article is protected by copyright. All rights reserved.
    Print ISSN: 0148-0227
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  • 17
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    Energy and heat fluxes due to vertically-propagating Yanai waves observed in the equatorial Indian ocean (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: Shipboard current measurements in the equatorial Indian Ocean in October and November of 2011 revealed oscillations in the meridional velocity with amplitude ~ 0.10m/s. These were clearest in a layer extending from ~300 to 600 m depth and had periods near 3 weeks. Phase propagation was upward. Measurements from two sequential time series at the equator, four meridional transects and one zonal transect are used to identify the oscillation as a Yanai wave packet and to establish its dominant frequency and vertical wavelength. The Doppler shift is accounted for, so that measured wave properties are translated into the reference frame of the mean zonal flow. We take advantage of the fact that, in the depth range where the wave signal was clearest, the time-averaged current and buoyancy frequency were nearly uniform with depth, allowing application of the classical theoretical representation of vertically propagating plane waves. Using the theory, we estimate wave properties that are not directly measured, such as the group velocity and the zonal wavelength and phase speed. The theory predicts a vertical energy flux that is comparable to that carried by midlatitude near-inertial waves. We also quantify the wave-driven meridional heat flux and the Stokes drift. This article is protected by copyright. All rights reserved.
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  • 18
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    Issue Information (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: No abstract is available for this article.
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  • 19
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    Impact of measurement uncertainties on determination of chlorophyll-specific absorption coefficient for marine phytoplankton (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: Understanding variability in the chlorophyll-specific absorption of marine phytoplankton, a ph * Chl (λ) , is essential for primary production modelling, calculation of underwater light field characteristics, and development of algorithms for remote sensing of chlorophyll concentrations. Previous field and laboratory studies have demonstrated significant apparent variability in a ph * Chl (λ) for natural samples and algal cultures. However, the potential impact of measurement uncertainties on derived values of a ph * Chl (λ) has received insufficient study. This study presents an analysis of measurement uncertainties for a data set collected in the Ligurian Sea in Spring and assesses the impact on estimates of a ph * Chl (λ) . It is found that a large proportion of apparent variability in this set of a ph * Chl (λ) can be attributed to measurement errors. Application of the same analysis to the global NOMAD data set suggests that a significant fraction of variability in a ph * Chl (λ) may also be due to measurement errors. This article is protected by copyright. All rights reserved.
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  • 20
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    An optimal XBT-based monitoring system for the South Atlantic Meridional Overturning Circulation at 34°S (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: The South Atlantic is an important pathway for the inter-basin exchanges of heat and freshwater with strong influence on the global meridional overturning stability and variability. Along the 34°S parallel, a quarterly, high resolution XBT transect (AX18) samples the temperature structure in the upper ocean. The AX18 transect has been shown to be a useful component of a meridional overturning monitoring system of the region. However, a feasible, cost-effective design for an XBT-based system has not yet been developed. Here we use a high-resolution ocean assimilation product to simulate an XBT-based observational system across the South Atlantic. The sensitivity of the meridional heat transport, meridional overturning circulation, and geostrophic velocities to key observational and methodological assumptions is studied. Key assumptions taken into account are horizontal and temporal sampling of the transect, salinity and deep temperature inference, as well as the level of reference for geostrophic velocities. With the current sampling strategy, the largest errors in the meridional overturning and heat transport estimations are the reference (barotropic) velocity and the western boundary resolution. We show how altimetry can be used along with hydrography to resolve the barotropic component of the flow. We use the results obtained by the state estimation under observational assumptions to make recommendations for potential improvements in the AX18 transect implementation. This article is protected by copyright. All rights reserved.
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  • 21
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    Atmospheric Response in Summer linked to Recent Arctic Sea Ice Loss (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: Since 2007 a large decline in Arctic sea ice has been observed. The large-scale atmospheric circulation response to this decline is investigated in ERA-Interim reanalyses and HadGEM3 climate model experiments. In winter, post-2007 observed circulation anomalies over the Arctic, North Atlantic and Eurasia are small compared to interannual variability. In summer, the post-2007 observed circulation is dominated by an anticyclonic anomaly over Greenland which has a large signal-to-noise ratio. Climate model experiments driven by observed SST and sea ice anomalies are able to capture the summertime pattern of observed circulation anomalies, although the magnitude is a third of that observed. The experiments suggest warm SSTs and reduced sea ice in the Labrador Sea lead to warm temperature anomalies in lower troposphere which weaken the westerlies over North America through thermal wind balance. The experiments also capture cyclonic anomalies over north-western Europe, which are consistent with downstream Rossby wave propagation.
    Print ISSN: 0035-9009
    Electronic ISSN: 1477-870X
    Topics: Geography , Physics
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  • 22
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    Variability of the subtropical highs, African easterly jet and easterly wave intensities over North Africa and Arabian Peninsula in late summer (2014)
    Wiley
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    Publication Date: 2014-12-18
    Description: ABSTRACT The North African climate is analysed for August during a 32-year period using the European Centre for Medium Range Weather Forecast (ECMWF) global data set to investigate the intensity variability at 600 mb of the subtropical highs, Africa easterly jet (AEJ) with two embedded local wind maxima, and African easterly waves over North Africa and the Arabian Peninsula. The variability of these synoptic weather systems is higher in East Africa. The most noticeable variability of intensity occurred with easterly waves. Maintenance of easterly waves from the Arabian Peninsula into East Africa is dependent on strong zonal gradients from the AEJ through shear vorticity. These zonal gradients were induced by the strengthening of the subtropical highs and the presence of a westerly jet in Central Africa and south of the Arabian Peninsula. During positive ENSO periods, these systems are generally weaker while in negative periods are stronger. The focus of this research is to investigate the role of the Arabian High and eastern local wind maximum (LWM E ) on complementing the Saharan High and western local wind maximum (LWM W ). It is found that an intense local wind maximum in East Africa helps maintain the easterly waves and their westward propagation from the Arabian Peninsula.
    Print ISSN: 0899-8418
    Electronic ISSN: 1097-0088
    Topics: Geosciences , Physics
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    Oceanic influence on the Baiu frontal zone in the East China Sea (2014)
    Wiley
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    Publication Date: 2014-12-18
    Description: A high-resolution transect of atmospheric soundings across the Kuroshio Current in the East China Sea was conducted onboard a ship in June 2012 with the objective of analyzing the influence of the complex sea surface temperature (SST) distribution on the Baiu frontal zone (BFZ). Expendable bathythermograph castings and continuous surface meteorological observations were also examined. Two distinct mesoscale atmospheric fronts, characterized by changes of wind direction in the lower troposphere and surface air temperature (SAT), were found in the BFZ. One (northern) atmospheric front was observed around the SST front in relation to a warm water tongue extending from the Kuroshio. A high SST region around the northern atmospheric front enhances unstable near surface stratification and intensifies turbulent heat flux. They help modify the marine atmospheric boundary layer in the BFZ. The other (southern) atmospheric front was at the southern end of the BFZ. Intense evaporation over the Kuroshio and moisture transport by southerly winds were important in forming the conditionally unstable air masses in the lower troposphere of the BFZ.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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    Variability of stratospheric mean age of air and of the local effects of residual circulation and eddy mixing (2014)
    Wiley
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    Publication Date: 2014-12-18
    Description: We analyze the variability of mean age of air (AoA) and of the local effects of the stratospheric residual circulation and eddy mixing on AoA within the framework of the isentropic zonal mean continuity equation. AoA for the period 1988–2013 has been simulated with the Lagrangian chemistry transport model CLaMS driven by ERA-Interim winds and diabatic heating rates. Model simulated AoA in the lower stratosphere shows good agreement with both in-situ observations and satellite observations from MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), even regarding interannual variability and changes during the last decade. The interannual variability throughout the lower stratosphere is largely affected by the QBO-induced circulation and mixing anomalies, with year-to-year AoA changes of about 0.5 years. The decadal 2002–2012 change shows decreasing AoA in the lowest stratosphere, below about 450 K. Above, AoA increases in the NH and decreases in the SH. Mixing appears to be crucial for understanding AoA variability, with local AoA changes resulting from a close balance between residual circulation and mixing effects. Locally, mixing increases AoA at low latitudes (40S-40N) and decreases AoA at higher latitudes. Strongest mixing occurs below about 500 K, consistent with the separation between shallow and deep circulation branches. The effect of mixing integrated along the air parcel path, however, significantly increases AoA globally, except in the polar lower stratosphere. Changes of local effects of residual circulation and mixing during the last decade are supportive of a strengthening shallow circulation branch in the lowest stratosphere and a southward shifting circulation pattern above.
    Print ISSN: 0148-0227
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    A stability dependent parametrization of transfer coefficients for momentum and heat over polar sea ice to be used in climate models (2014)
    Wiley
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    Publication Date: 2014-12-18
    Description: The interaction between sea ice and atmosphere depends strongly on the near-surface transfer coefficients for momentum and heat. A parametrization of these coefficients is developed on the basis of an existing parametrization of drag coefficients for neutral stratification that accounts for form drag caused by the edges of ice floes and melt ponds. This scheme is extended to better account for the dependence of surface wind on limiting cases of high and low ice concentration and to include near-surface stability effects over open water and ice on form drag. The stability correction is formulated on the basis of stability functions from Monin-Obukhov similarity theory and also using the Louis concept with stability functions depending on thebulk Richardson numbers. Furthermore, a parametrization is proposed that includes the effect of edge related turbulence also on heat transfer coefficients. The parametrizations are available in different levels of complexity. The lowest level only needs sea ice concentration and surface temperature as input while the more complex level needs additional sea ice characteristics. An important property of our parametrization is that form drag caused by ice edges depends on the stability over both ice and water which is in contrast to the skin drag over ice. Results of the parametrization show that stability has a large impact on form drag and, thereby, determines the value of sea ice concentration for which the transfer coefficients reach their maxima. Depending on the stratification, these maxima can occur anywhere between ice concentrations of 20 and 80%.
    Print ISSN: 0148-0227
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    A review of Stratospheric Sounding Unit radiance observations for climate trends and reanalyses (2014)
    Wiley
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    Publication Date: 2014-12-18
    Description: Stratospheric Sounding Units (SSU) on the NOAA polar orbiting satellites measured infrared radiances in the 15 micron CO 2 band between late 1978 and mid-2006. From these radiances a time series of layer mean stratospheric temperatures has been derived by several groups. Discrepancies in these temperature analyses have been highlighted recently and efforts are now underway to resolve the differences between them. This paper is the Met Office response summarising the issues to be resolved in creating a climate data record from the different SSUs, including corrections for radiometric, spectroscopic and tidal differences. Calibration issues identified include the SSU space view anomaly and radiometric anomalies in the NOAA-9 observations. The spectroscopic correction required for changing pressures in the pressure modulator cells is also outlined. The most important correction for the time series is for the solar diurnal and semi-diurnal tides as the satellite overpass local times change. Comparisons with other stratospheric temperature trend analyses are made and the reasons for the differences discussed. The time series presented here show sustained drops in stratospheric temperatures at all levels after the El Chichon and Pinatubo eruptions but only small trends to lower temperatures between eruptions.
    Print ISSN: 0035-9009
    Electronic ISSN: 1477-870X
    Topics: Geography , Physics
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    Buried Object Detection by Means of a Lp Banach-Space Inversion Procedure (2014)
    Wiley
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    Publication Date: 2014-12-18
    Description: Electromagnetic inspection techniques are becoming powerful tools for buried object detection and subsurface prospection in several applicative fields, such as civil engineering and archeology. However, the nonlinearity and ill-posedness of the underlying inverse problem make the development of efficient imaging techniques a very challenging task. In the present paper, an algorithm based on a regularizing approach in L p Banach spaces is proposed for tackling such problems. The effectiveness of the approach is verified by means of numerical simulations in a noisy environment, aimed at evaluating the reconstruction capabilities with respect to the choice of several model parameters. The reported results show that, for small targets, the use of L p Banach spaces with 1 〈  p  〈 2 allows to obtain a better localization of different buried scatterers.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
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    High-order multi-moment constrained finite volume global shallow water model on the Yin-Yang grid (2014)
    Wiley
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    Publication Date: 2014-12-18
    Description: A high-order global shallow water model on Yin-Yang grid has been developed by using the multi-moment constrained finite volume (MCV) method. Different from the traditional finite volume method, more degrees of freedom (DOFs) which are the values at the solution points within each mesh element are defined and updated in time. The time evolution equations for these point values are derived from a set constraint conditions in terms of the so-called multi-moment quantities, such as the point value (PV), the volume-integrated average (VIA) and derivative (DV). Different moments use different forms of equations which are all consistent with the shallow water equations, among which the VIA moment is computed from a finite volume formulation of flux form that guarantees the rigorous numerical conservation. A fourth-order formulation is devised with the third-order reconstruction built over each element using the DOFs locally available. A simple and orthogonal overset grid, the Yin-Yang grid, is used to represent the spherical geometry with quasi-uniform grid spacing. The resulting global shallow water model is attractive in algorithmic simplicity and computational efficiency. The model has been validated by widely used benchmark tests. The numerical results of the present model are competitive to most existing advanced models.
    Print ISSN: 0035-9009
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    Topics: Geography , Physics
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    External total alkalinity loads vs. internal generation - the influence of non-riverine alkalinity sources in the Baltic Sea (2014)
    Wiley
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    Publication Date: 2014-11-07
    Description: In this study we first present updated riverine total alkalinity (TA) loads to the various Baltic Sea sub-basins, based on monthly measurements in 82 of the major rivers that represent 85% of the total runoff. Simulations in the coupled physical-biogeochemical BALTSEM model show that these river loads together with North Sea water inflows are not sufficient to reproduce observed TA concentrations in the system, demonstrating the large influence from internal sources. Budget calculations indicate that the required internal TA generation must be similar to river loads in magnitude. The non-riverine source in the system amounts to about 2.4 mmol m -2 d -1 on average. We argue here that the majority of this source is related to denitrification together with unresolved sediment processes such as burial of reduced sulfur and/or silicate weathering. This hypothesis is supported by studies on sediment processes on a global scale, and also by data from sediment cores in the Baltic Sea. In a model simulation with all internal TA sources and sinks switched on, the net absorption of atmospheric CO 2 increased by 0.78 mol C m -2 y -1 compared to a simulation where TA was treated as a passive tracer. Our results clearly illustrate how pelagic TA sources together with anaerobic mineralization in coastal sediments generate a significant carbon sink along the aquatic continuum, mitigating CO 2 evasions from coastal and estuarine systems.
    Print ISSN: 0886-6236
    Electronic ISSN: 1944-9224
    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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    Error estimation of spectral parameters for high-resolution wind and turbulence measurements by wind profiler radars (2014)
    Wiley
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    Publication Date: 2014-11-07
    Description: Using numerical simulations, we investigated a method for calculating the spectral parameters from Doppler spectra collected by high-resolution wind profiler radars (WPRs). Because high-resolution WPRs collect a huge amount of Doppler spectra, calculations must be simple and fast. The proposed method has two steps. In the first step, the echo range ( R echo ), in which the Doppler spectrum point with peak intensity is contained and all the smoothed Doppler spectrum points have intensities that are greater than the noise intensity, was determined. For producing the smoothed Doppler spectrum, a running average with equal weight (RA) or multi-taper method (MTM) was used. In the second step, the spectral parameters were calculated using the Doppler spectrum points within R echo . By comparing the performance of the computation methods using RA and MTM, we concluded that the computation method using RA is more suitable because it has better estimation performance for small spectrum widths and the calculations are faster. Estimation error of the spectral parameters depends on the determination accuracy of the Doppler spectrum peak and R echo . Furthermore, for the case of a 512-point Doppler spectrum and 13-point RA, the estimation errors tend to be independent of the signal-to-noise ratio (SNR) when the peak level of the Doppler spectrum ( p est ) is ~8 dB or more greater than the noise intensity. For p est of 〈 ~8 dB, the estimation errors are well correlated to p est and the SNR. Therefore, the number of incoherent integration times should be determined by considering the SNR and p est .
    Print ISSN: 0048-6604
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    Tohoku-Oki Earthquake Caused Major Ionospheric Disturbances at 450 km Altitude Over Alaska (2014)
    Wiley
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    Publication Date: 2014-11-07
    Description: Ionospheric total electron content (TEC) and atmospheric density perturbations were derived from measurements made from instruments onboard the Gravity Recovery and Climate Experiment (GRACE) spacecraft. At the time of the Tohoku-Oki earthquake on March 11, 2011, the twin spacecraft were orbiting at an altitude of ~450 km over Alaska. Significant TEC fluctuations (up to 0.6 TEC units), atmospheric density perturbations (~3.6 · 10 −14  kg/m 3 ), and sudden changes in GRACE acceleration (~4 · 10 −8  m/s 2 ) were observed ~8 minutes after the arrival of seismic and infrasound waves on the ground in Alaska, ~20 minutes after the Tohoku-Oki main shock at 05:46:23 UTC. The results of three-dimensional ionospheric-thermospheric modeling and infrasound ray-tracing simulations are consistent with the arrival time and physical characteristics of the disturbances at GRACE. This is the first time that ionospheric disturbances associated with an earthquake are clearly attributable to perturbations at such high altitudes.
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    The Source Altitude, Electric Current, and Intrinsic Brightness of Terrestrial Gamma Ray Flashes (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: Many details of how thunderstorms generate terrestrial gamma-ray flashes (TGFs) and other forms of high energy radiation remain uncertain, including the basic question of where they are produced. We exploit the association of distinct low frequency radio emissions with generation of terrestrial gamma-ray flashes (TGFs) to directly measure for the first time the TGF source altitude. Analysis of two events reveals source altitudes of 11.8 ± 0.4 km and 11.9 ± 0.9 km. This places the source region in the interior of the thunderstorm between the two main charge layers, and implies an intrinsic TGF brightness of approximately 10 18 runaway electrons. The electric current in this non-traditional lightning process is found to be strong enough to drive nonlinear effects in the ionosphere, and in one case is comparable to the highest peak current lightning processes on the planet.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
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    Counteracting the climate effects of volcanic eruptions using short-lived greenhouse gases (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: A large volcanic eruption might constitute a climate emergency, significantly altering global temperature and precipitation for several years. Major future eruptions will occur, but their size or timing cannot be predicted. We show, for the first time, that it may be possible to counteract these climate effects through deliberate emissions of short-lived greenhouse gases, dampening the abrupt impact of an eruption. We estimate an emission pathway countering a hypothetical eruption three times the size of Mt Pinatubo in 1991. We use a global climate model to evaluate global and regional responses to the eruption, with and without counter emissions. We then raise practical, financial and ethical question related to such a strategy. Unlike the more commonly-discussed geoengineering to mitigate warming from long-lived greenhouse gases, designed emissions to counter temporary cooling would not have the disadvantage of needing to be sustained over long periods. Nevertheless, implementation would still face significant challenges.
    Print ISSN: 0094-8276
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    Rayleigh wave group velocity distributions for East Asia using ambient seismic noise (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: Vertical component data from 206 broadband seismometer stations from Korean networks KIGAM and KMA, the Japanese F-net network, and the Chinese IC and NECESSArray networks are collected for the year 2011 and the ambient seismic noise is analyzed. Rayleigh wave group velocity distribution maps are created in the period range 10 to 70 seconds. Our results are largely consistent with previous studies of the area, but provide greater detail in the Korean peninsula and the East Sea (Sea of Japan). Low group velocities are observed in the Ulleung basin, and the Chubu-Kanto and Kyushu regions in Japan. At 10 s period, sediment basins in the East Sea appear as low group velocity regions relative to higher group velocity continental regions. At periods longer than 40 s, a low group velocity region emerges in the Ulleung basin region, and is bounded by the Korean peninsula.
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    EGM TIM RL05: An Independent Geoid with Centimeter Accuracy Purely Based on the GOCE Mission (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: After more than 4.5 years in orbit, the Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission ended with the reentry of the satellite on 11 November 2013. This publication serves as a reference for the 5th gravity field model based on the time-wise approach (EGM_TIM_RL05), a global model only determined from GOCE observations. Due to its independence of any other gravity data, a consistent and homogeneous set of spherical harmonic coefficients up to degree and order 280 (corresponding to spatial resolution of 71.5 km on ground) is provided including a full covariance matrix characterizing the uncertainties of the model. The associated covariance matrix realistically describes the model quality. It is the first model which is purely based on GOCE including all observations collected during the entire mission. The achieved mean global accuracy is 2.4 cm in terms of geoid heights and 0.7 mGal for gravity anomalies at a spatial resolution of 100 km.
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    Modeling the spreading of glacial melt water from the Amundsen and Bellingshausen Seas (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: It has been suggested that an increased melting of continental ice in the Amundsen Sea (AS) and Bellinghausen Sea (BS) is a likely source of the observed freshening of Ross Sea (RS) water. To test this hypothesis, we simulate the spreading of glacial melt water using the Finite-Element Sea-ice/ice-shelf/Ocean Model. Based on the spatial distribution of simulated passive tracers, most of the basal melt water from AS ice shelves flows towards the RS with more than half of the melt originating from the Getz Ice Shelf. Further, the model results show that a slight increase of the basal mass loss can substantially intensify the transport of melt water into the RS due to a strengthening of the melt-driven shelf circulation and the westward flowing coastal current. This supports the idea that the basal melting of AS and BS ice shelves is one of the main sources for the RS freshening.
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    A low-frequency near-field interferometric-TOA 3D lightning mapping array (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: We report on the development of an easily deployable LF near-field Interferometric-TOA 3D Lightning Mapping Array applied to imaging of entire lightning flashes. An interferometric cross-correlation technique is applied in our system to compute windowed two-sensor time differences with sub-microsecond time resolution before TOA is used for source location. Compared to previously reported LF lightning location systems, our system captures many more LF sources. This is due mainly to the improved mapping of continuous lightning processes by using this type of hybrid interferometry/TOA processing method. We show with five station measurements that the array detects and maps different lightning processes, such as stepped and dart leaders, during both in-cloud and cloud-to-ground flashes. Lightning images mapped by our LF system are remarkably similar to those created by VHF mapping systems, which may suggest some special links between LF and VHF emission during lightning processes.
    Print ISSN: 0094-8276
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    Black Carbon Radiative Forcing over the Tibetan Plateau (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: We estimate the snow albedo forcing and direct radiative forcing (DRF) of black carbon (BC) in the Tibetan Plateau using a global chemical transport model in conjunction with a stochastic snow model and a radiative transfer model. The annual mean BC snow albedo forcing is 2.9 W m −2 averaged over snow-covered Plateau regions, which is a factor of three larger than the value over global land snowpack. BC-snow internal mixing increases the albedo forcing by 40-60% compared with external mixing and coated BC increases the forcing by 30-50% compared with uncoated BC aggregates, whereas Koch snowflakes reduce the forcing by 20-40% relative to spherical snow grains. The annual BC DRF at the top of the atmosphere is 2.3 W m −2 with uncertainties of −70-85% in the Plateau after scaling the modeled BC absorption optical depth to Aerosol Robotic Network (AERONET) observations. The BC forcings are attributed to emissions from different regions.
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    Low melt rates with seasonal variability at the base of Fimbul Ice Shelf, East Antarctica, revealed by in situ interferometric radar measurements. (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: ABSTRACT Basal melt is a major cause of ice shelf thinning affecting the stability of the ice shelf and reducing its buttressing effect on the inland ice. The Fimbul ice shelf (FIS) in Dronning Maud Land (DML), East Antarctica, is fed by the fast-flowing Jutulstraumen glacier, responsible for 10% of ice discharge from the DML sector of the ice sheet. Current estimates of the basal melt rates of the FIS come from regional ocean models, autosub measurements and satellite observations, which vary considerably. This discrepancy hampers evaluation of the stability of the Jutulstraumen catchment. Here, we present estimates of basal melt rates of the FIS using ground based interferometric radar. We find a low average basal melt rate on the order of 1 m/yr, with the highest rates located at the ice shelf front, which extends beyond the continental shelf break. Furthermore, our results provide evidence for a significant seasonal variability.
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    Thunderstorms enhance tropospheric ozone by wrapping and shedding stratospheric air (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: A significant source of ozone in the troposphere is transport from the stratosphere. The stratospheric contribution has been estimated mainly using global models that attribute the transport process largely to the global scale Brewer-Dobson circulation and synoptic scale dynamics associated with upper tropospheric jet streams. We report observations from research aircraft that reveal additional transport of ozone-rich stratospheric air downward into the upper troposphere by a leading-line-trailing-stratiform (LLTS) mesoscale convective system (MCS) with convection overshooting the tropopause altitude. The fine-scale transport demonstrated by these observations poses a significant challenge to global models that currently do not resolve storm scale dynamics. Thus the upper tropospheric ozone budget simulated by global chemistry-climate models where large-scale dynamics and photochemical production from lightning-produced NO are the controlling factors may require modification.
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    Van Allen Probe Observations of Periodic Rising Frequencies of the Fast Magnetosonic Mode (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: Near simultaneous periodic dispersive features of fast magnetosonic mode emissions are observed by both Van Allen Probes spacecraft while separated in magnetic local time by ~5 hours: Probe A at 15 and Probe B at 9–11 hours. Both spacecraft see similar frequency features, characterized by a periodic repetition at ~180 s. Each repetition is characterized by a rising frequency. Since no modulation is observed in the proton shell distribution, the plasma density, or in the background magnetic field at either spacecraft we conclude that these waves are not generated near the spacecraft but external to both spacecraft locations. Probe A while outside the plasmapause sees the start of each repetition ~40 s before probe B while deep inside the plasmasphere. We can qualitatively reproduce the dispersive features, but not the quantitative details. The cause for this phenomena remains to be identified.
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    Unsteady stress partitioning and momentum transfer in the wave bottom boundary layer over movable rippled beds (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: Observations of the nearbed velocity field over a rippled sediment bed under asymmetric wave forcing conditions were collected using a submersible particle image velocimetry (PIV) system. To examine the role of bedform-induced dynamics in the total momentum transfer, a double-averaging technique was implemented on the two-dimensional time-dependent velocity field by means of the full momentum equation. This approach allows for direct determination of the bedform-induced stresses, ie. stresses that arise due to the presence of bedforms, which are zero in flat bed conditions. This analysis suggests that bedform-induced stresses are closely related to the presence of coherent motions and may be partitioned from the turbulent stresses. Inferences of stress provided by a bedload transport model suggest that total momentum transfer obtained from the double-averaging technique is capable of reproducing bedform mobilization. Comparisons between the total momentum transfer and stress estimates obtained from local velocity profiles show significant variability across the ripple, and suggest that an array of sensors is necessary to reproduce bedform evolution. The imbalance of momentum obtained by resolving the different terms constituting the near-bed momentum balance (i.e. acceleration deficit, stress gradient, and bedform-induced skin friction) provides an estimate of the bedform-induced pressure that is consistent with flow separation. This analysis reveals three regions in the flow: the free-stream, where all terms are relatively balanced; the near-bed, where momentum imbalance is significant during flow weakening; and below ripple crests, where bedform-induced pressure is the leading order mechanism.
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    Multisensor observations of the Amazon-Orinoco River plume interactions with hurricanes (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: An analysis is presented for the spatial and intensity distributions of North Atlantic extreme atmospheric events crossing the buoyant Amazon-Orinoco freshwater plume. The sea surface cooling amplitude in the wake of an ensemble of storm tracks travelling in that region is estimated from satellite products for the period 1998-2012. For the most intense storms, cooling is systematically reduced by ~50% over the plume area compared to surroundings open ocean waters. Historical salinity and temperature observations from in situ profiles indicate that salt-driven vertical stratification, enhanced oceanic heat content and barrier-layer presence within the plume waters are likely key oceanic factors to explain these results. Satellite SMOS surface salinity data combined with in situ observations are further used to detail the oceanic response to Category 4 hurricane Igor in 2010. Argo and satellite measurements confirm the haline stratification impact on the cooling inhibition as the hurricane crossed the river plume. Over this region, the SSS mapping capability is further tested and demonstrated to monitor the horizontal distribution of the vertical stratification parameter. SMOS SSS data can thus be used to consistently anticipate the cooling inhibition in the wake of TCs travelling over the Amazon-Orinoco plume region.
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    Mixed layer salinity budget in the tropical Pacific Ocean estimated by a global GCM (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: The mixed layer salinity (MLS) budget of the tropical Pacific is investigated using results from a model of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO). The results focusing on the western Pacific freshwater pool indicate that the long-term averaged surface freshwater flux is well balanced by ocean dynamics, in which the subsurface processes account for the major part. The MLS budget shows significant seasonal and interannual variability, as a consequence of interplay among surface freshwater flux, advection, mixing, and vertical entrainment. On seasonal time scale, both the MLS and mixed layer depth are largely controlled by surface freshwater flux. The opposite phase between the subsurface processes and the barrier layer thickness confirms the important influence of the barrier layer on vertical mixing and entrainment from below. On interannual time scale, all the MLS budget terms show significant ENSO signal, which in turn is highly correlated with the salinity front and barrier layer thickness in the equatorial Pacific.
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    Solar heating of the Arctic Ocean in the context of ice-albedo feedback (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: To study the relationship of solar heat input into the Arctic open water and the variations of sea ice extent, improved satellite based estimates of shortwave radiative (SWR) fluxes and most recent observations of ice extent are used. The SWR flux estimates are based on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and from the Advanced Very High Resolution Radiometer (AVHRR) for the period of 1984-2009. Ice extent information at 25-km resolution comes from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data as generated with the NASA Team algorithm developed by the Oceans and Ice Branch, Laboratory for Hydrospheric Processes, NASA Goddard Space Flight Center. The trends of the solar heat input into the ocean and the open water fraction for 1984-2009 are found to be positive: 0.3%/year and 0.8% / year respectively at a 99% confidence level. There is an obvious transition region separating the 26 years into two periods: one with moderate change: 1984-2002; one with an abrupt growth in both solar heat input and open water fraction: 2003-2009. The impact of the observed changes on the reduction of winter ice growth in 2007 is estimated to be about 44 cm, and a delay in fall freeze-up as about 10~36 days.
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    Origin of fine-scale wind stress curl structures in the Benguela and Canary upwelling systems (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: Numerous studies have shown the primary importance of wind stress curl in coastal upwelling dynamics. The main goal of this new analysis is to describe the QuikSCAT surface wind stress curl at various scales in the Benguela and Canary upwelling systems. The dominant spatial pattern is characterized by cyclonic curl near continental boundaries and anticyclonic curl offshore, in association with equatorward alongshore (upwelling favorable) wind stress. At a smaller scale, we demonstrate the sensitivity of the QuikSCAT wind stress curl to coastal processes related to sea surface temperature (SST) mesoscale fluctuations by presenting a linear relationship between the curl and crosswind SST gradients. Despite the spatial and temporal sensitivity of the underlying thermal coupling coefficient, a local analysis of the fraction of the curl ascribed to SST variability shows that SST is a main driver of the wind stress curl variability and magnitude over the upwelling extension zone (~100 to 300 km from the coast) in both the Canary and Benguela systems. Closer to the shore, the curl patterns derived from QuikSCAT observations are only loosely related to SST-wind interactions. As a working hypothesis, they can also be associated with the coastline geometry and orographic effects that are likely to play an important role in local cooling processes.
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    Convective instability in sedimentation: 3-D numerical study (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: To provide a probable explanation on the field observed rapid sedimentation process near river mouths, we investigate the convective sedimentation in stably stratified saltwater using 3D numerical simulations. Guided by the linear stability analysis (Yu et al. 2013, J. Geophys. Res. Oceans, 118, 256-272), this study focuses on the nonlinear interactions of several mechanisms, which lead to various sediment finger patterns, and the effective settling velocity for sediment ranging from clay (single-particle settling velocity V 0 = 0.0036 and 0.0144 mm/s, or particle diameter d = 2 and 4 μ m) to silt ( V 0 =0.36 mm/s, or d =20 μ m). For very fine sediment with V 0 =0.0036 mm/s, the convective instability is dominated by double diffusion, characterized by millimeter-scale fingers. Gravitational settling slightly increases the growth rate; however, it has notable effect on the downward development of vertical mixing shortly after the sediment interface migrates below the salt interface. For sediment with V 0 = 0.0144 mm/s, Rayleigh-Taylor instabilities become dominant before double-diffusive modes grow sufficiently large. Centimeter-scale and highly asymmetric sediment fingers are obtained due to nonlinear interactions between different modes. For sediment with V 0 = 0.36 mm/s, Rayleigh-Taylor mechanism dominates and the resulting centimeter-scale sediment fingers show a plume-like structure. The flow pattern is similar to that without ambient salt stratification. Rapid sedimentation with effective settling velocity on the order of 1 cm/s is likely driven by convective sedimentation for sediment with V 0 greater than 0.1 mm/s at concentration greater than 10 to 20 g/L.
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    Heat and salinity budgets at the Stratus mooring in the southeast Pacific (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: The surface layer of the southeast Pacific Ocean (SEP) requires an input of cold, fresh water to balance heat gain and evaporation from air-sea fluxes. Models typically fail to reproduce the cool sea surface temperatures (SST) of the SEP, limiting our ability to understand the variability of this climatically important region. We estimate the annual heat budget of the SEP for the period 2004 - 2009, using data from the upper 250 m of the Stratus mooring, located at 85°W 20°S, and from Argo floats. The surface buoy measures meteorological conditions and air-sea fluxes; the mooring line is heavily instrumented, measuring temperature, salinity, and velocity at more than 15 depth levels. We use a new method for estimating the advective component of the heat budget that combines Argo profiles and mooring velocity data, allowing us to calculate monthly profiles of heat advection. Averaged over the 6 year study period, we estimate a cooling advective heat flux of -41 ± 29 W m-2, accomplished by a combination of the mean gyre circulation, Ekman transport, and eddies. This compensates for warming fluxes of 32 ± 4 W m-2 due to air-sea fluxes and 7 ± 9 W m-2 due to vertical mixing and Ekman pumping. A salinity budget exhibits a similar balance, with advection of freshwater (-60 psu m) replenishing the freshwater lost through evaporation (47 psu m) and Ekman pumping (14 psu m).
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    Laboratory constraints on models of earthquake recurrence (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: In this study, rock friction ‘stick-slip’ experiments are used to develop constraints on models of earthquake recurrence. Constant-rate loading of bare rock surfaces in high quality experiments produces stick-slip recurrence that is periodic at least to second order. When the loading rate is varied, recurrence is approximately inversely proportional to loading rate. These laboratory events initiate due to a slip rate-dependent process that also determines the size of the stress drop and as a consequence, stress drop varies weakly but systematically with loading rate. This is especially evident in experiments where the loading rate is changed by orders of magnitude, as is thought to be the loading condition of naturally occurring, small repeating earthquakes driven by afterslip, or low-frequency earthquakes loaded by episodic slip. As follows from the previous studies referred to above, experimentally observed stress drops are well described by a logarithmic dependence on recurrence interval that can be cast as a non-linear slip-predictable model. The fault's rate dependence of strength is the key physical parameter. Additionally, even at constant loading rate the most reproducible laboratory recurrence is not exactly periodic, unlike existing friction recurrence models. We present example laboratory catalogs that document the variance and show that in large catalogs, even at constant loading rate, stress drop and recurrence co-vary systematically. The origin of this covariance is largely consistent with variability of the dependence of fault strength on slip rate. Laboratory catalogs show aspects of both slip and time predictability and successive stress drops are strongly correlated indicating a ‘memory’ of prior slip history that extends over at least one recurrence cycle.
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    Effect of oil composition on fluid substitution in heavy oil reservoirs (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: ABSTRACT Unlike light oils, heavy oils do not have a well-established scheme for modelling elastic moduli from dynamic reservoir properties. One of the main challenges in the fluid substitution of heavy oils is their viscoelastic nature, which is controlled by temperature, pressure, and fluid composition. Here, we develop a framework for fluid substitution modelling that is reliable yet practical for a wide range of cold and thermal recovery scenarios in producing heavy oils and that takes into account the reservoir fluid composition, grounded on the effective-medium theories for estimating elastic moduli of an oil–rock system. We investigate the effect of fluid composition variations on oil–rock elastic moduli with temperature changes. The fluid compositional behaviour is determined by flash calculations. Elastic moduli are then determined using the double-porosity coherent potential approximation method and the calculated viscosity based on the fluid composition. An increase in temperature imposes two opposing mechanisms on the viscosity behaviour of a heavy-oil sample: gas liberation, which tends to increase the viscosity, and melting, which decreases the viscosity. We demonstrate that melting dominates gas liberation, and as a result, the viscosity and, consequently, the shear modulus of the heavy oils always decrease with increasing temperature. Furthermore, it turns out that one can disregard the effects of gas in the solution when modelling the elastic moduli of heavy oils. Here, we compare oil–rock elastic moduli when the rock is saturated with fluids that have different viscosity levels. The objective is to characterize a unique relation between the temperature, the frequency, and the elastic moduli of an oil–rock system. We have proposed an approach that takes advantage of this relation to find the temperature and, consequently, the viscosity in different regions of the reservoir.
    Print ISSN: 0016-8025
    Electronic ISSN: 1365-2478
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    Contemporary Ocean Warming Dissociates Cascadia Margin Gas Hydrates (2014)
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    Publication Date: 2014-11-05
    Description: Gas hydrates, pervasive in continental margin sediments, are expected to release methane in response to ocean warming, but the geographic range of dissociation and subsequent flux of methane to the ocean are not well constrained. Sediment column thermal models based on observed water column warming trends offshore Washington (USA) show that a substantial volume of gas hydrate along the entire Cascadia upper continental slope is vulnerable to modern climate change. Dissociation along the Washington sector of the Cascadia margin alone has the potential to release 45–80 Tg of methane by 2100. These results highlight the importance of lower latitude warming to global gas hydrate dynamics and suggest that contemporary warming and downslope retreat of the gas hydrate reservoir occurs along a larger fraction of continental margins worldwide than previously recognized.
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    Southward displacement of the upper atmosphere zonal Jet in the eastern north Pacific due to global warming (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: The jet stream over the eastern North Pacific (ENPJ) as a core of the atmospheric flow is known to strongly fluctuate meridionally, and its meridional displacement directly influences adjacent regional climate. Here, we investigate how this jet will be changed due to global warming. By analyzing the future scenario experiments of CMIP3 and 5, and found that both ENPJ and the eastern tropical Pacific ITCZ tend to move southward, which are closely related to the tropical eastern Pacific warming trend. Tropical eastern Pacific warming leads to not only the southward migration of ITCZ by southward-shifting the off-equatorial eastern Pacific warm pool, but also the southward shift of ENPJ by increasing baroclinic instability of the atmosphere in subtropical region through intensifying the meridional SST gradient. Not primary but yet secondly the southward shift of ITCZ contributes to the southward shift of ENPJ through a kinematic connection bridged by local Hadley circulation.
    Print ISSN: 0094-8276
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    Crab Burrows as Conduits for Groundwater-Surface water Exchange in Bangladesh (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: Groundwater recharge affects water budgets and groundwater quality on the deltas and floodplains of South and Southeast Asia. Rain and flooding rivers recharge groundwater during the monsoon; irrigated rice fields and surface-water bodies recharge aquifers during the dry season. Groundwater throughout the region is severely contaminated by arsenic and recent research suggests that quantifying and characterizing recharge is important to understand whether recharge flushes or mobilizes arsenic from aquifers. At a field site in Bangladesh, we found that burrows of terrestrial crabs short-circuit low-permeability surface sediments, providing the primary conduit for recharge. We combine field observations along with a model that couples isotope and water balances to quantify the effect of crab burrows on aquifer recharge. Given the wide distribution of burrowing crabs and the surficial geology, we suggest that crab burrows provide widespread conduits for groundwater recharge.
    Print ISSN: 0094-8276
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    Heating of the Plasma Sheet by Broadband Electromagnetic Waves (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: We demonstrate that broadband low frequency electromagnetic field fluctuations embedded within fast flows throughout the Earth's plasma sheet may drive significant ion heating. This heating is nearly entirely in the direction perpendicular to the background magnetic field and is estimated to occur at an average rate of ~1 eV/s with rates in excess of 10 eV/s within one standard deviation of the average value over all observed events. For an Earthward flow the total change in temperature along a flow path may exceed one keV and for ‘wave-rich’ flows can be comparable to that expected due to conservation of the first adiabatic invariant. The consequent increase in plasma pressure and flux tube entropy may lead to braking of inward motion and the suppression of plasma interchange.
    Print ISSN: 0094-8276
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    Global Estimate of Submarine Groundwater Discharge Based on an Observationally Constrained Radium Isotope Model (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: Along the continental margins, rivers and submarine groundwater supply nutrients, trace elements, and radionuclides to the coastal ocean, supporting coastal ecosystems and, increasingly, causing harmful algal blooms and eutrophication. While the global magnitude of gauged riverine water discharge is well known, the magnitude of submarine groundwater discharge (SGD) is poorly constrained. Using an inverse model combined with a global compilation of 228 Ra observations, we show that the SGD integrated over the Atlantic and Indo-Pacific Oceans between 60°S and 70°N is (12 ± 3) x 10 13  m 3  yr -1 , which is 3 to 4 times greater than the freshwater fluxes into the oceans by rivers. Unlike the rivers, where more than half of the total flux is discharged into the Atlantic, about 70% of SGD flows into the Indo-Pacific Oceans. We suggest that SGD is the dominant pathway for dissolved terrestrial materials to the global ocean, and this necessitates revisions for the budgets of chemical elements including carbon.
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    Generation and Propagation of Ion Cyclotron Waves in Non-Uniform Magnetic Field: Application to the Corona and Solar Wind (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: ABSTRACT With the objective to understand the generation, propagation and nonlinear evolution of ion cyclotron waves (ICWs) in the corona and solar wind, we use electromagnetic hybrid (kinetic ions, fluid electrons) simulations with a non-uniform magnetic field. ICWs are generated by the temperature anisotropy of O 5+ ions as minority species in a proton-electron plasma with uniform density. A number of magnetic field models are used including radial and spiral with field strength decreasing linearly or with the square of the radial distance. O 5+ ions with perpendicular temperature larger than parallel are initially placed in the high magnetic field regions. These ions are found to expand outward along the magnetic field. Associated with this expansion, ion cyclotron waves propagating along the magnetic field are also seen to expand outward. These waves are generated at frequencies below the local gyro-frequency of O 5+ ions propagating parallel and anti-parallel to the magnetic field. Through analysis of the simulation results we demonstrate that wave generation and absorption takes place at all radial distances. Comparing the simulation results to observations of ICWs in the solar wind shows some of the observed wave characteristics may be explained by the mechanism discussed in this paper.
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    Airglow observations of nighttime medium-scale traveling ionospheric disturbances from Yonaguni: Statistical characteristics and low latitude limit (2014)
    Wiley
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    Publication Date: 2014-11-04
    Description: The characteristics of nighttime medium-scale travelling ionospheric disturbance (MSTID) features observed over Yonaguni (24.5 o N, 123.0 o E; 19.3 o N dip latitude), Japan are studied using all-sky imaging of OI 630.0 nm airglow emission. The uniqueness of these observations is that the area observed by the imager covers the transition region between low to middle latitudes in the ionosphere. Typical low latitude limit of mid-latitude type nighttime MSTIDs possessing phase front alignments along the northwest to the southeast occurs in this region. These MSTID features are rarely sighted at dip latitudes below 15 o . We selected two year period for analysis in which one year corresponded to the solar minimum conditions and another year to the solar maximum conditions. The MSTIDs were observed to extend to farther lower latitudes during the solar minimum conditions than during the solar maximum periods. Their observed range of wavelengths, phase velocities, phase front alignment and propagation directions are similar to those observed at typical mid-latitude sites. However, on many occasions the phase fronts of the observed MSTIDs did not extend over the whole field of view of the imager indicating that some process inhibits their extension to further lower latitudes. Detailed investigation suggests that the poleward propagating enhancement of airglow intensity, probably associated with the midnight pressure bulge, causes the MSTID features to disappear when they reach lower latitudes later in the night. When the MSTIDs reach lower latitudes well before midnight, they are found to be inhibited by the equatorial ionization anomaly crest region.
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    Low- and mid-latitude ionospheric effects of energetic electrons during a recurrent magnetic storm (2014)
    Wiley
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    Publication Date: 2014-11-04
    Description: We study a magnetosphere-ionosphere coupling at low-latitudes during a moderate (CIR/HSS-driven) geomagnetic storm on 22 July 2009. Recently, it has been shown that during major (CME-driven) storms, quasi-trapped 〉30 keV electrons largely enhance below the radiation belt in the forbidden zone and produce an additional ionization in the topside ionosphere. In this work, we examine a case of the recurrent storm when the magnetosphere-ionosphere coupling through the quasi-trapped electrons also may take place. Data from NOAA/POES and Japanese GOSAT satellites were used to identify the forbidden electron enhancement (FEE). We find a positive vertical gradient of the electron fluxes that indicates to the radiation belt as a source of FEE. Using global ionospheric maps (GIM), radiotomography reconstructions from beacon data and COSMIC/FS3 radio occultation measurements, we have observed an unusually large area in the night-time ionosphere with increased total electron content (TEC) and prominent elevation of the F-layer at low-latitudes that coincides with FEEs spatially and temporarily. Ionizing particles are considered as an addition source of ionization along with generally accepted mechanisms for storm time TEC increase (a positive ionospheric storm). We discuss relative contributions of the FEE and disturbance dynamo electric field in the TEC increases during the storm recovery phase.
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    Linear Instability of Warm Core, Constant Potential Vorticity, Eddies in a Two Layer Ocean (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: Linear instability of warm core eddies of constant potential vorticity (PV) is studied in a two layer, finite depth, shallow water ocean. The basic state flow in the constant PV eddy that obeys the gradient balance cannot be described by explicit expressions and can only be solved numerically. The various cases of gradient balance are classified by constructing a canonical formulation that relates any PV value to a value of the angular velocity that has to prevail near the center of the constant PV eddy. The growth-rates of perturbations imposed on the basic state are calculated for a variety of values of the (constant) PV and the depth of the surrounding ocean. The growth-rates i.e. the eigenvalues are calculated numerically by employing a shooting to fitting point method that guarantees that the corresponding eigenfunctions are regular at all singular points. The maximal growth-rates are contoured as functions of PV and ocean depth for azimuthal wavenumber 2 and 3 and the maximum of these growth-rates is of the order of 1 day which is similar to that of a solidly rotating eddy. However, the range angular velocity and ocean depth where the constant PV eddy is unstable is greatly reduced compared to that of a solidly rotating eddy. The instabilities found here are classified in terms of wave-wave interactions by comparing our results in each PV value with the known instabilities of the solidly rotating eddy with the same angular velocity. In the constant PV eddy the Baroclinic instability is filtered out and the range of angular velocity where the Hybrid instability exists is significantly reduced. All instabilities decay monotonically with the increase in ocean depth.
    Print ISSN: 0035-9009
    Electronic ISSN: 1477-870X
    Topics: Geography , Physics
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    Microstructures, composition, and seismic properties of the Ontong Java Plateau mantle root (2014)
    Wiley
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    Publication Date: 2014-11-06
    Description: To study how an impacting plume modifies the mantle lithosphere, we analyzed the microstructures and crystal preferred orientations (CPO) of 29 peridotites and 37 pyroxenites that sample the mantle root of the Ontong Java Plateau (OJP) from 60 to 120 km depth. The peridotites show a strong compositional variability, but homogeneous coarse-granular to tabular microstructures, except for those equilibrated at the shallowest and deepest depths, which are porphyroclastic. All peridotites have clear olivine CPO, with dominant fiber-[010] patterns. Low intragranular misorientations and straight grain boundaries in olivine suggest that, above 100 km depth, annealing often followed deformation. Calculated density and P-wave velocities of the peridotites decrease weakly with depth. S-wave velocities decrease faster, resulting in increasing Vp/Vs ratio with depth. Calculated densities and seismic velocity profiles are consistent with those estimated for normal mantle compositions under a cold oceanic geotherm. Enrichment in pyroxenites may further increase seismic velocities. The calculated seismic properties cannot therefore explain the low S-waves velocities predicted by Rayleigh wave tomography and ScS data in the mantle beneath the OJP. Calculated P- and S-wave anisotropy is variable (2-12%). It is higher on average in the deeper section of the lithosphere. Because olivine has dominantly [010]-fiber CPO patterns, if foliations are horizontal, vertically propagating S-waves and Rayleigh waves will sample very weak anisotropy in the OJP mantle lithosphere. Moreover, if the orientation of the lineation changes with depth, the anisotropy-induced contrast in seismic properties might produce an intralithospheric reflector marking the stratification of the OJP mantle root.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Impact of Fluid Deformation on Mixing-Induced Chemical Reactions in Heterogeneous Flows (2014)
    Wiley
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    Publication Date: 2014-11-06
    Description: Fast chemical reactions in geophysical flows are controlled by fluid mixing, which perturbs local chemical equilibria and thus triggers chemical reactions. Spatial fluctuations in the flow velocity lead to deformation of material fluid elements, which form the support volumes of transported chemical species. We develop an approach based on a lamellar representation of fluid mixing that provides a direct link between fluid deformation, the distribution of concentration gradiens, and the upscaled reaction rates for fast reversible reactions. The temporal evolution of effective reaction rates are determined by the flow topology and the distribution of local velocity gradients. This leads to a significant increase of the reaction efficiency, which turns out to be orders of magnitude larger than in homogeneous flow. This approach allows for the systematic evaluation of the temporal evolution of equilibrium reaction rates, and establishes a direct link between the reaction efficiency and the spatial characteristics of the underlying flow field as quantified by the deformation of material fluid elements.
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    Tsunami surges around the Hawaiian Islands from the 1 April 2014 North Chile Mw 8.1 earthquake (2014)
    Wiley
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    Publication Date: 2014-11-06
    Description: The 1 April 2014 Iquique M w 8.1 earthquake ruptured a segment of the megathrust fault offshore of northern Chile and generated a moderate-size tsunami across the Pacific. Tide gauges in Hawaii recorded over 1 m of wave height despite the long distance from the source and position away from the main radiated energy lobe. Inversion of global teleseismic body wave observations combined with forward modeling of the tsunami at four near-field DART stations arrives iteratively at a self-consistent finite-fault model with very compact dimensions. The slip distribution produces a NNE-SSW trending seafloor uplift patch that enhances the tsunami directionality in the WNW, resulting in good matches to observed DART and tide gauge records around the Hawaiian Islands. The relatively large waves at selected locations in Hawaii can be attributed to a combination of the spatial slip distribution and the resulting short-period waves that triggered localized resonance over the insular shelves. This event highlights the importance of characterizing detailed slip distributions in analysis or forecasting of tsunamis even for a compact source.
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    Surface warming hiatus caused by increased heat uptake across multiple ocean basins (2014)
    Wiley
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    Publication Date: 2014-11-06
    Description: The first decade of the twenty-first century was characterised by a hiatus in global surface warming. Using ocean model hindcasts and reanalyses we show that heat uptake between the 1990s and 2000s increased by 0.7 ± 0.3 Wm −2 . Approximately 30% of the increase is associated with colder sea surface temperatures in the eastern Pacific. Other basins contribute via reduced heat loss to the atmosphere, in particular the Southern and subtropical Indian Oceans (30%), and the subpolar North Atlantic (40%). A different mechanism is important at longer timescales (1960s-present) over which the Southern Annular Mode trended upwards. In this period, increased ocean heat uptake has largely arisen from reduced heat loss associated with reduced winds over the Agulhas Return Current and southward displacement of Southern Ocean westerlies.
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    Soil CO2 emissions as a proxy for heat and mass flow assessment, Taupō Volcanic Zone, New Zealand (2014)
    Wiley
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    Publication Date: 2014-11-06
    Description: The quantification of heat and mass flow between deep reservoirs and the surface is important for understanding magmatic and hydrothermal systems. Here, we use high-resolution measurement of carbon dioxide flux (ϕCO 2 ) and heat flow at the surface to characterize the mass (CO 2 and steam) and heat released to the atmosphere from two magma-hydrothermal systems. Our soil gas and heat flow surveys at Rotokawa and White Island in the Taupō Volcanic Zone, New Zealand, include over 3,000 direct measurements of ϕCO 2 and soil temperature and 60 carbon isotopic values on soil gases. Carbon dioxide flux was separated into background and magmatic/hydrothermal populations based on the measured values and isotopic characterization. Total CO 2 emission rates (ΣCO 2 ) of 441 ± 84 t d -1 and 124 ± 18 t d -1 were calculated for Rotokawa (2.9 km 2 ) and for the crater floor at White Island (0.3 km 2 ), respectively. The total CO 2 emissions differ from previously published values by +386 t d -1 at Rotokawa and +25 t d -1 at White Island, demonstrating that earlier research underestimated emissions by 700% (Rotokawa) and 25% (White Island). These differences suggest that soil CO 2 emissions facilitate more robust estimates of the thermal energy and mass flux in geothermal systems than traditional approaches. Combining the magmatic/hydrothermal-sourced CO 2 emission (constrained using stable isotopes) with reservoir H 2 O:CO 2 mass ratios and the enthalpy of evaporation, the surface expression of thermal energy release for the Rotokawa hydrothermal system (226 MW t ) is 10 times greater than the White Island crater floor (22.5 MW t ).
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Anisotropy signature in reverse-time migration extended images (2014)
    Wiley
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    Publication Date: 2014-11-06
    Description: ABSTRACT Reverse-time migration can accurately image complex geologic structures in anisotropic media. Extended images at selected locations in the Earth, i.e., at common-image-point gathers, carry rich information to characterize the angle-dependent illumination and to provide measurements for migration velocity analysis. However, characterizing the anisotropy influence on such extended images is a challenge. Extended common-image-point gathers are cheap to evaluate since they sample the image at sparse locations indicated by the presence of strong reflectors. Such gathers are also sensitive to velocity error that manifests itself through moveout as a function of space and time lags. Furthermore, inaccurate anisotropy leaves a distinctive signature in common-image-point gathers, which can be used to evaluate anisotropy through techniques similar to the ones used in conventional wavefield tomography. It specifically admits a V-shaped residual moveout with the slope of the “V” flanks depending on the anisotropic parameter η regardless of the complexity of the velocity model. It reflects the fourth-order nature of the anisotropy influence on moveout as it manifests itself in this distinct signature in extended images after handling the velocity properly in the imaging process. Synthetic and real data observations support this assertion.
    Print ISSN: 0016-8025
    Electronic ISSN: 1365-2478
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    Deep ocean water transport by acoustic-gravity waves (2014)
    Wiley
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    Publication Date: 2014-11-06
    Description: Acoustic-gravity waves are compression-type waves propagating with amplitudes governed by the restoring force of gravity. They are generated, among others, by wind-wave interactions, surface waves interactions, and submarine earthquakes. We show that acoustic-gravity waves contribute to deep ocean currents and circulation; they cause chaotic flow trajectories of individual water parcels, which can be transported by a few centimetres per second.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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    The effects of sediment-transport, weathering and aeolian mechanisms on soil evolution (2014)
    Wiley
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    Publication Date: 2014-12-13
    Description: Aeolian-derived soils are found throughout the world. Soil evolution processes in aeolian-dominated landscapes differ from processes in bedrock-weathering landscapes by a number of key aspects including the lack of: (1) soil-production depth-dependency; (2) surface armoring; and (3) grain size self-organization in the soil profile. We use here a soil evolution model (mARM5D) to study the differences between aeolian and bedrock-weathering dominated landscapes by analyzing soil evolution on a hillslope under various aeolian and bedrock soil supply settings subject to fluvial and diffusive sediment transport. The model simulates spatial and temporal variation in soil particle size distribution (PSD) and profile depth for each grid-cell on the landscape, as a function of physical weathering, aeolian deposition, and diffusive and fluvial sediment transport. Our results indicate that surface armoring plays a major role in soil evolution. Under bedrock-weathering dominated conditions, armoring reduces soil erosion and in conjunction with depth-dependent soil production, leads to steady-state soil grading and depth and a relatively uniform soil distribution. In contrast, aeolian-dominated landscapes tend to have considerable spatial variability in soil depth and PSD. Our results also indicate that in contrast with diffusive transport, which is assumed to be PSD-independent, fluvial sediment transport is strongly influenced by the soil-production mechanism (aeolian or bedrock-weathering). Based on the results presented here we propose that aeolian-dominated landscapes are more responsive to environmental changes (e.g. climatic and anthropogenic) compared with bedrock-weathering landscapes. We further propose that this sensitivity may help explain the patchy soil distribution that is often observed in aeolian-dominated regions.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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    Generation of High-Frequency Electric Field Activity by Turbulence in the Earth's Magnetotail (2014)
    Wiley
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    Publication Date: 2014-12-13
    Description: Bursty bulk flow (BBF) events, frequently observed in the magnetotail, carry significant energy and mass from the tail region at distances that are often greater than 20 R E into the near-Earth plasma sheet at ~10 R E where the flow is slowed and/or diverted. This region at ~10 R E is referred to as the BBF braking region. A number of possible channels are available for the transfer or dissipation of energy in BBF events including adiabatic heating of particles, the propagation of Alfvén waves out of the BBF braking region and into the auroral region, diverted flow out of the braking region, and energy dissipation within the braking region itself. This study investigates the generation of intense high-frequency electric field activity observed within the braking region. When present, these intense electric fields have power above the ion cyclotron frequency and almost always contain nonlinear structures such as electron phase space holes and double layers, which are often associated with field-aligned currents. A hypothesis in which the observed high-frequency electric field activity is generated by field-aligned currents resulting from turbulence in the BBF braking region is considered. Although linear Alfvén waves can generate field-aligned currents, based on theoretical calculations, the required currents are likely not the result of linear waves. Observations from the THEMIS satellites support the picture of a turbulent plasma leading to the generation of nonlinear kinetic structures. This work provides a possible mechanism for energy dissipation in turbulent plasmas.
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    Ion reflection and acceleration near magnetotail dipolarization fronts associated with magnetic reconnection (2014)
    Wiley
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    Publication Date: 2014-12-13
    Description: Dipolarization fronts (DFs) are often associated with the leading edge of Earthward bursty bulk flows in the magnetotail plasma sheet. Here multi-spacecraft THEMIS observations are used to show that a spatially limited region of counter-propagating ion beams, whose existence is not evident in either the plasma moments or the electric field, is observed on the low density side of DFs. The THEMIS magnetic field data are used to establish appropriate comparison cuts through a particle-in-cell (PIC) simulation of reconnection, and very good agreement is found between the observed and simulated ion distributions on both sides of the DF. Self-consistent back-tracing shows that the ion beams originate from the thermal component of the pre-existing high density plasma into which the DF is propagating; they do not originate from the inflow region in the traditional sense. Forward tracing shows that some of these ions can subsequently overtake the DF and pass back into the high density pre-existing plasma sheet with an order-of-magnitude increase in energy; this process is distinct from other ion reflection processes that occur directly at the DF. The interaction of the reconnection jet with the pre-existing plasma sheet therefore occurs over a macroscopic region, rather than simply being limited to the thin DF interface. A more general consequence of this study is the conclusion that reconnection jets are not simply fed by plasma inflow across the separatrices, but are also fed by plasma from the region into which the jet is propagating; the implications of this finding are discussed.
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    Misidentification of the source of a neutron transient detected by MESSENGER on 2011 June 4 (2014)
    Wiley
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    Publication Date: 2014-12-13
    Description: Low energy (1-10 MeV) neutrons emanating from the Sun provide unique information about accelerated ions with steep energy spectra that may be produced in weak solar flares. However, observation of these solar neutrons can only be made in the inner heliosphere where measurement is difficult due to high background rates from neutrons produced by energetic ions interacting in the spacecraft. These ions can be from solar energetic particle events or produced in passing shocks associated with fast coronal mass ejections. Therefore, it is of the utmost importance that investigators rule out these secondary neutrons before making claims about detecting neutrons from the Sun. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging ( tect MESSENGER ) Neutron Spectrometer recorded an hour-long neutron transient beginning at 15:45 UTC on 2011 June 4 for which [13] claim there is “strong evidence" that the neutrons were produced by the interaction of ions in the solar atmosphere. We studied this event in detail using data from the MESSENGER neutron spectrometer, gamma-ray spectrometer, X-ray Spectrometer, and Energetic Particle Spectrometer, and from the particle spectrometers on STEREO A . We demonstrate that the transient neutrons were secondaries produced by energetic ions, probably accelerated by a passing shock, that interacted in the spacecraft. We also identify significant faults with the authors’ arguments in favor of a solar neutron origin for the transient.
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    Response of the low latitude D-region ionosphere to extreme space weather event of 14-16 December 2006 (2014)
    Wiley
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    Publication Date: 2014-12-13
    Description: The response of the D-region low latitude ionosphere has been examined for extreme space weather event of 14-16 December 2006 associated with a X1.5 solar flare and an intense geomagnetic storm ( Dst  = -146 nT) using VLF signals from NWC (19.8 kHz) and NPM (21.4 kHz) transmitters monitored at Suva (Geog. 18.10 ο S, 178.40 ο E), Fiji. Modeling of flare associated amplitude and phase enhancements of NWC (3.6 dB, 223 o ) and NPM (5 dB, 153 o ) using Long Wave Propagation Capability code shows reduction in the D-region reflection height ( H ') by 11.1 km and 9.4 km, and enhancement in ionization gradients described by increases in the exponential sharpness factor ( β ) by 0.122 and 0.126 km -1 , for the NWC and NPM paths, respectively. During the storm the daytime signal strengths of the NWC and NPM signals were reduced by 3.2 dB on 15 and 16 December (for about 46 hrs) and recovered by 17 December. Modelling for the NWC path shows that storm-time values of H ' and β were reduced by 1.2 km and 0.06 km -1 , respectively. Morlet wavelet analysis of signals amplitudes shows no clearly strong signatures of gravity wave propagation to low latitudes during the main and recovery phases. The reduction in VLF signal strength is due to increased signal attenuation and absorption by the Earth-ionosphere waveguide due to storm-induced D-region ionization changes and hence changes in D-region parameters. The long duration of the storm effect results from the slow diffusion of changed composition/ionization at D-region altitudes compared with higher altitudes in the ionosphere.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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    Two states of magnetotail dipolarization fronts: A statistical study (2014)
    Wiley
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    Publication Date: 2014-12-13
    Description: We study the ion density and temperature in the pre- and post-dipolarization plasma sheets in the Earth's magnetotail, using 9 years (2001–2009) of Cluster data. For our study we selected cases when Cluster observed dipolarization fronts (DFs) with an earthward plasma flow greater than 150 km/s. We perform a statistical study of the temperature and density variations during the DF crossings. Earlier studies concluded that on average the temperature increases while the densitydecreases across the DF. Our statistical results show a more diverse picture: While ~53% of the DFs follow this pattern (category A), for ~28% the temperature decreases while the density increases across the DF (category B). We found an overall decrease in thermal pressure for category A DFs with a more pronounced decrease at the DF duskside, while DFs of the category B showed no clear pattern in the pressure change. Both categories are associated with earthward plasma flows, but with some difference: (1) Category A flows are faster than category B flows. (2) The observations indicate that category B flows are directed perpendicular to the current in the near-Earth current sheet while category A flows are tilted slightly duskward from this direction. (3) The background B z of category B is higher than that of category A. Based on these results we hypothesize that after reconnection takes place, a BBF emerges with category A characteristics, and as it travels earthward it further evolves into category B characteristics, which is in a more dipolarized region with slower plasma flow (closer to the flow braking region).
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    Fluid substitution effects on seismic anisotropy (2014)
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    Publication Date: 2014-12-14
    Description: We derive equations for HTI and orthorhombic symmetries to analyze fluid substitution effects in porous fractured media. The derivations are based on the anisotropic Gassmann equation and linear slip theory. We assess the influence of fluid substitution (gas, brine, and oil), on elastic moduli, velocities, anisotropy, and azimuthal amplitude variations. We find that in the direction normal to fractures, P-wave moduli increase as much as 56% and P-wave velocity increases up to 19% for gas-to-brine substitution. For the direction parallel to fractures, P-wave velocity remains almost constant when porosity is low (5%), but can increase up to 4% if porosity is high (25%). Since P-waves in two different directions have different sensitivities to fluids and fractures, the Thomsen's parameters (defined for HTI and orthorhombic symmetries), ε and δ , are sensitive to fluid types and fractures. We also found that δ is sensitive to porosity for liquid saturation, but insensitive to porosity for the case of gas saturation. Gassmann assumes (and as has been observed) that shear modulus does not depend on fluids. And we observe no changes in shear-wave splitting ( γ ) for different fluids. The azimuthal amplitude variation is dependent on fluid types, fractures and porosity. We observe up to 12% increase in azimuthal amplitude variation for low porosity gas sands after brine saturation, and 6% decrease for high porosity gas sands. We find that the percentage changes in gas-to-oil substitution are about half that of the gas-to-brine case. The equations we have derived provide a useful tool to quantitatively evaluate the effects of fluid substitution on seismic anisotropy.
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    Glacial Isostatic Adjustment in the Static Gravity Field of Fennoscandia (2014)
    Wiley
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    Publication Date: 2014-12-14
    Description: In the central part of Fennoscandia the crust is currently rising, because of the delayed response of the viscous mantle to melting of the Late Pleistocene ice sheet. This process, called Glacial Isostatic Adjustment (GIA), causes a negative anomaly in the present-day static gravity field as isostatic equilibrium has not been reached yet. Several studies have tried to use this anomaly as a constraint on models of GIA, but the uncertainty in crustal and upper mantle structures has not been fully taken intoaccount. Therefore, our aim is to revisit this using improved crustal models and compensation techniques. We find that, in contrast with other studies, the effect of crustal anomalies on the gravity field cannot be effectively removed, because of uncertainties in the crustal and upper mantle density models. Our second aim is to estimate the effects on geophysical models, which assume isostatic equilibrium, after correcting the observed gravity field with numerical models for GIA. We show that correcting for GIA in geophysical modelling can give changes of several km in the thickness of structural layers of modeled lithosphere, which is a small but significant correction. Correcting the gravity field for GIA prior to assuming isostatic equilibrium and inferring density anomalies might be relevant in other areas with ongoing post-glacial rebound such as North America and the polar regions.
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    Variability of seismic source spectra, estimated stress drop and radiated energy, derived from cohesive-zone models of symmetrical and asymmetrical circular and elliptical ruptures (2014)
    Wiley
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    Publication Date: 2014-12-14
    Description: Large variability of earthquake stress drops and scaled energy has been commonly reported in the literature, but it is difficult to assess how much of this variability is caused by underlying physical source processes rather than simply observational uncertainties. Here, we examine a variety of dynamically realistic rupture scenarios for circular and elliptical faults and investigate to what extent the variability in seismically estimated stress drops and scaled energy comes from differences in source geometry, rupture directivity, and rupture speeds. We numerically simulate earthquake source scenarios using a cohesive-zone model with the small-scale yielding limit, where the solution approaches a singular crack model with spontaneous healing of slip. Compared to symmetrical circular source models, asymmetrical models result in larger variability of estimated corner frequencies and scaled energy over the focal sphere. The general behavior of the spherical averages of corner frequencies and scaled energy in the subshear regime extends to the supershear regime, although shear Mach waves generated by the propagation of supershear rupture lead to much higher corner-frequency and scaled-energy estimates locally. Our results suggest that at least a factor of two difference in the spherical average of corner frequencies is expected in observational studies simply from variability in source characteristics almost independent of the actual stress drops, translating into a factor ofeight difference in estimated stress drops. Furthermore, radiation efficiency estimates derived from observed seismic spectra should not be directly interpreted as describing rupture properties unless there are independent constraints on rupture speed and geometry.
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    Understanding Instrumental Stokes Leakage in Murchison Widefield Array Polarimetry (2014)
    Wiley
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    Publication Date: 2014-12-14
    Description: This paper offers an electromagnetic, more specifically array theory, perspective on understanding strong instrumental polarization effects for planar low-frequency “aperture arrays” with the Murchison Widefield Array (MWA) as an example. A long-standing issue that has been seen here is significant instrumental Stokes leakage after calibration, particularly in Stokes Q at high frequencies. A simple model that accounts for inter-element mutual coupling is presented which explains the prominence of Q leakage seen when the array is scanned away from zenith in the principal planes. On these planes, the model predicts current imbalance in the X (E-W) and Y (N-S) dipoles and hence the Q leakage. Although helpful in concept, we find that this model is inadequate to explain the full details of the observation data. This finding motivates further experimentation with more rigorous models that account for both mutual coupling and embedded element patterns. Two more rigorous models are discussed: the “full” and “average” embedded element patterns. The viability of the “full” model is demonstrated by simulating current MWA practice of using a Hertzian dipole model as a Jones matrix estimate. We find that these results replicate the observed Qleakage to approximately 2 to 5%. Finally, we offer more direct indication for the level of improvement expected from upgrading the Jones matrix estimate with more rigorous models. Using the “average” embedded pattern as an estimate for the “full” model, we find that Q leakage of a few percent is achievable.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
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    Bayesian inversion of time-lapse seismic data for the estimation of static reservoir properties and dynamic property changes (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: ABSTRACT Seismic conditioning of static reservoir model properties such as porosity and lithology has traditionally been faced as a solution of an inverse problem. Dynamic reservoir model properties have been constrained by time-lapse seismic data. Here, we propose a methodology to jointly estimate rock properties (such as porosity) and dynamic property changes (such as pressure and saturation changes) from time-lapse seismic data. The methodology is based on a full Bayesian approach to seismic inversion and can be divided into two steps. First we estimate the conditional probability of elastic properties and their relative changes; then we estimate the posterior probability of rock properties and dynamic property changes. We apply the proposed methodology to a synthetic reservoir study where we have created a synthetic seismic survey for a real dynamic reservoir model including pre-production and production scenarios. The final result is a set of point-wise probability distributions that allow us to predict the most probable reservoir models at each time step and to evaluate the associated uncertainty. Finally we also show an application to real field data from the Norwegian Sea, where we estimate changes in gas saturation and pressure from time-lapse seismic amplitude differences. The inverted results show the hydrocarbon displacement at the times of two repeated seismic surveys.
    Print ISSN: 0016-8025
    Electronic ISSN: 1365-2478
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    Revision of the EPICA Dome C CO2 record from 800 to 600 kyr before present (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: The EPICA Dome C (EDC) ice core has allowed for the reconstruction of atmospheric CO 2 concentrations for the last 800,000 years. Here, we revisit the oldest part of the EDC CO 2 record using different air extraction methods and sections of the core. For our established cracker system, we found an analytical artifact, which increases over the deepest 200 m and reaches 10.1 ± 2.4 ppm in the oldest/deepest part. The governing mechanism is not yet fully understood, but it is related to insufficient gas extraction in combination with ice relaxation during storage and ice structure. The corrected record presented here resolves partly the issue with a different correlation between CO 2 and Antarctic temperatures found in this oldest part of the records, however, this anomaly still persists. In addition we provide here an update of 800,000 years atmospheric CO 2 history including recent studies covering the last glacial cycle.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
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    A cautionary note on the use of EESC-based regression analysis for ozone trend studies (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: Equivalent effective stratospheric chlorine (EESC) construct of ozone regression models attributes ozone changes to EESC changes using a single value of the sensitivity of ozone to EESC over the whole period. Using space-based total column ozone (TCO) measurements, and a synthetic TCO time series constructed such that EESC does not fall below its late 1990s maximum, we demonstrate that the EESC-based estimates of ozone changes in the polar regions (70–90°) after 2000 may, falsely, suggest an EESC-driven increase in ozone over this period. An EESC-based regression of our synthetic “failed Montreal Protocol with constant EESC" time series suggests a positive TCO trend that is statistically significantly different from zero over 2001–2012 when in fact no recovery has taken place. Our analysis demonstrates that caution needs to be exercised when using explanatory variables, with a single fit coefficient, fitted to the entire data record, to interpret changes in only part of the record.
    Print ISSN: 0094-8276
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    Issue Information (2014)
    Wiley
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    Publication Date: 2014-12-16
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Field-aligned Currents' Scale Analysis Performed with the Swarm Constellation (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: We present a statistical study of the temporal and spatial scale characteristics of different field-aligned current (FAC) types derived with the Swarm satellite formation. We divide FACs into two classes: small-scale, up to some ten kilometer, which arecarried predominantly by kinetic Alfvén waves, and large-scale FACs with sizes of more than 150 km. For determining temporal variability we consider measurements at the same point, the orbital crossovers near the poles, but at different times. Fromcorrelation analysis we obtain a persistent period of small-scale FACs of order 10 s, while large-scale FACs can be regarded stationary for more than 60 s. For the first time we investigate the longitudinal scales. Large-scale FACs are different on dayside and nightside. On the nightside the longitudinal extension is on average 4 times the latitudinal width, while on the dayside, in particular in the cusp region, latitudinal and longitudinal scales are comparable.
    Print ISSN: 0094-8276
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    Predicting the long-term fate of buried organic carbon in colluvial soils (2014)
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    Publication Date: 2014-12-16
    Description: A significant part of the soil organic carbon that is eroded in uplands is deposited and buried in colluvial settings. Understanding the fate of this deposited soil organic carbon (SOC) is of key importance for the understanding of the role of (accelerated) erosion in the global C cycle: the residence time of the deposited carbon will determine if, and for how long, accelerated erosion due to human disturbance will induce sequestration of SOC from the atmosphere to the soil. Experimental studies may provide useful information, but, given the time scale under consideration, the response of the colluvial SOC can only be simulated using numerical models which need careful calibration using field data. In this study, we present a depth explicit SOC model (ICBM-DE) including soil profile evolution due to sedimentation to simulate the long-term C dynamics in colluvial soils. The SOC profile predicted by our model is in good agreement with field observations. The C burial efficiency (the ratio of current C content of the buried sediments to the original C content at the time of sedimentation) of deposited sediments exponentially decreases with time and gradually reached an equilibrium value. This equilibrium C burial efficiency is positively correlated with the sedimentation rate. The sedimentation rate is crucial for the long-term dynamics of the deposited SOC as it controls the time that buried sediments spend at a given soil depth, thereby determining its temporal evolution of C input and decomposition rate during the burial process: C input and decomposition rate vary with depth due to the vertical variation of root distribution and soil environmental factors such as (but not limited to) humidity, temperature and aeration. The model demonstrates that, for the profiles studied, it takes ca. 300 yr for the buried SOC to lose half of its C load. It would also take centuries for the SOC accumulated in colluvial soils over the past decades due to soil redistribution under mechanized agriculture to be released to the atmosphere after the application of soil conservation measures such as conservation tillage.
    Print ISSN: 0886-6236
    Electronic ISSN: 1944-9224
    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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    Coherent Time Reversal Microwave Imaging for the Detection and Localization of Breast Tissue Malignancies (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: This paper deals with coherent processing of time reversal operator for microwave imaging in the frequency-domain. In frequency-domain time-reversal imaging approach, images obtained for different frequency bins over ultrawideband are incoherently processed. In highly dense and cluttered medium the signal subspace over each narrow frequency bin varies from that obtained using the complete ultrawideband. As a result, the detection and localization from non-coherent imaging approach is often inconclusive. In order to improve the stability of time-reversal microwave imaging, we propose coherent processing using novel focusing matrix approach. The proposed focusing matrix makes possible the time-reversal imaging technique to coherently process each frequency bin to yield a consistent signal subspace. The performance of coherent focusing is investigated when combined with time-reversal robust capon beamformer (TR-RCB). We have used numerical experiments on breast cancer detection using FDTD employing anatomically realistic numerical breast phantoms that contain varying amounts of dense fibro-glandular tissue content. The imaging results indicate that the proposed Coherent-TR-RCB (C-TR-RCB) could overcome the limitations of time-reversal imaging in a highly heterogeneous and cluttered medium.
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    Design and Numerical Verification of Plasmonic Cross Antennas to Generate Localized Circularly Polarized Light for All-Optical Magnetic Recording (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: All-optical magnetic recording with localized circularly polarized light is studied for realizing ultra-fast and high-density magnetic recording. We design plasmonic cross antennas with bit-patterned media and evaluate the Stokes parameters. Using our proposed method, the magnetic recording speed is about 100,000 times faster than that for conventional methods and the recording density becomes over 2Tbit/inch 2 .
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
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    Unifying the relative hindered velocity in suspensions and emulsions of non-deformable particles (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Although the relative velocity of a single crystal or bubble in a quiescent fluid (melt) is well-characterized, the interplay of crystals/bubbles in multiparticle systems and its effect on their settling/rising velocity is poorly quantified. We propose a theoretical model for the hindered velocity of non-Brownian emulsions and suspensions of non-deformable fluid and solid particles in the creeping flow regime. The model is based on three sets of correction; two on the drag coefficient experienced by each particle to account for both return flow and Smoluchowski effects, and a correction on the rheology to account for non-local interactions introduced as a mean-field effective viscosity. Our model is tested against new and published experimental data over a wide range of particle volume fraction and viscosity ratio between the fluids. We find an excellent agreement between our model and experiments. The model is then applied to show that hindered settling can increase mineral residence time by up to an order of magnitude in convecting magma chambers.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    Cosmic ray event of AD 774-775 shown in quasi-annual 10Be data from the Antarctic Dome Fuji ice core (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: 14 C content in tree rings and 10 Be concentration records in polar ice core provide information about past cosmic ray intensities. The AD 774–775 cosmic ray event has been identified by 14 C measurement in several tree rings from all over the world. Although the quasi-decadal 10 Be Dome Fuji data in the Antarctic ice core also shows a sharp peak around AD 775, annual 10 Be variations in the Dome Fuji core or in other cores have not been revealed. We have measured quasi-annual 10 Be concentrations from approximately AD 763–794 in the Dome Fuji ice core, and detected a clear increase (~80% above the baseline) in 10 Be concentration around AD 775. However, an accurate height of this increase is not straightforwardly estimated due to the background variation in 10 Be concentration. The 10 Be increase can be due to the same cosmic ray event as shown in the 14 C content in AD 774–775.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    Gravity waves simulated by high-resolution Whole Atmosphere Community Climate Model (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: For the first time a mesoscale-resolving whole atmosphere general circulation model (GCM) has been developed, using the NCAR Whole Atmosphere Community Climate Model (WACCM) with ~0.25° horizontal resolution and 0.1 scale height vertical resolution above the middle stratosphere (higher resolution below). This is made possible by the high accuracy and high scalability of the spectral element dynamical core from the High-Order Method Modeling Environment (HOMME). For the simulated January-February period, the latitude-height structure and the magnitudes of the temperature variance compare well with those deduced from SABER observations. The simulation reveals the increasing dominance of gravity waves (GWs) at higher altitudes through both the height dependence of the kinetic energy spectra, which display a steeper slope (~-3) in the stratosphere and an increasingly shallower slope above, and the increasing spatial extent of GWs (including a planetary-scale extent of a concentric GW excited by a tropical cyclone) at higher altitudes. GW impacts on the large-scale flow is evaluated in terms of zonal mean zonal wind and tides: with no GW drag parameterized in the simulations, forcing by resolved GWs does reverse the summer mesospheric wind, albeit at an altitude higher than climatology, and only slows down the winter mesospheric wind without closing it. The hemispheric structures and magnitudes of diurnal and semidiurnal migrating tides compare favorably with observations.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    3-D Simulations of the Southern Polar Giant Impact Hypothesis for the Origin of the Martian Dichotomy. (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: We demonstrate via numerical simulations that the impact of a ~ lunar-sized body with Mars is capable of creating a hemispherical magma ocean that upon cooling and solidification resulted in the formation of the southern highlands and thus the Martian dichotomy. The giant impact may have contributed a significant amount of iron to the Martian core and generated a deep thermal anomaly that led to the onset and development of the volcanism in the southern highlands. Our model predicts several mantle plumes converging to the South Pole from the equatorial regions as well as new plumes forming in the equatorial region and also an absence of significant large-scale volcanism in the northern lowlands. The core heat flux evolution obtained from our numerical models is consistent with the decline of the magnetic field. We argue that such a scenario is more consistent with a range of observations than a northern giant impact (excavating the Borealis basin) for the formation of the Martian dichotomy.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    Greenland ice sheet annual motion insensitive to spatial variations in subglacial hydraulic structure (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: We present ice velocities observed with global positioning systems and TerraSAR-X/TanDEM-X in a land-terminating region of the south-west Greenland ice sheet (GrIS) during the melt-year 2012–2013, to examine the spatial pattern of seasonal and annual ice motion. We find that whilst spatial variability in the configuration of the subglacial drainage system controls ice motion at short timescales, this configuration has negligible impact on the spatial pattern of the proportion of annual motion which occurs during summer. Whilst absolute annual velocities vary substantially, the proportional contribution of summer motion to annual motion does not. These observations suggest that in land-terminating margins of the GrIS, subglacial hydrology does not significantly influence spatial variations in net summer speedup. Furthermore, our findings imply that not every feature of the subglacial drainage system needs to be resolved in ice sheet models.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    δ15N in laminated marine sediments provide a proxy for mixing between the California Undercurrent and the California Current: A proof of concept (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Measurements of particulate δ 15 N in coastal marine laminated sediments provide a high-resolution proxy for fluctuations in the intensity of denitrification in the water column. In the Eastern Tropical North Pacific oxygen minimum zone, this denitrification signal is transported northward by the California Undercurrent, thus serving as a tracer of ocean circulation. This is verified through comparisons between salinity in the thermocline off Southern California (Santa Monica Basin) and the difference between δ 15 N sed within age equivalent sediments from a southern (Pescadero Slope) and northern (Santa Monica Basin) site. Trends in this parameter, Δ δ 15 N sed , relate to Pacific Decadal Oscillation (PDO) phase changes between 1900 and 1990. We hypothesize that the decline in Δ δ 15 N sed during warm PDO phases is due to a strengthening of the California Undercurrent transporting 15  N-enriched nitrate from the ETNP northward. The deviation from this trend after 1990 suggests recent changes in circulation and/or California Current water nutrient biogeochemistry.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    Mechanical properties and processes of deformation in shallow sedimentary rocks from subduction zones: An experimental study (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: To better constrain the mechanical behavior of sediments accreted to accretionary prism, we conducted triaxial mechanical tests on natural samples from the Miura-Boso paleo-accretionary prism (Japan) in drained conditions with confining pressures up to 200 MPa as well as post-experiments P-wave velocity (V p ) measurements. During experiments, deformation is principally non-coaxial and accommodated by two successive modes of deformation, both associated with strain-hardening and velocity-strengthening behavior: (1) compaction-assisted shearing, distributed in a several mm-wide shear zone and (2) faulting, localized within a few tens of µm-wide, dilatant fault zone. Deformation is also associated with (1) a decrease in Young's modulus all over the tests, (2) anomalously low V p in the deformed samples compared to their porosity and (3) an increase in sensitivity of V p to effective pressure. We interpret this evolution of the poroelastic properties of the material as reflecting the progressive breakage of intergrain cement and the formation of microcracks along with macroscopic deformation. When applied to natural conditions, these results suggest that the deformation style (localized vs distributed) of shallow (z 〈 a few km) sediments is mainly controlled by the variations in stress/strain rate during the seismic cycle and is therefore independent of the porosity of sediments. Finally, we show that the effect of strain, through cement breakage and microcracks formation, may lower V p for effective pressure up to 40 MPa. As a consequence, the low V p anomalies observed in Nankai accretionary prisms by seismic imaging between 2 and 4km depth [ Kitajima and Saffer , 2012] could reflect sediment deformation rather than porosity anomalies. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Effects of Climate Oscillations on Wind Resource Variability in the United States (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Natural climate variations in the United States wind resource are assessed by using cyclostationary empirical orthogonal functions (CSEOFs) to decompose wind reanalysis data. Compared to approaches that average climate signals or assume stationarity of the wind resource on interannual timescales, the CSEOF analysis isolates variability associated with specific climate oscillations, as well as their modulation from year to year. Contributions to wind speed variability from the modulated annual cycle (MAC) and the El Niño-Southern Oscillation (ENSO) are quantified, and information provided by the CSEOF analysis further allows the spatial variability of these effects to be determined. The impacts of the MAC and ENSO on the wind resource are calculated at existing wind turbine locations in the United States, revealing variations in the wind speed of up to 30% at individual sites. The results presented here have important implications for predictions of wind plant power output and siting.
    Print ISSN: 0094-8276
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    Thermodynamic calculations of the polybaric melting phase relations of spinel lherzolite (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: This study presents a new thermodynamic model for the calculation of phase relations during the melting of anhydrous spinel lherzolite at pressures of 1–2.5 GPa. The model is based on the total energy minimization algorithm for calculating phase equilibria within multicomponent systems and the thermodynamic configuration of Ueki and Iwamori [2013]. The model is based on a SiO 2 – Al 2 O 3 – FeO–Fe 3 O 4 –MgO–CaO system that includes silicate melt, olivine, clinopyroxene, orthopyroxene, and spinel as possible phases. The molar Gibbs free energy of the melt phase is modeled quasi-empirically, and the thermodynamic parameters for silicate melt end-member components are calibrated with a polybaric calibration database. The temperatures and pressures used in this newly compiled calibration dataset are 1230–1600 ∘ C and 0.9–3 GPa, corresponding to the stability range of spinel lherzolite. The modeling undertaken during this study reproduces the general features of experimentally determined melting phase relations of spinel lherzolite at 1–2.5 GPa, including the solidus temperature, the melt composition, the chemical reaction during melting and the degree of melting. This new thermodynamic modeling also reproduces phase relations of various bulk compositions from fertile to deplete spinel lherzolite and can be used in the modeling of polybaric mantle melting within various natural settings. Comparing the results derived from this new modeling with those produced using previous models indicates that the new approach outlined here, involving a combination of total energy minimization and the direct calibration of melt thermodynamic parameters at pressure and temperature conditions corresponding to mantle melting with a relatively simple melt thermodynamic equation, can accurately model polybaric melting phase relations. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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    Issue Information (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: Cover. Evolving and increasingly hazardous radiation levels in space. (top) ACE dose rates (red) are based on fits to CRIS spectra [ O'Neill , 2006]; CRaTER measurements (green) from the zenith-facing D1/D2 detectors are used as proxies for lens dose rates behind 0.3 g/cm2 Al shielding Schwadron et al . [2012]. The sunspot number predictions (the lower black and blue dashed lines) show two cases based on a Gleissberg-like and a Dalton-like minimum, the results of which are similar. The dose predictions (solid blue line and the upper black and blue dashed lines) are from a sunspot-based model of the heliospheric magnetic field and the correlated variation in modulation of GCRs. The ACE data, CRaTER data, and model results are projected to the lunar surface. (bottom) Same as the top panel, but for a longer time span. See pp. 622–632.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
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    Relationship between sunshine duration and air temperature and contemporary global warming (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: ABSTRACT The purpose of this paper is to describe the variability of sunshine duration and air temperature and to assess the relationship between these variables. The paper is based on daily means of air temperature and daily sunshine duration totals from the 1884–2012 period. The variability analyses of these variables for the whole period is supplemented by additional trend analyses for two separate time periods described in the literature as dimming (1958–1985) and brightening (1985–2012). All the data come from the Jagiellonian University Research Station in Krakow. In Krakow, there is a decrease in the number of sunshine hours in the 1950s to 1980s period followed by an increase in the last two decades of the 20th century. The long-term course of air temperature in Krakow confirms the results obtained in other places in the world and exhibits increasing warming of the climate. The temperature increase is visible in each month but is highest in winter (exceeding 1.5 °C per 100 years). Therefore, it may be assumed that the increase in air temperature, which has been particularly strong since the 1980s, showed the full effect of global warming in the brightening period that was previously masked by global dimming as shown in the sunshine duration records. The time course of air temperature for the whole study period (1884–2012) shows a statistically significant positive linear trend. The present warming is influenced probably by the air-quality improvement (brightening period) and an increase in sunshine duration in the years 1980–2012.
    Print ISSN: 0899-8418
    Electronic ISSN: 1097-0088
    Topics: Geosciences , Physics
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    Long-term changes in summer temperature anomaly patterns in Japan since the early 20th century (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: The relationship between long-term variations in summer temperature in Japan since the early 20th century and the large-scale atmospheric circulation field was analysed. The combined influence of various circulation predictors on temperature variations was analysed via a multicomponent canonical correlation analysis (CCA). The positive phase of the first CCA mode is related to a positive temperature anomaly across Japan, characterized by a weak blocking high over the Okhotsk Sea, and a strong North Pacific subtropical high (NPSH) over Japan. The positive phase of the second CCA mode corresponds to a positive temperature anomaly in southwestern Japan, and a negative anomaly in northern Japan, characterized by an anticyclonic circulation anomaly over eastern China and a cyclonic anomaly over the northwestern Pacific. By investigating the temporal changes in CCA scores, we detected an abrupt increase in the first CCA score in the early 1910s and a long-term increasing trend in the second CCA score since the early 20th century. The abrupt increase in the first CCA score indicates an abrupt increase in temperature throughout Japan. By investigating changes in the circulation field associated with this abrupt warming, we determined that intensification of zonal flow over the Okhotsk Sea was responsible. The increasing trend of the second CCA score indicates an increase in the regional difference in summer temperatures between northern and southwestern Japan. After investigating changes in the circulation field before and after the mid-20th century, we suggest that the southwestward shift of the NPSH and the weakening of anticyclonic circulation anomalies over the northwestern Pacific were responsible for this increase in the regional temperature difference.
    Print ISSN: 0899-8418
    Electronic ISSN: 1097-0088
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    Wind forcing controls on river plume spreading on a tropical continental shelf (2014)
    Wiley
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    Publication Date: 2014-12-17
    Description: The Berau Continental Shelf is located close to the Equator in the Indonesian Archipelago, hosting a complex of coral reefs along its oceanic edge. The Berau coral reefs have a very high biodiversity, but the area is under serious risk due to river-derived nutrients and sediments. The region is characterized by weak winds, moderate tides and almost absent Coriolis forcing. Existing knowledge about river plume behaviour in tropical environments is limited. The aim of this paper is to investigate the influence of the subtle physical forcing on the dynamics of the Berau river plume. A three-dimensional model (ECOMSED) was calibrated with observational data. The model was forced by freshwater input from the Berau river distributaries, tides at the open boundaries and measured hourly wind. The model reproduces the freshwater dynamics on the shelf adequately and highlights that the river plume spreads symmetrically for river forcing only. Tides cause vertical mixing and suppress the cross-shelf spreading of the river plume. However, the spreading of the river plume over the shelf is mainly controlled by the weak monsoonal winds, resulting in a seasonal development. During the Southeast Monsoon, the southerly winds push the plume northeastward and cause a stratified water column in the northern part of the continental shelf. Northerly winds during the Northwest Monsoon disperse the plume to the south, promoting a vertically well-mixed water column. The results can be used to predict the possible impact of land-use changes in the steadily developing Berau region on coral reef health. This article is protected by copyright. All rights reserved.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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    Kinetics and thermodynamics of reversible disproportionation–comproportionation in redox triad oxoammonium cations – nitroxyl radicals – hydroxylamines (2014)
    Wiley
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    Publication Date: 2014-12-06
    Description: Kinetics and equilibrium of the acid-catalyzed disproportionation of cyclic nitroxyl radicals R 2 NO • to oxoammonium cations R 2 NO + and hydroxylamines R 2 NOH is defined by redox and acid–base properties of these compounds. In a recent work ( J. Phys. Org. Chem . 2014, 27, 114-120), we showed that the kinetic stability of R 2 NO • in acidic media depends on the basicity of the nitroxyl group. Here, we examined the kinetics of the reverse comproportionation reaction of R 2 NO + and R 2 NOH to R 2 NO • and found that increasing in – I -effects of substituents greatly reduces the overall equilibrium constant of the reaction K 4 . This occurs because of both the increase of acidity constants of hydroxyammonium cations K 3H+ and the difference between the reduction potentials of oxoammonium cations E R2NO+/R2NO• and nitroxyl radicals E R2NO•/R2NOH . pH dependences of reduction potentials of nitroxyl radicals to hydroxylamines E 1/3Σ and bond dissociation energies D(O–H) for hydroxylamines R 2 NOH in water were determined. For a wide variety of piperidine- and pyrrolidine-1-oxyls values of p K 3H+ and E R2NO+/R2NO• correlate with each other, as well as with the equilibrium constants K 4 and the inductive substituent constants σ I . The correlations obtained allow prediction of the acid–base and redox characteristics of redox triads R 2 NO • –R 2 NO + –R 2 NOH. Copyright © 2014 John Wiley & Sons, Ltd. The overall equilibrium constant K 4 of disproportionation of nitroxyl radicals 1 depend on the difference of their pH-dependent reduction potentials to hydroxyammonium cations 3 H + and reduction potentials of oxoammonium cations 2 to 1 : E 1 / 3 Σ  −  E 2 / 1  = ( RT / F )ln K 4 . – I -Effects of substituents R and heterocycle contraction increase the rate constant of comproportionation k − 2 , acidity constants of protonated nitroxyl radicals K 1 H+ and hydroxylamines K 3 H+ , thus reducing the constant K 4  =  k 2  /  (k − 2  ×  K 1 H+  ×  K 3 H+ ) in the studied series of compounds a–i by more than seven orders of magnitude.
    Print ISSN: 0894-3230
    Electronic ISSN: 1099-1395
    Topics: Chemistry and Pharmacology , Physics
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    Dimethyl Sulfide in the Amazon Rain Forest (2014)
    Wiley
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    Publication Date: 2014-12-09
    Description: Surface-to-atmosphere emissions of dimethyl sulfide (DMS) may impact global climate through the formation of gaseous sulfuric acid, which can yield secondary sulfate aerosols and contribute to new particle formation. While oceans are generally considered the dominant source of DMS, a shortage of ecosystem observations prevents an accurate analysis of terrestrial DMS sources. Using mass spectrometry, we quantified ambient DMS mixing ratios within and above a primary rainforest ecosystem in the central Amazon Basin in real-time (2010–2011) and at high vertical resolution (2013–2014). Elevated but highly variable DMS mixing ratios were observed within the canopy, showing clear evidence of a net ecosystem source to the atmosphere during both day and night in both the dry and wet seasons. Periods of high DMS mixing ratios lasting up to 8 hours (up to 160 ppt) often occurred within the canopy and near the surface during many evenings and nights. Daytime gradients showed mixing ratios (up to 80 ppt) peaking near the top of the canopy as well as near the ground following a rain event. The spatial and temporal distribution of DMS suggests that ambient levels and their potential climatic impacts are dominated by local soil and plant emissions. A soil source was confirmed by measurements of DMS emission fluxes from Amazon soils as a function of temperature and soil moisture. Furthermore, light and temperature dependent DMS emissions were measured from seven tropical tree species. Our study has important implications for understanding terrestrial DMS sources and their role in coupled land-atmosphere climate feedbacks.
    Print ISSN: 0886-6236
    Electronic ISSN: 1944-9224
    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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    The effect of stratospheric sulfur from Mount Pinatubo on tropospheric oxidizing capacity and methane (2014)
    Wiley
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    Publication Date: 2014-12-09
    Description: The eruption of Mount Pinatubo in 1991 injected a large amount of SO 2 into the stratosphere, which formed sulfate aerosols. Increased scattering and absorption of UV radiation by the enhanced stratospheric SO 2 and aerosols decreased the amount of UV radiation reaching the troposphere, causing changes in tropospheric photochemistry. These changes affected the oxidizing capacity of the atmosphere and the removal rate of CH 4 in the years following the eruption. We use the three-dimensional chemistry transport model TM5 coupled to the aerosol microphysics module M7 to simulate the evolution of SO 2 and sulfate aerosols from the Pinatubo eruption. Their effect on tropospheric photolysis frequencies and concentrations of OH and CH 4 are quantified for the first time. We find that UV attenuation by stratospheric sulfur decreased the photolysis frequencies of both ozone and NO 2 by about 2% globally, decreasing global OH concentrations by a similar amount in the first two years after the eruption. SO 2 absorption mainly affects OH primary production by ozone photolysis, while aerosol scattering also alters OH recycling. The effect of stratospheric sulfur on global OH and CH 4 is dominated by the effect of aerosol extinction, while SO 2 absorption contributes by 12.5% to the overall effect in the first year after the eruption. The reduction in OH concentrations causes an increase in the CH 4 growth rate of 4 and 2 ppb/yr in the first and second year after the eruption, respectively, contributing 11 Tg to the 27 Tg observed CH 4 burden change in late 1991 and early 1992.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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