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  • Articles  (4,699)
  • Wiley  (4,699)
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  • Geosciences  (4,699)
  • 101
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    The two types of El-Nino and their impacts on the Length-of-day (2014)
    Wiley
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    Publication Date: 2014-05-01
    Description: At the interannual to decadal timescale, the changes in the Earth rotation rate are linked with the El-Niño Southern Oscillation phenomena through changes in the Atmospheric Angular Momentum. As climatic studies demonstrate that there were two types of El-Niño events, namely Eastern Pacific (EP) and Central Pacific (CP) events, we investigate how each of them affect the Atmospheric Angular Momentum. We show in particular that EP events are associated with stronger variations of the Atmospheric Angular Momentum and length-of-day. We explain this difference by the stronger pressure gradient over the major mountain ranges, due to a stronger and more efficiently localized pressure dipole over the Pacific Ocean in the case of EP events.
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  • 102
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    Usefulness of Ensemble Forecasts from NCEP Climate Forecast System in Sub-seasonal to Intra-annual Forecasting (2014)
    Wiley
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    Publication Date: 2014-05-01
    Description: Typically, sub-seasonal to intra-annual climate forecasts are based on ensemble mean (EM) predictions. The EM prediction provides only a part of the information available from the ensemble forecast. Here we test the null hypothesis that the observations are randomly distributed about the EM predictions using a new metric that quantifies the distance between the EM predictions from the National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2) and the observations represented by CFSv2 Reanalysis. The null hypothesis cannot be rejected in this study. Hence, we argue that the higher order statistics such as ensemble standard deviation are also needed to describe the forecast. We also show that removal of systematic errors that are a function of the forecast initialization month and lead-time is a necessary pre-processing step. Finally, we show that CFSv2 provides useful ensemble climate forecasts from zero to nine months lead-time in several regions.
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  • 103
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    Explosion depths for phreatomagmatic eruptions (2014)
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    Publication Date: 2014-05-01
    Description: Subsurface phreatomagmatic explosions can result from the interaction of ascending magma with groundwater. Experiments over a wide range of energies show that for a given energy there is a depth below which an explosion will be contained within the subsurface (not erupt), and there is a corresponding shallower depth that will optimize ejecta dispersal. We combine these relationships with constraints on the energies of phreatomagmatic explosions at maar-diatreme volcanoes, and show that most eruptions are likely sourced by explosions in the uppermost ~200 m, and even shallower ones (〈100 m) are likely to dominate deposition onto tephra rings. Most explosions below ~200 m will not erupt, but contribute to formation of, and to the vertical mixing of materials within, a diatreme (vent structure), with only rare very high energy explosions between ~200 – 500 m erupting. Similar constraints likely apply at other volcanoes that experience phreatomagmatic explosions.
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  • 104
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    20th Century sea ice trends in the Ross Sea from a high resolution, coastal ice core record (2014)
    Wiley
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    Publication Date: 2014-05-01
    Description: We present the first proxy record of sea ice area (SIA) in the Ross Sea, Antarctica, from a 130-year coastal ice core record. High-resolution deuterium excess data show prevailing stable SIA from the 1880 s until the 1950s, a 2–5% reduction from the mid-1950s to the early-1990s, and a 5% increase after 1993. Additional support for this reconstruction is derived from ice core methanesulphonic acid (MSA) concentrations and whaling records. While SIA has continued to decline around much of the West Antarctic coastline since the 1950s, concurrent with increasing air and ocean temperatures, the underlying trend is masked in the Ross Sea by a switch to positive SIA anomalies since the early-1990s. This increase is associated with a strengthening of southerly winds and the enhanced northwards advection of sea ice.
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  • 105
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    Annual Amphidromic Columns of Sea Temperature in Global Oceans from Argo Data (2014)
    Wiley
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    Publication Date: 2014-03-05
    Description: Nearly a decade ago, it was recognized that “amphidrome” is not a tide-only feature. Although more than a dozen annual amphidromes have been reported for several oceanic/atmospheric variables, little has been done so far to identify non-tidal amphidromes under the sea surface. In this study, analysis of annual phase properties of sea temperature at different depths has been conducted using Argo data. As a result, four annual amphidromic columns of sea temperature (AACST) with different thicknesses have been identified across the tropical oceans. The concept of “amphidromic column” is defined for the first time as the water column where an amphidromic point extends vertically for a given depth in the ocean. Given the fact that it is coupled with the lower limit of the oceanic mixed layer and provides a direct measure of the solar penetration, the determination of AACST has many potential implications ranging from a more precise monitoring of the interannual to multidecadal temperature changes under global warming, to a better understanding of the thermal-dynamical, biological, and fishery processes in the tropical oceans.
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  • 106
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    Observed Changes in False Springs over the Contiguous United States (2014)
    Wiley
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    Publication Date: 2014-03-05
    Description: [1]  Climate warming fosters an earlier spring green-up that may bring potential benefits to agricultural systems. However, advances in green-up timing may leave early-stage vegetation growth vulnerable cold damage when hard freezes follow green-up resulting in a false spring. Spatiotemporal patterns of green-up dates, last spring freezes, and false springs were examined across the contiguous United States from 1920 to 2013. Results indicate widespread earlier green-up and last spring freeze dates over the period. Observed changes in these dates were asymmetric with the last spring freeze date advancing to earlier in the year relative to green-up date. Although regionally variable, these changes resulted in a reduction in false springs, notably over the past 20 years, except across the intermountain western United States where the advance in green-up timing outpaced that of the last spring freeze. A sensitivity experiment shows that observed decreases in false springs are consistent with a warming climate.
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  • 107
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    A Southern Hemisphere Booster of Super El Niño (2014)
    Wiley
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    Publication Date: 2014-03-05
    Description: [1]  A distinct class of El Niño events with extreme magnitude (termed “super El Niño” events in this study) is identified after removing decadal variation. These events occurred in 1972/73, 1982/83 and 1997/98. They are distinguished not only by their size but also by associated features such as a Southern Hemispheric transverse circulation that is not similarly robust in other El Niño events. This transverse circulation is characterized by a low-level equatorward flow, which spins off from a high sea-level-pressure anomaly around Australia and then merges into the deep convection anomalies over the central Pacific, and by a upper-level southward divergent flow, which branches off from the convection center and connects to the subsidence of the Australia high. It is suggested that this transverse cell, peaking in boreal summer, serves as an effective booster during the developing stage of a super El Niño by intensifying tropical Pacific low-level westerly winds.
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  • 108
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    Tropical Cyclones in Reanalysis Datasets (2014)
    Wiley
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    Publication Date: 2014-03-05
    Description: [1]  This study evaluates and compares tropical cyclones (TCs) in state-of-the-art reanalysis datasets including: the Japanese 55-year Reanalysis (JRA-55); Japanese 25-year Reanalysis (JRA-25); ECMWF Reanalysis-40 (ERA-40); Interim Reanalysis (ERA-Interim); NCEP Climate Forecast System Reanalysis (CFSR); and NASA's Modern Era Retrospective Analysis for Research and Application (MERRA). Most of the reanalyses reproduce a reasonable global spatial distribution of observed TCs and temporal interannual variation of total TC frequency.Of the six reanalysis datasets, JRA-55 appears to be the best in terms of: the highest skill for spatial and temporal distribution of TC frequency of occurrence; highest TC hitting rate, lower false-alarm rate; reasonable TC structure in terms of the relationship between maximum surface wind speed and sea level pressure; and higher correlation coefficients for interannual variations of TC frequency. These results also suggest that the finest resolution reanalysis datasets, like MERRA, are not always the best in terms of TC climatology.
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  • 109
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    Magnetotelluric imaging of upper crustal partial melt at Tendaho graben in Afar, Ethiopia (2014)
    Wiley
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    Publication Date: 2014-04-26
    Description: We report on a recent magnetotelluric (MT) survey across the Manda Hararo magmatic segment (MHMS) within the Tendaho graben in the Afar Depression in north eastern Ethiopia. Twenty-two broadband MT sites with ~ 1 km station spacing were deployed along a profile with the recorded data covering a period range from 0.003 s to 1000 s. A two dimensional (2D) resistivity model reveals an upper crustal fracture zone (fault) and partial melt with resistivity of 1-10 Ω m at a depth of 〉 1 km. The partial melt has a maximum horizontal width of 15 km and extends to a depth of 15 km within the Afar Stratoid Series basalts. We estimate a melt fraction of about 13% based on geochemical and borehole data, and bulk resistivity from the 2D MT inversion model. The interpreted upper crustal partial melt may have been formed by either a magma intrusion from mantle sources or a large volume of continental crust that has been fluxed by a small amount of mantle melt and heat. Within the MHMS and Tendaho graben, a magma intrusion is a plausible explanation for the upper crustal conductor. The inferred presence of a conductive fracture zone or fault with hydrothermal fluid and shallow heat sourcing magma reservoir also make the Tendaho graben a promising prospect for the development of conventional hydrothermal geothermal energy.
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  • 110
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    Imprint of long-term solar signal in groundwater recharge fluctuation rates from North West China. (2014)
    Wiley
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    Publication Date: 2014-04-26
    Description: Multiple spectral and statistical analyses of a 700-year long temporal record of groundwater recharge from the dry lands, Badain Jaran Desert (Inner Mongolia) of North West China reveal a stationary harmonic cycle at ~200 ± 20 year. Interestingly, the underlying periodicity in groundwater recharge fluctuations is similar to those of solar induced climate cycle “Suess wiggles” and appears to be coherent with phases of the climate fluctuations and solar cycles. Matching periodicity of groundwater recharge rates and solar and climate cycles renders a strong impression that solar induced climate signals may act as a critical amplifier for driving the underlying hydrographic cycle through the common coupling of long-term Sun-climate groundwater linkages.
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  • 111
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    Postseismic deformation following the Mw 7.2 October 23, 2011 Van earthquake (Turkey): Evidence for aseismic fault reactivation (2014)
    Wiley
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    Publication Date: 2014-03-20
    Description: Geodetic measurements following the 23 October 2011, Mw = 7.2 Van (E. Turkey) Earthquake reveal that a fault splay on the footwall block of the coseismic thrust fault was reactivated and slipped aseismically for more than 1.5 years following the earthquake. Although long-lasting aseismic slip on coseismic ruptures has been documented following many large earthquakes, long-lasting, triggered slip on neighboring faults that did not rupture during the earthquake has not been reported previously. Elastic dislocation and Coulomb stress modeling indicate that the postseismic deformation can be adequately explained by shallow slip on both the coseismic and splay fault, and is likely driven mostly by coseismic stress changes. Thus the slip deficit on the shallow section of the coseismic fault indicated by InSAR-based models has been partially filled by aseismic slip, suggesting a lower likelihood for a large earthquake on the shallow section of the Van fault than suggested by previous studies.
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  • 112
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    Role of mixed layer depth in surface frontogensis: the Agulhas Return Current front (2014)
    Wiley
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    Publication Date: 2014-03-20
    Description: Air-sea interaction processes that modify the sea surface temperature (SST) front in the Agulhas Return Current region (between 40°E and 55°E) during austral summer and winter are examined using observational data and output from a high-resolution ocean general circulation model. While the air-sea heat flux frontal variations tend to relax the SST front, the frontolysis is amplified (damped) in summer (winter) when frontal variations in the mixed layer depth (MLD) are incorporated. The stronger (weaker) frontolysis associated with the MLD variations is due to the fact that the warming (cooling) by the surface heat flux is amplified south of the front where the MLD is shallower, and is reduced north of the front where the MLD is deeper. This study is the first to show that the MLD variations play an important role in affecting the strength of the SST front.
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  • 113
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    Global characteristics of porosity and density stratification within the lunar crust from GRAIL gravity and LOLA topography data (2014)
    Wiley
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    Publication Date: 2014-03-05
    Description: The Gravity Recovery And Interior Laboratory (GRAIL) mission is providing unprecedentedly high-resolution gravity data. The gravity signal in relation to topography decreases from 100 km to 30 km wavelength, equivalent to a uniform crustal density of 2450 kg/m 3 that is 100 kg/m 3 smaller than the density required at 100 km. To explain such frequency-dependent behavior, we introduce rock compaction models under lithostatic pressure that yield radially-stratified porosity (and thus density) and examine the depth extent of porosity. Our modeling and analysis support the assertion that the crustal density must vary from surface to deep crust by up to 500 kg/m 3 . We found that the surface density of megaregolith is around 2400 kg/m 3 with an initial porosity of 10–20%, and this porosity is eliminated at 10–20 km depth due to lithostatic overburden pressure. Our stratified density models provide improved fits to both GRAIL primary and extended mission data.
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  • 114
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    Slab detachment in laterally varying subduction zones: 3D numerical modeling (2014)
    Wiley
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    Publication Date: 2014-03-05
    Description: [1]  Understanding the three-dimensional (3D) dynamics of subduction-collision systems is a longstanding challenge in geodynamics. We investigate the impact of slab detachment in collision systems that are subjected to along-trench variations. High resolution thermo-mechanical numerical models, encompassing experimentally-derived flow laws and a pseudo free surface, are employed to unravel lithospheric and topographic evolutions. Firstly, we consider coeval subduction of adjacent continental and oceanic lithospheres (SCO). This configuration yields to two-stage slab detachment during collision, topographic buildup and extrusion, variable along-trench convergence rates and associated trench deformation. The second setting considers a convergent margin, which is laterally limited by a transform boundary (STB). Such collisional system is affected by a single slab detachment, little trench deformation and moderately confined upper plate topography. The effect of initial thermal slab age on SCO and STB models are explored. Similarities with natural analogues along the Arabia-Eurasia collision are discussed.
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  • 115
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    Issue Information (2014)
    Wiley
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    Publication Date: 2014-04-29
    Description: No abstract is available for this article.
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  • 116
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    Enhanced ionization of the Martian nightside ionosphere during solar energetic particle events (2014)
    Wiley
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    Publication Date: 2014-01-25
    Description: [1]  Electron densities in the Martian nightside ionosphere are more than 90% of time too low to be detected by the MARSIS radar sounder on board the Mars Express spacecraft. However, the relative number of ionograms with peak electron density high enough to be detected represents a good statistical proxy of the ionospheric density. We focus on solar energetic particle (SEP) events, and we analyze their effects on ionospheric formation. SEP time intervals were identified in-situ using the background counts recorded by the ion sensor of the ASPERA-3 instrument on board Mars Express. We show that peak electron densities during the SEP events are large enough to be detected in more than 30% of measurements, and, moreover, the reflections of the sounding signal from the ground almost entirely disappear. Nightside electron densities during SEP events are thus substantially increased as compared to normal nightside conditions.
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  • 117
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    The dependence of transient climate sensitivity and radiative feedbacks on the spatial pattern of ocean heat uptake (2014)
    Wiley
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    Publication Date: 2014-01-26
    Description: [1]  The effect of ocean heat uptake (OHU) on transient global warming is studied in a multi-model framework. Simple heat sinks are prescribed in shallow aquaplanet ocean mixed layers underlying atmospheric general circulation models, independently and combined with CO 2 forcing. Sinks are localized to either tropical or high latitudes, representing distinct modes of OHU found in coupled simulations. Tropical OHU produces modest cooling at all latitudes, offsetting only a fraction of CO 2 warming. High-latitude OHU produces three times more global-mean cooling in a strongly polar-amplified pattern. Global sensitivities in each scenario are set primarily by large differences in local shortwave cloud feedbacks, robust across models. Differences in atmospheric energy transport set the pattern of temperature change. Results imply that global and regional warming rates depend sensitively on regional ocean processes setting the OHU pattern, and that equilibrium climate sensitivity cannot be reliably estimated from transient observations.
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  • 118
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    Simulated Magnetopause Losses and Van Allen Probe Flux Dropouts (2014)
    Wiley
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    Publication Date: 2014-01-30
    Description: [1]  Three radiation belt flux dropout events seen by the Relativistic Electron Proton Telescope soon after launch of the Van Allen Probes in 2012 (Baker et al., 2013a) have been simulated using the Lyon-Fedder-Mobarry MHD code coupled to the Rice Convection Model, driven by measured upstream solar wind parameters. MHD results show inward motion of the magnetopause for each event, along with enhanced ULF wave power affecting radial transport. Test particle simulations of electron response on 8 October, prior to the strong flux enhancement on 9 October, provide evidence for loss due to magnetopause shadowing, both in energy and pitch angle dependence. Severe plasmapause erosion occurred during ~ 14 hours of strongly southward IMF Bz beginning 8 October coincident with the inner boundary of outer zone depletion.
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  • 119
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    An efficient tracer test for time-variable transit time distributions in periodic hydrodynamic systems (2014)
    Wiley
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    Publication Date: 2014-01-31
    Description: [1]  The time-varying transport dynamics of complex hydrodynamic systems with long transit times are difficult to observe due to the need for multiple tracer injections. Where only one or two distinct tracers are available, overprinting in the output concentrations limits the injection frequency. In this letter we propose an experimental method (the PERiodic Tracer Hierarchy - PERTH) that allows overprinted breakthrough curves to be decomposed into contributions from multiple injections of the same tracer,so long as the transporting flow is periodic. This method allows the time varying transit time distributions to be observed efficiently while making no a priori assumptions about the transport processes operating in the system. Simulations of transport through a soil column subject to a periodic sequence of irrigation events demonstrate that the distinct transit time distributions associated with each irrigation event can be retrieved almost exactly.
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  • 120
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    What is Responsible for the Strong Observed Asymmetry in Teleconnections Between El Niño and La Niña? (2014)
    Wiley
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    Publication Date: 2014-01-31
    Description: [1]  A large asymmetric component (El Niño + La Niña) of ENSO-related teleconnections over North America is found during 1984-2009 that is comparable in strength to the commonly studied symmetric component (El Niño – La Niña). Climate reforecasts spanning this period are diagnosed in order to understand the processes responsible for the observed asymmetry. It is confirmed that an asymmetric component is indeed a fundamental property of atmospheric responses to recent ENSO forcing. Each and every composite of a 16-member reforecast ensemble has appreciable asymmetry in tropical Pacific rainfall, upper tropospheric Pacific-North American circulation patterns, and contiguous U.S. surface temperatures. There is considerable sampling variability in the magnitude of this asymmetric component among individual reforecast composites. We argue therefore that the true SST boundary forced signal of ENSO teleconnections is likely comprised of a symmetric component having greater magnitude than its asymmetric component, though the latter is an important property of how ENSO affects North American climate.
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  • 121
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    Quantitative estimate of the palaeo-Agulhas leakage (2014)
    Wiley
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    Publication Date: 2014-01-31
    Description: [1]  The Indian-Atlantic water exchange south of Africa (Agulhas Leakage) is a key component of the global ocean circulation. No quantitative estimation of the palaeo-Agulhas leakage exists. We quantify the variability in inter-ocean exchange over the past 640,000 years, using planktic foraminiferal assemblage data from two marine sediment records to define an Agulhas Leakage Efficiency (ALE) index. We confirm the validity of our new approach with a numerical ocean model that realistically simulates the modern Agulhas leakage changes. Our results suggest that, during the past several glacial-interglacial cycles, the Agulhas leakage varied by ~10 Sv and more during major climatic transitions. This lends strong credence to the hypothesis that modifications in the leakage played a key role in changing the overturning circulation to full-strength mode. Our results are instrumental for validating and quantifying the contribution of the Indian-Atlantic water leakage to global climate changes.
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  • 122
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    Wind-blown sandstones cemented by sulfate and clay minerals in Gale Crater, Mars (2014)
    Wiley
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    Publication Date: 2014-02-01
    Description: [1]  Gale crater contains Mt. Sharp, a ~5 km thick stratigraphic record of Mars’ early environmental history. The strata comprising Mt. Sharp are believed to be sedimentary in origin, but the specific depositional environments recorded by the rocks remain speculative. We present orbital evidence for the occurrence of eolian sandstones within Gale Crater and the lower reaches of Mt. Sharp, including preservation of wind-blown sand dune topography in sedimentary strata—a phenomenon that is rare on Earth and typically associated with stabilization, rapid sedimentation, transgression and submergence of the land surface. The preserved bedforms in Gale are associated with clay minerals and elsewhere accompanied by typical dune cross-stratification marked by bounding surfaces whose lateral equivalents contain sulfate salts. These observations extend the range of possible habitable environments that may be recorded within Gale Crater and provide hypotheses that can be tested in situ by the Curiosity rover payload.
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  • 123
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    Wind Stress and Near-Surface Shear in the Equatorial Atlantic Ocean (2014)
    Wiley
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    Publication Date: 2014-02-01
    Description: [1]  The upper ocean response to wind stress is examined using eight months of unique near-surface moored velocity, temperature, and salinity data at 0°N, 23°W in the equatorial Atlantic. The effects of wind stress and shear on the time-varying eddy viscosity are inferred using the surface shear-stress boundary condition. Parameterizations of eddy viscosity as a function of wind stress and shear versus wind-stress alone are then examined. In principle, eddy viscosity should be proportional to the inverse shear but how it is represented implicitly or explicitly can affect estimates of the near-surface flow field. This result may explain some discrepancies that have arisen from using wind stress dependent parameterizations to characterize the effects of turbulent momentum mixing.
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  • 124
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    Cluster observations of fast magnetosonic waves in the heliosphere current sheet (2014)
    Wiley
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    Publication Date: 2014-02-01
    Description: [1]  We present Cluster spacecraft observations of large-amplitude ( δ | B |/| B | ~ 1) fast mode magnetosonic waves in the heliospheric current sheet (HCS). The crossings of the HCS and the associated heliospheric plasma sheet (HPS) are encountered by Cluster in the near Earth solar wind and by ACE upstream of Earth. Multiple current layers are detected in correspondence with small-scale discontinuities in the regime of open magnetic field lines within the HCS. Fast magnetosonic waves are observed at one current layer, accompanying the phase-steeped edge of a large amplitude transverse Alfvén wave. The observed fast mode waves are in the frequency range 0.01 Hz-0.2 Hz, characterized by a strong correlation between variations of plasma density and magnetic field strength. Analysis of ratio δE / δB indicate that the fast mode wave packet consists of an anti-sunward propagating component and a larger sunward propagating components in the rest frame of the solar wind. The fast mode waves are not observed by ACE in the upstream solar wind. The generation of the fast mode waves may relate to the development of the phase-steeped Alfvén wave and have profound effects on the evolution of the solar wind plasma.
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    Stopping flow bursts and their role in the generation of the substorm current wedge. (2014)
    Wiley
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    Publication Date: 2014-02-01
    Description: [1]  During two flow burst events, earthward-propagating dipolarization/injection fronts (DF) fortuitously stopped at ∼ 9 Re within a unique, compact multi-spacecraft constellation for the duration of a 30 min -long substorm current wedge-related dipolarization. Observations inside and outside the halted flow burst indicate that it retained properties (including a narrow DF, a localized compression region ahead of it and its structured, low density, low entropy ( pV 5/3 ) content) when arrived at its stopping point, where the entropy of the ambient plasma was nearly equal to that of the flow burst. We show that even short duration flow bursts can significantly modify pressure and entropy distributions in the inner magnetosphere. The new distribution takes a long time to relax (a few tens of minutes, consistent with substorm recovery time scales). We argue that these pressure and entropy changes resulting from the incoming flow bursts can be responsible for the support/generation of a substorm current wedge.
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    Quiet-time Observations of He-ions in the Inner Magnetosphere as Observed from the RBSPICE Instrument Aboard the Van Allen Probes Mission (2014)
    Wiley
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    Publication Date: 2014-02-01
    Description: [1]  He-ions contribute to Earth's ring current energy and species population density, and are important in understanding ion transport and charge exchange processes in the inner magnetosphere. He-ion flux measurements made by the Van Allen Probes RBSPICE instrument are presented in this paper. Particular focus is centered on geomagnetically quiet intervals in late 2012 and 2013 that show the flux, L-shell, and energy (65-keV to 518-keV) morphology of ring current He-ions between geomagnetic storm injection events. The overall He-ion abundance during the first nine months of RBSPICE observations, the appearance of a persistent high energy, low L-shell He-ion population, and the temporal evolution of this population all provide new insights into trapped ring current energy He-ions. These data provide a unique resource that will be used to provide verifications of, and improvements to, models of He-ion transport and loss in Earth's ring current region.
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    Topographic vorticity waves forced by Antarctic dense shelf water outflows (2014)
    Wiley
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    Publication Date: 2014-02-05
    Description: We use numerical simulations to investigate excitation of topographic vorticity waves (TVWs) along the Antarctic continental slope by outflows of dense shelf water through troughs. Idealized models show that wave frequency depends on the amount of stretching in the ambient fluid over the outflow and on background along-slope mean flow. Frequency is higher for steeper bottom slope, larger outflow density anomaly, and stronger westward mean flow. For weak stratification and weak westward along-slope flows typical of the Antarctic slope, wave energy propagates eastward, in the opposite direction from phase velocity. Our results are consistent with recent observations of TVWs in the southern Weddell Sea. In a realistic simulation of the Ross Sea, TVW properties are modulated on seasonal and shorter time scales as background ocean state varies. We expect these waves to contribute to mixing, cross-slope exchanges and sea ice concentration in the vicinity of sources of dense water outflows.
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    Distinguishing high surf from volcanic long-period earthquakes (2014)
    Wiley
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    Publication Date: 2014-02-05
    Description: Repeating long-period (LP) earthquakes are observed at active volcanoes worldwide and are typically attributed to unsteady pressure fluctuations associated with fluid migration through the volcanic plumbing system. Non-volcanic sources of LP signals include ice movement and glacial outburst floods, and the waveform characteristics and frequency content of these events often make them difficult to distinguish from volcanic LP events. We analyze seismic and infrasound data from an LP swarm recorded at Pagan volcano on 12-14 October 2013, and compare the results to ocean wave data from a nearby buoy. We demonstrate that although the events show strong similarity to volcanic LP signals, the events are not volcanic but due to intense surf generated by a passing typhoon. Seismo-acoustic methods allow for rapid distinction of volcanic LP signals from those generated by large surf and other sources, a critical task for volcano monitoring.
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    Global Distribution and Seasonal Dependence of Satellite-based Whitecap Fraction (2014)
    Wiley
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    Publication Date: 2014-02-05
    Description: [1]  We present the first study of global seasonal distributions of whitecap fraction, W , obtained from satellite-based radiometric observations. Satellite-based W incorporates variability from forcings other than wind speed, and can capture differences in W in initial and late lifetime stages. The satellite-based W is more uniform latitudinally than predictions from a widely used wind speed dependent parameterization, W ( U 10 ), formulated from in situ observations, being on average higher than the W ( U 10 ) predictions at low latitudes and lower at mid and high latitudes. This difference provides an explanation for the consistent geographical biases in sea spray aerosol concentration found in a number of large scale models. Satellite estimates of W would benefit air-sea interaction and remote sensing applications that use parameterizations in terms of W such as sea spray flux, gas transfer, and surface winds.
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    The role of ring current particle injections: Global Simulations and Van Allen Probes Observations during March 17, 2013 Storm (2014)
    Wiley
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    Publication Date: 2014-02-05
    Description: We simulate substorm injections observed by the Van Allen Probes during the March 17, 2013 storm using a self-consistent coupling between the ring current model RAM-SCB and the global MHD model BATS-R-US. This is a significant advancement compared to previous studies that used artificially imposed electromagnetic field pulses to mimic substorm dipolarization and associated inductive electric field. Several substorm dipolarizations and injections are reproduced in the MHD model, in agreement with the timing of shape changes in the AE/AL index. The associated inductive electric field transports plasmasheet plasma to geostationary altitudes, providing the boundary plasma source to the ring current model. It is found that impulsive plasmasheet injections, together with a large-scale convection electric field, are necessary to develop a strong ring current. Comparisons with Van Allen Probes observations show that our model reasonably well captures dispersed electron injections and the global Dst index.
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    Modeling the seismic cycle in subduction zones: the role and spatiotemporal occurrence of off-megathrust earthquakes (2014)
    Wiley
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    Publication Date: 2014-02-05
    Description: abstract [1]  Shallow off-megathrust subduction events are important in terms of hazard assessment and coseismic energy budget. Their role and spatiotemporal occurrence, however, remain poorly understood. We simulate their spontaneous activation and propagationusing a newly developed 2D, physically consistent, continuum, visco-elasto-plastic seismo-thermo-mechanical (STM) modeling approach. The characteristics of simulated normal events within the outerrise and splay and normal antithetic events within the wedge resemble seismic and seismological observations in terms of location, geometry, and timing. Their occurrence agrees reasonably well with both long-term analytical predictions based on dynamic Coulomb wedge theory and short-term quasi-static stress changes resulting from the typically triggering megathrust event. The impact of off-megathrust faulting on the megathrust cycle is distinct, as more both shallower and slower megathrust events arise due to occasional off-megathrust triggering and increased updip locking. This also enhances tsunami hazards, which are amplified due to the steeply dipping fault planes of especially outerrise events.
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    The impact of climate-vegetation interactions on the onset of the Antarctic ice sheet (2014)
    Wiley
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    Publication Date: 2014-02-05
    Description: [1]  A global coupled atmosphere/vegetation model and a dynamic ice-sheet model were employed to study the impact of climate-vegetation interactions on the onset of the Antarctic ice sheet during the Eocene-Oligocene transition. We found that the CO 2 threshold for Antarctic glaciation is highly sensitive to the prevailing vegetation. In our experiments, the CO 2 threshold is less than 280 ppm if the Antarctic vegetation is dominated by forests, and between 560 and 1120 ppm for tundra and bare ground conditions. The large impact of vegetation on inception is attributed to the ability of canopies to shade the snow-covered ground, which leads to a weaker snow-albedo feedback and higher summer temperatures. However, the overall effect ofcanopy shading on the Antarctic climate also depends on features like local cloudiness and atmospheric meridional heat transport. Our results suggest that vegetation feedbacks on climate are crucial for the timing of the Antarctic glaciation.
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    On the ‘bowl-shaped’ deformation of planetary equatorial current sheets (2014)
    Wiley
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    Publication Date: 2014-02-05
    Description: [1]   Arridge et al . [2008] reported evidence for the formation of a ‘bowl-shaped’ equatorial current disk in Saturn's magnetosphere during epochs with large planetary dipole tilt angle from perpendicularity with the Sun–planet axis. Using a large multi-year set of Geotail, Polar, and Time History of Events and Macroscale Interactions (THEMIS) spacecraft data and a simple model of the magnetic field reversal surface, we found that a similar kind of the tilt-related magnetic field deformation is present in the Earth's magnetosphere. Based on the properties of a tilted vacuum magnetic field configuration, we conclude that the bowl-shaped distortion is a universal feature of magnetospheres with a tilted planetary dipole, resulting from the joint effect of the north-south asymmetry due to the tilt and of the day-night asymmetry imposed by the solar wind flow.
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    Coherent structure generated in the boundary layer of a laboratory-created ionospheric depletion (2014)
    Wiley
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    Publication Date: 2014-02-24
    Description: [1]  Laboratory experiments have been conducted to simulate the boundary processes of ionospheric depletion. The ionospheric depletion was modeled through releasing depletion chemical( SF 6 ) into the ambient plasmas. These plasmas were segregated into tworegions by a boundary layer of width electric scale length. In the localized boundary layer, the electron density decreased sharply that yielded steep density gradients. Meanwhile, the floating potential increased in the time-scales of the lower hybrid(LH) period, which produced strong sheared electron flows. The shear frequency ω s  =  V E / L E , which characterizes the sheared flow, is much larger than the LH frequency ω LH . A coherent structure was observed when the floating potential fluctuations were analyzed using digital spectral analysis techniques. Comparison with the theory indicated that the structure is driven by the electron-ion hybrid(EIH) instability which is generated owing to the nonlinear coupling between the electron density gradient and the sheared electron flow. Our results are important to study the early phase nonlinear evolution of the ionospheric depletion, especially in the development of plasma irregularities and turbulence in the boundary layer.
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    The seasonal variation of the CO2 flux over Tropical Asia estimated from GOSAT, CONTRAIL and IASI (2014)
    Wiley
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    Publication Date: 2014-02-25
    Description: [1]  We estimate the CO 2 flux over Tropical Asia in 2009, 2010 and 2011 using GOSAT total column CO 2 (XCO 2 ) and in situ measurements of CO 2 . Compared to flux estimates from assimilating surface measurements of CO 2 , GOSAT XCO 2 estimates a more dynamic seasonal cycle and a large source in March – May 2010. The more dynamic seasonal cycle is consistent with earlier work by Patra et al . [2011], and the enhanced 2010 source is supported by independent upper air CO 2 measurements from the CONTRAIL project. Using IASI measurements of total column CO (XCO), we show that biomass burning CO 2 can explain neither the dynamic seasonal cycle nor the 2010 source. We conclude that both features must come from the terrestrial biosphere. In particular the 2010 source points to biosphere response to above-average temperatures that year.
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    Summertime maximum and minimum temperature coupling asymmetry over Australia determined using WRF (2014)
    Wiley
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    Publication Date: 2014-02-12
    Description: [1]  Using the Weather and Research Forecasting model we derive the first estimates for intraseasonal soil moisture–atmosphere coupling strength for the Australian summer climate using methodology adapted from the Global Land–Atmosphere Coupling Experiment. We examine the variations in coupling strength by perturbing the background climate (dry vs. wet year) and the model physics (planetary boundary layer or cumulus scheme). For all choices of model physics, results identify Australia as a “hot spot”of soil moisture–atmosphere coupling for both mean and maximum temperature. For the wet case, results are consistent for maximum temperature for all physics choices. Results diverge more for maximum temperature in the chosen dry year. The coupling of soil moisture with minimum temperature is weaker but consistent for all choices of model physics or whether a wet or dry year is used. Coupling strength for precipitation is weak and not statistically significant irrespective of the choice of model physics.
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    Observed change in plate coupling close to the rupture initiation area before the occurrence of the 2011 Tohoku earthquake: implications from an earthquake cycle model (2014)
    Wiley
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    Publication Date: 2014-02-13
    Description: [1]  Changes in plate coupling off the coast of Fukushima have been detected by GPS since 2000. These changes occurred close to the rupture initiation area of the Mw9.0 2011 Tohoku earthquake, and possibly initiated the earthquake. We investigated these changes with quasi-dynamic earthquake cycle simulations using a hierarchical asperity model. We modeled the entire rupture region as rate-weakening but conditionally stable, while areas with huge slips and Mw7 asperities were modeled as strong and ordinary rate-weakening unstable friction, respectively. The following observed characteristics were reproduced: long recurrence time, large rupture region (including a localized huge coseismic slip area and source area of recurring Mw7 earthquakes), and a Mw7-foreshock triggering the Tohoku earthquake. Off Fukushima, repeated aseismic slips propagating northward appeared in the last half of the cycle and possibly caused the decrease in plate coupling. However, it is not necessarily related to the immediate occurrence of the giant earthquake.
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    Unusually large shear wave anisotropy for chlorite in subduction zone settings (2014)
    Wiley
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    Publication Date: 2014-02-14
    Description: [1]  Using first principle simulations we calculated the elasticity of chlorite. At a density ρ ~ 2.60 gm cm -3 , the elastic constant tensor reveals significant elastic anisotropy: V P ~27 % , V S1 ~56 %, and V S2 ~43 %. The shear anisotropy is exceptionally large for chlorite and enhances upon compression. Upon compression, the shear elastic constant component C 44 and C 55 decreases whereas C 66 shear component stiffens. The softening in C 44 and C 55 is reflected in shear modulus, G and the shear wave velocity, V S . Our results on elastic anisotropy at conditions relevant to the mantle wedge indicates that a 10-20 km layer of hydrated peridotite with serpentine and chlorite could account for the observed shear polarization anisotropy and associated large delay times of 1-2 sec observed in some subduction zone settings. In addition, chlorite could also explain the low V P /V S ratios that have been observed in recent high-resolution seismological studies.
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    Thermodynamic and dynamic effects on regional monsoon rainfall changes in a warmer climate (2014)
    Wiley
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    Publication Date: 2014-02-14
    Description: [1]  This study investigates thermodynamic and dynamic effects on regional monsoon rainfall changes in a warmer climate, by using 20 climate model outputs in the Coupled Model Intercomparison Project (CMIP) Phase 5. In a warmer climate, rainfall is projected to increase in most monsoon regions. However, the rates of increase are not uniform among the monsoon regions. A diagnosis based on a linearized moisture budget equation reveals that the regional differences of the rainfall change are largely explained by differences in the dynamic effect. In the Asian monsoon regions, monsoon circulation slows down at a much lower rate than in the other monsoon regions, in addition, surface evaporation increases at a higher rate, resulting in a much larger increase in rainfall. These features are commonly found in both CMIP Phase 3 and Phase 5 model projections.
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    Evidence for a differential sea level rise between hemispheres over the 20th century (2014)
    Wiley
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    Publication Date: 2014-02-14
    Description: [1]  Tide gauge records are the primary source of sea level information over multi-decadal to century timescales. A critical issue in using this type of data to determine global climate-related contributions to sea level change concerns the vertical motion of the land upon which the gauges are grounded. Here we use observations from the Global Positioning System for the correction of this vertical land motion. As a result, the spatial coherence in the rates of sea level change during the 20th century is highlighted at the local and the regional scales, ultimately revealing a clearly distinct behavior between the northern and the southern hemispheres with values of 2.0 mm/year and 1.1 mm/year, respectively. Our findings challenge the widely accepted value of global sea level rise for the 20th century.
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    The Pace of East African Monsoon Evolution during the Holocene (2014)
    Wiley
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    Publication Date: 2014-02-22
    Description: [1]  African monsoon precipitation experienced a dramatic change in the course of the Holocene. The pace with which the African monsoon shifted from a strong early-mid to a weak late Holocene is critical for our understanding of climate dynamics, hydroclimate-vegetation interaction, and shifts of pre-historic human settlements, yet it is controversially debated. On the basis of planktonic foraminiferal Ba/Ca time-series from the Eastern Mediterranean Sea, here we present a proxy record of Nile River runoff that provides a spatially integrated measure of changes in East African Monsoon (EAM) precipitation. The runoff record indicates a markedly gradual mid-late Holocene EAM transition that lasted over 3500 years. The timing and pace of runoff change parallels those of insolation and vegetation changes over the Nile basin, indicating orbitally forced variation of insolation as the main EAM forcing and the absence of a non-linear precipitation-vegetation feedback. A tight correspondence between a threshold level of Nile River runoff and the timing of occupation/abandonment of settlements suggests that, along with climate changes in the eastern Sahara, the level of Nile River and intensity of summer floods were likely critical for the habitability of the Nile Valley (Egypt).
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    Recent and future trends in synthetic greenhouse gas radiative forcing (2014)
    Wiley
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    Publication Date: 2014-02-19
    Description: [1]  Atmospheric measurements show that emissions of hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are now the primary drivers of the positive growth in synthetic greenhouse gas (SGHG) radiative forcing. We infer recent SGHG emissions and examine the impact of future emissions scenarios, with a particular focus on proposals to reduce HFC use under the Montreal Protocol. If these proposals are implemented, overall SGHG radiative forcing could peak at around 355 mW m -2 in 2020, before declining by approximately 26% by 2050, despite continued growth of fully fluorinated greenhouse gas emissions. Compared to “no HFC policy” projections, this amounts to a reduction in radiative forcing of between 50 and 240 mW m -2 by 2050, or a cumulative emissions saving equivalent to 0.5 to 2.8 years of CO 2 emissions at current levels. However, more complete reporting of global HFC emissions is required, as less than half of global emissions are currently accounted for.
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    Direct evidence of double slope power spectra in the high latitude ionospheric plasma (2014)
    Wiley
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    Publication Date: 2014-02-19
    Description: [1]  We report direct observations of the double slope power spectra for plasma irregularities in the F-layer of the polar ionosphere. The ICI-2 sounding rocket, which was launched into the polar cusp ionosphere, intersected enhanced plasma density regions with decameter scale irregularities. Density measurements at unprecedented high resolution with multi-Needle Langmuir Probes allowed for a detailed study of the plasma irregularities down to kinetic scales. Spectral analysis reveals double-slope power spectra for regions of enhanced fluctuations associated mainly with density gradients, with the steepening of the spectra occuring close to the oxygen gyro-frequency. These findings are further supported with the first results from the ICI-3 rocket, which flew through regions with strong precipitation and velocity shears. Previously, double-slope spectra have been observed in the equatorial ionosphere. The present work gives a direct evidence that the double slope power spectra can be common in the high latitude ionosphere.
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    Cusp observation at Saturn's high latitude magnetosphere by the Cassini spacecraft (2014)
    Wiley
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    Publication Date: 2014-02-19
    Description: [1]  We report on the first analysis of magnetospheric cusp observations at Saturn by multiple in-situ instruments onboard the Cassini spacecraft. Using this we infer the process of reconnection was occurring at Saturn's magnetopause. This agrees with remote observations that showed the associated auroral signatures of reconnection. Cassini crossed the northern cusp around noon local time along a poleward trajectory. The spacecraft observed ion energy-latitude dispersions - a characteristic signature of the terrestrial cusp. This ion dispersion is ‘stepped’, which shows that the reconnection is pulsed. The ion energy-pitch angle dispersions suggest that the field-aligned distance from the cusp to the reconnection site varies between ∼27 - 51 R S . An intensification of lower frequencies of the Saturn kilometric radiation emissions suggests the prior arrival of a solar wind shock front, compressing the magnetosphere and providing more favourable conditions for magnetopause reconnection.
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    MHD Analysis of the Velocity Oscillations in the Outer Heliosphere (2014)
    Wiley
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    Publication Date: 2014-02-26
    Description: [1]  We carried out a MHD simulation of deep interplanetary space from 2001 to 2009 with a realistic inner boundary condition obtained from a time-dependent 3D ideal MHD model incorporating interplanetary scintillation and synoptic solar magnetic field data. As a result, we found that wavy solar wind structure with amplitudes of 10-50 km/s and a monochromatic wavelength of ~ 2 AU filled deep interplanetary space beyond ~ 50 AU from the Sun, which corresponds to a velocity oscillation detected by Voyager 2. The phase of the velocity oscillation is anticorrelated with density fluctuations in our simulation, which suggests that the velocity oscillation should propagate inward towards the Sun in the frame moving with the solar-wind, but is advected outward by the solar wind in the rest frame of the Sun.
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    Robust spring drying in the Southwestern US and seasonal migration of wet/dry patterns in a warmer climate (2014)
    Wiley
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    Publication Date: 2014-02-26
    Description: [1]  This study compares climate simulations over North America produced by a regional climate model with the driving global climate simulations as well as a multi-model ensemble of global climate simulations to investigate robust changes in water availability (precipitation (P) – evapotranspiration (E)). A robust spring drying signal across multiple models is identified in the Southwest that results from a decrease in P and an increase in E in the future. In the boreal winter and summer, the prominent changes in P – E are associated with a north – south dipole pattern, while in spring, the prominent changes in P – E appear as an east – west dipole pattern. The progression of the north – south and east – west dipole patterns through the seasons manifests clearly as a seasonal “clockwise” migration of wet/dry patterns, which is a robust feature of water availability changes in North America consistent across regional and global climate simulations.
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    Improved Constraints on Seismic Source Parameters of the 2011 Tohoku Earthquake from GRACE Gravity and Gravity Gradient Changes (2014)
    Wiley
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    Publication Date: 2014-02-27
    Description: [1]  A new approach of using only the north component of gravity change from Gravity Recovery And Climate Experiment (GRACE) data reveals that a substantially higher spatial resolution of the observed seismic deformation following the 2011 Tohoku earthquake is achievable at 333 km or longer. Here we show that GRACE observed north component of gravity change, –17.6 ± 1.1 μGal, and the corresponding gravity gradient change, e.g., Txz at 1.25 ± 0.09 mEötvös, agree well with seismic/GPS model predictions. Localized Slepian spectrum analysis further confirms that the GRACE gravity and gravity gradient changes agree well with seismic model spectra and have powers up to the limit of the GRACE solution complete to spherical harmonic degree 60. Using the gravity observations for the fault parameter inversion via simulated annealing algorithm, we show that the estimated slip orientation and centroid location are different from GPS/seismic solutions and potentially due to the additional offshore constraint from GRACE data.
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    Lee wave generation rates in the deep ocean (2014)
    Wiley
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    Publication Date: 2014-02-27
    Description: [1]  Using the world's largest dataset of in-situ ocean current measurements, combined with a high resolution topography roughness dataset, we use a model-assisted hierarchical clustering methodology to estimate the global lee wave generation rate at the oceanfloor. Our analysis suggests that internal wave generation contributes 0.75 ± 0.19 TW (± 2 st. dev.) to the oceanic energy budget, but with a strong dependence on the Brunt-Väisäla (buoyancy) frequency climatology used. This estimate is higher than previous calculations, and suggests that internal wave generation may be a much more significant contributor to the global oceanic mechanical energy budget than had previously been assumed. Our results imply that lee wave generation and propagation may be a dominant sink of as at least half and potentially the overwhelming majority of ocean surface wind work on the geostrophic circulation.
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    HFC-43-10mee atmospheric abundances and global emission estimates (2014)
    Wiley
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    Publication Date: 2014-02-27
    Description: [1]  We report in situ atmospheric measurements of HFC-43-10mee (C 5 H 2 F 10 , 1,1,1,2,2,3,4,5,5,5-decafluoropentane) from seven observatories at various latitudes, together with measurements of archived air samples and recent Antarctic flask air samples. The global mean tropospheric abundance was 0.21 ± 0.05 ppt (parts-per-trillion, dry air mole fraction) in 2012, rising from 0.04 ± 0.03 ppt in 2000. We combine the measurements with a model and inverse method to estimate rising global emissions –– from 0.43 ± 0.34 Gg yr -1 in 2000 to 1.13 ± 0.31 Gg yr -1 in 2012 (~1.9 Tg CO 2 -eq yr -1 based on a 100-yr global warming potential of 1,660). HFC-43-10mee –– a cleaning solvent used in the electronics industry –– is currently a minor contributor to global radiative forcing relative to total HFCs; however, our calculated emissions highlight a significant difference from the available reported figures and projected estimates.
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    Long-Period Seismic Amplification in the Kanto Basin from the Ambient Seismic Field (2014)
    Wiley
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    Publication Date: 2014-03-22
    Description: Tokyo, like many seismically threatened cities, is situated atop a sedimentary basin that has the potential to trap and amplify seismic waves from earthquakes. We study amplification in the Kanto Basin by exploiting the information carried by the ambient seismic field. We use 375 seismic stations from the High Sensitivity Seismograph network across central Honshu as virtual sources and 296 seismic stations of the Metropolitan Seismic Observation network shallow-borehole seismometers within the basin as receivers to map the basin response. We find a linear relationship between ground motion and basin depth at periods of 2 – 10 seconds that could be used to represent 3D basin effects in ground motion prediction equations. We also find that the strength of basin seismic amplification depends strongly on the direction of illumination by seismic waves.
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    Estimated likelihood of observing a large earthquake on a continental low-angle normal fault, and implications for low-angle normal fault activity (2014)
    Wiley
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    Publication Date: 2014-03-22
    Description: The lack of observed continental earthquakes that clearly occurred on low-angle normal faults (LANFs) may indicate that these structures are not seismically active, or that these earthquakes are simply rare events. To address this, we compile all potentially active continental LANFs (twenty-four in total) and calculate the likelihood of observing a significant earthquake on them over periods of 1-100 years. This probability depends on several factors including the frequency-magnitude distribution. For either a characteristic or Gutenberg-Richter distribution, we calculate a probability of about 0.5 that an earthquake greater than M 6.5 (large enough to avoid ambiguity in dip angle) will be observed on any LANF in a period of 35 years, which is the current length of the Global CMT catalog. We then use Bayes’ Theorem to illustrate how the absence of observed significant LANF seismicity over the catalog period moderately decreases the likelihood that the structures generate large earthquakes.
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    Large-scale impact of the island mass effect through nitrogen fixation in the western South Pacific Ocean (2014)
    Wiley
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    Publication Date: 2014-03-23
    Description: We describe a new mechanism for the island mass effect fueled by nitrogen fixation. The nitrogen fixation activities and δ 15 N of suspended particles in the surface water in the South Pacific were examined. Active nitrogen fixation and abundant Trichodesmium spp. were observed near islands in the western subtropical region, which was attributable to the material supplied by land runoff. High primary production was extensively centered around the islands and was characterized by low δ 15 N of suspended particles and a reduction in phosphate concentrations at the surface compared with the subtropical gyre and eastern equatorial upwelling. This suggested that Trichodesmium spp. were advected to areas remote from these islands, and consequently, the elevated primary production fueled by nitrogen fixation extended over a large area around them. Because the proposed island mass effect is triggered by a terrigenous nutrient supply, this ecosystem is potentially vulnerable to human activity on small islands.
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    Tracing the long-term microbial production of recalcitrant fluorescent dissolved organic matter in seawater (2014)
    Wiley
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    Publication Date: 2014-03-23
    Description: The majority of dissolved organic matter (DOM) in the ocean is resistant to microbial degradation (recalcitrant), yet its formation remains poorly understood. The fluorescent fraction of DOM can be used to trace the formation of recalcitrant DOM (RDOM). A long-term (〉 1 year) experiment revealed 27-52 % removal of dissolved organic carbon and a nonlinear increase in RDOM fluorescence associated with microbial turnover of semi-labile DOM. This fluorescence was also produced using glucose as the only initial carbon source, suggesting that degradation of prokaryote remnants contributes to RDOM. Our results indicate that the formation of a fluorescent RDOM component depends on the bioavailability of the substrate: the less labile, the larger the production of fluorescent RDOM relative to organic carbon remineralized. The anticipated increase in microbial carbon demand due to ocean warming can potentially force microbes to degrade less labile substrates, thereby increasing RDOM production and stimulating ocean carbon storage.
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    Rapid Changes in Biomass Burning Aerosols by Atmospheric Oxidation (2014)
    Wiley
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    Publication Date: 2014-03-23
    Description: Primary and secondary aerosol particles originating from biomass burning contribute significantly to the atmospheric aerosol budget and thereby to both direct and indirect radiative forcing. Based on detailed measurements of a large number of biomass burning plumes of variable age in southern Africa, we show that the size distribution, chemical composition, single scattering albedo and hygroscopicity of biomass burning particles change considerably during the first 2–4 hours of their atmospheric transport. These changes, driven by atmospheric oxidation and subsequent secondary aerosol formation, may reach a factor of 6 for the aerosol scattering coefficient and a factor 〉10 for the cloud condensation nuclei concentration. Since the observed changes take place over the spatial and temporal scales that are neither covered by emission inventories nor captured by large-scale model simulations the findings reported here point out a significant gap in our understanding on the climatic effects of biomass burning aerosols.
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    Distinguishing the impacts of ozone depleting substances and well-mixed greenhouse gases on Arctic stratospheric ozone and temperature trends (2014)
    Wiley
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    Publication Date: 2014-03-23
    Description: Whether stratospheric cooling due to increases in well-mixed greenhouse gases (WMGHG) could increase the depletion of Arctic stratospheric ozone has been the subject of scientific and public attention for decades. Here we provide evidence that changes in the concentrations of ozone depleting substances (ODS), not WMGHG, have been the primary driver of observed Arctic lower stratospheric trends in both ozone and temperature. We do so by analyzing polar cap ozone and temperature trends in reanalysis data: these clearly suggest that both trends are mainly driven by ODS in the lower stratosphere. This observation-based finding is supported by results from a stratosphere-resolving chemistry-climate model driven with time-varying ODS and WMGHG, specified in isolation and in combination. Taken together, these results provide strong evidence that ODS are the main driver of changes in the Arctic lower stratospheric temperatures and ozone, whereas WMGHG are the primary driver of changes in the upper stratosphere.
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    The five stable noble gases are sensitive unambiguous tracers of glacial meltwater. (2014)
    Wiley
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    Publication Date: 2014-03-23
    Description: The five inert noble gases – He, Ne, Ar, Kr and Xe – exhibit a unique dissolved gas saturation pattern resulting from the formation and addition of glacial meltwater to seawater. He and Ne become oversaturated, and Ar, Kr and Xe become undersaturated to varying percentages. For example, addition of 10‰ glacial meltwater to seawater results in a saturation anomaly of ΔHe = 12.8%, ΔNe = 8.9%, ΔAr = -0.5%, ΔKr = -2.2% and ΔXe = -3.3%. This pattern in noble gas saturation reflects a unique meltwater signature that is distinct from the other major physical processes that modify the gas concentration and saturation – namely seasonal changes in temperature at the ocean surface and bubble mediated gas exchange. We use OMP analysis to illustrate how all five noble gases can help distinguish glacial meltwater from wind-driven bubble injection, making them a potentially valuable suite of tracers for glacial melt and its concentration in the deep waters of the world ocean.
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    Historical and future learning about climate Sensitivity (2014)
    Wiley
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    Publication Date: 2014-03-23
    Description: Equilibrium climate sensitivity measures the long-term response of surface temperature to changes in atmospheric CO 2 . The range of climate sensitivities in the IPCC AR5 Report is unchanged from that published almost 30 years earlier in the Charney Report. We conduct perfect-model experiments using an energy balance model to study the rate at which uncertainties might be reduced by observation of global temperature and ocean heat uptake. We find that a climate sensitivity of 1.5 °C can be statistically distinguished from 3 °C by 2030; 3 °C from 4.5 °C by 2040; and 4.5 °C from 6 °C by 2065. Learning rates are slowest in the scenarios of greatest concern (high sensitivities), due to a longer ocean response time, which may have bearing on a wait-and-see vs. precautionary mitigation policies. Learning rates are optimistic in presuming the availability of whole-ocean heat data, but pessimistic by using simple aggregated metrics and model physics.
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    Dynamic Variability of Biogeochemical Ratios in the Southern California Current System (2014)
    Wiley
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    Publication Date: 2014-03-23
    Description: We use autonomous nitrate (NO 3 - ), oxygen (O 2 ), and dissolved inorganic carbon (DIC) observations to examine the relationship between ratios of C:N:O at an upwelling site in the southern California Current System. Mean ratios and 95% CIs observed by sensors over eight months were NO 3 - :O 2  = -0.11 ± 0.002, NO 3 - :DIC = 0.14 ± 0.001, DIC:O 2  = -0.83 ± 0.01, in good agreement with Redfield ratios. Variability in the ratios on the weekly timescale is attributable to shifts in biological demand and nutrient availability and shown to exhibit a spectrum of values ranging from near 100% New Production to 100% Regenerated Production.
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    Recent Antarctic sea ice trends in the context of Southern Ocean surface climate variations since 1950 (2014)
    Wiley
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    Publication Date: 2014-03-23
    Description: This study compares the distribution of surface climate trends over the Southern Ocean in austral summer between 1979-2011 and 1950-1978, using a wide variety of data sets including un-interpolated gridded marine archives, land station data, reanalysis and satellite products. Apart from the Antarctic Peninsula and adjacent regions, sea surface temperatures (SSTs) and surface air temperatures decreased during 1979-2011, consistent with the expansion of Antarctic sea ice. In contrast, the Southern Ocean and coastal Antarctica warmed during 1950-1978. Sea level pressure (SLP) and zonal wind trends provide additional evidence for a sign reversal between the two periods, with cooling (warming) accompanied by stronger (weaker) westerlies and lower (higher) SLP at polar latitudes in the early (late) period. Such physically consistent trends across a range of independently measured parameters provides robust evidence for multi-decadal climate variability over the Southern Ocean and places the recent Antarctic sea ice trends into a broader context.
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    Strong responses of Southern Ocean phytoplankton communities to volcanic ash (2014)
    Wiley
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    Publication Date: 2014-03-23
    Description: Volcanic eruptions have been hypothesized as an iron supply mechanism for phytoplankton blooms, however little direct evidence of stimulatory responses has been obtained in the field. Here we present the results of 21 1-2 day bottle enrichment experiments from cruises in the South Atlantic and Southern Ocean which conclusively demonstrated a photophysiological and biomass stimulation of phytoplankton communities following supply of basaltic or rhyolitic volcanic ash. Furthermore, experiments in the Southern Ocean demonstrated significant phytoplankton community responses to volcanic ash supply in the absence of responses to addition of dissolved iron alone. At these sites, dissolved manganese concentrations were amongst the lowest ever measured in seawater and we therefore suggest that the enhanced response to ash may have been a result of the relief of manganese (co-)limitation. Our results imply that volcanic ash deposition events could trigger extensive phytoplankton blooms, potentially capable of significant impacts on regional carbon cycling.
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    The Thermal Conductivity of Earth's Lower Mantle (2014)
    Wiley
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    Publication Date: 2014-03-26
    Description: We assess the thermal conductivity of the Earth's lower mantle anchored on our first-principles calculations of lattice thermal conductivity of MgSiO 3 perovskite. Our calculations agree with measurements of iron-free perovskite at ambient conditions, and show a lower pressure dependence compared with other recent calculations. In addition, we show the effect of iron on the lattice thermal conductivity of silicate perovskite is likely to be small at high temperatures. To provide an assessment of thermal conductivity throughout the lower mantle, we reevaluate existing high-pressure optical absorption data to constrain an upper-bound radiative contribution to diffusive heat transfer and examine a composite model for combining thermal conductivity of oxide and perovskite phases in the lower mantle. We find that the overall thermal conductivity of the lower mantle is approximately constant between 2.5 and 3.5 W/m/K. These values imply that the mantle has a blanketing effect on heat flow across the core-mantle boundary.
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    Influence of solar variability on the infrared radiative cooling of the thermosphere from 2002 to 2014 (2014)
    Wiley
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    Publication Date: 2014-03-26
    Description: Infrared radiative cooling of the thermosphere by carbon dioxide (CO 2 , 15 µm) and by nitric oxide (NO, 5.3 µm) has been observed for 12 years by the SABER instrument on the TIMED satellite. For the first time we present a record of the two most important thermospheric infrared cooling agents over a complete solar cycle. SABER has documented dramatic variability in the radiative cooling on timescales ranging from days to the 11-year solar cycle. Deep minima in global mean vertical profiles of radiative cooling are observed in 2008–2009. Current solar maximum conditions, evidenced in the rates of radiative cooling, are substantially weaker than prior maximum conditions in 2002–2003. The observed changes in thermospheric cooling correlate well with changes in solar ultraviolet irradiance and geomagnetic activity during the prior maximum conditions. NO and CO 2 combine to emit 7 x 10 18 more Joules annually at solar maximum than at solar minimum.
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    Statistical cyclicity of the supercontinent cycle (2014)
    Wiley
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    Publication Date: 2014-03-26
    Description: Supercontinents like Pangea impose a first-order control on Earth's evolution as they modulate global heat loss, sea level, climate and biodiversity. In a traditional view, supercontinents form and break-up in a regular, perhaps periodic, manner in a cycle lasting several 100 Myr as reflected in the assembly times of Earth's major continental aggregations: Columbia, Rodinia and Pangea. However, modern views of the supercontinent cycle propose a more irregular evolution on thebasis of an improved understanding of the Precambrian geologic record. Here, we use fully dynamic spherical mantle convection models featuring plate-like behavior and continental drift to investigate supercontinent formation and break-up. We further dismiss the concept of regularity, but suggest a statistical cyclicity in which the supercontinent cycle may have a characteristic period imposed by mantle and lithosphere properties, but this is hidden in immense fluctuations between different cycles that arise from the chaotic nature of mantle flow.
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    On vertical electric fields at lunar magnetic anomalies (2014)
    Wiley
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    Publication Date: 2014-03-26
    Description: We study the interaction between a magnetic dipole mimicking the Gerasimovich magnetic anomaly on the lunar surface and the solar wind in a self-consistent 3-D quasi-neutral hybrid simulation where ions are modelled as particles and electrons as a charge-neutralizing fluid. Especially, we consider the origin of the recently observed electric potentials at lunar magnetic anomalies. An anti-moonward Hall electric field forms in our simulation resulting in a potential difference of 〈300 V on the lunar surface, which value is similar to observations. Since the hybrid model assumes charge neutrality, our results suggest that the electric potentials at lunar magnetic anomalies can be formed by decoupling of ion and electron motion even without charge separation.
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    Consideration of tidal variations in the geocenter on satellite altimeter observations of ocean tides (2014)
    Wiley
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    Publication Date: 2014-03-26
    Description: Tidal geocenter motion has been previously ignored when developing ocean tide models from satellite altimetry. Accounting for tidal geocenter motion is necessary because the best orbit determinations for altimetric satellites position sea-surface heights relative to the center of mass of the total Earth system, including the ocean tides. But the ocean tides are presumed relative to the Earth's crust, and thus are effectively relative to the center of figure. By accounting for this effect, we find improved agreement between an altimeter-based ocean tide model and bottom pressure recorder observations. The variance of differences between these two observations is reduced by 31% and 43% for the two tidal constituents with the largest contributions to geocenter variations, O 1 and K 1 respectively. With this accommodation the predicted contribution from altimeter-based ocean tide models to geocenter variations is amplified by 15–22%, providing improved agreement with observations, especially for the K 1 component.
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    How deep is deep enough? Ocean iron fertilization and carbon sequestration in the Southern Ocean (2014)
    Wiley
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    Publication Date: 2014-03-26
    Description: Artificial ocean iron fertilization (OIF) enhances phytoplankton productivity and is being explored as a means of sequestering anthropogenic carbon within the deep ocean. To be considered successful, carbon should be exported from the surface ocean and isolated from the atmosphere for an extended period (e.g. the IPCCÕs standard 100-year time-horizon). This study assesses the impact of deep circulation on carbon sequestered by OIF in the Southern Ocean, a high-nutrient-low-chlorophyll region known to be iron-stressed. A Lagrangian particle-tracking approach is employed to analyze water mass trajectories over a 100-year simulation. By the end of the experiment, for a sequestration depth of 1000 m, 66% of the carbon had been re-exposed to the atmosphere, taking an average of 37.8 years. Upwelling occurs predominately within the Antarctic Circumpolar Current due to Ekman suction and topography. These results emphasize that successful OIF requires consideration of the role of circulation as well asbiogeochemistry.
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    The influence of different El Niño types on global average temperature (2014)
    Wiley
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    Publication Date: 2014-03-27
    Description: [1]  The El Niño/ Southern Oscillation (ENSO) is known to influence surface temperatures worldwide. El Niño conditions are thought to lead to anomalously warm global average surface temperature, absent other forcings. Recent research has identified distinct possible types of El Niño events based on the location of peak sea surface temperature anomalies. Here we analyze the relationship between the type of El Niño event and the global average temperature anomaly, using three historical temperature datasets. Separating El Niño events into types reveals that the global average surface temperatures are anomalously warm during and after traditional Eastern Pacific El Niño events, but not Central Pacific or Mixed events. Historical analysis indicated that slowdowns in the rate of global surface warming since the late-1800 s may be related to decadal variability in the frequency of different types of El Niño events.
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    Numerical Tsunami Simulation Including Elastic Loading And Sea Water Density Stratification (2014)
    Wiley
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    Publication Date: 2014-03-28
    Description: Systemic discrepancies between observed and modelled tsunami wave speeds where previously identified for two recent major tsunamis, the 2010 Maule and 2011 Tohoku events. To account for these discrepancies, we developed a numerical tsunami propagation code solving the shallow water equation and including the effects of elastic loading of the sea-floor by the tsunami as well as a linear density profile in the sea water column. We show here that both effects are important to explain the commonly observed difference between observations and simulations. We conclude that the density variation in the sea water column affects the wave speed without changing the waveform, whereas the loading effect has an effect on the wave speed and the waveform showing a negative phase before the main arrival due to the depression of the sea floor surrounding the tsunami wave. The combination of both effects is needed to achieve a better match between observations and simulations.
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    Systematic model forecast error in Rossby wave Structure (2014)
    Wiley
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    Publication Date: 2014-03-28
    Description: Diabatic processes can alter Rossby wave structure; consequently errors arising from model processes propagate downstream. However, the chaotic spread of forecasts from initial condition uncertainty renders it difficult to trace back from root mean square forecast errors to model errors. Here diagnostics unaffected by phase errors are used, enabling investigation of systematic errors in Rossby waves in winter-season forecasts from three operational centers. Tropopause sharpness adjacent to ridges decreases with forecast lead time. It depends strongly on model resolution, even though models are examined on a common grid. Rossby wave amplitude reduces with lead time up to about five days, consistent with under-representation of diabatic modification and transport of air from the lower troposphere into upper-tropospheric ridges, and with too weak humidity gradients across the tropopause. However, amplitude also decreases when resolution is decreased. Further work is necessary to isolate the contribution from errors in the representation of diabatic processes.
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    Little late Holocene strain accumulation and release on the Aleutian megathrust below the Shumagin Islands, Alaska (2014)
    Wiley
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    Publication Date: 2014-03-28
    Description: Can a predominantly creeping segment of a subduction zone generate a great (M 〉 8) earthquake? Despite Russian accounts of strong shaking and high tsunamis in 1788, geodetic observations above the Aleutian megathrust indicate creeping subduction across the Shumagin Islands segment, a well-known seismic gap. Seeking evidence for prehistoric great earthquakes, we investigated Simeonof Island, the archipelago's easternmost island, and found no evidence for uplifted marine terraces or subsided shorelines. Instead, we found freshwater peat blanketing lowlands, and organic-rich silt and tephra draping higher glacially-smoothed bedrock. Basal peat ages place glacier retreat prior to 10.4 ka and imply slowly rising (〈0.2 m/ka) relative sea level since ~3.4 ka. Storms rather than tsunamis probably deposited thin, discontinuous deposits in coastal sites. If rupture of the megathrust beneath Simeonof Island produced great earthquakes in the late Holocene, then coseismic uplift or subsidence was too small (≤0.3 m) to perturb the onshore geologic record.
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    Mineral assemblage anomalies in the slip zone of the 1999 Taiwan Chi-Chi earthquake: Ultrafine particles preserved only in the latest slip zone (2014)
    Wiley
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    Publication Date: 2014-03-28
    Description: We determined mineral assemblages of samples from the Taiwan Chelungpu fault and from milling and heating experiments by using X-ray diffraction and scanning and transmission electron microscopy. The fault system contains three dominant fault zones, the shallowest of which slipped during the 1999 Chi-Chi earthquake. The quartz and clay mineral contents of the primary slip zone were low, and it contained partly amorphous ultrafine particles (several tens of nanometers). Up to 30 weight percent of materials in that zone could not be fit to standard diffraction patterns, whereas nearly 100 weight percent of those in surrounding samples could be. The unfitted component could be attributed to the observed ultrafine particles produced by comminution during the earthquake, because weak diffraction intensities are caused from mineral lattice distortion, granulation, and amorphous coatings. Such particles are a potential proxy for identifying the slip zone of the most recent earthquake along a fault.
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    Light-Enhanced Primary Marine Aerosol Production from Biologically Productive Seawater (2014)
    Wiley
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    Publication Date: 2014-03-28
    Description: Physical and biogeochemical processes in seawater controlling primary marine aerosol (PMA) production and composition are poorly understood and associated with large uncertainties in estimated fluxes into the atmosphere. PMA production was investigated in the biologically-productive NE Pacific Ocean and in biologically-productive and oligotrophic regions of the NW Atlantic Ocean. Physicochemical properties of model PMA, produced by aeration of fresh seawater under controlled conditions, were quantified. Diel variability in model PMA mass and number fluxes was observed in biologically productive waters, increasing following sunrise and decreasing to pre-dawn levels overnight. Such variability was not seen in oligotrophic waters. Surfactant scavenging by aeration in the aerosol generator without replenishing the seawater in the reservoir during daytime reduced model PMA production in productive waters to nighttime levels but had no influence on production from oligotrophic waters. Results suggest bubble-plume interactions with sunlight-mediated biogenic surfactants in productive seawater significantly enhanced model PMA production.
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    On Forced Temperature Changes, Internal Variability and the AMO (2014)
    Wiley
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    Publication Date: 2014-03-28
    Description: We estimate the low-frequency internal variability of Northern Hemisphere (NH) mean temperature using observed temperature variations, which include both forced and internal variability components, and several alternative model simulations of the (natural + anthropogenic) forced component alone. We then generate an ensemble of alternative historical temperature histories based on the statistics of the estimated internal variability. Using this ensemble, we show, firstly, that recent NH mean temperatures fall within the range of expected multidecadal variability. Using the synthetic temperature histories, we also show that certain procedures used in past studies to estimate internal variability, and in particular, an internal multidecadal oscillation termed the “Atlantic Multidecadal Oscillation” or “AMO”, fail to isolate the true internal variability when it is a priori known. Such procedures yield an AMO signal with an inflated amplitude and biased phase, attributing some of the recent NH mean temperature rise to the AMO. The true AMO signal, instead, appears likely to have been in a cooling phase in recent decades, offsetting some of the anthropogenic warming. Claims of multidecadal “stadium wave” patterns of variation across multiple climate indices are also shown to likely be an artifact of this flawed procedure for isolating putative climate oscillations.
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    Peculiar pitch angle distribution of relativistic electrons in the inner radiation belt and slot region (2014)
    Wiley
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    Publication Date: 2014-03-28
    Description: The relativistic electrons in the inner radiation belt have received little attention in the past due to sparse measurements and unforgiving contamination from the inner belt protons. The high quality measurements of the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard Van Allen Probes provides a great opportunity to investigate the dynamics of relativistic electrons in the low L region. In this letter, we report the newly unveiled pitch angle distribution (PAD) of the energetic electrons with minima at 90° near the magnetic equator in the inner belt and slot region. Such a PAD is persistently present throughout the inner belt and appears in the slot region during storms. One hypothesis for 90°-minimum PADs is that off-90° electrons are preferentially heated by chorus waves just outside the plasmapause (which can be at very low L during storms) and/or fast magnetosonic waves which exist both inside and outside the plasmasphere.
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  • 175
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    Uplift and subsidence reveal a non-persistent megathrust rupture boundary (Sitkinak Island, Alaska) (2014)
    Wiley
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    Publication Date: 2014-03-28
    Description: We report stratigraphic evidence of land-level change and tsunami inundation along the Alaska-Aleutian megathrust during prehistoric and historical earthquakes west of Kodiak Island. On Sitkinak Island, cores and tidal outcrops fringing a lagoon reveal five sharp lithologic contacts that record coseismic land-level change. Radiocarbon dates, 137 Cs profiles, computerized tomography scans, and microfossil assemblages are consistent with rapid uplift ca. 290-0, 520-300, and 1050-790 cal yr BP, and subsidence in AD 1964 and ca. 640-510 cal yr BP. Radiocarbon, 137 Cs, and 210 Pb ages bracketing a sand bed traced 1.5 km inland and evidence for sudden uplift are consistent with Russian accounts of an earthquake and tsunami in AD 1788. The mixed uplift and subsidence record suggests that Sitkinak Island sits above a non-persistent boundary near the southwestern limit of the AD 1964 Mw 9.2 megathrust rupture.
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    Skilful Long Range Prediction of European and North American Winters (2014)
    Wiley
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    Publication Date: 2014-03-29
    Description: Until recently, long range forecast systems showed only modest levels of skill in predicting surface winter climate around the Atlantic basin and associated fluctuations in the North Atlantic Oscillation at seasonal lead times. Here we use a new forecast system to assess seasonal predictability of winter north Atlantic climate. We demonstrate that key aspects of European and North American winter climate and the surface North Atlantic Oscillation are highly predictable months ahead. We demonstrate high levels of prediction skill in retrospective forecasts of the surface North Atlantic Oscillation, winter storminess, near surface temperature and wind speed; all of which have high value for planning and adaptation to extreme winter conditions. Analysis of forecast ensembles suggests that while useful levels of seasonal forecast skill have now been achieved, key sources of predictability are still only partially represented and there is further untapped predictability.
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    Quantifying anthropogenic and natural contributions to thermosteric sea level rise (2014)
    Wiley
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    Publication Date: 2014-03-30
    Description: Changes in thermosteric sea level at decadal and longer time scales respond to anthropogenic forcing and natural variability of the climate system. Disentangling these contributions is essential to quantify the impact of human activity in the past and to anticipate thermosteric sea level rise under global warming. Climate models, fed with radiative forcing, display a large spread of outputs with limited correspondence with the observationally based estimates of thermosteric sea level during the last decades of the 20 th century. Here we extract the common signal of climate models from CMIP5 using a signal-to-noise maximizing EOF technique for the period 1950-2005. Our results match the observed trends, improving the widely used approach of multi-model ensemble averaging. We then compute the fraction of the observed thermosteric sea level rise of anthropogenic origin and conclude that 87% of the observed trend in the upper 700m since 1970 is induced by human activity.
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    RECONSTRUCTION OF PACIFIC OCEAN BOTTOM WATER SALINITY DURING THE LAST GLACIAL MAXIMUM (2014)
    Wiley
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    Publication Date: 2014-03-30
    Description: ABSTRACT Knowledge of salinity in the deep ocean is important for understanding past ocean circulation and climate. Based on sedimentary pore fluid chloride measurements of a single Pacific site, Adkins et al. (2002) suggested that, during the last glacial maximum (LGM), the Pacific deep bottom water was saltier than expected based on lower sea level alone. Here we present high-resolution salinity profiles from five sites in the South, Equatorial and North Pacific Ocean. Our study greatly constrains understanding of LGM salinity in the Pacific Ocean. Our results show that LGM chloride concentrations of deep Pacific bottom water were 4.09 ± 0.4% greater than today's values. Pacific Ocean bottom water salinity was also indistinguishable from being homogeneous across the wide range of latitudes studied here. These LGM salinity reconstructions are on average slightly higher (~1.4 to 1% higher) than expected from sea level of the time, which is generally inferred to have been ~120 to ~135 m lower than today.
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    Is the 2004–2012 reduction of the Atlantic meridional overturning circulation significant? (2014)
    Wiley
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    Publication Date: 2014-04-01
    Description: The Atlantic meridional overturning circulation (AMOC) at 26.5°N weakened by −0.53 Sv/yr between April 2004 and October 2012. To assess whether this trend is consistent with the expected ‘noise’ in the climate system, we compare the observed trend with estimates of internal variability derived from 14 control simulations from the Climate Model Intercomparison Project 5 (CMIP5). Eight-year trends of −0.53 Sv/yr are relatively common in two models but are extremely unusual (or out of range) in the other twelve. However, all 14 models underestimate AMOC variability on interannual time scales. To account for this bias, we estimate plausible upper-limits of internal AMOC variability by combining the temporal correlation characteristics of the AMOC from CMIP5 models with an observational estimate of interannual variability. We conclude that the observed AMOC trend is not significantly different ( p  〉 0.01) from plausible estimates of internal variability. Detecting the influence of external climate forcings on the AMOC will require more than one decade of continuous observations.
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    Real-time measurements of the concentration and isotope composition of atmospheric and volcanic CO2 at Mt. Etna (Italy) (2014)
    Wiley
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    Publication Date: 2014-04-01
    Description: We present unprecedented data of real-time measurements of the concentration and isotope composition of CO 2 in air and in fumarole-plume gases collected in 2013 during two campaigns at Mt. Etna volcano, which were made using a laser-based isotope-ratio infrared-spectrometer. We performed approximately 360 measurements/hour, which allowed calculation of the δ 13 C-values of volcanic CO 2 . The fumarole gases of Torre del Filosofo (2,900 m a.s.l.) range from –3.24 ± 0.06‰ to –3.71 ± 0.09‰, comparable to isotope-ratio mass-spectrometry (IRMS) measurements of discrete samples collected on the same dates. Plume gases sampled more than 1 km from the craters show a δ 13 C = –2.2 ± 0.4‰, in agreement with the crater fumarole gases analyzed by IRMS. Measurements performed along ~17 km driving-track from Catania to Mt. Etna show negative δ 13 C-values when passing through populated centers due to anthropogenic-derived CO 2 inputs (e.g., car exhaust). The reported results demonstrate that this technique may represent an important advancement for volcanic and environmental monitoring.
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    Field identification of groundwater flow systems and hydraulic traps in drainage basins using a geophysical method (2014)
    Wiley
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    Publication Date: 2014-04-01
    Description: Groundwater flow systems and stagnant zones in drainage basins are critical to a series of geologic processes. Unfortunately, the difficulty of mapping flow system boundaries and no field example of detected stagnant zones restrict the application of the concept of nested flow systems. By assuming the variation in bulk resistivity of an aquifer with uniform porosity is mainly caused by groundwater salinity, the magnetotelluric technique is used to obtain the apparent resistivity of a profile across a groundwater-fed river in the Ordos Plateau, China. Based on the variations in apparent resistivity of the Cretaceous sandstone aquifer, the basin-bottom hydraulic trap below the river has been detected for the first time, and its size is found to be large enough for possible deposition of large ore bodies. The boundary between local and regional flows has also been identified, which would be useful for groundwater exploration and calibration of large-scale groundwater models.
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    Reconstruction of gaps in the past history of solar wind parameters (2014)
    Wiley
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    Publication Date: 2014-04-01
    Description: The historical (1972–2013) gaps of the solar wind parameters are filled-in by smooth modes of co-variability using the continuous geomagnetic indices with Singular Spectrum Analysis (SSA). A systematic study using experiments with synthetic gaps has been performed to determine optimal SSA parameters for reconstruction of gaps over full solar cycle, and to assess the reconstruction skill. We assessed the accuracy of the SSA gap-filling for solar wind reconstruction in T96 and TS05 empirical magnetic field models by using GOES measurements at geostationary orbit, and compared it to results based on parameters from existing Qin and Denton interpolation. The SSA gap-filling method does improve accuracy of empirical magnetic field models, especially when gaps are large and in particular for the TS05 magnetic field model.
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    Seismic anisotropy and subduction-induced mantle fabrics beneath the Arabian and Nubian plates adjacent to the Red Sea (2014)
    Wiley
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    Publication Date: 2014-04-01
    Description: For most continental areas, the mechanisms leading to mantle fabrics responsible for the observed anisotropy remain ambiguous, partially due to the lack of sufficient spatial coverage of reliable seismological observations. Here we report the first joint analysis of shear-wave splitting measurements obtained at stations on the Arabian and Nubian plates adjacent to the Red Sea. More than 1100 pairs of high-quality splitting parameters show dominantly N-S fast orientations at all 47 stations and larger-than-normal splitting times beneath the Afro-Arabian Dome (AAD). The uniformly N-S fast orientations and large splitting times up to 1.5 s are inconsistent with significant contributions from the lithosphere, which is about 50-80 km thick beneath the AAD and even thinner beneath the Red Sea. The results can best be explained by simple shear between the lithosphere and the asthenosphere associated with northward subduction of the African/Arabian plates over the past 150 Ma.
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    Decrypting geophysical signals at Stromboli Volcano (Italy): integration of seismic and Ground-Based InSAR displacement data (2014)
    Wiley
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    Publication Date: 2014-04-01
    Description: We present the integration of seismic and GBInSAR displacement data at Stromboli volcano. Ground deformation in the area of summit vents is positively correlated with both seismic tremor amplitude and cumulative amplitudes of very-long-period (VLP) signals associated with Strombolian explosions. Changes in VLP amplitudes precede by a few days the variations in ground deformation and seismic tremor. We propose a model where the arrival of fresh, gas-rich magma from depth enhances gas slug formation, promoting convection and gas transfer throughout the conduit system. At the shallowest portion of the conduit, an increase in volatile content causes a density decrease, expansion of the magmatic column and augmented degassing activity, which respectively induce inflation of the conduit, and increased tremor amplitudes. The temporal delay between increase of VLP and tremor amplitudes / conduit inflation can be interpreted in terms of the different time scales characterizing bulk gas transfer versus slug formation and ascent.
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    Predictions of electron temperatures in the Mars ionosphere and their effects on electron densities (2014)
    Wiley
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    Publication Date: 2014-04-01
    Description: Observations of peak electron densities in the Mars ionosphere are well fit by a simplistic theory that assumes the electron temperature, T e , at the peak remains constant as solar zenith angle, χ , changes. However, T e ought to vary withboth altitude and χ . Here we use an existing numerical model of ionospheric energetics, which includes both vertical and diurnal variations in temperatures, to predict that T e at the ionospheric peak is relatively independent of χ . This model accurately predicts the observed dependence of peak electron density on χ , whereas predictions using Viking-based electron temperatures that are held constant with timedo not. A simplified analytic model is developed to interpret these results further. It predicts that the difference between electron and neutral temperatures is proportional to the ratio of electron heating rate to electron production rate and proportional to the square root of solar irradiance.
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  • 186
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    Impact of acetone (photo)oxidation on HOx production in the UT/LMS based on CARIBIC passenger aircraft observations and EMAC simulations (2014)
    Wiley
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    Publication Date: 2014-04-02
    Description: Until a decade ago acetone was assumed to be a dominant HO x source in the dry extra-tropical upper troposphere (ex-UT). New photodissociation quantum yields of acetone and the lack of representative data from the ex-UT challenged that assumption. Regular mass spectrometric observations onboard the CARIBIC passenger aircraft deliver the first representative distribution of acetone in the UT/LMS (UT / lowermost stratosphere). Based on diverse CARIBIC trace gas data and non-observed parameters taken from the model EMAC, we quantify the HO x source in the UT/LMS from (photo)oxidation of acetone. The findings are contrasted to HO x production from ozone photolysis, overall the dominant tropospheric HO x source. It is shown that HO x production from acetone (photo)oxidation reaches up to 95% of the HO x source from ozone photolysis in autumn in the UT and on average ~61% in summer. That is, acetone is a significant source of HO x in the UT/LMS.
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  • 187
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    Aquarius Surface Salinity and the Madden-Julian Oscillation: the Role of Salinity in Surface Layer Density and Potential Energy (2014)
    Wiley
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    Publication Date: 2014-04-02
    Description: Sea surface salinity (SSS) data from the Aquarius satellite are analyzed along with auxiliary data to investigate the SSS signature of the Madden-Julian Oscillation (MJO) in the equatorial Indian and Pacific Oceans, the effect of evaporation-minus-precipitation (E-P), the implication for the role of ocean dynamics, and the SSS influence on surface density and potential energy. MJO-related SSS changes are consistent with E-P forcing in the western Indian Ocean throughout the MJO cycle and in the central Indian Ocean during the wet phase of the MJO cycle. However, SSS changes cannot be explained by E-P in the central Indian Ocean during the dry phase and in the eastern Indian and western Pacific Oceans throughout the MJO cycle, implying the importance of ocean dynamics. SSS has an overall larger contribution to MJO-related surface density and potential energy anomalies than SST. It partially offsets the SST effect in the western-to-central Indian Ocean and reinforces the SST effect in the eastern Indian and western Pacific Oceans. Ocean modeling and assimilation need to properly account for salinity effects in order to correctly represent mixed layer variability associated with the MJO. Our results also clarify some discrepancy in previous studies about the E-P effect on MJO-related SSS variations.
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    Hydrodynamic provinces and oceanic connectivity from a transport network help designing marine reserves (2014)
    Wiley
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    Publication Date: 2014-04-02
    Description: Oceanic dispersal and connectivity have been identified as crucial factors for structuring marine populations and designing Marine Protected Areas (MPAs). Focusing on larval dispersal by ocean currents, we propose an approach coupling Lagrangian transport and new tools from Network Theory to characterize marine connectivity in the Mediterranean basin. Larvae of different pelagic durations and seasons are modeled as passive tracers advected in a simulated oceanic surface flow from which a network of connected areas is constructed. Hydrodynamical provinces extracted from this network are delimited by frontiers which match multi-scale oceanographic features. By examining the repeated occurrence of such boundaries, we identify the spatial scales and geographic structures that would control larval dispersal across the entire seascape. Based on these hydrodynamical units, we study novel connectivity metrics for existing reserves. Our results are discussed in the context of ocean biogeography and MPAs design, having ecological and managerial implications.
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  • 189
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    Impact of increased water vapor on precipitation efficiency over northern Eurasia (2014)
    Wiley
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    Publication Date: 2014-04-03
    Description: This study investigates the relationships among water vapor, precipitation efficiency, precipitation amount, and air temperature anomalies on monthly time scales over northern Eurasia for winter and summer 2003-2010. Daily precipitation and temperature records at 505 historical stations, and atmospheric total precipitable water vapor and relative humidity data from Atmospheric Infrared Sounders (AIRS), are used for analysis. Results show that higher atmospheric precipitable water associated with warmer temperature directly contributes to winter precipitation amount, but has little impact on winter precipitation efficiency. However, accelerated decreasing relative humidity associated with higher temperature is the primary factor in the reduction of precipitation efficiency and precipitation amount regardless of higher precipitable water in summer. This study suggests that there are evident seasonal differences in precipitation trend associated with air temperature changes over the study region. Air temperature modifies a key atmospheric water variable that directly controls precipitation for that particular season.
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  • 190
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    Combined satellite and radar retrievals of drop concentration and CCN at convective cloud base (2014)
    Wiley
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    Publication Date: 2014-04-03
    Description: ABSTRACT The number of activated cloud condensation nuclei (CCN) into cloud drops at the base of convective clouds (N a ) is retrieved based on high resolution (375 m) satellite retrievals of vertical profiles of convective cloud drop effective radius (r e ). The maximum cloud base supersaturation (S) is calculated when N a is combined with radar measured updraft, and yields CCN(S), which was validated well against ground base CCN measurements during conditions of well mixed boundary layer over the DOE/ASR SGP site. Satellite retrieving N a is a new capability, which is one essential component of simultaneous measurements of cloud microstructure and CCN from space by using clouds as natural CCN chambers. This has to be complemented by a methodology for satellite estimates of cloud base updraft, which is yet to be developed and demonstrated. In the mean time, the retrieved N a can be used for assimilation of the combined CCN and updraft effects on clouds in models.
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  • 191
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    Anomalous Expansion of Coronal Mass Ejections during Solar Cycle 24 and its Space Weather Implications (2014)
    Wiley
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    Publication Date: 2014-04-03
    Description: The familiar correlation between the speed and angular width of coronal mass ejections (CMEs) is also found in solar cycle 24, but the regression line has a larger slope: for a given CME speed, cycle 24 CMEs are significantly wider than those in cycle 23. The slope change indicates a significant change in the physical state of the heliosphere, due to the weak solar activity. The total pressure in the heliosphere (magnetic + plasma) is reduced by ~40%, which leads to the anomalous expansion of CMEs explaining the increased slope. The excess CME expansion contributes to the diminished effectiveness of CMEs in producing magnetic storms during cycle 24, both because the magnetic content of the CMEs is diluted and also because of the weaker ambient fields. The reduced magnetic field in the heliosphere may contribute to the lack of solar energetic particles accelerated to very high energies during this cycle.
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    Issue Information (2014)
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    Publication Date: 2014-04-03
    Description: No abstract is available for this article.
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  • 193
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    Coupling dry deposition to vegetation phenology in the Community Earth System Model: Implications 3 for the simulation of surface O3 (2014)
    Wiley
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    Publication Date: 2014-04-03
    Description: Dry deposition is an important removal process controlling surface ozone. We examine the representation of this ozone loss mechanism in the Community Earth System Model (CESM). We first correct the dry deposition parameterization by coupling the leaf and stomatal vegetation resistances to the leaf area index, an omission which has adversely impacted over a decade of ozone simulations using both the MOZART and CAM-Chem global models. We show that this correction increases O 3 dry deposition velocities over vegetated regions and improves the simulated seasonality in this loss process. This enhanced removal reduces the previously reported bias in summertime surface O 3 simulated over eastern U.S. and Europe. We further optimize the parameterization by scaling down the stomatal resistance used in the Community Land Model (CLM) to observed values. This in turn further improves the simulation of dry deposition velocity of O 3 , particularly over broadleaf forested regions. The summertime surface O 3 bias is reduced from 30 ppb to 14 ppb over eastern U.S. and 13 ppb to 5 ppb over Europe from the standard to the optimized scheme, respectively. O 3 deposition processes must therefore be accurately coupled to vegetation phenology within 3D atmospheric models, as a first step towards improving surface O 3 and simulating O xh responses to future and past vegetation changes.
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    Response of the Bering Sea to 11-year solar irradiance cycles during the Bølling-Allerød (2014)
    Wiley
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    Publication Date: 2014-04-03
    Description: ABSTRACT Previous studies find decadal climate variability possibly related to solar activity, although the details regarding the feedback with the ocean environment and ecosystem remain unknown. Here, we explore the feedback system of solar irradiance change during the Bølling-Allerød period, based on laminated sediments in the northern Bering Sea. During this period, well-ventilated water was restricted to the upper intermediate layer, and oxygen-poor lower intermediate water preserved the laminated sediment. An 11-year cycle of diatom and radiolarian flux peaks was identified from the laminated interval. Increased fresh meltwater input and early sea-ice retreat in spring under the solar irradiance maximum follow the positive phase of Arctic Oscillation which impacted the primary production and volume of upper intermediate water production in the following winter. Strength of this 11-year solar irradiance effect might be further regulated by the pressure patterns of PDO and/or ENSO variability.
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    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    Paradox in South Asian summer monsoon circulation change: Lower tropospheric strengthening and upper tropospheric weakening (2014)
    Wiley
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    Publication Date: 2014-04-03
    Description: In the literature, there exist contradictory conclusions on the South Asian summer monsoon (SASM) precipitation and circulation changes: whether the circulation change contributes positively by strengthening or negatively by weakening to the rainfall enhancement, on a background of moisture content increase. Based on Representative Concentration Pathway 4.5 simulations by eighteen Coupled Model Intercomparison Project phase 5 models, this study explains these puzzles by illustrating that the SASM circulation changes oppositely between the lower and upper troposphere, with tipping point at 450 hPa. However, this indicates a new paradox, created by competing mechanisms. By analyzing the inter-model variability, we determine that the mean advection of stratification change mechanism weakens the upper tropospheric circulation, while the enhanced surface land-sea thermal contrast strengthens the lower-level and surface winds. Our moisture budget analysis shows that the SASM precipitation enhancement (8% K -1 ) attributes to moisture increase (5% K -1 ) and lower tropospheric circulation strengthening (3% K -1 ).
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    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    Propagation of lower-band whistler-mode waves in the outer Van Allen belt: systematic analysis of 11 years of multicomponent data from the Cluster spacecraft (2014)
    Wiley
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    Publication Date: 2014-04-04
    Description: Lower-band whistler-mode emissions can influence the dynamics of the outer Van Allen radiation belts. We use 11 years of measurements of the STAFF-SA instruments onboard the four Cluster spacecraft to systematically build maps of wave propagation parameters as a function of position. We determine probability distributions of wave vector angle weighted by the wave intensity. The results show that wave vector directions of intense waves are close to a Gaussian-shaped peak centered on the local magnetic field line. The width of this peak is between 10-20 degrees. The cumulative percentage of oblique waves is below 10-15%. This result is especially significant for an important class of whistler-mode emissions of lower band chorus at higher latitudes, well outside their source region, where a simple ray tracing model fails and another mechanism is necessary to keep the wave vectors close to the field-aligned direction.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
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    Quantifying the Radiation Belt Seed Population in the March 17, 2013 Electron Acceleration Event (2014)
    Wiley
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    Publication Date: 2014-04-04
    Description: We present phase space density (PSD) observations using data from the MagEIS instrument on the Van Allen Probes for the 17 March 2013 electron acceleration event. We confirm previous results and quantify how PSD gradients depend on the first adiabatic invariant. We find a systematic difference between the lower energy electrons (〈few 100 keV) originating from the plasma sheet and higher energy electrons 〉1 MeV with a source region within the radiation belts. Our observations show that the source process begins with enhancements to the 50-100's keV energy seed population, followed by enhancements to the 〉1 MeV population and eventually leading to enhancements in the multi-MeV electron population. These observations provide the clearest evidence to date of the timing and nature of the radial transport of a 100's keV electron seed population into the heart of the outer belt and subsequent local acceleration of those electrons to higher radiation belt energies.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    Time-causal operational estimation of electric fields induced in the Earth's lithosphere during magnetic storms (2014)
    Wiley
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    Publication Date: 2014-04-04
    Description: In support of projects for monitoring geomagnetic hazards for electric power grids, we develop a simple mathematical formalism, consistent with the time-causality of deterministic physics, for estimating electric fields that are induced in the Earth's lithosphere during magnetic storms. For an idealized model of the lithosphere, an infinite half-space having uniform electrical conductivity properties described by a galvanic tensor, we work in the Laplace-transformed frequency domain to obtain a transfer function which, when convolved with measured magnetic field time series, gives an estimated electric field time series. Using data collected at the Kakioka, Japan observatory, we optimize lithospheric conductivity parameters by minimizing the discrepancy between model-estimated electric field variation and that actually measured. With our simple model, we can estimate 87% of the variance in storm time Kakioka electric field data; a more complicated model of lithospheric conductivity would be required to estimate the remaining 13% of the variance. We discuss how our estimation formalism might be implemented for geographically coordinated real-time monitoring of geoelectric fields.
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    Topics: Geosciences , Physics
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    Simulations of inner magnetosphere dynamics with an expanded RAM-SCB model and comparisons with Van Allen Probes observations (2014)
    Wiley
    Details
    Publication Date: 2014-04-04
    Description: Simulations from our newly expanded RAM-SCB model, now valid out to 9 R E , are compared for the first time with Van Allen Probes observations. The expanded model reproduces the storm-time ring current buildup due to the increased convection and inflowof plasma from the magnetotail. It matches MagEIS observations of the trapped high-energy (〉50 keV) ion flux; however, it underestimates the low-energy (〈10 keV) HOPE observations. The dispersed injections of ring current ions observed with the ECT suite at high (〉20 keV) energy are better reproduced using a high-resolution convection model. In agreement with EMFISIS observations, RAM-SCB indicates that the large-scale magnetic field is depressed as close as ∼4.5 R E during even a moderate storm. Regions of electromagnetic ion cyclotron (EMIC) instability are predicted on the duskside from ∼6 to ∼9 R E , indicating that previous studies confined to geosynchronous orbit may have underestimated their scattering effect on the energetic particles.
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    The Destructive 1946 Unimak Near-Field Tsunami: New Evidence for a Submarine Slide Source from reprocessed Marine Geophysical Data (2014)
    Wiley
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    Publication Date: 2014-09-16
    Description: The Mw 8.6 earthquake in 1946 off the Pacific shore of Unimak Island at the end of the Alaska Peninsula generated a far-field tsunami that crossed the Pacific to Antarctica. Its tsunami magnitude, 9.3, is comparable to the 9.1 magnitude of the 2011 Tohoku tsunami. On Unimak Island's Pacific shore, a run-up of 42 m destroyed the lighthouse at Scotch Cap. Elsewhere, localized tsunamis with such high run-ups have been interpreted as caused by large submarine landslides. However, previous to this study, no landslide large enough to generate this run-up was found in the area that is limited by the time interval between earthquake shaking and tsunami inundation at Scotch Cap. Reworking of a seismic reflection transect and co-located multibeam bathymetric surveys reveal a landslide block that may explain the 1946 high run-up. It is seaward of Scotch Cap on the mid-slope terrace and within the time-limited area.
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    Topics: Geosciences , Physics
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