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  • Articles  (1,141)
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  • Wiley  (1,141)
  • AGU (American Geophysical Union)
  • 2010-2014  (1,141)
  • 1980-1984
  • 2011  (1,141)
  • Geophysical Research Letters  (1,141)
  • 4905
  • 1
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    The composition and structure of the Enceladus plume (2011)
    Wiley
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    Publication Date: 2011-06-09
    Description: The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed an occultation of the Sun by the water vapor plume at the south polar region of Saturn's moon Enceladus. The Extreme Ultraviolet (EUV) spectrum is dominated by the spectral signature of H2O gas, with a nominal line-of-sight column density of 0.90 ± 0.23 × 1016 cm−2 (upper limit of 1.0 × 1016 cm−2). The upper limit for N2 is 5 × 1013 cm−2, or
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  • 2
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    What caused a recent increase in dust outbreaks over East Asia? (2011)
    Wiley
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    Publication Date: 2011-06-10
    Description: A decadal change in the frequency of dust outbreaks for April over East Asia using World Meteorological Organization (WMO) synoptic data was identified. The causes of the decadal change can be defined in terms of aeolian erosivity (i.e., ability of wind to cause erosion represented by wind speed) and erodibility (i.e., susceptibility of soil and land surface to wind erosion represented by the threshold wind speed for dust outbreak). Dust outbreak frequency (fDO) increased at many stations in Mongolia, Inner Mongolia, northeastern China, and the Hexi Corridor from the 1990s to the 2000s. Especially, remarkable increases were observed for Mongolia. Strong wind frequency (fu〉ut5%) decreased or changed little over the period of study, and the 5th percentile of the threshold wind speed for dust outbreak (ut5%) decreased at many stations in Mongolia, eastern Inner Mongolia, and northeastern China. This suggests changes in erodibility factors increased fDO in these regions. There are stations where fu〉ut5% increased and ut5% remained relatively constant in western Inner Mongolia and the Hexi Corridor. This suggests changes in the erosivity factor increased fDO at these locations. The change in erodibility is linked, in part, to the effect of dead leaves of grasses in spring, which are the residue of vegetation in summer from the previous year, which can cause an increase in the threshold wind speed.
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  • 3
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    The impact of Asian and non-Asian anthropogenic aerosols on 20th century Asian summer monsoon (2011)
    Wiley
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    Publication Date: 2011-06-11
    Description: Studies into the impact of anthropogenic aerosols on regional summer monsoon rainfall have focused on a black carbon-induced enhancement and a sulfate-induced suppression. The latter encompasses significant Asian and non-Asian sources, but their relative roles in forcing historical global and regional monsoon trends are largely unexplored. Using targeted 20th century coupled climate simulations, designed to isolate the impact from anthropogenic aerosols, we show that Asian aerosols induce a weak suppression of global summer monsoon, confined to the East Asian region. The addition of non-Asian aerosols generates an enhancement and broadening of cooler temperatures over Europe and Asia relative to the ambient oceans, supporting stronger northerly flows that further suppress Asian monsoon rainfall. Furthermore, atmospheric convection is directed away from the Asian monsoon regions, resulting in an equatorward shift in rainfall. Our results highlight the importance of the non-Asian aerosols in exacerbating the impact of Asian aerosols on global monsoon rainfall, particularly across Asia.
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  • 4
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    Differences in future recharge estimates due to GCMs, downscaling methods and hydrological models (2011)
    Wiley
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    Publication Date: 2011-06-14
    Description: The impact of climate change upon groundwater has an increasing profile in the literature but there is little guidance on selecting Global Climate Models (GCMs), downscaling methods or hydrological models. This paper quantifies the relative uncertainties inherent in projections of future recharge contributed by multiple GCMs, downscaling methods and hydrological models at three locations across southern Australia. Results highlight that the choice of GCM is the largest source of uncertainty, with a median range between the highest and lowest GCM of 53% of the historical recharge for a given downscaling method and hydrological model. The downscaling method is the next largest source of uncertainty with a median range of 44% and the choice of hydrological model is the source of the least uncertainty with a median range of 24%. These results strongly suggest that impact studies should use multiple GCMs and give careful consideration to the choice of downscaling methods.
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  • 5
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    Drivers of past and future Southern Ocean change: Stratospheric ozone versus greenhouse gas impacts (2011)
    Wiley
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    Publication Date: 2011-06-16
    Description: We study the separate impacts of changing atmospheric greenhouse gas (GHG) and stratospheric ozone concentrations on past (1960–2010) and future (2010–2100) Southern Ocean conditions. To this end we employ a coupled atmosphere-ocean model with interactive stratospheric chemistry. In our model we separately prescribe i) GHGs that monotonically increase to 2100 and ii) ozone depleting substances (ODSs) that rapidly increase to a maximum in 1995 and then slowly return to 1960 values around 2100, following moderate emission scenarios. Past GHG and ODS changes in our model drive, in about equal measure in the annual mean, poleward intensified surface winds which act to strengthen the sub polar meridional ocean circulation cell and Antarctic Circumpolar Current (ACC). Future GHG-induced oceanic changes continue nearly monotonically to 2100, while the ODS-induced ACC transport peaks, and then reverses, a couple of decades after the ODS maximum in 1995. The ODS impact on ACC transport exceeds the corresponding GHG impact up to the second quarter of the 21st century, a result that highlights the importance of often-neglected stratospheric ozone trends for the simulation of the ocean circulation.
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  • 6
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    Mass balance of Greenland's three largest outlet glaciers, 2000–2010 (2011)
    Wiley
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    Publication Date: 2011-06-19
    Description: Acceleration of Greenland's three largest outlet glaciers, Helheim, Kangerdlugssuaq and Jakobshavn Isbræ, accounted for a substantial portion of the ice sheet's mass loss over the past decade. Rapid changes in their discharge, however, make their cumulative mass-change uncertain. We derive monthly mass balance rates and cumulative balance from discharge and surface mass balance (SMB) rates for these glaciers from 2000 through 2010. Despite the dramatic changes observed at Helheim, the glacier gained mass over the period, due primarily to the short-duration of acceleration and a likely longer-term positive balance. In contrast, Jakobshavn Isbræ lost an equivalent of over 11 times the average annual SMB and loss continues to accelerate. Kangerdlugssuaq lost over 7 times its annual average SMB, but loss has returned to the 2000 rate. These differences point to contrasts in the long-term evolution of these glaciers and the danger in basing predictions on extrapolations of recent changes.
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  • 7
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    Contribution of Pacific wind stress to multi-decadal variations in upper-ocean heat content and sea level in the tropical south Indian Ocean (2011)
    Wiley
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    Publication Date: 2011-06-19
    Description: Reconstructions of the spatial pattern of recent multi-decadal sea level trends in the Indian Ocean (IO) indicate a zonally-extended band in the southern tropics where sea level has substantially fallen between the 1960s and 1990s; the decline is consistent with the observed subsurface cooling associated with a shoaling thermocline in this region. Here the origin and spatio-temporal characteristics of these trends are elucidated by a sequence of ocean model simulations. Whereas interannual variability in the southwestern tropical IO appears mainly governed by IO atmospheric forcing, longer term changes in the south tropical IO involve a strong contribution from the western Pacific via wave transmission of thermocline anomalies through the Indonesian Archipelago, and their subsequent westward propagation by baroclinic Rossby waves. The late 20th-century IO subsurface cooling trend reversed in the 1990s, reflecting the major regime shift in the tropical Pacific easterlies associated with the Pacific Decadal Oscillation.
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  • 8
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    HF-driven currents in the polar ionosphere (2011)
    Wiley
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    Publication Date: 2011-06-21
    Description: Polar ionospheric heaters have generated ULF/ELF/VLF waves by modulating the auroral electrojet at D/E region altitudes. We present theoretical/computational results indicating that modulated F-region HF heating can generate ionospheric currents even in the absence of electrojet currents. The ELF currents are driven in a two-step process. First, the pressure gradient associated with F-region electron heating drives a local diamagnetic current. This acts as an antenna to inject Magneto-Sonic (MS) waves in the ionospheric plasma. Second, the electric field of the magneto-sonic wave drives Hall currents when it reaches the E region of the ionosphere. The Hall currents act as a secondary antenna that injects waves in the Earth-Ionosphere Waveguide below and Shear Alfven waves upwards to the conjugate regions. The paper examines the scaling and limitations of the concept and suggests proof-of-principle experiments using the HAARP ionospheric heater.
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  • 9
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    Strong thermospheric cooling during the 2009 major stratosphere warming (2011)
    Wiley
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    Publication Date: 2011-06-21
    Description: Thermospheric density simultaneously observed by the CHAMP and GRACE satellites in both the pre-dawn and afternoon local time sectors undergoes significant decrease across both hemispheres during the major stratospheric sudden warming (SSW) in January 2009. This decrease is largest in the equatorial region near the subsolar latitude, reaching ∼30% at 325 km, and 45% at 475 km altitude in the afternoon sector. This large density drop demonstrates a substantial cooling of about 50 Kelvin in the equatorial upper thermosphere. Furthermore, the cooling varies clearly with longitude in terms of magnitude and the timing of the maximum cooling. Thermosphere cooling can have important impact on the ionosphere, as indicated by simultaneous plasma observations. Though many questions remain about what causes the cooling, our results open a new perspective for investigating the global coupling of the lower and upper atmosphere during SSWs.
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  • 10
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    The drought of 2010 in the context of historical droughts in the Amazon region (2011)
    Wiley
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    Publication Date: 2011-06-22
    Description: The year 2010 featured a widespread drought in the Amazon rain forest, which was more severe than the “once-in-a-century” drought of 2005. Water levels of major Amazon tributaries fell drastically to unprecedented low values, and isolated the floodplain population whose transportation depends upon on local streams which completely dried up. The drought of 2010 in Amazonia started in early austral summer during El Niño and then was intensified as a consequence of the warming of the tropical North Atlantic. An observed tendency for an increase in dry and very dry events, particularly in southern Amazonia during the dry season, is concomitant with an increase in the length of the dry season. Our results suggest that it is by means of a longer dry season that warming in the tropical North Atlantic affects the hydrology of the Amazon Rivers at the end of the recession period (austral spring). This process is, sometimes, further aggravated by deficient rainfall in the previous wet season.
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  • 11
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    Influence of subtropical and polar sea-surface temperature anomalies on temperatures in Eurasia (2011)
    Wiley
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    Publication Date: 2011-06-23
    Description: In summer 2010 an exceptional heatwave occurred over western Russia. At the same time sea-surface temperatures (SSTs) were anomalously warm in the Barents Sea and the Arabian Sea. We investigate a possible link between these two SST anomalies by prescribing SST anomalies separately and combined in an ensemble of climate model simulations. The positive surface air temperature response over western Russia is strengthened if both SST forcings are combined. While the SST anomalies in the Arabian Sea are likely due to natural variability the sea surface in the Barents Sea is expected to warm in future and the sea-ice cover to decline enhancing the warming. Thus, we hypothesize that heatwaves over Europe and Russia will likely become more frequent as a result of the dynamic response of the atmosphere in addition to what is expected from the change in mean temperature.
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  • 12
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    Influence of non-Newtonian rheology on magma degassing (2011)
    Wiley
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    Publication Date: 2011-06-24
    Description: Many volcanoes exhibit temporal changes in their degassing process, from rapid gas puffing to lava fountaining and long-lasting quiescent passive degassing periods. This range of behaviors has been explained in terms of changes in gas flux and/or magma input rate. We report here a simple laboratory experiment which shows that the non-Newtonian rheology of magma can be responsible, alone, for such intriguing behavior, even in a stationary gas flux regime. We inject a constant gas flow-rate Q at the bottom of a non-Newtonian fluid column, and demonstrate the existence of a critical flow rate Q* above which the system spontaneously alternates between a bubbling and a channeling regime, where a gas channel crosses the entire fluid column. The threshold Q* depends on the fluid rheological properties which are controlled, in particular, by the gas volume fraction (or void fraction) $\phi$. When $\phi$ increases, Q* decreases and the degassing regime changes. Non-Newtonian properties of magma might therefore play a crucial role in volcanic eruption dynamics.
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  • 13
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    Micromechanics of asperity rupture during laboratory stick slip experiments (2011)
    Wiley
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    Publication Date: 2011-06-28
    Description: Acoustic emissions and tremor-like signals are widely recorded in laboratory experiments. We are able to isolate the physical origins of these signals using high resolution nanoseismic analysis. The use of a picometer-sensitive, wide-band sensor array permits us to determine force-time functions and focal mechanisms for discrete events found amid the “noise” of friction, similar to how low frequency earthquakes are found buried within tremor. We interpret these localized events to be the rupture of μm-sized contacts, known as asperities. We performed stick-slip experiments on plastic/plastic and rock/rock interfaces and found a systematic difference between the nano earthquakes: the rock interface produces very rapid (
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  • 14
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    Arctic cyclogenesis at the marginal ice zone: A contributory mechanism for the temperature amplification? (2011)
    Wiley
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    Publication Date: 2011-06-28
    Description: Rapid sea-ice retreat over the Arctic Ocean has a leading role in Arctic amplification. The sea-ice extent dramatically recovers during every freezing season, so despite the recent summer sea-ice retreat, there must be extraordinary heat exchange between the lower atmosphere and upper ocean. However, the underlying mechanisms for this remain uncertain. Here we show that autumn frontal cyclogenesis is a crucial event in the Arctic air-sea coupled system. Our shipboard Doppler radar and intensive radiosonde observations at the marginal ice zone detected an explosive frontal cyclogenesis, with coupling between upper and lower tropospheric vortices. The thermal contrast between ocean and ice surfaces is likely favorable to cyclogenesis with an identical life-cycle to that at mid-latitudes. This suggests a northward shift of meridional heat transport. The 1.5 K temperature decrease in the upper ocean after the cold front has passed reveals that a large amount of heat is transported into the atmosphere. This is an invaluable example of the fact that sea ice retreat contributes to polar amplification of surface air temperature increase.
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  • 15
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    Global energy transfer during a magnetospheric field line resonance (2011)
    Wiley
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    Publication Date: 2011-06-19
    Description: Field line resonances (FLRs) are important for transferring energy from fast mode waves to shear Alfvén waves in the Earth's magnetosphere. Using simultaneous multi-satellite observations from THEMIS and the IMAGE ground magnetometer array, we report on the transfer of energy from compressional magnetopause undulations through an FLR to the ionosphere. Energy diversion from the magnetosphere to the ionosphere took place at the FLR: we find net energy flux there to have comparable values in the radial and the field-aligned directions. The field-aligned energy flux, when mapped to the ionosphere, was 0.70 mW/m2 and consistent with the inferred Joule dissipation rate at that time. IMAGE's regional monitoring of wave activity reveals that the temporal evolution of the FLR wave power and energy transfer were correlated with the amplitude profile of magnetopause undulations, confirming these waves to be the FLR driver.
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  • 16
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    Lightning development associated with two negative gigantic jets (2011)
    Wiley
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    Publication Date: 2011-06-21
    Description: We report observations of two negative polarity gigantic jets sufficiently near very high-frequency (VHF) lightning mapping networks that the associated lightning characteristics and charge transfer could be investigated. In both cases the gigantic jet-producing flash began as ordinary intracloud lightning with upper level channels attempting to exit the cloud, and then produced the upward gigantic jet. Neither flash had developed channels to ground, confirming that the major charge transfer during gigantic jets occurred between the cloud and ionosphere. The leader progression of one event was detected at altitudes above 20 km, demonstrating the possibility of detecting and tracking the propagation of negative jets above the cloud with VHF techniques.
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  • 17
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    Characteristics of the TIGGE multimodel ensemble prediction system in representing forecast variability associated with extratropical transition (2011)
    Wiley
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    Publication Date: 2011-06-22
    Description: The characteristics of the extratropical transition (ET) of tropical cyclones and its impact on the midlatitude flow are examined in the THORPEX Interactive Grand Global Ensemble (TIGGE) multimodel ensemble prediction system (EPS). Ten ensemble forecasts initialized prior to ET for five tropical cyclones in 2008 are investigated using an empirical orthogonal function analysis and fuzzy clustering methodology. Each forecast contains 231 members from eight different global EPS. The EPS contributing to TIGGE differ in their spread and their contributions to the different scenarios. Some of the individual EPS are generally confined to only a few scenarios, whereas others contribute regularly to almost all. TIGGE contains more development scenarios than European Centre for Medium Range Weather Forecast (ECMWF) EPS but the full range of development scenarios is only found with the ECMWF included in the multimodel EPS.
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  • 18
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    The 3D model of the plasmasphere coupled to the ionosphere (2011)
    Wiley
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    Publication Date: 2011-06-25
    Description: The 3D dynamic model of the plasmasphere developed by Pierrard and Stegen (2008) has been coupled with the ionospheric IRI model. In addition to the electron number density, the plasmaspheric model is also developed to include the temperature profiles of the different particles and ion composition at altitudes from 60 to 2000 km. Results of the model for the F region trough are compared with coincident observations of middle and top ionosphere by means of satellite tomography and radar measurements. A good match between the model and observations supports the idea that the present model is useful for investigating physical mechanism involved in the plasmasphere-ionosphere coupling and for acquiring information about the plasmaspheric behaviour based on ionospheric observations.
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  • 19
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    Transport and coastal zooplankton communities in the northern California Current system (2011)
    Wiley
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    Publication Date: 2011-06-25
    Description: Alongshore transport was estimated from the gridded AVISO altimeter data and water level data from NOAA tide gauges (1993–2010) for the northern California Current (NCC) system. The biomass of the cold neritic copepods including Calanus marshallae, Pseudocalanus mimus and Acartia longiremis (dominants in the eastern Bering Sea, coastal Gulf of Alaska, and NCC) was estimated from a 15 year time series of zooplankton samples (1996–2010) collected biweekly at a coastal station 9 km off Newport Oregon U.S.A. The alongshore currents and the biomass of the cold neritic copepods exhibit a strong seasonal pattern and fluctuate in opposite phase: positive alongshore current (from south) leads to low biomass in winter and negative alongshore current (from north) leads to high biomass in summer. When the Pacific Decadal Oscillation (PDO) is positive, i.e., warm conditions around the northeast Pacific, there is more movement of water from the south in the NCC during winter. When the PDO is negative, there is more movement of water from the north during summer. The mean biomass of cold neritic copepods was positively correlated with the survival rate of juvenile coho salmon and cumulative transport was negatively correlated with coho salmon survival, i.e., in years when a greater portion of the source waters feeding the NCC enters from the north, the greater the salmon survival. We conclude that alongshore transport manifests PDO signals and serves as a linkage between large scale forcing to local ecosystem dynamics.
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  • 20
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    The average influence of decadal solar forcing on the atmosphere in the South Pacific region (2011)
    Wiley
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    Publication Date: 2011-06-25
    Description: Composite mean difference analyses are applied to historical sea level pressure (SLP) and sea surface temperature (SST) data to investigate the spatial dependence of the Pacific climate system response to 11-year solar forcing. Previous work has found that the SST and SLP responses are most clearly detected near the times of sunspot maxima, which occur as much as two years prior to the centers of the broad decadal solar cycle maxima. In January–February, the SLP response at sunspot maximum is nearly the same on either side of the equator, although the amplitude is larger in the winter hemisphere. The solar influence is seen as above normal SLP in the sub-Arctic Pacific, as found previously, and as corresponding positive SLP anomalies in the sub-Antarctic Pacific, as shown here for the first time. These SLP anomalies are associated with previously documented signals at sunspot maxima of greater ocean upwelling and cooling along the Pacific equator, and a poleward extension of the tropical convergence zones in both hemispheres. Previous studies using multiple linear regression methods show the broad decadal solar maxima being associated with the lagged warm response in equatorial Pacific SSTs seen in the composites, which is not inconsistent with the present results. In the South Pacific Ocean, the solar effect is visible in the southern summer in the year before the sunspot number peak. The SST and SLP anomalies in the South Pacific in the solar peaks differ markedly from those in Cold Events (La Niña events) of the Southern Oscillation.
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  • 21
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    Evaluation of velocity-related approximations in the nonlinear shallow water equations for the Kuril Islands, 2006 tsunami event at Honolulu, Hawaii (2011)
    Wiley
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    Publication Date: 2011-06-29
    Description: The Nonlinear Shallow Water Equations (NSWE) provide a model for long wave behavior commonly used in tsunami modeling problems in which the scale associated with the surface wavelength is much greater than the ocean depth. This approximation allows for the derivation of the NSWE under the assumptions that the pressure is hydrostatic and the horizontal velocity components uniform along the water column. The present study uses current velocity data acquired by the Kilo Nalu Near-Shore Reef Observatory Acoustic Doppler Current Profiler (ADCP) on the south shore of Oahu (21.288°N, 157.865°W) in the aftermath of the 2006 Kuril tsunami to assess the validity of the NSWE assumptions on the velocity components. ADCP measurements provide information on all three velocity components along the water column, allowing the calculation of terms discarded in the NSWE approximation, containing temporal or vertical derivatives. Comparison of the relative magnitude of terms retained and neglected in the NSWE reveals that in shallow waters (10 m) the size of discarded terms remains smaller, but approaches the order of magnitude of retained terms in the momentum equations. Not all terms present in the equations can be evaluated from the available data, so verification of the assumptions investigated here represents a necessary but not sufficient condition for the validity of the NSWE approximation to model tsunami waves in shallow coastal waters.
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  • 22
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    Water vapor intrusions into the High Arctic during winter (2011)
    Wiley
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    Publication Date: 2011-06-29
    Description: The meridional transport of water vapor into the High Arctic, accompanied by dry enthalpy and clouds, impacts the surface radiative forcing. The evolution of one such moist intrusion over 9–11 February 2010 is presented. The event is analyzed using a unique blend of measurements including a new pan-Arctic retrieval of column water vapor from the Microwave Humidity Sounders, water vapor profiles from a Raman lidar and a ground-based microwave radiometer at the Polar Environment Atmospheric Research Laboratory (PEARL), in Eureka (80°N, 86°W), on Ellesmere Island in the Canadian High Arctic. A radiation model reveals the intrusion is associated with a 17 W m−2 average increase in downwelling longwave irradiance. Optically thin clouds, as observed by the lidar, contribute a further 20 W m−2 to the downwelling longwave irradiance at their peak. Intrusion events are shown to be a regular occurrence in the Arctic winter with implications for the understanding of the mechanisms driving Arctic Amplification.
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  • 23
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    Probabilistic multi-model ensemble predictions of nitrogen concentrations in river systems (2011)
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    Publication Date: 2011-06-29
    Description: Because of an increasing human pressure on the naturally balanced nitrogen cycle, eutrophication-driven hypoxia and corresponding dead zones have multiplied over the last years. Models are used to predict the nitrogen balances and develop mitigation scenarios for such systems. Due to the very complex interaction of water and nitrogen fluxes, no single model structure has been commonly adopted to describe the fluxes best and modeling results often differ substantially. Relying on a single model prediction only can be therefore highly uncertain. Here we illustrate the potential advantage of using a probabilistic multi-model ensemble approach in comparison to predictions of each of its members. Evaluation of corresponding skills and potential economic values to correctly predict a 2 mg N/L target of total nitrogen concentration in the water flowing out of a catchment in south-west Western Australia is conducted. Results show that the ensemble has always more skill and almost always more potential economic value than any of its four members and that it therefore constitutes a more reliable choice in the decision-making process.
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    Aqueous OH oxidation of ambient organic aerosol and cloud water organics: Formation of highly oxidized products (2011)
    Wiley
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    Publication Date: 2011-06-10
    Description: Aqueous chemistry can play a vital role in secondary organic aerosol (SOA) formation and aging. A novel analytical approach that allows for simultaneous photo-oxidation and atomization of reacting bulk solutions coupled to an aerosol mass spectrometer (AMS) investigates aqueous OH oxidation of ambient biogenic SOA, cloud water from a biogenic environment, glyoxal, and mixtures of glyoxal with α-pinene SOA components. This is the first study of aqueous oxidative aging of ambient SOA and cloud water organics. Starting with an AMS-based observational framework, we show that aqueous oxidation of biogenic SOA in the presence of glyoxal can better represent observed atmospheric aging than when glyoxal is absent. Oxidation of glyoxal alongside semi-volatile SOA components leads to the production of highly oxidized SOA.
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    The stratigraphic filter and bias in measurement of geologic rates (2011)
    Wiley
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    Publication Date: 2011-06-10
    Description: Erosion and deposition rates estimated from the stratigraphic record frequently exhibit a power-law dependence on measurement interval. This dependence can result from a power-law distribution of stratigraphic hiatuses. By representing the stratigraphic filter as a stochastic process called a reverse ascending ladder, we describe a likely origin of power-law hiatuses, and thus, rate scaling. While power-law hiatuses in certain environments can be a direct result of power-law periods of stasis (no deposition or erosion), they are more generally the result of randomness in surface fluctuations irrespective of mean subsidence or uplift. Autocorrelation in fluctuations can make hiatuses more or less heavy-tailed, but still exhibit power-law characteristics. In addition we show that by passing stratigraphic data backward through the filter, certain statistics of surface kinematics from their formative environments can be inferred.
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    Aftershock study and seismotectonic implications of the 8 March 2010 Kovancılar (Elazığ, Turkey) earthquake (MW = 6.1) (2011)
    Wiley
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    Publication Date: 2011-06-11
    Description: A destructive earthquake (Mw = 6.1) occurred on the northeast end of the Palu segment of the East Anatolian Fault System (EAFS, eastern Turkey) on 8 March 2010. The spatial distribution of aftershocks suggests that the Palu segment does not terminate to the east of Palu town but extends in the N50°E direction where it has produced a 30 km right stepover. Aftershock depths indicate a seismogenic brittle zone of about 15 km depth. The stress changes on the segments due to recorded events might have loaded more than 0.5 bars of stress on the NE end of the Palu segment and the SW end of the Göynük segment. Therefore, a future earthquake on both segments may occur sooner than was expected before the occurrence of the Mw 6.1 earthquake on 8 March 2010.
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    Extinction and optical depth of contrails (2011)
    Wiley
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    Publication Date: 2011-06-14
    Description: One factor limiting the understanding of the climate impact from contrails and aircraft induced cloud modifications is the accurate determination of their optical depth. To this end, 14 contrails were sampled for 2756 s with instruments onboard the research aircraft Falcon during the CONCERT (CONtrail and Cirrus ExpeRimenT) campaign in November 2008. The young (
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    Melt extraction pathways at segmented oceanic ridges: Application to the East Pacific Rise at the Siqueiros transform (2011)
    Wiley
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    Publication Date: 2011-06-15
    Description: Throughout the global mid-ocean ridge system, transform faults offset spreading centers. Conductive cooling may be more efficient beneath transform faults, producing a thickened lithosphere that directs melt away from the transform. However, recent observations of thickened crust along transform faults at fast ridges suggest melt redistribution toward transforms, intra-crustal melt production, or efficient extraction of melt. We apply a 3-D model of melt migration and extraction along an oceanic transform domain bounded by ridge segments. Melt is assumed to travel vertically before collecting and migrating beneath a low-permeability boundary inclined towards the ridge axis. A melt extraction zone, which may be geologically interpreted as the presence of faults and/or dikes leading to rapid lateral and vertical melt migration toward plate boundaries, affects the pattern of crustal accretion at segmented ridges. First, we examine a generic ridge-transform-ridge geometry and then a model that represents the Siqueiros transform on the East Pacific Rise. On the basis of crustal thickness variations within the intra-transform spreading centers along the fast-slipping Siqueiros fault, we constrain the presence of a melt extraction zone within 10 km of the transform zone.
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    Did aboriginal vegetation burning impact on the Australian summer monsoon? (2011)
    Wiley
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    Publication Date: 2011-06-15
    Description: Aboriginal vegetation burning practices and their role in the Australian environment remains a central theme of Australian environmental history. Previous studies have identified a decline in the Australian summer monsoon during the late Quaternary and attributed it to land surface-atmosphere feedbacks, related to Aboriginal burning practices. Here we undertake a comprehensive, ensemble model evaluation of the effects of a decrease in vegetation cover over the summer monsoon region of northern Australia. Our results show that the climate response, while relatively muted during the full monsoon, was significant for the pre-monsoon season (austral spring), with decreases in precipitation, higher surface and ground temperatures, and enhanced atmospheric stability. These early monsoon season changes can invoke far-reaching ecological impacts and set-up land surface-atmosphere feedbacks that further accentuate atmospheric stability.
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    Spatial performance of four climate field reconstruction methods targeting the Common Era (2011)
    Wiley
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    Publication Date: 2011-06-15
    Description: The spatial skill of four climate field reconstruction (CFR) methods is investigated using pseudoproxy experiments (PPEs) based on two millennial-length general circulation model simulations. Results indicate that presently available global and hemispheric CFRs for the Common Era likely suffer from spatial uncertainties not previously characterized. No individual method produced CFRs with universally superior spatial error statistics, making it difficult to advocate for one method over another. Northern Hemisphere means are shown to be insufficient for evaluating spatial skill, indicating that the spatial performance of future CFRs should be rigorously tested for dependence on proxy type and location, target data and employed methodologies. Observed model-dependent methodological performance also indicates that CFR methods must be tested across multiple models and conclusions from PPEs should be carefully evaluated against the spatial statistics of real-world climatic fields.
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    European cold winter 2009–2010: How unusual in the instrumental record and how reproducible in the ARPEGE-Climat model? (2011)
    Wiley
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    Publication Date: 2011-06-15
    Description: Boreal winter 2009–2010 made headlines for cold anomalies in many countries of the northern mid-latitudes. Northern Europe was severely hit by this harsh winter in line with a record persistence of the negative phase of the North Atlantic Oscillation (NAO). In the present study, we first provide a wider perspective on how unusual this winter was by using the recent 20th Century Reanalysis. A weather regime analysis shows that the frequency of the negative NAO was unprecedented since winter 1939–1940, which is then used as a dynamical analog of winter 2009–2010 to demonstrate that the latter might have been much colder without the background global warming observed during the twentieth century. We then use an original nudging technique in ensembles of global atmospheric simulations driven by observed sea surface temperature (SST) and radiative forcings to highlight the relevance of the stratosphere for understanding if not predicting such anomalous winter seasons. Our results demonstrate that an improved representation of the lower stratosphere is necessary to reproduce not only the seasonal mean negative NAO signal, but also its intraseasonal distribution and the corresponding increased probability of cold waves over northern Europe.
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    Atmospheric chemistry in stereo: A new look at secondary organic aerosols from isoprene (2011)
    Wiley
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    Publication Date: 2011-06-15
    Description: Isoprene, a compound emitted by vegetation, could be a major contributor to secondary organic aerosols (SOA) in the atmosphere. The main evidence for this contribution were the 2-methylbutane-1,2,3,4-tetraols, or 2-methyltetrols (2-methylerythritol and 2-methylthreitol) present in ambient aerosols. In this work, the four stereoisomers of these tetraols were analyzed in aerosols from Aspvreten, Sweden. 2-C-methyl-D-erythritol was found in excess over its enantiomer in the Spring/Summer, by up to 29% in July. This clearly indicated some biological origins for this enantiomer, consistent with its well-documented production by plants and other living organisms. In addition, a minimum of 20 to 60% of the mass of racemic tetraols appeared from biological origin. Thus, the SOA mass produced by isoprene in the atmosphere is less than what indicated by the 2-methyltetrols in aerosols. Our results also demonstrate that stereochemical speciation can distinguish primary and secondary organic material in atmospheric aerosols.
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    Relation between single very-long-period pulses and volcanic gas emissions at Mt. Asama, Japan (2011)
    Wiley
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    Publication Date: 2011-06-15
    Description: Multiple volcanic observations conducted at Mt. Asama, Japan, provide evidence of a link between single very-long-period (VLP) seismic pulses and volcanic gas emissions. SO2 flux measurements were conducted on 2 June 2009, when Mt. Asama was producing ash-free eruptions with VLP pulses. Gas bursts from a vent at the crater bottom following the VLP pulses provided an excellent opportunity to examine the relation directly. The SO2 emission for each eruption was calculated by integrating high temporal SO2 flux data obtained by the SO2 imaging system and subtracting the contribution from quiescent degassing from fumaroles around the crater bottom. A seismic moment of VLP pulse was estimated by the waveform inversion. We observed seven eruptions and obtained the proportional relation between VLP pulse moment and SO2 emission. The relation determined is consistent with the VLP source model; these observational results are the first report of a quantitative comparison between single VLP pulse moment and volcanic gas emission.
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    Influence of pre-existing volcanic edifice geometry on caldera formation (2011)
    Wiley
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    Publication Date: 2011-06-15
    Description: Volcanic edifice construction at the Earth's surface significantly modifies the stress field within the underlying crust with two main implications for caldera formation. First, tensile rupture at the Earth's surface is favored at the periphery, which enables ring fault formation. Second, edifice formation amplifies the amount of pressure decrease occurring within a magma reservoir before the eruption stops. Taking into account both of these effects, caldera formation can be initiated during a central eruption of a pre-existing volcano even when assuming elastic behaviour for the surrounding crust. Providing the roof aspect ratio is small enough, conditions for caldera formation by reservoir withdrawal can be reached whatever the reservoir shape is. However ring fault initiation is easier for laterally elongated reservoirs.
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    Direct measurements of CO2 flux in the Greenland Sea (2011)
    Wiley
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    Publication Date: 2011-06-22
    Description: During summer 2006 eddy correlation CO2 fluxes were measured in the Greenland Sea using a novel system set-up with two shrouded LICOR-7500 detectors. One detector was used exclusively to determine, and allow the removal of, the bias on CO2 fluxes due to sensor motion. A recently published correction method for the CO2-H2O cross-correlation was applied to the data set. We show that even with shrouded sensors the data require significant correction due to this cross-correlation. This correction adjusts the average CO2 flux by an order of magnitude from −6.7 × 10−2 mol m−2 day−1 to −0.61 × 10−2 mol m−2 day−1, making the corrected fluxes comparable to those calculated using established parameterizations for transfer velocity.
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    Sea-level rise and impacts projections under a future scenario with large greenhouse gas emission reductions (2011)
    Wiley
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    Publication Date: 2011-06-23
    Description: Using projections from two coupled climate models (HadCM3C and HadGEM2-AO), we consider the effect on 21st century sea-level rise (SLR) of mitigation policies relative to a scenario of business-as-usual (BAU). Around a third of the global-mean SLR over the century is avoided by a mitigation scenario under which global-mean near surface air temperature stabilises close to the Copenhagen Accord limit of a 2°C increase. Under BAU (a variant of the A1B scenario) the model-averaged projected SLR for 2090–2099 relative to 1980–1999 is 0.29 m–0.51 m (5%–95% uncertainties from treatment of land-based ice melt); under mitigation (E1 scenario) it is 0.17 m–0.34 m. This reduction is primarily from reduced thermal expansion. The spatial patterns of regional SLR are fairly dissimilar between the models, but are qualitatively similar across scenarios for a particular model. An impacts model suggests that by the end of the 21st century and without upgrade in defences around 55% of the 84 million additional people flooded per year globally under BAU (from SLR alone) could be avoided under such mitigation. The above projections of SLR follow the methodology of the IPCC Fourth Assessment. We have, however, also conducted a sensitivity study of SLR and its impacts where the possibility of accelerated ice sheet dynamics is accounted for.
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    Shelf resonance and impact of near-field tsunami generated by the 2010 Chile earthquake (2011)
    Wiley
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    Publication Date: 2011-06-23
    Description: The 2010 Chile earthquake of Mw 8.8 generated a destructive tsunami in the near field that resulted in warnings across the Pacific. Numerical modeling shows trapping and amplification of the energy over the continental shelf and slope. A spectral analysis of the computed surface elevation reveals resonance oscillations with periods up to 129 min along the central Chile coast. The temporal and spectral data provides an explanation for the long-period waves recorded by DART buoys and the prolonged wave activities and belated impacts reported by residents and survey teams. The present study of the 2010 Chile tsunami together with those of the 2006 Kuril and 2009 Samoa tsunamis has directly associated shelf resonance with impacts on insular and continental coasts and provided a tool to identify at-risk localities in tsunami hazard assessment.
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    Infrasonic detection of a near-Earth object impact over Indonesia on 8 October 2009 (2011)
    Wiley
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    Publication Date: 2011-06-24
    Description: We present analysis of infrasonic signals produced by a large Earth-impacting fireball, believed to be among the most energetic instrumentally recorded during the last century that occurred on 8 October, 2009 over Indonesia. This extraordinary event, detected by 17 infrasonic stations of the global International Monitoring Network, generated stratospherically ducted infrasound returns at distances up to 17 500 km, the greatest range at which infrasound from a fireball has been detected since the 1908 Tunguska explosion. From these infrasonic records, we find the total source energy for this bolide as 8–67 kilotons of TNT equivalent explosive yield, with the favored best estimate near ∼50 kt. Global impact events of such energy are expected only once per decade and study of their impact effects can provide insight into the impactor threshold levels for ground damage and climate perturbations.
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    Low seismic resolution cannot explain S/P decorrelation in the lower mantle (2011)
    Wiley
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    Publication Date: 2011-06-29
    Description: Inverted models of the deep mantle show a decorrelation between maps of shear VS and compressional VP wave velocities, an anti-correlation between the bulk sound velocity V$\phi$ and VS and a much larger variability of VS with respect to VP, expressed by large values of the ratio of their relative lateral variations. We carried out synthetic tests to verify if these features could be artifacts, explained by limits in tomographic resolution: synthetic data are calculated for an “input” model, and linearly inverted, as in tomography, to find an “output” model. Comparing the values of the aforementioned parameters for two different chemically homogeneous input models with the associated reconstructed output ones, we found that artifacts caused by realistic data noise and the nonuniform distribution of seismic sources and stations over the globe are not sufficient to introduce the features previously described. We confirm that compositional effects are required to explain them.
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    Extreme melt on Canada's Arctic ice caps in the 21st century (2011)
    Wiley
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    Publication Date: 2011-06-09
    Description: Canada's Queen Elizabeth Islands contain ∼14% of Earth's glacier and ice cap area. Snow accumulation on these glaciers is low and varies little from year to year. Changes in their surface mass balance are driven largely by changes in summer air temperatures, surface melting and runoff. Relative to 2000–2004, strong summer warming since 2005 (1.1 to 1.6°C at 700 hPa) has increased summer mean ice surface temperatures and melt season length on the major ice caps in this region by 0.8 to 2.2°C and 4.7 to 11.9 d respectively. 30–48% of the total mass lost from 4 monitored glaciers since 1963 has occurred since 2005. The mean rate of mass loss from these 4 glaciers between 2005 and 2009 (−493 kg m−2 a−1) was nearly 5 times greater than the 1963–2004 average. In 2007 and 2008, it was 7 times greater (−698 kg m−2 a−1). These changes are associated with a summer atmospheric circulation configuration that favors strong heat advection into the Queen Elizabeth Islands from the northwest Atlantic, where sea surface temperatures have been anomalously high.
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    A 5200-year record of freshwater availability for regions in western North America fed by high-elevation runoff (2011)
    Wiley
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    Publication Date: 2011-06-09
    Description: Shrinking glaciers and snowpacks are reducing discharge in rivers that drain the central Rocky Mountain region – water that supports downstream societies and ecosystems of western North America. However, a new 5200-year record of Lake Athabasca water-level variations, which serves as a sensitive gauge of past changes in alpine-sourced river discharge, reveals that western Canadian society has developed during a rare period of unusually abundant water ‘subsidized’ by prior glacier expansion. As the ‘alpine water tap’ closes, much drier times are ahead. Future water availability is likely to become similar to the mid-Holocene when Lake Athabasca dropped 2–4 m below the twentieth-century mean. Regions dependent on high-elevation runoff (i.e., western North America) must prepare to cope with impending water scarcity of magnitude not yet experienced since European settlement.
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    The reversal of the multi-decadal trends of the equatorial Pacific easterly winds, and the Indonesian Throughflow and Leeuwin Current transports (2011)
    Wiley
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    Publication Date: 2011-06-11
    Description: Multi-decadal weakening trend of the equatorial Pacific easterly winds since 1960 has reversed after 1993. The trend reversal has induced cooling (shallow thermocline) trend in the equatorial western Pacific before 1993, followed by a warming (deep thermocline) trend from 1993 to the present. All available atmospheric reanalysis products corroborate the trend reversal during the two multi-decadal periods. The magnitudes of the multi-decadal trends of the easterly winds, however, differ among the reanalysis products. The trend reversals of regional ocean circulations are assessed using linear regressions between wind and transport anomalies in an eddy-permitting numerical model, suggesting that since 1993 the Indonesian Throughflow and the Leeuwin Current transports have also reversed their multi-decadal weakening trends.
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    Impacts of wind stress on the Antarctic Circumpolar Current fronts and associated subduction (2011)
    Wiley
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    Publication Date: 2011-06-14
    Description: Recent studies suggest that the overturning circulation in the Antarctic Circumpolar Current (ACC) region shows a weak sensitivity to overlying wind stress changes, due to balancing of changes in the eddy-induced and Eulerian mean transports. Using an eddy-permitting coupled climate model, we analyze the response of the ACC transport, and associated water mass subduction rates, in response to an idealized poleward shift and intensification of the westerlies. As in previous studies, we find a small increase in the net ACC transport, and a poleward shift in the mean position of the ACC flow. However, the ACC is restructured, with the Subantarctic Front (SAF) and Polar Front (PF) branches shifting poleward by between 0.9° and 2.5° of latitude, resulting in a weaker ACC flow in both the SAF and PF zones. The wind stress anomaly drives a stronger northward Ekman transport of cool surface waters, deepening the winter mixed layer and causing a 12.7 Sv increase in the subduction of Subantarctic Mode Water (SAMW) north of the SAF zone and a 6.5 Sv increase in the subduction of Antarctic Intermediate Water (AAIW) within the SAF and PF zones. Our results suggest that changes in the wind stress restructure the Southern Ocean large-scale circulation, including the flow of the ACC in its primary jets, and that this affects the formation rates of SAMW and AAIW in this complex region.
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    Oceanic forcing for the East Asian precipitation in pacemaker AGCM experiments (2011)
    Wiley
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    Publication Date: 2011-06-17
    Description: The impact of the tropical oceanic forcing on the East Asian winter-to-early spring climate is investigated by implementing the pacemaker technique in the slab mixed-layer version of the Geophysical Fluid Dynamics Laboratory AM2.1 atmospheric general circulation model. The results demonstrate that oceanic forcing from the deep tropical eastern Pacific (DTEP) can instigate the Pacific-East Asia teleconnection and that the Philippine Sea anticyclone and the associated air-sea interaction are crucial for the realization of the impact of the teleconnection over East Asia. Comparison among cases in which the pace-maker is designated over the DTEP region, DTEP plus the Indian Ocean, and the whole tropical oceans indicates that tropical oceanic forcing outside of the Niño regions can also exert significant influence on East Asian climate. As a result, a total of 30% of the variance of the East Asian precipitation index can be accounted for by the tropical oceanic forcing.
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    North Tropical Atlantic influence on western Amazon fire season variability (2011)
    Wiley
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    Publication Date: 2011-06-17
    Description: The prevailing wet climate in the western Amazon is not favorable to the natural occurrence of fires. Nevertheless, the current process of clearing of humid forests for agriculture and cattle ranching has increased the vulnerability of the region to the spread of fires. Using meteorological stations precipitation and the Moderate Resolution Spectroradiometer (MODIS) Active-Fires (AF) during 2000–2009, we show that fire anomalies vary closely with July-August-September (JAS) precipitation variability as measured by the Standardized Precipitation Index (SPI). The precipitation variability is, in turn, greatly determined by sea surface temperature (SST) anomalies in the North Tropical Atlantic (NTA). We develop a linear regression model to relate local fire activity to an index of the NTA-SST. By using seasonal forecasts of SST from a coupled model, we are able to predict anomalous JAS fire activity as early as April. We applied the method to predict the severe 2010 JAS season, which indicated strongly positive seasonal fire anomalies within the 95% prediction confidence intervals in most western Amazon. The spatial distribution of predicted SPI was also in accordance with observed precipitation anomalies. This three months lead time precipitation and fire prediction product in the western Amazon could help local decision makers to establish an early warning systems or other appropriate course of action before the fire season begins.
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    Does positioning of the North Pacific Current affect downstream ecosystem productivity? (2011)
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    Publication Date: 2011-06-24
    Description: Fluctuations in the positioning of major ocean currents can influence ecosystem dynamics, but previously the technology has been lacking to make direct observational assessments. Here, we test the hypothesis that positioning of the North Pacific Current (NPC) is related to biological attributes of the central-northern California Current Ecosystem (CCE). To test this hypothesis we use newly available data from the Argo array and compare it with a suite of well-known ecosystem indicators over 6 years, 2002 through 2007. We found increased biomass and productivity when the NPC was shifted poleward, and suggest that positioning influences advective transport of nutrients and perhaps key planktonic organisms from the sub-arctic domain thereby enhancing mid to upper trophic level species. This study is significant because climate change is predicted to cause poleward shifts in the westerlies that drive ocean currents and positioning of large marine gyre systems. Rather than reducing ecosystem productivity, poleward shifts in positioning of the NPC may be beneficial for many species of the central-northern CCE.
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    The vertical distribution of cloud feedback in coupled ocean-atmosphere models (2011)
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    Publication Date: 2011-06-28
    Description: We assess the vertical distribution of cloud feedbacks in coupled climate models, taking care to distinguish between cloud feedbacks and a change in cloud forcing. We show that the effect of cloud changes on the longwave fluxes provides a strong positive feedback that is broadly consistent across models. In contrast, the effect of cloud changes on the shortwave fluxes ranges from a modest negative to a strong positive feedback, and is responsible for most of the intermodel spread in net cloud feedback. The feedback from high clouds is positive in all models, and is consistent with that anticipated by the Proportionately Higher Anvil Temperature hypothesis over the tropics. In contrast, low cloud cover is responsible for roughly three-quarters of the difference in global mean net cloud feedback among models, with the largest contributions from regions associated with low-level subtropical marine cloud systems.
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    The Greenland Sea Jet: A mechanism for wind-driven sea ice export through Fram Strait (2011)
    Wiley
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    Publication Date: 2011-06-28
    Description: We present a mechanism for wind-driven sea ice export from the Arctic Ocean through Fram Strait for the period 1979–2007, using the output of a high-resolution regional atmospheric climate model. By explicitly calculating the components of the atmospheric momentum budget, we show that not large scale synoptic forcing (LSC), but mainly thermal wind forcing (THW) causes the persistent northerly jet (the Greenland Sea Jet) over Fram Strait. The jet results from horizontal temperature gradients in the atmospheric boundary layer (ABL), set up between cold ABL-air over the sea ice covered western Greenland Sea and the relatively warmer ABL over the ice-free eastern Greenland Sea. From 1993 onwards we find a negative trend in THW, due to a stronger response to climate warming of the ABL over the sea ice covered ocean, compared to that over the ice free ocean. Although on average LSC is smaller than THW, year to year variations in LSC explain most of the interannual variability in the sea ice area flux through Fram Strait (R = 0.81). A small positive trend is found for LSC, partly compensating the decrease in THW in recent years.
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    Electrical properties of polycrystalline methane hydrate (2011)
    Wiley
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    Publication Date: 2011-05-11
    Description: Electromagnetic (EM) remote-sensing techniques are demonstrated to be sensitive to gas hydrate concentration and distribution and complement other resource assessment techniques, particularly seismic methods. To fully utilize EM results requires knowledge of the electrical properties of individual phases and mixing relations, yet little is known about the electrical properties of gas hydrates. We developed a pressure cell to synthesize gas hydrate while simultaneously measuring in situ frequency-dependent electrical conductivity (σ). Synthesis of methane (CH4) hydrate was verified by thermal monitoring and by post run cryogenic scanning electron microscope imaging. Impedance spectra (20 Hz to 2 MHz) were collected before and after synthesis of polycrystalline CH4 hydrate from polycrystalline ice and used to calculate σ. We determined the σ of CH4 hydrate to be 5 × 10−5 S/m at 0°C with activation energy (Ea) of 30.6 kJ/mol (−15 to 15°C). After dissociation back into ice, σ measurements of samples increased by a factor of ∼4 and Ea increased by ∼50%, similar to the starting ice samples.
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    A new model for the evolution of La Réunion volcanic complex from complete marine geophysical surveys (2011)
    Wiley
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    Publication Date: 2011-05-11
    Description: Results from recent marine geophysical surveys offer a new perspective for characterizing the evolution processes of volcanic islands. In 2006, cruises FOREVER and ERODER 1 investigated the submarine flanks and the surrounding abyssal plain of La Réunion (Indian Ocean) to obtain for the first time a complete geophysical survey of the area. Combined analyses of these data reveal major differences in the evolution of the two emerged volcanoes, Piton des Neiges and Piton de la Fournaise. We show that debris avalanche deposits extend on the abyssal plain only offshore the active Piton de la Fournaise volcano attesting the occurrence of large flank-collapse events. The absence of such deposits offshore Piton des Neiges and the presence of compressive structures within the sedimentary unit below the edifice support a mechanism of slow deformation of this volcano, such as sliding or spreading. The slow deformation of Piton des Neiges has led to numerous secondary submarine slope instabilities and favored some unconfined turbidity flows which generated large sediment waves running downward all around the island. This study proposes a new model using the most complete marine data set available: slow deformation controls the evolution of Piton des Neiges whereas Piton de la Fournaise (formed on the flanks of a pre-existing edifice) experienced catastrophic, large flank-collapse events.
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    Aerosol nucleation induced by a high energy particle beam (2011)
    Wiley
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    Publication Date: 2011-05-12
    Description: We have studied sulfuric acid aerosol nucleation in an atmospheric pressure reaction chamber using a 580 MeV electron beam to ionize the volume of the reaction chamber. We find a clear contribution from ion-induced nucleation and consider this to be the first unambiguous observation of the ion-effect on aerosol nucleation using a particle beam under conditions that resemble the Earth's atmosphere. By comparison with ionization using a gamma source we further show that the nature of the ionizing particles is not important for the ion-induced component of the nucleation. This implies that inexpensive ionization sources - as opposed to expensive accelerator beams - can be used for investigations of ion-induced nucleation.
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    Formation dynamics of subsurface hydrocarbon intrusions following the Deepwater Horizon blowout (2011)
    Wiley
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    Publication Date: 2011-05-12
    Description: Hydrocarbons released following the Deepwater Horizon (DH) blowout were found in deep, subsurface horizontal intrusions, yet there has been little discussion about how these intrusions formed. We have combined measured (or estimated) observations from the DH release with empirical relationships developed from previous lab experiments to identify the mechanisms responsible for intrusion formation and to characterize the DH plume. Results indicate that the intrusions originate from a stratification-dominated multiphase plume characterized by multiple subsurface intrusions containing dissolved gas and oil along with small droplets of liquid oil. Unlike earlier lab measurements, where the potential density in ambient water decreased linearly with elevation, at the DH site it varied quadratically. We have modified our method for estimating intrusion elevation under these conditions and the resulting estimates agree with observations that the majority of the hydrocarbons were found between 800 and 1200 m.
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    Characteristics of neutral calcium and Ca+ near the mesopause, and their relationship with sporadic ion/electron layers at Arecibo (2011)
    Wiley
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    Publication Date: 2011-05-14
    Description: We present the first simultaneous observations of mesospheric neutral calcium, its ion, and electron density over Arecibo obtained on adjacent nights in December 2009. A good correlation is seen between Sporadic-E and ion layers while the neutrals have different characteristics. The measurements reveal that average abundances in electron densities and ions to be at a maximum on 9 December with values ∼4.8 × 109 electrons cm−2 and 3.4 × 107 ions cm−2, while those of the neutrals to be low, of the order of 9.7 × 106 atoms cm−2. The neutral abundance maximizes with values of 1.8 × 107 atoms cm−2 on 10 December, when both ions and electrons display weak layers with abundances almost 3 times lower than on the previous night. A significant correlation between the ions and neutrals indicates that sudden enhancements are caused by advection while a poor correlation is related to chemistry. This study reveals that the ion-neutral coupling is most efficient between 92 and 95 km. Also, the ion-to-neutral ratio is dependent on the strength of the ions or Sporadic-E and can exceed 3.0 during such events. We present evidence of neutralization of the ionized layers occurring on scales of a few tens of minutes for the first time. This agrees fairly well with the lifetimes of Ca+ against neutralization as predicted by recent ion-neutral reactions.
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    Determination of the friction law parameters of the Mw 6.7 Michilla earthquake in northern Chile by dynamic inversion (2011)
    Wiley
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    Publication Date: 2011-05-14
    Description: We perform a full dynamic inversion at low frequencies of the 16 December 2007 (Mw = 6.7) northern Chile earthquake that we model as a simple elliptical patch. We use two different stress-friction end-member models: asperities and barriers, finding similar results. The inversions are performed for strong motion data filtered between 0.02 and 0.5 Hz. Eleven geometrical and stress and friction parameters are inverted using the neighbourhood algorithm. The optimum solutions have relative errors lower than 0.21. The earthquake rupture has duration of less than 5 s and propagates at sub-shear speed. The rupture area is similar to that of the aftershock distribution and the seismic moment is 0.95 · 1019 Nm. We derive the friction law parameters from the models situated close to the optimum solution using a Monte Carlo technique. The results show a strong trade-off between applied stress and frictional resistance. We find that the distribution of friction models collapses into a finite zone of the space of moment and non-dimensional parameter $\kappa$. We conclude that it is possible to determine the friction law from near field seismograms, but there is a strong trade-off between friction and initial stress.
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    A numerical investigation of supercells in landfalling tropical cyclones (2011)
    Wiley
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    Publication Date: 2011-05-17
    Description: Supercell thunderstorms are known to occur in the outer rainbands of tropical cyclones (TCs), and these cells are of interest because they are occasionally tornadic. A series of multi-scale quasi-idealized TC simulations are used to study the development of outer rainband supercells. Within the environment of landfalling TCs, the sea-to-land transition of the rainbands is found to locally benefit supercell development onshore due in part to (1) enhanced surface-based destabilization during the day and (2) increased surface friction slowing and backing the low-level winds. Landfall time of day is shown to have a substantial impact on the average number, strength, and lifetime of the ensemble of simulated supercells. These idealized simulations represent a first attempt at controlled hypothesis tests for supercell and tornado ingredients within the context of a highly resolved parent TC.
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    Very recent and wide-spread basaltic volcanism on Mars (2011)
    Wiley
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    Publication Date: 2011-05-17
    Description: New spacecraft data provide increasing evidence for a dynamic environment on present-day Mars. Exogenic processes such as impact cratering, mass wasting processes, and active dune migration have all been observed to modify the surface. No traces of current endogenic activity have been found yet, but some studies point to very localized volcanism in the last few millions of years. However, no systematic study of young volcanic surfaces had been performed so far. We present absolute model age determinations of plains volcanism on Mars as derived from impact crater size-frequency distributions. Extended areas in Tharsis, the largest volcano-tectonic region on Mars, have been resurfaced by lava flows in the last few tens of millions of years. We also present results on the rheologic properties of these lava flows, inferred from morphometric measurements. Yield strengths are in the range of 100–300 Pa, and viscosities reach values of 102 to 103 Pa s, indicating basaltic compositions. The results imply that Mars retained until recently, and probably still retains, enough internal heat to produce wide-spread plain-style volcanism, producing low-viscosity lava flows throughout large parts of Tharsis.
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    Connecting microstructural attributes and permeability from 3D tomographic images of in situ shear-enhanced compaction bands using multiscale computations (2011)
    Wiley
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    Publication Date: 2011-05-17
    Description: Tomographic images taken inside and outside a compaction band in a field specimen of Aztec sandstone are analyzed by using numerical methods such as graph theory, level sets, and hybrid lattice Boltzmann/finite element techniques. The results reveal approximately an order of magnitude permeability reduction within the compaction band. This is less than the several orders of magnitude reduction measured from hydraulic experiments on compaction bands formed in laboratory experiments and about one order of magnitude less than inferences from two-dimensional images of Aztec sandstone. Geometrical analysis concludes that the elimination of connected pore space and increased tortuosities due to the porosity decrease are the major factors contributing to the permeability reduction. In addition, the multiscale flow simulations also indicate that permeability is fairly isotropic inside and outside the compaction band.
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    Toward the predictability of meteotsunamis in the Balearic Sea using regional nested atmosphere and ocean models (2011)
    Wiley
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    Publication Date: 2011-05-18
    Description: Meteotsunamis are oceanic waves that possess tsunami-like characteristics but are meteorological in origin. In the western Mediterranean, travelling atmospheric pressure oscillations generate these long oceanic surface waves that can become amplified and produce strong seiche oscillations inside harbors. We analyze a June 2006 meteotsunami event in Ciutadella harbor (Menorca Island, Spain), studying numerically the phenomenon during its full life cycle, from the early atmospheric stages to the atmosphere-ocean resonant phase and the final highly amplified harbor oscillation. The Weather Research Forecast (WRF) atmospheric model adequately reproduces the development of a convective nucleus and also reproduces the induced atmospheric pressure oscillations moving at a speed of 27 m/s. The oceanic response is studied using the Regional Ocean Modeling System (ROMS), forced by the WRF pressure field. It shows an inverse barometer wave front in the open ocean progressively amplified through resonant interactions in the different shelf and coastal regions. The predictive capability of this new WRF/ROMS modeling approach is then discussed.
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    Consistency and complementarity of different coastal ocean observations: A neural network-based analysis for the German Bight (2011)
    Wiley
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    Publication Date: 2011-05-18
    Description: HF radar measurements in the German Bight and their consistency with other available observations were analyzed. First, an empirical orthogonal function (EOF) analysis of the radial component of the surface current measured by one radar was performed. Afterwards, Neural Networks (NNs) were trained to now- and forecast the first five EOFs from tide gauge measurements. The inverse problem, i.e., to forecast a sea level from these EOFs was also solved using NNs. For both problems, the influence of wind measurements on the nowcast/forecast accuracy was quantified. The forecast improves if HF radar data are used in combination with wind data. Analysis of the upscaling potential of HF radar measurements demonstrated that information from one radar station in the German Bight is representative of an area larger than the observational domain and could contribute to correcting information from biased observations or numerical models.
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    Mapping sea surface oil slicks using RADARSAT-2 quad-polarization SAR image (2011)
    Wiley
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    Publication Date: 2011-05-18
    Description: Polarimetric SAR decomposition parameters, average alpha angle ($\overline{\alpha}$) and entropy (H) are estimated for oil-slick contaminated sea surfaces and slick-free conditions using a RADARSAT-2 quad-polarization SAR image. The values of H and $\overline{\alpha}$ within oil slick areas are significantly higher than those of the ambient sea surface, indicating the dominance of Bragg scattering for the slick-free ocean and non-Bragg scattering for the oil-slick area. In land classification, the conformity coefficient (μ) is often used to discriminate surface scattering with double-bounce or volume scattering. Based on this rationale, we also develop a method using μ as a logical scalar descriptor to map oil slicks under low-to-moderate wind conditions. The proposed method is assessed using a RADARSAT-2 quad-polarization SAR image of oil slicks in the Gulf of Mexico. Analysis shows that when μ is positive the sea surface is slick-free, whereas μ is negative for oil-slick areas. This method provides a simple and effective mapping technique for oil slick detection.
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    Evidence of atmospheric gravity waves during the 2008 eruption of Okmok volcano from seismic and remote sensing observations (2011)
    Wiley
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    Publication Date: 2011-05-18
    Description: Okmok volcano erupted on July 12, 2008, following an 11-year hiatus. Detailed inspection of the syn-eruptive seismograms revealed the presence of an ultra long-period mode at a frequency of 1.7 mHz, which is not a characteristic of the background seismic noise at Okmok. Data collected by the National Oceanic and Atmospheric Administration Geostationary Operational Environmental Satellite (GOES) and National Aeronautical and Space Administration Moderate Resolution Imaging Spectroradiometer (MODIS) sensors displayed the propagation of a vigorous ash-and-steam plume up to about 17 km above sea level. We suggest that the observed ultra long-period signals represent the response of the seismometer to changes in gravity associated with buoyancy oscillations set off in the lower atmosphere above Okmok by the emplacement of the eruption column. Calculations based on simple modeling of these effects allowed estimation of peak atmospheric pressure perturbations associated with the eruption of less than 1 mbar.
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    Correction to “Multiple melt injection along a spreading segment at Askja, Iceland” (2011)
    Wiley
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    Publication Date: 2011-05-19
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    Combining geothermal energy capture with geologic carbon dioxide sequestration (2011)
    Wiley
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    Publication Date: 2011-05-19
    Description: Geothermal energy offers clean, renewable, reliable electric power with no need for grid-scale energy storage, yet its use has been constrained to the few locations worldwide with naturally high geothermal heat resources and groundwater availability. We present a novel approach with the potential to permit expansion of geothermal energy utilization: heat extraction from naturally porous, permeable formations with CO2 as the injected subsurface working fluid. Fluid-mechanical simulations reveal that the significantly higher mobility of CO2, compared to water, at the temperature/pressure conditions of interest makes CO2 an attractive heat exchange fluid. We show numerically that, compared to conventional water-based and engineered geothermal systems, the proposed approach provides up to factors of 2.9 and 5.0, respectively, higher geothermal heat energy extraction rates. Consequently, more regions worldwide could be economically used for geothermal electricity production. Furthermore, as the injected CO2 is eventually geologically sequestered, such power plants would have negative carbon footprints.
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    A note on equations for steady-state optimal landscapes (2011)
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    Publication Date: 2011-05-19
    Description: Based on the optimality principle (that the global energy expenditure rate is at its minimum for a given landscape under steady state conditions) and calculus of variations, we have derived new governing equations for describing steady-state optimal landscapes. Other than building on the well-established Manning's equation, this work does not rely on any empirical relationships (such as those relating hydraulic parameters to local slopes). Using additional constraints, we also theoretically demonstrate that discharge is a power function of local slope, which is consistent with field data and previous studies.
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    Inflation and deflation at the steep-sided Llaima stratovolcano (Chile) detected by using InSAR (2011)
    Wiley
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    Publication Date: 2011-05-19
    Description: Llaima volcano, Chile, is a typical basaltic-to-andesitic stratovolcano in the southcentral Andes. Llaima had at least four explosive eruptions in the decade 2000 – 2010, however little is known about the physical processes and magma storage at this volcano. In this study we present an InSAR deformation field at Llaima from 2003 – 2008, covering both the post-eruptive and syn-eruptive periods. The satellite InSAR data are significantly affected by environmental decorrelation due to steep topography, snow and vegetation; because of this, we applied a model-assisted phase unwrapping approach. The analysis of these data suggests two main deformation episodes: subsidence associated with the post-eruptive period, and uplift associated with the syn-eruptive period. Maximum summit subsidence and uplift are ∼10 cm and ∼8 cm, respectively. Through inverse modeling of both periods, a deflating and inflating magma body can be inferred, located at a depth of 4 – 12 km, subject to a volume decrease of 10 – 46 × 106 m3 during the subsidence period, followed by a volume increase of 6 – 20 × 106 m3 during the uplift period. Therefore, this study presents the first evidence of magma-driven deformation at Llaima volcano, and suggests that eruption periods are associated with the inflation and deflation of a deep magma body that can be monitored by using space geodesy.
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    Volume variability diagnostic for 4D datasets (2011)
    Wiley
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    Publication Date: 2011-05-19
    Description: Recent advances in the compilation of the climate science databases, derived from observations and model simulations, help in estimating trends and variability of current and past atmospheric conditions. However, an exponential increase in the amount of information limits its comprehension. In this paper we present a simple diagnostic, named as “volume variability”, that allows for analysis of multidimensional fields of climate variables. The simplicity inherent in the new diagnostic, calculated as a product of variances along a set of directions, and its transparent interpretation thereof will enable its use as a metric for the quick and easy comparison of the 3D time series of models and observations. Using this diagnostic, we explore the evolution of the temperature and geopotential height fields over the Southern Hemisphere Cap region by way of four reanalysis systems.
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    Simulation of long eccentricity (400-kyr) cycle in ocean carbon reservoir during Miocene Climate Optimum: Weathering and nutrient response to orbital change (2011)
    Wiley
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    Publication Date: 2011-05-20
    Description: Deep-sea foraminiferal δ13C records contain abundant 400-kyr cycles indicating a link between eccentricity forcing and ocean carbon reservoir change. Here we simulate the oceanic δ13C maxima events forced by the Earth's orbital geometry during the Miocene Climate Optimum (MCO, 17-14 Ma) using a box model. The simulated results of both surface and deep water δ13C display co-varying 400-kyr cycle. Modulated by orbital parameters, weathering induced carbon input will change the burial ratio of carbonates to organic carbon and further result in periodic changes in the oceanic δ13C. The increase of riverine nutrient input, which is synchronous with riverine carbon input, often stimulates primary productivity and burial of organic carbon. Our results support that eccentricity maxima (minima) enhance (reduce) weathering intensity and nutrient supply, which lead to minima (maxima) of δ13C. The prominent 400-kyr cycle of ocean carbon reservoir is interpreted as likely caused by a long memory of carbon in the ocean.
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    Increasing trends for the surface heat flux and fresh water flux in the North Pacific eastern subtropical region (2011)
    Wiley
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    Publication Date: 2011-05-20
    Description: We investigated trends for surface net heat flux (NET) and fresh water flux (FWF) over the eastern subtropical region in the North Pacific for the period of 1988–2005 using satellite-based data set. It is concluded that NET and FWF remarkably increased during this period. Latent heat flux (LHF) mainly contributes to the increase in NET. Moreover, in order to clarify which flux has larger contribution to the increase in LHF, we investigated four LHF products. Both of the increase in the wind speed (U) and the decrease in the air specific humidity (Qa) have critical contribution to the increase in LHF. In contrast, the decrease of the sea surface temperature (SST) which contributes the decrease in LHF is also found. Also, we proposed a possible mechanism that the increase in U and the decrease in Qa are induced by the strengthened descending branch of local Hadley circulation, associated with change of SST around this region.
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    Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport (2011)
    Wiley
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    Publication Date: 2011-05-25
    Description: The possibility of a rapid collapse in the strength of the Atlantic meridional overturning circulation (AMOC), with associated impacts on climate, has long been recognized. The suggested basis for this risk is the existence of two stable regimes of the AMOC (‘on’ and ‘off’), and such bistable behaviour has been identified in a range of simplified climate models. However, up to now, no state-of-the-art atmosphere-ocean coupled global climate model (AOGCM) has exhibited such behaviour, leading to the interpretation that the AMOC is more stable than simpler models indicate. Here we demonstrate AMOC bistability in the response to freshwater perturbations in the FAMOUS AOGCM - the most complex AOGCM to exhibit such behaviour to date. The results also support recent suggestions that the direction of the net freshwater transport at the southern boundary of the Atlantic by the AMOC may be a useful physical indicator of the existence of bistability. We also present new estimates for this net freshwater transport by the AMOC from a range of ocean reanalyses which suggest that the Atlantic AMOC is currently in a bistable regime, although with large uncertainties. More accurate observational constraints, and an improved physical understanding of this quantity, could help narrow uncertainty in the future evolution of the AMOC and to assess the risk of a rapid AMOC collapse.
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    Antarctic grounding line mapping from differential satellite radar interferometry (2011)
    Wiley
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    Publication Date: 2011-05-26
    Description: The delineation of an ice sheet grounding line, i.e., the transition boundary where ice detaches from the bed and becomes afloat in the ocean, is critical to ice sheet mass budget calculations, numerical modeling of ice sheet dynamics, ice-ocean interactions, oceanic tides, and subglacial environments. Here, we present 15 years of comprehensive, high-resolution mapping of grounding lines in Antarctica using differential satellite synthetic-aperture radar interferometry (DInSAR) data from the Earth Remote Sensing Satellites 1–2 (ERS-1/2), RADARSAT-1 and 2, and the Advanced Land Observing System (ALOS) PALSAR for years 1994 to 2009. DInSAR directly measures the vertical motion of floating ice shelves in response to tidal oceanic forcing with millimeter precision, at a sample spacing better than 50 m, simultaneously over areas several 100 km wide; in contrast with earlier methods that detect abrupt changes in surface slope in satellite visible imagery or altimetry data. On stagnant and slow-moving areas, we find that breaks in surface slope are reliable indicators of grounding lines; but on most fast-moving glaciers and ice streams, our DInSAR results reveal that prior mappings have positioning errors ranging from a few km to over 100 km. A better agreement is found with ICESat's data, also based on measurements of vertical motion, but with a detection noise one order of magnitude larger than with DInSAR. Overall, the DInSAR mapping of Antarctic grounding lines completely redefines the coastline of Antarctica.
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    Spurious trends in global surface drifter currents (2011)
    Wiley
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    Publication Date: 2011-05-26
    Description: The Global Drifter Program (GDP) has been measuring near-surface ocean currents with surface drifters since 1979. At least half of the World Ocean now has drifter velocity time series longer than 15 years. The availability of this data opens new opportunities to explore observationally how ocean circulation responds to changing surface forcing. In this paper we report evidence of an apparently spurious acceleration of global surface drifter currents. This rapid acceleration occurs in a pattern reflecting the geographic distribution of mean surface winds. For example, in the westerly wind region of the Southern Ocean this strengthening is at least 0.5 cm/s per year eastward, while in the easterly trade wind region of the tropics this strengthening is on average 0.25 cm/s per year westward. One possible explanation we explore is that the bias is due to the presence of some undrogued drifters whose frequency of occurrence changes in time and whose windage is significantly greater than that of the drogued drifters. This paper is dedicated to the memory of Professor Peter Niiler, who first suggested this explanation.
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    Improving the predictability of global CO2 assimilation rates under climate change (2011)
    Wiley
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    Publication Date: 2011-05-27
    Description: Feedbacks between the terrestrial carbon cycle and the atmosphere have the potential to greatly modify expected rates of future climate change. This makes it all the more urgent to exploit all existing data for the purpose of accurate modelling of the underlying processes. Here we use a Bayesian random sampling method to constrain parameters of the Farquhar model of leaf photosynthesis and a model of leaf respiration against a comprehensive set of plant trait data at the leaf level. The resulting probability density function (PDF) of model parameters is contrasted with a PDF derived using a conventional “expert knowledge” approach. When running the Biosphere Energy Transfer Hydrology (BETHY) scheme with a 1000- member sub-sample of each of the two PDFs for present climate and a climate scenario, we find that the use of plant trait data is able to reduce the uncertainty range of simulated net leaf assimilation (NLA) by more than a factor of two. Most of the remaining variability is caused by only four parameters, associated with the acclimation of photosynthesis to plant growth temperature and to how leaf stomata react to atmospheric CO2 concentration. We suggest that this method should be used extensively to parameterize Earth system models, given that data bases on plant traits are increasingly being made available to the modelling community.
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    ENSO regimes: Reinterpreting the canonical and Modoki El Niño (2011)
    Wiley
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    Publication Date: 2011-05-27
    Description: We propose that the first two empirical orthogonal function (EOF) modes of tropical Pacific sea surface temperature (SST) anomalies do not describe different phenomena (i.e., El Niño-Southern Oscillation (ENSO) and “El Niño Modoki”) but rather the nonlinear evolution of ENSO. We introduce two new uncorrelated indices (E and C), based on the leading EOFs, that respectively account for extreme warm events in the eastern and cold/moderate warm events in the central equatorial Pacific, corresponding to regimes with different evolution. Recent trends in ENSO can be described as an increase in the central Pacific (C) variability that is associated with stronger cold events, as well as a reduction in the eastern Pacific (E) variability within the cold/moderate warm regime, consistent with model projections. However, little can be said observationally with respect to the extreme warm regime.
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    VLF wave generation by beating of two HF waves in the ionosphere (2011)
    Wiley
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    Publication Date: 2011-05-27
    Description: Theory of a beat-wave mechanism for very low frequency (VLF) wave generation in the ionosphere is presented. The VLF current is produced by beating two high power HF waves of slightly different frequencies through the nonlinearity and inhomogeneity of the ionospheric plasma. Theory also shows that the density irregularities can enhance the beat-wave generation. An experiment was conducted by transmitting two high power HF waves of 3.2 MHz and 3.2 MHz + f, where f = 5, 8, 13, and 2.02 kHz, from the HAARP transmitter. In the experiment, the ionosphere was underdense to the O-mode heater, i.e., the heater frequency f0 〉 foF2, and overdense or slightly underdense to the X-mode heater, i.e., f0 〈 fxF2 or f0 ≥ fxF2. The radiation intensity increased with the VLF wave frequency, was much stronger with the X-mode heaters, and was not sensitive to the electrojet. The strongest VLF radiation of 13 kHz was generated when the reflection layer of the X-mode heater was just slightly below the foF2 layer and the spread of the O-mode sounding echoes had the largest enhancement, suggesting an optimal setting for beat-wave generation of VLF waves by the HF heaters.
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    Didymosphenia geminata: Algal blooms in oligotrophic streams and rivers (2011)
    Wiley
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    Publication Date: 2011-05-28
    Description: In recent decades, the diatom Didymosphenia geminata has emerged as nuisance species in river systems around the world. This periphytic alga forms large “blooms” in temperate streams, presenting a counterintuitive result: the blooms occur primarily in oligotrophic streams and rivers, where phosphorus (P) availability typically limits primary production. The goal of this study is to examine how high algal biomass is formed under low P conditions. We reveal a biogeochemical process by which D. geminata mats concentrate P from flowing waters. First, the mucopolysaccaride stalks of D. geminata adsorb both iron (Fe) and P. Second, enzymatic and bacterial processes interact with Fe to increase the biological availability of P. We propose that a positive feedback between total stalk biomass and high growth rate is created, which results in abundant P for cell division. The affinity of stalks for Fe in association with iron-phosphorus biogeochemistry suggest a resolution to the paradox of algal blooms in oliogotrophic streams and rivers.
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    Estimating the risk of glacier cavity collapse during artificial drainage: The case of Tête Rousse Glacier (2011)
    Wiley
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    Publication Date: 2011-05-28
    Description: During the summer of 2010, the presence of a pressurized water-filled subglacial-cavity of at least 50,000 m3 was detected within the Tête Rousse Glacier (French Alps). Artificial drainage was started to avoid an uncontrolled rupture of the ice dam, but was interrupted soon after to evaluate the capacity of the cavity-roof to bear itself. The risk was that the release of pressure within the cavity during the artificial drainage would precipitate the collapse of the cavity roof and potentially flush out the remaining water flooding the valley below. An unprecedented modeling effort was deployed to answer the question of the cavity roof stability. We set up a model of the glacier with its water cavity, solved the three-dimensional full-Stokes problem, predicted the upper surface and cavity surface displacements for various drainage scenarios, and quantified the risk of the cavity failure during artificial drainage. We found that the maximum tensile stress in the cavity roof was below the rupture value, indicating a low risk of collapse. A post drainage survey of the glacier surface displacements has confirmed the accuracy of the model prediction. This practical application demonstrates that ice flow models have reached sufficient maturity to become operational and assist policy-makers when faced with glaciological hazards, thus opening new perspectives in risk management of glacier hazards in high mountain regions.
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    Dense shelf water formation along the south-west Australian inner shelf (2011)
    Wiley
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    Publication Date: 2011-05-28
    Description: Hydrological data from a repeated cross-shore transect obtained using Teledyne Webb Research Slocum Electric gliders offshore Two Rocks in south-western Australia over 13 months are presented. The data revealed that formation of dense water inshore and its transport across the shelf as a near bed gravity current (defined as Dense Shelf Water Cascade, DSWC) was a regular occurrence, particularly during autumn and winter months. In autumn, the dense water is mainly formed through changes in salinity resulting from evaporation, whilst in winter; temperature change through surface cooling was the dominant factor. The mean wind speeds also decrease during the transition during autumn. The speed of the DSWC was estimated to be 0.01–0.02 m s−1, and similar to that measured in other selected regions globally. The offshore transport from the shelf is a significant component of the alongshore wind-driven transport.
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    Another potential source of destructive earthquakes and tsunami offshore of Sumatra (2011)
    Wiley
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    Publication Date: 2011-05-28
    Description: We link geodetic data from the Sumatran GPS Array (SuGAr) and earthquake focal mechanisms to show that a 900-km long backthrust, arising from the Sunda megathrust offshore of Sumatra, has recently become active following the series of great megathrust earthquakes of this past decade. Shallow failure of the Mentawai backthrust explains coseismic displacements during moderate-earthquake clusters in 2005 and 2009. These two clusters represent the first activity on the backthrust in more than 30 years. Existing paleogeodetic evidence of vertical deformation in past centuries is too sparse to characterize earlier major activity, but leaves open the possibility of historic great backthrust earthquakes. Our geodetic evidence for rupture of the Mentawai backthrust during the two recent earthquake clusters suggests that this large fault may well pose an additional seismic and tsunami hazard to the coastal communities of central Sumatra.
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    Start and end of bedload transport in gravel-bed streams (2011)
    Wiley
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    Publication Date: 2011-02-23
    Description: The threshold of incipient bedload motion, expressed either as a critical force or as a critical water discharge, is a key parameter in bedload transport prediction. Measuring the threshold of motion is difficult, and reliable data from natural streams are rare. By recording the vibrations triggered by bedload particles when moving over a steel plate mounted in the channel bed, we determined the time at start and end of bedload transport in four streams, where discharge is continuously monitored. The threshold discharge scatters over approximately one order of magnitude for each stream, reinforcing previous observations that critical discharge is characterized by a distribution of values. We interpret a strong correlation between the discharge at the start of transport and the discharge at the end of transport of the previous event to reflect temporal changes in bed structure and consequent effects on the driving and resisting forces acting on the bed.
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    Delayed triggering of microearthquakes by multiple surface waves circling the Earth (2011)
    Wiley
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    Publication Date: 2011-02-24
    Description: It is well known that direct surface waves of large earthquakes are capable of triggering shallow earthquakes and deep tremor at long-range distances. However, it is not clear whether multiple surface waves circling the Earth could also trigger/modulate seismic activities. Here we conduct a systematic search of remotely triggered microearthquakes near the Coso Geothermal Field in central California following the 2010 Mw 8.8 Chile earthquake. We find a statistically significant increase of microearthquakes in the first few hours after the Chile mainshock. These observations of apparently delayed earthquake triggering do not follow the Omori-law decay with time since the largest ML 3.5 event occurred during the large-amplitude Love waves. Instead, they are better correlated with the first three groups of multiple surface waves (G1 − R1, G2 − R2, and G3). Our observation provides an alternative explanation of delayed triggering of microearthquakes at long-range distances, at least in the first few hours after large earthquakes.
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    The influence of large dams on surrounding climate and precipitation patterns (2011)
    Wiley
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    Publication Date: 2011-02-24
    Description: Understanding the forcings exerted by large dams on local climate is key to establishing if artificial reservoirs inadvertently modify precipitation patterns in impounded river basins. Using a 30 year record of reanalysis data, the spatial gradients of atmospheric variables related to precipitation formation are identified around the reservoir shoreline for 92 large dams of North America. Our study reports that large dams influence local climate most in Mediterranean, and semi-arid climates, while for humid climates the influence is least apparent. Clear spatial gradients of convective available potential energy, specific humidity and surface evaporation are also observed around the fringes between the reservoir shoreline and farther from these dams. Because of the increasing correlation observed between CAPE and extreme precipitation percentiles, our findings point to the possibility of storm intensification in impounded basins of the Mediterranean and arid climates of the United States.
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    Cropland for sub-Saharan Africa: A synergistic approach using five land cover data sets (2011)
    Wiley
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    Publication Date: 2011-02-23
    Description: This paper presents a methodology for the creation of a cropland map for Africa through the combination of five existing land cover products: GLC-2000, MODIS Land Cover, GlobCover, MODIS Crop Likelihood and AfriCover. A synergy map is created in which the products are ranked by experts, which reflects the likelihood or probability that a given pixel is cropland. The cropland map is then calibrated with national and sub-national crop statistics using a novel approach. Preliminary validation of the map was undertaken and the results are presented. The resulting cropland map has an accuracy of 83%, which is higher than the accuracy of any of the individual maps. The cropland map is freely available at agriculture.geo-wiki.org.
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    Bipolar seesaw in the northeastern tropical Atlantic during Heinrich stadials (2011)
    Wiley
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    Publication Date: 2011-02-23
    Description: Two SST records based on Mg/Ca of G. ruber (pink) from the continental slope off West Africa at 15°N and 12°N shed new light on the thermal bipolar seesaw pattern in the northeastern tropical Atlantic during periods of reduced Atlantic Meridional Overturning Circulation (AMOC) associated with Heinrich stadials H1 to H6. The two records indicate that the latitudinal position of the bipolar seesaw's zero-anomaly line, between cooling in the North and warming in the South, gradually shifted southward from H6 to H1. A conceptual model is presented that aims to provide a physically consistent mechanism for the southward migration of the seesaw's fulcrum. The conceptual model suggests latitudinal movements of the Intertropical Convergence Zone, driven by a combination of orbital-forced changes in the meridional temperature gradient within the realm of the Hadley cell and the expansion of the Northern Hemisphere cryosphere, as a major factor.
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    Stress drop variations of induced earthquakes at the Basel geothermal site (2011)
    Wiley
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    Publication Date: 2011-05-06
    Description: We determine stress drops from P-wave spectra of about 1000 earthquakes induced by hydraulic stimulation in crystalline rock for a deep heat mining project in Basel, Switzerland. We observe an increase in stress drop by about a factor of five with radial distance from 10 m to 300 m, which suggests that stress drop correlates with pore pressure perturbations due to the injection. We test this hypothesis by calculating the injection-related pore pressure perturbation based on a simple linear pore pressure diffusion model and find a good correlation of the expected pore pressure perturbation with the estimated stress drops.
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    Coseismic slip distribution of the February 27, 2010 Mw 8.8 Maule, Chile earthquake (2011)
    Wiley
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    Publication Date: 2011-05-06
    Description: Static offsets produced by the February 27, 2010 Mw = 8.8 Maule, Chile earthquake as measured by GPS and InSAR constrain coseismic slip along a section of the Andean megathrust of dimensions 650 km (in length) × 180 km (in width). GPS data have been collected from both campaign and continuous sites sampling both the near-field and far field. ALOS/PALSAR data from several ascending and descending tracks constrain the near-field crustal deformation. Inversions of the geodetic data for distributed slip on the megathrust reveal a pronounced slip maximum of order 15 m at ∼15–25 km depth on the megathrust offshore Lloca, indicating that seismic slip was greatest north of the epicenter of the bilaterally propagating rupture. A secondary slip maximum appears at depth ∼25 km on the megathrust just west of Concepción. Coseismic slip is negligible below 35 km depth. Estimates of the seismic moment based on different datasets and modeling approaches vary from 1.8 to 2.6 × 1022 N m. Our study is the first to model the static displacement field using a layered spherical Earth model, allowing us to incorporate both near-field and far-field static displacements in a consistent manner. The obtained seismic moment of 1.97 × 1022 N m, corresponding to a moment magnitude of 8.8, is similar to that obtained by previous seismic and geodetic inversions.
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    Rebound of Antarctic ozone (2011)
    Wiley
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    Publication Date: 2011-05-06
    Description: Restrictions on CFCs have led to a gradual decline of Equivalent Effective Stratospheric Chlorine (EESC). A rebound of Antarctic ozone, however, has remained elusive, masked by large interannual changes that dominate its current evolution. A positive response of ozone is not expected to emerge for at least 1–2 decades, possibly not for half a century. We show that interannual changes of the Antarctic ozone hole are accounted for almost perfectly by changes in dynamical forcing of the stratosphere. The close relationship enables dynamically-induced changes of ozone to be removed, unmasking the climate signal associated with CFCs. The component independent of dynamically-induced changes exhibits a clear upward trend over the last decade - the first signature of a rebound in Antarctic ozone. It enables ozone to be tracked relative to CFCs and other changes of climate.
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    Correction to “Increasing atmospheric poleward energy transport with global warming” (2011)
    Wiley
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    Publication Date: 2011-05-07
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    Does earlier snowmelt lead to greater CO2 sequestration in two low Arctic tundra ecosystems? (2011)
    Wiley
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    Publication Date: 2011-05-07
    Description: Some studies have reported that spring warming and earlier snowmelt leads to increased CO2 sequestration in Arctic terrestrial ecosystems. We measured tundra-atmosphere CO2 exchange via eddy covariance at two low Arctic sites (mixed upland tundra and sedge fen) in central Canada over multiple snow-free periods to assess this hypothesis. Both sites were net sinks for atmospheric CO2 in all years (2004–2010), but with high interannual variability. Despite a large range in snowmelt date (30 days), we did not find a statistically significant correlation between seasonal accumulated net ecosystem production (NEP) and snowmelt for either site. Although many factors can influence seasonal total NEP, our analysis shows that annual variations in photosynthetic capacity, likely driven by changes in leaf area, is a dominating control at these Arctic sites. At the upland tundra site, protection of overwintering buds by a longer duration of deep snow appears to be linked to greater photosynthetic capacity and NEP. Whereas at the fen site, sedge growth benefits from earlier snowmelt resulting in a strong correlation with early season NEP and an increase in total study period NEP with increasing growing degree days. These results highlight the complexity of interannual variation in ecosystem CO2 exchange in Arctic tundra and suggest that snowmelt date alone cannot predict seasonal, or annual, NEP.
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    Ice flux divergence anomalies on 79north Glacier, Greenland (2011)
    Wiley
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    Publication Date: 2011-05-07
    Description: The ice flux divergence of a glacier is an important quantity to examine because it determines the rate of temporal change of its thickness. Here, we combine high-resolution ice surface velocity observations of Nioghalvfjerdsfjorden (79north) Glacier, a major outlet glacier in north Greenland, with a dense grid of ice thickness data collected with an airborne radar sounder in 1998, to examine its ice flux divergence. We detect large variations, up to 100 m/yr, in flux divergence on grounded ice that are incompatible with what we know of the glacier surface mass balance, basal mass balance and thinning rate. We examine the hypothesis that these anomalies are due to the three-dimensional flow of ice around and atop bumps and hollows in basal topography by comparing the flux divergence of three-dimensional numerical models with its surface equivalent. We find that three-dimensional effects have only a small contribution to the observed anomalies. On the other hand, if we degrade the spatial resolution of the data to 10 km the anomalies disappear. Further analysis shows that the source of the anomalies is not the ice velocity data but the interpolation of multiple tracks of ice thickness data onto a regular grid using a scheme (here block kriging) that does not conserve mass or ice flux. This problem is not unique to 79north Glacier but is common to all conventional ice thickness surveys of glaciers and ice sheets; and fundamentally limits the application of ice thickness grids to high-resolution numerical modeling of glacier flow.
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    Beryllium-7 in soils and vegetation along an arid precipitation gradient in Owens Valley, California (2011)
    Wiley
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    Publication Date: 2011-05-07
    Description: Beryllium-7 is a potentially powerful tracer of atmospheric deposition and recent sediment transport, but the quantity and distribution of 7Be on arid landscapes have not been described. We measured 7Be in soil, vegetation, and dust in Owens Valley, California, and describe its distribution in aridisols and mollisols to evaluate its potential as a sediment tracer in desert environments. Beryllium-7 in vegetation and the upper few cm of soil is low but detectable (〉20 Becquerels [Bq] m−2). Surface inventories of 7Be at sites on the valley floor vary by a factor of five between the end of the rainy season (April) and the end of the dry season (November). In mollisols, live grasses hold ∼50 Bq 7Be m−2, which is on the order of half of the total springtime surface inventory. We find that within-site variability at the 5 m scale is 5 to 22% (1 relative standard error) and can be explained by localized rain shadowing and erosion, but between site variability at the km scale can be explained by differences in rainfall. Our alpine site has more than triple the inventory that is predicted from the rainfall-7Be flux relationship that we generate using our springtime soil measurements and previously reported deposition data. Dust deposition does not appear to contribute significantly to 7Be inventories, but anomalously high 7Be on the eastern flank of the Sierras may be explained by a higher altitude air mass source and better scavenging efficiency of snow.
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    Statistics between mainshocks and foreshocks in Italy and Southern California (2011)
    Wiley
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    Publication Date: 2011-05-10
    Description: The most used and accepted models for daily forecasts are based on short-term space and time earthquake clustering for occurrence rates and on the Gutenberg-Richter law for the frequency-magnitude. These models have been demonstrated to produce reliable prospective space-time-magnitude forecasts during an aftershock sequence, but their skill in forecasting mainshocks is still under discussion. This paper studies the foreshock statistics of the Italian and Californian seismicity in two ways: i) we compare the foreshock activity observed in real seismic catalogs and in synthetic catalogs derived from a pure Epidemic-Type Aftershock Sequence (ETAS) model; ii) we analyze the triggering capability of earthquakes using different ETAS parameterizations, in order to check whether large events are triggered in the same way as regular earthquakes. The results indicate that the foreshock activity observed in the real catalogs is compatible with what is expected by the ETAS model. Moreover, we find that the empirical foreshock rates have an intrinsic variability due to limited sampling that may explain most of the differences found so far in different seismic catalogs.
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    Migration properties of non-volcanic tremor in Shikoku, southwest Japan (2011)
    Wiley
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    Publication Date: 2011-05-10
    Description: Based on the precise locations of deep non-volcanic tremor in southwest Japan, which show a depth-dependent behavior, we clarified characteristic properties of tremor migration. Major tremor swarms are mainly initiated at deep levels of the tremor zone, corresponding to areas of frequent minor tremor, and migrate up-dip along the subduction interface. In central and eastern Shikoku, tremor occurs in two clusters, separated by a small gap, with the timing of tremor activity in the clusters differing by 1 or 2 days. The time evolution of tremor activity in western Shikoku reveals that the migration front of tremor propagates continuously and radially, even in the case of a tremor gap. These findings indicate that the rupture front of slow slip events propagates continuously and radially from the deep levels through the area without tremor activity. The occurrence of tremor may depend on the existence of inhomogeneous patches upon the plate interface.
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    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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    The 1992–2009 transport variability of the East Greenland-Irminger Current at 60°N (2011)
    Wiley
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    Publication Date: 2011-04-01
    Description: The East Greenland Irminger Current (EGIC) decadal transport variability likely influences deep convection intensity in the Labrador and Irminger Seas but is poorly known yet. The EGIC transport west of the 2000 m isobath was estimated, for the first time, between 1992 and 2009 by combining surface geostrophic velocities derived from altimetry with an estimate of the vertical structure of the transport variability statistically determined from a moored array deployed in 2004–2006. The reconstructed 17-year time series of the EGIC transport was then validated against independent estimates confirming that, indeed, the vertical distribution of the EGIC variability has not changed significantly over the last two decades. The 1992–2009 mean transport is 19.5 Sv with a standard error of 0.3 Sv (1 Sv = 106 m3 s−1). In 1992–1996, the EGIC transport was close to the average. Over the following decade (1997–2005), the EGIC transport declined by 3 Sv (15%) so that the 2004–2006 mean transport inferred from the moored array is 2.2 Sv (10%) less than the 1992–2009 mean. It was followed by a period of higher transport. The seasonal to interannual transport variability is coherent with the variability of the windstress curl at the center of the Irminger Sea.
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    Shortwave spectral radiative forcing of cumulus clouds from surface observations (2011)
    Wiley
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    Publication Date: 2011-04-02
    Description: The spectral changes of the shortwave total, direct and diffuse cloud radiative forcing (CRF) at surface are examined for the first time using spectrally resolved all-sky flux observations and clear-sky fluxes. The latter are computed applying a physically based approach, which accounts for the spectral changes of aerosol optical properties and surface albedo. Application of this approach to 13 summertime days with single-layer continental cumuli demonstrates: (i) the substantial contribution of the diffuse component to the total CRF, (ii) the well-defined spectral variations of total CRF in the visible spectral region, and (iii) the strong statistical relationship between spectral (500 nm) and shortwave broadband values of total CRF. Our results suggest that the framework based on the visible narrowband fluxes can provide important radiative quantities for rigorous evaluation of radiative transfer parameterizations and also can be applied for estimation of the shortwave broadband CRF.
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  • 95
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    CRRES observation and STEERB simulation of the 9 October 1990 electron radiation belt dropout event (2011)
    Wiley
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    Publication Date: 2011-03-31
    Description: We examine and simulate the electron radiation belt dropout event on 9 October 1990. CRRES observations show that significant depletions of electron fluxes occurred at energies ∼0.1–1.0 MeV beyond 6 RE and at energies 〉 ∼0.4 MeV within 6 RE. The three-dimensional kinetic radiation belt model STEERB is used to simulate this dropout event, taking into account the magnetopause shadowing, adiabatic transport, radial diffusion, and plume and chorus wave-particle interactions. Our results show that STEERB code can basically reproduce the observed depletion of ∼0.1–1.0 MeV electron fluxes throughout the outer radiation belt, suggesting that the competition and combination of all these physical mechanisms can well explain this electron radiation belt dropout event.
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  • 96
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    Pollution from China increases cloud droplet number, suppresses rain over the East China Sea (2011)
    Wiley
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    Publication Date: 2011-05-10
    Description: Rapid economic growth over the last 30 years in China has led to a significant increase in aerosol loading, which is mainly due to the increased emissions of its precursors such as SO2 and NOx. Here we show that these changes significantly affect wintertime clouds and precipitation over the East China Sea downwind of major emission sources. Satellite observations show an increase of cloud droplet number concentration from less than 200 cm−3 in the 1980s to more than 300 cm−3 in 2005. In the same time period, precipitation frequency reported by voluntary ship observers was reduced from more than 30% to less than 20% of the time. A back trajectory analysis showed the pollution in the investigation area to originate from the Shanghai-Nanjing and Jinan industrial areas. A model sensitivity study was performed, isolating the effects of changes in emissions of the aerosol precursors SO2 and NOx on clouds and precipitation using a state-of-the-art mesocale model including chemistry and aerosol indirect effects. Similar changes in cloud droplet number concentration over the East China Sea were obtained when the current industrial emissions in China were reduced to the 1980s levels. Simulated changes in precipitation were somewhat smaller than the observed changes but still significant.
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  • 97
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    Subterranean CO2 ventilation and its role in the net ecosystem carbon balance of a karstic shrubland (2011)
    Wiley
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    Publication Date: 2011-05-10
    Description: Recent studies of carbonate ecosystems suggest a possible contribution of subterranean ventilation to the net ecosystem carbon balance. However, both the overall importance of such CO2 exchange processes and their drivers remain unknown. Here we analyze several dry-season episodes of net CO2 emissions to the atmosphere, along with soil and borehole CO2 measurements. Results highlight important events where rapid decreases of underground CO2 molar fractions correlate well with sizeable CO2 release to the atmosphere. Such events, with high friction velocities, are attributed to ventilation processes, and should be accounted for by predictive models of surface CO2 exchange.
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  • 98
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    Coastal sustainability depends on how economic and coastline responses to climate change affect each other (2011)
    Wiley
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    Publication Date: 2011-04-05
    Description: Human-induced climate change is predicted to accelerate sea level rise and alter storm frequency along the US east coast. Rising sea level will enhance shoreline erosion, and recent work indicates changing storm patterns and associated changes in wave conditions can intensify coastal erosion along parts of a coastline. Investigations of coastal response to climate change typically consider natural processes in isolation — neglecting repeated changes to the coastline from human actions, primarily through shoreline nourishment projects, which add sand to the shoreline to counteract erosion. In a model coupling economically driven shoreline nourishment with wave- and sea level rise-driven coastline change, and accounting for dwindling sediment resources for nourishment, coastline response depends dramatically on the relationship between patterns of property value and erosion. Simulations show that when nourishment costs rise with depletion of sand resources, coastline change is tied to the interaction between patterns of erosion and property value. Simulations show that when high property values align with highly erosive locations, sand resources are depleted rapidly and nourishment in lower property value towns is quickly abandoned. Although our model simulates a particular coastal morphology, the result that future behavior of the coastline and the economic viability of nourishment in a given town depend on the regional interaction between patterns of property value and erosion is likely applicable to many coastal configurations. More broadly, coupling economic and physical models reveals equity and sustainability implications of coastal climate adaptation as well as patterns of coastline change that a physical model alone would overlook.
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  • 99
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    Secondary eyewall formation in WRF simulations of Hurricanes Rita and Katrina (2005) (2011)
    Wiley
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    Publication Date: 2011-04-07
    Description: An analysis is presented of two high-resolution hurricane simulations of Katrina and Rita (2005) that exhibited secondary eyewall formation (SEF). The results support the notion of vortex Rossby waves (VRWs) having an important role in SEF and suggest that VRW activity is a defining aspect of the moat. SEF occurs at a radius of ∼65 (80) km in Katrina (Rita), close to the hypothesized stagnation radius of VRWs. VRW activity appears to be the result of eye-eyewall mixing events, themselves a product of the release of barotropic instability. The convection in the radial region that becomes the moat is mainly in the form of VRWs propagating radially outward from the primary eyewall until the negative radial gradient of potential vorticity is no longer conducive for their propagation. These convectively coupled waves, originating and being expelled from the eyewall, are rotation dominated and have the coherency necessary to survive their passage through the strain-dominated region outside the eyewall.
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    Evidence for the chaotic origin of Northern Annular Mode variability (2011)
    Wiley
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    Publication Date: 2011-08-03
    Description: Exponential spectra are found to characterize variability of the Northern Annular Mode (NAM) for periods less than 36 days. This corresponds to the observed rounding of the autocorrelation function at lags of a few days. The characteristic persistence timescales during winter and summer is found to be ∼5 days for these high frequencies. Beyond periods of 36 days the characteristic decorrelation timescale is ∼20 days during winter and ∼6 days in summer. We conclude that the NAM cannot be described by autoregressive models for high frequencies; the spectra are more consistent with low-order chaos. We also propose that the NAM exhibits regime behaviour, however the nature of this has yet to be identified.
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