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  • Articles  (4,699)
  • Wiley  (4,699)
  • 2010-2014  (4,699)
  • 1985-1989
  • 1950-1954
  • 1945-1949
  • Geophysical Research Letters  (4,699)
  • 4905
  • 1
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    Differentiating flavors of the Indian Ocean Dipole using dominant modes in tropical Indian Ocean rainfall (2014)
    Wiley
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    Publication Date: 2014-12-07
    Description: Phase-locked to austral winter and spring, canonical positive Indian Ocean Dipoles (pIODs) generally peak in spring. In recent decades, there has been an increase in unseasonable pIODs which, different from canonical pIODs, peak and decay by September. Distinguishing unseasonable pIODs from canonical pIODs is important, as conditions leading to more frequent unseasonable events are projected to persist in a warming climate. Here using superimposition of the first two seasonally evolving dominant modes of tropical Indian Ocean rainfall variability, we differentiate these types of pIODs. The first mode reflects characteristics of canonical pIODs, in which anomalies intensify with seasonal evolution. However, the second mode, with cool and dry anomalies extending from the eastern pole, reverses from winter to spring, signifying the demise of unseasonable pIODs. Processes embedded in the second mode reflect timing of propagation in equatorial Kelvin waves, and their relative importance to the first mode, in generating different pIODs.
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  • 2
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    Tracking major storms from microseismic and hydroacoustic observations on the seafloor (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Ocean wave activity excites seismic waves that propagate through the solid earth, known as microseismic noise. Here we use a network of 57 ocean-bottom seismometers (OBS) deployed around La Réunion Island in the southwest Indian Ocean to investigate the noise generated in the secondary microseismic band as a tropical cyclone moved over the network. Spectral and polarization analyses show that microseisms strongly increase in the 0.1 - 0.35 Hz frequency band as the cyclone approaches, and that this noise is composed of both compressional and surface waves, confirming theoretical predictions. We infer the location of maximum noise amplitude in space and time, and show that it coincides roughly with the location of maximum ocean-wave interactions. Although this analysis was performed retrospectively, microseisms recorded on the seafloor can be considered a novel source of information for future real-time tracking and monitoring of major storms, complementing atmospheric, oceanographic and satellite observations.
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  • 3
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    Galápagos Coral Reef Persistence after ENSO Warming Across an Acidification Gradient (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Anthropogenic CO 2 is causing warming and ocean acidification. Coral reefs are being severely impacted, yet confusion lingers regarding how reefs will respond to these stressors over this century. Since the 1982-83 El Niño-Southern Oscillation warming event, the persistence of reefs around the Galápagos Islands has differed across an acidification gradient. Reefs disappeared where pH 〈 8.0 and aragonite saturation state (Ω arag ) ≤ 3 and have not recovered, whereas one reef has persisted where pH 〉 8.0 and Ω arag  〉 3. Where upwelling is greatest, calcification by massive Porites is higher than predicted by a published relationship with temperature despite high CO 2 , possibly due to elevated nutrients. However, skeletal P/Ca, a proxy for phosphate exposure, negatively correlates with density ( R  = -0.822, p  〈 0.0001). We propose that elevated nutrients have the potential to exacerbate acidification by depressing coral skeletal densities and further increasing bioerosion already accelerated by low pH.
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  • 4
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    Changes to the temporal distribution of daily precipitation (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Changes to the temporal distribution of daily precipitation were investigated using a dataset of 12,513 land-based stations from the Global Historical Climatology Network (GHCN). The distribution of precipitation was measured using the Gini index (which describes how uniformly precipitation is distributed throughout a year) and the annual number of wet days. The Mann-Kendall test and a regression analysis were used to assess the direction and rate of change to both indices. Over the period of 1976-2000, East Asia, central America and Brazil exhibited a decrease in the number of both wet and light precipitation days, and eastern Europe exhibited a decrease in the number of both wet and moderate precipitation days. In contrast, the USA, southern South America, western Europe and Australia exhibited an increase in the number of both wet and light precipitation days. Trends in both directions were field significant at the global scale.
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  • 5
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    Effects of the post-perovskite phase change on the observed geoid (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: In the lowermost mantle, seismic velocity variations beneath Pacific margins have been related to the perovskite (pv) to post-perovskite (pPv) phase transition. We investigate the influence of this phase transformation on the geoid using 3D spherical mantle circulation models based on a seismic tomography model and strong lateral viscosity variations in the lower mantle. We demonstrate that the geoid anomalies are strongly affected by the presence of pPv because of phase-dependent viscosity changes relative to the surrounding mantle. Whereas geoid heights above subduction zones are increased for high-viscosity pPv, the presence of weak pPv reduces them, thereby improving the fit to the observed geoid. An investigation using two different tomography models, different pPv density contrasts, and the presence or absence of a global thermal boundary layer and of lateral viscosity variations in the lower mantle demonstrate the various effects of weak pPv on the geoid.
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  • 6
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    Interdecadal Pacific variability and eastern Australian mega-droughts over the last millennium (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: The Interdecadal Pacific Oscillation (IPO) influences multidecadal drought risk across the Pacific, but there are no millennial-length, high resolution IPO reconstructions for quantifying long-term drought risk. In Australia, drought risk increases in positive phases of the IPO, yet few suitable rainfall proxies and short (~100 y) instrumental records mean large uncertainties remain around drought frequency and duration. Likewise, it is unknown whether mega-droughts have occurred in Australia's past. In this study, an atmospheric teleconnection in the Indian Ocean mid-latitudes linking East Antarctica and Australia is exploited to produce the first accurate, annually dated millennial-length IPO reconstruction from the Law Dome (East Antarctica) ice core. Combined with an eastern Australian rainfall proxy from Law Dome, the first millennial-length Australian mega-drought (〉5 y duration) reconstruction is presented. Eight mega-droughts are identified including one 39 y drought (AD 1174–1212), which occurred during an unprecedented century of aridity (AD 1102–1212).
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  • 7
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    The Source Altitude, Electric Current, and Intrinsic Brightness of Terrestrial Gamma Ray Flashes (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: Many details of how thunderstorms generate terrestrial gamma-ray flashes (TGFs) and other forms of high energy radiation remain uncertain, including the basic question of where they are produced. We exploit the association of distinct low frequency radio emissions with generation of terrestrial gamma-ray flashes (TGFs) to directly measure for the first time the TGF source altitude. Analysis of two events reveals source altitudes of 11.8 ± 0.4 km and 11.9 ± 0.9 km. This places the source region in the interior of the thunderstorm between the two main charge layers, and implies an intrinsic TGF brightness of approximately 10 18 runaway electrons. The electric current in this non-traditional lightning process is found to be strong enough to drive nonlinear effects in the ionosphere, and in one case is comparable to the highest peak current lightning processes on the planet.
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  • 8
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    Counteracting the climate effects of volcanic eruptions using short-lived greenhouse gases (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: A large volcanic eruption might constitute a climate emergency, significantly altering global temperature and precipitation for several years. Major future eruptions will occur, but their size or timing cannot be predicted. We show, for the first time, that it may be possible to counteract these climate effects through deliberate emissions of short-lived greenhouse gases, dampening the abrupt impact of an eruption. We estimate an emission pathway countering a hypothetical eruption three times the size of Mt Pinatubo in 1991. We use a global climate model to evaluate global and regional responses to the eruption, with and without counter emissions. We then raise practical, financial and ethical question related to such a strategy. Unlike the more commonly-discussed geoengineering to mitigate warming from long-lived greenhouse gases, designed emissions to counter temporary cooling would not have the disadvantage of needing to be sustained over long periods. Nevertheless, implementation would still face significant challenges.
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  • 9
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    Rayleigh wave group velocity distributions for East Asia using ambient seismic noise (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: Vertical component data from 206 broadband seismometer stations from Korean networks KIGAM and KMA, the Japanese F-net network, and the Chinese IC and NECESSArray networks are collected for the year 2011 and the ambient seismic noise is analyzed. Rayleigh wave group velocity distribution maps are created in the period range 10 to 70 seconds. Our results are largely consistent with previous studies of the area, but provide greater detail in the Korean peninsula and the East Sea (Sea of Japan). Low group velocities are observed in the Ulleung basin, and the Chubu-Kanto and Kyushu regions in Japan. At 10 s period, sediment basins in the East Sea appear as low group velocity regions relative to higher group velocity continental regions. At periods longer than 40 s, a low group velocity region emerges in the Ulleung basin region, and is bounded by the Korean peninsula.
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  • 10
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    EGM TIM RL05: An Independent Geoid with Centimeter Accuracy Purely Based on the GOCE Mission (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: After more than 4.5 years in orbit, the Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission ended with the reentry of the satellite on 11 November 2013. This publication serves as a reference for the 5th gravity field model based on the time-wise approach (EGM_TIM_RL05), a global model only determined from GOCE observations. Due to its independence of any other gravity data, a consistent and homogeneous set of spherical harmonic coefficients up to degree and order 280 (corresponding to spatial resolution of 71.5 km on ground) is provided including a full covariance matrix characterizing the uncertainties of the model. The associated covariance matrix realistically describes the model quality. It is the first model which is purely based on GOCE including all observations collected during the entire mission. The achieved mean global accuracy is 2.4 cm in terms of geoid heights and 0.7 mGal for gravity anomalies at a spatial resolution of 100 km.
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  • 11
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    Modeling the spreading of glacial melt water from the Amundsen and Bellingshausen Seas (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: It has been suggested that an increased melting of continental ice in the Amundsen Sea (AS) and Bellinghausen Sea (BS) is a likely source of the observed freshening of Ross Sea (RS) water. To test this hypothesis, we simulate the spreading of glacial melt water using the Finite-Element Sea-ice/ice-shelf/Ocean Model. Based on the spatial distribution of simulated passive tracers, most of the basal melt water from AS ice shelves flows towards the RS with more than half of the melt originating from the Getz Ice Shelf. Further, the model results show that a slight increase of the basal mass loss can substantially intensify the transport of melt water into the RS due to a strengthening of the melt-driven shelf circulation and the westward flowing coastal current. This supports the idea that the basal melting of AS and BS ice shelves is one of the main sources for the RS freshening.
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  • 12
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    A low-frequency near-field interferometric-TOA 3D lightning mapping array (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: We report on the development of an easily deployable LF near-field Interferometric-TOA 3D Lightning Mapping Array applied to imaging of entire lightning flashes. An interferometric cross-correlation technique is applied in our system to compute windowed two-sensor time differences with sub-microsecond time resolution before TOA is used for source location. Compared to previously reported LF lightning location systems, our system captures many more LF sources. This is due mainly to the improved mapping of continuous lightning processes by using this type of hybrid interferometry/TOA processing method. We show with five station measurements that the array detects and maps different lightning processes, such as stepped and dart leaders, during both in-cloud and cloud-to-ground flashes. Lightning images mapped by our LF system are remarkably similar to those created by VHF mapping systems, which may suggest some special links between LF and VHF emission during lightning processes.
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  • 13
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    Black Carbon Radiative Forcing over the Tibetan Plateau (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: We estimate the snow albedo forcing and direct radiative forcing (DRF) of black carbon (BC) in the Tibetan Plateau using a global chemical transport model in conjunction with a stochastic snow model and a radiative transfer model. The annual mean BC snow albedo forcing is 2.9 W m −2 averaged over snow-covered Plateau regions, which is a factor of three larger than the value over global land snowpack. BC-snow internal mixing increases the albedo forcing by 40-60% compared with external mixing and coated BC increases the forcing by 30-50% compared with uncoated BC aggregates, whereas Koch snowflakes reduce the forcing by 20-40% relative to spherical snow grains. The annual BC DRF at the top of the atmosphere is 2.3 W m −2 with uncertainties of −70-85% in the Plateau after scaling the modeled BC absorption optical depth to Aerosol Robotic Network (AERONET) observations. The BC forcings are attributed to emissions from different regions.
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  • 14
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    Low melt rates with seasonal variability at the base of Fimbul Ice Shelf, East Antarctica, revealed by in situ interferometric radar measurements. (2014)
    Wiley
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    Publication Date: 2014-11-08
    Description: ABSTRACT Basal melt is a major cause of ice shelf thinning affecting the stability of the ice shelf and reducing its buttressing effect on the inland ice. The Fimbul ice shelf (FIS) in Dronning Maud Land (DML), East Antarctica, is fed by the fast-flowing Jutulstraumen glacier, responsible for 10% of ice discharge from the DML sector of the ice sheet. Current estimates of the basal melt rates of the FIS come from regional ocean models, autosub measurements and satellite observations, which vary considerably. This discrepancy hampers evaluation of the stability of the Jutulstraumen catchment. Here, we present estimates of basal melt rates of the FIS using ground based interferometric radar. We find a low average basal melt rate on the order of 1 m/yr, with the highest rates located at the ice shelf front, which extends beyond the continental shelf break. Furthermore, our results provide evidence for a significant seasonal variability.
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  • 15
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    Thunderstorms enhance tropospheric ozone by wrapping and shedding stratospheric air (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: A significant source of ozone in the troposphere is transport from the stratosphere. The stratospheric contribution has been estimated mainly using global models that attribute the transport process largely to the global scale Brewer-Dobson circulation and synoptic scale dynamics associated with upper tropospheric jet streams. We report observations from research aircraft that reveal additional transport of ozone-rich stratospheric air downward into the upper troposphere by a leading-line-trailing-stratiform (LLTS) mesoscale convective system (MCS) with convection overshooting the tropopause altitude. The fine-scale transport demonstrated by these observations poses a significant challenge to global models that currently do not resolve storm scale dynamics. Thus the upper tropospheric ozone budget simulated by global chemistry-climate models where large-scale dynamics and photochemical production from lightning-produced NO are the controlling factors may require modification.
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  • 16
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    Van Allen Probe Observations of Periodic Rising Frequencies of the Fast Magnetosonic Mode (2014)
    Wiley
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    Publication Date: 2014-11-09
    Description: Near simultaneous periodic dispersive features of fast magnetosonic mode emissions are observed by both Van Allen Probes spacecraft while separated in magnetic local time by ~5 hours: Probe A at 15 and Probe B at 9–11 hours. Both spacecraft see similar frequency features, characterized by a periodic repetition at ~180 s. Each repetition is characterized by a rising frequency. Since no modulation is observed in the proton shell distribution, the plasma density, or in the background magnetic field at either spacecraft we conclude that these waves are not generated near the spacecraft but external to both spacecraft locations. Probe A while outside the plasmapause sees the start of each repetition ~40 s before probe B while deep inside the plasmasphere. We can qualitatively reproduce the dispersive features, but not the quantitative details. The cause for this phenomena remains to be identified.
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  • 17
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    Contemporary Ocean Warming Dissociates Cascadia Margin Gas Hydrates (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: Gas hydrates, pervasive in continental margin sediments, are expected to release methane in response to ocean warming, but the geographic range of dissociation and subsequent flux of methane to the ocean are not well constrained. Sediment column thermal models based on observed water column warming trends offshore Washington (USA) show that a substantial volume of gas hydrate along the entire Cascadia upper continental slope is vulnerable to modern climate change. Dissociation along the Washington sector of the Cascadia margin alone has the potential to release 45–80 Tg of methane by 2100. These results highlight the importance of lower latitude warming to global gas hydrate dynamics and suggest that contemporary warming and downslope retreat of the gas hydrate reservoir occurs along a larger fraction of continental margins worldwide than previously recognized.
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  • 18
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    Southward displacement of the upper atmosphere zonal Jet in the eastern north Pacific due to global warming (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: The jet stream over the eastern North Pacific (ENPJ) as a core of the atmospheric flow is known to strongly fluctuate meridionally, and its meridional displacement directly influences adjacent regional climate. Here, we investigate how this jet will be changed due to global warming. By analyzing the future scenario experiments of CMIP3 and 5, and found that both ENPJ and the eastern tropical Pacific ITCZ tend to move southward, which are closely related to the tropical eastern Pacific warming trend. Tropical eastern Pacific warming leads to not only the southward migration of ITCZ by southward-shifting the off-equatorial eastern Pacific warm pool, but also the southward shift of ENPJ by increasing baroclinic instability of the atmosphere in subtropical region through intensifying the meridional SST gradient. Not primary but yet secondly the southward shift of ITCZ contributes to the southward shift of ENPJ through a kinematic connection bridged by local Hadley circulation.
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  • 19
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    Crab Burrows as Conduits for Groundwater-Surface water Exchange in Bangladesh (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: Groundwater recharge affects water budgets and groundwater quality on the deltas and floodplains of South and Southeast Asia. Rain and flooding rivers recharge groundwater during the monsoon; irrigated rice fields and surface-water bodies recharge aquifers during the dry season. Groundwater throughout the region is severely contaminated by arsenic and recent research suggests that quantifying and characterizing recharge is important to understand whether recharge flushes or mobilizes arsenic from aquifers. At a field site in Bangladesh, we found that burrows of terrestrial crabs short-circuit low-permeability surface sediments, providing the primary conduit for recharge. We combine field observations along with a model that couples isotope and water balances to quantify the effect of crab burrows on aquifer recharge. Given the wide distribution of burrowing crabs and the surficial geology, we suggest that crab burrows provide widespread conduits for groundwater recharge.
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  • 20
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    Heating of the Plasma Sheet by Broadband Electromagnetic Waves (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: We demonstrate that broadband low frequency electromagnetic field fluctuations embedded within fast flows throughout the Earth's plasma sheet may drive significant ion heating. This heating is nearly entirely in the direction perpendicular to the background magnetic field and is estimated to occur at an average rate of ~1 eV/s with rates in excess of 10 eV/s within one standard deviation of the average value over all observed events. For an Earthward flow the total change in temperature along a flow path may exceed one keV and for ‘wave-rich’ flows can be comparable to that expected due to conservation of the first adiabatic invariant. The consequent increase in plasma pressure and flux tube entropy may lead to braking of inward motion and the suppression of plasma interchange.
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  • 21
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    Global Estimate of Submarine Groundwater Discharge Based on an Observationally Constrained Radium Isotope Model (2014)
    Wiley
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    Publication Date: 2014-11-05
    Description: Along the continental margins, rivers and submarine groundwater supply nutrients, trace elements, and radionuclides to the coastal ocean, supporting coastal ecosystems and, increasingly, causing harmful algal blooms and eutrophication. While the global magnitude of gauged riverine water discharge is well known, the magnitude of submarine groundwater discharge (SGD) is poorly constrained. Using an inverse model combined with a global compilation of 228 Ra observations, we show that the SGD integrated over the Atlantic and Indo-Pacific Oceans between 60°S and 70°N is (12 ± 3) x 10 13  m 3  yr -1 , which is 3 to 4 times greater than the freshwater fluxes into the oceans by rivers. Unlike the rivers, where more than half of the total flux is discharged into the Atlantic, about 70% of SGD flows into the Indo-Pacific Oceans. We suggest that SGD is the dominant pathway for dissolved terrestrial materials to the global ocean, and this necessitates revisions for the budgets of chemical elements including carbon.
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  • 22
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    Impact of Fluid Deformation on Mixing-Induced Chemical Reactions in Heterogeneous Flows (2014)
    Wiley
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    Publication Date: 2014-11-06
    Description: Fast chemical reactions in geophysical flows are controlled by fluid mixing, which perturbs local chemical equilibria and thus triggers chemical reactions. Spatial fluctuations in the flow velocity lead to deformation of material fluid elements, which form the support volumes of transported chemical species. We develop an approach based on a lamellar representation of fluid mixing that provides a direct link between fluid deformation, the distribution of concentration gradiens, and the upscaled reaction rates for fast reversible reactions. The temporal evolution of effective reaction rates are determined by the flow topology and the distribution of local velocity gradients. This leads to a significant increase of the reaction efficiency, which turns out to be orders of magnitude larger than in homogeneous flow. This approach allows for the systematic evaluation of the temporal evolution of equilibrium reaction rates, and establishes a direct link between the reaction efficiency and the spatial characteristics of the underlying flow field as quantified by the deformation of material fluid elements.
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  • 23
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    Tsunami surges around the Hawaiian Islands from the 1 April 2014 North Chile Mw 8.1 earthquake (2014)
    Wiley
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    Publication Date: 2014-11-06
    Description: The 1 April 2014 Iquique M w 8.1 earthquake ruptured a segment of the megathrust fault offshore of northern Chile and generated a moderate-size tsunami across the Pacific. Tide gauges in Hawaii recorded over 1 m of wave height despite the long distance from the source and position away from the main radiated energy lobe. Inversion of global teleseismic body wave observations combined with forward modeling of the tsunami at four near-field DART stations arrives iteratively at a self-consistent finite-fault model with very compact dimensions. The slip distribution produces a NNE-SSW trending seafloor uplift patch that enhances the tsunami directionality in the WNW, resulting in good matches to observed DART and tide gauge records around the Hawaiian Islands. The relatively large waves at selected locations in Hawaii can be attributed to a combination of the spatial slip distribution and the resulting short-period waves that triggered localized resonance over the insular shelves. This event highlights the importance of characterizing detailed slip distributions in analysis or forecasting of tsunamis even for a compact source.
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  • 24
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    Surface warming hiatus caused by increased heat uptake across multiple ocean basins (2014)
    Wiley
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    Publication Date: 2014-11-06
    Description: The first decade of the twenty-first century was characterised by a hiatus in global surface warming. Using ocean model hindcasts and reanalyses we show that heat uptake between the 1990s and 2000s increased by 0.7 ± 0.3 Wm −2 . Approximately 30% of the increase is associated with colder sea surface temperatures in the eastern Pacific. Other basins contribute via reduced heat loss to the atmosphere, in particular the Southern and subtropical Indian Oceans (30%), and the subpolar North Atlantic (40%). A different mechanism is important at longer timescales (1960s-present) over which the Southern Annular Mode trended upwards. In this period, increased ocean heat uptake has largely arisen from reduced heat loss associated with reduced winds over the Agulhas Return Current and southward displacement of Southern Ocean westerlies.
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    Revision of the EPICA Dome C CO2 record from 800 to 600 kyr before present (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: The EPICA Dome C (EDC) ice core has allowed for the reconstruction of atmospheric CO 2 concentrations for the last 800,000 years. Here, we revisit the oldest part of the EDC CO 2 record using different air extraction methods and sections of the core. For our established cracker system, we found an analytical artifact, which increases over the deepest 200 m and reaches 10.1 ± 2.4 ppm in the oldest/deepest part. The governing mechanism is not yet fully understood, but it is related to insufficient gas extraction in combination with ice relaxation during storage and ice structure. The corrected record presented here resolves partly the issue with a different correlation between CO 2 and Antarctic temperatures found in this oldest part of the records, however, this anomaly still persists. In addition we provide here an update of 800,000 years atmospheric CO 2 history including recent studies covering the last glacial cycle.
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    A cautionary note on the use of EESC-based regression analysis for ozone trend studies (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: Equivalent effective stratospheric chlorine (EESC) construct of ozone regression models attributes ozone changes to EESC changes using a single value of the sensitivity of ozone to EESC over the whole period. Using space-based total column ozone (TCO) measurements, and a synthetic TCO time series constructed such that EESC does not fall below its late 1990s maximum, we demonstrate that the EESC-based estimates of ozone changes in the polar regions (70–90°) after 2000 may, falsely, suggest an EESC-driven increase in ozone over this period. An EESC-based regression of our synthetic “failed Montreal Protocol with constant EESC" time series suggests a positive TCO trend that is statistically significantly different from zero over 2001–2012 when in fact no recovery has taken place. Our analysis demonstrates that caution needs to be exercised when using explanatory variables, with a single fit coefficient, fitted to the entire data record, to interpret changes in only part of the record.
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    Field-aligned Currents' Scale Analysis Performed with the Swarm Constellation (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: We present a statistical study of the temporal and spatial scale characteristics of different field-aligned current (FAC) types derived with the Swarm satellite formation. We divide FACs into two classes: small-scale, up to some ten kilometer, which arecarried predominantly by kinetic Alfvén waves, and large-scale FACs with sizes of more than 150 km. For determining temporal variability we consider measurements at the same point, the orbital crossovers near the poles, but at different times. Fromcorrelation analysis we obtain a persistent period of small-scale FACs of order 10 s, while large-scale FACs can be regarded stationary for more than 60 s. For the first time we investigate the longitudinal scales. Large-scale FACs are different on dayside and nightside. On the nightside the longitudinal extension is on average 4 times the latitudinal width, while on the dayside, in particular in the cusp region, latitudinal and longitudinal scales are comparable.
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    Unifying the relative hindered velocity in suspensions and emulsions of non-deformable particles (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Although the relative velocity of a single crystal or bubble in a quiescent fluid (melt) is well-characterized, the interplay of crystals/bubbles in multiparticle systems and its effect on their settling/rising velocity is poorly quantified. We propose a theoretical model for the hindered velocity of non-Brownian emulsions and suspensions of non-deformable fluid and solid particles in the creeping flow regime. The model is based on three sets of correction; two on the drag coefficient experienced by each particle to account for both return flow and Smoluchowski effects, and a correction on the rheology to account for non-local interactions introduced as a mean-field effective viscosity. Our model is tested against new and published experimental data over a wide range of particle volume fraction and viscosity ratio between the fluids. We find an excellent agreement between our model and experiments. The model is then applied to show that hindered settling can increase mineral residence time by up to an order of magnitude in convecting magma chambers.
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    Cosmic ray event of AD 774-775 shown in quasi-annual 10Be data from the Antarctic Dome Fuji ice core (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: 14 C content in tree rings and 10 Be concentration records in polar ice core provide information about past cosmic ray intensities. The AD 774–775 cosmic ray event has been identified by 14 C measurement in several tree rings from all over the world. Although the quasi-decadal 10 Be Dome Fuji data in the Antarctic ice core also shows a sharp peak around AD 775, annual 10 Be variations in the Dome Fuji core or in other cores have not been revealed. We have measured quasi-annual 10 Be concentrations from approximately AD 763–794 in the Dome Fuji ice core, and detected a clear increase (~80% above the baseline) in 10 Be concentration around AD 775. However, an accurate height of this increase is not straightforwardly estimated due to the background variation in 10 Be concentration. The 10 Be increase can be due to the same cosmic ray event as shown in the 14 C content in AD 774–775.
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    Gravity waves simulated by high-resolution Whole Atmosphere Community Climate Model (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: For the first time a mesoscale-resolving whole atmosphere general circulation model (GCM) has been developed, using the NCAR Whole Atmosphere Community Climate Model (WACCM) with ~0.25° horizontal resolution and 0.1 scale height vertical resolution above the middle stratosphere (higher resolution below). This is made possible by the high accuracy and high scalability of the spectral element dynamical core from the High-Order Method Modeling Environment (HOMME). For the simulated January-February period, the latitude-height structure and the magnitudes of the temperature variance compare well with those deduced from SABER observations. The simulation reveals the increasing dominance of gravity waves (GWs) at higher altitudes through both the height dependence of the kinetic energy spectra, which display a steeper slope (~-3) in the stratosphere and an increasingly shallower slope above, and the increasing spatial extent of GWs (including a planetary-scale extent of a concentric GW excited by a tropical cyclone) at higher altitudes. GW impacts on the large-scale flow is evaluated in terms of zonal mean zonal wind and tides: with no GW drag parameterized in the simulations, forcing by resolved GWs does reverse the summer mesospheric wind, albeit at an altitude higher than climatology, and only slows down the winter mesospheric wind without closing it. The hemispheric structures and magnitudes of diurnal and semidiurnal migrating tides compare favorably with observations.
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    3-D Simulations of the Southern Polar Giant Impact Hypothesis for the Origin of the Martian Dichotomy. (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: We demonstrate via numerical simulations that the impact of a ~ lunar-sized body with Mars is capable of creating a hemispherical magma ocean that upon cooling and solidification resulted in the formation of the southern highlands and thus the Martian dichotomy. The giant impact may have contributed a significant amount of iron to the Martian core and generated a deep thermal anomaly that led to the onset and development of the volcanism in the southern highlands. Our model predicts several mantle plumes converging to the South Pole from the equatorial regions as well as new plumes forming in the equatorial region and also an absence of significant large-scale volcanism in the northern lowlands. The core heat flux evolution obtained from our numerical models is consistent with the decline of the magnetic field. We argue that such a scenario is more consistent with a range of observations than a northern giant impact (excavating the Borealis basin) for the formation of the Martian dichotomy.
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    Greenland ice sheet annual motion insensitive to spatial variations in subglacial hydraulic structure (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: We present ice velocities observed with global positioning systems and TerraSAR-X/TanDEM-X in a land-terminating region of the south-west Greenland ice sheet (GrIS) during the melt-year 2012–2013, to examine the spatial pattern of seasonal and annual ice motion. We find that whilst spatial variability in the configuration of the subglacial drainage system controls ice motion at short timescales, this configuration has negligible impact on the spatial pattern of the proportion of annual motion which occurs during summer. Whilst absolute annual velocities vary substantially, the proportional contribution of summer motion to annual motion does not. These observations suggest that in land-terminating margins of the GrIS, subglacial hydrology does not significantly influence spatial variations in net summer speedup. Furthermore, our findings imply that not every feature of the subglacial drainage system needs to be resolved in ice sheet models.
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    δ15N in laminated marine sediments provide a proxy for mixing between the California Undercurrent and the California Current: A proof of concept (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Measurements of particulate δ 15 N in coastal marine laminated sediments provide a high-resolution proxy for fluctuations in the intensity of denitrification in the water column. In the Eastern Tropical North Pacific oxygen minimum zone, this denitrification signal is transported northward by the California Undercurrent, thus serving as a tracer of ocean circulation. This is verified through comparisons between salinity in the thermocline off Southern California (Santa Monica Basin) and the difference between δ 15 N sed within age equivalent sediments from a southern (Pescadero Slope) and northern (Santa Monica Basin) site. Trends in this parameter, Δ δ 15 N sed , relate to Pacific Decadal Oscillation (PDO) phase changes between 1900 and 1990. We hypothesize that the decline in Δ δ 15 N sed during warm PDO phases is due to a strengthening of the California Undercurrent transporting 15  N-enriched nitrate from the ETNP northward. The deviation from this trend after 1990 suggests recent changes in circulation and/or California Current water nutrient biogeochemistry.
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    Effects of Climate Oscillations on Wind Resource Variability in the United States (2014)
    Wiley
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    Publication Date: 2014-12-15
    Description: Natural climate variations in the United States wind resource are assessed by using cyclostationary empirical orthogonal functions (CSEOFs) to decompose wind reanalysis data. Compared to approaches that average climate signals or assume stationarity of the wind resource on interannual timescales, the CSEOF analysis isolates variability associated with specific climate oscillations, as well as their modulation from year to year. Contributions to wind speed variability from the modulated annual cycle (MAC) and the El Niño-Southern Oscillation (ENSO) are quantified, and information provided by the CSEOF analysis further allows the spatial variability of these effects to be determined. The impacts of the MAC and ENSO on the wind resource are calculated at existing wind turbine locations in the United States, revealing variations in the wind speed of up to 30% at individual sites. The results presented here have important implications for predictions of wind plant power output and siting.
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    Full-Wave Multiscale Anisotropy Tomography in Southern California (2014)
    Wiley
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    Publication Date: 2014-12-09
    Description: Understanding the spatial variation of anisotropy in the upper mantle is important for characterizing the lithospheric deformation and mantle flow dynamics. In this study, we apply a full-wave approach to image the upper-mantle anisotropy in Southern California using 5954 SKS splitting data. Three-dimensional sensitivity kernels combined with a wavelet-based model parameterization are adopted in a multiscale inversion. Spatial resolution lengths are estimated based on a statistical resolution matrix approach, showing a finest resolution length of ~25 km in regions with densely-distributed stations. The anisotropic model displays structural fabric in relation to surface geologic features such as the Salton Trough, the Transverse Ranges and the San Andreas Fault. The depth variation of anisotropy does not suggest a lithosphere-asthenosphere decoupling. At long-wavelengths, the fast directions of anisotropy are aligned with the absolute plate motion inside the Pacific and North American plates.
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    Carbon-cycle extremes during the 21st century in CMIP5 models: future evolution and attribution to climatic drivers (2014)
    Wiley
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    Publication Date: 2014-12-02
    Description: Climate extremes such as droughts and heatwaves affect terrestrial ecosystems and may alter local carbon budgets. However, it still remains uncertain to what degree extreme impacts in the carbon cycle influence the carbon cycle-climate feedback both today and the near future. Here, we analyze spatiotemporally contiguous negative extreme anomalies in gross primary production (GPP) and net ecosystem production (NEP) in model output of the Coupled Model Intercomparison Project Phase 5 (CMIP5) ensemble and investigate their future development and attribution to climatic drivers. We find that relative to the overall increase in global carbon uptake, negative extremes in GPP and NEP lose importance towards the end of the 21st century. This effect can be related to elevated CO 2 concentrations and higher amounts of available water at the global scale, partially mitigating the impacts of droughts and heatwaves, respectively. Overall, based on CMIP5 models we hypothesize that terrestrial ecosystems might be more resilient against future climate extremes than previously thought. Future work will have to further scrutinize these results considering that various biological and biogeochemical feedbacks are not yet integrated within earth system models.
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    Laboratory Experiments of Forced Plumes in a Density-Stratified Crossflow and Implications for Volcanic Plumes (2014)
    Wiley
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    Publication Date: 2014-12-02
    Description: The mass eruption rate feeding a volcanic plume is commonly estimated from its maximum height. Winds are known to affect the column dynamics causing bending and hence reducing the maximum plume height for a given mass eruption rate. However, the quantitative predictions including wind effects on mass eruption rate estimates are not well constrained. To fill this gap, we present a series of new laboratory experiments on forced plumes rising in a density-stratified crossflow. We identify three dynamical regimes corresponding to increasing effect of wind on the plume rise. The transition from one regime to another is governed by two dimensionless velocity scales defined as a function of source and environmental parameters. The results are found consistent with the conditions of historical eruptions and provide new empirical relationships to estimate mass eruption rate from plume height in windy conditions, leading to valuable tools for eruption risk assessment.
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    Global Model of Low Frequency Chorus (fLHR  (2014)
    Wiley
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    Publication Date: 2014-01-11
    Description: [1]  Whistler mode chorus is an important magnetospheric emission, playing a dual role in the acceleration and loss of relativistic electrons in the Earth's outer radiation belt. Chorus is typically generated in the equatorial region in the frequencyrange 0.1 - 0.8 f ce , where f ce is the local electron gyrofrequency. However, as the waves propagate to higher latitudes, significant wave power can occur at frequencies below 0.1 f ce . Since this wave power is largely omitted in current radiation belt models we construct a global model of low frequency chorus, f LHR  〈  f  〈 0.1 f ce , using data from six satellites. We find that low frequency chorus is strongest, with an average intensity of 200 pT 2 , in the pre-noon sector during active conditions at mid latitudes (20 o  〈 | λ m | 〈 50 o ) from 4 〈  L ∗  〈 8. Such mid-latitude, low frequency chorus wave power will contribute to the acceleration and loss of relativistic electrons and should be taken into account in radiation belt models.
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    Mechanisms limiting the growth of aeolian Megaripples (2014)
    Wiley
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    Publication Date: 2014-01-11
    Description: [1]  Megaripples are distinguished from regular ripples by their larger size and bimodal sediment distribution. The interplay between wind, grain size and morphology controls their development, but the exact mechanisms that limit the size of megaripples have been unclear. Using wind tunnel experiments we found two main mechanisms that limit the height of megaripples. The first mechanism is megaripple flattening due to strong enough winds that drive the coarse grains into saltation; the second mechanism is megaripple deflation by impacts of faster saltation grains. In this latter mechanism, the coarse grains are propelled by the impacts of fine saltating grains. The occurrence of both these mechanisms depends on the grain size distribution, and increases with both megaripple height and wind speed. Thus, for a given wind environment and grain size distribution, there exists a limit on the size of megaripples, which is determined by these two mechanisms.
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    Observation of Local and Conjugate Ionospheric Perturbations from Individual Oceanic Lightning Flashes (2014)
    Wiley
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    Publication Date: 2014-01-11
    Description: [1]  Very low frequency (VLF) remote sensing observations of multifaceted local and conjugate ionospheric perturbations from geographically identified and well characterized oceanic lightning discharges are presented for the first time. Lightning-induced electron precipitation (LEP) events are shown to produce disturbances first in the conjugate hemisphere and subsequently in the hemisphere of the causative lightning discharge in agreement with theoretical predictions. A rough threshold peak current of ~100 kA is identified for lightning discharges to generate LEP events for the geomagnetic conditions present during observations. The occurrence of early VLF events and the subsequent duration of their recovery does not seem to fit any simple metric of lightning discharge peak current or proximity to great circle path. Knowledge of the full spectral density of the lightning EMP not just its peak current and the subionospheric mode structure are likely necessary to determine if a specific lightning discharge will generate an early VLF perturbation.
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    Splay fault triggering by great subduction earthquakes inferred from finite-element models (2014)
    Wiley
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    Publication Date: 2014-01-14
    Description: [1]  We have investigated the influence that megathrust earthquake slip has on the activation of splay faults using a 2D Finite Element Method (FEM), taking into account the effects of gravity and variations in the frictional strength properties of splay faults. We simulated both landward-dipping and seaward-dipping splay fault geometries, and imposed depth-variable slip distributions of subduction events. Our results indicate that the two types of splay fault exhibit a similar behavior, with variations in frictional properties along the faults affecting only the seismic magnitude. The triggering process is controlled by a critical depth. Megathrust slip concentrated at depths shallower than the critical depth will favor normal displacement, while megathrust slip concentrated at depths deeper than the critical depth is likely to result in reverse motion. Our results thus provide a useful tool for predicting the activation of secondary faults and may have direct implications for tsunami hazard research.
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    Evanescent wave coupling in a geophysical system: Airborne acoustic signals from the Mw 8.1 Macquarie Ridge earthquake (2014)
    Wiley
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    Publication Date: 2014-01-15
    Description: [1]  Atmospheric low frequency sound, i.e., infrasound, from underwater events has not been considered thus far, due to the high impedance contrast of the water-air interface making it almost fully reflective. Here, we report for the first time on atmospheric infrasound from a large underwater earthquake (Mw 8.1) near the Macquarie Ridge, which was recorded at 1,325 km from the epicenter. Seismic waves coupled to hydro-acoustic waves at the ocean floor, after which the energy entered the SOund Fixing And Ranging (SOFAR) channel and was detected on a hydrophone array. The energy was diffracted by a sea mount and an oceanic ridge, which acted as a secondary source, into the water column followed by coupling into the atmosphere. The latter results from evanescent wave coupling and the attendant anomalous transparency of the sea surface for very low frequency acoustic waves.
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    Persistent flow acceleration within the interior of the Greenland Ice Sheet (2014)
    Wiley
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    Publication Date: 2014-01-16
    Description: [1]  We present surface velocity measurements from a high-elevation site located 140 km from the western margin of the Greenland Ice Sheet, and ~ 50 km into its accumulation area. Annual velocity increased each year from 51.78 ± 0.01 m yr -1 in 2009 to 52.92 ± 0.01 m yr -1 in 2012 – a net increase of 2.2 %. These data also reveal a strong seasonal velocity cycle of up to 8.1 % above the winter mean, driven by seasonal melt and supraglacial lake drainage. Sole et al. (2013) recently argued that ice motion in the ablation area is mediated by reduced winter flow following the development of efficient subglacial drainage during warmer, faster, summers. Our data complement this analysis and reveal a year-on-year increase in annual velocity above the ELA, where despite surface melt increasing, it is still sufficiently low to hinder the development of efficient drainage under thick ice.
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    Gradual Diffusion and Punctuated Phase Space Density Enhancements of Highly Relativistic Electrons: Van Allen Probes Observations (2014)
    Wiley
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    Publication Date: 2014-01-16
    Description: [1]  The dual-spacecraft Van Allen Probes mission has provided a new window into megaelectron Volt (MeV) particle dynamics in the Earth's radiation belts. Observations (up to E ~10 MeV) show clearly the behavior of the outer electron radiation belt at different time scales: months-long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. We present analysis of multi-MeV electron flux and phase space density (PSD) changes during March 2013 in the context of the first year of Van Allen Probes operation. This March period demonstrates the classic signatures both of inward radial diffusive energization as well as abrupt localized acceleration deep within the outer Van Allen zone (L ~4.0 ± 0.5). This reveals graphically that both “competing” mechanisms of multi-MeV electron energization are at play in the radiation belts, often acting almost concurrently or at least in rapid succession.
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    A 3400-year lacustrine paleoseismic record from the North Anatolian Fault, Turkey: Implications for bimodal recurrence behavior (2014)
    Wiley
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    Publication Date: 2014-01-16
    Description: [1]  High-resolution physical, geochemical and geochronological analyses on the sedimentary sequence of Yeniçağa Lake, located in a fault-bounded basin along the North Anatolian Fault, reveal fingerprints of paleoearthquakes. A robust sediment chronology, spanning the last 3400 years, is constructed by radiocarbon dating and time-stratigraphical correlation with the precisely dated Sofular Cave speleothem record. Yeniçağa sedimentary sequence contains eleven seismically induced event deposits characterized by siliciclastic-enriched intervals. Some of the event deposits are also associated with implications of sudden lake deepening, which may be related to co-seismic subsidence. The paleoearthquake series having an average recurrence interval of ca. 260 years are interrupted by two possible seismic gaps of ca. 420 and 540 years.
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    Melt water input from the Bering Glacier Watershed into the Gulf of Alaska (2014)
    Wiley
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    Publication Date: 2014-01-17
    Description: [1]  The annual runoff from the melting of large glaciers and snow fields along the northern perimeter of the Gulf of Alaska is a critical component of marine physical and biological systems; yet, most of this freshwater is not measured. Here we show estimates of melt for the watershed that contains the largest and longest glacier in North America, the Bering Glacier. The procedure combines in-situ observations of snow and ice melt acquired by a long-term monitoring program, multi-spectral satellite observations and nearby temperature measurements. The estimated melt is 40 km 3 per melt season, ± 3.0 km 3 , observed over the decadal period, 2002-2012. As a result of climate change these estimates could increase to 60 km 3 per year by 2050. This technique and the derived melt coefficients can be applied to estimate melt from Alaska to Washington glaciers.
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    Mobilization of optically invisible dissolved organic matter (iDOM) in response to rainstorm events in a tropical forest headwater river (2014)
    Wiley
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    Publication Date: 2014-01-19
    Description: [1]  This study emphasizes the importance of rainstorm events in mobilizing carbon at the soil-stream interface from tropical rainforests. Half-hourly geochemical/isotopic records over a 13.5 hour period from a 20 km 2 tropical rainforest headwater in Guyana show an order of magnitude increase in dissolved organic carbon (DOC) concentration in less than 30 minutes (10.6-114 mg/L). The composition of DOC varies significantly and includes optically invisible dissolved organic matter (iDOM) that accounts for a large proportion (4-89%) of the total DOC, quantified using size exclusion chromatography (SEC). SEC suggests that iDOM is comprised of low molecular weight organic moieties, which are likely sourced from fresh leaf litter and/or topsoil, as shown in soils from the surrounding environment. Although poorly constrained at present, the presence of iDOM further downstream during the wet season suggests that this organic matter fraction may represent an un-quantified source of riverine CO 2 outgassing in tropical headwaters, requiring further consideration.
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    Tracking high-frequency seismic source evolution: 2004 Mw 8.1 Macquarie event (2014)
    Wiley
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    Publication Date: 2014-01-19
    Description: [1]  The 2004 Mw 8.1 event on 2004 December 23 near the Macquarie Ridge is a very large intraplate event that has been overshadowed by the Mw 9.3 Sumatra-Andaman event only three days later. We are able to track the progress of source evolution by estimating the progression of the points of energy emission, exploiting the good azimuthal distribution of available stations. The results indicate that this event ruptured on two nearby fault systems reactivating fossil fracture zones, with the second sub-event to the west triggered by the first. The total duration of high-frequency radiation is quite short, about 60 s, for such a large event. Much of the high-frequency radiation occurs on a fault sub-parallel to that inferred from long-period studies. This composite fault behaviour with displaced triggered failure appears to be a characteristic of large intraplate events in the oceans.
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    Why did the 2011-12 La Niña cause a severe drought in the Brazilian Northeast? (2014)
    Wiley
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    Publication Date: 2014-01-23
    Description: [1]  The Brazilian Northeast (NE) is strongly affected by ENSO. During La Niña events, the precipitation over the NE is generally above average. However, during the last La Niña event in 2011-12, the NE went through its worst drought in the last 30 years. In this study, observations and numerical simulations are used to determine what made the 2011-12 event different from other events. We find that eastern Pacific (canonical) La Niña events cause a cooling of the tropical North Atlantic and warming of the tropical South Atlantic that lead to a southward migration of the ITCZ, which in turn brings rain to the NE. On the other hand, La Niña events with the cooling concentrated in the central Pacific cause the opposite meridional SST gradient in the tropical Atlantic, leading to droughts over the NE. The 2011-12 event was of the latter type. This study also shows that it is possible to predict the sign of the NE rainfall anomaly during ENSO events using a simple SST index.
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    Observational quantification of the separation of simple and complex atmospheric ice particles (2014)
    Wiley
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    Publication Date: 2014-01-23
    Description: [1]  The impact of ice clouds on weather and climate is a function of ice particle shape through light scattering properties and cloud lifetime through ice particle sedimentation rates. Many weather forecast and climate models use two categories to represent ice cloud particles: cloud ice, and snow, though the distinction between particle categories is generally without observational justification. Improved characterization of cloud ice and snow as well as the transition between them will make models more realistic. An analysis of particle imagery data from high resolution aircraft particle imaging probes indicates that atmospheric ice particles can easily be separated by particle complexity. In this work, a technique is described which enables the clear separation of vapor grown particles from aggregates of particles. When applied to two example datasets, the technique shows that the separation between these categories occurs at 150 and 250 microns, for two example datasets.
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    First Satellite-detected Perturbations of Outgoing Longwave Radiation Associated with Blowing Snow Events over Antarctica (2014)
    Wiley
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    Publication Date: 2014-01-16
    Description: [1]  We present the first satellite-detected perturbations of the outgoing longwave radiation (OLR) associated with blowing snow events over the Antarctic ice sheet using data from CALIOP and CERES. Significant cloud-free OLR differences are observed between the clear and blowing snow sky, with the sign and magnitude depending on season and time of the day. During nighttime, OLRs are usually larger when blowing snow is present; the average difference in OLRs between without and with blowing snow over the East Antarctica ice sheet is about -5.2 W/m 2 for the winter months of 2009. During daytime, in contrast, the OLR perturbation is usually smaller or even has the opposite sign. The observed seasonal variations and day-night differences in the OLR perturbation are consistent with theoretical calculations of the influence of blowing snow on OLR. Detailed atmospheric profiles are needed to quantify the radiative effect of blowing snow from the satellite observations.
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    Intensified land surface control on boundary layer growth in a changing climate (2014)
    Wiley
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    Publication Date: 2014-01-17
    Description: [1]  Data from the Coupled Model Intercomparison Project Phase 5 (CMIP5) for historical and future climate scenarios are examined for changes in the energy cycle component of land surface feedbacks on the atmosphere, namely through the linkages from soil moisture to sensible heat flux to the height of the lifting condensation level marking the cloud base. Climate models project heightened sensitivity in both of these segments of the feedback pathway over most of the globe. This is in agreement with studies showing similar increases in land-atmosphere feedbacks through the water cycle, despite different physical processes, and may contribute to prevalent droughts and floods found in most climate change forecasts.
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    Disruptions of El Niño-Southern Oscillation Teleconnections by the Madden Julian Oscillation (2014)
    Wiley
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    Publication Date: 2014-01-17
    Description: [1]  The El Niño-Southern Oscillation (ENSO) is the leading mode of interannual variability, with global impacts on weather and climate that have seasonal predictability. Research on the link between interannual ENSO variability and the leading mode of intraseasonal variability, the Madden-Julian Oscillation (MJO), has focused mainly on the role of MJO initiating or terminating ENSO. We use observational analysis and modeling to show that the MJO has an important simultaneous link to ENSO: strong MJO activity significantly weakens the atmospheric branch of ENSO. For weak MJO conditions relative to strong MJO conditions, the average magnitude of ENSO-associated tropical precipitation anomalies increases by 63% and the strength of hemispheric teleconnections increases by 58%. Since the MJO has predictability beyond three weeks, the relationships shown here suggest that there may be subseasonal predictability of the ENSO teleconnections to continental circulation and precipitation.
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    The Pacific Quasi-Decadal Oscillation (QDO) – An important precursor towards anticipating major flood events in the Missouri River Basin? (2014)
    Wiley
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    Publication Date: 2014-01-17
    Description: [1]  Measurements taken by the Gravity Recovery And Climate Experiment (GRACE) satellites indicated a continued water storage increase over the Missouri River Basin (MRB) prior to the 2011 flood event. An analysis of the major hydrologic variables in the MRB, i.e. those of soil moisture, streamflow, groundwater storage and precipitation, show a marked variability at the 10-15 year timescale coincident with the water storage increase. A climate diagnostic analysis was conducted to determine what climate forcing conditions preceded the long-term changes in these variables. It was found that precipitation over the MRB undergoes a profound modulation during the transition points of the Pacific Quasi-Decadal Oscillation (QDO) and associated teleconnections. The results infer a prominent teleconnection forcing in driving the wet/dry spells in the MRB, and this connection implies the potential for climate prediction of future wet/dry extreme events.
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    Arctic Ocean basin liquid freshwater storage trend 1992 - 2012 (2014)
    Wiley
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    Publication Date: 2014-01-18
    Description: Freshwater in the Arctic Ocean plays an important role in the regional ocean circulation, sea ice and global climate. From salinity observed by a variety of platforms we are able, for the first time, to estimate a statistically reliable liquid freshwater trend from monthly gridded fields over all upper Arctic Ocean basins. From 1992 to 2012 this trend was 600 ± 300  km 3 yr − 1 . A numerical model agrees very well with the observed freshwater changes. A decrease in salinity made up about 2/3 of the freshwater trend and a thickening of the upper layer 1/3. The Arctic Ocean Oscillation index, a measure for the regional wind stress curl, correlated well with our freshwater timeseries. No clear relation to Arctic Oscillation or Arctic Dipole indices could be found. Following other observational studies, an increased Bering Strait freshwater import to the Arctic Ocean, a decreased Davis Strait export and enhanced net sea ice melt could have played an important role in the freshwater trend we observed.
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    Evidence for Coral Island Formation During Rising Sea Level in the Central Pacific Ocean (2014)
    Wiley
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    Publication Date: 2014-01-18
    Description: [1]  The timing and evolution of Jabat Island, Marshall Islands, was investigated using morphostratigraphic analysis and radiometric dating. Results show the first evidence of island building in the Pacific during latter stages of Holocene sea-level rise. A three-phase model of development of Jabat is presented. Initially, rapid accumulation of coarse sediments on Jabat occurred 4,800-4,000 yrs BP across a reef flat higher than present level, as sea level continued to rise. During the highstand, island margins and particularly the western margin, accreted vertically to 2.5-3.0 m above contemporary ridge elevations. This accumulation phase was dominated by sand-size sediments. Phase three involved deposition of gravel ridges on the northern reef, as sea level fell to present position. Jabat has remained geomorphically stable for the past 2,000 years. Findings suggest reef platforms may accommodate the oldest reef islands in atoll systems, which may have profound implications for questions of prehistoric migration through Pacific archipelagos.
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    The impact of different external sources of iron on the global carbon cycle (2014)
    Wiley
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    Publication Date: 2014-01-18
    Description: [1]  Variable supply of iron to the ocean is often invoked to explain part of past changes in atmospheric CO 2 (CO 2atm ). Using model simulations we find that CO 2atm is sensitive on the order 15, 2 and 1 ppm to sedimentary, dust and hydrothermal iron input. CO 2atm is insensitive to dust because it is not the major iron input to the Southern Ocean. Modifications to the relative export of Si(OH) 4 to low latitudes are opposite to those predicted previously. Although hydrothermalism is the major control on the iron inventory in ~25% of the ocean, it remains restricted to the deep ocean, with minor effects on CO 2atm . Nevertheless, uncertainties regarding the iron binding ligand pool can have significant impacts on CO 2atm . Ongoing expansion of iron observations as part of GEOTRACES will be invaluable in refining these results.
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    The importance of shallow hydrothermal island arc systems in ocean biogeochemistry (2014)
    Wiley
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    Publication Date: 2014-01-18
    Description: [1]  Hydrothermal venting often occurs at submarine volcanic calderas on island arc chains, typically at shallower depths than mid-ocean ridges. The effect of these systems on ocean biogeochemistry has been under-investigated to date. Here we show that hydrothermal effluent from an island arc caldera was rich in Fe(III) colloids (0.02 – 0.2 µm; 46% of total Fe), contributing to a fraction of hydrothermal Fe that was stable in ocean water. Iron(III) colloids from island arc calderas may be transferred into surrounding waters (generally 0-1500 m depth) by ocean currents, thereby potentially stimulating surface ocean primary productivity. Hydrothermal Fe oxy-hydroxide particles (〉0.2 µm) were also pervasive in the studied caldera and contained high concentrations of oxyanions of phosphorus (P), vanadium (V), arsenic (As), and manganese (Mn). Hydrothermal island arcs may be responsible for 〉 50% of global hydrothermal P scavenging and 〉 40% V scavenging, despite representing 〈10% of global hydrothermal fluid flow.
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    Sea surface topography retrieved from GNSS reflectometry phase data of the GEOHALO flight mission (2014)
    Wiley
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    Publication Date: 2014-01-18
    Description: [1]  Sea surface topography observations are deduced from an airborne reflectometry experiment. The GORS (GNSS Occultation Reflectometry Scatterometry) receiver was set up aboard the German HALO (High Altitude LOng range) research aircraft. Flights were conducted over the Mediterranean Sea about 3500 m above sea level. A signal path model divided into large and small scale contributions is used for phase altimetry. The results depict geoid undulations and resolve anomalies of the sea surface topography.For the whole experiment 65 tracks over the Mediterranean sea are retrieved and compared with a topography model. Tracks distinguish between RHCP/LHCP. The differnce, however, is not significant for this study. Precision and spatial resolution decrease disproposionately at low elevations. Eight tracks with centimeter precision are obtained between 11 ∘ and 33 ∘ of elevation. At higher elevation angles the number of tracks is significantly reduced due to surface roughness. In future such retrievals could contribute to ocean eddy detection.
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    Pressure-induced enhancement of proton conduction in brucite (2014)
    Wiley
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    Publication Date: 2014-01-19
    Description: [1]  The electrical conductivity of single-crystal brucite was measured as a function of pressure and temperature (P-T) using impedance spectroscopy (IS). IS measurements demonstrated that electrical conductivity perpendicular to the c -axis is nearly half order of magnitude higher than that parallel to the c -axis under the same P-T conditions. Electrical conductivity increased by two orders of magnitude during compression from 3.7 to 11 GPa, irrespective of crystallographic direction. However, the conductivity increase with pressure became less significant upon further compression from 11 to 13 GPa. The pressure effect is closely related to the interactions between neighboring hydroxyls. The ratio of free protons to total hydrogen increases from 2% to 33% when pressure increases from 3.7 to 13 GPa at 950 K. This indicates that the most of protons are bound within the crystal structure at low pressures, whereas more protons become free and mobile at higher pressures.
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    A ubiquitous low velocity layer at the base of the mantle transition zone (2014)
    Wiley
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    Publication Date: 2014-01-19
    Description: [1]  Global stacks of receiver functions clearly exhibit the upper mantle stratification. Besides the most prominent seismic discontinuities, such as the Moho and the 410 and 660 km discontinuities, a negative discontinuity is detected at a depth of ~600 km, indicating a low velocity layer at the base of the mantle transition zone. The slant slack technique helps to identify the primary conversions from the multiple reverberations. Presence of the negative 600 km discontinuity underneath both continent and ocean island stations, where the crustal thickness significantly differs, also precludes the possible cause of crustal reverberations. We conclude that the negative 600 km discontinuity could be a global feature, possibly resulted from accumulation of ancient subducted oceanic crust. The X-discontinuity at ~300 km depth is also observed in our global stacks, which can be explained by the coesite-stishovite phase transformation.
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    Passive margin subduction and the dynamics of collisional orogenesis (2014)
    Wiley
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    Publication Date: 2014-01-19
    Description: [1]  Continental passive margins are characterized by a wide variety of geometries and widths. Whether these variations have an influence on subsequent dynamics of orogenesis is unresolved. To investigate, a series of upper mantle numerical experiments were performed with systematically varied continental margin widths and geometries. Results show that the vertical geometry of subducting continental margin crust controls both crustal and mantle lithosphere deformation. On both scales, deformational end-members can be identified. Namely break-off vs. delamination of continental mantle lithosphere, and double vergence vs. single vergence of crustal thrust fronts form as a direct result of passive margin geometry. We find that the subduction of upper crust to depths 〉 100 km promotes lithospheric delamination and is facilitated by an extended passive margin. Modelled orogens show decreasing upper plate deformation with increasing margin width. These results suggest that along-strike deformational variation within orogens may develop due to pre-collision passive margin geometry.
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    Effects of Mackenzie River discharge and bathymetry on sea ice in the Beaufort Sea (2014)
    Wiley
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    Publication Date: 2014-01-21
    Description: [1]  Mackenzie River discharge and bathymetry effects on sea ice in the Beaufort Sea are examined in 2012 when Arctic sea ice extent hit a record low. Satellite-derived sea surface temperature revealed warmer waters closer to river mouths. By 5 July 2012, Mackenzie warm waters occupied most of an open-water area about 316,000 km 2 . Surface temperature in a common open-water area increased by 6.5 °C between 14 June and 5 July 2012, before and after the river waters broke through a recurrent landfast ice barrier formed over the shallow seafloor offshore the Mackenzie Delta. In 2012, melting by warm river waters was especially effective when the strong Beaufort Gyre fragmented sea ice into unconsolidated floes. The Mackenzie and other large rivers can transport an enormous amount of heat across immense continental watersheds into the Arctic Ocean, constituting a stark contrast to the Antarctic that has no such rivers to affect sea ice.
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    Exploring recent trends in Northern Hemisphere blocking (2014)
    Wiley
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    Publication Date: 2014-01-11
    Description: [1]  Observed blocking trends are diagnosed to test the hypothesis that recent Arctic warming and sea ice loss has increased the likelihood of blocking over the Northern Hemisphere. To ensure robust results, we diagnose blocking using three unique blocking identification methods from the literature, each applied to four different reanalyses. No clear hemispheric increase in blocking is found for any blocking index, and while seasonal increases and decreases are found for specific isolated regions and time periods, there is no instance where all three methods agree on a significant trend. Blocking is shown to exhibit large interannual and decadal variability, highlighting the difficulty in separating any potentially forced response from natural variability.
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    Assessing urbanization impacts on catchment transit times (2014)
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    Publication Date: 2014-01-11
    Description: [1]  Stable isotopes have potential for assessing the hydrologic impacts of urbanization, though it is unclear whether established methods of isotope modeling translate to such disturbed environments. We tested two transit time modeling approaches (using a gamma distribution and a two-parallel linear reservoir (TPLR) model) in a rapidly urbanizing catchment. Isotopic variability in precipitation was damped in streams with attenuation inversely correlated with urban cover. The models captured this reasonably well, though the TPLR better represented the integrated dual response of urban and non-urban areas with reduced uncertainty. Percent urban cover influenced the shape of the catchment transit time distribution. Total urban cover reduced mean transit time to 〈10 days compared with ~1 yr and ~2-3 yrs with 63% and 13% urbanization respectively, whilst it was at 〉4 yrs for non-urban sites.
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    North Pacific Dissolved Inorganic Carbon Variations Related to the Pacific Decadal Oscillation (2014)
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    Publication Date: 2014-01-17
    Description: [1]  We elucidate multi-year variations of sea surface dissolved inorganic carbon (DIC) concentrations in the North Pacific from 2002 to 2008 by using monthly DIC maps derived from partial pressure CO 2 observations. The Pacific Decadal Oscillation (PDO) was related to an east-west seesaw pattern in the North Pacific DIC anomaly field. In the western North Pacific, DIC concentrations were relatively high from mid-2002 to mid-2005 and low after late 2007 compared with climatological values, and in the eastern North Pacific the opposite change was observed. Changes of the forcing factors associated with the PDO could explain the DIC east-west seesaw pattern; horizontal advection, freshwater fluxes and vertical mixing in most regions, CO 2 fluxes south of 40°N, and biological production in the subarctic.
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    Biogeochemical context impacts seawater pH changes resulting from atmospheric sulfur and nitrogen deposition (2014)
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    Publication Date: 2014-01-17
    Description: [1]  Seawater acidification can be induced both by absorption of atmospheric carbon dioxide (CO 2 ) and by atmospheric deposition of sulfur and nitrogen oxides and ammonia. Their relative significance, interplay and dependency on water-column biogeochemistry are not well understood. Using a simple biogeochemical model we show that the initial conditions of coastal systems are not only relevant for CO 2 -induced acidification, but also for additional acidification due to atmospheric acid deposition. Coastal areas undersaturated with respect to CO 2 are most vulnerable to CO 2 -induced acidification, but are relatively least affected by additional atmospheric deposition-induced acidification. In contrast, the pH of CO 2 -supersaturated systems is most sensitive to atmospheric deposition. The projected increment in atmospheric CO 2 by 2100 will increase the sensitivity of coastal systems to atmospheric deposition-induced acidification by up to a factor 4, but the additional annual change in proton concentration is at most 28%.
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    An Improved Mass Budget for the Greenland Ice Sheet (2014)
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    Publication Date: 2014-01-17
    Description: [1]  Extensive ice thickness surveys by NASA's Operation IceBridge enable over a decade of ice discharge measurements at high precision for the majority of Greenland's marine-terminating outlet glaciers, prompting a reassessment of the temporal and spatial distribution of glacier change. Annual measurements for 178 outlet glaciers reveal that, despite widespread acceleration, only 15 glaciers accounted for 77% of the 739 ± 29 Gt of ice lost due to acceleration since 2000, and four accounted for ~50%. Among the top sources of loss are several glaciers that have received little scientific attention. The relative contribution of ice discharge to total loss decreased from 58% before 2005 to 32% between 2009-2012. As such, 84% of the increased in mass loss after 2009 was due to increased surface runoff. These observations support recent model projections that surface mass balance, rather than ice dynamics, will dominate the ice sheet's contribution to 21 st -century sea level rise.
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    Morphology of Asteroid (4179) Toutatis as Imaged by Chang'E-2 Spacecraft (2014)
    Wiley
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    Publication Date: 2014-01-17
    Description: [1]  Close observations by the Chang'E-2 spacecraft reveal that the surface of asteroid (4179) Toutatis is characterized by abundant impact craters with most of them being degraded by surface resetting. The less degraded large crater with a diameter of ~800 m at the south pole is estimated to be produced by an impactor with a diameter of ~50 m from strength crater scaling relations. From the analysis of large impact events on highly porous targets, we argue that Toutatis is likely a rubble-pile body and its two lobes are contact binaries. The fact that Toutatis suffered plenty of impacts with seismic shaking resetting the initial surface features but not resulting in catastrophic disruption is probably because of the material's high attenuation of shock wave.
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    Spatial Localization and Ducting of EMIC Waves: Van Allen Probes and Ground-based Observations (2014)
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    Publication Date: 2014-01-17
    Description: [1]  On 11 th October 2012, during the recovery phase of a moderate geomagnetic storm, an extended interval (〉 18 hours) of continuous EMIC waves was observed by CARISMA and STEP induction coil magnetometers in North America. At around 14:15 UT, both Van Allen Probes B and A (65 degrees magnetic longitude apart) in conjunction with the ground array observed very narrow (ΔL ~ 0.1-0.4) left-hand polarized EMIC emission confined to regions of mass density gradients at the outer edge of the plasmasphere at L ~ 4. EMIC waves were seen with complex polarization patterns on the ground, in good agreement with model results from Woodroffe and Lysak [2012] and consistent with Earth's rotation sweeping magnetometer stations across multiple polarization reversals in the fields in the Earth-ionosphere duct. The narrow L-widths explain the relative rarity of space-based EMIC occurrence, ground-based measurements providing better estimates of global EMIC wave occurrence for input into radiation belt dynamical models.
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    Structure and impact of atmospheric blocking over the Euro-Atlantic region in present day and future simulations (2014)
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    Publication Date: 2014-01-17
    Description: [1]  The spatial structure of winter atmospheric blocking and its impact on the surface temperatures are analysed for the current climate and a strong CO2 emission scenario over the Euro-Atlantic sector, using four different global circulation models. The models perform very well in describing the spatial pattern of meteorological fields associated with blocking, despite the well known negative bias associated with the European blocking frequency. While a slight increase in the frequency of the Atlantic blocking is observed for the future climate, the European blocking frequency remains unchanged, with a net eastward shift apparent for the European warm blocking events. Under enhanced CO2 forcing, Atlantic blocking is associated with reduced amplitudes for positive and negative anomalies both in the geopotential height at 500 hPa and in the surface temperature, in particular for the latter. The anomalies associated with the occurrence of the two types of European blocking (those dominated by warm and cold air masses) exhibit changed shapes and locations in both the geopotential height and surface temperature fields, with only the cold cases leading to severe cold weather conditions over Europe and most of the polar region. Moreover, the eastward shift and amplification of the anticyclone associated with the warm events in the future is found to generate strong positive surface temperature anomalies over the entire polar cap. As a whole, the results show a marked increase in the sensitivity of Arctic temperatures to blocking in the future.
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    Global Time-dependent Chorus Maps from Low-Earth-Orbit Electron Precipitation and Van Allen Probes Data (2014)
    Wiley
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    Publication Date: 2014-01-17
    Description: [1]  Substorm injected electrons (several-100 s keV) produce whistler-mode chorus waves that are thought to have a major impact on the radiation belts by causing both energization and loss of relativistic electrons in the outer belt. High-altitude measurements, such as those from the Van Allen Probes, provide detailed wave measurements at a few points in the magnetosphere. But physics-based models of radiation-belt dynamics require knowledge of the global distribution of chorus waves. We demonstrate that time-dependent, global distributions of near-equatorial chorus wave intensities can be inferred from low-Earth-orbit (LEO) measurements of precipitating low-energy electrons. We compare in-situ observations of near-equatorial chorus waves with LEO observations of precipitating electrons and derive a heuristic formula that relates, quantitatively, electron precipitation fluxes to chorus wave intensities. Finally, we demonstrate how that formula can be applied to LEO precipitation measurements and in-situ Van Allen Probes wave measurements to provide global, data-driven inputs for radiation belt models.
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    The response to MJO-like forcing in a nonlinear shallow-water model (2014)
    Wiley
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    Publication Date: 2014-01-22
    Description: [1]  This study examines the response to Madden-Julian Oscillation (MJO)-like heat forcing in a nonlinear shallow-water model, including monopolar heating source traveling eastward with an around the world period of 48 days and dipolar heating with zonal wave period of 48 days, with zonal wavenumber 2 confined in longitude to the MJO active regions. A jet localized in the Pacific is compared to a zonally uniform boreal basic flow. The results show that the Rossby wave response downstream exhibits intensified quasi-stationary anomalies in the Pacific jet exit region when the MJO-like heat forcing passes the Maritime Continent, in accord with the observational analysis by Adames and Wallace (2014). The dynamical mechanism suggested in this study can be used to interpret the intraseasonal MJO-PNA (Pacific North American Pattern) coherence and other extratropical intraseasonal events.
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    Seasonal to interannual Arctic sea-ice predictability in current GCMs (2014)
    Wiley
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    Publication Date: 2014-01-24
    Description: [1]  We establish the first inter-model comparison of seasonal to interannual predictability of present-day Arctic climate by performing coordinated sets of idealized ensemble predictions with four state-of-the-art global climate models. For Arctic sea-ice extent and volume, there is potential predictive skill for lead times of up to three years, and potential prediction errors have similar growth rates and magnitudes across the models. Spatial patterns of potential prediction errors differ substantially between the models, but some features are robust. Sea-ice concentration errors are largest in the marginal ice zone, and in winter they are almost zero away from the ice edge. Sea-ice thickness errors are amplified along the coasts of the Arctic Ocean, an effect that is dominated by sea-ice advection. These results give an upper bound on the ability of current global climate models to predict important aspects of Arctic climate.
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    Recent Shift in Eurasian Boreal Forest Greening Response may be Associated with Warmer and Drier Summers (2014)
    Wiley
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    Publication Date: 2014-03-12
    Description: Terrestrial ecosystems in the northern high-latitudes are currently experiencing drastic warming and recent studies suggest that boreal forests may be increasingly vulnerable to warming-related factors, including temperature-induced drought stress as well as shifts in fire regimes and insect outbreaks. Here, we analyze interannual relationships in boreal forest greening and climate over the last three decades using newly available satellite vegetation data. Our results suggest that due to continued summer warming in the absence of sustained increases in precipitation a turning point has been reached around the mid-1990s that shifted western central Eurasian boreal forests into a warmer and drier regime. This may be the leading cause for the emergence of large-scale negative correlations between summer temperatures and forest greenness. If such a regime shift would be sustained, the dieback of the boreal forest induced by heat and drought stress as predicted by vegetation models may proceed more rapidly than anticipated.
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    Infrasonic component of volcano-seismic eruption tremor (2014)
    Wiley
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    Publication Date: 2014-03-12
    Description: Air-ground and ground-air elastic wave coupling are key processes in the rapidly developing field of seismo-acoustics and are particularly relevant for volcanoes. During a sustained explosive volcanic eruption, it is typical to record a sustained broadband signal on seismometers, termed eruption tremor. Eruption tremor is usually attributed to a subsurface seismic source process, such as the upward migration of magma and gases through the shallow conduit and vent. However, it is now known that sustained explosive volcanic eruptions also generate powerful tremor signals in the atmosphere, termed infrasonic tremor. We investigate infrasonic tremor coupling down into the ground and its contribution to the observed seismic tremor. Our methodology builds on that proposed by Ichihara et al. (2012) and involves cross correlation, coherence, and cross-phase spectra between waveforms from nearly collocated seismic and infrasonic sensors; we apply it to datasets from Mount St. Helens, Tungurahua, and Redoubt volcanoes.
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    Ocean-atmosphere forcing of centennial hydroclimate variability in the Pacific Northwest (2014)
    Wiley
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    Publication Date: 2014-03-12
    Description: Reconstructing centennial timescale hydroclimate variability during the late Holocene is critically important for understanding large-scale patterns of drought and their relationship with climate dynamics. We present sediment oxygen isotope records spanning the last two millennia from 10 lakes, as well as climate model simulations, indicating that the Little Ice Age was dry relative to the Medieval Climate Anomaly in much of the Pacific Northwest of North America. This pattern is consistent with observed associations between the El Niño Southern Oscillation (ENSO), the Northern Annular Mode and drought as well as with proxy-based reconstructions of Pacific ocean-atmosphere variations over the past 1000 years. The large amplitude of centennial variability indicated by the lake data suggests that regional hydroclimate is characterized by longer-term shifts in ENSO-like dynamics, and that an improved understanding of the centennial timescale relationship between external forcing and drought conditions is necessary for projecting future hydroclimatic conditions in western North America.
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    Fusion characteristics of volcanic ash relevant to aviation hazards (2014)
    Wiley
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    Publication Date: 2014-03-13
    Description: The fusion dynamics of volcanic ash strongly impacts deposition in hot parts of jet engines. In this study, we investigate the sintering behavior of volcanic ash using natural ash of intermediate composition, erupted in 2012 at Santiaguito volcano, Guatemala. A material science procedure was followed in which we monitored the geometrical evolution of cylindrical-shape volcanic ash compact upon heating from 50 to 1400 °C in a heating microscope. Combined morphological, mineralogical and rheological analyses helped define the evolution of volcanic ash during fusion and sintering, and constrain their sticking potential as well as their ability to flow at characteristic temperatures. For the ash investigated, 1240 °C marks the onset of adhesion and flowability. The much higher fusibility of ash compared to that of typical test sands demonstrates for the need of a more extensive fusion characterization of volcanic ash in order to mitigate the risk posed on jet engine operation.
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    Seasonal variation in total water storage in California inferred from GPS observations of vertical land motion (2014)
    Wiley
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    Publication Date: 2014-03-13
    Description: GPS is accurately recording vertical motion of Earth's surface in elastic response to seasonal changes in surface water storage in California. California's mountains subside up to 12 mm in the fall and winter due to the load of snow and rain, then rise an identical amount in the spring and summer when the snow melts, the rain runs off, and soil moisture evaporates. We invert the GPS observations of seasonal vertical motions to infer changes in equivalent water thickness. GPS resolves the distribution of change in total water across California's physiographic provinces at a resolution of 50 km, compared to 200 km resolution from GRACE. The seasonal surface water thickness change is 0.6 m in the Sierra Nevada, Klamath, and southern Cascade Mountains and decreases sharply to about 0.1 m east into the Great Basin and west toward the Pacific coast. GPS provides an independent inference of change in total surface water, indicating water storage to be on average 50 per cent larger than in the NLDAS–Noah hydrology model, likely due to larger changes in snow and reservoir water than in the model.
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    A quantitative analysis of stratospheric HCl, HNO3 and O3 in the tropopause region near the subtropical jet (2014)
    Wiley
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    Publication Date: 2014-03-13
    Description: The effects of chemical two-way mixing on the Extratropical Transition Layer (ExTL) near the subtropical jet (STJ) is investigated by stratospheric tracer-tracer correlations. To this end, in-situ measurements were performed west of Africa (25- 32°N) during the TACTS/ESMVal mission in August/September 2012. The Atmospheric chemical Ionization Mass Spectrometer AIMS sampling HCl and HNO 3 was for the first time deployed on the new German High Altitude and LOng range research aircraft HALO. Measurements of O 3 , CO, ECMWF analysis and the tight correlation of the unambiguous tracer HCl to O 3 and HNO 3 in the lower stratosphere were used to quantify the stratospheric content of these species in the ExTL. With increasing distance from the tropopause the stratospheric content increased from 10% to 100% with differing profiles for HNO 3 and O 3 . Tropospheric fractions of 20% HNO 3 and 40% O 3 were detected up to a distance of 30 K above the tropopause.
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    Internal Solitary Wave Propagation Observed by Tandem Satellites (2014)
    Wiley
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    Publication Date: 2014-03-13
    Description: Internal solitary waves (ISWs) are observed two times within 30 minutes in synthetic aperture radar (SAR) image pairs from the Envisat and ERS-2 tandem satellites. Three pairs of SAR images were acquired in the South China Sea (SCS) in April 2007, August 2008 and March 2009, and 13 ISWs were tracked between the image pair in an arcGIS environment. The phase speeds of these ISWs are calculated from their spatial displacement and time interval. The resultant ISW speeds agree well with the theoretical values estimated from the Sturm-Louisville equation using local bathymetric and monthly climatology ocean stratification data. This technique reveals the spatial variation in the ISWs speed in the water depth between 100 and 4000 m in the SCS. The study shows that ISWs speed is mainly affected by bottom topography, and generally decreases from deep to shallow water from east to west and from south to north.
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    Attribution of extreme weather to anthropogenic greenhouse gas emissions: sensitivity to spatial and temporal scales (2014)
    Wiley
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    Publication Date: 2014-03-13
    Description: Recent studies have examined the anthropogenic contribution to specific extreme weather events, such as the European (2003) and Russian (2010) heat waves. While these targeted studies examine the attributable risk of an event occurring over a specified temporal and spatial domain, it is unclear how effectively their attribution statements can serve as a proxy for similar events occurring at different temporal and spatial scales. Here we test the sensitivity of attribution results to the temporal and spatial scales of extreme precipitation and temperature events by applying a probabilistic event attribution framework to output of two global climate models, each run with and without anthropogenic greenhouse gas emissions. Attributable risk tends to be more sensitive to the temporal than spatial scale of the event, increasing as event duration increases. Globally, correlations between attribution statements at different spatial scales are very strong for temperature extremes and moderate for heavy precipitation extremes.
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    Sahel Rainfall in Multimodel CMIP5 Decadal Hindcasts (2014)
    Wiley
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    Publication Date: 2014-03-14
    Description: This study assesses the capability of CMIP5 decadal hindcasts to represent Sahel rainfall and a relative sea surface temperature (SST) index (RSI). The RSI measures the relative difference between subtropical North Atlantic SST and tropical SST and is highly correlated with Sahel rainfall. Ten-year predictions from 15 models initialized every five years (six initialized every year) beginning in 1960 are evaluated. The hindcasts show increased decadal variability compared to the uninitialized historical simulations, with a larger magnitude drought and smaller RSI in the 1970s and 1980s. The multimodel ensemble mean shows skillful predictions that are more accurate than persistence and historical simulations. Models that were able to realistically simulate the correlation between the RSI and Sahel rainfall in uninitialized historical simulations produced more skillful decadal hindcasts of Sahel rainfall as these models were able to successfully translate the SST that was improved through initialization, to more skillful Sahel rainfall predictions.
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    First optical observations of energetic electron precipitation at 4278 Å caused by a powerful VLF transmitter (2014)
    Wiley
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    Publication Date: 2014-03-14
    Description: A summary is presented of experimental optical observations at 4278 Å from close to a powerful (~150 kW) VLF transmitter (call-sign JXN) with a transmission frequency of 16.4 kHz. Approximately 2.5 seconds after transmitter turn-on, a sudden increase in optical emissions at 4278 Å was detected using a dedicated camera/CCD monitoring system recording at a frequency of 10 Hz. The optical signal is interpreted as a burst of electron precipitation lasting ~0.5 seconds, due to gyro-resonant wave-particle interactions between the transmitted wave and the magnetospheric electron population. The precipitation was centered on the zenith and had no detectable spatial structure. The timing of this sequence of events is in line with theoretical predictions and previous indirect observations of precipitation. This first direct measurement of VLF-induced precipitation at 4278 Å reveals the spatial and temporal extent of the resulting optical signal close to the transmitter.
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    Influence of climate variability on near-surface ozone depletion events in the Arctic spring (2014)
    Wiley
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    Publication Date: 2014-03-14
    Description: Near-surface ozone depletion events (ODEs) generally occur in the Arctic spring, and the frequency shows large inter-annual variations. We use surface ozone measurements at Barrow, Alert, and Zeppelinfjellet to analyze if their variations are due to climate variability. In years with frequent ODEs at Barrow and Alert, the Western Pacific (WP) teleconnection pattern is usually in its negative phase, during which the Pacific jet is strengthened but the storm track originated over western Pacific is weakened. Both factors tend to reduce the transport of ozone-rich air-mass from mid-latitudes to the Arctic, creating a favorable environment for the ODEs. Correlation of ODE frequencies at Zeppelinfjellet with WP indices is higher in the 2000s, reflecting stronger influence of the WP pattern in recent decade to cover ODEs in broader Arctic regions. We find that the WP pattern can be used to diagnose ODE changes and subsequent environmental impacts in the Arctic spring.
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    Global and regional variability in marine surface temperatures (2014)
    Wiley
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    Publication Date: 2014-03-14
    Description: The temperature variability simulated by climate models is generally consistent with that observed in instrumental records at the scale of global averages, but further insight can also be obtained from regional analysis of the marine temperature record. A protocol is developed for comparing model simulations to observations that accounts for observational noise and missing data. General consistency between CMIP5 model simulations and regional sea surface temperature variability is demonstrated at interannual timescales. At interdecadal timescales, however, the variability diagnosed from observations is significantly greater. Discrepancies are greatest at low-latitudes, with none of the 41 models showing equal or greater interdecadal variability. The pattern of suppressed variability at longer timescales and smaller spatial scales appears consistent with models generally being too diffusive. Suppressed variability of low-latitude marine temperatures points to underestimation of intrinsic variability and may help explain why few models reproduce the observed temperature trends during the last fifteen years.
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    Isolating the Anthropogenic Component of Arctic Warming (2014)
    Wiley
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    Publication Date: 2014-05-03
    Description: Structural equation modeling is used in statistical applications as both confirmatory and exploratory modeling: to test models and to suggest the most plausible explanation for a relationship between the independent and the dependent variables. Although structural analysis cannot prove causation, it can suggest the most plausible set of factors that influence the observed variable. We apply structural model analysis to the annual mean Arctic surface air temperature from 1900-2012 to find the most effective set of predictors, and to isolate the anthropogenic component of the recent Arctic warming by subtracting the effects of natural forcing and variability from the observed temperature. We find that anthropogenic greenhouse gases and aerosols radiative forcing, and the Atlantic Multi-decadal Oscillation internal mode dominate Arctic temperature variability. Our structural model analysis of observational data suggests that about half of the recent Arctic warming of 0.64 K/decade may have anthropogenic causes.
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    Pacific plate-motion change at the time of the Hawaiian-Emperor bend constrains the viscosity of Earth's asthenosphere (2014)
    Wiley
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    Publication Date: 2014-05-02
    Description: Important constraints on asthenospheric viscosity come primarily from modelling the glacial rebound of the past 20 kyr, but remain somewhat loose because of the intrinsic resolving power of these models. We obtain narrower bounds by building on the notion that the asthenosphere also controls the ability to change plate motions over Myr. We focus on the Pacific kinematic change at the time of the Hawaiian–Emperor bend event, which is linked to the coeval inception of subduction in the Western Pacific. We sample plausible asthenospheric viscosity and thickness values by requiring the rate at which torque varied to generate the observed kinematics consistent with the nature of subduction initiation. Uncertainties on the bend–event duration and the occurrence of Pacific hotspots drift do not hamper our results that suggest that the asthenosphere viscous response to vertical shear over kyr is consistent with that to horizontal shear over Myr.
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    Symmetry, Randomness, and Process in the Structure of Branched Channel Networks (2014)
    Wiley
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    Publication Date: 2014-05-04
    Description: The branched structure of channel networks has a primary impact on the spatial distribution of elevation, water, and life across Earth's surface from the hillslope to the continental scale, and is also observed on other planets. However, the link between this dendritic multi-scale structure and the erosional processes that sculpt it has remained elusive for more than six decades. In fact, many topologic measures fail to distinguish natural networks from those generated by random walks. Here we show that a fundamental multi-scale topologic symmetry is ingrained into the structure of these networks and reflects the equal elevation drop spanned by flows that split at the drainage divide and meet again downslope. We demonstrate that this symmetry distinguishes random-walk networks from natural ones, captures the temporal evolution of these networks, and divulges information about the processes that shape them.
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    Tropical influence independent of ENSO on the austral summer Southern Annular Mode (2014)
    Wiley
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    Publication Date: 2014-05-04
    Description: A link between atmospheric variability in the Tropics independent of ENSO and the Southern Annular Mode (SAM) is found based on seasonal mean data for austral summer. Variations associated with El Niño Southern Oscillation (ENSO) are removed usinga linear method and a Tropics Index (TI) is defined as the zonal average of the ENSO-removed 500 hPa geopotential height between 10° S and 10° N . Since the detrended TI shows no link to SST variability in the Tropics, it appears to be related to internal atmospheric variability. We find that the TI can explain about 40% variance of the SAM interannual variability and about 75% of the SAM long term trend between 1957/58 and 2001/02, where here the SAM includes the ENSO signal. Positive/negative values of the TI are associated with the positive/negative SAM. A possible link between the TI and global warming is noted.
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    Efflorescence upon humidification? X-ray microspectroscopic in-situ observation of changes in aerosol microstructure and phase state upon hydration (2014)
    Wiley
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    Publication Date: 2014-05-06
    Description: The phase and mixing state of atmospheric aerosols is a central determinant of their properties and, thus, their role in atmospheric cycling and climate. Particularly, the hygroscopic response of aerosol particles to relative humidity (RH) variation is a key aspect of their atmospheric life cycle and impacts. Here, we applied X-ray microspectroscopy under variable RH conditions to internally mixed aerosol particles from the Amazonian rainforest collected during periods with anthropogenic pollution. Upon hydration, we observed substantial and reproducible changes in particle microstructure, which appear as mainly driven by efflorescence and recrystallization of sulfate salts. Multiple solid and liquid phases were found to coexist, especially in intermediate humidity regimes. We show that X-ray microspectroscopy under variable RH is a valuable technique to analyze the hygroscopic response of individual ambient aerosol particles. Our initial results underline that RH changes can trigger strong particle restructuring, in agreement with previous studies on artificial aerosols.
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    The Robustness of the Atmospheric Circulation and Precipitation Response to Future Anthropogenic Surface Warming (2014)
    Wiley
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    Publication Date: 2014-03-20
    Description: The impact of long-term sea surface temperature (SST) change on the atmospheric circulation is studied by comparing atmospheric general circulation model (AGCM) simulations forced with a spatially uniform SST increase and a structured SST increase. The structured SST increase is calculated from the response of an ensemble of coupled ocean-atmosphere models to increased CO 2 . Most of the impact of SST pattern change is confined to equatorial Indo-Pacific. However, the circulation change under the two types of SST forcing is similar over the rest of the tropics and almost identical in the extratropics, indicating that the pattern of future SST change has overall little impact on the response of the atmospheric circulation and, in turn, on the resulting changes in precipitation. The tropical similarity is argued to result from energetic constraints that weaken the atmospheric circulation, whereas the extratropical similarity likely results from the insensitivity of Rossby Wave generation to the changes in near-equatorial upper-level divergence. A comparison of the AGCM simulations with those from externally-forced coupled ocean-atmosphere models suggest that ocean coupling or the direct effect of radiative forcing has a larger impact on the projected changes in circulation and precipitation than the pattern of SST change over most regions.
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    How Will Orographic Precipitation Respond to Surface Warming?—An Idealized Thermodynamic Perspective (2014)
    Wiley
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    Publication Date: 2014-03-20
    Description: Future changes in orographic precipitation will have important consequences for societies and ecosystems near mountain ranges. Here we use a simple numerical model to evaluate the response of orographic precipitation to surface warming under idealized conditions representative of the strongest orographic storms. We find an upward shift in the pattern of condensation with warming, caused by larger fractional changes in condensation at low temperature and amplified warming aloft. As a result, the distribution of precipitation shifts downwind, causing larger fractional changes in precipitation on the lee slope than on the windward slope. Total precipitation is found to increase by a smaller fraction than near-surface water vapor, in contrast to expected changes in other types of extreme precipitation. Factors limiting the increase in orographic precipitation include the pattern of windward ascent, lee-side evaporation, and thermodynamic constraints on the change in condensation with temperature.
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    Elevated rates of organic carbon, nitrogen and phosphorus accumulation in a highly impacted mangrove wetland (2014)
    Wiley
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    Publication Date: 2014-03-20
    Description: The effect of nutrient enrichment on mangrove sediment accretion and carbon accumulation rates is poorly understood. Here we quantify sediment accretion through radionuclide tracers to determine organic carbon (OC), total nitrogen (TN) and total phosphorus (TP) accumulation rates during the previous 60 years in both a nutrient-enriched and a pristine mangrove forest within the same geomorphological region of south-eastern Brazil. The forest receiving high nutrient loads has accumulated OC, TN and TP at rates that are 4, 2 and 8 fold higher than those from the undisturbed mangrove. Organic carbon and TN stable isotopes (δ 13 C and δ 15 N) signal an increased presence of organic matter (OM) originating with either phytoplankton, benthic algae or another allochthonous source within the more rapidly-accumulated sediments of the impacted mangrove. This suggests that the accumulation rate of OM in eutrophic mangrove systems may be enhanced through the addition of autochthonous and allochthonous non-mangrove material.
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    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    Impact of the Sea Surface Temperature Rise on Storm-Track Clouds in Global Non-hydrostatic Aqua-Planet Simulations (2014)
    Wiley
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    Publication Date: 2014-04-30
    Description: Aqua-planet experiments were conducted to investigate storm-track cloud change due to sea surface temperature (SST) rise. 14 km-mesh global non-hydrostatic model was employed with an explicit cloud microphysical process, and the model output data were composited to the cyclone center. Both the column-integrated liquid and ice cloud contents are significantly increased around the cyclone center due to the SST rise. The occurrence of low-level liquid clouds becomes more frequent not only near the cyclone center, but also for all of the higher latitudes, which cannot be seen in low-resolution models. This as well as thicker liquid clouds enhances shortwave cooling. Upper-level ice clouds occur more frequently on the east side of the cyclone center, and they partly offset the enhanced shortwave cooling through longwave warming. These results may imply an importance of the cloud-scale process with cloud microphysics on the storm-track clouds and their radiative forcing.
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    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    Response of the Mantle to Flat Slab Evolution: Insights from Local S Splitting beneath Peru (2014)
    Wiley
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    Publication Date: 2014-05-01
    Description: The dynamics of flat-subduction, particularly the interaction between a flat slab and the overriding plate, are poorly understood. Here we study the (seismically) anisotropic properties and deformational regime of the mantle directly above the Peruvian flat-slab. We analyze shear wave splitting from 370 local S events at 49 stations across southern Peru. We find that the mantle above the flat slab appears to be anisotropic, with modest average delay times (~0.28 s) that are consistent with ~4% anisotropy in a ~30 km thick mantle layer. The most likely mechanism is the lattice preferred orientation (LPO) of olivine, which suggests that the observed splitting pattern preserves information about the mantle deformation. We observe a pronounced change in anisotropy along strike, with predominately trench-parallel fast directions in the north and more variable orientations in the south, which we attribute to the ongoing migration of the Nazca Ridge through the flat-slab system.
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    tFISH/RAPiD: rapid improvement of near-field tsunami forecasting based on offshore tsunami data by incorporating onshore GNSS data (2014)
    Wiley
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    Publication Date: 2014-05-01
    Description: We have developed a new algorithm that improves near-field tsunami forecasting based on offshore tsunami data soon after an earthquake by incorporating real-time onshore Global Navigation Satellite System (GNSS) data. In our algorithm, called tFISH/RAPiD, the initial sea-surface height distribution estimated from rapidly acquired GNSS data provides robust finite source size information that is incorporated into an offshore tsunami data inversion for reliable tsunami predictions along the near-field coast. Our algorithm can be applicable to arbitrary types of large tsunamigenic earthquakes when the static displacements are substantial enough to be detected at onshore GNSS stations. We retrospectively applied our algorithm to the 2011 M w 9.0 Tohoku earthquake and demonstrated its ability to provide information about disastrous tsunamis approaching wide areas along the near-field coast. Furthermore, arrival times and wave heights of large-amplitude, short-period tsunamis affecting specific near-field coasts can be predicted at least 5 minutes before the actual tsunami arrivals.
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    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    What causes the location of the air-sea turbulent heat flux maximum over the Labrador Sea? (2014)
    Wiley
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    Publication Date: 2014-05-01
    Description: The Labrador Sea is a region of climatic importance as a result of the occurrence of oceanic wintertime convection, a process that is integral to the Atlantic Meridional Overturning Circulation. This process requires large air-sea heat fluxes that result in a loss of surface buoyancy, triggering convective overturning of the water column. The Labrador Sea wintertime turbulent heat flux maximum is situated downstream of the ice edge, a location previously thought to be causal. Here we show that there is considerable similarity in the characteristics of the regional mean atmospheric circulation and high heat flux events over the Labrador Sea during early-winter, when the ice is situated to the north, and mid-winter, when it is near the region of maximum heat loss. This suggests that other factors, including the topography of the nearby upstream and downstream landmasses, contribute to the location of the heat flux maximum.
    Print ISSN: 0094-8276
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    Climate variability features of the last interglacial in the East Antarctic EPICA Dome C ice core (2014)
    Wiley
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    Publication Date: 2014-05-01
    Description: Whereas millennial to sub-millennial climate variability has been identified during the current interglacial period, past interglacial variability features remain poorly explored because of lacking data at sufficient temporal resolutions. Here, we present new deuterium data from the EPICA Dome C ice core, documenting at decadal resolution temperature changes occurring over the East Antarctic plateau during the warmer-than-today last interglacial. Expanding previous evidence of instabilities during the last interglacial, multi-centennial sub-events are identified and labelled for the first time in a past interglacial context. A variance analysis further reveals two major climatic features. First, an increase in variability is detected prior to the glacial inception, as already observed at the end of Marine Isotopic Stage 11 in the same core. Second, the overall variance level is systematically higher during the last interglacial than during the current one, suggesting that a warmer East Antarctic climate may also be more variable.
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    To what extent can interannual CO2 variability constrain carbon cycle sensitivity to climate change in CMIP5 Earth System Models? (2014)
    Wiley
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    Publication Date: 2014-05-01
    Description: We analyze the carbon–climate feedback in eight Earth System Models (ESMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5). We focus on tropical land carbon change and find decreases (−31.02 to −169.32 GtC K −1 ), indicating tropical ecosystems will release carbon as temperature warms, thus contributing to a positive feedback identified in earlier studies. We further investigate the relationship between tropical land carbon change and sensitivity of historical atmospheric CO 2 growth rate to tropical temperature variability, and find a weak linear relationship. This sensitivity for most models is stronger than observed. We further use this " emergent constraint" [ Cox et al. , 2013] to constrain uncertainties in model-projected future carbon–climate changes, and find little effect in narrowing the model-spread, but the mean sensitivity is slightly smaller. This contrasts with earlier C 4 MIP results, highlighting the challenge in constraining future projections by modern observations and necessity for evaluating such relationships continuously.
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