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Journal Cover Geophysical Research Letters
  [SJR: 3.323]   [H-I: 185]   [130 followers]  Follow
    
   Full-text available via subscription Subscription journal
   ISSN (Print) 0094-8276 - ISSN (Online) 1944-8007
   Published by AGU Homepage  [17 journals]
  • Climate Impacts of CALIPSO-Guided Corrections to Black Carbon Aerosol
           Vertical Distributions in a Global Climate Model
    • Authors: Mahesh Kovilakam; Salil Mahajan, R. Saravanan, Ping Chang
      Abstract: We alleviate the bias in the tropospheric vertical distribution of black carbon aerosols (BC) in the Community Atmosphere Model (CAM4) using the Cloud-Aerosol and Infrared Pathfinder Satellite Observations (CALIPSO)-derived vertical profiles. A suite of sensitivity experiments are conducted with 1x, 5x, and 10x the present-day model estimated BC concentration climatology, with (corrected, CC) and without (uncorrected, UC) CALIPSO-corrected BC vertical distribution. The globally averaged top of the atmosphere radiative flux perturbation of CC experiments is ∼8–50% smaller compared to uncorrected (UC) BC experiments largely due to an increase in low-level clouds. The global average surface temperature increases, the global average precipitation decreases, and the ITCZ moves northward with the increase in BC radiative forcing, irrespective of the vertical distribution of BC. Further, tropical expansion metrics for the poleward extent of the Northern Hemisphere Hadley cell (HC) indicate that simulated HC expansion is not sensitive to existing model biases in BC vertical distribution.
      PubDate: 2017-10-17T01:10:36.390462-05:
      DOI: 10.1002/2017GL074652
       
  • How Sharp Is the Cratonic Lithosphere-Asthenosphere Transition'
    • Authors: Nicholas J. Mancinelli; Karen M. Fischer, Colleen A. Dalton
      Abstract: Earth's cratonic mantle lithosphere is distinguished by high seismic wave velocities that extend to depths greater than 200 km, but recent studies disagree on the magnitude and depth extent of the velocity gradient at their lower boundary. Here we analyze and model the frequency dependence of Sp waves to constrain the lithosphere-asthenosphere velocity gradient at long-lived stations on cratons in North America, Africa, Australia, and Eurasia. Beneath 33 of 44 stations, negative velocity gradients at depths greater than 150 km are less than a 2–3% velocity drop distributed over more than 80 km. In these regions the base of the typical cratonic lithosphere is gradual enough to be explained by a thermal transition. Vertically sharper lithosphere-asthenosphere transitions are permitted beneath 11 stations, but these zones are spatially intermittent. These results demonstrate that lithosphere-asthenosphere viscosity contrasts and coupling fundamentally differ between cratons and younger continents.
      PubDate: 2017-10-17T00:05:46.186266-05:
      DOI: 10.1002/2017GL074518
       
  • Weak thermocline mixing in the North Pacific low-latitude western boundary
           current system
    • Authors: Zhiyu Liu; Qiang Lian, Fangtao Zhang, Lei Wang, Mingming Li, Xiaolin Bai, Jianing Wang, Fan Wang
      Abstract: Despite its potential importance in the global climate system, mixing properties of the North Pacific low-latitude western boundary current system (LLWBC) remained unsampled until very recently. We report here on the first measurements of turbulence microstructure associated with these currents, made in the western boundary region of the tropical North Pacific east of the Philippines. The results suggest that thermocline mixing in the North Pacific LLWBC is generally weak with the diapycnal diffusivity κρ∼ O(10−6) m2 s−1. This is consistent with predictions from internal wave-wave interaction theory that mixing due to internal wave breaking is significantly reduced at low latitudes. Enhanced mixing is found to be associated with a permanent cyclonic eddy, the Mindanao Eddy, but mainly at its south and north flanks. There, κρ is elevated by an order of magnitude due to eddy-induced geostrophic shear. Mixing in the eddy core is at the background level with no indication of enhancement.
      PubDate: 2017-10-16T18:40:27.631668-05:
      DOI: 10.1002/2017GL075210
       
  • Attribution of Observed Streamflow Changes in Key British Columbia
           Drainage Basins
    • Authors: Mohammad Reza Najafi; Francis Zwiers, Nathan Gillett
      Abstract: We study the observed decline in summer streamflow in four key river basins in British Columbia (BC) Canada using a formal detection and attribution (D&A) analysis procedure. Reconstructed and simulated streamflow is generated using the semi-distributed Variable Infiltration Capacity (VIC) hydrologic model, which is driven by 1/16° gridded observations and downscaled climate model data from the Coupled Model Intercomparison Project Phase 5 (CMIP5), respectively. The internal variability of the regional hydrologic components using ~ 5,100 years of streamflow simulated using CMIP5 preindustrial control runs (CTL). Results show that the observed changes in summer streamflow are inconsistent with simulations representing the responses to natural forcing factors alone, while the response to anthropogenic and natural forcing factors combined is detected in these changes. A two-signal D&A analysis indicates that the effect of anthropogenic forcing factors (ANT) are discernable from NAT in BC, albeit with large uncertainties.
      PubDate: 2017-10-16T13:35:38.536235-05:
      DOI: 10.1002/2017GL075016
       
  • How might recharge change under projected climate change in the western
           US'
    • Authors: R. Niraula; T. Meixner, F. Dominguez, N. Bhattarai, M. Rodell, H. Ajami, D. Gochis, C. Castro
      Abstract: Although groundwater is a major water resource in the western US, little research has been done on the impacts of climate change on groundwater storage and recharge in the West. Here we assess the impact of projected changes in climate on groundwater recharge in the near (2021-2050) and far (2071-2100) future across the western US. VIC model was run with RCP 6.0 forcing from 11 GCMs and ‘subsurface runoff’ output was considered as recharge. Recharge is expected to decrease in the West (-5.8±14.8 %) and Southwest (-4.0±6.7%) regions in the near future and in the South region (-9.5±24.3%) in the far future. The Northern Rockies region is expected to get more recharge in the near (+5.3±6.3%) and far (+11.8±8.3%) future. Overall, southern portions of the western US are expected to get less recharge in the future and northern portions will get more. Climate change interacts with land surface properties to affect the amount of recharge that occurs in the future. Effects on recharge due to change in vegetation response from projected changes in climate and CO2 concentration, though important, are not considered in this study.
      PubDate: 2017-10-16T13:35:30.221385-05:
      DOI: 10.1002/2017GL075421
       
  • Seismic observations indicating that the 2015 Ogasawara (Bonin) earthquake
           ruptured beneath the 660-km discontinuity
    • Authors: Keiko Kuge
      Abstract: The termination of deep earthquakes at a depth of ~700 km is a key feature for understanding the physical mechanism of deep earthquakes. The ~680 km deep May 30, 2015, Ogasawara (Bonin) earthquake (Mw7.9) and its aftershocks were recorded by seismic stations at distances from 7° to 19°. Synthetic seismograms indicate that the P waveforms depend on whether the earthquake is located above or below the 660-km discontinuity. In this study, I show that broadband recordings indicate that the 2015 earthquake may have occurred below the 660-km velocity discontinuity. Recordings of the P wave from the strongest aftershock lack evidence for wave triplication expected when a subhorizontal discontinuity underlies the hypocenter. Theoretical waveforms computed with a 660-km discontinuity above the aftershock and mainshock match the observed waveforms more accurately. These observations may indicate earthquake ruptures due to mantle minerals other than olivine, or strong deformation of the 660-km phase transition.
      PubDate: 2017-10-16T13:35:27.787796-05:
      DOI: 10.1002/2017GL074469
       
  • Theoretical Solution and Applications of Ocean-bottom Pressure Induced by
           Seismic Seafloor Motion
    • Authors: Chao An; Chen Cai, Yong Zheng, Lingsen Meng, Philip Liu
      Abstract: Seismic signals captured by ocean-bottom pressure sensors, which are designed to record tsunami waves, are largely ignored. In this paper, we derive a simple theoretical solution of the ocean-bottom pressure as a function of prescribed seafloor motion. All the assumptions are clearly stated and analyzed. The solution is checked by comparing the seafloor displacement and pressure from three M7+ earthquakes, recorded by ocean-bottom seismometers and pressure gauges located off the Japanese coast. We then show two applications. First, using the seafloor displacement data recorded by an ocean-bottom seismometer, the pressure amplitude recorded by the associated pressure gauge is corrected, and vice versa. Second, pressure recordings from DART during the 2011 Tohoku earthquake are converted to seafloor displacements, which are then utilized to estimate the earthquake focal mechanism. Thus, we demonstrate that seismic signals recorded by pressure sensors have great potential for fast estimate of earthquake source parameters.
      PubDate: 2017-10-16T13:31:35.448258-05:
      DOI: 10.1002/2017GL075137
       
  • The transient response of ice volume to orbital forcing during the warm
           Late Pliocene
    • Authors: B. de Boer; Alan M. Haywood, Aisling M. Dolan, Stephen J. Hunter, Caroline L. Prescott
      Abstract: Examining the nature of ice-sheet and sea-level response to past episodes of enhanced greenhouse gas forcing may help constrain future sea-level change. Here, for the first time, we present the transient nature of ice sheets and sea level during the Late Pliocene. The transient ice-sheet predictions are forced by multiple climate snapshots derived from a climate model set up with Late Pliocene boundary conditions, forced with different orbital forcing scenarios appropriate to two Marine Isotope Stages (MIS), MIS KM5c and K1. Our results indicate that during MIS KM5c both the Antarctic and Greenland ice sheets contributed to sea-level rise relative to present and were relatively stable. Insolation forcing between the hemispheres was out of phase during MIS K1 and led to an asynchronous response of ice volume globally. Therefore, when variations of precession were high, inferring the behavior of ice sheets from benthic isotope or sea-level records is complex.
      PubDate: 2017-10-16T13:31:28.39295-05:0
      DOI: 10.1002/2017GL073535
       
  • Tropical forcing of the Summer East Atlantic pattern
    • Authors: C. Ole Wulff; Richard J. Greatbatch, Daniela I.V. Domeisen, Gereon Gollan, Felicitas Hansen
      Abstract: The Summer East Atlantic (SEA) mode is the second dominant mode of summer low-frequency variability in the Euro-Atlantic region. Using reanalysis data, we show that SEA-related circulation anomalies significantly influence temperatures and precipitation over Europe. We present evidence that part of the interannual SEA variability is forced by diabatic heating anomalies of opposing signs in the tropical Pacific and Caribbean that induce an extratropical Rossby wave train. This precipitation dipole is related to SST anomalies characteristic of the developing ENSO phases. Seasonal hindcast experiments forced with observed sea surface temperatures (SST) exhibit skill at capturing the interannual SEA variability corroborating the proposed mechanism and highlighting the possibility for improved prediction of boreal summer variability. Our results indicate that tropical forcing of the SEA likely played a role in the dynamics of the 2015 European heat wave.
      PubDate: 2017-10-16T13:30:52.236269-05:
      DOI: 10.1002/2017GL075493
       
  • Joint modulation of intraseasonal rainfall in tropical Australia by the
           Madden-Julian Oscillation and El Niño-Southern Oscillation
    • Authors: Roohi P. S. Ghelani; Eric C. J. Oliver, Neil J. Holbrook, Matthew C. Wheeler, Philip J. Klotzbach
      Abstract: Rainfall in tropical Australia is a critical resource for the agricultural sector. However, its high variability implores improvements in our understanding of its variability. Australian tropical rainfall is influenced by both the Madden-Julian Oscillation (MJO) on intraseasonal time scales and El Niño-Southern Oscillation (ENSO) on interannual time scales. This study examines the joint relationship between the MJO, ENSO and tropical Australian rainfall variability. We analyse daily precipitation data from stations across tropical Australia during the wet season (November to April). The wet season rainfall response to the MJO is found to be greater during El Niño than La Niña. We demonstrate that this relationship is not due to the statistical relationship between the MJO and ENSO indices but instead due to differences in how the MJO modulates the large scale circulation during El Niño versus during La Niña.
      PubDate: 2017-10-16T13:30:39.172304-05:
      DOI: 10.1002/2017GL075452
       
  • Significant contributions of volcanic aerosols to decadal changes in the
           stratospheric circulation
    • Authors: M. Diallo; F. Ploeger, P. Konopka, T. Birner, R. Müller, M. Riese, H. Garny, B. Legras, E. Ray, G. Berthet, F. Jegou
      Abstract: The stratospheric circulation is an important element of climate as it determines the concentration of radiatively active species like water vapour and aerosol above the tropopause. Climate models predict that increasing greenhouse gas levels speed up the stratospheric circulation. However, these results have been challenged by observational estimates of the circulation strength, constituting an uncertainty in current climate simulations. Here, we quantify the effect of volcanic aerosol on the stratospheric circulation focussing on the Mt. Pinatubo eruption and discussing further the minor extratropical volcanic eruptions after 2008. We show that the observed pattern of decadal circulation change over the past decades is substantially driven by volcanic aerosol injections. Thus, climate model simulations need to realistically take into account the effect of volcanic eruptions, including the minor eruptions after 2008, for a reliable reproduction of observed stratospheric circulation changes.
      PubDate: 2017-10-16T13:30:26.252738-05:
      DOI: 10.1002/2017GL074662
       
  • Southern Ocean Origin of Multidecadal Variability in the North Brazil
           Current
    • Authors: René M. van Westen; Henk A. Dijkstra
      Abstract: Analysis of model data from a long (200 years) simulation of a high-resolution version of the Parallel Ocean Program indicates a connection between a mode of multidecadal variability in the Southern Ocean, the so-called Southern Ocean Mode, and multidecadal variability in the North Brazil Current. The multidecadal sea surface height variability in the Southern Ocean, propagates northwards, and submerges at about 40°S. Northward propagating anomalies in ocean heat content are found between 5°– 40°S at depths down to 1 km and affect the North Brazil Current. Similar variability and connections between Southern Ocean and North Brazil Current are also found in a (200 years) simulation of a high-resolution global version of the Community Earth System Model. The results provide a new mechanism for the low-frequency variability of the North Brazil Current.
      PubDate: 2017-10-16T13:26:05.603174-05:
      DOI: 10.1002/2017GL074815
       
  • Prolongation of SMAP to Spatio-temporally Seamless Coverage of Continental
           US Using a Deep Learning Neural Network
    • Authors: Kuai Fang; Chaopeng Shen, Daniel Kifer, Xiao Yang
      Abstract: The Soil Moisture Active Passive (SMAP) mission has delivered valuable sensing of surface soil moisture since 2015. However, it has a short time span and irregular revisit schedules. Utilizing a state-of-the-art time-series deep learning neural network, Long Short-Term Memory (LSTM), we created a system that predicts SMAP level-3 moisture product with atmospheric forcings, model-simulated moisture, and static physiographic attributes as inputs. The system removes most of the bias with model simulations and improves predicted moisture climatology, achieving small test root-mean-squared errors (0.87 for over 75% of Continental United States, including the forested Southeast. As the first application of LSTM in hydrology, we show the proposed network avoids overfitting and is robust for both temporal and spatial extrapolation tests. LSTM generalizes well across regions with distinct climates and environmental settings. With high fidelity to SMAP, LSTM shows great potential for hindcasting, data assimilation, and weather forecasting.
      PubDate: 2017-10-16T13:25:40.114339-05:
      DOI: 10.1002/2017GL075619
       
  • Evidence of tropospheric 90-day oscillations in the thermosphere
    • Authors: F. Gasperini; M. E. Hagan, Y. Zhao
      Abstract: In the last decade evidence demonstrated that terrestrial weather greatly impacts the dynamics and mean state of the thermosphere via small-scale gravity waves and global-scale solar tidal propagation and dissipation effects. While observations have shown significant intra-seasonal variability in the upper mesospheric mean winds, relatively little is known about this variability at satellite altitudes (c.a., 250-400 km). Using cross-track wind measurements from the CHAMP and GOCE satellites, winds from a MERRA/TIME-CGM simulation, and outgoing long-wave radiation (OLR) data, we demonstrate the existence of a prominent and global-scale 90-day oscillation in the thermospheric zonal mean winds and in the diurnal eastward-propagating tide with zonal wavenumber 3 (DE3) during 2009-2010 and present evidence of its connection to variability in tropospheric convective activity. This study suggests that strong coupling between the troposphere and the thermosphere occurs on intra-seasonal time scales.
      PubDate: 2017-10-16T13:25:24.655904-05:
      DOI: 10.1002/2017GL075445
       
  • State-dependence of the Climate Sensitivity in Earth System Models of
           Intermediate Complexity
    • Authors: Patrik L. Pfister; Thomas F. Stocker
      Abstract: Growing evidence from General Circulation Models (GCMs) indicates that the equilibrium climate sensitivity (ECS) depends on the magnitude of forcing, which is commonly referred to as state-dependence. We present a comprehensive assessment of ECS state-dependence in Earth System Models of Intermediate Complexity (EMICs) by analyzing millennial simulations with sustained 2×CO2 and 4×CO2 forcings. We compare different extrapolation methods and show that ECS is smaller in the higher-forcing scenario in 12 out of 15 EMICs, in contrast to the opposite behavior reported from GCMs. In one such EMIC, the Bern3D-LPX model, this state-dependence is mainly due to the weakening sea ice-albedo feedback in the Southern Ocean, which depends on model configuration. Due to ocean mixing adjustments, state-dependence is only detected hundreds of years after the abrupt forcing, highlighting the need for long model integrations. Adjustments to feedback parametrizations of EMICs may be necessary if GCM intercomparisons confirm an opposite state-dependence.
      PubDate: 2017-10-16T13:25:21.659028-05:
      DOI: 10.1002/2017GL075457
       
  • The Importance of Upper Mantle Heterogeneity in Generating the Indian
           Ocean Geoid Low
    • Authors: Attreyee Ghosh; G. Thyagarajulu, Bernhard Steinberger
      Abstract: One of the most pronounced geoid lows on Earth lies in the Indian Ocean just south of the Indian peninsula. Several theories have been proposed to explain this geoid low, most of which invoke past subduction. Some recent studies have also argued that high-velocity anomalies in the lower mantle coupled with low-velocity anomalies in the upper mantle are responsible for these negative geoid anomalies. However, there is no general consensus regarding the source of this particular anomaly. We investigate the source of this geoid low by using models of density-driven mantle convection. Our study is the first to successfully explain the occurrence of this anomaly using a global convection model driven by present-day density anomalies derived from tomography. We test various tomography models in our flow calculations with different radial and lateral viscosity variations. Some of them produce a fairly high correlation to the global geoid, but only a few (SMEAN2, GyPSuM, SEMUCB, and LLNL-JPS) could match the precise location and pattern of the geoid low in the Indian Ocean. The source of this low stems from a low-density anomaly stretching from a depth of 300 km down to ∼900 km in the northern Indian Ocean region. This density anomaly potentially originates from material rising along the edge of the African Large Low Shear Velocity Province and moving toward the northeast, facilitated by the movement of the Indian plate in the same direction.
      PubDate: 2017-10-14T19:11:02.333502-05:
      DOI: 10.1002/2017GL075392
       
  • The Enhancement of Neutral Metal Na Layer Above Thunderstorms
    • Authors: Bingkun Yu; Xianghui Xue, Gaopeng Lu, Cheng-Ling Kuo, Xiankang Dou, Qi Gao, Xiushu Qie, Jianfei Wu, Shican Qiu, Yutian Chi, Yihuan Tang
      Abstract: Atomic sodium (Na) is one of the major meteoric species existing as layers of atoms in the mesosphere/lower thermosphere (MLT) at altitudes of 80–105 km and atomic ions at higher altitudes. As the boundary between neutral atmosphere and ionosphere, the MLT region is of particular interest because it is influenced by the mesoscale convective weather and thunderstorms from below and exposed to solar photons from above. There has been great interest on the atmosphere-ionosphere coupling, and numerous studies have been reported on the connection between thunderstorms and the ionosphere. However, the influence of thunderstorms on metallic species, which would significantly enhance our understanding of how Earth's atmosphere interacts with its ionosphere and studying the chemistry and physics of the MLT region, has rarely been studied. Here we present observational results on a statistical basis showing evidence that thunderstorm activities can affect the metal layer, by identifying a statistically significant enhancement of the neutral metal Na layer above thunderstorms at Haikou, China (20.0°N, 110.3°E). The thunderstorm-generated gravity waves and electric field effects could be the mechanisms responsible for the lightning-associated enhancement of Na layer.
      PubDate: 2017-10-14T19:00:33.256798-05:
      DOI: 10.1002/2017GL074977
       
  • First observations of short-period eastward propagating planetary waves
           from the stratosphere to the lower thermosphere (110 km) in winter
           Antarctica
    • Authors: Xian Lu; Xinzhao Chu, Cao Chen, Vu Nguyen, Anne K. Smith
      Abstract: Unique Fe lidar observations in May 2014 at McMurdo, combined with Aura-Microwave Limb Sounder (MLS) measurements, lead to a new discovery that the amplitudes of 4-day and 2.5-day planetary waves (PWs) grow rapidly from 1–2 K at 100 km to over 10 K at 110 km. This report is also the first observation of short-period (1–5 days) eastward propagating PWs from 30 km all the way to 110 km. The Specified Dynamics-Whole Atmosphere Community Climate Model reproduces the observed three dominant peaks of amplitudes in temperature and coherent vertical phase structures. The data-model comparison indicates a possible mechanism: After PWs originated from the stratosphere dissipate along the critical level, the surviving waves are amplified by in-situ instability in the mesosphere and lower thermosphere, resulting in the second (third) peak in geopotential (temperature). This third peak in temperature explains the PW amplitude growth from 100 to 110 km.
      PubDate: 2017-10-13T19:05:43.153991-05:
      DOI: 10.1002/2017GL075641
       
  • MJO-Related Tropical Convection Anomalies Lead to More Accurate
           Stratospheric Vortex Variability in Subseasonal Forecast Models
    • Authors: C. I. Garfinkel; C. Schwartz
      Abstract: The effect of the Madden-Julian Oscillation (MJO) on the Northern Hemisphere wintertime stratospheric polar vortex in the period preceding stratospheric sudden warmings is evaluated in operational subseasonal forecasting models. Reforecasts which simulate stronger MJO-related convection in the Tropical West Pacific also simulate enhanced heat flux in the lowermost stratosphere and a more realistic vortex evolution. The time scale on which vortex predictability is enhanced lies between 2 and 4 weeks for nearly all cases. Those stratospheric sudden warmings that were preceded by a strong MJO event are more predictable at ∼20 day leads than stratospheric sudden warmings not preceded by a MJO event. Hence, knowledge of the MJO can contribute to enhanced predictability, at least in a probabilistic sense, of the Northern Hemisphere polar stratosphere.
      PubDate: 2017-10-13T12:00:27.900223-05:
      DOI: 10.1002/2017GL074470
       
  • Are Simulated and Observed Twentieth Century Tropical Pacific Sea Surface
           Temperature Trends Significant Relative to Internal Variability'
    • Authors: S. Coats; K. B. Karnauskas
      Abstract: Historical trends in the tropical Pacific zonal sea surface temperature gradient (SST gradient) are analyzed herein using 41 climate models (83 simulations) and 5 observational data sets. A linear inverse model is trained on each simulation and observational data set to assess if trends in the SST gradient are significant relative to the stationary statistics of internal variability, as would suggest an important role for external forcings such as anthropogenic greenhouse gasses. None of the 83 simulations have a positive trend in the SST gradient, a strengthening of the climatological SST gradient with more warming in the western than eastern tropical Pacific, as large as the mean trend across the five observational data sets. If the observed trends are anthropogenically forced, this discrepancy suggests that state-of-the-art climate models are not capturing the observed response of the tropical Pacific to anthropogenic forcing, with serious implications for confidence in future climate projections. There are caveats to this interpretation, however, as some climate models have a significant strengthening of the SST gradient between 1900 and 2013 Common Era, though smaller in magnitude than the observational data sets, and the strengthening in three out of five observational data sets is insignificant. When combined with observational uncertainties and the possibility of centennial time scale internal variability not sampled by the linear inverse model, this suggests that confident validation of anthropogenic SST gradient trends in climate models will require further emergence of anthropogenic trends. Regardless, the differences in SST gradient trends between climate models and observational data sets are concerning and motivate the need for process-level validation of the atmosphere-ocean dynamics relevant to climate change in the tropical Pacific.
      PubDate: 2017-10-13T11:35:39.223124-05:
      DOI: 10.1002/2017GL074622
       
  • The mechanisms underpinning Cenozoic intraplate volcanism in eastern
           Australia: Insights from seismic tomography and geodynamic modeling
    • Authors: N. Rawlinson; D. R. Davies, S. Pilia
      Abstract: Cenozoic intraplate volcanism is widespread throughout much of eastern Australia and manifests as both age-progressive volcanic tracks and non-age-progressive lava fields. Various mechanisms have been invoked to explain the origin and distribution of the volcanism, but a broad consensus remains elusive. We use results from seismic tomography to demonstrate a clear link between lithospheric thickness and the occurrence, composition, and volume of volcanic outcrop. Furthermore, we find that non-age-progressive lava fields overlie significant cavities in the base of the lithosphere. Based on numerical simulations of mantle flow, we show that these cavities generate vigorous mantle upwellings, which likely promote decompression melting. However, due to the intermittent nature of the lava field volcanics over the last 50 Ma, it is probable that transient mechanisms also operate to induce or enhance melting. In the case of the Newer Volcanics Province, the passage of a nearby plume appears to be a likely candidate. Our results demonstrate why detailed 3-D variations in lithospheric thickness, plate motion, and transient sources of mantle heterogeneity need to be considered when studying the origin of non age-progressive volcanism in continental interiors.
      PubDate: 2017-10-13T11:06:00.714527-05:
      DOI: 10.1002/2017GL074911
       
  • Gamma Ray Signatures of Neutrons From a Terrestrial Gamma Ray Flash
    • Authors: G. S. Bowers; D. M. Smith, G. F. Martinez-McKinney, M. Kamogawa, S. A. Cummer, J. R. Dwyer, D. Wang, M. Stock, Z. Kawasaki
      Abstract: Following a lightning strike to a wind turbine in Japan, we have observed a large burst of neutrons lasting 100 ms with a ground fluence of ~1,000 n cm−2, thousands of times greater than the peak neutron flux associated with the largest ground level solar particle event ever observed. This is the first detection of an unequivocal signature of neutrons from a terrestrial gamma ray flash, consisting of a 2.223 MeV gamma-ray spectral line from a neutron-capture on hydrogen reaction occurring in our detector, and is shown to be consistent with the production of 1012–1013 photoneutrons from a downward terrestrial gamma ray flash (TGF) at 1.0 km, with a gamma ray brightness typical of upward TGFs observed by satellites.
      PubDate: 2017-10-13T10:50:36.632242-05:
      DOI: 10.1002/2017GL075071
       
  • Advance, Retreat, and Halt of Abrupt Gravel-Sand Transitions in Alluvial
           Rivers
    • Authors: Astrid Blom; Víctor Chavarrías, Robert I. Ferguson, Enrica Viparelli
      Abstract: Downstream fining of bed sediment in alluvial rivers is usually gradual, but often an abrupt decrease in characteristic grain size occurs from about 10 to 1 mm, i.e., a gravel-sand transition (GST) or gravel front. Here we present an analytical model of GST migration that explicitly accounts for gravel and sand transport and deposition in the gravel reach, sea level change, subsidence, and delta progradation. The model shows that even a limited gravel supply to a sand bed reach induces progradation of a gravel wedge and predicts the circumstances required for the gravel front to advance, retreat, and halt. Predicted modern GST migration rates agree well with measured data at Allt Dubhaig and the Fraser River, and the model qualitatively captures the behavior of other documented gravel fronts. The analysis shows that sea level change, subsidence, and delta progradation have a significant impact on the GST position in lowland rivers.
      PubDate: 2017-10-13T10:48:20.079433-05:
      DOI: 10.1002/2017GL074231
       
  • Highly temporally resolved response to seasonal surface melt of the
           Zachariae and 79N outlet glaciers in northeast Greenland
    • Authors: N. M. Rathmann; C. S. Hvidberg, A. M. Solgaard, A. Grinsted, G. H. Gudmundsson, P. L. Langen, K. P. Nielsen, A. Kusk
      Abstract: The seasonal response to surface melting of the Northeast Greenland Ice Stream outlets, Zachariae and 79N, is investigated using new highly temporally resolved surface velocity maps for 2016 combined with numerical modeling. The seasonal speedup at 79N of 0.15 km/yr is suggested to be driven by a decrease in effective basal pressure induced by surface melting, whereas for Zachariae its 0.11 km/yr seasonal speedup correlates equally well with the breakup of its large ice mélange. We investigate the influence 76 km long floating tongue at 79N, finding it provides little resistance and that most of it could be lost without impacting the dynamics of the area. Furthermore, we show that reducing the slipperiness along the tongue-wall interfaces produces a velocity change spatially inconsistent with the observed seasonal speedup. Finally, we find that subglacial sticky spots such as bedrock bumps play a negligible role in the large-scale response to a seasonally enhanced basal slipperiness of the region.
      PubDate: 2017-10-13T10:36:52.321497-05:
      DOI: 10.1002/2017GL074368
       
  • Quantifying CO2 Emissions From Individual Power Plants From Space
    • Authors: Ray Nassar; Timothy G. Hill, Chris A. McLinden, Debra Wunch, Dylan B. A. Jones, David Crisp
      Abstract: In order to better manage anthropogenic CO2 emissions, improved methods of quantifying emissions are needed at all spatial scales from the national level down to the facility level. Although the Orbiting Carbon Observatory 2 (OCO-2) satellite was not designed for monitoring power plant emissions, we show that in some cases, CO2 observations from OCO-2 can be used to quantify daily CO2 emissions from individual middle- to large-sized coal power plants by fitting the data to plume model simulations. Emission estimates for U.S. power plants are within 1–17% of reported daily emission values, enabling application of the approach to international sites that lack detailed emission information. This affirms that a constellation of future CO2 imaging satellites, optimized for point sources, could monitor emissions from individual power plants to support the implementation of climate policies.
      PubDate: 2017-10-13T09:01:25.625421-05:
      DOI: 10.1002/2017GL074702
       
  • Critical role of snow on sea ice growth in the Atlantic sector of the
           Arctic Ocean
    • Authors: Ioanna Merkouriadi; Bin Cheng, Robert M. Graham, Anja Rösel, Mats A. Granskog
      Abstract: During the Norwegian young sea ICE (N-ICE2015) campaign in early 2015, a deep snow pack was observed, almost double the climatology for the region north of Svalbard. There were significant amounts of snow-ice in second-year ice (SYI), while much less in first-year ice (FYI). Here we use a 1-D snow/ice thermodynamic model, forced with reanalyses, to show that snow-ice contributes to thickness growth of SYI in absence of any bottom growth, due to the thick snow. Growth of FYI is tightly controlled by the timing of growth onset relative to precipitation events. A later growth-onset can be favorable for FYI growth due to less snow accumulation, which limits snow-ice formation. We surmise these findings are related to a phenomenon in the Atlantic sector of the Arctic, where frequent storm events bring heavy precipitation during autumn and winter, in a region with a thinning ice cover.
      PubDate: 2017-10-12T18:35:30.859368-05:
      DOI: 10.1002/2017GL075494
       
  • Understanding the Origin of Jupiter's Diffuse Aurora Using Juno's First
           Perijove Observations
    • Authors: W. Li; R. M. Thorne, Q. Ma, X.-J. Zhang, G. R. Gladstone, V. Hue, P. W. Valek, F. Allegrini, B. H. Mauk, G. Clark, W. S. Kurth, G. B. Hospodarsky, J. E. P. Connerney, S. J. Bolton
      Abstract: Juno observed the low-altitude polar region during perijove 1 on 27 August 2016 for the first time. Auroral intensity and false-color maps from the UVS instrument show extensive diffuse aurora observed equatorward of the main auroral oval. Juno passed over the diffuse auroral region near the System III longitude of 120°–150° (90°–120°) in the northern (southern) hemisphere. In the region where these diffuse auroral emissions were observed, the JEDI and JADE instruments measured nearly full loss cone distributions for the downward-going electrons over energies of 0.1–700 keV, but very few upward-going electrons. The false-color maps from UVS indicate more energetic electron precipitation at lower latitudes than less energetic electron precipitation, consistent with observations of precipitating electrons measured by JEDI and JADE. The comparison between particle and aurora measurements provide first direct evidence that these precipitating energetic electrons are mainly responsible for the diffuse auroral emissions at Jupiter.
      PubDate: 2017-10-12T18:35:26.569322-05:
      DOI: 10.1002/2017GL075545
       
  • First Application of the Zeeman Technique to Remotely Measure Auroral
           Electrojet Intensity from Space
    • Authors: J. H. Yee; J. Gjerloev, D. Wu, M. J. Schwartz
      Abstract: Using the O2 118 GHz spectral radiance measurements obtained by the Microwave Limb Sounder (MLS) instrument onboard the Aura spacecraft, we demonstrate that the Zeeman effect can be used to remotely measure the magnetic field perturbations produced by the auroral electrojet near the Hall current closure altitudes. Our derived current-induced magnetic field perturbations are found to be highly correlated with those coincidently obtained by ground magnetometers. These perturbations are also found to be linearly correlated with auroral electrojet strength. The statistically-derived polar maps of our measured magnetic field perturbation reveal a spatial-temporal morphology consistent with that produced by the Hall current during substorms and storms. With today's technology, a constellation of compact, low-power, high-spectral-resolution cubesats would have the capability to provide high precision and spatiotemporal magnetic-field samplings needed for auroral electrojet measurements to gain insights into the spatiotemporal behavior of the auroral electrojet system.
      PubDate: 2017-10-12T18:35:24.683373-05:
      DOI: 10.1002/2017GL074909
       
  • Permeability evolution in variably glassy basaltic andesites measured
           under magmatic conditions
    • Authors: A. R. L. Kushnir; C. Martel, R. Champallier, F. B. Wadsworth
      Abstract: Heat from inflowing magma may act to seal permeable networks that assist passive outgassing at volcanic conduit margins and in overlying domes, reducing the efficiency of overpressure dissipation. Here we present a study of the evolution of permeability – measured under magmatic conditions - with increasing temperature in glassy and glass-poor basaltic andesites from Merapi volcano (Indonesia). Whereas the permeability of glass-poor rocks decreases little up to a temperature of 1010°C, glassy specimens experience a pronounced decrease in permeability above the glass transition once the viscosity of the crystal suspension is low enough to relax under external stresses. Changes in temperature alone are thus not enough to significantly modify the permeability of the glass-poor rocks that commonly form Merapi's dome. However, the presence of glass-rich domains in a dome may lead to local sealing of the volcanic plumbing between eruptions, exacerbating localized overpressure development that could contribute to explosivity.
      PubDate: 2017-10-12T18:35:22.299879-05:
      DOI: 10.1002/2017GL074042
       
  • A Near Global Atmospheric Distribution of N2O Isotopologues
    • Authors: Peter F. Bernath; Mahdi Yousefi, Eric Buzan, Chris D. Boone
      Abstract: The distributions of the four most abundant isotopologues and isotopomers (N2O, 15NNO, N15NO, and NN18O) of nitrous oxide have been measured in the Earth's stratosphere by infrared remote sensing with the Atmospheric Chemistry Experiment (ACE) Fourier transform spectrometer. These satellite observations have provided a near global picture of N2O isotopic fractionation. The relative abundances of the heavier species increase with altitude and with latitude in the stratosphere as the air becomes older. The heavy isotopologues are enriched by 20-30% in the upper stratosphere and even more over the poles. These observations are in general agreement with model predictions made with the Whole Atmosphere Community Climate Model (WACCM). A detailed 3-D chemical transport model is needed to account for the global isotopic distributions of N2O, and to infer sources and sinks.
      PubDate: 2017-10-12T18:35:19.552828-05:
      DOI: 10.1002/2017GL075122
       
  • Nucleation phase and dynamic inversion of the Mw 6.9 Valparaíso 2017
           earthquake in Central Chile
    • Authors: S. Ruiz; F. Aden-Antoniow, J. C. Baez, C. Otarola, B. Potin, F. Campo, P. Poli, C. Flores, C. Satriano, F. Leyton, R. Madariaga, P. Bernard
      Abstract: The Valparaiso 2017 sequence occurred in the Central Chile mega-thrust, an active zone where the last mega-earthquake occurred in 1730. Intense seismicity started 2 days before the Mw 6.9 main-shock, a slow trench-ward movement was observed in the coastal GPS antennas and was accompanied by foreshocks and repeater-type seismicity. To characterize the rupture process of the main-shock, we perform a dynamic inversion using the strong-motion records and an elliptical patch approach. We suggest that a slow slip event preceded and triggered the Mw 6.9 earthquake, which ruptured an elliptical asperity (semi-axis of 10 km and 5 km, with a sub-shear rupture, stress drop of 11.71 MPa, yield stress of 17.21 MPa, slip weakening of 0.65 m and kappa value of 1.98). This earthquake could be the beginning of a long-term nucleation phase to a major rupture, within the highly coupled Central Chile zone where a mega-thrust earthquake like 1730 is expected.
      PubDate: 2017-10-12T18:25:22.06504-05:0
      DOI: 10.1002/2017GL075675
       
  • Green's Function-based Tsunami Data Assimilation (GFTDA): A fast data
           assimilation approach toward tsunami early warning
    • Authors: Yuchen Wang; Kenji Satake, Takuto Maeda, Aditya Riadi Gusman
      Abstract: We propose a new tsunami data assimilation approach based on Green's functions to reduce the computation time for tsunami early warning. Green's Function-based Tsunami Data Assimilation (GFTDA) forecasts the waveforms at points of interest (PoIs) by superposition of Green's functions between observation stations and PoIs. Unlike the previous assimilation approach, GFTDA does not require the calculation of the tsunami wavefield for the whole region during the assimilation process, because the Green's functions have been calculated in advance. The forecasted waveforms can be calculated by a simple matrix manipulation. The application to the tsunami waveforms recorded by the bottom pressure gauges of the Cascadia Initiative from the 2012 Haida Gwaii earthquake reveals that GFTDA achieves the same accuracy as the previous assimilation approach while reducing the time required to issue a valid tsunami warning.
      PubDate: 2017-10-12T18:25:19.571417-05:
      DOI: 10.1002/2017GL075307
       
  • Using Noble Gas Measurements to Derive Air-Sea Process Information and
           Predict Physical Gas Saturations
    • Authors: Roberta C. Hamme; Steven R. Emerson, Jeffrey P. Severinghaus, Matthew C. Long, Igor Yashayaev
      Abstract: Dissolved gas distributions are important because they influence oceanic habitats and Earth's climate, yet competing controls by biology and physics make gas distributions challenging to predict. Bubble-mediated gas exchange, temperature change, and varying atmospheric pressure all push gases away from equilibrium. Here we use new noble gas measurements from the Labrador Sea to demonstrate a technique to quantify physical processes. Our analysis shows that water-mass formation can be represented by a quasi steady state in which bubble fluxes and cooling push gases away from equilibrium balanced by diffusive gas exchange forcing gases toward equilibrium. We quantify the rates of these physical processes from our measurements, allowing direct comparison to gas exchange parameterizations, and predict the physically driven saturation of other gases. This technique produces predictions that reasonably match N2/Ar observations and demonstrates that physical processes should force SF6 to be ∼6% more supersaturated than CFC-11 and CFC-12, impacting ventilation age calculations.
      PubDate: 2017-10-12T12:25:30.709563-05:
      DOI: 10.1002/2017GL075123
       
  • Distinguishing remobilized ash from erupted volcanic plumes using
           space-borne multi-angle imaging.
    • Authors: Verity J. B. Flower; Ralph A. Kahn
      Abstract: Volcanic systems are comprised of a complex combination of ongoing eruptive activity and secondary hazards, such as remobilized ash plumes. Similarities in the visual characteristics of remobilized and erupted plumes, as imaged by satellite-based remote sensing, complicate the accurate classification of these events. The stereo imaging capabilities of the Multi-angle Imaging SpectroRadiometer (MISR) were used to determine the altitude and distribution of suspended particles. Remobilized ash shows distinct dispersion, with particles distributed within ~1.5 km of the surface. Particle transport is consistently constrained by local topography, limiting dispersion pathways downwind. The MISR Research Aerosol (RA) retrieval algorithm was used to assess plume particle microphysical properties. Remobilized ash plumes displayed a dominance of large particles with consistent absorption and angularity properties, distinct from emitted plumes. The combination of vertical distribution, topographic control, and particle microphysical properties makes it possible to distinguish remobilized ash flows from eruptive plumes, globally.
      PubDate: 2017-10-11T15:03:01.402016-05:
      DOI: 10.1002/2017GL074740
       
  • Revisiting the cause of the 1989-2009 Arctic surface warming using the
           surface energy budget: downward infrared radiation dominates the surface
           fluxes
    • Authors: Sukyoung Lee; Tingting Gong, Steven B. Feldstein, James Screen, Ian Simmonds
      Abstract: The Arctic has been warming faster than elsewhere, especially during the cold season. According to the leading theory, ice-albedo feedback warms the Arctic Ocean during the summer, and the heat gained by the ocean is released during the winter, causing the cold-season warming. Screen and Simmonds (2010; SS10) concluded that the theory is correct by comparing trend patterns in surface air temperature (SAT), surface turbulence heat flux (HF) and net surface infrared radiation (IR). However, in this comparison, downward IR is more appropriate to use. By analyzing the same data used in SS10 using the surface energy budget, it is shown here that over most of the Arctic the skin temperature trend, which closely resembles the SAT trend, is largely accounted for by the downward IR, not the HF, trend.
      PubDate: 2017-10-11T15:02:25.187995-05:
      DOI: 10.1002/2017GL075375
       
  • Constraining the Global Ocean Heat Content Through Assimilation of CERES
           derived TOA Energy Imbalance Estimates
    • Authors: Andrea Storto; Chunxue Yang, Simona Masina
      Abstract: The Earth's Energy Imbalance (EEI) is stored in the oceans for the most part. Thus, estimates of its variability can be ingested in ocean retrospective analyses to constrain the global ocean heat budget. Here, we propose a scheme to assimilate top of the atmosphere global radiation imbalance estimates from CERES in a coarse-resolution variational ocean reanalysis system (2000-2014). The methodology proves able to shape the heat content tendencies according to the EEI estimates, without compromising the reanalysis accuracy. Spurious variability and under- (over-) estimation present in experiments with in-situ (no) data assimilation disappear when EEI data are assimilated. The warming hiatus present without the assimilation of EEI data is mitigated, inducing ocean warming at depths below 1500 m and slightly larger in the Southern Hemisphere, in accordance with recent studies. Furthermore, the methodology may be applied to Earth System reanalyses and climate simulations to realistically constrain the global energy budget.
      PubDate: 2017-10-11T15:02:18.787906-05:
      DOI: 10.1002/2017GL075396
       
  • Stratospheric ozone depletion: an unlikely driver of the regional trends
           in Antarctic sea ice in austral fall in the late 20th Century
    • Authors: Laura Landrum; Marika Holland, Marilyn Raphael, Lorenzo Polvani
      Abstract: It has been suggested that recent regional trends in Antarctic sea ice might have been caused by the formation of the ozone hole in the late 20th century. Here we explore this by examining two ensembles of a climate model over the ozone hole formation period (1955-2005). One ensemble includes all known historical forcings; the other is identical except for ozone levels, which are fixed at 1955 levels. We demonstrate that the model is able to capture, on interannual and decadal time scales, the observed statistical relationship between summer Amundsen Sea Low strength (when ozone loss causes a robust deepening) and fall sea-ice concentrations (when observed trends are largest). In spite of this, the modeled regional trends caused by ozone depletion are found to be almost exactly opposite to the observed ones. We deduce that the regional character of observed sea ice trends is likely not caused by ozone depletion.
      PubDate: 2017-10-11T14:53:59.224565-05:
      DOI: 10.1002/2017GL075618
       
  • Successful Sampling Strategy Advances Laboratory Studies of NMR Logging in
           Unconsolidated Aquifers
    • Authors: Ahmad A. Behroozmand; Rosemary Knight, Mike Müller-Petke, Esben Auken, Adrian A. S. Barfod, Ty P. A. Ferré, Troels N. Vilhelmsen, Carole D. Johnson, Anders V. Christiansen
      Abstract: The nuclear magnetic resonance (NMR) technique has become popular in groundwater studies because it responds directly to the presence and mobility of water in a porous medium. There is a need to conduct laboratory experiments to aid in the development of NMR-hydraulic conductivity models, as is typically done in the petroleum industry. However, the challenge has been obtaining high-quality laboratory samples from unconsolidated aquifers. At a study site in Denmark, we employed sonic drilling, which minimizes the disturbance of the surrounding material, and extracted twelve 7.6-cm diameter samples for laboratory measurements. We present a detailed comparison of the acquired laboratory- and logging-NMR data. The agreement observed between the laboratory and logging data suggests that the methodologies proposed in this study provide good conditions for studying NMR measurements of unconsolidated near-surface aquifers. Finally, we show how laboratory sample size and condition impact the NMR measurements.
      PubDate: 2017-10-11T10:55:20.765832-05:
      DOI: 10.1002/2017GL074999
       
  • Implications of detection methods on characterizing atmospheric river
           contribution to seasonal snowfall across Sierra Nevada, USA
    • Authors: Laurie S. Huning; Steven A. Margulis, Bin Guan, Duane E. Waliser, Paul J. Neiman
      Abstract: This study investigates the extent to which the diagnosed contribution of atmospheric rivers (ARs) to the seasonal cumulative snowfall (CS) is related to the AR detection approach utilized. Using both satellite integrated water vapor (IWV)-based and reanalysis integrated vapor transport (IVT)-based methodologies, the corresponding AR-derived CS distributions were characterized over the Sierra Nevada (USA) from 1998-2015. AR detection methods indicated that ARs yield greater orographic enhancement of the seasonal CS than non-AR storms above ~2100-2300 m for the IWV-based approach and over all elevations for the IVT-based detection approach across the western (i.e. windward) Sierra Nevada. Due to differences in the methodologies, the IWV-based approach diagnosed 2.1 times fewer ARs than the IVT-based approach. As a result, the ARs diagnosed using the IWV-based detection method yielded an average 33% of the total range-wide CS annually as opposed to 56% from the IVT-based detection method.
      PubDate: 2017-10-10T18:08:57.53895-05:0
      DOI: 10.1002/2017GL075201
       
  • Channelized Melting Drives Thinning Under a Rapidly Melting Antarctic Ice
           Shelf
    • Authors: Noel Gourmelen; Dan N. Goldberg, Kate Snow, Sian F. Henley, Robert G. Bingham, Satoshi Kimura, Anna E. Hogg, Andrew Shepherd, Jeremie Mouginot, Jan T. M. Lenaerts, Stefan R. M. Ligtenberg, Willem Jan Berg
      Abstract: Ice shelves play a vital role in regulating loss of grounded ice and in supplying freshwater to coastal seas. However, melt variability within ice shelves is poorly constrained and may be instrumental in driving ice shelf imbalance and collapse. High-resolution altimetry measurements from 2010 to 2016 show that Dotson Ice Shelf (DIS), West Antarctica, thins in response to basal melting focused along a single 5 km-wide and 60 km-long channel extending from the ice shelf's grounding zone to its calving front. If focused thinning continues at present rates, the channel will melt through, and the ice shelf collapse, within 40–50 years, almost two centuries before collapse is projected from the average thinning rate. Our findings provide evidence of basal melt-driven sub-ice shelf channel formation and its potential for accelerating the weakening of ice shelves.
      PubDate: 2017-10-10T16:00:02.817297-05:
      DOI: 10.1002/2017GL074929
       
  • Dominating roles of ice sheets and insolation in variation of tropical
           cyclone genesis potential over the North Atlantic during the last 21,000
           years
    • Authors: Qing Yan; Zhongshi Zhang
      Abstract: To advance our knowledge on tropical cyclone (TC)’s sensitivity to various external forcings, we investigate evolution of TC genesis potential over the North Atlantic during the last 21,000 years that experienced more varied forcings than present-day using transient simulations. Abrupt large amplitude of fluctuations in genesis potential during the studied period were dominated by meltwater discharge, whereas insolation controlled the long-term trend. Increased (decreased) meltwater discharge from northern hemisphere ice sheets leads to detrimental (conducive) conditions for genesis via altering the Atlantic Meridional Overturning Circulation and hence three-dimensional temperature structure and Hadley circulation. This effect is larger than the orographic effect of retreated ice sheets that favors TC formation. Decreasing (increasing) insolation broadly results in more (less) favorable conditions for genesis arising from larger (smaller) local vertical temperature contrast and stronger (weaker) meridional circulation. Our study highlights potential changes of ice sheets and insolation in affecting future long-term TC activity.
      PubDate: 2017-10-10T15:21:08.304356-05:
      DOI: 10.1002/2017GL075786
       
  • Unified spectrum of tropical rainfall and waves in a simple stochastic
           model
    • Authors: Samuel N. Stechmann; Scott Hottovy
      Abstract: In the tropics, rainfall is coupled with atmospheric dynamics in ways that are not fully understood, and often different mechanisms are proposed to underlie different modes of variability. Here, it is shown that a unified model with a simple form can produce many different modes of variability. In particular, this includes the Madden–Julian oscillation and convectively coupled equatorial waves. The model predicts the length scales, time scales, structures, and spatiotemporal variability of these modes reasonably well for a simple model. Furthermore, the model produces a background spectrum of rainfall that resembles spatiotemporal red noise and is only weakly coupled with wave dynamics. The full spectrum is also shown to be shaped by antiresonance, whereby rainfall oscillations are prevented from occurring at the oscillation frequencies of dry waves. To produce all of these aspects simultaneously, a key factor is differing roles of lower and middle tropospheric water vapor.
      PubDate: 2017-10-10T15:15:48.436634-05:
      DOI: 10.1002/2017GL075754
       
  • Relative Importance of Nitric Oxide Physical Drivers in the Lower
           Thermosphere
    • Authors: Koen Hendrickx; Linda Megner, Daniel R. Marsh, Jorg Gumbel, Rickard Strandberg, Felix Martinsson
      Abstract: Nitric Oxide (NO) observations from the Solar Occultation for Ice Experiment (SOFIE) and Student Nitric Oxide Explorer (SNOE) satellite instruments are investigated to determine the relative importance of drivers of short term NO variability. We study the variations of de-seasonalized NO anomalies by removing a climatology, which explains between approximately 70% and 90% of the total NO budget, and relate them to variability in geomagnetic activity and solar radiation. Throughout the lower thermosphere geomagnetic activity is the dominant process at high latitudes, while in the equatorial region solar radiation is the primary source of short term NO changes. Consistent results are obtained on estimated geomagnetic and radiation contributions of NO variations in the two datasets, which are nearly a decade apart in time. The analysis presented here can be applied to model simulations of NO to investigate the accuracy of the parametrized physical drivers.
      PubDate: 2017-10-10T13:46:14.016473-05:
      DOI: 10.1002/2017GL074786
       
  • Dam construction in Lancang-Mekong River Basin could mitigate future flood
           risk from warming-induced intensified rainfall
    • Authors: Wei Wang; Hui Lu, L. Ruby Leung, Hongyi Li, Jianshi Zhao, Fuqiang Tian, Kun Yang, Khem Sothea
      Abstract: Water resources management, in particular flood control, in the Lancang-Mekong River Basin (LMRB) faces two key challenges in the 21st century: climate change and dam construction. A large scale distributed Geomorphology-Based Hydrological Model coupled with a simple reservoir regulation model (GBHM-LMK-SOP) is used to investigate the relative effects of climate change and dam construction on the flood characteristics in the LMRB. Results suggest an increase in both flood magnitude and frequency under climate change, which is more severe in the upstream basin and increases over time. However, stream regulation by dam reduce flood risk consistently throughout this century, with more obvious effects in the upstream basin where larger reservoirs will be located. The flood mitigation effect of dam regulation dominates over the flood intensification effect of climate change before 2060, but the latter emerges more prominently after 2060 and dominates the flood risk especially in the lower basin.
      PubDate: 2017-10-09T13:20:34.298024-05:
      DOI: 10.1002/2017GL075037
       
  • Global floods and water availability driven by atmospheric rivers
    • Authors: Homero Paltan; Duane Waliser, Wee Ho Lim, Bin Guan, Dai Yamazaki, Raghav Pant, Simon Dadson
      Abstract: Whilst emerging regional evidence shows that atmospheric rivers (ARs) can exert strong impacts on local water availability and flooding, their role in shaping global hydrological extremes has not yet been investigated. Here we quantify the relative contribution of ARs variability to both flood hazard and water availability. We find that globally, precipitation from ARs contributes 22% of total global runoff, with a number of regions reaching 50% or more. In areas where their influence is strongest, ARs may increase the occurrence of floods by 80%, whilst absence of ARs may increase the occurrence of hydrological droughts events by up to 90%. We also find that ~300 million people are exposed to additional floods and droughts due the occurrence of ARs. ARs provide a source of hydro-climatic variability whose beneficial or damaging effects depend on the capacity of water resources managers to predict and adapt to them.
      PubDate: 2017-10-09T13:20:30.753298-05:
      DOI: 10.1002/2017GL074882
       
  • Future changes to El Niño-Southern Oscillation temperature and
           precipitation teleconnections
    • Authors: Sarah J. Perry; Shayne McGregor, Alex Sen Gupta, Matthew H. England
      Abstract: Potential changes to the El Niño-Southern Oscillation (ENSO) resulting from climate change may have far reaching impacts through atmospheric teleconnections. Here, ENSO temperature and precipitation teleconnections between the historical and high-emission future simulations are compared in 40 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Focusing on the global land area only, we show that there are robust increases in the spatial extent of ENSO teleconnections during austral summer in 2040-2089 of ~19% for temperature and ~12% for precipitation in the multi-model mean (MMM), relative to the 1950-1999 period. The MMM further shows the expansion of ENSO teleconnection extent is at least partly related to a strengthening of ENSO teleconnections over continental regions, however, a consistent strengthening is not found across the individual models. This suggests that while more land may be affected by ENSO, the existing teleconnections may not be simply strengthened.
      PubDate: 2017-10-09T13:20:27.173875-05:
      DOI: 10.1002/2017GL074509
       
  • ENSO weather and coral bleaching on the Great Barrier Reef, Australia
    • Authors: Hamish McGowan; Alison Theobald
      Abstract: The most devastating mass coral bleaching has occurred during El Niño events, with bleaching reported to be a direct result of increased sea surface temperatures (SSTs). However, El Niño itself does not cause SSTs to rise in all regions that experience bleaching. Nor is the upper-ocean warming trend of 0.11°C per decade since 1971, attributed to global warming, sufficient alone to exceed the thermal tolerance of corals. Here we show that weather patterns during El Niño that result in reduced cloud cover, higher than average air temperatures and higher than average atmospheric pressures play a crucial role in determining the extent and location of coral bleaching on the world's largest coral reef system, the World Heritage Great Barrier Reef (GBR), Australia. Accordingly, synoptic scale weather patterns and local atmosphere-ocean feedbacks related to ENSO and not large scale SST warming due to El Niño alone and/or global warming, are often the cause of coral bleaching on the GBR.
      PubDate: 2017-10-09T13:20:24.134638-05:
      DOI: 10.1002/2017GL074877
       
  • Ice and snow thickness variability and change in the high Arctic Ocean
           observed by in-situ measurements
    • Authors: Christian Haas; Justin Beckers, Josh King, Arvids Silis, Julienne Stroeve, Jeremy Wilkinson, Bernice Notenboom, Axel Schweiger, Stefan Hendricks
      Abstract: In April 2017 we collected unique, extensive in-situ data of sea ice and snow thickness. At ten sampling sites, located under a CryoSat-2 overpass, between Ellesmere Island and 87.1°N mean and modal total ice thicknesses ranged between 2 to 3.4 m and 1.8 to 2.9 m respectively. Coincident snow thicknesses ranged between 0.3 to 0.47 m (mean), and 0.1 to 0.5 m (mode). The profile spanned the complete multiyear ice zone in the Lincoln Sea, into the first-year ice zone further north. Complementary snow thickness measurements near the North Pole showed a mean thickness of 0.31 m. Compared with scarce measurements from other years, multiyear ice was up to 0.75 m thinner than in 2004, but not significantly different from 2011 and 2014. We found excellent agreement with a commonly used snow climatology and with published long-term ice thinning rates. There was reasonable agreement with CryoSat-2 thickness retrievals.
      PubDate: 2017-10-09T13:15:33.508173-05:
      DOI: 10.1002/2017GL075434
       
  • A Hydro-meteorological Perspective on the Karakoram Anomaly using Unique
           Valley-based Synoptic Weather Observations
    • Authors: Furrukh Bashir; Xubin Zeng, Hoshin Gupta, Pieter Hazenberg
      Abstract: Glaciers in the eastern Hindukush, western Karakoram and northwestern Himalayan mountain ranges of Northern Pakistan are not responding to global warming in the same manner as their counterparts elsewhere. Their retreat rates are less than the global average, and some are either stable or growing. Various investigations have questioned the role of climatic factors in regards to this anomalous behavior, widely referred to as ‘The Karakoram Anomaly’. Here, for the first time, we present a hydro-meteorological perspective based on five decades of synoptic weather observations collected by the meteorological network of Pakistan. Analysis of this unique data set indicates that increased regional scale humidity, cloud cover, and precipitation, along with decreased net radiation, near-surface wind speed, potential evapotranspiration and river flow, especially during the summer season, represent a substantial change in the energy, mass and momentum fluxes that are facilitating the establishment of the Karakoram Anomaly.
      PubDate: 2017-10-09T13:15:31.235188-05:
      DOI: 10.1002/2017GL075284
       
  • Time-dependent freshwater input from ice shelves: impacts on Antarctic sea
           ice and the Southern Ocean in an Earth System Model
    • Authors: Andrew G. Pauling; Inga J. Smith, Patricia J. Langhorne, Cecilia M. Bitz
      Abstract: Earth System Models do not reproduce the observed increase in Antarctic sea ice extent which may be due to the unrealistic representation of ice shelves. Here we investigate the response of sea ice to increasing freshwater input from ice shelves using the Community Earth System Model with the Community Atmosphere Model version 5 [CESM1(CAM5)]. Including the effect of heat loss from the ocean to melt ice shelves resulted in significantly more positive trends in sea ice area. We have conducted model experiments adding fresh water as if from ice shelf melt with a linear increase in the rate of input over the period 1980-2013. We found that an increase in the rate of change of freshwater input of ∼45 Gt yr−2 was sufficient to offset the negative trend in sea ice area in CESM1(CAM5), although the freshwater input by the end of the experiment was larger than observed at that time.
      PubDate: 2017-10-09T13:15:24.309063-05:
      DOI: 10.1002/2017GL075017
       
  • On the dependence of cloud feedbacks on physical parameterizations in WRF
           aquaplanet simulations
    • Authors: Gregory Cesana; Kay Suselj, Florent Brient
      Abstract: We investigate the effects of physical parameterizations on cloud feedback uncertainty in response to climate change. For this purpose, we construct an ensemble of eight aquaplanet simulations using the Weather Research and Forecasting (WRF) model. In each WRF-derived simulation, we replace only one parameterization at a time while all other parameters remain identical. By doing so, we aim to (i) reproduce cloud feedback uncertainty from state-of-the-art climate models and (ii) understand how parametrizations impact cloud feedbacks. Our results demonstrate that this ensemble of WRF simulations, which differ only in physical parameterizations, replicates the range of cloud feedback uncertainty found in state-of-the-art climate models. We show that microphysics and convective parameterizations govern the magnitude and sign of cloud feedbacks, mostly due to tropical low-level clouds in subsidence regimes. Finally, this study highlights the advantages of using WRF to analyze cloud feedback mechanisms owing to its plug-and-play parameterization capability.
      PubDate: 2017-10-09T13:10:26.362234-05:
      DOI: 10.1002/2017GL074820
       
  • Drivers of continued surface warming after cessation of carbon emissions
    • Authors: Richard G. Williams; Vassil Roussenov, Thomas L. Frölicher, Philip Goodwin
      Abstract: The climate response after cessation of carbon emissions is examined here, exploiting a single equation connecting surface warming to cumulative carbon emissions. The multi-centennial response to an idealized pulse of carbon is considered by diagnosing a 1000 year integration of an Earth system model (GFDL ESM2M) and an ensemble of efficient Earth system model simulations. After emissions cease, surface temperature evolves according to (i) how much of the emitted carbon remains in the atmosphere and (ii) how much of the additional radiative forcing warms the surface rather than the ocean interior. The peak in surface temperature is delayed in time after carbon emissions cease through the decline in ocean heat uptake, which in turn increases the proportion of radiative forcing warming the surface. Eventually, after many centuries, surface temperature declines as the radiative forcing decreases through the excess atmospheric CO2 being taken up by the ocean and land.
      PubDate: 2017-10-09T13:01:14.02077-05:0
      DOI: 10.1002/2017GL075080
       
  • Conversion of wet glass to melt at lower seismogenic zone conditions:
           implications for pseudotachylyte creep
    • Authors: B. P. Proctor; D. A. Lockner, J. B. Lowenstern, N. M. Beeler
      Abstract: Coseismic frictional melting and the production of quenched glass called pseudotachylyte is a recurring process during earthquakes. To investigate how glassy materials affect the postseismic strength and stability of faults, obsidian gouges were sheared under dry and wet conditions from 200 °C to 300 °C at ~150 MPa effective normal stress. Dry glass exhibited a brittle rheology at all conditions tested, exhibiting friction values and microstructures consistent with siliciclastic materials. Likewise, wet glass at 200 °C exhibited a brittle rheology. In contrast, wet gouges at 300 °C transitioned from brittle sliding to linear-viscous (Newtonian) flow at strain rates < 3x10-4 s-1, indicating melt-like behavior. The viscosity ranged from 2x1011 to 7.8x1011 Pa-s. Microstructures show viscous gouges were fully welded with rod-shaped microlites rotated into the flow direction. Fourier transform infrared spectroscopy along with electron backscatter imaging demonstrate that hydration of the glass by diffusion of pore water was the dominant process reducing the viscosity and promoting viscous flow. As much as 5 wt% water diffused into the glass. These results may provide insight into postseismic-slip behaviors and challenge some interpretations of fault kinematics based on studies assuming pseudotachylyte formation and flow is solely coseismic.
      PubDate: 2017-10-09T13:00:27.733151-05:
      DOI: 10.1002/2017GL075344
       
  • Getting it right matters – temperature goal interpretations in
           geoscience research
    • Authors: Joeri Rogelj; Carl-Friedrich Schleussner, William Hare
      Abstract: The adoption of the 1.5°C long-term warming limit in the Paris Agreement made 1.5°C a ‘hot topic’ in the scientific community, with researchers eager to address this issue [Rogelj and Knutti, 2016]. Long-term warming limits have a decades-long history in international policy [Randalls, 2010]. To effectively inform the climate policy debate, geoscience research hence needs a core understanding of their legal and policy context. Here, we describe this context in detail, and illustrate its importance by showing the impact it can have on global carbon budget estimates. We show that definitional clarity is essential on this important matter.
      PubDate: 2017-10-09T12:40:39.695935-05:
      DOI: 10.1002/2017GL075612
       
  • A new mechanism for upper crustal fluid flow driven by solitary porosity
           waves in rigid reactive media'
    • Authors: Sumit Chakraborty
      Abstract: The equations governing the interaction of viscous deformation with porous flow of fluids gives rise to wave-like solutions. Such solutions have been explored in the context of melt and fluid flow in the mantle and crust at high temperatures, where ductile behavior occurs. Now it has been shown (Omlin et al., 2017) that the coupling of the kinetics of chemical reactions with fluid flow may give rise to similar solutions. This opens intriguing new possibilities. Porosity waves may arise in low temperature regimes, and may become more accessible to observation; or, they may remain mathematical curiosities because other modes of transport dominate in such settings. A number of possibilities, questions, and future courses of research have been opened up by Omlin et al. (2017).
      PubDate: 2017-10-09T12:35:52.634311-05:
      DOI: 10.1002/2017GL075798
       
  • Differential radiative heating drives tropical atmospheric circulation
           weakening
    • Authors: Yan Xia; Yi Huang
      Abstract: The tropical atmospheric circulation is projected to weaken during global warming, although the mechanisms that cause the weakening remain to be elucidated. We hypothesize that the weakening is related to the inhomogeneous distribution of the radiative forcing and feedback, which heats the tropical atmosphere in the ascending and subsiding regions differentially and thus requires the circulation to weaken due to energetic constraints. We test this hypothesis in a series of numerical experiments using a fully coupled general circulation model (GCM), in which the radiative forcing distribution is controlled using a novel method. The results affirm the effect of inhomogeneous forcing on the tropical circulation weakening and this effect is greatly amplified by radiative feedbacks, especially that of clouds. In addition, we find that differential heating explains the inter-model differences in tropical circulation response to CO2 forcing in the GCM ensemble of the Climate Model Intercomparison project.
      PubDate: 2017-10-09T12:35:51.047086-05:
      DOI: 10.1002/2017GL075678
       
  • Evidence of the lower thermospheric winter-to-summer circulation from
           SABER CO2 observations
    • Authors: Liying Qian; Alan Burns, Jia Yue
      Abstract: Numerical studies have shown that there is a lower thermospheric winter-to-summer circulation that is driven by wave dissipation; that it plays a significant role in trace gas distributions in the mesosphere and lower thermosphere (MLT), and in the composition of the thermosphere. However, the characteristics of this circulation are poorly known. Direct observations of it are difficult, but it leaves clear signatures in tracer distributions. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite has obtained CO2 concentration from 2002 to present. This dataset, combined with simulations by the Whole Atmosphere Community Climate Model (WACCM), provides an unprecedented opportunity to infer the morphology of this circulation in both the summer and winter hemispheres. Our study show that there exists a maximum vertical gradient of CO2 at summer high latitudes, driven by the convergence of the upwelling of the mesospheric circulation and the downwelling of the lower thermospheric circulation; in the winter hemisphere, the maximum vertical gradient of CO2 is located at a higher altitude, driven by the convergence of the upwelling of the lower thermospheric circulation and the downwelling of the solar-driven thermospheric circulation; the bottom of the lower thermospheric circulation is located between ~ 95 km and 100 km, and it has a vertical extent of ~ 10 km. Analysis of the SABER CO2 and temperature at summer high latitudes showed that the bottom of this circulation is consistently higher than the mesopause height by ~ 10 km.
      PubDate: 2017-10-09T12:35:40.417724-05:
      DOI: 10.1002/2017GL075643
       
  • Highly variable latest Pleistocene–Holocene incremental slip rates on
           the Awatere fault at Saxton River, South Island, New Zealand, revealed by
           lidar mapping and luminescence dating
    • Authors: Robert Zinke; James F. Dolan, Edward J. Rhodes, Russ Van Dissen, Christopher P. McGuire
      Abstract: Geomorphic mapping using high-resolution lidar imagery and luminescence dating reveal highly variable incremental Holocene–latest Pleistocene slip rates at the well-known Saxton River site along the Awatere fault, a dextral strike-slip fault in the Marlborough Fault System, South Island, New Zealand. Using lidar and field observations, we measured seven fault offsets recorded by fluvial terraces and bedrock markers. Improved dating of the offsets is provided by post-IR-IRSL225 luminescence ages. Incremental slip rates varied from < 2 mm/yr to> 15 mm/yr over intervals of thousands of years and 10s of m of slip, demonstrating order-of-magnitude temporal variations in rate at a single site. These observations have basic implications for earthquake fault behavior, lithospheric mechanics, discrepancies between geodetic and geologic slip rates, and probabilistic seismic hazard assessment.
      PubDate: 2017-10-09T12:35:38.08001-05:0
      DOI: 10.1002/2017GL075048
       
  • Rapid pitch angle evolution of suprathermal electrons behind
           dipolarization fronts
    • Authors: C. M. Liu; H. S. Fu, J. B. Cao, Y. Xu, Y. Q. Yu, E. A. Kronberg, P. W. Daly
      Abstract: The pitch angle distribution (PAD) of suprathermal electrons can have both spatial and temporal evolution in the magnetotail and theoretically can be an indication of electron energization/cooling processes there. So far, the spatial evolution of PAD has been well studied, leaving the temporal evolution as an open question. To reveal the temporal evolution of electron PAD, spacecraft should monitor the same flux tube for a relatively long period, which is not easy in the dynamic magnetotail. In this study, we present such an observation by Cluster spacecraft in the magnetotail behind a dipolarization front (DF). We find that the PAD of suprathermal electrons can evolve from pancake-type to butterfly-type during
      PubDate: 2017-10-09T12:30:31.301794-05:
      DOI: 10.1002/2017GL075007
       
  • The attenuation of swell waves by rain
    • Authors: Luigi Cavaleri; Luciana Bertotti
      Abstract: Within the progressive improvement in wave modeling we focus on the attenuation of swell waves by rain. Till now ignored, the process is shown to be relevant, especially for the correct estimate of swell. Following the practical impossibility of devoted field experiments, we exploit the global model results over a period of more than four years to extract the tiny signal associated to the attenuation by rain. A direct comparison of the ratio model by altimeter significant wave heights versus the encountered rain amount hints to a marked related dependence. A proper quantification of the related physical effect requires a multiple step procedure that we describe in detail. We check the reliability of the results, and we provide the related source function ready for implementation in operational wave models.
      PubDate: 2017-10-09T12:30:29.697174-05:
      DOI: 10.1002/2017GL075458
       
  • Florida Current salinity and salinity transport: mean and decadal changes
    • Authors: Zoltan B. Szuts; Christopher S. Meinen
      Abstract: The Florida Current (FC) contributes to Atlantic circulation by carrying the western boundary flow of the subtropical gyre and the upper branch of meridional overturning circulation. Repeated FC hydrographic (velocity, salinity, and temperature) sections during 1982-1987 and 2001-2015 characterize its water mass structure and associated transport variability. On average, FC volume transport comes from subtropical North Atlantic water (NAW, 44%), Antarctic Intermediate Water (AAIW, 14%), surface water (SW, 27%), and an indistinct source (Rem 15%), while salinity transport relative to the average salinity along 26°N comes from NAW (55%), AAIW (0.2%), SW (30%), and Rem (15%). From 1982-1987 to 2001-2015, NAW, AAIW, and Rem salinified by 0.03-0.16 g kg-1 and increased the salinity anomaly transport by 3%. These patterns imply that advective salt transport by the FC (1): is sensitive to subtropical North Atlantic variability; and (2) is partially decoupled from the volumetric pathway of the upper overturning branch.
      PubDate: 2017-10-09T12:30:28.103196-05:
      DOI: 10.1002/2017GL074538
       
  • Low-δ18O rhyolites from the Malani Igneous Suite: a positive test for
           South China and NW India linkage in Rodinia
    • Authors: Wei Wang; Peter A. Cawood, Mei-Fu Zhou, Manoj K. Pandit, Xiao-Ping Xia, Jun-Hong Zhao
      Abstract: The Malani Igneous Suite (MIS) in NW India represents one of the best preserved silicic large igneous provinces. Voluminous silicic lavas of the MIS erupted between ca. 780-750 Ma. Zircon grains from rhyolite and dacite lavas have oxygen isotopic compositions that include depleted (δ18O = 4.12 to -1.11‰) and enriched (δ18O = 8.23-5.12‰) signatures. The low-δ18O zircon grains have highly radiogenic Hf isotopic compositions (ƐHf(t)= +13.0 to +3.6), suggesting high temperature bulk cannibalization of upper level juvenile mafic crust as an essential mechanism to produce the low-δ18O felsic magma. Xenocrystic zircon grains in dacites have high δ18O and low ƐHf(t) values for magmas older than 800 Ma, reflecting a dramatic transition in tectono-thermal regime in NW India during 800-780 Ma. A synchronous transition also occurred in South China and Madagascar, suggesting a spatially linked geodynamic system. NW India and South China together with Madagascar and the Seychelles lay either along the periphery of Rodinia or outboards of the supercontinent with the age of convergent plate margin magmatism coinciding with breakup of the supercontinent.
      PubDate: 2017-10-09T12:30:24.89123-05:0
      DOI: 10.1002/2017GL074717
       
  • Depth of origin of the peak (inner) ring in lunar impact basins
    • Authors: Katarina Miljković; Myriam Lemelin, Paul Lucey
      Abstract: Numerical modeling of the peak-ring basin formation showed that the peak-ring forms from the material that is part of the central uplift outwardly thrust over the inwardly collapsing transient crater rim. Simulations of the lunar basin formation showed that the peak or inner ring in peak-ring or multi-ring basins, respectively, is composed of the overturned crust and deep-seated material, possibly from the upper mantle. Numerical impact simulations were used to trace the depth of origin of material exposed within the peak (or inner) ring. We estimate the scaling trends between basin size and the depth of origin of material exposed within the ring. We also report on the likely crust, mantle and projectile abundances exposed within the ring. Quantifying the excavation depths during the formation of the peak or inner ring provides a step towards understanding the lunar crust and mantle stratigraphy.
      PubDate: 2017-10-09T12:25:19.208675-05:
      DOI: 10.1002/2017GL075207
       
  • Dissolution-Assisted Pattern Formation During Olivine Carbonation
    • Authors: Harrison Lisabeth; Wenlu Zhu, Tiange Xing, Vincent De Andrade
      Abstract: Olivine and pyroxene-bearing rocks in the oceanic crust react with hydrothermal fluids producing changes in the physical characteristics and behaviors of the altered rocks. Notably, these reactions tend to increase solid volume, reducing pore volume, permeability, and available reactive surface area, yet entirely hydrated and/or carbonated rocks are commonly observed in the field. We investigate the evolution of porosity and permeability of fractured dunites reacted with CO2-rich solutions in laboratory experiments. The alteration of crack surfaces changes the mechanical and transport properties of the bulk samples. Analysis of three-dimensional microstructural data shows that although precipitation of secondary minerals causes the total porosity of the sample to decrease, an interconnected network of porosity is maintained through channelized dissolution and coupled carbonate precipitation. The observed microstructure appears to be the result of chemo-mechanical coupling, which may provide a mechanism of porosity maintenance without the need to invoke reaction-driven cracking.
      PubDate: 2017-10-09T03:21:24.799338-05:
      DOI: 10.1002/2017GL074393
       
  • Evidence for the interior evolution of Ceres from geologic analysis of
           fractures
    • Authors: J. E. C. Scully; D. L. Buczkowski, N. Schmedemann, C. A. Raymond, J. C. Castillo-Rogez, S. D. King, M. T. Bland, A. I. Ermakov, D. P. O'Brien, S. Marchi, A. Longobardo, C. T. Russell, R. R. Fu, M. Neveu
      Abstract: Ceres is the largest asteroid-belt object and the Dawn spacecraft observed Ceres since 2015. Dawn observed two morphologically distinct linear features on Ceres’ surface: secondary crater chains and pit chains. Pit chains provide unique insights into Ceres’ interior evolution. We interpret pit chains called the Samhain Catenae as the surface expression of subsurface fractures. Using the pit chains’ spacings, we estimate that the localized thickness of Ceres’ fractured, outer layer is approximately ≥58 km, at least ~14 km greater than the global average. We hypothesize that extensional stresses, induced by a region of upwelling material arising from convection/diapirism, formed the Samhain Catenae. We derive characteristics for this upwelling material, which can be used as constraints in future interior modeling studies. For example, its predicted location coincides with Hanami Planum, a high-elevation region with a negative residual gravity anomaly, which may be surficial evidence for this proposed region of upwelling material.
      PubDate: 2017-10-09T03:21:07.896921-05:
      DOI: 10.1002/2017GL075086
       
  • Estimating morning change in land surface temperature from MODIS day/night
           observations: Applications for surface energy balance modeling
    • Authors: Christopher R. Hain; Martha C. Anderson
      Abstract: Observations of land surface temperature (LST) are crucial for the monitoring of surface energy fluxes from satellite. Methods that require high temporal resolution LST observations (e.g., from geostationary orbit) can be difficult to apply globally because several geostationary sensors are required to attain near-global coverage (60°N to 60°S). While these LST observations are available from polar-orbiting sensors, providing global coverage at higher spatial resolutions, the temporal sampling (twice daily observations) can pose significant limitations. For example, the Atmosphere Land Exchange Inverse (ALEXI) surface energy balance model, used for monitoring evapotranspiration and drought, requires an observation of the morning change in LST—a quantity not directly observable from polar-orbiting sensors. Therefore, we have developed and evaluated a data-mining approach to estimate the midmorning rise in LST from a single sensor (two observations per day) of LST from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on the Aqua platform. In general, the data-mining approach produced estimates with low relative error (5 to 10%) and statistically significant correlations when compared against geostationary observations. This approach will facilitate global, near-real-time applications of ALEXI at higher spatial and temporal coverage from a single sensor than currently achievable with current geostationary data sets.
      PubDate: 2017-10-09T03:20:51.46195-05:0
      DOI: 10.1002/2017GL074952
       
  • A new approach developed to study variability in North African dust
           transport routes over the Atlantic during 2001–2015
    • Authors: L. Meng; H. W. Gao, Y. Yu, X. H. Yao, Y. Gao, C. Zhang, Lei Fan
      Abstract: We investigated the variability in the North African dust transport routes over the Atlantic (NAD routes) by extracting the dust transport central axis using Moderate Resolution Imaging Spectroradiometer aerosol data for 2001–2015. The results showed that the NAD routes can be classified into two regimes, a southern route centered at the southernmost position of 6.08 ± 1.12°N during November to March and a northern route centered at the northernmost position of 18.21 ± 1.04°N during April to October. In the southern route, large intervariation was correlated with the Intertropical Convergence Zone (ITCZ) and North Atlantic Oscillation (NAO), but the ITCZ and NAO jointly explained only 38% of the variation. In the northern route, the ITCZ alone explained 67% of the intervariation. The extracted trends during 2001–2015 exhibited a northward shift of 1.68° for the southern route and of 0.52° for the northern route. The causes for the shift were also examined.
      PubDate: 2017-10-09T03:20:27.983032-05:
      DOI: 10.1002/2017GL074478
       
  • Large-scale seasonal changes in glacier thickness across High Mountain
           Asia
    • Authors: Qiuyu Wang; Shuang Yi, Le Chang, Wenke Sun
      Abstract: Recently, increased efforts have been made to estimate the mass budgets of glaciers in High Mountain Asia (HMA). However, seasonal changes in glaciers are poorly understood, despite the fact that seasonal meltwater released from glaciers is a crucial local water resource in HMA. Utilizing satellite altimetry and gravimetry data, we constructed annual changes in glacier elevation and identified two general patterns of the seasonality of glacier elevation changes. Glaciers in the periphery of HMA (except for those in the eastern Himalayas) thicken from ~December to April-June, thus exhibiting winter and spring accumulation. Glaciers in the inner Tibetan Plateau, especially those in Western Kunlun and Tanggula, accumulate from ~March to ~August, thus exhibiting spring and summer accumulation. The amounts of seasonal glacier ablation were obtained using a new approach of direct observations of glacier changes, rather than inferring changes using a climate model.
      PubDate: 2017-10-09T03:20:18.48274-05:0
      DOI: 10.1002/2017GL075300
       
  • Rivers and floodplains as key components of global terrestrial water
           storage variability
    • Authors: Augusto Getirana; Sujay Kumar, Manuela Girotto, Matthew Rodell
      Abstract: This study quantifies the contribution of rivers and floodplains to terrestrial water storage (TWS) variability. We use state-of-the-art models to simulate land surface processes and river dynamics, and to separate TWS into its main components. Based on a proposed impact index, we show that surface water storage (SWS) contributes 8% of TWS variability globally, but that contribution differs widely among climate zones. Changes in SWS are a principal component of TWS variability in the tropics, where major rivers flow over arid regions, and at high latitudes. SWS accounts for ~22-27% of TWS variability in both the Amazon and Nile basins. Changes in SWS are negligible in the Western U.S., Northern Africa, Middle-East, and central Asia. Based on comparisons with GRACE-based TWS, we conclude that accounting for SWS improves simulated TWS in most of South America, Africa, and Southern Asia, confirming that SWS is a key component of TWS variability.
      PubDate: 2017-10-09T03:20:14.405077-05:
      DOI: 10.1002/2017GL074684
       
  • Reversal of long-term trend in baseline ozone concentrations at the North
           American west coast
    • Authors: D. D. Parrish; I. Petropavlovskikh, S. J. Oltmans
      Abstract: Changes in baseline (here understood as representative of continental to hemispheric scales) tropospheric ozone concentrations that have occurred over western North American and eastern North Pacific are analyzed based on data from three measurement records: 1) sites in the U.S. Pacific coast marine boundary layer, 2) an inland, higher altitude site at Lassen Volcanic National Park, CA, and 3) springtime airborne measurements in the free troposphere between 3 and 8 km altitude. Consistent with previously published results, we find increasing ozone prior to the year 2000, but that rate of increase has slowed and now reversed in these data sets in all seasons. The past ozone increase has been identified as a significant difficulty to overcome in achieving U.S. air quality goals; this difficulty has now eased. Global models only poorly reproduce the observed baseline ozone and trends; policy guidance from such models must be considered very cautiously.
      PubDate: 2017-10-09T03:20:13.401418-05:
      DOI: 10.1002/2017GL074960
       
  • Effect of topography degradation on crater size-frequency distributions:
           Implications for populations of small craters and age dating
    • Authors: Minggang Xie; Meng-Hua Zhu, Zhiyong Xiao, Yunzhao Wu, Aoao Xu
      Abstract: Whether or not background secondary craters dominate populations of small impact craters on terrestrial bodies is a half-century controversy. It has been suggested that small craters on some planetary bodies are dominated by background secondary craters based partly on the steepened slope of crater size-frequency distribution (CSFD) towards small diameters, such as the less than ~1 km diameter crater population on the lunar mare. Here we show that topography degradation enlarges craters and increases CSFD slopes with time. When topography degradation is taken into account, for various-aged crater populations, the observed steep CSFD at small diameters is uniformly consistent with an originally shallower CSFD, whose slope is undifferentiated from the CSFD slope estimated from near-Earth objects and terrestrial bolides. The results show that the effect of topography degradation on CSFD is important in dating planetary surfaces, and the steepening of CSFD slopes is not necessarily caused by secondary cratering, but rather a natural consequence of topography degradation.
      PubDate: 2017-10-06T17:20:35.447316-05:
      DOI: 10.1002/2017GL075298
       
  • Quasi-additivity of the radiative effects of marine cloud brightening and
           stratospheric sulfate aerosol injection
    • Authors: Olivier Boucher; Christoph Kleinschmitt, Gunnar Myhre
      Abstract: Stratospheric sulfate aerosol injection (SAI) and marine cloud brightening (MCB) are the two most studied solar radiation management techniques. For the first time we combine them in a climate model to investigate their complementarity in terms of both instantaneous and effective radiative forcings. The effective radiative forcing induced by SAI is significantly stronger than its instantaneous counterpart evaluated at the top of atmosphere. Radiative kernel calculations indicate that this occurs because of a significant stratospheric warming and despite a large increase in stratospheric water vapor that strengthens the greenhouse effect. There is also a large decrease in high-level cloudiness induced by a stratification of the upper tropopause. Our model experiments also show that the radiative effects of SAI and MCB are quasi additive and have fairly complementary patterns in the Tropics. This results in less spatial and temporal variability in the radiative forcing for combined SAI and MCB as compared to MCB alone.
      PubDate: 2017-10-06T17:16:19.955279-05:
      DOI: 10.1002/2017GL074647
       
  • Effect of snow salinity on CryoSat-2 Arctic first-year sea ice freeboard
           measurements
    • Authors: Vishnu Nandan; Torsten Geldsetzer, John Yackel, Mallik Mahmud, Randall Scharien, Stephen Howell, Joshua King, Robert Ricker, Brent Else
      Abstract: The European Space Agency's CryoSat-2 satellite mission provides radar altimeter data that are used to derive estimates of sea ice thickness and volume. These data are crucial to understanding recent variability and changes in Arctic sea ice. Sea ice thickness retrievals at the CryoSat-2 frequency require accurate measurements of sea ice freeboard, assumed to be attainable when the main radar scattering horizon is at the snow/sea ice interface. Using an extensive snow thermophysical property dataset from late winter conditions in the Canadian Arctic, we examine the role of saline snow on first-year sea ice (FYI), with respect to its effect on the location of the main radar scattering horizon, its ability to decrease radar penetration depth, and its impact on FYI thickness estimates. Based on the dielectric properties of saline snow commonly found on FYI, we quantify the vertical shift in the main scattering horizon. This is found to be approximately 0.07 m. We propose a thickness-dependent snow salinity correction factor for FYI freeboard estimates. This significantly reduces CryoSat-2 FYI retrieval error. Relative error reductions of ~ 11% are found for an an ice thickness of 0.95 m and ~ 25% for 0.7 m. Our method also helps to close the uncertainty gap between SMOS and CryoSat-2 thin ice thickness retrievals. Our results indicate that snow salinity should be considered for FYI freeboard estimates.
      PubDate: 2017-10-06T17:16:10.356929-05:
      DOI: 10.1002/2017GL074506
       
  • First observation of the Earth's permanent free oscillations on Ocean
           Bottom Seismometers
    • Authors: M. Deen; E. Wielandt, E. Stutzmann, W. Crawford, G. Barruol, K. Sigloch
      Abstract: The Earth's hum is the permanent free oscillations of the Earth recorded in the absence of earthquakes, at periods above 30 seconds. We present the first observations of its fundamental spheroidal eigenmodes on broadband Ocean Bottom Seismometers (OBS) in the Indian Ocean. At the ocean bottom, the effects of ocean infragravity waves (compliance) and seafloor currents (tilt) overshadow the hum. In our experiment, data are also affected by electronic glitches. We remove these signals from the seismic trace by subtracting average glitch signals; performing a linear regression and using frequency-dependent response functions between pressure, horizontal and vertical seismic components. This reduces the long period noise on the OBS to the level of a good land station. Finally, by windowing the autocorrelation to include only the direct arrival, the first and second orbit around the Earth, and by calculating its Fourier transform, we clearly observe the eigenmodes at the ocean bottom.
      PubDate: 2017-10-06T17:16:07.127881-05:
      DOI: 10.1002/2017GL074892
       
  • The magnetospheric source region of the bright proton aurora
    • Authors: E. Spanswick; E. Donovan, L. Kepko, V. Angelopoulos
      Abstract: The bright proton aurora is generally understood to be the projection of the Central Plasma Sheet (CPS) where there is sufficient particle energy to cause auroral luminosity and strong pitch angle scattering (presumably due to field line curvature). This region is often interpreted as the transition region between dipolar and tail-like magnetic topologies. The location of auroral features relative to the peak in the proton aurora has been used, for example, to argue that the substorm onset arc lies on field lines that thread this transition region. In this paper, we present statistics of proton aurora luminosity computed from THEMIS ESA measurements for various radial distances in the magnetotail. These results are compared to ground observations of proton auroral luminosity and used to derive a statistical source region of the bright proton aurora.
      PubDate: 2017-10-06T17:10:36.695098-05:
      DOI: 10.1002/2017GL074956
       
  • A twentieth century perspective on summer Antarctic pressure change and
           variability and contributions from tropical SSTs and ozone depletion
    • Authors: Ryan L. Fogt; Chad A. Goergens, Julie M. Jones, David P. Schneider, Julien P. Nicolas, David H. Bromwich, Hallie E. Dusselier
      Abstract: During the late twentieth century, the Antarctic atmospheric circulation has changed and significantly influenced the overall Antarctic climate, through processes including a poleward shift of the circumpolar westerlies. However, little is known about the full spatial pattern of atmospheric pressure over the Antarctic continent prior to 1979. Here we investigate surface pressure changes across the entire Antarctic continent back to 1905 by developing a new summer pressure reconstruction poleward of 60°S. We find that only across East Antarctica are the recent pressures significantly lower than pressures in the early twentieth century; we also discern periods of significant positive pressure trends in the early twentieth century across the coastal South Atlantic sector of Antarctica. Climate model simulations reveal that both tropical sea surface temperature variability and other radiative forcing mechanisms, in addition to ozone depletion, have played an important role in forcing the recent observed negative trends.
      PubDate: 2017-10-06T14:45:53.858498-05:
      DOI: 10.1002/2017GL075079
       
  • Revisiting the observed correlation between weekly averaged Indian monsoon
           precipitation and Arabian Sea aerosol optical depth
    • Authors: Disha Sharma; Ron L. Miller
      Abstract: Dust influences the Indian summer monsoon on seasonal time scales by perturbing atmospheric radiation. On weekly time scales, aerosol optical depth retrieved by satellite over the Arabian Sea is correlated with Indian monsoon precipitation. This has been interpreted to show the effect of dust radiative heating on Indian rainfall on synoptic (few-day) time scales. However, this correlation is reproduced by Earth System Model simulations, where dust is present but its radiative effect is omitted. Analysis of daily variability suggests that the correlation results from the effect of precipitation on dust through the associated cyclonic circulation. Boundary layer winds that deliver moisture to India are responsible for dust outbreaks in source regions far upwind, including the Arabian Peninsula. This suggests that synoptic variations in monsoon precipitation over India enhance dust emission and transport to the Arabian Sea. The effect of dust radiative heating upon synoptic monsoon variations remains to be determined.
      PubDate: 2017-10-05T17:51:26.340946-05:
      DOI: 10.1002/2017GL074373
       
  • Can atmospheric reanalysis datasets be used to reproduce flooding over
           large scales'
    • Authors: Konstantinos M. Andreadis; Guy J-P. Schumann, Dimitrios Stampoulis, Paul D. Bates, G. Robert Brakenridge, Albert J. Kettner
      Abstract: Floods are costly to global economies and can be exceptionally lethal. The ability to produce consistent flood hazard maps over large areas could provide a significant contribution to reducing such losses, as the lack of knowledge concerning flood risk is a major factor in the transformation of river floods into flood disasters. In order to accurately reproduce flooding in river channels and floodplains, high spatial resolution hydrodynamic models are needed. Despite being computationally expensive, recent advances have made their continental to global implementation feasible, although inputs for long-term simulations may require the use of reanalysis meteorological products especially in data-poor regions. We employ a coupled hydrologic/hydrodynamic model cascade forced by the 20CRv2 reanalysis dataset and evaluate its ability to reproduce flood inundation area and volume for Australia during the 1973-2012 period. Ensemble simulations using the reanalysis data were performed to account for uncertainty in the meteorology, and compared with a validated benchmark simulation. Results show that the reanalysis ensemble capture the inundated areas and volumes relatively well, with correlations for the ensemble mean of 0.82 and 0.85 for area and volume respectively, although the meteorological ensemble spread propagates in large uncertainty of the simulated flood characteristics.
      PubDate: 2017-10-04T21:10:50.329788-05:
      DOI: 10.1002/2017GL075502
       
  • Midlatitude summer drying : An underestimated threat in CMIP5 models'
    • Authors: H. Douville; M. Plazzotta
      Abstract: Early assessments of the hydrological impacts of global warming suggested both an intensification of the global water cycle and an expansion of dry areas. Yet, these alarming conclusions were challenged by a number of latter studies emphasizing the lack of evidence in observations and historical simulations, as well as the large uncertainties in climate projections from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Here, several aridity indices and a two-tier attribution strategy are used to demonstrate that a summer mid-latitude drying has recently emerged over the northern continents, which is mainly attributable to anthropogenic climate change. This emerging signal is shown to be the harbinger of a long-term drying in the CMIP5 projections. Linear trends in the observed aridity indices can therefore be used as observational constraints and suggest that the projected mid-latitude summer drying was underestimated by most CMIP5 models. Mitigating global warming therefore remains a priority to avoid dangerous impacts on global water and food security.
      PubDate: 2017-10-04T21:10:30.566721-05:
      DOI: 10.1002/2017GL075353
       
  • Illuminating the voluminous subsurface structures of Old Faithful geyser,
           Yellowstone national Park
    • Authors: Shaul Hurwitz; David R. Shelly
      Abstract: Old Faithful geyser in Yellowstone National Park has attracted scientific research for almost a century and a half. Temperature and pressure measurements and video recordings in the geyser's conduit led to proposals of many quantitative eruption models. Nevertheless, information on the processes that initiate the geyser's eruption in the subsurface remained limited. Two new studies, specifically Wu et al. (2017) and Ward and Lin (2017) take advantage of recent developments in seismic data-acquisition technology and processing methods to illuminate subsurface structures. Using a dense array of three-component nodal geophones, these studies delineate subsurface structures on a scale larger than previously realized, which exert control on the spectacular eruptions of Old Faithful geyser.
      PubDate: 2017-10-03T17:21:03.469647-05:
      DOI: 10.1002/2017GL075833
       
  • Anatomy of Old Faithful from subsurface seismic imaging of the Yellowstone
           Upper Geyser Basin
    • Authors: Sin-Mei Wu; Kevin M. Ward, Jamie Farrell, Fan-Chi Lin, Marianne Karplus, Robert B. Smith
      Abstract: The Upper Geyser Basin in Yellowstone National Park contains one of the highest concentrations of hydrothermal features on Earth including the iconic Old Faithful geyser. Although this system has been the focus of many geological, geochemical, and geophysical studies for decades, the shallow (
      PubDate: 2017-10-03T17:21:01.768061-05:
      DOI: 10.1002/2017GL075255
       
  • Projected response of low-level convergence and associated precipitation
           to greenhouse warming
    • Authors: Evan Weller; Christian Jakob, Michael J. Reeder
      Abstract: The parameterization of convection in climate models is a large source of uncertainty in projecting future precipitation changes. Here, an objective method to identify organized low-level convergence lines has been used to better understand how atmospheric convection is organized and projected to change, as low-level convergence plays an important role in the processes leading to precipitation. The frequency and strength of convergence lines over both ocean and land in current climate simulations is too low compared to reanalysis data. Projections show a further reduction in the frequency and strength of convergence lines over the mid-latitudes. In the tropics, the largest changes in frequency are generally associated with shifts in major low-latitude convergence zones, consistent with changes in the precipitation. Further, examining convergence lines when in the presence or absence of precipitation results in large spatial contrasts, providing a better understanding of regional changes in terms of thermodynamic and dynamic effects.
      PubDate: 2017-10-03T17:20:47.235076-05:
      DOI: 10.1002/2017GL075489
       
  • The Response of Local Precipitation and Sea Level Pressure to Hadley Cell
           Expansion
    • Authors: Daniel F. Schmidt; Kevin M. Grise
      Abstract: Numerous lines of observational evidence suggest that Earth's tropical belt has expanded over the past 30–40 years. It is natural to expect that this poleward displacement should be associated with drying on the poleward margins of the subtropics, but it is less clear to what degree the drying should be zonally symmetric. This study tests the degree to which poleward motion of the Hadley cell boundary is associated with changes in local precipitation or sea level pressure, and the degree to which those changes are zonally symmetric. Evidence from both reanalysis data and global climate models reveals that the local changes associated with Hadley cell expansion are mostly confined to certain centers of action which lie primarily over oceans. Consequently, the tropical expansion measured by zonally averaged variables is not associated with systematic drying over subtropical land regions, as is often assumed.
      PubDate: 2017-10-03T17:20:35.870485-05:
      DOI: 10.1002/2017GL075380
       
  • Australia's unprecedented future temperature extremes under Paris limits
           to warming
    • Authors: Sophie C. Lewis; Andrew D. King, Daniel M. Mitchell
      Abstract: Record-breaking temperatures can detrimentally impact ecosystems, infrastructure, and human health. Previous studies show that climate change has influenced some observed extremes, which are expected to become more frequent under enhanced future warming. Understanding the magnitude, as a well as frequency, of such future extremes is critical for limiting detrimental impacts. We focus on temperature changes in Australian regions, including over a major coral reef-building area, and assess the potential magnitude of future extreme temperatures under Paris Agreement global warming targets (1.5°C and 2°C). Under these limits to global mean warming, we determine a set of projected high-magnitude unprecedented Australian temperature extremes. These include extremes unexpected based on observational temperatures, including current record-breaking events. For example, while the difference in global-average warming during the hottest Australian summer and the 2°C Paris target is 1.1°C, extremes of 2.4°C above the observed summer record are simulated. This example represents a more than doubling of the magnitude of extremes, compared with global mean change, and such temperatures are unexpected based on the observed record alone. Projected extremes do not necessarily scale linearly with mean global warming and this effect demonstrates the significant potential benefits of limiting warming to 1.5°C, compared to 2°C or warmer.
      PubDate: 2017-10-03T09:00:01.815155-05:
      DOI: 10.1002/2017GL074612
       
  • Turbulent entrainment into volcanic plumes: New constraints from
           laboratory experiments on buoyant jets rising in a stratified crossflow
    • Authors: T. J. Aubry; G. Carazzo, A. M. Jellinek
      Abstract: Predictions for the heights and downwind trajectories of volcanic plumes using integral models are critical for the assessment of risks and climate impacts of explosive eruptions, but are strongly influenced by parameterizations for turbulent entrainment. We compare four popular parameterizations using small scale laboratory experiments spanning the large range of dynamical regimes in which volcanic eruptions occur. We reduce uncertainties on the wind entrainment coefficient β which quantifies the contribution of wind-driven radial velocity shear to entrainment and is a major source of uncertainty for predicting plume height. We show that models better predict plume trajectories if: i) β is constant or increases with the plume buoyancy to momentum flux ratio; ii) the superposition of the axial and radial velocity shear contributions to the turbulent entrainment is quadratic rather than linear. Our results have important implications for predicting the heights and likelihood of collapse of volcanic columns.
      PubDate: 2017-10-02T17:45:52.875902-05:
      DOI: 10.1002/2017GL075069
       
  • On the yield strength of oceanic lithosphere
    • Authors: Chhavi Jain; Jun Korenaga, Shun-ichiro Karato
      Abstract: The yield strength of oceanic lithosphere determines the mode of mantle convection in a terrestrial planet, and low-temperature plasticity in olivine aggregates is generally believed to govern the plastic rheology of the stiffest part of lithosphere. Because so far proposed flow laws for this mechanism exhibit nontrivial discrepancies, we revisit the recent high-pressure deformation data of ('M10) with a comprehensive inversion approach based on Markov chain Monte Carlo sampling. Our inversion results indicate that the uncertainty of the relevant flow-law parameters is considerably greater than previously thought. Depending on the choice of flow-law parameters, the strength of oceanic lithosphere would vary substantially, carrying different implications for the origin of plate tectonics on Earth. To reduce the flow-law ambiguity, we suggest that it is important to establish a theoretical basis for estimating macroscopic stress in high-pressure experiments, and also to better utilize marine geophysical observations.
      PubDate: 2017-10-02T17:30:40.446292-05:
      DOI: 10.1002/2017GL075043
       
  • Stochastic dynamics of intermittent pore-scale particle motion in
           three-dimensional porous media: Experiments and theory
    • Authors: V. L. Morales; M. Dentz, M. Willmann, M. Holzner
      Abstract: We study the evolution of velocity in time, which fundamentally controls the way dissolved substances are transported and spread in porous media. Experiments are conducted that use tracer particles to track the motion of substances in water, as it flows through transparent, 3-D synthetic sandstones. Particle velocities along streamlines are found to be intermittent and strongly correlated, while their probability density functions are lognormal and nonstationary. We demonstrate that these particle velocity characteristics can be explained and modeled as a continuous time random walk that is both Markovian and mean reverting toward the stationary state. Our model accurately captures the fine-scale velocity fluctuations observed in each tested sandstone, as well as their respective dispersion regime progression from initially ballistic, to superdiffusive, and finally Fickian. Model parameterization is based on the correlation length and mean and standard deviation of the velocity distribution, thus linking pore-scale attributes with macroscale transport behavior for both short and long time scales.
      PubDate: 2017-09-29T15:40:01.118151-05:
      DOI: 10.1002/2017GL074326
       
  • Durable terrestrial bedrock predicts submarine canyon formation
    • Authors: M. Elliot Smith; Noah J. Finnegan, Erich R. Mueller, Rebecca J. Best
      Abstract: Though submarine canyons are first order topographic features of Earth, the processes responsible for their occurrence remains poorly understood. Potentially analogous studies of terrestrial rivers show that the flux and caliber of transported bedload are significant controls on bedrock incision. Here we hypothesize that coarse sediment load could exert a similar role in the formation of submarine canyons. We conducted a comprehensive empirical analysis of canyon occurrence along the west coast of the contiguous United States which indicates that submarine canyon occurrence is best predicted by the occurrence of durable crystalline bedrock in adjacent terrestrial catchments. Canyon occurrence is also predicted by the flux of bed sediment to shore from terrestrial streams. Surprisingly, no significant correlation was observed between canyon occurrence and the slope or width of the continental shelf. These findings suggest that canyon incision is promoted by a greater yields of durable terrestrial clasts to the shore.
      PubDate: 2017-09-26T17:46:12.812938-05:
      DOI: 10.1002/2017GL075139
       
  • Tidal response in shallow tectonic tremors
    • Authors: S. Katakami; Y. Yamashita, H. Yakihara, H. Shimizu, Y. Ito, K. Ohta
      Abstract: Various types of slow earthquakes (e.g., tectonic tremors and slow slip events) have been reported in tectonic zones, especially in the subduction zone. The tidal response of a tremor is considered to be strongly related to the weak friction state of the plate interface, and many studies have reported observational evidence of such correlation between tides and deep tremor activity. Here, we used the modified frequency scanning method at a single station to detect micro tectonic tremors that have not been previously reported in southern Kyushu. In the early stage of the tremor activity, tremors are mostly modulated by slow slip events. In contrast, we found a seismic response to ocean tides during the later stage in the shallower part of the subduction zone. This might indicate that the tremors are triggered by tidal changes caused by fault weakening due to slow slip events as same as deeper condition.
      PubDate: 2017-09-26T17:46:09.891787-05:
      DOI: 10.1002/2017GL074060
       
  • Expanding Greenland Ice Sheet Enhances Sensitivity of Plio-Pleistocene
           Climate to Obliquity Forcing in the Kiel Climate Model
    • Authors: Zhaoyang Song; Mojib Latif, Wonsun Park
      Abstract: Proxy data suggest the onset of Northern Hemisphere glaciation during the Plio-Pleistocene transition from 3.2 to 2.5 Ma resulted in enhanced climate variability at the obliquity (41 kyr) frequency. Here, we investigate the influence of the expanding Greenland ice sheet (GrIS) on the mean climate and obliquity-related variability in a series of climate model simulations. These suggest that an expanding GrIS weakens the Atlantic Meridional Overturning Circulation (AMOC) by ~1 Sv, mainly due to reduced heat loss in the Greenland-Iceland-Norwegian Sea. Moreover, the growing GrIS amplifies the Hadley circulation response to obliquity forcing driving variations in freshwater export from the tropical Atlantic and in turn variations of the AMOC. The stronger AMOC response to obliquity forcing, by about a factor of two, results in a stronger global-mean near-surface temperature response. We conclude that the AMOC response to obliquity forcing is important to understand the enhanced climate variability at the obliquity frequency during the Plio-Pleistocene transition.
      PubDate: 2017-09-26T17:46:05.322456-05:
      DOI: 10.1002/2017GL074835
       
  • Spatially variable geothermal heat flux in West Antarctica: evidence and
           implications
    • Authors: Carolyn Branecky Begeman; Slawek M. Tulaczyk, Andrew T. Fisher
      Abstract: Geothermal heat flux (GHF) is an important part of the basal heat budget of continental ice sheets. The difficulty of measuring GHF below ice sheets has directly hindered progress in understanding of ice sheet dynamics. We present a new GHF measurement from below the West Antarctic Ice Sheet, made in subglacial sediment near the grounding zone of the Whillans Ice Stream. The measured GHF is 88 ± 7 mW m-2, a relatively high value compared to other continental settings and to other GHF measurements along the eastern Ross Sea of 55 mW m-2 and 69 ± 21 mW m-2, but within the range of regional values indicated by geophysical estimates. The new GHF measurement was made ~100 km from the only other direct GHF measurement below the ice sheet, which was considerably higher at 285 ± 80 mW m-2, suggesting spatial variability that could be explained by shallow magmatic intrusions or the advection of heat by crustal fluids. Analytical calculations suggest that spatial variability in GHF exceeds spatial variability in the conductive heat flux through ice along the Siple Coast. Accurate GHF measurements and high-resolution GHF models may be necessary to reliably predict ice sheet evolution, including responses to ongoing and future climate change.
      PubDate: 2017-09-26T17:45:58.14547-05:0
      DOI: 10.1002/2017GL075579
       
  • Large Eddy Simulation of Flow and Turbulence at the Steep Topography of
           Luzon Strait
    • Authors: M. Jalali; S. Sarkar
      Abstract: Steep generation sites for topographic internal gravity waves can also be sites of turbulence. The present work uses turbulence-resolving Large Eddy Simulation (LES) for tidal flow over multiscale, steep bathymetry patterned after a portion of the west ridge of Luzon Strait at 20.6∘N. Oceanic values of the slope criticality, Froude number, excursion number, and aspect ratio are matched. The amplitude and phasing of velocity, overturns, and dissipation are similar to observations made at a deep mooring. However a tidal phase is noted in which there is a discrepancy that could arise from the resonant interaction with waves generated at the east ridge that is not included in the model. Turbulent dissipation originates from several mechanisms including breaking lee waves during flow reversal, downslope jets, critical-slope boundary layer, high-mode internal wave beams, off-slope lee wave breaking and wave breaking in a valley between subridges. The model baroclinic energy budget is closed with negligible residual and the local turbulence loss of 23.5% is large.
      PubDate: 2017-09-25T20:26:25.868442-05:
      DOI: 10.1002/2017GL074119
       
  • Systematic Observations of the Slip-pulse Properties of Large Earthquake
           Ruptures
    • Authors: Diego Melgar; Gavin P. Hayes
      Abstract: In earthquake dynamics there are two end member models of rupture: propagating cracks and self-healing pulses. These arise due to different properties of faults and have implications for seismic hazard; rupture mode controls near-field strong ground motions. Past studies favor the pulse-like mode of rupture; however, due to a variety of limitations, it has proven difficult to systematically establish their kinematic properties. Here, we synthesize observations from a database of>150 rupture models of earthquakes spanning M7-M9 processed in a uniform manner and show the magnitude scaling properties of these slip pulses indicates self-similarity. Further, we find that large and very large events are statistically distinguishable relatively early (at ~15 seconds) in the rupture process. This suggests that with dense regional geophysical networks strong ground motions from a large rupture can be identified before their onset across the source region.
      PubDate: 2017-09-25T14:10:26.972826-05:
      DOI: 10.1002/2017GL074916
       
  • Sensitivity of clay suspension rheological properties to pH, temperature,
           salinity and smectite-quartz ratio
    • Authors: Jun Kameda; Tomonori Morisaki
      Abstract: Understanding the rheological properties of clay suspensions is critical to assessing the behaviour of sediment gravity flows such as debris flow or turbidity current. We conducted rheological measurements of composite smectite–quartz suspensions at a temperature of 7 °C and a salt concentration of 0.6 M. This is representative of smectite-bearing sediments under conditions on the seafloor. The flow curves obtained were fitted by the Bingham fluid model, from which we determined the Bingham yield stress and dynamic viscosity of each suspension. At a constant smectite–quartz mixing ratio, the yield stress and the dynamic viscosity tend to increase as the solid/water ratio of the suspension is increased. In the case of a constant solid/water ratio, these values increase with increasing smectite content in the smectite–quartz mixture. Additional experiments exploring differing physicochemical conditions (pH 1.0–9.0; temperature 2–30 °C; electrolyte (NaCl) concentration 0.2–0.6 M) revealed that the influence of temperature is negligible, while pH moderately affects the rheology of the suspension. More significantly, the electrolyte concentration greatly affects the flow behaviour. These variations can be explained by direct and/or indirect (double-layer) interactions between smectite–smectite particles as well as between smectite–quartz particles in the suspension. Although smectite is known as a frictionally weak material, our experimental results suggest that its occurrence can reduce the likelihood that slope failure initiates. Furthermore, smectite can effectively suppress the spreading distance once the slope has failed.
      PubDate: 2017-09-25T14:10:23.250838-05:
      DOI: 10.1002/2017GL075334
       
  • Capturing the acoustic radiation pattern of strombolian eruptions using
           infrasound sensors aboard a tethered aerostat, Yasur volcano, Vanuatu
    • Authors: Arthur D. Jolly; Robin S. Matoza, David Fee, Ben M. Kennedy, Alexandra M. Iezzi, Rebecca H. Fitzgerald, Allison C. Austin, Richard Johnson
      Abstract: We obtained an unprecedented view of the acoustic radiation from persistent strombolian volcanic explosions at Yasur volcano, Vanuatu from the deployment of infrasound sensors attached to a tethered aerostat. While traditional ground-based infrasound arrays may sample only a small portion of the eruption pressure wavefield, we were able to densely sample angular ranges of ~200o in azimuth and ~50o in take-off angle by placing the aerostat at 38 tethered loiter positions around the active vent. The airborne data joined contemporaneously collected ground-based infrasound and video recordings over the period 29 July to 1 August 2016. We observe a persistent variation in the acoustic radiation pattern with average eastward-directed root-mean squared pressures more than 2 times larger than in other directions. The observed radiation pattern may be related to both path effects from the crater walls, and source directionality.
      PubDate: 2017-09-25T14:06:58.366999-05:
      DOI: 10.1002/2017GL074971
       
  • Removing Circulation Effects to Assess Central US Land-Atmosphere
           Interactions in the CESM Large Ensemble
    • Authors: Anna Merrifield; Flavio Lehner, Shang-Ping Xie, Clara Deser
      Abstract: Interannual variability of summer surface air temperature (SAT) in the central United States (US) is influenced by atmospheric circulation and land surface feedbacks. Here a method of dynamical adjustment is used to remove the effects of circulation on summer SAT variability over North America in the Community Earth System Model Large Ensemble. The residual SAT variability is shown to reflect thermodynamic feedbacks associated with land surface conditions. In particular, the central US is a "hotspot" of land-atmosphere interaction, with residual SAT accounting for more than half of the total SAT variability. Within the "hotspot", residual SAT anomalies show higher month-to-month persistence through the warm season and a redder spectrum than dynamically-induced SAT anomalies. Residual SAT variability in this region is also shown to be related to preseason soil moisture conditions, surface flux variability, and local atmospheric pressure anomalies.
      PubDate: 2017-09-22T17:40:39.426061-05:
      DOI: 10.1002/2017GL074831
       
  • Can measurements of the near-infrared solar spectral irradiance be
           reconciled' A new ground-based assessment between 4000-10000 cm-1
    • Authors: Jonathan Elsey; Marc D. Coleman, Tom Gardiner, Keith P. Shine
      Abstract: The near-infrared solar spectral irradiance (SSI) is of vital importance for understanding the Earth's radiation budget, and in Earth observation applications. Differences between previously published solar spectra (including the commonly-used ATLAS3 spectrum) reach up to 10% at the low-wavenumber end of the 4000-10000 cm-1 (2.5 – 1 μm) spectral region. The implications for the atmospheric sciences are significant, since this spectral region contains 25% of the incoming total solar irradiance. This work details an updated analysis of the CAVIAR SSI, featuring additional analysis techniques and an updated uncertainty budget using a Monte Carlo method. We report good consistency with ATLAS3 in the 7000-10000 cm-1 region where there is confidence in these results due to agreement with other spectra, but ~7% lower in the 4000-7000 cm-1 region, in general agreement with several other analyses.
      PubDate: 2017-09-22T17:40:35.827133-05:
      DOI: 10.1002/2017GL073902
       
  • A Madden-Julian Oscillation event remotely accelerates ocean upwelling to
           abruptly terminate the 1997/1998 super El Niño
    • Authors: T. Miyakawa; H. Yashiro, T. Suzuki, H. Tatebe, M. Satoh
      Abstract: The termination of the superintense 1997/1998 El Niño was extraordinarily abrupt. The May 1998 Madden-Julian Oscillation (MJO), a massive complex of stormy tropical clouds, is among possible contributors to the abrupt termination. Despite having been sensationally proposed 18 years ago, the role of the MJO remained controversial and speculative because of the difficulty of sufficiently simulating the El Niño and MJO simultaneously. An ensemble simulation series using a newly developed, fully ocean-coupled version of a global cloud system resolving numerical model replicated the specific atmosphere and ocean conditions of May 1998 in unprecedented detail, extending the prediction skill of the MJO to 46 days. Simulation ensemble members with stronger MJO activities over the Maritime Continent experienced quicker sea surface temperature drop in the eastern Pacific, confirming that the easterly winds associated with the remote MJO accelerated ocean upwelling to abruptly terminate the El Niño.
      PubDate: 2017-09-22T17:00:40.578052-05:
      DOI: 10.1002/2017GL074683
       
  • Post-stagnation retreat of Kamb Ice Stream's grounding zone
    • Authors: H. J. Horgan; C. Hulbe, R. B. Alley, S. Anandakrishnan, B. Goodsell, S. Taylor-Offord, M. J. Vaughan
      Abstract: Despite the importance of grounding zone sedimentation for ice sheet stability and ice sheet history, evidence for sedimentary deposits beneath West Antarctica's modern grounding zone remains sparse. Seismic surveying shows that Kamb Ice Stream has no detectable grounding zone deposit. As grounding zone deposition relies strongly on ice flow, the absence of a deposit suggests that the transition from the ice stream to the ice shelf has moved after stagnation of Kamb Ice Stream. Further support for a recent grounding-zone occupation comes from satellite imagery of sub ice-shelf channel features that likely originated at previous grounding-zone locations. These features begin 25 km seaward of the current grounding zone and cut across ice-flow streak lines. We estimate that retreat to the modern grounding-zone position was abrupt at rates>0.2 km a−1.
      PubDate: 2017-09-22T02:02:06.126113-05:
      DOI: 10.1002/2017GL074986
       
  • Bounce resonance scattering of radiation belt electrons by low-frequency
           hiss: Comparison with cyclotron and Landau resonances
    • Authors: Xing Cao; Binbin Ni, Danny Summers, Zhengyang Zou, Song Fu, Wenxun Zhang
      Abstract: Bounce-resonant interactions with magnetospheric waves have been proposed as important contributing mechanisms for scattering near-equatorially mirroring electrons by violating the second adiabatic invariant associated with the electron bounce motion along a geomagnetic field line. This study demonstrates that low-frequency plasmaspheric hiss with significant wave power below 100 Hz can bounce-resonate efficiently with radiation belt electrons. By performing quantitative calculations of pitch-angle scattering rates, we show that low-frequency hiss induced bounce-resonant scattering of electrons has a strong dependence on equatorial pitch-angle αeq. For electrons with αeq close to 90°, the timescale associated with bounce resonance scattering can be comparable to or even less than 1 hour. Cyclotron- and Landau-resonant interactions between low-frequency hiss and electrons are also investigated for comparisons. It is found that while the bounce and Landau resonances are responsible for the diffusive transport of near-equatorially mirroring electrons to lower αeq, pitch-angle scattering by cyclotron resonance could take over to further diffuse electrons into the atmosphere. Bounce resonance provides a more efficient pitch-angle scattering mechanism of relativistic (≥ 1 MeV) electrons than Landau resonance due to the stronger scattering rates and broader resonance coverage of αeq, thereby demonstrating that bounce resonance scattering by low-frequency hiss can contribute importantly to the evolution of the electron pitch-angle distribution and the loss of radiation belt electrons.
      PubDate: 2017-09-21T19:45:52.472611-05:
      DOI: 10.1002/2017GL075104
       
  • Investigating the capability to extract impulse response functions (IRFs)
           from ambient seismic noise using a mine collapse event
    • Authors: Sangmin Kwak; Seok Goo Song, Geunyoung Kim, Changsoo Cho, Jin Soo Shin
      Abstract: Using recordings of a mine collapse event (Mw 4.2) in South Korea in January 2015, we demonstrated that the phase and amplitude information of impulse response functions (IRFs) can be effectively retrieved using seismic interferometry. This event is equivalent to a single downward force at shallow depth. Using quantitative metrics, we compared three different seismic interferometry techniques—deconvolution, coherency and cross-correlation—to extract the IRFs between two distant stations with ambient seismic noise data. The azimuthal dependency of the source distribution of the ambient noise was also evaluated. We found that deconvolution is the best method for extracting IRFs from ambient seismic noise within the period band of 2 – 10 s. The coherency method is also effective if appropriate spectral normalization or whitening schemes are applied during the data processing.
      PubDate: 2017-09-21T19:41:18.509478-05:
      DOI: 10.1002/2017GL075532
       
  • Can the ocean's heat engine control horizontal circulation' Insights
           from the Caspian Sea
    • Authors: N. Bruneau; J. Zika, R. Toumi
      Abstract: We investigate the role of the ocean's heat engine in setting horizontal circulation using a numerical model of the Caspian Sea. The Caspian Sea can be seen as a virtual laboratory - a compromise between realistic global models which are hampered by long equilibration times and idealized basin geometry models which are not constrained by observations. We find that increases in vertical mixing drive stronger thermally direct overturning and consequent conversion of available potential to kinetic energy. Numerical solutions with water mass structures closest to observations overturn 0.02 − 0.04 x 106m3/s(Sv) representing the first estimate of Caspian Sea overturning. Our results also suggest that the overturning is thermally-forced increasing in intensity with increasing vertical diffusivity. Finally, stronger thermally direct overturning is associated with a stronger horizontal circulation in the Caspian Sea. This suggests the ocean's heat engine can strongly impact broader horizontal circulations in the ocean.
      PubDate: 2017-09-21T19:41:02.948234-05:
      DOI: 10.1002/2017GL075182
       
  • Enhanced Decadal Warming of the Southeast Indian Ocean during the Recent
           Global Surface Warming Slowdown
    • Authors: Yuanlong Li; Weiqing Han, Lei Zhang
      Abstract: The rapid Indian Ocean warming during the early-21th century was a major heat sink for the recent global surface warming slowdown. Analysis of observational data and ocean model experiments reveals that during 2003-2012 more than half of the increased upper Indian Ocean heat content was concentrated in the southeast Indian Ocean (SEIO), causing a warming “hotspot” of 0.8-1.2 K decade-1 near the west coast of Australia. This SEIO warming was primarily induced by the enhancements of the Pacific trade winds and Indonesian through-flow associated with the Interdecadal Pacific Oscillation (IPO)’s transition to its negative phase, and to a lesser degree by local atmospheric forcing within the Indian Ocean. Large-ensemble climate model simulations suggest that this warming event was likely also exacerbated by anthropogenic forcing and thus unprecedentedly strong as compared to previous IPO transition periods. Climate model projections suggest an increasing possibility of such strong decadal warming in future.
      PubDate: 2017-09-21T19:40:54.770841-05:
      DOI: 10.1002/2017GL075050
       
  • Relative humidity has uneven effects onshifts from snow to rain over the
           Western U.S.
    • Authors: A. A. Harpold; S. Rajagopal, J. B. Crews, T. Winchell, R. Schumer
      Abstract: Predicting the phase of precipitation is fundamental to water supply and hazard forecasting.Phaseprediction methods (PPMs) are used to predict snow fraction, or the ratio of snowfall to total precipitation. Common temperature-based regression (Dai method) and threshold at freezing (0 C)PPMs had comparable accuracyto a humidity-based PPM (TRHmethod)using 6- and 24-hour observations. Using a daily climate dataset from 1980-2015,the TRHmethodestimates14% and 6% greater precipitation-weighted snow fraction than the 0Cand Dai methods, respectively. The TRH method predicts four times less area with declining snow fraction than the Dai method (2.1% and 8.1% of the study domain, respectively) from 1980-2015, with the largest differences in the Cascade and Sierra Nevada mountains and Southwestern U.S. Future RCP8.5 projections suggest warming temperatures of 4.2°C and declining relative humidity of 1% over the 21st century. The TRHmethod predicts a smaller reduction in snow fraction than temperature-only PPMs by 2100, consistent withlower humidity bufferingdeclines insnow fraction caused byregional warming.
      PubDate: 2017-09-21T19:40:51.363011-05:
      DOI: 10.1002/2017GL075046
       
  • Rupture Depth-Varying Seismicity Patterns for Major and Great (MW ≥ 7.0)
           Megathrust Earthquakes
    • Authors: Nadav Wetzler; Thorne Lay, Emily E. Brodsky, Hiroo Kanamori
      Abstract: Large earthquakes on subduction zone plate boundary megathrusts result from intervals of strain accumulation and release. The mechanism diversity and spatial distribution of moderate size aftershocks is influenced by the mainshock rupture depth extent. Mainshocks that rupture across the shallow megathrust to near the trench have greater intraplate aftershock faulting diversity than events with rupture confined to deeper portions of the megathrust. Diversity of intraplate aftershock faulting also increases as the size of the mainshock approaches the largest size event to have ruptured that region of the megathrust. Based on these tendencies, we identify ‘breakthrough' ruptures as those involving shallow rupture of the megathrust with volumetrically extensive elastic strain drop around the plate boundary that allows activation of diverse intraplate faulting influenced by long-term ambient deformation stresses. In contrast, homogeneity of the aftershock faulting mechanisms indicates only partial release of elastic strain energy, and remaining potential for another large rupture.
      PubDate: 2017-09-21T19:40:45.692563-05:
      DOI: 10.1002/2017GL074573
       
  • Multi-decadal weakening of Indian summer monsoon circulation induces an
           increasing northern Indian Ocean sea level
    • Authors: P. Swapna; J. Jyoti, R. Krishnan, N. Sandeep, S. M. Griffies
      Abstract: North Indian Ocean sea level has shown significant increase during last 3-4 decades. Analyses of long-term climate datasets and ocean model sensitivity experiments identify a mechanism for multi-decadal sea level variability relative to global mean. Our results indicate that north Indian Ocean sea level rise is accompanied by a weakening summer monsoon circulation. Given that Indian Ocean meridional heat transport is primarily regulated by the annual cycle of monsoon winds, weakening of summer monsoon circulation has resulted in reduced upwelling off Arabia and Somalia and decreased southward heat-transport, and corresponding increase of heat storage in the north Indian Ocean. These changes in-turn lead to increased retention of heat and increased thermosteric sea level rise in the north Indian Ocean, especially in the Arabian Sea. These findings imply that rising north Indian Ocean sea level due to weakening of monsoon circulation demand adaptive strategies to enable a resilient South Asian population.
      PubDate: 2017-09-19T01:50:21.929774-05:
      DOI: 10.1002/2017GL074706
       
  • Driving Roles of Tropospheric and Stratospheric Thermal Anomalies in
           Intensification and Persistence of the Arctic Superstorm in 2012
    • Authors: Wei Tao; Jing Zhang, Yunfei Fu, Xiangdong Zhang
      Abstract: Intense synoptic-scale storms have been more frequently observed over the Arctic during recent years. Specifically, a superstorm hit the Arctic Ocean in August 2012 and preceded a new record low Arctic sea ice extent. In this study, the major physical processes responsible for the storm's intensification and persistence are explored through a series of numerical modeling experiments with the Weather Research and Forecasting model. It is found that thermal anomalies in troposphere as well as lower stratosphere jointly lead to the development of this superstorm. Thermal contrast between the unusually warm Siberia and the relatively cold Arctic Ocean results in strong troposphere baroclinicity and upper-level jet, which contribute to the storm intensification initially. On the other hand, Tropopause Polar Vortex (TPV) associated with the thermal anomaly in lower stratosphere further intensifies the upper-level jet and accordingly contributes to a drastic intensification of the storm. Stacking with the enhanced surface low, TPV intensifies further, which sustains the storm to linger over the Arctic Ocean for an extended period.
      PubDate: 2017-09-18T15:45:37.492504-05:
      DOI: 10.1002/2017GL074778
       
  • Wastewater disposal and the earthquake sequences during 2016 near
           Fairview, Pawnee, and Cushing, Oklahoma
    • Authors: A. McGarr; Andrew J. Barbour
      Abstract: Each of the three earthquake sequences in Oklahoma in 2016 – Fairview, Pawnee, and Cushing – appears to have been induced by high-volume wastewater disposal within 10 km. The Fairview M5.1 mainshock was part of a two-year sequence of more than 150 events of M3, or greater; the mainshock accounted for about half of the total moment. The foreshocks and aftershocks of the M5.8 Pawnee earthquake were too small and too few to contribute significantly to the cumulative moment; instead, nearly all of the moment induced by wastewater injection was focused on the mainshock. The M5.0 Cushing event is part of a sequence that includes 48 earthquakes of M3, or greater, that are mostly foreshocks. The cumulative moment for each of the three sequences during 2016, as well as that for the 2011 Prague, Oklahoma, and nine other sequences representing a broad range of injected volume, were all limited by the total volumes of wastewater injected locally.
      PubDate: 2017-09-18T15:40:27.563871-05:
      DOI: 10.1002/2017GL075258
       
  • Spatial Distribution and Properties of 0.1 – 100 keV Electrons in
           Jupiter's Polar Auroral Region
    • Authors: R. W. Ebert; F. Allegrini, F. Bagenal, S. J. Bolton, J. E. P. Connerney, G. Clark, G. R. Gladstone, V. Hue, W. S. Kurth, S. Levin, P. Louarn, B. H. Mauk, D. J. McComas, C. Paranicas, M. Reno, J. Saur, J. R. Szalay, M. F. Thomsen, P. Valek, S. Weidner, R. J. Wilson
      Abstract: We present observations of 0.1 – 100 keV electrons from Juno's Jovian Auroral Distributions Experiment Electron instrument (JADE-E) over Jupiter's polar auroral region for periods around four Juno perijoves (PJ1, PJ3, PJ4, PJ5). The observations reveal regions containing magnetic field aligned beams of bi-directional electrons having broad energy distributions interspersed between beams of upward electrons with narrow, peaked energy distributions, regions void of these electrons, and regions dominated by penetrating radiation. The electrons show evidence of acceleration via parallel electric fields (inverted-V structures) and via stochastic processes (bi-directional distributions). The inverted-V structures shown here were observed from ~1.4 – 2.9 RJ and had spatial scales of 100s to 1000s of kilometers along Juno's trajectory. The upward electron energy flux was typically greater than the downward flux, the latter ranging between ~0.01 – 5 mW m-2 for two cases shown here which we estimate could produce ~0.1 – 50 kilorayleighs of ultraviolet emission.
      PubDate: 2017-09-18T15:40:22.932411-05:
      DOI: 10.1002/2017GL075106
       
  • Wave-related Reynolds number parameterizations of CO2 and DMS transfer
           velocities
    • Authors: Sophia E. Brumer; Christopher J. Zappa, Byron W. Blomquist, Christopher W. Fairall, Alejandro Cifuentes-Lorenzen, James B. Edson, Ian M. Brooks, Barry J. Huebert
      Abstract: Predicting future climate hinges on our understanding of, and ability to quantify air-sea gas transfer. The latter relies on parameterizations of the gas transfer velocity k, which represents physical mass transfer mechanisms and is usually parameterized as a non-linear function of wind forcing. In an attempt to reduce uncertainties in k, this study explores empirical parameterizations that incorporate both wind speed and sea state dependence via wave-wind and breaking Reynolds numbers, RH and RB. Analysis of concurrent eddy covariance gas transfer and measured wave field statistics supplemented by wave model hindcasts shows for the first time that wave-related Reynold numbers collapse four open ocean datasets that have a wind speed dependence of CO2 transfer velocity ranging from lower than quadratic to cubic. Wave-related Reynolds number and wind speed show comparable performance for parametrizing DMS which, because of its higher solubility, is less affected by bubble-mediated exchange associated with wave breaking.
      PubDate: 2017-09-18T15:40:19.738569-05:
      DOI: 10.1002/2017GL074979
       
  • Quick release of internal water storage in a glacier leads to
           underestimation of the hazard potential of glacial lake outburst floods
           from Lake Merzbacher in central Tian shan Mountains
    • Authors: Donghui Shangguan; Yongjian Ding, Shiyin Liu, Zunyi Xie, Tino Pieczonka, Junli Xu, Bolot Moldobekov
      Abstract: Glacial meltwater and ice calving contribute to the flood volume of glacial lakes such as Lake Merzbacher in the Tian Shan Mountains of Central Asia. In this study, we simulated the lake's volume by constructing an empirical relationship between the area of Lake Merzbacher, determined from satellite images, and the lake's water storage, derived from Digital Elevation Models (DEMs). Results showed that the lake water supply rate before Glacial Lake Outburst Floods (GLOFs) generally agreed well with those during the GLOFs from 2009 to 2012, but not in 2008 and 2015. Furthermore, we found the combination of glacial meltwater and ice calving is not enough to fully explain the supply rate during GLOFs in 1996 and 1999, suggesting other factors affect the supply rate during GLOFs as well. To examine this further we compared the water supply rate before and during GLOF events in 1999 and 2008. We inferred that quickly released short-term and intermediate-term water storage by glaciers have likely contributed to both flood events in those years. This study highlights the need to improve our understanding of the supply component of outburst floods, such as; irregularly released stored water may lead to GLOF events with generally three different types: cases I (Singular event triggered englacial water release), case II (Glacier melt due to temperature changes), and case III (Englacial water release mixed with glacier melt).
      PubDate: 2017-09-18T15:38:23.684923-05:
      DOI: 10.1002/2017GL074443
       
  • Stress- and structure-induced anisotropy in Southern California from
           two-decades of shear-wave splitting measurements
    • Authors: Zefeng Li; Zhigang Peng
      Abstract: We measure shear-wave splitting (SWS) parameters (i.e. fast direction and delay time) using 330,000 local earthquakes recorded by more than 400 stations of the Southern California Seismic Network (1995-2014). The resulting 232,000 SWS measurements (90,000 high-quality ones) provide a uniform and comprehensive database of local SWS measurements in Southern California. The fast directions at many stations are consistent with regional maximum compressional stress σHmax. However, several regions show clear deviations from the σHmax directions. These include linear sections along the San Andreas Fault and the Santa Ynez Fault, geological blocks NW to the Los Angeles Basin, regions around the San Jacinto Fault, the Peninsular Ranges near San Diego, and the Coso volcanic field. These complex patterns show that regional stresses and active faults cannot adequately explain the upper crustal anisotropy in Southern California. Other types of local structures, such as local rock types or tectonic features, also play significant roles.
      PubDate: 2017-09-18T15:36:32.786552-05:
      DOI: 10.1002/2017GL075163
       
  • Seasonal noise vs. subseasonal signal: forecasts of California
           precipitation during the unusual winters of 2015-16 and 2016-17
    • Authors: Shuguang Wang; Alek Anichowski, Michael K. Tippett, Adam H. Sobel
      Abstract: Subseasonal forecasts of California precipitation during the unusual winters of 2015-16 and 2016-17 are examined in this study. It is shown that two different ensemble forecast systems were able to predict monthly precipitation anomalies in California during these periods with some skill in forecasts initialized near or at the start of the month. The unexpected anomalies in February 2016, as well as in January and February 2017, were associated with shifts in the position of the jet stream over the northeast Pacific in a manner broadly consistent with associations found in larger ensembles of forecasts. These results support the broader notion that what is unpredictable atmospheric noise at the seasonal time scale can become predictable signal at the subseasonal time scale, despite that the lead times and verification averaging times associated with these forecasts are outside the predictability horizons of canonical mid-range weather forecasting.
      PubDate: 2017-09-18T15:32:23.452687-05:
      DOI: 10.1002/2017GL075052
       
  • Quantifying the zonal-mean structure of tropical precipitation
    • Authors: M. Popp; N. J. Lutsko
      Abstract: The tropical zonal-mean precipitation in climate models is well known to have substantial biases such as an erroneous double intertropical convergence zone in the Pacific, but a comprehensive quantification of these biases is currently missing. Therefore, we introduce a set of nine indicators that fully characterize the position and magnitude of the tropical extrema in zonal-mean precipitation. An analysis of the CMIP-historical and AMIP simulations reveals large biases in the position and, especially, in the magnitude of the zonal-mean precipitation extrema in both sets of simulations relative to observations. We find some of the nine indicators to be correlated, and that the structure of tropical precipitation can be well represented using four indicators, though these indicators are different in AMIP and CMIP. Previously defined indicators can only partly explain the biases and so the more comprehensive terminology introduced here is a useful tool for characterizing tropical precipitation.
      PubDate: 2017-09-15T19:30:45.884997-05:
      DOI: 10.1002/2017GL075235
       
  • Effects of Australian Summer Monsoon on Sea Surface Temperature Diurnal
           Variation over the Australian North-Western Shelf
    • Authors: Xiao Hua Wang; Haifeng Zhang
      Abstract: Five-year (2010 to 2014) sea surface temperature (SST) data from the Advanced Very High Resolution Radiometer (AVHRR) are used to study the effects of the Australian summer monsoon on SST diurnal variation (DV) over the Australian North-Western shelf (NWS, defined as 105°E-125°E, 25°S-10°S). Strong DV events identified with amplitude of 0.5-2K were observed over the NWS. A double-peak seasonal pattern of DV is obtained, with the strongest DV occurring in February/March and October/November. This seasonal DV pattern over the NWS is largely due to the Australian summer monsoon, which reduces the easterly trade wind during the summer monsoonal period, favouring the development of SST DV. Since the monsoon region is distributed globally in the tropical oceans, in the western and eastern North Pacific, as well as in the southern Indian Ocean, we anticipate that strong SST DV may also exist in these parts of coastal oceans.
      PubDate: 2017-09-15T19:25:56.697243-05:
      DOI: 10.1002/2017GL075008
       
  • Measured horizontal temperature gradients constrain heat transfer
           mechanisms in Greenland ice
    • Authors: Benjamin H. Hills; Joel T. Harper, Neil F. Humphrey, Toby W. Meierbachtol
      Abstract: Ice in the ablation zone of the Greenland Ice Sheet is known to contain vertical temperature gradients that arise from conduction at the boundaries, the addition of strain and latent heat, and advective heat transport. A three-dimensional array of temperature measurements in a grid of boreholes reveals horizontal ice temperature gradients that challenge the present conceptualization of heat transfer. We measure two distinct types of temperature variability in the horizontal direction, one impacting a confined region where ice temperatures span a range of 5°C, and another with temperatures consistently varying by approximately 2°C across the entire 3D block. We suggest the first demonstrates the localized and limited nature of latent heat input, and the second demonstrates that vertical heat advection outpaces diffusion. These findings imply that ice flow is highly variable over sub-ice-thickness length scales, which in turn generates contrasts in ice temperature that may impact ice deformation and fracturing.
      PubDate: 2017-09-14T17:51:19.586526-05:
      DOI: 10.1002/2017GL074917
       
  • Complete implementation of the Green's Function Based Time Reverse Imaging
           and sensitivity analysis of reversed-time tsunami source inversion
    • Authors: M. J. Hossen; Phil Cummins, Kenji Satake
      Abstract: In recent studies, the Time Reverse Imaging (TRI) method has been implemented in tsunami science to estimate tsunami source models. A TRI algorithm called Green's Function Based Time Reverse Imaging (GFTRI) was previously developed to reconstruct the source model by using source inversion as a guide to appropriately scale time-reversed images of the tsunami source. In this article, we consider a more complete approach to source inversion using reversed time images of the tsunami source by considering cross-correlations among Green's functions. As a consequence, the performance of the method improves significantly. We tested this new algorithm using data from the 2011 Japan tsunami. Our results show that the method is capable of extracting more details of the source and providing excellent waveform fits at all stations, including those not used in the source imaging. We have also studied the sensitivity analysis of reversed-time tsunami source inversion and found that the method is less sensitive to the number of stations, once a minimum number of stations is utilized. Moreover, this new approach is able to estimate the tsunami source with reasonable accuracy using data available soon after an earthquake, which indicates that it has potential to be used in both near- and far-field tsunami forecasting.
      PubDate: 2017-09-13T15:24:57.535023-05:
      DOI: 10.1002/2017GL074528
       
  • Local scale advection of sensible and latent heat during snowmelt
    • Authors: Phillip Harder; John W. Pomeroy, Warren Helgason
      Abstract: The breakup of snowcover into patches during snowmelt leads to a dynamic, heterogeneous land surface composed of melting snow, and wet and dry soil and plant surfaces. Energy exchange with the atmosphere is therefore complicated by horizontal gradients in surface temperature and humidity as snow surface temperature and humidity are regulated by the phase change of melting snow unlike snow-free areas. Airflow across these surface transitions results in local-scale advection of energy that has been documented as sensible heat during snowmelt, while latent heat advection has received scant attention. Herein, results are presented from an experiment measuring near-surface profiles of air temperature and humidity across snow-free to snow-covered transitions that demonstrates that latent heat advection can be the same order of magnitude as sensible heat advection and is therefore an important source of snowmelt energy. Latent heat advection is conditional on an upwind source of water vapor from a wetted snow-free surface.
      PubDate: 2017-09-13T15:15:41.500884-05:
      DOI: 10.1002/2017GL074394
       
  • Long-lead prediction of the 2015 fire and haze episode in Indonesia
    • Authors: Dilshad Shawki; Robert D. Field, Michael K. Tippett, Bambang Hero Saharjo, Israr Albar, Dwi Atmoko, Apostolos Voulgarakis
      Abstract: We conducted a case study of NCEP CFSv2 seasonal model forecast performance over Indonesia in predicting the dry conditions in 2015 that led to severe fire, in comparison to the non-El Niño dry season conditions of 2016. Forecasts of the Drought Code (DC) component of Indonesia's Fire Danger Rating System were examined across the entire Equatorial Asia region and for the primary burning regions within it. Our results show that early warning lead times of high observed DC in September and October 2015 varied considerably for different regions. High DC over Southern Kalimantan and Southern New Guinea were predicted with 180-day lead times, whereas Southern Sumatra had lead times of up to only 60 days, which we attribute to the absence in the forecasts of an eastward decrease in Indian Ocean SSTs. This case study provides the starting point for longer-term evaluation of seasonal fire danger rating forecasts over Indonesia.
      PubDate: 2017-09-13T15:15:39.116619-05:
      DOI: 10.1002/2017GL073660
       
  • Local and remote impacts of atmospheric cloud radiative effects onto the
           eddy-driven jet
    • Authors: O. Watt-Meyer; D. M. W. Frierson
      Abstract: This study examines the cause of the spread of extratropical circulation responses to the inclusion of atmospheric cloud radiative effects (ACRE) across atmospheric general circulation models. The ensemble of Clouds On-Off Klimate Intercomparison Experiment aquaplanet simulations shows that these responses include both equatorward and poleward shifts of the eddy-driven jet of varying magnitudes. These disparate extratropical responses occur despite the relatively consistent response in the tropics: a heating in the upper troposphere, which leads to a strengthening of the Hadley cell. It is argued that the eddy-driven jet response is a competition between two effects: the local influence of clouds driving shifts of the jet through meridional gradients in ACRE and the remote impact of a strengthened Hadley cell causing an equatorward shift of the eddy-driven jet. Simulations in which cloud radiative effects are separately turned on in the tropics and extratropics demonstrate this explicitly.
      PubDate: 2017-09-13T15:00:31.965855-05:
      DOI: 10.1002/2017GL074901
       
  • Mechanistic drivers of re-emergence of anthropogenic carbon in the
           Equatorial Pacific
    • Authors: Ping Zhai; Keith B. Rodgers, Stephen M. Griffies, Richard D. Slater, Daniele Iudicone, Jorge L. Sarmiento, Laure Resplandy
      Abstract: Relatively rapid re-emergence of anthropogenic carbon (Cant) in the Equatorial Pacific is of potential importance for its impact on the carbonate buffering capacity of surface seawater, and thereby impeding the ocean's ability to further absorb Cant from the atmosphere. We explore the mechanisms sustaining Cant re-emergence (upwelling) from the thermocline to surface layers by applying water mass transformation diagnostics to a global ocean/sea-ice/biogeochemistry model. We find that the upwelling rate of Cant (0.4 PgC yr-1) from the thermocline to the surface layer is almost twice as large as air-sea Cant fluxes (0.203 PgC yr-1). The upwelling of Cant from the thermocline to the surface layer can be understood as a two-step process: the first being due to diapycnal diffusive transformation fluxes and the second due to surface buoyancy fluxes. We also find that this re-emergence of Cant decreases dramatically during the 1982/1983 and 1997/1998 El Niño events.
      PubDate: 2017-09-12T18:45:43.487526-05:
      DOI: 10.1002/2017GL073758
       
  • In-situ stress and pore pressure in the deep interior of the Nankai
           accretionary prism, IODP Site C0002
    • Authors: Hiroko Kitajima; Demian Saffer, Hiroki Sone, Harold Tobin, Takehiro Hirose
      Abstract: Strength and slip behaviors of subduction megathrusts are controlled by the physical properties and stress state not only of the fault zones themselves but also the adjacent wall rocks in overriding and subducting plates. In the Nankai Trough, a 3 km-deep riser borehole at Integrated Ocean Drilling Program Site C0002 was drilled into the Kumano forearc basin and the underlying accretionary prism. We quantify the full in-situ stress tensor and pore pressure at Site C0002 by integrating drilling data, geophysical logging, and downhole measurements using empirical relations that describe sediment compaction behavior as a function of loading path. The Kumano Basin is loaded in uniaxial vertical strain conditions, whereas the prism below is loaded with greater differential stress, has a modestly elevated pore pressure, and lies in a strike-slip faulting regime. This can be reconciled with slip on the megathrust fault if the horizontal stresses vary with time or depth, and/or if shear stress along the megathrust is low.
      PubDate: 2017-09-11T18:28:13.787584-05:
      DOI: 10.1002/2017GL075127
       
  • Oxygen ebullition from lakes
    • Authors: M. Koschorreck; I. Hentschel, B. Boehrer
      Abstract: The exchange of oxygen between lakes and the atmosphere is assumed to be driven by diffusion. Here we show that lakes can emit significant amounts of O2 by emerging gas bubbles - a process called ebullition. We found very high proportions of 17±10 % O2 (maximum34 %) in emerging gas bubbles in two shallow eutrophic reservoirs. In the studied reservoirs, O2 emission by ebullition was of similar magnitude as diffusive O2 fluxes. By re-analyzing previous studies we show that the process is ubiquitous and probably quantitatively relevant in many places. We present evidence that O2 in bubbles originates both from photosynthetic oxygen production and hence bubble formation in the oxic water as well as from stripping by emerging methane bubbles. Ebullition can turn lakes under saturated in respect to the atmosphere into a net O2 source. Neglecting O2 ebullition leads to an over estimation of lake internal respiration.
      PubDate: 2017-09-11T18:27:52.91965-05:0
      DOI: 10.1002/2017GL074591
       
  • Interannual variability of methane and nitrous oxide in the North Pacific
           Subtropical Gyre
    • Authors: Samuel T. Wilson; Sara Ferrón, David M. Karl
      Abstract: The temporal variability of two important greenhouse gases, methane (CH4) and nitrous oxide (N2O), is reported for the upper water column at Station ALOHA in the North Pacific Subtropical Gyre (NPSG). Measured concentrations of N2O conform to predicted values with an increase in saturation during the summer period. In contrast, CH4 is less predictable and shows an approximate two year transition from a state of oversaturation in surface waters to equilibrium values in 2015, implying a change in net CH4 production. The decrease in CH4 followed on from fluctuations in phosphate concentrations supporting the hypothesized link between microbial metabolism of phosphorus and the global biogeochemical cycle of CH4. At this current time, future trends in the net CH4 production in the NPSG are uncertain and specifically whether the surface ocean will be a net source or sink for CH4.
      PubDate: 2017-09-11T18:27:45.17961-05:0
      DOI: 10.1002/2017GL074458
       
  • Decadal modulation of repeating slow slip event activity in the
           southwestern Ryukyu Arc possibly driven by rifting episodes at the Okinawa
           Trough
    • Authors: Yoko Tu; Kosuke Heki
      Abstract: We studied 38 slow slip events (SSEs) in 1997-2016 beneath the Iriomote Island, southwestern Ryukyu Arc, Japan, using continuous Global Navigation Satellite System (GNSS) data. These SSEs occur biannually on the same fault patch at a depth of ~30 km on the subducting Philippine Sea Plate slab with average moment magnitudes (Mw) of ~6.6. Here we show that the slip accumulation rate (cumulative slip/lapse time) of these SSEs fluctuated over a decadal time scale. The rate increased twice around 2002 and 2013 concurrently with earthquake swarms in the Okinawa Trough. This suggests that episodic activations of the back-arc spreading at the Okinawa Trough caused extra southward movement of the block south of the trough and accelerated convergence at the Ryukyu Trench.
      PubDate: 2017-09-11T14:35:31.21715-05:0
      DOI: 10.1002/2017GL074455
       
  • Topography of the overriding plate during progressive subduction: A
           dynamic model to explain forearc subsidence
    • Authors: Zhihao Chen; Wouter P. Schellart, João C. Duarte, Vincent Strak
      Abstract: Overriding plate topography provides constrains on subduction zone geodynamics. We investigate its evolution using fully dynamic laboratory models of subduction with techniques of stereoscopic photogrammetry and Particle Image Velocimetry. Model results show that the topography is characterized by an area of forearc dynamic subsidence, with a magnitude scaling to 1.44-3.97 km in nature, and a local topographic high between the forearc subsided region and the trench. These topographic features rapidly develop during the slab free sinking phase and gradually decrease during the steady-state slab rollback phase. We propose that they result from the variation of the vertical component of the trench suction force along the subduction zone interface, which gradually increases with depth and results from the gradual slab steepening during the initial transient slab sinking phase. The downward mantle flow in the nose of the mantle wedge plays a minor role in driving forearc subsidence.
      PubDate: 2017-09-11T14:35:26.124726-05:
      DOI: 10.1002/2017GL074672
       
  • The influence of gravity on the displacement field produced by fault slip
    • Authors: Demián D. Gómez; Michael Bevis, Ernian Pan, Robert Smalley
      Abstract: We calculated surface displacements produced by a synthetic megathrust earthquake using two spherical, layered, elastic dislocation models which differ only in that one model accounts for the coupling between elasticity and gravity and the other does not. We show that including gravity perturbs the displacement field differently in the near-, medium- and far-fields. As a result, slip inversions based on an Earth model without gravity cannot simultaneously fit the near-, medium- and far-field displacements generated using a forward model including gravity. This suggests that the spatially systematic misfits between observations and dislocation predictions seen in the literature arise, at least in part, because these studies are based on models that neglect gravity. Although the magnitude of the far-field displacements is small compared to those of the near-field, our slip inversions show the most improvement when we both up-weight the far-field observations and use a physically consistent model in the inversion.
      PubDate: 2017-09-11T14:35:23.060823-05:
      DOI: 10.1002/2017GL074113
       
  • GPS Signal Corruption by the Discrete Aurora: Precise Measurements From
           the Mahali Experiment
    • Authors: Joshua Semeter; Sebastijan Mrak, Michael Hirsch, John Swoboda, Hasan Akbari, Gregory Starr, Don Hampton, Philip Erickson, Frank Lind, Anthea Coster, Victor Pankratius
      Abstract: Measurements from a dense network of GPS receivers have been used to clarify the relationship between substorm auroras and GPS signal corruption as manifested by loss-of-lock on the received signal. A network of nine receivers was deployed along roadways near the Poker Flat Research Range in central Alaska, with receiver spacing between 15 and 30 km. Instances of large-amplitude phase fluctuations and signal loss-of-lock were registered in space and time with auroral forms associated with a sequence of westward traveling surges associated with a substorm onset over central Canada. The following conclusions were obtained: 1) The signal corruption originated in the ionospheric E-region, between 100 and 150 km altitude, 2) The GPS links suffering loss-of-lock were confined to a narrow band (
      PubDate: 2017-09-11T14:30:41.863743-05:
      DOI: 10.1002/2017GL073570
       
  • Nanoscale roughness of natural fault surfaces controlled by
           scale-dependent yield strength
    • Authors: C. A. Thom; E. E. Brodsky, R. W. Carpick, G. M. Pharr, W. C. Oliver, D. L. Goldsby
      Abstract: Many natural fault surfaces exhibit remarkably similar scale-dependent roughness, which may reflect the scale-dependent yield strength of rocks. Using atomic force microscopy (AFM), we show that a sample of the Corona Heights Fault exhibits isotropic surface roughness well-described by a power law, with a Hurst exponent of 0.75 +/- 0.05 at all wavelengths from 60 nm to 10 μm. The roughness data and a recently proposed theoretical framework predict that yield strength varies with length scale as λ-0.25+/-0.05. Nanoindentation tests on the Corona Heights sample and another fault sample whose topography was previously measured with AFM (the Yair Fault) reveal a scale-dependent yield stress with power-law exponents of -0.12 +/- 0.06 and -0.18 +/- 0.08, respectively. These values are within one to two standard deviations of the predicted value, and provide experimental evidence that fault roughness is controlled by intrinsic material properties, which produces a characteristic surface geometry.
      PubDate: 2017-09-11T14:30:26.277624-05:
      DOI: 10.1002/2017GL074663
       
  • Low evapotranspiration enhances the resilience of peatland carbon stocks
           to fire
    • Authors: N. Kettridge; M. C. Lukenbach, K. J. Hokanson, C. Hopkinson, K. J. Devito, R. M. Petrone, C. A. Mendoza, J. M. Waddington
      Abstract: Boreal peatlands may be vulnerable to projected changes in the wildfire regime under future climates. Extreme drying during the sensitive post-fire period may exceed peatland ecohydrological resilience, triggering long-term degradation of these globally significant carbon stocks. Despite these concerns, we show low peatland evapotranspiration at both the plot and landscape scale post-fire, in water-limited peatlands dominated by feather moss that are ubiquitous across continental western Canada. Low post-fire evapotranspiration enhance the resilience of carbon stocks in such peatlands to wildfire disturbance and reinforces their function as a regional source or water. Near-surface water repellency may provide an important, previously unexplored, regulator of peatland evapotranspiration that can induce low evapotranspiration in the initial post-fire years by restricting the supply of water to the peat surface.
      PubDate: 2017-09-08T16:30:30.092839-05:
      DOI: 10.1002/2017GL074186
       
  • Ionospheric gravity waves driven by oceanic gravity waves in resonance: A
           modeling study in search of their spectra
    • Authors: Yonghui Yu; Michael P. Hickey
      Abstract: Ionospheric observations associated with the 2011 Tohoku tsunami have revealed gravity waves having spectral characteristics that depend on their proximity to the epicenter. There is a preponderance of medium scale waves in the vicinity of the epicenter, a significant bifurcation into short and long period waves over the Hawaiian archipelago, and a narrow and rich spectrum of waves over the west coast and inland of the United States (US). Guided by these previous observations, we consider wave sources as triads of nonlinearly interacting oceanic gravity waves, whose wave parameters satisfy resonant conditions. These waves are simulated using a 2-D nonlinear model describing gravity wave propagation in order to explain the observations of tsunamigenic traveling ionospheric disturbances (TIDs) associated with the Tohoku event.
      PubDate: 2017-09-08T16:30:27.339476-05:
      DOI: 10.1002/2017GL074417
       
  • Temperature covariance in tree-ring reconstructions and model simulations
           over the past millennium
    • Authors: C. T. M. Hartl-Meier; U. Büntgen, J. E. Smerdon, E. Zorita, P. J. Krusic, F. C. Ljungqvist, L. Schneider, J. Esper
      Abstract: Spatial covariance in the simulated temperature evolution over the past millennium has been reported to exceed that of multi-proxy-based reconstructions. Here, we use tree ring-based temperature reconstructions and state-of-the-art climate model simulations to assess temporal changes in Northern Hemisphere inter-continental temperature covariance during the last 1000 years. Tree ring-only approaches reveal stronger agreement with model simulations compared to multi-proxy networks. Although simulated temperatures exhibit a substantial spread among individual models, inter-continental temperature coherency is mainly driven by the cooling of large volcanic eruptions in 1257, 1452, 1600, and 1815 CE. The coherence of these synchronizing events appears to be elevated in several climate simulations relative to their own unforced covariance baselines and in comparison to the proxy reconstructions. This suggests that some models likely overestimate the amplitude of abrupt summer cooling in response to volcanic eruptions, particularly at larger spatial scales.
      PubDate: 2017-09-07T18:16:48.66876-05:0
      DOI: 10.1002/2017GL073239
       
  • Annual irrigation dynamics in the US Northern High Plains derived from
           Landsat satellite data
    • Authors: Jillian M. Deines; Anthony D. Kendall, David W. Hyndman
      Abstract: Sustainable management of agricultural water resources requires improved understanding of irrigation patterns in space and time. We produced annual, high resolution (30 m) irrigation maps for 1999-2016 by combining all available Landsat satellite imagery with climate and soil covariables in Google Earth Engine. Random forest classification had accuracies from 92-100% and generally agreed with county statistics (r2 = 0.88-0.96). Two novel indices which integrate plant greenness and moisture information show promise for improving satellite classification of irrigation. We found considerable interannual variability in irrigation location and extent, including a near doubling between 2002 and 2016. Statistical modeling suggested precipitation and commodity price influenced irrigated extent through time. High prices incentivized expansion to increase crop yield and profit, but dry years required greater irrigation intensity, thus reducing area in this supply-limited region. Datasets produced with this approach can improve water sustainability by providing consistent, spatially explicit tracking of irrigation dynamics over time.
      PubDate: 2017-09-07T18:16:35.347617-05:
      DOI: 10.1002/2017GL074071
       
  • Significant aerosol influence on the recent decadal decrease in tropical
           cyclone activity over the western North Pacific
    • Authors: Chiharu Takahashi; Masahiro Watanabe, Masato Mori
      Abstract: Over the past two decades, the number of Tropical cyclone (TCs) has decreased markedly in the southeastern part of the western North Pacific (WNP) as a component of the interdecadal variation. This decrease has partially been explained by an internal low-frequency variability of sea surface temperature (SST) in the Pacific, but influences of external forcing remain unclear. Here, we show that past changes in sulphate aerosol emissions contributed approximately 60 % of the observed decreasing trends in TC genesis frequency in the southeastern WNP for 1992–2011, using multiple simulations by a global climate model. This decrease was mainly attributed to the increased vertical wind shear and decreased low-level vorticity, associated with a trans-basin multidecadal SST change driven by aerosol forcing. The near-future projection shows that the aerosol forcing still has some potential influence on decadal TC change, but the projected decreasing frequency is mainly due to increasing greenhouse gases forcing.
      PubDate: 2017-09-05T18:41:20.671297-05:
      DOI: 10.1002/2017GL075369
       
  • Thaw depth determines dissolved organic carbon concentration and
           biodegradability on the northern Qinghai-Tibetan Plateau
    • Authors: C. C. Mu; B. W. Abbott, X. D. Wu, Q. Zhao, H. J. Wang, H. Su, S. F. Wang, T. G. Gao, H. Guo, X. Q. Peng, T. J. Zhang
      Abstract: The response of dissolved organic carbon (DOC) flux to permafrost degradation is one of the major sources of uncertainty in predicting the permafrost carbon feedback. We investigated DOC export and properties over two complete flow seasons in a catchment on the northern Qinghai-Tibetan Plateau. DOC concentration and biodegradability decreased systematically as thaw depth increased through the season, attributable to changing carbon sources and degree of microbial processing. Increasing DOC aromaticity and δ13C-DOC indicated shifts towards more recalcitrant carbon sources and greater residence time in soils prior to reaching the stream network. These strong and consistent seasonal trends suggest that gradual active layer deepening may decrease DOC export and biodegradability from permafrost catchments. Because these patterns are opposite observations from areas experiencing abrupt permafrost collapse (thermokarst), the overall impact of permafrost degradation on DOC flux and biodegradability may depend on the proportion of the landscape experiencing gradual thaw versus thermokarst.
      PubDate: 2017-09-05T18:40:48.975435-05:
      DOI: 10.1002/2017GL075067
       
  • Multiple-satellite observation of magnetic dip event during the substorm
           on 10 October, 2013
    • Authors: Zhaoguo He; Lunjin Chen, Hui Zhu, Zhiyang Xia, G. D. Reeves, Ying Xiong, Lun Xie, Yong Cao
      Abstract: We present a multiple-satellite observation of the magnetic dip event during the substorm on October 10, 2013. The observation illustrates the temporal and spatial evolution of the magnetic dip and gives a compelling evidence that ring current ions induce the magnetic dip by enhanced plasma beta. The dip moves with the energetic ions in a comparable drift velocity and affects the dynamics of relativistic electrons in the radiation belt. In addition, the magnetic dip provides a favorable condition for the EMIC wave generation based on the linear theory analysis. The calculated proton diffusion coefficients show that the observed EMIC wave can lead to the pitch angle scattering losses of the ring current ions, which in turn partially relax the magnetic dip in the observations. This study enriches our understanding of magnetic dip evolution and demonstrates the important role of the magnetic dip for the coupling of radiation belt and ring current.
      PubDate: 2017-09-05T18:36:09.377731-05:
      DOI: 10.1002/2017GL074869
       
  • Bullen's parameter as a seismic observable for spin crossovers in the
           lower mantle
    • Authors: Juan J. Valencia-Cardona; Quentin Williams, Gaurav Shukla, Renata M. Wentzcovitch
      Abstract: Elastic anomalies produced by the spin crossover in ferropericlase have been documented by both first principles calculations and high pressure-temperature experiments. The predicted signature of this spin crossover in the lower mantle is, however, subtle and difficult to geophysically observe within the mantle. Indeed, global seismic anomalies associated with spin transitions have not yet been recognized in seismologic studies of the deep mantle. A sensitive seismic parameter is needed to determine the presence and amplitude of such a spin crossover signature. The effects of spin crossovers on Bullen's parameter, η, are assessed here for a range of compositions, thermal profiles, and lateral variations in temperature within the lower mantle. Velocity anomalies associated with the spin crossover in ferropericlase span a depth range near 1,000 km for typical mantle temperatures. Positive excursions of Bullen's parameter with a maximum amplitude of ∼ 0.03 are calculated to be present over a broad depth range within the mid-to-deep lower mantle: these are largest for peridotitic and harzburgitic compositions. These excursions are highest in amplitude for model lower mantles with large lateral thermal variations, and with cold downwellings having longer lateral length-scales relative to hot upwellings. We conclude that predicted deviations in Bullen's parameter due to the spin crossover in ferropericlase for geophysically relevant compositions may be sufficiently large to resolve in accurate seismic inversions of this parameter, and could shed light on both the lateral variations in temperature at depth within the lower mantle, and the amount of ferropericlase at depth.
      PubDate: 2017-09-05T18:35:53.966883-05:
      DOI: 10.1002/2017GL074666
       
  • Generation of Highly Oblique Lower-band Chorus via Nonlinear Three-wave
           Resonance
    • Authors: Xiangrong Fu; S. Peter Gary, Geoffrey D. Reeves, Dan Winske, Jesse R. Woodroffe
      Abstract: Chorus in the inner magnetosphere has been observed frequently at geomagnetically active times, typically exhibiting a two-band structure with a quasi-parallel lower-band and an upper-band with a broad range of wave normal angles. But recent observations by Van Allen Probes confirm another type of lower-band chorus, which has a large wave normal angle close to the resonance cone angle. It has been proposed that these waves could be generated by a low-energy beam-like electron component or by temperature anisotropy of keV electrons in the presence of a low-energy plateau-like electron component. This paper, however, presents an alternative mechanism for generation of this highly oblique lower-band chorus. Through a nonlinear three-wave resonance, a quasi-parallel lower-band chorus wave can interact with a mildly oblique upper-band chorus wave, producing a highly oblique quasi-electrostatic lower-band chorus wave. This theoretical analysis is confirmed by 2D electromagnetic particle-in-cell simulations. Furthermore, as the newly generated waves propagate away from the equator, their wave normal angle can further increase and they are able to scatter low-energy electrons to form a plateau-like structure in the parallel velocity distribution. The three-wave resonance mechanism may also explain the generation of quasi-parallel upper-band chorus which has also been observed in the magnetosphere.
      PubDate: 2017-09-05T18:35:39.553635-05:
      DOI: 10.1002/2017GL074411
       
  • Periodic emission within Jupiter's main auroral oval
    • Authors: J. D. Nichols; T. K. Yeoman, E. J. Bunce, M. N. Chowdhury, S. W. H. Cowley, T. R. Robinson
      Abstract: We have discovered pulsating emission within Jupiter's main auroral oval, providing evidence of the auroral signature of jovian ULF wave processes. The form comprises a 1°×2° spot located directly on the main emission, whose intensity oscillates with a period of ∼10 min throughout the 45 min observation. The feature appears on the duskward edge of the discontinuity, maps to ∼13-14 h LT and ∼20-50 RJ, and rotates at around a half of rigid corotation. We show that the period of the oscillation is similar to the expected Alfvén travel time between the ionosphere and the upper edge of the equatorial plasma sheet in the middle magnetosphere, and we thus suggest that the pulsating aurora is driven by a mode confined to the low density region outside the plasma sheet. This significant new observation shows that Jupiter's auroras present an important remote sensing window on jovian magnetospheric wave processes.
      PubDate: 2017-09-01T17:58:19.76521-05:0
      DOI: 10.1002/2017GL074824
       
  • Acoustic and seismic fields of hydraulic jumps at varying Froude numbers
    • Authors: Timothy J. Ronan; Jonathan M. Lees, T. Dylan Mikesell, Jacob F. Anderson, Jeffrey B. Johnson
      Abstract: Mechanisms that produce seismic and acoustic wavefields near rivers are poorly understood because of a lack of observations relating temporally dependent river conditions to the near-river seismo-acoustic fields. This controlled study at the Harry W. Morrison Dam (HWMD) on the Boise River, Idaho explores how temporal variation in fluvial systems affects surrounding acoustic and seismic fields. Adjusting the configuration of the HWMD changed the river bathymetry and therefore the form of the standing wave below the dam. The HWMD was adjusted to generate four distinct wave regimes that were parameterized through their dimensionless Froude numbers (Fr) and observations of the ambient seismic and acoustic wavefields at the study site. To generate detectable and coherent signals a standing wave must exceed a threshold Fr value of 1.7, where a non-breaking undular jump turns into a breaking weak hydraulic jump. Hydrodynamic processes may partially control the spectral content of the seismic and acoustic energies. Furthermore, spectra related to reproducible wave conditions can be used to calibrate and verify fluvial seismic and acoustic models.
      PubDate: 2017-09-01T17:57:25.38111-05:0
      DOI: 10.1002/2017GL074511
       
  • Modulation of the seasonal cycle of Antarctic sea ice extent related to
           the Southern Annular Mode
    • Authors: Edward W. Doddridge; John Marshall
      Abstract: Through analysis of remotely-sensed sea surface temperature (SST) and sea ice concentration data we investigate the impact of winds related to the Southern Annular Mode (SAM) on sea ice extent around Antarctica. We show that positive SAM anomalies in the austral summer are associated with anomalously cold SSTs that persist and lead to anomalous ice growth in the following autumn, while negative SAM anomalies precede warm SSTs and a reduction in sea ice extent during autumn. The largest effect occurs in April, when a unit change in the detrended summertime SAM is followed by a 1.8±0.6 ×105 km2 change in detrended sea ice extent. We find no evidence that sea ice extent anomalies related to the summertime SAM affect the wintertime sea ice extent maximum. Our analysis shows that the wind anomalies related to the negative SAM during the 2016/17 austral summer contributed to the record minimum Antarctic sea ice extent observed in March 2017.
      PubDate: 2017-09-01T17:57:15.759741-05:
      DOI: 10.1002/2017GL074319
       
  • Issue Information
    • Pages: 9113 - 9115
      Abstract: No abstract is available for this article.
      PubDate: 2017-10-16T21:05:34.519571-05:
      DOI: 10.1002/grl.54868
       
  • Dynamic response of the Martian ionosphere to an interplanetary shock:
           Mars Express and MAVEN observations
    • Authors: Y. Harada; D. A. Gurnett, A. J. Kopf, J. S. Halekas, S. Ruhunusiri, C. O. Lee, T. Hara, J. Espley, G. A. DiBraccio, D. L. Mitchell, C. Mazelle, D. E. Larson, B. M. Jakosky
      Pages: 9116 - 9123
      Abstract: Multipoint observations from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on board Mars Express and the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission reveal a dynamic response of the Martian ionosphere to abrupt variations in the upstream solar wind plasma. On 2 February 2017, MAVEN, located upstream from the Martian bow shock, encountered a corotating interaction region-related interplanetary shock with a sudden enhancement in the dynamic pressure. MARSIS, operating in the upper ionosphere at ∼478 km altitudes and ∼78° solar zenith angles, observed a sharp increase in the local magnetic field magnitude ∼1 min after the shock passage at MAVEN. The time lag is roughly consistent with the expected propagation time of a pressure pulse from the bow shock to the upper ionosphere at the fast magnetosonic speed. Subsequently, remote soundings recorded disturbed signatures of the topside ionosphere below Mars Express.
      PubDate: 2017-09-18T05:20:37.22424-05:0
      DOI: 10.1002/2017GL074897
       
  • Aspect dependence of Langmuir parametric instability excitation observed
           by EISCAT
    • Authors: Xiang Wang; Chen Zhou
      Pages: 9124 - 9133
      Abstract: Recent studies of X-mode wave heating experiments observed by EISCAT (European Incoherent Scatter Scientific Association UHF radar indicate that parametric instability (PI) can be excited unexpectedly. In this paper, we reinvestigate the ionospheric heating experiment on 21 and 22 October 2012 in details. The observed HF-enhanced ion lines (HFILs) and HF-enhanced plasma lines (HFPLs) observed during the experiments present dependence on the heating incidence angles. The observational evidence and numerical calculation that the parallel electric field of X-mode heating wave can exceed the PI excitation threshold at certain angles indicate the decisive role of parallel electric field for the PI excitation. The excited HFPLs and HFILs during X-mode heating cycles also present the evidence for the non-Maxwellian distribution of the electrons in the heated region.
      PubDate: 2017-09-18T07:25:33.41486-05:0
      DOI: 10.1002/2017GL074743
       
  • Subauroral Polarization Streams (SAPS) Duration as Determined From Van
           Allen Probe Successive Electric Drift Measurements
    • Authors: Solène Lejosne; F. S. Mozer
      Pages: 9134 - 9141
      Abstract: We examine a characteristic feature of the magnetosphere-ionosphere coupling, namely, the persistent and latitudinally narrow bands of rapid westward ion drifts called the subauroral polarization streams (SAPS). Despite countless works on SAPS, information relative to their durations is lacking. Here we report on the first statistical analysis of more than 200 near-equatorial SAPS observations based on more than 2 years of Van Allen Probe electric drift measurements. First, we present results relative to SAPS radial locations and amplitudes. Then, we introduce two different ways to estimate SAPS durations. In both cases, SAPS activity is estimated to last for about 9 h on average. However, our estimates for SAPS duration are limited either by the relatively long orbital periods of the spacecraft or by the relatively small number of observations involved. Fifty percent of the events fit within the time interval [0;18] hours.
      PubDate: 2017-09-18T05:55:48.044645-05:
      DOI: 10.1002/2017GL074985
       
  • Responses of Quasi 2 Day Waves in the MLT Region to the 2013 SSW Revealed
           by a Meteor Radar Chain
    • Authors: Zheng Ma; Yun Gong, Shaodong Zhang, Qihou Zhou, Chunming Huang, Kaiming Huang, You Yu, Guozhu Li, Baiqi Ning, Chun Li
      Pages: 9142 - 9150
      Abstract: We present an analysis of the responses of quasi 2 day waves (QTDWs) to the 2013 sudden stratosphere warming (SSW) in the mesosphere and lower thermosphere (MLT) region. The study is based on data collected by a meteor radar chain along the 120°E meridian in the Northern Hemisphere which consists of four stations at Mohe (52.5°N, 122.3°E), Beijing (40.3°N, 116.2°E), Wuhan (30.5°N, 114.6°E), and Sanya (18.3°N, 109.6°E). It is the first time that an enhancement of the QTDW in the neutral wind during the 2013 SSW is observed in the midlatitudes in the Northern Hemisphere. During the SSW, the amplification of the QTDW in the low latitudes is the most prominent and the direction of the mean zonal wind shows clear reversion from eastward to westward. The consistent variations of the QTDWs and the mean neutral wind at the four stations are very likely associated with the SSW.
      PubDate: 2017-09-18T07:26:27.572455-05:
      DOI: 10.1002/2017GL074597
       
  • Experimental Evidence of Arctic Summer Mesospheric Upwelling and Its
           Connection to Cold Summer Mesopause
    • Authors: F. I. Laskar; J. L. Chau, J. P. St.-Maurice, G. Stober, C. M. Hall, M. Tsutsumi, J. Höffner, P. Hoffmann
      Pages: 9151 - 9158
      Abstract: Common volume mesospheric meteor detections from two radar stations separated by about 130 km were used to retrieve horizontal wind fields between 82 and 96 km altitudes at high latitudes, near 69°N. The horizontal wind divergence was estimated from the gradients of the wind fields. This determination is the first of its kind for the mesosphere. Twelve years of nearly continuous data sets reveal systematic summer signatures in the horizontal wind divergence field, namely, a transition from negative to positive with increasing altitude while moving across the mesopause. There are indications that the reversal altitude is also anticorrelated with the mesopause temperature in addition for a tendency for the altitude of the reversal to increase over the years. We show that the reversal in the horizontal wind divergence at the mesosphere is consistent with upward winds peaking near the mesopause. These winds indicate that adiabatic cooling was strongest at the region of the deep temperature minimum seen in the summer mesopause.
      PubDate: 2017-09-22T15:50:29.826787-05:
      DOI: 10.1002/2017GL074759
       
  • What Happens Before a Southward IMF Turning Reaches the Magnetopause'
    • Authors: A. A. Samsonov; D. G. Sibeck, N. P. Dmitrieva, V. S. Semenov
      Pages: 9159 - 9166
      Abstract: Previous observations have shown a ∼10–15 min time delay in the ionospheric response to solar wind directional discontinuities marked by either southward or northward interplanetary magnetic field (IMF) turnings. We have studied one southward IMF turning observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) and GOES in the dayside magnetosphere. Using a global MHD model, we have reproduced the magnetopause motion in this event. We find that the observed delay in the ground response can be completely explained by deceleration of the directional discontinuity in the subsolar magnetosheath. We show that the speed of the discontinuity significantly decreases in the vicinity of the magnetopause where the magnetic barrier formed during the previous northward IMF interval. The southward turning can reach the magnetopause only after complete disruption of the magnetic barrier. The disruption or dissipation occurs via magnetosheath reconnection, as confirmed by high-speed jets in the magnetosheath. The magnetopause moves sunward as the directional discontinuity transits the magnetosheath. This sunward motion is followed by the earthward motion when the discontinuity strikes the magnetopause and magnetopause reconnection begins.
      PubDate: 2017-09-22T16:10:31.186801-05:
      DOI: 10.1002/2017GL075020
       
  • Global Distribution of Nighttime Medium-Scale Traveling Ionospheric
           Disturbances Seen by Swarm Satellites
    • Authors: Hyosub Kil; Larry J. Paxton
      Pages: 9176 - 9182
      Abstract: The activity of nighttime medium-scale traveling ionospheric disturbance (MSTID) in middle latitudes (±25°–40° magnetic latitudes) is investigated using the measurements of the electron density by Swarm satellites. Fractional electron density fluctuation is used as a proxy for MSTIDs, and the MSTID distribution is derived from the observations in December 2013–January 2017. The MSTID activity shows a semiannual variation in most longitudes with the primary and secondary peaks during the June and December solstices, respectively. The distinguishing feature in the MSTID distribution is the high occurrence rate of MSTIDs around 150°E longitude in June–August. The longitudinal and annual distribution of MSTIDs shows a hemispheric symmetry, which we interpret as the demonstration of conjugacy in MSTIDs.
      PubDate: 2017-09-22T16:25:30.191069-05:
      DOI: 10.1002/2017GL074750
       
  • Radar sounder evidence of thick, porous sediments in Meridiani Planum and
           implications for ice-filled deposits on Mars
    • Authors: Thomas R. Watters; Carl J. Leuschen, Bruce A. Campbell, Gareth A. Morgan, Andrea Cicchetti, John A. Grant, Roger J. Phillips, Jeffrey J. Plaut
      Pages: 9208 - 9215
      Abstract: Meridiani Planum is one of the most intensely studied regions on Mars, yet little is known about the physical properties of the deposits below those examined by the Opportunity rover. We report the detection of subsurface echoes within the Meridiani Planum deposits from data obtained by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument. The delay time between the surface and subsurface returns is indicative of materials with a real dielectric constant of 3.6 ± 0.6. The real dielectric constant is strongly modulated by bulk density. Newly derived compaction relationships for Mars indicate that the relatively low dielectric constant of the Meridiani Planum deposits is consistent with a thick layer of ice-free, porous, basaltic sand. The unique physiographic and hydrologic setting of Meridiani Planum may have provided an ideal sediment trap for eolian sands. The relatively low gravity and the cold, dry climate that has dominated Mars for billions of years may have allowed thick eolian sand deposits to remain porous and only weakly indurated. Minimally compacted sedimentary deposits may offer a possible explanation for other nonpolar region units with low apparent bulk dielectric constants.
      PubDate: 2017-09-19T09:00:02.807431-05:
      DOI: 10.1002/2017GL074431
       
  • Newly Discovered Ring-Moat Dome Structures in the Lunar Maria: Possible
           Origins and Implications
    • Authors: Feng Zhang; James W. Head, Alexander T. Basilevsky, Roberto Bugiolacchi, Goro Komatsu, Lionel Wilson, Wenzhe Fa, Meng-Hua Zhu
      Pages: 9216 - 9224
      Abstract: We report on a newly discovered morphological feature on the lunar surface, here named Ring-Moat Dome Structure (RMDS). These low domes (a few meters to ~20 m height with slopes
      PubDate: 2017-09-19T02:42:15.185972-05:
      DOI: 10.1002/2017GL074416
       
  • Io-Jupiter decametric arcs observed by Juno/Waves compared to ExPRES
           simulations
    • Authors: C. K. Louis; L. Lamy, P. Zarka, B. Cecconi, M. Imai, W. S. Kurth, G. Hospodarsky, S. L. G. Hess, X. Bonnin, S. Bolton, J. E. P. Connerney, S. M. Levin
      Pages: 9225 - 9232
      Abstract: We compare observations from the Juno/Waves radio experiment with simulations of radio «arcs» in the time-frequency plane resulting from the Io-Jupiter interaction, performed with the ExPRES code. We identify the hemisphere of origin of the observed arcs directly from simulations and confirm this identification through comparison with Juno, Nançay, and Wind observations. The occurrence and shape of observed arcs are well modeled, at low latitudes with their usual shapes as seen from Earth, as well as at high latitudes with longer, bowl-shaped, arcs observed for the first time. Predicted emission is actually observed only when the radio beaming angle θ = (k,B) ≥ 70° ± 5°, providing new constraints on the generation of the decameter emission by the Cyclotron Maser Instability. Further improvements of ExPRES are outlined, which will then be applied to Juno and Earth-based observations of radio emissions induced by other Galilean satellites or associated to the main auroral oval.
      PubDate: 2017-09-22T12:40:42.665374-05:
      DOI: 10.1002/2017GL073036
       
  • New evidence for surface water ice in small-scale cold traps and in three
           large craters at the north polar region of Mercury from the Mercury Laser
           Altimeter
    • Authors: Ariel N. Deutsch; Gregory A. Neumann, James W. Head
      Pages: 9233 - 9241
      Abstract: The Mercury Laser Altimeter (MLA) measured surface reflectance, rs, at 1064 nm. On Mercury, most water-ice deposits have anomalously low rs values indicative of an insulating layer beneath which ice is buried. Previous detections of surface water ice (without an insulating layer) were limited to seven possible craters. Here we map rs in three additional permanently shadowed craters that host radar-bright deposits. Each crater has a mean rs value>0.3, suggesting that water ice is exposed at the surface without an overlying insulating layer. We also identify small-scale cold traps (0.3 and permanent shadows have biannual maximum surface temperatures
      PubDate: 2017-09-22T15:26:26.917496-05:
      DOI: 10.1002/2017GL074723
       
  • Thermal State, Slab Metamorphism, and Interface Seismicity in the Cascadia
           Subduction Zone Based On 3-D Modeling
    • Authors: Yingfeng Ji; Shoichi Yoshioka, Yuval A. Banay
      Pages: 9242 - 9252
      Abstract: Giant earthquakes have repeatedly ruptured the Cascadia subduction zone, and similar earthquakes will likely also occur there in the near future. We employ a 3-D time-dependent thermomechanical model that incorporates an up-to-date description of the slab geometry to study the Cascadia subduction thrust. Results show a distinct band of 3-D slab dehydration that extends from Vancouver Island to the Seattle Basin and farther southward to the Klamath Mountains in northern California, where episodic tremors cluster. This distribution appears to include a region of increased dehydration in northern Cascadia. The phenomenon of heterogeneous megathrust seismicity associated with oblique subduction suggests that the presence of fluid-rich interfaces generated by slab dehydration favors megathrust seismogenesis in the northern part of this zone. The thin, relatively weakly metamorphosed Explorer, Juan de Fuca, and Gorda Plates are associated with an anomalous lack of thrust earthquakes, and metamorphism that occurs at temperatures of 500–700°C near the Moho discontinuity may represent a key factor in explaining the presence of the associated episodic tremor and slip (ETS), which requires a young oceanic plate to subduct at a small dip angle, as is the case in Cascadia and southwestern Japan. The 3-D intraslab dehydration distribution suggests that the metamorphosed plate environment is more complex than had previously been believed, despite the existence of channeling vein networks. Slab amphibolization and eclogitization near the continental Moho depth is thus inferred to account for the resultant overpressurization at the interface, facilitating the generation of ETS and the occurrence of small to medium thrust earthquakes beneath Cascadia.
      PubDate: 2017-09-18T06:41:43.162691-05:
      DOI: 10.1002/2017GL074826
       
  • Ancient Continental Lithosphere Dislocated Beneath Ocean Basins Along the
           Mid-Lithosphere Discontinuity: A Hypothesis
    • Authors: Zhensheng Wang; Timothy M. Kusky, Fabio A. Capitanio
      Pages: 9253 - 9260
      Abstract: The documented occurrence of ancient continental cratonic roots beneath several oceanic basins remains poorly explained by the plate tectonic paradigm. These roots are found beneath some ocean-continent boundaries, on the trailing sides of some continents, extending for hundreds of kilometers or farther into oceanic basins. We postulate that these cratonic roots were left behind during plate motion, by differential shearing along the seismically imaged mid-lithosphere discontinuity (MLD), and then emplaced beneath the ocean-continent boundary. Here we use numerical models of cratons with realistic crustal rheologies drifting at observed plate velocities to support the idea that the mid-lithosphere weak layer fostered the decoupling and offset of the African continent's buoyant cratonic root, which was left behind during Meso-Cenozoic continental drift and emplaced beneath the Atlantic Ocean. We show that in some cratonic areas, the MLD plays a similar role as the lithosphere-asthenosphere boundary for accommodating lateral plate tectonic displacements.
      PubDate: 2017-09-18T06:15:58.657646-05:
      DOI: 10.1002/2017GL074686
       
  • Mapping subduction interface coupling using magnetotellurics: Hikurangi
           margin, New Zealand
    • Authors: W. Heise; T. G. Caldwell, S. Bannister, E. A. Bertrand, Y. Ogawa, S. L. Bennie, H. Ichihara
      Pages: 9261 - 9266
      Abstract: The observation of slow-slip, seismic tremor, and low-frequency earthquakes at subduction margins has provided new insight into the mechanisms by which stress accumulates between large subduction (megathrust) earthquakes. However, the relationship between the physical properties of the subduction interface and the nature of the controls on interplate seismic coupling is not fully understood. Using magnetotelluric data, we show in situ that an electrically resistive patch on the Hikurangi subduction interface corresponds with an area of increased coupling inferred from geodetic data. This resistive patch must reflect a decrease in the fluid or sediment content of the interface shear zone. Together, the magnetotelluric and geodetic data suggest that the frictional coupling of this part on the Hikurangi margin may be controlled by the interface fluid and sediment content: the resistive patch marking a fluid- and sediment-starved area with an increased density of small, seismogenic-asperities, and therefore a greater likelihood of subduction earthquake nucleation.
      PubDate: 2017-09-19T03:05:49.671519-05:
      DOI: 10.1002/2017GL074641
       
  • Machine Learning Predicts Laboratory Earthquakes
    • Authors: Bertrand Rouet-Leduc; Claudia Hulbert, Nicholas Lubbers, Kipton Barros, Colin J. Humphreys, Paul A. Johnson
      Pages: 9276 - 9282
      Abstract: We apply machine learning to data sets from shear laboratory experiments, with the goal of identifying hidden signals that precede earthquakes. Here we show that by listening to the acoustic signal emitted by a laboratory fault, machine learning can predict the time remaining before it fails with great accuracy. These predictions are based solely on the instantaneous physical characteristics of the acoustical signal and do not make use of its history. Surprisingly, machine learning identifies a signal emitted from the fault zone previously thought to be low-amplitude noise that enables failure forecasting throughout the laboratory quake cycle. We infer that this signal originates from continuous grain motions of the fault gouge as the fault blocks displace. We posit that applying this approach to continuous seismic data may lead to significant advances in identifying currently unknown signals, in providing new insights into fault physics, and in placing bounds on fault failure times.
      PubDate: 2017-09-22T16:40:33.210685-05:
      DOI: 10.1002/2017GL074677
       
  • Long-Term Interactions Between Intermediate Depth and Shallow Seismicity
           in North Chile Subduction Zone
    • Authors: Jorge Jara; Anne Socquet, David Marsan, Michel Bouchon
      Pages: 9283 - 9292
      Abstract: We document interactions between intermediate depth and interplate seismicity in the North Chile subduction zone, over a 25 year period (1990–2015). We show that the 2005 Mw 7.8 Tarapaca slab-pull earthquake was followed by 9 years of enhanced deep and shallow seismicity, together with the decrease of eastward average GPS velocities and associated interplate coupling, eventually leading to the 2014 Mw 8.1 Iquique megathrust earthquake. In contrast, megathrust ruptures (e.g., Mw 8.0 Antofagasta in 1995, or Mw 8.1 Iquique in 2014) initiate several years of silent background seismicity in the studied area, both at shallow and intermediate depths. The plunge of a rigid slab into a viscous asthenospheric mantle could explain the observed synchronization between deep and shallow seismicity and their long-term interactions.
      PubDate: 2017-09-22T16:21:05.227194-05:
      DOI: 10.1002/2017GL075029
       
  • Generic Algorithms for Estimating Foliar Pigment Content
    • Authors: Anatoly Gitelson; Alexei Solovchenko
      Pages: 9293 - 9298
      Abstract: Foliar pigment contents and composition are main factors governing absorbed photosynthetically active radiation, photosynthetic activity, and physiological status of vegetation. In this study the performance of nondestructive techniques based on leaf reflectance were tested for estimating chlorophyll (Chl) and anthocyanin (AnC) contents in species with widely variable leaf structure, pigment content, and composition. Only three spectral bands (green, red edge, and near-infrared) are required for nondestructive Chl and AnC estimation with normalized root-mean-square error (NRMSE) below 4.5% and 6.1%, respectively. The algorithms developed are generic, not requiring reparameterization for each species allowing for accurate nondestructive Chl and AnC estimation using simple handheld field/lab instrumentation. They also have potential in interpretation of airborne and satellite data.
      PubDate: 2017-09-22T16:15:25.926129-05:
      DOI: 10.1002/2017GL074799
       
  • Mapping fluids to subduction megathrust locking and slip behavior
    • Authors: Demian M. Saffer
      Pages: 9337 - 9340
      Abstract: In subduction zones, high fluid content and pore pressure are thought to promote aseismic creep, whereas well-drained conditions are thought to promote locking and failure in earthquakes. However, observations directly linking fluid content and seismic coupling remain elusive. Heise et al. (2017) use a magnetotelluric survey to image the electrical resistivity structure of the northern Hikurangi subduction thrust to ~30 km depth, as an indicator of interconnected fluid content. The authors document a clear correlation between high resistivity and a distinct geodetically locked patch and between conductive areas and weak coupling. Their study, together with other recent geophysical investigations, provides new evidence for the role of fluids in governing subduction thrust locking.
      PubDate: 2017-09-19T03:05:23.039314-05:
      DOI: 10.1002/2017GL075381
       
  • Spatial Patterns of Summer Speedup on South Central Alaska Glaciers
    • Authors: William H. Armstrong; Robert S. Anderson, Mark A. Fahnestock
      Pages: 9379 - 9388
      Abstract: Seasonal changes in glacier basal motion are attributable to variations in subglacial hydrology and cause variations in both ice discharge and glacier erosion. We develop a novel workflow based upon Landsat 8 feature tracking to document differences between spatial patterns of summer and winter glacier surface speed, which reflect changes in the distribution of basal motion. We identify and characterize summer speedups on 13 of 19 land-terminating glaciers in Alaska's Wrangell-St Elias Ranges. The speedups are relatively uniform over much of the ablation zones, and the speedup magnitudes vary by only a factor of ~2 between glaciers whose velocities span an order of magnitude. Summer speedups extend up to ~30 km up glacier from termini and often end at the bases of icefalls. These data provide systematic observation of the spatial pattern of enhanced summer glacier basal motion and suggest the possibility of its parameterization in glacier models.
      PubDate: 2017-09-18T06:06:06.32503-05:0
      DOI: 10.1002/2017GL074370
       
  • Anticyclonic precession of a plume in a rotating environment
    • Authors: D. Frank; J. R. Landel, S. B. Dalziel, P. F. Linden
      Pages: 9400 - 9407
      Abstract: Motivated by potential effects of the Earth's rotation on the Deepwater Horizon oil plume, we conducted laboratory experiments on saltwater point plumes in a homogeneous rotating environment across a wide range of Rossby numbers 0.02≤Ro≤1.3. We report a striking physical instability in the plume dynamics near the source: after approximately one rotation period, the plume tilts laterally and starts to precess anticyclonically. The mean precession frequency ω̄ scales linearly with the rotation rate Ω as ω̄≈0.4Ω. We find no evidence of a critical Rossby number above which precession ceases. We infer that a conventionally defined Rossby number is not an appropriate parameter when the plume is maintained over a long time: provided Ω ≠ 0, rotation is always important to the dynamics. This indicates that precession may occur in persistent oceanic or atmospheric plumes even at low latitudes.
      PubDate: 2017-09-18T07:22:03.466885-05:
      DOI: 10.1002/2017GL074191
       
  • Prefrontal bore mixing
    • Authors: Hans Haren; Gerard Duineveld, Henko Stigter
      Pages: 9408 - 9415
      Abstract: Rainbow Ridge, a 1950 m deep upthrusted ultramafic block along the axis of the Mid-Atlantic Ridge, has an active hydrothermal vent system at 2400 m on its western slope. However, within 1 km from the vent excessive temperatures are barely measurable, probably due to strong turbulent mixing. This mixing is studied here using a 400 m long high-resolution temperature sensor array moored with a 600 m ranging 75 kHz acoustic Doppler current profiler. Rich internal wave turbulence was recorded, characterized by 100–200 m upshoots and>200 m large overturning in particular near the end of the warming phase of the up and down moving tide. These highly nonlinear internal waves of tides interacting with buoyancy frequency waves extend up to 400 m above the sloping bottom of the ridge. While a turbulent “bottom boundary layer” could barely be defined, the more intense turbulence higher up in the water column is suggested to lead to the strong dispersion of the hydrothermal plume.
      PubDate: 2017-09-18T07:11:50.232605-05:
      DOI: 10.1002/2017GL074384
       
  • The Response of the Aegean Sea (Eastern Mediterranean) to the Extreme
           2016–2017 Winter
    • Authors: Dimitris Velaoras; Vassilis P. Papadopoulos, Harilaos Kontoyiannis, Dimitra K. Papageorgiou, Alexandra Pavlidou
      Pages: 9416 - 9423
      Abstract: The exceptionally cold December 2016 over the Aegean Sea—one of the most important dense water formation areas of the eastern Mediterranean Sea—resulted in winter heat loss comparable to the peak Eastern Mediterranean Transient winters (1992–1993). Hydrological data sampled in March/April 2017 showed that newly produced dense waters ventilated the North Aegean deep basins up to density horizons of σθ ~ 29.35 kg/m3. The water column ventilation was unable to reach the bottom as the present upper thermohaline circulation of the eastern Mediterranean does not favor the salinity preconditioning of the Aegean Sea. In the southwest Aegean, the Myrtoan basin was ventilated by dense waters traceable farther south to the West Cretan Straits. Export of these masses from the Aegean Sea can potentially have a broader impact on the thermohaline circulation of the eastern Mediterranean.
      PubDate: 2017-09-18T05:10:56.732678-05:
      DOI: 10.1002/2017GL074761
       
  • Bacteria-induced mixing in natural waters
    • Authors: T. Sommer; F. Danza, J. Berg, A. Sengupta, G. Constantinescu, T. Tokyay, H. Bürgmann, Y. Dressler, O. Sepúlveda Steiner, C. J. Schubert, M. Tonolla, A. Wüest
      Pages: 9424 - 9432
      Abstract: Swimming organisms can enhance mixing in their natural environments by creating eddies in their wake and by dragging water along. However, these mixing mechanisms are inefficient for microorganisms, because swimming-induced variations in velocity, temperature, and dissolved substances are evened out before they can be advected. In bioconvection, however, microorganisms induce water movement not by propulsion directly but by locally changing the fluid density, which drives convection. Observations of bioconvection have so far mainly been limited to laboratory settings. We report the first observation and quantification of bioconvection within a stratified natural water body. Using in situ measurements, laboratory experiments, and numerical simulations, we demonstrate that the bacterium Chromatium okenii is capable of mixing 0.3 to 1.2 m thick water layers at around 12 m water depth in the Alpine Lake Cadagno (Switzerland). As many species are capable of driving bioconvection, this phenomenon potentially plays a role in species distributions and influences large-scale phenomena like algal blooms.
      PubDate: 2017-09-22T16:46:10.538476-05:
      DOI: 10.1002/2017GL074868
       
  • Connecting tropical climate change with Southern Ocean heat uptake
    • Authors: Yen-Ting Hwang; Shang-Ping Xie, Clara Deser, Sarah M. Kang
      Pages: 9449 - 9457
      Abstract: Under increasing greenhouse gas forcing, climate models project tropical warming that is greater in the Northern than the Southern Hemisphere, accompanied by a reduction in the northeast trade winds and a strengthening of the southeast trades. While the ocean-atmosphere coupling indicates a positive feedback, what triggers the coupled asymmetry and favors greater warming in the northern tropics remains unclear. Far away from the tropics, the Southern Ocean (SO) has been identified as the major region of ocean heat uptake. Beyond its local effect on the magnitude of sea surface warming, we show by idealized modeling experiments in a coupled slab ocean configuration that enhanced SO heat uptake has a profound global impact. This SO-to-tropics connection is consistent with southward atmospheric energy transport across the equator. Enhanced SO heat uptake results in a zonally asymmetric La-Nina-like pattern of sea surface temperature change that not only affects tropical precipitation but also has influences on the Asian and North American monsoons.
      PubDate: 2017-09-22T15:10:45.45947-05:0
      DOI: 10.1002/2017GL074972
       
  • Lightning NOx Emissions: Reconciling Measured and Modeled Estimates With
           Updated NOx Chemistry
    • Authors: B. A. Nault; J. L. Laughner, P. J. Wooldridge, J. D. Crounse, J. Dibb, G. Diskin, J. Peischl, J. R. Podolske, I. B. Pollack, T. B. Ryerson, E. Scheuer, P. O. Wennberg, R. C. Cohen
      Pages: 9479 - 9488
      Abstract: Lightning is one of the most important sources of upper tropospheric NOx; however, there is a large spread in estimates of the global emission rates (2–8 Tg N yr−1). We combine upper tropospheric in situ observations from the Deep Convective Clouds and Chemistry (DC3) experiment and global satellite-retrieved NO2 tropospheric column densities to constrain mean lightning NOx (LNOx) emissions per flash. Insights from DC3 indicate that the NOx lifetime is ~3 h in the region of outflow of thunderstorms, mainly due to production of methyl peroxy nitrate and alkyl and multifunctional nitrates. The lifetime then increases farther downwind from the region of outflow. Reinterpreting previous analyses using the 3 h lifetime reduces the spread among various methods that have been used to calculate mean LNOx emissions per flash and indicates a global LNOx emission rate of ~9 Tg N yr−1, a flux larger than the high end of recent estimates.
      PubDate: 2017-09-18T05:05:27.510102-05:
      DOI: 10.1002/2017GL074436
       
  • Why Flash Type Matters: A Statistical Analysis
    • Authors: Retha M. Mecikalski; Phillip M. Bitzer, Lawrence D. Carey
      Pages: 9505 - 9512
      Abstract: While the majority of research only differentiates between intracloud (IC) and cloud-to-ground (CG) flashes, there exists a third flash type, known as hybrid flashes. These flashes have extensive IC components as well as return strokes to ground but are misclassified as CG flashes in current flash type analyses due to the presence of a return stroke. In an effort to show that IC, CG, and hybrid flashes should be separately classified, the two-sample Kolmogorov-Smirnov (KS) test was applied to the flash sizes, flash initiation, and flash propagation altitudes for each of the three flash types. The KS test statistically showed that IC, CG, and hybrid flashes do not have the same parent distributions and thus should be separately classified. Separate classification of hybrid flashes will lead to improved lightning-related research, because unambiguously classified hybrid flashes occur on the same order of magnitude as CG flashes for multicellular storms.
      PubDate: 2017-09-22T16:35:27.582222-05:
      DOI: 10.1002/2017GL075003
       
 
 
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