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Wiley Online Library : Geophysical Research Letters
Updated: 22 weeks 20 hours ago

Commentary on: “Effects of friction and plastic deformation in shock-comminuted damaged rocks on impact heating” By K. Kurosawa and H. Genda.

Thu, 03/15/2018 - 18:25
Abstract:

An important new paper by Kurosawa and Genda [2017] reports a previously overlooked source of heating in low velocity meteorite impacts. Plastic deformation of the pressure-strengthened rocks behind the shock front dissipates energy, which appears as heat in addition to that generated across the shock wave itself. This heat source has surprisingly escaped explicit attention for decades: First, because it is minimized in the geometry typically chosen for laboratory experiments; and second because it is most important in rocks, and less so for the metals usually used in experiments. Nevertheless, modern numerical computer codes that include strength do compute this heating correctly. This raises the philosophical question of whether we can claim to understand some process just because our computer codes compute the results correctly.

Coseismic slip deficit of the 2017 Mw 6.5 Ormoc Earthquake that occurred along a creeping segment and geothermal field of the Philippine Fault

Thu, 03/15/2018 - 18:25

Abstract: Coseismic surface deformation imaged through Interferometric Synthetic Aperture Radar (InSAR) measurements was used to estimate the fault geometry and slip distribution of the 2017 Mw 6.5 Ormoc Earthquake along a creeping segment of the Philippine Fault on Leyte Island. Our best fitting faulting model suggests that the coseismic rupture occurred on a fault plane with high dip angle of 78.5° and strike angle of 325.8°, and the estimated maximum fault slip of 2.3 m is located at 6.5 km east-northeast of the town of Kananga. The recognized insignificant slip in the Tongonan geothermal field zone implies that the plastic behavior caused by high geothermal gradient underneath the Tongonan geothermal field could prevent the coseismic failure in heated rock mass in this zone. The predicted Coulomb failure stress (CFS) change shows that a significant positive CFS change occurred along the SE segment of central Philippine fault with insignificant coseismic slip and infrequent aftershocks, which suggests an increasing risk for future seismic hazard.

Snowmelt-driven tradeoffs between early and late season productivity negatively impact forest carbon uptake during drought

Thu, 03/15/2018 - 18:25
Abstract

Future projections of declining snowpack and increasing potential evaporation are predicted to advance the timing of snowmelt in mountain ecosystems globally with unknown implications for snowmelt-driven forest productivity. Accordingly, this study combined satellite- and tower-based observations to investigate the forest productivity response to snowpack and potential evaporation variability between 1989 and 2012 throughout the Southern Rocky Mountain ecoregion, USA. Our results show that early and late season productivity were significantly and inversely related, and that future shifts toward earlier and/or reduced snowmelt could decrease snowmelt water use efficiency and thus restrict productivity despite a longer growing season. This was explained by increasing snow aridity, which incorporated evaporative demand and snow water supply, and was modified by summer precipitation to determine total annual productivity. The combination of low snow accumulation and record high potential evaporation in 2012 resulted in the 34-year minimum ecosystem productivity that could be indicative of future conditions.

Contribution of Multi-GNSS Constellation to SLR-Derived Terrestrial Reference Frame

Thu, 03/15/2018 - 17:20
Abstract

All satellites of new Global Navigation Satellite Systems (GNSS) are equipped with laser retroreflectors dedicated to Satellite Laser Ranging (SLR). This paper demonstrates the contribution of SLR tracking of multi-GNSS constellations to the improved SLR-derived reference frame and scientific products. We show a solution strategy with estimating satellite orbits, SLR station coordinates, geocenter coordinates, and Earth rotation parameters using SLR observations to 2 Laser Geodynamics Satellites (LAGEOS) and 55 GNSS satellites: 1 GPS, 31 Globalnaya Navigatsionnaya Sputnikovaya Sistema, 18 Galileo, 3 BeiDou Inclined Geosynchronous Orbit, 1 BeiDou Medium Earth Orbit, and 1 Quasi-Zenith Satellite System satellite for the period 2014.0–2017.4. Due to a substantial number of GNSS observations, the number of weekly solutions for some SLR stations, for example, Arkhyz, Komsomolsk, Altay, and Brasilia, is larger up to 41% in the combined LAGEOS + GNSS solution when compared to the LAGEOS-only solution. The SLR observations to GNSS can transfer the orientation of the reference frame from GNSS to SLR solutions. As a result, the SLR-derived pole coordinates and length-of-day estimates become more consistent with GNSS microwave-based results. The root-mean-square errors of length-of-day are reduced from 122.5 μs/d to 43.0 μs/d, whereas mean offsets are reduced from −81.6 μs/d to 0.5 μs/d in LAGEOS only and in the combined LAGEOS + GNSS solutions, respectively.

Constraints on Transient Viscoelastic Rheology of the Asthenosphere From Seasonal Deformation

Thu, 03/15/2018 - 17:16
Abstract

We discuss the constraints on short-term asthenospheric viscosity provided by seasonal deformation of the Earth. We use data from 195 globally distributed continuous Global Navigation Satellite System stations. Surface loading is derived from the Gravity Recovery and Climate Experiment and used as an input to predict geodetic displacements. We compute Green's functions for surface displacements for a purely elastic spherical reference Earth model and for viscoelastic Earth models. We show that a range of transient viscoelastic rheologies derived to explain the early phase of postseismic deformation may induce a detectable effect on the phase and amplitude of horizontal displacements induced by seasonal loading at long wavelengths (1,300–4,000 km). By comparing predicted and observed seasonal horizontal motion, we conclude that transient asthenospheric viscosity cannot be lower than 5 × 1017 Pa.s, suggesting that low values of transient asthenospheric viscosities reported in some postseismic studies cannot hold for the seasonal deformation global average.

Seasonal Surface Loading Helps Constrain Short-Term Viscosity of the Asthenosphere

Thu, 03/15/2018 - 17:15
Abstract

Earth materials may display a range of rheological behaviors at different depths and over different timescales. The situation is particularly complex for postseismic relaxation in the uppermost mantle and lower crust, where it can be difficult to distinguish widespread viscous behavior from earthquake afterslip or localized deformation in shear zones over timescales of weeks to decades. By analyzing geodetic observations of seasonal surface mass loads and Earth's surface deformation in response, Chanard et al. (2018, https://doi.org/10.1002/2017GL076451) have established a globally averaged lower bound of 5 × 1017 Pa s for the transient viscosity of a Burgers-rheology asthenosphere. This implies that lower viscosities inferred by some studies of postseismic relaxation must result from local departures from this global value, or be an artifact of additional afterslip or shear zone deformation.

Photometric Modeling and VIS-IR Albedo Maps of Dione From Cassini-VIMS

Thu, 03/15/2018 - 17:10
Abstract

We report about visible and infrared albedo maps and spectral indicators of Dione's surface derived from the complete Visual and Infrared Mapping Spectrometer (VIMS) data set acquired between 2004 and 2017 during the Cassini tour in Saturn's system. Maps are derived by applying a photometric correction necessary to disentangle the intrinsic albedo of the surface from illumination and viewing geometry occurring at the time of the observation. The photometric correction is based on the Shkuratov et al. (2011, https://doi.org/10.1016/j.pss.2011.06.011) method which yields values of the surface equigonal albedo. Dione's surface albedo maps are rendered at five visible (VIS: 0.35, 0.44, 0.55, 0.7, and 0.95 μm) and five infrared (IR: 1.046, 1.540, 1.822, 2.050, and 2.200 μm) wavelengths in cylindrical projection with a 0.5° × 0.5° angular resolution in latitude and longitude, corresponding to a spatial resolution of 4.5 km/bin. Apart from visible and infrared albedo maps, we report about the distribution of the two visible spectral slopes (0.35–0.55 and 0.55–0.95 μm) and water ice 2.050 μm band depth computed after having applied the photometric correction. The derived spectral indicators are employed to trace Dione's composition variability on both global and local scales allowing to study the dichotomy between the bright-leading and dark-trailing hemispheres, the distribution of fresh material on the impact craters and surrounding ejecta, and the resurfacing of the bright material within the chasmata caused by tectonism.

Linking Low-Frequency Large-Scale Circulation Patterns to Cold Air Outbreak Formation in the Northeastern North Atlantic

Thu, 03/15/2018 - 17:06
Abstract

The regional variability of wintertime marine cold air outbreaks (CAOs) in the northeastern North Atlantic is studied focusing on the role of weather regimes in modulating the large-scale circulation. Each regime is characterized by a typical CAO frequency anomaly pattern and a corresponding imprint in air-sea heat fluxes. Cyclonically dominated regimes, Greenland blocking and the Atlantic ridge regime are found to provide favorable conditions for CAO formation in at least one major sea of the study region; CAO occurrence is suppressed, however, by blocked regimes whose associated anticyclones are centered over northern Europe (European / Scandinavian blocking). Kinematic trajectories reveal that strength and location of the storm tracks are closely linked to the pathways of CAO air masses and, thus, CAO occurrence. Finally, CAO frequencies are also linked to the strength of the stratospheric polar vortex, which is understood in terms of associated variations in the frequency of weather regimes.

A First Look at Decadal Hydrological Predictability by Land Surface Ensemble Simulations

Thu, 03/15/2018 - 17:05
Abstract

The prediction of terrestrial hydrology at the decadal scale is critical for managing water resources in the face of climate change. Here we conducted an assessment by global land model simulations following the design of the fifth Coupled Model Intercomparison Project (CMIP5) decadal hindcast experiments, specifically testing for the sensitivity to perfect initial or boundary conditions. The memory for terrestrial water storage (TWS) is longer than 6 years over 11% of global land areas where the deep soil moisture and aquifer water have a long memory and a nonnegligible variability. Ensemble decadal predictions based on realistic initial conditions are skillful over 31%, 43%, and 59% of global land areas for TWS, deep soil moisture, and aquifer water, respectively. The fraction of skillful predictions for TWS increases by 10%–16% when conditioned on Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation indices. This study provides a first look at decadal hydrological predictability, with an improved skill when incorporating low-frequency climate information.

Seasonal Sensitivity of the Hadley Cell and Cross-Hemispheric Responses to Diabatic Heating in an Idealized GCM

Thu, 03/15/2018 - 16:56
Abstract

The seasonal sensitivity of the Hadley cell to localized diabatic forcing is studied using a dry idealized atmospheric general circulation model. Sensitivities are broadly consistent with Hadley cell responses in observations and climate models to El Niño–Southern Oscillation and global warming-like forcings. However, the exact seasonal sensitivity patterns highlight the importance of reducing the uncertainty in the size and position of expected anthropogenic forcings to understand how the atmospheric circulation will respond. The sensitivities reveal cross-hemispheric Hadley cell responses that project onto the eddy-driven jets and storm tracks. For summer hemisphere heating, the winter Hadley cell extent and jet latitude responses are highly correlated. For winter hemisphere heating, the summer Hadley cell extent and jet speed responses are highly correlated. These seasonal differences arise due to the contrast between the dominant winter Hadley cell and weaker summer Hadley cell.

Circulation of Venusian Atmosphere at 90–110 km Based on Apparent Motions of the O2 1.27 μm Nightglow From VIRTIS-M (Venus Express) Data

Thu, 03/15/2018 - 16:53
Abstract

The paper is devoted to the investigation of Venus mesosphere circulation at 90–110 km altitudes, where tracking of the O2(a1Δg) 1.27 μm nightglow is practically the only method of studying the circulation. The images of the nightglow were obtained by VIRTIS-M on Venus Express over the course of more than 2 years. The resulting global mean velocity vector field covers the nightside between latitudes 75°S–20°N and local time 19–5 h. The main observed mode of circulation is two opposite flows from terminators to midnight; however, the wind speed in the eastward direction from the morning side exceeds the westward (evening) by 20–30 m/s, and the streams “meet” at 22.5 ± 0.5 h. The influence of underlying topography was suggested in some cases: Above mountain regions, flows behave as if they encounter an “obstacle” and “wrap around” highlands. Instances of circular motion were discovered, encompassing areas of 1,500–4,000 km.

Issue Information

Thu, 03/15/2018 - 16:47

No abstract is available for this article.

Increasing Mobility of High Arctic Sea Ice Increases Marine Hazards Off the East Coast of Newfoundland

Thu, 03/15/2018 - 14:15
Abstract

Heavy ice conditions along Canada's east coast during spring 2017 presented hazardous conditions for the maritime industry and required the Canadian Coast Guard to pull its research icebreaker, CCGS Amundsen, off its scientific cruise to provide ice escort services and conduct search and rescue operations along Newfoundland's northeast coast. Greater ice concentrations and a thicker ice pack than are typical of this area created the anomalous ice cover. Within this paper we present in situ observations of the ice cover, confirming that pieces of multiyear sea ice from the high Arctic were present within the ice cover, and subsequently examine the transport pathway that connects the export of thick multiyear sea ice from the Lincoln Sea and Canadian Arctic Archipelago to coastal communities in Newfoundland. We conclude with a discussion on how an increasingly mobile Arctic sea ice cover may increase these ice hazards in the south.

On the Chaotic Variability of Deep Convection in the Mediterranean Sea

Thu, 03/15/2018 - 13:51
Abstract

Chaotic intrinsic variability is a fundamental driver of the oceanic variability. Its understanding is key to interpret observations, evaluate numerical models, and predict the future ocean and climate. Here we study intrinsic variability of deep convection in the northwestern Mediterranean Sea using an ensemble eddy-resolving hindcast simulation over the period 1979–2013. We find that the variability of deep convection is mostly forced but also, to a considerable extent, intrinsic. The intrinsic variability can dominate the total convection variability locally and over a single winter. It also makes up a significant fraction of its interannual variability but has only modest impacts on the long-term mean state. We find that the occurrence of deep convection is random 18% of years at the basin scale, and 29% locally at the LION observational site. Spatially, the intrinsic variability is highest far from the continental shelf. We relate this pattern to baroclinic instability theory that takes bottom stabilization into account.

Assessing the Decadal Predictability of Land and Ocean Carbon Uptake

Thu, 03/15/2018 - 13:51
Abstract

The decadal predictability of carbon fluxes has been examined over continents and oceans using a “perfect model” approach based on a 400 year preindustrial simulation and five 10-member ensembles from the Centre National de Recherches Météorologiques-Earth System Model version 1. From these experiments, we find that the global land uptake and ocean carbon uptake are potentially predictable by up to six years, with a median predictability horizon of four years. Predictability of global carbon uptake is prominently driven by the ocean's predictability. The difference in predictability between ocean and land carbon fluxes stems from the relative capability of ocean or land to generate low-frequency fluctuations in carbon flux. Indeed, ocean carbon fluxes display low-frequency variability that emerges from the year-to-year variability in the North Atlantic, the North Pacific, and the Southern Ocean. The Southern Ocean carbon uptake can be predicted up to six years in advance and explains most of the global carbon uptake predictability.

Nonlinear Electrostatic Steepening of Whistler Waves: The Guiding Factors and Dynamics in Inhomogeneous Systems

Thu, 03/15/2018 - 13:47
Abstract

Whistler mode chorus waves are particularly important in outer radiation belt dynamics due to their key role in controlling the acceleration and scattering of electrons over a very wide energy range. The efficiency of wave-particle resonant interactions is defined by whistler wave properties which have been described by the approximation of plane linear waves propagating through the cold plasma of the inner magnetosphere. However, recent observations of extremely high-amplitude whistlers suggest the importance of nonlinear wave-particle interactions for the dynamics of the outer radiation belt. Oblique chorus waves observed in the inner magnetosphere often exhibit drastically nonsinusoidal (with significant power in the higher harmonics) waveforms of the parallel electric field, presumably due to the feedback from hot resonant electrons. We have considered the nature and properties of such nonlinear whistler waves observed by the Van Allen Probes and Time History of Events and Macroscale Interactions define during Substorms in the inner magnetosphere, and we show that the significant enhancement of the wave electrostatic component can result from whistler wave coupling with the beam-driven electrostatic mode through the resonant interaction with hot electron beams. Being modulated by a whistler wave, the electron beam generates a driven electrostatic mode significantly enhancing the parallel electric field of the initial whistler wave. We confirm this mechanism using a self-consistent particle-in-cell simulation. The nonlinear electrostatic component manifests properties of the beam-driven electron acoustic mode and can be responsible for effective electron acceleration in the inhomogeneous magnetic field.

The Influence of Water Storage in Marine Sediment on Sea-Level Change

Thu, 03/15/2018 - 13:47
Abstract

Sea-level changes are of wide interest because they provide information about Earth's internal structure and the sensitivity of ice sheets to climate change. Here we illustrate the sensitivity of sea level to marine sedimentary water storage by modeling sea-level responses to a synthetic global sediment redistribution history in which rates and patterns of erosion and deposition are similar to those at present and steady in time from the Last Interglacial to present. Our simulations show that if sediment redistribution were accounted for but sedimentary water storage were neglected, modeled sea-level changes could be overestimated by ~2 ± 1 m of global mean sea-level equivalent, a significant fraction of published estimates of 6–9 m of global mean sea-level change since the Last Interglacial. These results show that sedimentary water storage may significantly contribute to changes in Earth's long-term seawater budget over >105 year timescales and underscore the importance of accounting for it in modeling long-term sea-level changes.

Disentangling Global Warming, Multidecadal Variability, and El Niño in Pacific Temperatures

Thu, 03/15/2018 - 11:57
Abstract

A key challenge in climate science is to separate observed temperature changes into components due to internal variability and responses to external forcing. Extended integrations of forced and unforced climate models are often used for this purpose. Here we demonstrate a novel method to separate modes of internal variability from global warming based on differences in time scale and spatial pattern, without relying on climate models. We identify uncorrelated components of Pacific sea surface temperature variability due to global warming, the Pacific Decadal Oscillation (PDO), and the El Niño–Southern Oscillation (ENSO). Our results give statistical representations of PDO and ENSO that are consistent with their being separate processes, operating on different time scales, but are otherwise consistent with canonical definitions. We isolate the multidecadal variability of the PDO and find that it is confined to midlatitudes; tropical sea surface temperatures and their teleconnections mix in higher-frequency variability. This implies that midlatitude PDO anomalies are more persistent than previously thought.

Volcanic Structures within Niger and Dao Valles, Mars, and Implications for Outflow Channel Evolution and Hellas Basin Rim Development

Wed, 03/14/2018 - 21:11
Abstract

Outflow channel formation on the eastern Hellas rim region is traditionally thought to have been triggered by activity phases of the nearby volcanoes Hadriacus and Tyrrhenus Montes: As a result of volcanic heating subsurface volatiles were mobilized. It is however under debate, whether eastern Hellas volcanism was in fact more extensive, and if there were volcanic centers separate from the identified central volcanoes.

This work describes previously unrecognized structures in the Niger–Dao Valles outflow channel complex. We interpret them as volcanic edifices: cones, a shield, and a caldera. The structures provide evidence of an additional volcanic center within the valles, and indicate volcanic activity both prior to and following the formation of the outflow events. They expand the extent, type and duration of volcanic activity in the Circum-Hellas Volcanic Province, and provide new information on interaction between volcanism and fluvial activity.

Analysis of intense Z-mode emission observed during the Cassini proximal orbits

Wed, 03/14/2018 - 21:11
Abstract

The role of Z-mode emission in the diffusive scattering and resonant acceleration of electrons is believed to be important at Saturn. A survey of the "5 kHz" component of this emission at Saturn earlier reported strong intensity in the lower density regions where the ratio of plasma frequency to cyclotron frequency, fp/fc < 1. At Saturn this occurs along the inner edge of the Enceladus torus near the equator and at higher latitudes. Using the Cassini radio and plasma wave science (RPWS) instrument observations during the Cassini proximal orbits we have now identified these emissions extending down to and within the ionosphere. Wave polarization measurements and unique frequency cutoffs are used to positively identify the wave mode. Analogous to the role of whistler mode chorus at Earth, Saturn Z-mode emissions may interact with electrons contributing to the filling or depleting of Saturn's inner radiation belts.

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