ALBERT

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.

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.

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.

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.

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.

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).

Description: ABSTRACT A reactive transport modeling framework is presented that allows simultaneous assessment of groundwater flow, water quality evolution including δ 13 C, and 14 C activity or “age”. Through application of this framework, simulated 14 C activities can be directly compared with measured 14 C activities. This bypasses the need for interpretation of a 14 C age prior to flow simulation through factoring out processes other than radioactive decay, which typically involves simplifying assumptions regarding spatial and temporal variability in reactions, flow, and mixing. The utility of the approach is demonstrated for an aquifer system with spatially variable carbonate mineral distribution, multiple organic carbon sources, and transient boundary conditions for 14 C activity in the recharge water. In this case the simulated 14 C age was shown to be relatively insensitive to isotopic fractionation during DOC oxidation and variations in assumed DOC degradation behaviour. We demonstrate that the model allows quantitative testing of hypotheses regarding controls on groundwater age and water quality evolution for all three carbon isotopes. The approach also facilitates incorporation of multiple environmental tracers and combination with parameter optimization techniques. This article is protected by copyright. All rights reserved.

Description: Water scarcity is likely to increase in the coming years, making improvements in irrigation efficiency increasingly important. An emerging technology that promises to increase irrigation efficiency substantially is a wireless irrigation sensor network that uploads sensor data into irrigation management software, creating an integrated system that allows real-time monitoring and control of moisture status that has been shown in experimental settings to reduce irrigation costs, lower plant loss rates, shorten production times, decrease pesticide application, and increase yield, quality, and profit. We use an original survey to investigate likely initial acceptance, ceiling adoption rates, and profitability of this new sensor network technology in the nursery and greenhouse industry. We find that adoption rates for a base system and demand for expansion components are decreasing in price, as expected. The price elasticity of the probability of adoption suggests that sensor networks are likely to diffuse at a rate somewhat greater than that of drip irrigation. Adoption rates for a base system and demand for expansion components are increasing in specialization in ornamental production: Growers earning greater shares of revenue from greenhouse and nursery operations are willing to pay more for a base system and are willing to purchase larger numbers of expansion components at any given price. We estimate that growers who are willing to purchase a sensor network expect investment in this technology to generate significant profit, consistent with findings from experimental studies. This article is protected by copyright. All rights reserved.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Description: This technical note presents a useful methodology for studying how the variance of hydraulic and/or reactive attributes of an aquifer are linked to the multi-scaled and hierarchical sedimentary architecture of the aquifer. A new recursive equation is derived which quantitatively describes how the variance is related to sedimentary facies defined at all scales across an entire stratal hierarchy. As compared to prior published equations that emphasize differences in means among facies populations within a hierarchical level, it emphasizes differences across levels. Because of the hierarchical relationships among the terms of the equation, we find it to be useful for conducting a holistic analysis of the relative contributions to the variance arising from all facies types defined across all scales. The methodology is demonstrated using appropriate field data, and is shown to be useful in defining parsimonious classification systems.

Description: ABSTRACT A primary concern for geologic carbon storage is the potential for leakage of stored carbon dioxide (CO 2 ) into the shallow subsurface where it could degrade the quality of groundwater and surface water. In order to predict and mitigate the potentially negative impacts of CO 2 leakage, it is important to understand the physical processes that CO 2 will undergo as it moves through naturally heterogeneous porous media formations. Previous studies have shown that heterogeneity can enhance the evolution of gas phase CO 2 in some cases, but the conditions under which this occurs have not yet been quantitatively defined, nor tested through laboratory experiments. This study quantitatively investigates the effects of geologic heterogeneity on the process of gas phase CO 2 evolution in shallow aquifers through an extensive set of experiments conducted in a column that was packed with layers of various test sands. Soil moisture sensors were utilized to observe the formation of gas phase near the porous media interfaces. Results indicate that the conditions under which heterogeneity controls gas phase evolution can be successfully predicted through analysis of simple parameters, including the dissolved CO 2 concentration in the flowing water, the distance between the heterogeneity and the leakage location, and some fundamental properties of the porous media. Results also show that interfaces where a less permeable material overlies a more permeable material affect gas phase evolution more significantly than interfaces with the opposite layering.

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.

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.

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.

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.

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.

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.

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.

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.

Description: Fluvial sediment loads are frequently calculated with rating curves fit to measured sediment transport rates. Rating curves are often treated as statistical representations in which the fitted parameters have little or no physical meaning. Such models, however, may produce large errors when extrapolation is needed, and they provide no insight into the sediment transport process. It is shown that log-linear least squares, the usual method for fitting rating curves, does not generally produce physically meaningful parameter values. In addition, it cannot accommodate data that include zero-transport samples. Alternative fitting methods based non-linear least squares and on maximum likelihood parameter estimation are described and evaluated. The maximum likelihood approach is shown to fit synthetic data better than linear or non-linear least squares, and to perform well with data that include zero-transport samples. In contrast, non-linear least squares methods produce large errors in the parameter estimates when zero-transport samples are present or when the variance structure of the data is incorrectly specified. Analyses with fractional bedload data from a mountain stream suggest that bedload transport rates are gamma distributed, that the arrivals of bedload particles in a sampler conform to a Poisson distribution, and that the variance of non-zero samples can be expressed as a power function of the mean. Preliminary physical interpretations of variations in the rating curve parameters fit to fractional bedload data with the maximum likelihood method are proposed, and their relation to some previous interpretations of rating curve parameters are briefly discussed. This article is protected by copyright. All rights reserved.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Description: This paper presents the results of a comprehensive model-based analysis of a uranyl [U(VI)] tracer test conducted at the U.S. DOE Hanford 300 Area (300A) IFRC. Despite the highly complex field conditions the numerical three-dimensional multi-component reactive transport model was able to capture most of the spatiotemporal variations of the observed U(VI) concentrations. A multi-model analysis was performed to interrogate the relative importance of various processes and factors for controlling field-scale reactive transport during the uranyl tracer test. The results indicate that multi-rate sorption/desorption, surface complexation reactions, and initial concentrations were the most important processes and factors controlling U(VI) migration. On the other hand, cation exchange reactions, the choice of the surface complexation model, and dual-domain mass transfer processes played less important roles under the prevailing field-test condition. Further analysis of the modeling results demonstrates that these findings are conditioned to the relatively stable groundwater chemistry and the selected length of the field experimental duration (16 days). The model analysis also revealed the crucial role of the intraborehole flow that occurred within the long-screened monitoring wells and thus affected both field measurements and simulated U(VI) concentrations as a combined effect of aquifer heterogeneity and dynamic flow conditions. This study provides the first highly data-constrained uranium transport simulations under highly dynamic flow conditions. It illustrates the value of reactive transport modeling for elucidating the relative importance of individual processes in controlling uranium transport under specific field-scale conditions.

Description: The dynamics of drying processes from porous media are critically influenced by the intensity of an adjacent free flow and by processes at the interface between free flow and the porous medium. In this paper, the influence of hydraulic properties of a porous medium and of the interaction between fluids and porous medium on the drying dynamics during the capillary-flow dominated stage-1 and transition to the diffusion-dominated stage-2 are studied using a coupled free-flow - porous-medium flow model on the REV scale. We present a detailed model concept that considers mass balance equations, an energy balance equation and the coupling to the adjacent free flow. Key microscale processes are identified and incorporated in the macroscale description of the evaporation process. Own experimental results are used to illustrate main features of the modeling framework. We demonstrate that the use of a homogeneous distribution of soil parameters without consideration of pore-scale induced nonlinearities in the numerical simulations results in a rather constant drying rate in stage-1, which was not observed for the high evaporative demand in the experiments. To account for the dependency of the drying rate on the surface moisture content, special conditions based on the work of Haghighi et al. [2013] and Schlünder [1988] are analyzed for their applicability on the REV scale. Typical features of a drying process, such as different stages of the drying rate, could be reproduced.

Description: Making useful predictions in ungauged basins is an incredibly difficult task given the limitations of hydrologic models to represent physical processes appropriately across the heterogeneity within and among different catchments. Here, we introduce a new method for this challenge, Bayes empirical Bayes, that allows for the statistical pooling of information from multiple donor catchments and provides the ability to transfer parametric distributions rather than single parameter sets to the ungauged catchment. Further, the methodology provides an efficient framework with which to formally assess predictive uncertainty at the ungauged catchment. We investigated the utility of the methodology under both synthetic and real data conditions, and with respect to its sensitivity to the number and quality of the donor catchments used. This study highlighted the ability of the hierarchical Bayes empirical Bayes approach to produce expected outcomes in both the synthetic and real data applications. The method was found to be sensitive to the quality (hydrologic similarity) of the donor catchments used. Results were less sensitive to the number of donor catchments, but indicated that predictive uncertainty was best constrained with larger numbers of donor catchments (but still adequate with fewer donors)

Description: Autumn is a season of dynamic change in forest streams of the northeastern USA due to effects of leaf fall on both hydrology and biogeochemistry. Few studies have explored how interactions of biogeochemical transformations, various nitrogen sources, and catchment flowpaths affect stream nitrogen variation during autumn. To provide more information on this critical period, we studied 1) the timing, duration, and magnitude of changes to stream nitrate, dissolved organic nitrogen (DON), and ammonium concentrations; 2) changes in nitrate sources and cycling; and 3) source areas of the landscape that most influence stream nitrogen. We collected samples at higher temporal resolution for a longer duration than typical studies of stream nitrogen during autumn. This sampling scheme encompassed the patterns and extremes that occurred during baseflow and stormflow events of autumn. Baseflow nitrate concentrations decreased by an order of magnitude from 5.4 to 0.7 μmol L -1 during the week when most leaves fell from deciduous trees. Changes to rates of biogeochemical transformations during autumn baseflow explained the low nitrate concentrations; in-stream transformations retained up to 72% of the nitrate that entered a stream reach. A decrease of in-stream nitrification coupled with assimilatory nitrate uptake was a primary factor in the seasonal nitrate decline. The period of low nitrate concentrations ended with a storm event in which stream nitrate concentrations increased by 25 fold. In the ensuing weeks, stormflow nitrate concentrations progressively decreased over closely-spaced, yet similarly sized events. Most stormflow nitrate originated from nitrification in near-stream areas with occasional, large inputs of unprocessed atmospheric nitrate, which has rarely been reported for non-snowmelt events. A maximum input of 33% unprocessed atmospheric nitrate to the stream occurred during one event. The large inputs of unprocessed atmospheric nitrate show direct and rapid effects on forest streams that may be widespread, although undocumented, throughout nitrogen-polluted temperate forests. In contrast to a week-long nitrate decline during peak autumn litterfall, baseflow DON concentrations increased after leaf fall and remained high for two months. Dissolved organic nitrogen was hydrologically flushed to the stream from riparian soils during stormflow. In contrast to distinct seasonal changes in baseflow nitrate and DON concentrations, ammonium concentrations were typically at or below detection limit, similar to the rest of the year. Our findings reveal couplings among catchment flow paths, nutrient sources and transformations that control seasonal extremes of stream nitrogen in forested landscapes.

Description: During the recent years there has been an increasing interest in multivariate frequency analysis of hydrological variables, e.g. those describing extreme events like rainfall, floods or droughts. The multivariate analysis provides a better understanding of the phenomena under investigation and an additional insight about the interrelationships between the different variables (e.g. peak, volume and duration of the flood), exploiting the complete structure of the problem and making a full use of the available data. However, while the developments on multivariate analysis of hydrological data has produced a large body of literature, a clear assessment of the use of these methods in the design and risk assessment of hydraulic structures is still a matter of debate. In the present work we illustrate a general, structure-based framework for the design and/or risk assessment of hydraulic structures in a bivariate environment; we also compare it to recently proposed methods which are based on the assumption of hydrological design events (as is customary in the univariate context). For illustration purposes, both the structure-based and the design event-based approaches are applied to the design of an idealized structure, thus exploring the differences among the methods as function of the parameters involved. Our work highlights that the return period of structure failure in a multivariate environment strictly depends on the particular structure under design, and in most cases the design of an hydraulic structure cannot be based on a single, hydrological multivariate design event. This acts as a warning for practitioners against the use of design methods based on single hydrological events, as usually done in the context of univariate hydrology, thus neglecting the interplay between the structure and the hydrological loads acting on it.

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.

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.

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.

Description: Proliferation of evapotranspiration (ET) products warrants comparison of these products. The study objective was to assess uncertainty in ET output from four land surface models (LSMs), Noah, Mosaic, VIC, and SAC in NLDAS-2, two remote sensing-based products, MODIS and AVHRR, and GRACE-inferred ET from a water budget with precipitation from PRISM, monitored runoff, and total water storage change (TWSC) from GRACE satellites. The three cornered hat method, which does not require a priori knowledge of the true ET value, was used to estimate ET uncertainties. In addition, TWSC or total water storage anomaly (TWSA) from GRACE was compared with water budget estimates of TWSC from a flux-based approach or TWSA from a storage-based approach. The analyses were conducted using data from three regions (humid – arid) in the South Central US as case studies. Uncertainties in ET are lowest in LSM ET (~5 mm/month), moderate in MODIS- or AVHRR-based ET (10 – 15 mm/month), and highest in GRACE-inferred ET (20 – 30 mm/month). There is a tradeoff between spatial resolution and uncertainty, with lower uncertainty in the coarser-resolution LSM ET (~14 km) relative to higher uncertainty in the finer-resolution (~ 1 ‒ 8 km) RS ET. Root-mean-square (RMS) of uncertainties in water budget estimates of TWSC is about half of RMS of uncertainties in GRACE-derived TWSC for each of the regions. Future ET estimation should consider a hybrid approach that integrates strengths of LSMs and satellite-based products to constrain uncertainties.

Description: It is well documented that deforestation results in an increase in landslide frequency due to the control that forest roots have on slope stability. The loss of forest vegetation leads to a reduction in soil cohesion and a decrease in the shear strength of the soil profile. As a result, the slope becomes more susceptible to landsliding and the return time of landslides decreases. When a landslide removes the soil profile, there may not be adequate time for seedlings to grow and enhance soil stability. In this study, we investigate whether bistable dynamics emerge from the interaction of forest vegetation with the formation and accumulation of colluvial deposits in soil-mantled landscapes. To that end, we develop deterministic and stochastic models of landslide occurrence with a dynamic vegetation component. Results show that bistability exists for the deterministic case for both steep and shallow hollows under event and supply limited conditions. However, for the stochastic case, the randomness of landslide occurrence largely changed the states of the system such that the system only exhibited one stable state, which was the fully vegetated condition. Examining different management practices under stochastic conditions showed that the system eventually recovered; however, management practices influenced the recovery time of the forest. Thus, different management practices could render the land in a state of low vegetation over economically significant time periods.

Description: Seepage flux from ephemeral streams can be an important component of the water balance in arid and semi-arid regions. An emerging technique for quantifying this flux involves the measurement and simulation of a flood wave as it moves along an initially dry channel. This study investigates the usefulness of including surface water and groundwater data to improve model calibration when using this technique. We trialed this approach using a controlled flow event along a 1387 m reach of artificial stream channel. Observations were then simulated using a numerical model that combines the diffusion wave approximation of the Saint-Vénant equations for streamflow routing, with Philips’ infiltration equation and the groundwater flow equation. Model estimates of seepage flux for the upstream segments of the study reach, where streambed hydraulic conductivities were approximately 10 1 m d -1 , were on the order of 10 -4 m 3 d -1 m -2 . In the downstream segments, streambed hydraulic conductivities were generally much lower but highly variable (~10 -3 – 10 -7 m d -1 ). The Latin Hypercube Monte Carlo sensitivity analysis showed that the flood front timing, surface water stage, groundwater heads and the predicted stream flow seepage were most influenced by specific yield. Furthermore, inclusion of groundwater data resulted in a higher estimate of total seepage estimates than if the flood front timing were used alone.

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.

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.

Description: We present a probabilistic sediment cascade model to simulate sediment transfer in a mountain basin (Illgraben, Switzerland) where sediment is produced by hillslope landslides and rockfalls and exported out of the basin by debris flows and floods. The model conceptualizes the fluvial system as a spatially lumped cascade of connected reservoirs representing hillslope and channel storages where sediment goes through cycles of storage and remobilization by surface runoff. The model includes all relevant hydrological processes that lead to runoff formation in an Alpine basin, such as precipitation, snow accumulation, snow melt, evapotranspiration, and soil water storage. Although the processes of sediment transfer and debris flow generation are described in a simplified manner, the model produces complex sediment discharge behavior which is driven by the availability of sediment and antecedent wetness conditions (system memory) as well as the triggering potential (climatic forcing). The observed probability distribution of debris flow volumes and their seasonality in 2000-2009 are reproduced. The stochasticity of hillslope sediment input is important for reproducing realistic sediment storage variability, although many details of the hillslope landslide triggering procedures are filtered out by the sediment transfer system. The model allows us to explicitly quantify the division into transport and supply-limited sediment discharge events. We show that debris flows may be generated for a wide range of rainfall intensities because of variable antecedent basin wetness and snowmelt contribution to runoff, which helps to understand the limitations of methods based on a single rainfall threshold for debris flow initiation in Alpine basins.

Description: Characterizing the complex geometries and the heterogeneity of the deposits in meandering river systems is a long-standing issue for the 3D modeling of alluvial formations. Such deposits are important sources of accessible groundwater in alluvial aquifers throughout the world and also play a major role as hydrocarbons reservoirs. In this paper we present a method to generate meandering river centerlines that are stochastic, geologically realistic, connected and conditioned to local observations or global geomorphological characteristics. The method is based on fast 1D multiple-point statistics in a transformed curvilinear domain: the succession in directions observed in a real world meandering river (the analog) is considered as statistical model for multiple-point statistics simulation. The integration of local data is accomplished by an inverse procedure ensuring that the channels pass through a given set of locations while conserving the high-order spatial characteristics of an analog. The methodology is applied on seven real world case studies. This work demonstrates the flexibility and the applicability of multiple-point statistics outside the standard paradigm that considers the simulation of a 2D or 3D variable with spatial coordinates.

Description: Large asymmetric bedforms commonly develop in rivers. The turbulence associated with flow separation that develops over their steep lee side is responsible for the form shear stress which can represent a substantial part of total shear stress in rivers. This paper uses the Delft3D modeling system to investigate the effects of bedform geometry and forcing conditions on flow separation length and associated turbulence, and bedform shear stress over angle-of-repose (30° lee side angle) bedforms. The model was validated with laboratory measurements that showed sufficient agreement to be used for a systematic analysis. The influence of flow velocity, bed roughness, relative height (bedform height / water depth) and aspect ratio (bedform height / length) on the variations of the normalized length of the flow separation zone, the extent of the wake region (where the turbulent kinetic energy (TKE) was more than 70% of the maximum TKE), the average TKE within the wake region and the form shear stress were investigated. Form shear stress was found not to scale with the size of the flow separation zone but to be related to the product of the normalized extent of the wake region (extent of the wake region / extent of water body above the bedform) and the average TKE within the wake region. The results add to understanding of the hydrodynamics of bedforms and may be used for the development of better parameterizations of small-scale processes for application in large-scale studies.

Description: We explore the bankfull width ( W bf ) vs. drainage area ( A da ) relationship across a range of climatic and geologic environments, and ask (1) is the relationship between ln ( W bf ) and ln ( A da ) best described by a linear function and (2) can a reliable relationship be developed for predicting W bf with A da as the only independent variable. The principal dataset for this study was compiled from regional curve studies and other reports that represent 1,018 sites (1 m ≤ W bf ≤ 110 m and 0.50 km 2 ≤ A da ≤ 22,000 km 2 ) in the continental U.S. Two additional datasets were used for validation. After dividing the data into small-, medium-, and large-size basins which, respectfully, correspond to A da 〈 4.95 km 2 , 4.95 km 2 ≤ A da 〈 337 km 2 , and A da ≥ 337 km 2 , regression lines from each dataset were compared using one-way analysis of covariance (ANCOVA). A second ANCOVA was performed to determine if mean annual precipitation ( P ) is an extraneous factor in the W bf vs. A da relationship. The ANCOVA results reveal that using A da alone does not yield a reliable W bf vs. A da relationship that is applicable across a wide range of environments and that P is a significant extraneous factor in the relationship. Considering data for very small basins ( A da ≤ 0.49 km 2 ) and very large basins ( A da ≥ 1.0×10 5 km 2 ) we conclude that a two-segment linear model is the most probable form of the ln ( W bf ) vs. ln ( A da ) relationship. This study provides useful information for building complex multivariate models for predicting W bf .

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Description: Recent studies show that multimodel combinations improve hydroclimatic predictions by reducing model uncertainty. Given that climate forecasts are available from multiple climate models, which could be ingested with multiple watershed models, what is the best strategy to reduce the uncertainty in streamflow forecasts? To address this question, we consider three possible strategies: (1) reduce the input uncertainty first by combining climate models and then use the multimodel climate forecasts with multiple watershed models (MM-P) (2) ingest the individual climate forecasts (without multimodel combination) with various watershed models and then combine the streamflow predictions that arise from all possible combinations of climate and watershed models (MM-Q)(3) combine the streamflow forecasts obtained from multiple watershed models based on strategy (1) to develop a single streamflow prediction that reduces uncertainty in both climate forecasts and watershed models (MM-PQ) . For this purpose, we consider synthetic schemes that generate streamflow and climate forecasts, for comparing the performance of three strategies with the true streamflow generated by a given hydrologic model. Results from the synthetic study show that reducing input uncertainty first ( MM-P ) by combining climate forecasts results in reduced error in predicting the true streamflow compared to the error of multimodel streamflow forecasts obtained by combining streamflow forecasts from all-possible combination of individual climate model with various hydrologic models ( MM-Q ). Since the true hydrologic model structure is unknown, it is desirable to consider MM-PQ as an alternate choice that reduces both input uncertainty and hydrologic model uncertainty. Application on two watersheds in NC also indicates that reducing the input uncertainty first is critical before reducing the hydrologic model uncertainty.

Description: The understanding of reasons leading to non-uniqueness of soil erosion susceptibility is still inadequate, yet indispensable for establishing general relations between runoff volume and sediment yield. To obtain relevant insights, we performed a series of numerical simulations with a detailed hydrodynamic model using synthetic storms of varying intensity, duration, and lag time between events as representations of different hydrologic response conditions in a zero-order catchment. The design targeted to generate surface flow and ‘perturb’ soil substrate by a first rainfall event, creating a set of initial conditions in terms of flow and deposited sediment prior to the onset of a subsequent rainfall event. Due to the differential effect of (re)detachment and (re)entrainment processes on soil particles of varying sizes, the deposited sediment mass formed shielding layer. One of the essential results is that unless the initial condition of flow and sediment is identical, the same volume of runoff can generate different total sediment yields and their variation can reach up to ~200%. The effect is attributed to two major conflicting effects exerted by the deposited ‘initialization’ (soil antecedent condition) sediment mass: erosion enhancement, because of supply of highly erodible sediment, and erosion impediment, because of constrain on the availability of lighter particles by heavier sediment. Consistently with this inference, long-term simulations with continuous rainfall show that a peculiar feature of sediment yield series is the existence of maximum before the steady-state is reached. The two characteristic time scales, the time to peak and the time to steady-state, separate three characteristic periods that correspond to flow-limited, source-limited, and steady-state regimes. These time scales are log-linearly and negatively related to the spatially averaged Shields parameter: the smaller the rainfall input and the heavier a given particle is, the larger the two scales are. The results provide insights on how the existence of shield operates on erosion processes, possibly implying that accurate short-term predictions of geomorphic events from headwater areas may never become a tractable problem: the latter would require a detailed spatial characterization of particle size distribution prior to precipitation events.

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.

Description: Aquifer hydraulic properties such as hydraulic conductivity ( K ) are ubiquitously heterogeneous and typically only a statistical characterization can be sought. Additionally statistical anisotropy at typical characterization scales is the rule. Thus, regardless of the processes governing solute transport at the local (pore) scale, transport becomes non-Fickian. Mass-transfer models provide an efficient tool that reproduces observed anomalous transport; in some cases though, these models lack predictability as model parameters cannot readily be connected to the physical properties of aquifers. In this study we focus on a multi-rate mass-transfer model (MRMT), and in particular the apparent capacity coefficient (β), which is a strong indicator of the potential of immobile zones to capture moving solute. We aim to find if the choice of an apparent β can be phenomenologically related to measures of statistical anisotropy. We analyzed an ensemble of random simulations of three-dimensional log-transformed multi-Gaussian permeability fields with stationary anisotropic correlation under convergent flow conditions. It was found that apparent β also displays an anisotropic behavior, physically controlled by the aquifer directional connectivity, which in turn is controlled by the anisotropic correlation model. A high hydraulic connectivity results in large β values. These results provide new insights into the practical use of mass-transfer models for predictive purposes.

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.

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.

Description: In this paper, an operational algorithm is proposed for the mapping of surface moisture over the northern and central parts of Tunisia, in North Africa. A change detection approach is applied, using 160 multi-incidence Envisat ASAR Wide Swath images acquired in the horizontal polarization over a 7-year period. Parameterization of this algorithm is considered for three classes of vegetation cover density (NDVI〈0.25, 0.25〈NDVI〈0.5 and NDVI〉0.5), retrieved from SPOT-VGT decadal images. A relative soil moisture index, ranging between 0 (for the driest surfaces) and 1 (for saturated soils), is proposed for each date, with a resolution of 1 km. The retrieved soil moistures are validated by means of ground measurements based on continuous thetaprobe measurements, as well as low resolution (25 km) ERS and ASCAT soil moisture products from the Vienna University of Technology (TU Wien). A qualitative relationship between spatio-temporal variations of moisture and precipitation is also discussed.

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.

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%.

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.

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.

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.

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.

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.

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.

Description: This paper presents the results of a data based comparative study of several hundred catchments across continental United States belonging to the MOPEX dataset to systematically explore the connection between the flood frequency curve and mean annual water balance. Mean annual water balance is expressed in terms of two similarity measures: (i) the climatic aridity index, AI , which is a measure of the competition between energy and water availability; and (ii) the baseflow index, BFI , which is a measure of total runoff partitioning into surface and subsurface components at the annual time scale. The data analyses showed that the aridity index, AI , has a first order control on the shape of the flood frequency curve (also known as the growth curve), as expressed in terms of both the mean and coefficient of variation ( C v ) of the annual maximum floods, once normalized by catchment size (i.e., specific flood discharge) While the mean annual (specific) flood discharge decreases with increasing aridity, C v increases with increasing aridity. On the other hand, the BFI was found to be a second order control on the flood frequency curve. Higher BFI , meaning higher contributions of subsurface flow to total streamflow, leads to a decrease of the mean annual (specific) flood discharge, and vice versa. The statistically significant relationship between AI and the flood frequency curve and the consistent shift of the growth curves with AI support the use of AI as a similarity measure for regionalization of flood frequency.

Description: The performance of glacio-hydrological models which simulate catchment response to climate variability depends to a large degree on the data used to force the models. The forcing data become increasingly important in high elevation, glacierised catchments where the interplay between extreme topography, climate and the cryosphere is complex. It is challenging to generate a reliable forcing dataset that captures this spatial heterogeneity. In this paper, we analyze the results of a one year field campaign focusing on air temperature and precipitation observations in the Langtang Valley in the Nepalese Himalayas. We use the observed time series to characterize both temperature lapse rates (LRs) and precipitation gradients (PGs). We study their spatial and temporal variability, and we attempt to identify possible controlling factors. We show that very clear LRs exist in the valley and that there are strong seasonal differences related to the water vapor content in the atmosphere. Results also show that the LRs are generally shallower than the commonly used environmental lapse rates. The analysis of the precipitation observations reveal that there is great variability in precipitation over short horizontal distances. A uniform valley wide PG cannot be established and several scale-dependent mechanisms may explain our observations. We complete our analysis by showing the impact of the observed LRs and PGs on the outputs of the TOPKAPI-ETH glacio-hydrological model. We conclude that LRs and PGs have a very large impact on the water balance composition and that short-term monitoring campaigns have the potential to improve model quality considerably.

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.

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.

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.

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.