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  • Articles  (1,141)
  • Other Sources
  • Wiley  (1,141)
  • AGU (American Geophysical Union)
  • 2010-2014  (1,141)
  • 1980-1984
  • 2011  (1,141)
  • Geophysical Research Letters  (1,141)
  • 4905
  • 101
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    The transient response of the Southern Ocean pycnocline to changing atmospheric winds (2011)
    Wiley
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    Publication Date: 2011-08-04
    Description: The vertical density structure of the Southern Ocean is dynamically linked to wind stress at the surface, but the nature of this coupling is not fully understood. Observations from the last several decades show a significant increase in the strength of westerly winds over the Southern Ocean, but an appreciable change in the tilt of constant density surfaces (isopycnals) has not yet been detected there. Using a combination of theory and idealized numerical simulations, we show that the response of the density structure occurs on centennial timescales, making it difficult to detect significant changes with a few decades of hydrographic observations. Dynamic coupling between the circumpolar current and northern basins regulates the slow adjustment of the density structure. Our results provide a new interpretation for recent observations and highlight the importance of the interaction between regional Southern Ocean dynamics and global ocean circulation.
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  • 102
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    Non-volcanic tremor and discontinuous slab dehydration (2011)
    Wiley
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    Publication Date: 2011-08-04
    Description: Non-volcanic tremor is a recently discovered fault slip style occurring with remarkable regularity in space near the down-dip end of the locked zone on several subduction thrust interfaces. The physical mechanisms and the controls on the location of tremor have not yet been determined. We calculate the stable mineral assemblages and their water content in the subducting slab, and find that slab dehydration is not continuous, but rather restricted to a few reactions localised in pressure-temperature space. Along geothermal gradients applicable to Shikoku and Cascadia - where tremor has been relatively easy to detect - tremor locations correlate with discontinuous and localised voluminous water release from the breakdown of lawsonite and chlorite + glaucophane respectively. The shape of the pressure-temperature path for subducting slabs prevents fluid release at depths above and below where these dehydration reactions occur. We conclude that abundant tremor activity requires metamorphic conditions where localised dehydration occurs during subduction, and this may explain why tremor appears more abundant in some subduction zones than others.
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  • 103
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    A shallow strong patch model for the 2011 great Tohoku-oki earthquake: A numerical simulation (2011)
    Wiley
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    Publication Date: 2011-08-09
    Description: A numerical simulation is conducted for understanding the mechanics of the 2011 great Tohoku-oki earthquake (Mw = 9.0), which widely broke the plate interface at the Pacific plate subducting beneath northern Honshu (Tohoku), Japan. In the model, frictional stress on the plate interface is assumed to obey a rate- and state-dependent friction law. A strong patch (asperity) with higher effective normal stress and a large value of characteristic slip distance is assumed at a shallower part of the plate interface. This strong patch controls the occurrence of great earthquakes that broke the entire seismogenic plate interface with recurrence intervals of several hundred years. The present model explains large coseismic slip at a shallower part of the 2011 great earthquake and accumulation of slip deficit at deeper parts, where smaller M7 class earthquakes repeatedly occurred before the great earthquake.
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  • 104
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    The impact of ENSO and the Arctic Oscillation on winter temperature extremes in the southeast United States (2011)
    Wiley
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    Publication Date: 2011-08-11
    Description: Interannual variations of the winter mean temperature and the number of days of warm and cold extremes were investigated for the southeast United States to identify the relative influence of El Niño-Southern Oscillation (ENSO) and the Arctic Oscillation (AO). Generalized extreme value theory was used to estimate the probability distribution function (PDF) of warm and cold extremes and their return values for different phases of ENSO and the AO. An analysis of the temperature observations for the past 58 years (1951–2008) reveals that both the winter mean temperature anomalies and the number of days of extreme cold are most closely linked to variations in the AO especially in the recent past (1981–2008). In contrast, the number of days of extreme warmth are linked to both ENSO and the AO.
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  • 105
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    Frontal wedge deformation near the source region of the 2011 Tohoku-Oki earthquake (2011)
    Wiley
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    Publication Date: 2011-08-12
    Description: We report an uplift of 5 m with a horizontal displacement of more than 60 m due to the 2011 Tohoku-Oki earthquake. The uplift was measured by an ocean-bottom pressure gauge installed before the earthquake on a frontal wedge, which formed an uplift system near the Japan Trench. Horizontal displacements of the frontal wedge were measured using local benchmark displacements obtained by acoustic ranging before and after the earthquake. The average displacements at the frontal wedge were 58 m east and 74 m east-southeast. These results strongly suggest a huge coseismic slip beneath the frontal wedge on the plate boundary. The estimated magnitude of the slip along the main fault was 80 m near the trench. Our results suggest that the horizontal and vertical deformations of the frontal wedge due to the slip generated the tremendous tsunami that struck the coastal area of northeastern Japan.
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  • 106
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    Sulfide melts and long-term low seismic wavespeeds in lithospheric and asthenospheric mantle (2011)
    Wiley
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    Publication Date: 2011-06-01
    Description: Some studies of lithospheric and asthenospheric seismic structure, report mantle velocities as low as ∼4% below the reference models used. While these low wavespeeds may be attributed to thermal effects in tectonically young or actively volcanic regions, in older, tectonically stable regions low velocity anomalies apparently persist even past the decay time of any thermal perturbation, rendering such a mechanism implausible. Low volume melts can also reduce wavespeeds, but their buoyancy should drain them upward away from source regions, preventing significant accumulation if they are able to segregate. Sulfide, ubiquitous as inclusions in lithospheric mantle xenoliths, forms dense, non-segregating melts at temperatures and volatile fugacities characteristic of even old lithospheric mantle. We show that 1–5 volume percent sulfide melts can act to permanently create reductions up to 5.5% in seismic wavespeeds in areas of the lithosphere and the asthenosphere disturbed by prior melting events that carry and concentrate sulfide.
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  • 107
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    Verification of medium-range MJO forecasts with TIGGE (2011)
    Wiley
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    Publication Date: 2011-06-01
    Description: The Madden–Julian oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics. Accurate simulations of the MJO are important for studies of weather and climate in the tropics and extratropics. This study assesses the forecast performance of operational medium-range ensemble forecasts, available at THe Observing system Research and Predictability EXperiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) data portal, regarding the MJO for the past 3 years. The results indicate that ECMWF (European Centre for Medium-range Weather Forecasts) and UKMO (United Kingdom Meteorological Office) generally yield the best performances in predicting the MJO; however, they do not always show similar skills. ECMWF performs well in simulating the maintenance and onset of the MJO in phases 1–4, whereas UKMO and NCEP (National Centers for Environmental Prediction) perform well in simulating the maintenance and onset of the MJO in phases 5–8. Thus, the best-performing numerical weather prediction (NWP) centre varies with the phase of the MJO. With advance knowledge of the forecast characteristics of each NWP centre, we can ensure more reliable forecasts of the MJO in operational uses, based on the MJO phase. This represents an advantage of the multi-centre grand ensemble approach.
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  • 108
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    Laboratory realization of an ion-ion hybrid Alfvén wave resonator (2011)
    Wiley
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    Publication Date: 2011-06-01
    Description: In a magnetized plasma with two ion species, shear Alfvén waves (or guided electromagnetic ion cyclotron waves) have zero parallel group velocity and experience a cut-off near the ion-ion hybrid frequency. Since the ion-ion hybrid frequency is proportional to the magnetic field, it is possible, in principle, for a magnetic well configuration to behave as an Alfvén wave resonator in a two-ion plasma. This study demonstrates such a resonator in a controlled laboratory experiment using a H+-He+ mixture. The resonator response is investigated by launching monochromatic waves and sharp tone-bursts from a magnetic loop antenna. The observed frequency spectra are found to agree with predictions of a theoretical model of trapped eigenmodes.
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  • 109
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    East-West Coast differences in total electron content over the continental US (2011)
    Wiley
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    Publication Date: 2011-10-07
    Description: Total electron content (TEC) measurements made by a network of dense GPS receivers over the continental US are used to investigate ionospheric longitudinal differences. We find that the evening TEC is substantially higher on the US east coast than on the west, and vice versa for the morning TEC; the longitudinal difference displays a clear diurnal variation. Through an analysis of morning-evening variability in the east-west TEC difference, minimum variability is found to coincide with the longitudes of zero magnetic declination over the continental US. We suggest that these new findings of longitudinal differences in ionospheric TEC at midlatitudes are caused by the longitudinal difference in magnetic declination combined with the effects of thermospheric zonal winds which are subject to directional reversal over the course of a day. This study indicates that longitudinal variations in TEC measurements contain critical information on thermospheric zonal winds. The proposed declination-zonal wind mechanism may also provide a new insight into longitude/UT changes at midlatitudes on a global scale, as well as into some geospace disturbances.
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  • 110
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    Biological implications of high-energy cosmic ray induced muon flux in the extragalactic shock model (2011)
    Wiley
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    Publication Date: 2011-10-07
    Description: A ∼62 My periodicity in fossil biodiversity has been observed in independent studies of paleontology databases over ∼0.5Gy. The period and phase of this biodiversity cycle coincides with the oscillation of our solar system normal to the galactic disk with an amplitude ∼70 parsecs and a period ∼64 My. Our Galaxy is falling toward the Virgo cluster, forming a galactic shock at the north end of our galaxy due to this motion, capable of accelerating particles and exposing our galaxy's northern side to a higher flux of cosmic rays. These high-energy particles strike the Earth's atmosphere initiating extensive air showers, ionizing the atmosphere by producing charged secondary particles. Secondary particles such as muons produced as a result of nuclear interactions are able to reach the ground and enhance the biological radiation dose. Using a Monte Carlo simulation package CORSIKA, we compute the biological dose resulting from enhanced muon exposure from cosmic rays and discuss their implications for terrestrial biodiversity variations.
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  • 111
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    Vegetation dynamics contributes to the multi-decadal variability of precipitation in the Amazon region (2011)
    Wiley
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    Publication Date: 2011-10-07
    Description: Precipitation in most of the Amazon shows multi-decadal fluctuations that were linked to oceanic forcing in the Atlantic. This modeling study shows that vegetation dynamics may play a major role in such low-frequency variability in the Amazon. Despite the large amount of annual precipitation, the presence of a dry season (albeit short) facilitates a strong impact of dynamic vegetation on precipitation persistence in the model. The year-to-year variation of net primary productivity (NPP) is dominated by that of the dry season NPP. As a result, above-normal (below-normal) precipitation in a particular year can enhance (suppress) vegetation growth, leading to widespread increase (decrease) of vegetation density in the subsequent year. Precipitation in the subsequent year is therefore more likely to be above (below) normal. This damping effect of vegetation enhances low-frequency variability of precipitation and leads to recurrent droughts or floods, a result previously considered characteristic of arid and semi-arid regions.
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  • 112
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    The role of storm-relative advection of absolute angular momentum in strengthening of Atlantic tropical cyclones (2011)
    Wiley
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    Publication Date: 2011-10-07
    Description: This study examined absolute angular momentum tendency in Atlantic tropical cyclones. Each advective and torque term in the storm-relative Eulerian absolute angular momentum tendency equation was calculated in a storm relative reference frame using modeled and observational data. 18 storms between 2004 and 2006 were simulated using the hurricane weather research and forecast model. In addition, the storm-relative advection of absolute angular momentum was calculated using reconnaissance aircraft wind data from 12 Atlantic hurricanes which occurred between 2003 and 2007. Through methods of statistical correlation, categorical composition and linear regression, it was found that mid-level horizontal advection of relative angular momentum was most relevant to 12 hour strength change in the modeled tropical cyclones. Mid-level horizontal advection of relative angular momentum was most relevant to intensity change in the observed hurricanes, while mid-level advection of Earth's angular momentum was found to be most closely related to strength change in observed hurricanes.
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  • 113
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    Impact of the Atlantic warm pool on United States landfalling hurricanes (2011)
    Wiley
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    Publication Date: 2011-10-07
    Description: The 2010 Atlantic hurricane season was extremely active, but no hurricanes made landfall in the United States, raising a question of what dictated the hurricane track. Here we use observations from 1970–2010 (also extending back to 1950) and numerical model experiments to show that the Atlantic warm pool (AWP) – a large body of warm water comprised of the Gulf of Mexico, the Caribbean Sea and the western tropical North Atlantic – plays an important role in the hurricane track. An eastward expansion of the AWP shifts the hurricane genesis location eastward, decreasing the possibility for a hurricane to make landfall. A large AWP also induces barotropic stationary wave patterns that weaken the North Atlantic subtropical high and produce the eastward steering flow anomalies along the eastern seaboard of the United States. Due to these two mechanisms, hurricanes are steered toward the northeast without making landfall in the United States. Although the La Niña event in the Pacific may be associated with the increased number of Atlantic hurricanes, its relationship with landfalling activity has been offset in 2010 by the effect of the extremely large AWP.
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  • 114
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    How does local tropical deforestation affect rainfall? (2011)
    Wiley
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    Publication Date: 2011-10-07
    Description: The aim of this study is to investigate the potential impacts of vegetation-breezes on locally-generated rainfall and its distribution on the mesoscale. Ensembles of simulations with a 2D large-eddy model were performed using various heterogeneous land surfaces. Rainfall was found to be 4–6 times higher over warmer surface anomalies, associated with cropland, compared to a homogeneous surface, but rainfall was reduced to half or less over the forest. While the suppression of rainfall tended to occur throughout the forest with an intensity comparable to the surface anomaly, the exact location of the maximum in rainfall was less predictable. The location of peak rainfall depended on an interplay between the size of the heat flux gradient (governing the vegetation-breeze strength), the size of the anomaly (as vegetation-breezes organize in certain preferential length-scales), and the distance to other anomalies (since convection in one location could suppress it elsewhere). The presence of surface heterogeneity also increased the total rainfall in the domain by 13% on average, with higher increases in the presence of more intense surface variabilities.
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  • 115
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    Solar wind access to lunar polar craters: Feedback between surface charging and plasma expansion (2011)
    Wiley
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    Publication Date: 2011-10-05
    Description: Determining the plasma environment within permanently shadowed lunar craters is critical to understanding local processes such as surface charging, electrostatic dust transport, volatile sequestration, and space weathering. In order to investigate the nature of this plasma environment, the first two-dimensional kinetic simulations of solar wind expansion into a lunar crater with a self-consistent plasma-surface interaction have been undertaken. The present results reveal how the plasma expansion into a crater couples with the electrically-charged lunar surface to produce a quasi-steady wake structure. In particular, there is a negative feedback between surface charging and ambipolar wake potential that allows an equilibrium to be achieved, with secondary electron emission strongly moderating the process. A range of secondary electron yields is explored, and two distinct limits are highlighted in which either surface charging or ambipolar expansion is responsible for determining the overall wake structure.
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  • 116
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    Refugium for surface life on Snowball Earth in a nearly-enclosed sea? A first simple model for sea-glacier invasion (2011)
    Wiley
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    Publication Date: 2011-10-08
    Description: The existence of photosynthetic eukaryotic algae during the so-called Snowball Earth events presents a conundrum. If thick ice covered the oceans, where could such life persist? Here we explore the possibility that photosynthetic life persisted at the end of long narrow seas, analogous to the modern-day Red Sea. In this first analytical model, we test the ability of the global sea glacier to penetrate a Red Sea analogue under climatic conditions appropriate during a Snowball Earth event. We find the Red Sea is long enough to provide a refugium only if certain ranges of climatic conditions are met. These ranges would likely expand if the restrictive effect of a narrow entrance strait is also considered.
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  • 117
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    The role of ocean acidification in systemic carbonate mineral suppression in the Bering Sea (2011)
    Wiley
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    Publication Date: 2011-10-06
    Description: Ocean acidification driven by absorption of anthropogenic carbon dioxide (CO2) from the atmosphere is now recognized as a systemic, global process that could threaten diverse marine ecosystems and a number of commercially important species. The change in calcium carbonate (CaCO3) mineral saturation states (Ω) brought on by the reduction of seawater pH is most pronounced in high latitude regions where unique biogeochemical processes create an environment more susceptible to the suppression of Ω values for aragonite and calcite, which are critical to shell building organisms. New observations from the eastern Bering Sea shelf show that remineralization of organic matter exported from surface waters rapidly increases bottom water CO2 concentrations over the shelf in summer and fall, suppressing Ω values. The removal of CO2 from surface waters by high rates of phytoplankton primary production increases Ω values between spring and summer, but these increases are partly counteracted by mixing with sea ice melt water and terrestrial river runoff that have low Ω values. While these environmental processes play an important role in creating seasonally low saturation states, ocean uptake of anthropogenic CO2 has shifted Ω values for aragonite to below the saturation horizon in broad regions across the shelf for at least several months each year. Furthermore, we also report that calcite became undersaturated in September of 2009 in the bottom waters over the shelf. The reduction in CaCO3 mineral saturation states could have profound implications for several keystone calcifying species in the Bering Sea, particularly the commercially important crab fisheries.
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  • 118
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    Determination of the most pertinent EUV proxy for use in thermosphere modeling (2011)
    Wiley
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    Publication Date: 2011-10-07
    Description: Two major issues in the specification of the thermospheric density are the definition of proper solar inputs and the empirical modeling of thermosphere response to solar and to geomagnetic forcings. This specification is crucial for the tracking of low Earth orbiting satellites. Here we address both issues by using 14 years of daily density measurements made by the Stella satellite at 813 km altitude and by carrying out a multiscale statistical analysis of various solar inputs. First, we find that the spectrally integrated solar emission between 26–34 nm offers the best overall performance in the density reconstruction. Second, we introduce linear parametric transfer function models to describe the dynamic response of the density to the solar and geomagnetic forcings. These transfer function models lead to a major error reduction and in addition open new perspectives in the physical interpretation of the thermospheric dynamics.
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  • 119
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    How important is biogenic isoprene in an urban environment? A study in London and Paris (2011)
    Wiley
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    Publication Date: 2011-10-08
    Description: As the emissions of anthropogenic volatile organic compounds (VOCs) are reduced through regulatory measures and improved control technologies, biogenic VOCs could gain in importance in terms of reactivity, especially in urban areas. Here we investigate a 12 year record of non-methane hydrocarbons (NMHCs) in central London and the importance of biogenics (in the form of isoprene) for ozone formation through the contribution to OH reactivity. Significant reductions in NMHCs were observed from 1998 through 2009 at an urban traffic site (−13% per year) and suburban background site (−5% per year) in London. Total isoprene levels decreased similarly and the relative contribution of isoprene to the total NMHC OH reactivity did not change. Furthermore, a dataset for Paris showed strong similarities to the data from London, which would indicate that these results are not limited to London. Interestingly, a rural site to the east of London, Harwell, showed similar contributions of isoprene to the total NMHC reactivity, which may indicate the need for measurements of other biogenic species, such as monoterpenes, in some areas to reliably capture the importance of biogenics in the region. These results would indicate that the influence of biogenic isoprene in London, and likely other low isoprene emitter urban areas have a long way to go before the importance of biogenic VOCs equals or exceeds that of anthropogenic contributions.
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  • 120
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    Correction to “Energetics of a global ocean circulation model compared to observations” (2011)
    Wiley
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    Publication Date: 2011-10-08
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  • 121
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    Proper estimation of marine gross O2 production with 17O/16O and 18O/16O ratios of dissolved O2 (2011)
    Wiley
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    Publication Date: 2011-10-12
    Description: Recently, the method for estimation of marine gross production from measurements of 17O/16O and 18O/16O of dissolved O2 has been improved by obtaining a rigorous equation for evaluation of gross production, and it has been suggested that small errors associated with approximations resulted in significant underestimation of gross production rates. The new equation requires accurate knowledge of the isotopic composition of photosynthetic and atmospheric oxygen. While the values for the latter have been determined very precisely, those of the former have never been measured. Here we present the first experimentally derived values of δ17Op and δ18Op of marine photosynthetic O2 (−10.126 and −20.014 ‰). Based on these, we show that the suggested underestimation was not the result of approximation errors, rather it was caused by inaccurate estimated values of δ17Op and δ18Op.
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  • 122
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    Cloud variations and the Earth's energy budget (2011)
    Wiley
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    Publication Date: 2011-10-01
    Description: The question of whether clouds are the cause of surface temperature changes, rather than acting as a feedback in response to those temperature changes, is explored using data obtained between 2000 and 2010. An energy budget calculation shows that the radiative impact of clouds accounts for little of the observed climate variations. It is also shown that observations of the lagged response of top-of-atmosphere (TOA) energy fluxes to surface temperature variations are not evidence that clouds are causing climate change.
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  • 123
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    High strain rate damage of Carrara marble (2011)
    Wiley
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    Publication Date: 2011-10-08
    Description: Several cases of rock pulverization have been observed along major active faults in granite and other crystalline rocks. They have been interpreted as due to coseismic pervasive microfracturing. In contrast, little is known about pulverization in carbonates. With the aim of understanding carbonate pulverization, we investigate the high strain rate (c. 100 s−1) behavior of unconfined Carrara marble through a set of experiments with a Split Hopkinson Pressure Bar. Three final states were observed: (1) at low strain, the sample is kept intact, without apparent macrofractures; (2) failure is localized along a few fractures once stress is larger than 100 MPa, corresponding to a strain of 0.65%; (3) above 1.3% strain, the sample is pulverized. Contrary to granite, the transition to pulverization is controlled by strain rather than strain rate. Yet, at low strain rate, a sample from the same marble displayed only a few fractures. This suggests that the experiments were done above the strain rate transition to pulverization. Marble seems easier to pulverize than granite. This creates a paradox: finely pulverized rocks should be prevalent along any high strain zone near faults through carbonates, but this is not what is observed. A few alternatives are proposed to solve this paradox.
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  • 124
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    Daily and intraseasonal relationships between lightning and NO2 over the Maritime Continent (2011)
    Wiley
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    Publication Date: 2011-10-08
    Description: The relationship between lightning and NO2 over Indonesia is examined on daily and intraseasonal time scales based on lightning observations from the World Wide Lightning Location Network (WWLLN) and tropospheric NO2 column densities from the Global Ozone Monitoring Experiment (GOME-2) satellite mission. Composites of the daily NO2 observations regressed onto lightning frequency reveal a plume of enhanced NO2 following a day of enhanced lightning. Lightning and NO2 also vary coherently with the intraseasonal Madden-Julian Oscillation (MJO) in a manner distinct from the cloudiness signature, with variations of up to ∼50% of the annual mean.
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    Time corrections to teleseismic P delays derived from SKS splitting parameters and implications for western U.S. P-wave tomography (2011)
    Wiley
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    Publication Date: 2011-10-11
    Description: Upper mantle anisotropy will affect teleseismic P-wave arrival times and create artifacts in isotropic travel-time tomography. Compiled SKS splitting results indicate variations in the strength and orientation of azimuthal anisotropy beneath the western U.S. We use SKS splitting parameters and a hexagonally anisotropic elastic tensor (fast symmetry axis) to estimate azimuthal anisotropy contributions to P-wave delay times and evaluate the effects on isotropic P-wave tomography. Estimated anisotropy correction times have a root-mean-square (RMS) value of 0.16 s, which is 37% of P-wave delay time RMS. The magnitude of azimuthal anisotropy, rather than its azimuth, has the strongest effect on P delay times; however, if the anisotropy symmetry axis has a plunge, P-wave travel times may be strongly affected by both magnitude and azimuth. Applying azimuthal anisotropy corrections significantly increases tomographic P-wave velocity beneath the High Lava Plains and central California coast, and decreases velocity beneath the Great Basin.
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    Lithospheric control on the spatial pattern of Azores hotspot seafloor anomalies: Constraints from a model of plume-triple junction interaction (2011)
    Wiley
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    Publication Date: 2011-10-11
    Description: The Azores hotspot is located near a plate boundary triple junction (TJ) consisting of the Terceira Rift (TER) and two branches of the Mid-Atlantic Ridge (MAR). The seafloor expression of the Azores hotspot has a complex spatial pattern. Latitudinal anomalies in seafloor depth and other data along the MAR extend farther to the south of the inferred location of the mantle heterogeneity than to the north. Longitudinal anomalies span a greater distance to the east of the MAR (along the TER) than to the west. A finite element model is used to investigate how the divergence of three plates away from a TJ may affect the spatial dispersion of thermally buoyant material simulating a mantle plume. Prescribed plate motion vectors approximate the kinematics of the Azores TJ during a main phase of plateau formation ∼7 Ma. The plume is located off axis to the southeast of the simulated triple junction, following several studies that suggest that the present-day conduit is located near the islands of Faial and Pico. Asymmetry in the divergence of the three plates with respect to the triple junction tends to drive plume material preferentially southward and eastward, consistent with observed anomalies.
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    The climate influence on the mid-depth Northeast Atlantic gyres viewed by cold-water corals (2011)
    Wiley
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    Publication Date: 2011-10-11
    Description: The neodymium (Nd) isotopic composition (expressed in epsilon units, $\varepsilon$Nd) of reef framework-forming cold-water corals provides unique measures of water mass provenance and mixing within the Northeast Atlantic today and in the past. A reconstruction of near thermocline water $\varepsilon$Nd from cold-water corals of the Gulf of Cádiz and Porcupine Seabight spanning over the past 300,000 years, now revealed that climate cooling during Marine Isotope Stages (MIS) 7.2 and MIS 8/9 led to a retraction of the mid-depth Subpolar Gyre (mSPG) to the west. Conversely, Northern Hemisphere warming and increasing fresh water fluxes to the northwest (Labrador Sea) favor a stronger eastward extension of the mSPG blocking the northward flow of temperate Atlantic water as observed during the early MIS 1 and the early stage MIS 5.5. These changes are likely the result of large-scale south-north displacement of the westerlies similar to present-day observations that the North Atlantic Oscillation (NAO) is linked with mid-depth ocean circulation. Based on these observations, we hypothesize that further climate warming will also strengthen the mSPG leading to a salt and temperature decrease in the Northeast Atlantic whereas salinity and temperature will increase in the temperate Atlantic. However, the amplitude of such changes on North Atlantic overturning remains to be tested.
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    Plate tectonics may control geomagnetic reversal frequency (2011)
    Wiley
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    Publication Date: 2011-10-11
    Description: The discovery of the reversals of Earth's magnetic field and the description of plate tectonics are two of the main breakthroughs in geophysics in the 20th century. We claim that these two phenomena are correlated and that plate tectonics controls long-term changes in geomagnetic reversal frequency. More precisely, geological intervals characterized by an asymmetrical distribution of the continents with respect to the equator are followed by intervals of high reversal frequency. We speculate that the distribution and symmetry of mantle structures driving continental motions at the surface influence the equatorial symmetry of the flow within the core and thus change the coupling between the dipolar and quadrupolar modes which controls the occurrence of reversals.
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    Magnetic susceptibility as a proxy for investigating microbially mediated iron reduction (2011)
    Wiley
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    Publication Date: 2011-11-09
    Description: We investigated magnetic susceptibility (MS) variations in hydrocarbon contaminated sediments. Our objective was to determine if MS can be used as an intrinsic bioremediation indicator due to the activity of iron-reducing bacteria. A contaminated and an uncontaminated core were retrieved from a site contaminated with crude oil near Bemidji, Minnesota and subsampled for MS measurements. The contaminated core revealed enriched MS zones within the hydrocarbon smear zone, which is related to iron-reduction coupled to oxidation of hydrocarbon compounds and the vadose zone, which is coincident with a zone of methane depletion suggesting aerobic or anaerobic oxidation of methane is coupled to iron-reduction. The latter has significant implications for methane cycling. We conclude that MS can serve as a proxy for intrinsic bioremediation due to the activity of iron-reducing bacteria iron-reducing bacteria and for the application of geophysics to iron cycling studies.
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    The three-dimensional distribution of clouds around Southern Hemisphere extratropical cyclones (2011)
    Wiley
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    Publication Date: 2011-11-09
    Description: The organization and, for the first time, the three-dimensional structure of clouds associated with the Southern Hemisphere cyclones are studied using active observations from the CloudSat and CALIPSO satellites. First, a composite cyclone is constructed from more than 800 individual cases in the years 2007 and 2008 using the cyclone centre as the composite reference point. It is shown that the three-dimensional cloud distribution around the composite cyclone agrees well with conceptual models of extratropical cyclones. Composite mean fields of sea level pressure, vertical motion, potential temperature and relative humidity are superposed on the three-dimensional cloud structure to better define the relationship between the clouds and dynamical properties of extratropical cyclones. The methodology used here reveals the relationship between dynamical and cloud processes in three dimensions around cyclones and provides the foundation for in-depth evaluations of the ability of climate models to simulate the cloud and dynamical structures of Southern Hemisphere extratropical cyclones.
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    Co-existence of temperature and amount effects on precipitation δ18O in the Asian monsoon region (2011)
    Wiley
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    Publication Date: 2011-11-09
    Description: Whether stable oxygen isotope (δ18O) in precipitation obeys the temperature effect and/or amount effect in the monsoon region has long been controversial. An intensive, precipitation event-based sampling project has been carried out at Guangzhou and Changsha stations in southeast China under the Asian monsoon influence. By dividing a year into summer and winter half years at respective station, we find prevalence of amount effect at both stations throughout the year. δ18O-temperature presents complex correlations, with the positive correlation significant at Guangzhou during the summer half year and at Changsha during the winter half year, but vague at either station during the rest of the year; the former attributable to a third mode of convection, while the latter indicative of the weakening monsoon influence accompanied by intensified local recycling. Our high-resolution data demonstrate a significant coexistence of temperature and amount effects of precipitation δ18O in the monsoon domain, conducive to climatic interpretation of δ18O in paleoclimate proxies in mid/low latitudes.
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    3D modeling of the cycle of a great Tohoku-oki earthquake, considering frictional behavior at low to high slip velocities (2011)
    Wiley
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    Publication Date: 2011-11-09
    Description: We perform 3D quasi-dynamic modeling of the cycle of a megathrust earthquake in the offshore Tohoku region, Japan, using a rate- and state-dependent friction law with two state variables that exhibits strong velocity weakening at high slip velocities. We set several asperities where velocity weakening occurs at low to intermediate slip velocities. Outside of the asperities, velocity strengthening occurs at low to intermediate slip velocities. At high slip velocities, strong velocity weakening with large displacements occurs both within and outside of the asperities. The rupture of asperities occurs at intervals of several tens of years, whereas megathrust events occur at much longer intervals (several hundred years). Megathrust slips occur even in regions where velocity strengthening occurs at low to intermediate slip velocities, but where velocity weakening is dominant at high slip velocities. The proposed model explains that megathrust earthquakes occur in the same subduction zone as large thrust earthquakes.
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    Estimating the effects of ENSO upon the observed freshening trends of the western tropical Pacific Ocean (2011)
    Wiley
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    Publication Date: 2011-11-09
    Description: A significant surface freshening trend and an eastward expansion of fresh surface waters have been documented in the western tropical Pacific, consistent with the expected effects of climate change. The highest El Niño Southern Oscillation (ENSO) variability in Sea Surface Salinity (SSS) has been also documented in that region, with different quantitative signatures for the Eastern and Central Pacific ENSO events (EP and CP ENSO, respectively). This study hence analyses to what extent have the EP and CP ENSO events contributed to the long-term freshening trends, relying on 1955–2008 in situ SSS data and on EP and CP ENSO main features. We show the influence of EP ENSO events to be negligible, while CP El Niño events contribute to enhance the long-term freshening trend (up to 30%) in the far western equatorial Pacific and moderately reduce that freshening (up to 10%) in the South Pacific Convergence Zone (SPCZ). Our results thus suggest that the observed eastward expansion of the surface covered by low-salinity waters in the western half of the tropical Pacific is mostly due to climate change rather than to the documented possible increased occurrence and intensity of CP El Niño events. The sensitivity of the trend estimates to the different periodicity of the SSS data records is also discussed.
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    Amazon vegetation greenness as measured by satellite sensors over the last decade (2011)
    Wiley
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    Publication Date: 2011-10-12
    Description: During the last decade two major drought events, one in 2005 and another in 2010, occurred in the Amazon basin. Several studies have claimed the ability to detect the effect of these droughts on Amazon vegetation response, measured through satellite sensor vegetation indices (VIs). Such monitoring capability is important as it potentially links climate changes (increasing frequency and severity of drought), vegetation response as observed through vegetation greenness, and land-atmosphere carbon fluxes which directly feedback into global climate change. However, we show conclusively that it is not possible to detect the response of vegetation to drought from space using VIs. We analysed 11 years of dry season (July–September) Moderate Resolution Imaging Spectroradiometer (MODIS) enhanced vegetation index (EVI) and normalised difference vegetation index (NDVI) images. The VI standardised anomaly was analysed alongside the absolute value of EVI and NDVI, and the VI values for drought years were compared with those for non-drought years. Through a series of analyses, the standardised anomalies and VI values for drought years were shown to be of similar magnitude to those for non-drought years. Thus, while Amazon vegetation may respond to drought, this is not detectable through satellite-observed changes in vegetation greenness. A significant long-term decadal decline in VI values is reported, which is independent of the occurrence of drought. This trend may be caused by environmental or noise-related factors which require further investigation.
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    On the feasibility of magma fracture within volcanic conduits: Constraints from earthquake data and empirical modelling of magma viscosity (2011)
    Wiley
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    Publication Date: 2011-10-15
    Description: In the last decade several studies have indicated that magma within volcanic conduits can undergo repeated failure and healing, thus, providing a realistic source mechanism for the peculiar sequences of low-frequency earthquakes that often announce eruptions. Although geological observations, laboratory experiments, and numerical modelling support such a hypothesis, the links between geophysical observations and the proposed models remain qualitative. This study focuses on providing constraints to the relationship between the occurrence of repeating earthquakes and magma fracture at andesite volcanoes. Empirical modelling of viscosity is incorporated into the fundamental physics of magma fracture in order to assess whether and where the conditions for brittle failure are met within volcanic conduits. A case study from the 1995-ongoing eruption of Soufrière Hills Volcano, Montserrat, is presented. The locations of earthquakes from a pre-eruptive low-frequency seismic swarm illuminate a relatively compact source volume at depths of 100–300 meters below sea level. Viscosity modelling confirms that the Soufrière Hills magma could rupture at those depths.
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    Mapping near-inertial variability in the SE Bay of Biscay from HF radar data and two offshore moored buoys (2011)
    Wiley
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    Publication Date: 2011-10-15
    Description: HF radar surface current data together with data from two operational offshore oceanographic buoys located over the slope are used to map the variability associated with the near-inertial waves, during a target year (2009), in the SE Bay of Biscay. The results obtained show the complex 4D distribution of inertial oscillations in this area. We find a very pronounced horizontal structure across the area with ranges of a factor 5 in near-inertial kinetic energy. This pattern presents also strong seasonal variability, with a peak in KE closer to the shelf-break in summer, whereas winter maximum is weaker and located further to the north-east. The mooring data indicate more trapping near the surface in summer. These patterns are discussed in relation to the known seasonal differences in atmospheric/buoyancy forcing and the characteristics of the sub-inertial surface velocity field.
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    Imaging thunder (2011)
    Wiley
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    Publication Date: 2011-10-15
    Description: We use a network of broadband microphones, including a 4-element array, to locate the sources of thunder occurring during an electrical storm in central New Mexico on July 24th, 2009. Combined slowness search and distance ranging are used to identify thunder regions in three dimensions (out to 12 km) and for two overlapping frequency bands (1–10 and 4–40 Hz). Distinct thunder pulses are locatable and used to predict time-of-arrival to neighboring stations and to identify correlated phases across the network. Spatial correlation is also found between the thunder source regions and regions of very high frequency (VHF) radiation as located by the New Mexico Lightning Mapping Array (LMA). Some of the misfit between the LMA and thunder locations is attributable to differences in excitation mechanisms of the respective radiation, which is related to current impulses in lightning channels (for thunder) and incremental ionization of the atmosphere (for VHF emissions).
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    The rotation of the plasmapause-like boundary at high latitudes in Saturn's magnetosphere and its relation to the eccentric rotation of the northern and southern auroral ovals (2011)
    Wiley
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    Publication Date: 2011-11-09
    Description: Here we present a study of the rotation of the plasmapause-like density boundary discovered by the Cassini spacecraft at high latitudes in the Saturnian magnetosphere, and compare the results with previously published studies of high-latitude magnetic field perturbations and the eccentric rotation of the auroral ovals. Near the planet the density boundary is located at dipole L values ranging from about 8 to 15, and separates a region of very low densities at high latitudes from a region of higher densities at lower latitudes. We show that the density boundary rotates at different rates in the northern and southern hemispheres, and that the periods are the same as the modulation periods of Saturn kilometric radiation in those hemispheres. We also show that the phase of rotation in a given hemisphere is closely correlated with the phase of the high-latitude magnetic field perturbations observed by Cassini in that hemisphere, and also with the phase of the eccentric rotation of the auroral oval observed by the Hubble Space Telescope.
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    A slow slip event triggered by teleseismic surface waves (2011)
    Wiley
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    Publication Date: 2011-11-10
    Description: In recent years slow slip events (SSE) have been observed to occur at regular intervals on the deep portions of subduction zone interfaces. These are accompanied by seismic tremor that occurs over their duration. It has been observed that tremor activity shows transient modulations in response to earth tides and the passage of seismic waves from distant earthquakes. Here we show, for the first time, geodetic evidence for the triggering of an interplate SSE itself by teleseismic surface waves. This SSE, in southwest Japan, which had an equivalent magnitude Mw 5.3 and duration of 1.5 days, was triggered by the surface waves of a Mw 7.6 earthquake in Tonga. This evidence was captured by a newly deployed sensitive strainmeter network. The triggered SSE occurred on a place on the plate interface where the recurrence time for such events had almost expired, whereas other regions, at up to 90% of the recurrence time, were not triggered. This provides information for the conditions for triggering and generation of SSEs and, perhaps, for regular earthquakes.
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    A mechanism for preseismic steady rupture fronts observed in laboratory experiments (2011)
    Wiley
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    Publication Date: 2011-11-10
    Description: It has been shown that the onset of frictional instability is characterized by a transition from stable, quasi-static rupture growth to unstable, inertially-controlled high-speed rupture. In particular, slow rupture fronts propagating at a steady speed Vslow of the order of 5% of the S-wave speed have been observed prior to the onset of dynamic rupture in recent fault-friction laboratory experiments. However, the precise mechanism governing this Vslow stage is unknown. Here we reproduce this phenomenon in numerical simulations of earthquake sequences that incorporate laboratory-derived rate-and-state friction laws. Our simulations show that the Vslow stage originates from a stress concentration inherited from the coalescence of interseismic slow creep fronts. Its occurrence is limited to a narrow range of the parameter space but is found in simulations with two commonly-used state-variable evolution laws in the rate-and-state formulation. The sensitivity of the speed Vslow to the model parameters suggests that the propagation speed Vslow reported in laboratory experiments may also be sensitive to parameters of friction and stress conditions. Our results imply that time and space dimensions associated with the propagation of Vslow on natural faults can be as much as a few seconds and several hundred meters, respectively. Hence the detection of such preseismic signals may be possible with near-field high-resolution observations.
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    Observed volume fluxes in the Bosphorus Strait (2011)
    Wiley
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    Publication Date: 2011-11-10
    Description: Pairs of moorings containing acoustic Doppler current profilers (ADCPs) were deployed at each end of the Bosphorus Strait as a part of the United States Naval Research Laboratory's “Exchange Processes in Ocean Straits (EPOS)” project. The moorings were deployed in September 2008 and remained in place for about half a year. For the first time, current velocity profiles were collected concurrently at both ends of the strait. They well resolved the two-layer exchange flow and were used to estimate volume flux time series. These estimates clearly indicated that there was a mean net volume flux of over 100 km3/yr directed from the Black Sea to the Sea of Marmara over this period. The upper-layer fluxes showed distinct temporal variability whereas the lower-layer fluxes varied less. Volume fluxes were maximized at over 1500 km3/yr in the upper layer and over 1100 km3/yr in the lower layer, with an upper-layer mean of about 400 km3/yr and a lower-layer mean of about 300 km3/yr. Fluxes in both layers were highly coherent with the bottom pressure difference between the southern and northern ends of Bosphorus Strait, and they can be fairly well predicted from this pressure difference. The fluxes in the upper layer were also influenced by atmospheric forcing, but generally less so than by the bottom pressure difference.
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    Residual CO2 imaged with X-ray micro-tomography (2011)
    Wiley
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    Publication Date: 2011-11-10
    Description: Carbon capture and storage (CCS), where CO2 is injected into geological formations, has been identified as an important way to reduce CO2 emissions to the atmosphere. While there are several aquifers worldwide into which CO2 has been injected, there is still uncertainty in terms of the long-term fate of the CO2. Simulation studies have proposed capillary trapping – where the CO2 is stranded as pore-space droplets surrounded by water – as a rapid way to secure safe storage. However, there has been no direct evidence of pore-scale trapping. We imaged trapped super-critical CO2 clusters in a sandstone at elevated temperatures and pressures, representative of storage conditions using computed micro-tomography (μ-CT) and measured the distribution of trapped cluster size. The clusters occupy 25% of the pore space. This work suggests that locally capillary trapping is an effective, safe storage mechanism in quartz-rich sandstones.
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    Sensitivity of the attribution of near surface temperature warming to the choice of observational dataset (2011)
    Wiley
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    Publication Date: 2011-11-10
    Description: A number of studies have demonstrated that much of the recent warming in global near surface temperatures can be attributed to increases in anthropogenic greenhouse gases. While this conclusion has been shown to be robust in analyses using a variety of climate models there have not been equivalent studies using different available observational datasets. Here we repeat the analyses as reported previously using an updated observational dataset and other independently processed datasets of near surface temperatures. We conclude that the choice of observational dataset has little impact on the attribution of greenhouse gas warming and other anthropogenic cooling contributions to observed warming on a global scale over the 20th century, however this robust conclusion may not hold for other periods or for smaller sub-regions. Our results show that the dominant contributor to global warming over the last 50 years of the 20th century is that due to greenhouse gases.
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    High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core (2011)
    Wiley
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    Publication Date: 2011-11-10
    Description: Greenland recently incurred record high temperatures and ice loss by melting, adding to concerns that anthropogenic warming is impacting the Greenland ice sheet and in turn accelerating global sea-level rise. Yet, it remains imprecisely known for Greenland how much warming is caused by increasing atmospheric greenhouse gases versus natural variability. To address this need, we reconstruct Greenland surface snow temperature variability over the past 4000 years at the GISP2 site (near the Summit of the Greenland ice sheet; hereafter referred to as Greenland temperature) with a new method that utilises argon and nitrogen isotopic ratios from occluded air bubbles. The estimated average Greenland snow temperature over the past 4000 years was −30.7°C with a standard deviation of 1.0°C and exhibited a long-term decrease of roughly 1.5°C, which is consistent with earlier studies. The current decadal average surface temperature (2001–2010) at the GISP2 site is −29.9°C. The record indicates that warmer temperatures were the norm in the earlier part of the past 4000 years, including century-long intervals nearly 1°C warmer than the present decade (2001–2010). Therefore, we conclude that the current decadal mean temperature in Greenland has not exceeded the envelope of natural variability over the past 4000 years, a period that seems to include part of the Holocene Thermal Maximum. Notwithstanding this conclusion, climate models project that if anthropogenic greenhouse gas emissions continue, the Greenland temperature would exceed the natural variability of the past 4000 years sometime before the year 2100.
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    Ultra-low clouds over the southern West African monsoon region (2011)
    Wiley
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    Publication Date: 2011-11-09
    Description: New ground- and space-based observations show that summertime southern West Africa is frequently affected by an extended cover of shallow, non-precipitating clouds only few hundred meters above the ground. These clouds are associated with nocturnal low-level wind speed maxima and frequently persist into the day, considerably reducing surface solar radiation. While the involved phenomena are well represented in re-analysis data, climate models show large errors in low-level wind, cloudiness, and solar radiation of up to 90 W m−2. Errors of such a magnitude could strongly affect the regional energy and moisture budgets, which might help to explain the notorious difficulties of many models to simulate the West African climate. More effort is needed in the future to improve the monitoring, modeling, and physical understanding of these ultra-low clouds and their importance for the West African monsoon system.
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    Light-absorbing soluble organic aerosol in Los Angeles and Atlanta: A contrast in secondary organic aerosol (2011)
    Wiley
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    Publication Date: 2011-11-09
    Description: Light absorption spectra and carbon mass of fine particle water-soluble components were measured during the summer of 2010 in the Los Angeles (LA) basin, California, and Atlanta, Georgia. Fresh LA secondary organic carbon had a consistent brown color and a bulk absorption per soluble carbon mass at 365 nm that was 4 to 6 times higher than freshly-formed Atlanta soluble organic carbon. Radiocarbon measurements of filter samples show that LA secondary organic aerosol (SOA) was mainly from fossil carbon and chemical analysis of aqueous filter extracts identified nitro-aromatics as one component of LA brown SOA. Interpreting soluble brown carbon as a property of freshly-formed anthropogenic SOA, the difference in absorption per carbon mass between these two cities suggests most fresh secondary water-soluble organic carbon formed within Atlanta is not from an anthropogenic process similar to LA. Contrasting emissions of biogenic volatile organic compounds may account for these differences.
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    The Earth's electromagnetic environment (2011)
    Wiley
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    Publication Date: 2011-11-09
    Description: It is found that the energy density of electromagnetic fields at the surface of the Earth follow a scaling law that extends over ∼16 orders of magnitude from ∼10−9 Hz to ∼107 Hz. The temporal variability of the field can be described with an ∼1/f2, or Brownian, noise power spectrum which reflects the superposition of numerous transient source processes. To the best of our knowledge, the spectral extent of this straightforward scaling law is unparalleled and outperforms any other scaling law in physics which describes a time dependent observable. The frequency dependence of the energy density can be approximated with the analytic description u(f) = u0(f0/f)2 where u0 = 10−16 Jm−3Hz−1, f0 = 1 Hz is a scaling constant, and f is the frequency of the electromagnetic field. The corresponding frequency dependence of the magnetic field is B(f) = B0(f0/f) where B0 = 10−11 T/Hz1/2.
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    Primary marine organic aerosol: A dichotomy of low hygroscopicity and high CCN activity (2011)
    Wiley
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    Publication Date: 2011-11-09
    Description: High-time resolution measurements of primary marine organic sea-spray physico-chemical properties reveal an apparent dichotomous behavior in terms of water uptake: specifically sea-spray aerosol enriched in organic matter possesses a low hydroscopic Growth Factor (GF∼1.25) while simultaneously having a cloud condensation nucleus/condensation nuclei (CCN/CN) activation efficiency of between 83% at 0.25% supersaturation and 100% at 0.75%. In contrast, the activation efficiency of particles dominated by non-sea-salt (nss)-sulfate ranged between 48–100% over supersaturation range of 0.25%–1%. Simultaneous retrieval of Cloud Droplet Number Concentration (CDNC) during primary organic aerosol plumes reveals CDNC concentrations of 350 cm−3 for organic mass concentrations 3–4 μg m−3. It is demonstrated that the retrieved high CDNCs under clean marine conditions can only be explained by organic sea-spray and corroborates the high CCN activation efficiency associated with primary organics. It is postulated that marine hydrogels are responsible for this dichotomous behavior.
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    Glaciation temperatures of convective clouds ingesting desert dust, air pollution and smoke from forest fires (2011)
    Wiley
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    Publication Date: 2011-11-09
    Description: Heavy aerosol loads have been observed to suppress warm rain by reducing cloud drop size and slowing drop coalescence. The ice forming nuclei (IFN) activity of the same aerosols glaciate the clouds and create ice precipitation instead of the suppressed warm rain. Satellite observations show that desert dust and heavy air pollution over East Asia have similar ability to glaciate the tops of growing convective clouds at glaciation temperature of Tg 〈 ∼ −20°C, whereas similarly heavy smoke from forest fires in Siberia without dust or industrial pollution glaciated clouds at Tg ≤ −33°C. The observation that both smoke and air pollution have same effect on reducing cloud drop size implies that the difference in Tg is due to the IFN activity. This dependence of Tg on aerosol types appears only for clouds with re-5 〈 12 μm (re-5 is the cloud drop effective radius at the −5°C isotherm, above which ice rarely forms in cloud tops). For the rest of the clouds the glaciation temperature increases strongly with re-5 with little relation to the aerosol types, reaching Tg〉 ∼ −15°C for the largest re-5, which are typical to marine clouds in pristine atmosphere.
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    Estimating eruption temperature from thermal emission spectra of lava fountain activity in the Erta'Ale (Ethiopia) volcano lava lake: Implications for observing Io's volcanoes (2011)
    Wiley
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    Publication Date: 2011-11-11
    Description: We have analysed high-spatial-resolution and high-temporal-resolution temperature measurements of the active lava lake at Erta'Ale volcano, Ethiopia, to derive requirements for measuring eruption temperatures at Io's volcanoes. Lava lakes are particularly attractive targets because they are persistent in activity and large, often with ongoing lava fountain activity that exposes lava at near-eruption temperature. Using infrared thermography, we find that extracting useful temperature estimates from remote-sensing data requires (a) high spatial resolution to isolate lava fountains from adjacent cooler lava and (b) rapid acquisition of multi-color data. Because existing spacecraft data of Io's volcanoes do not meet these criteria, it is particularly important to design future instruments so that they will be able to collect such data. Near-simultaneous data at more than two relatively short wavelengths (shorter than 1 μm) are needed to constrain eruption temperatures. Resolving parts of the lava lake or fountains that are near the eruption temperature is also essential, and we provide a rough estimate of the required image scale.
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    An evaluation of subsidence rates and sea-level variability in the northern Gulf of Mexico (2011)
    Wiley
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    Publication Date: 2011-11-11
    Description: While subsidence is widely recognized as a driver of geomorphic change in the northern Gulf of Mexico (GOM), there is considerable disagreement over the rates of subsidence and the interpreted variability in these rates, which leads to controversies over the impacts of subsidence on surface land area change. Here we present a new method to calculate subsidence rates from the tide gauge record that is based on an understanding of the meteorological drivers of inter-annual sea-level change. In Grand Isle, LA and Galveston, TX, we explicitly show that temporal patterns of subsidence are closely linked to subsurface fluid withdrawal and coastal land loss, and suggest changes in withdrawal rates can both increase and decrease rates of subsidence and wetland loss. Our results also imply that the volume of sediment needed to rebuild GOM wetlands may currently fall within the low end of some restoration scenarios.
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    Directional spectra of hurricane-generated waves in the Gulf of Mexico (2011)
    Wiley
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    Publication Date: 2011-10-15
    Description: Hurricane-induced directional wave spectra in the Gulf of Mexico are investigated based on the measurements collected at 12 buoys during 7 hurricane events in recent years. Focusing on hurricane-generated wave spectra, we only consider the wave measurements at the buoys within eight times the radius of the hurricane maximum wind speed (Rmax) from the hurricane center. A series of numerical experiments using a third-generation spectral wave prediction model were carried out to gain insight into the mechanism controlling the directional and frequency distributions of hurricane wave energy. It is found that hurricane wave spectra are almost swell-dominated except for the right-rear quadrant of a hurricane with respect to the forward direction, where the local strong winds control the spectra. Despite the complexity of a hurricane wind field, most of the spectra are mono-modal, similar to those under fetch-limited, unidirectional winds. However, bi-modal spectra were also found in both measurements and model results. Four types of bi-modal spectra have been observed. Type I happens far away (〉6 × Rmax) from a hurricane. Type II is bi-modal in frequency with significant differences in direction. It happens in the two left quadrants when the direction of hurricane winds deviates considerably from the swell direction. Type III is bi-modal in frequency in almost the same wave direction with two close peaks. It occurs when the energy of locally-generated wind-sea is only partially transferred to the swell energy by non-linear wave-wave interactions. Type IV was observed in shallow waters owing to coastal effects.
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    Simultaneous observations at Darwin of equatorial bubbles by ionosonde-based range/time displays and airglow imaging (2011)
    Wiley
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    Publication Date: 2011-12-07
    Description: Airglow observations of ionospheric electron density depletions made at Darwin, Australia have demonstrated that the tree-like structure of bubbles developed at the magnetic equator are mapped along magnetic field lines with considerable accuracy to the base of the ionosphere at higher latitudes. Ionosonde range-time displays made at Darwin and other equatorial sites in the Australian region show characteristic approaching and receding echoes which converge on a typical spread-F event. These off-angle echoes have often been referred to in the literature as satellite traces and associated with spread F with little recognition of their true significance. All four optical depletions previously reported in the literature as being seen at Darwin are found in this paper to be accompanied by such typical off-angle/spread F events. The zonal drift velocity of the moving reflectors can be measured from the speed at which such echoes approach and recede. Since digital ionosondes in equatorial sites have existed for many years, existing ionogram data, when suitably processed into range-time displays, may allow the occurrence of such events over several sunspot cycles to be found. A question remains as to whether all or only some of such equatorial range-time events correspond to electron density depletions.
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    San Andreas Fault tremor and retrograde metamorphism (2011)
    Wiley
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    Publication Date: 2011-12-08
    Description: Tectonic tremor is an enigmatic low-frequency seismic phenomenon mainly observed in subduction zones, but also documented along the deep extension of the central San Andreas Fault. The physical mechanisms behind this unusual seismic event are not yet determined for any tectonic setting; however, low effective stress conditions arising from metamorphic fluid production are commonly inferred for subduction-related tremor. We investigate the petrologic conditions at which the San Andreas tectonic tremor is inferred to occur through calculations of the pressure – temperature – time evolution of stable mineral assemblages and their water content in the dominant lithologies of the Franciscan Complex. We find that tremor locations around Parkfield and Cholame are currently experiencing retrograde metamorphic conditions. Within the temperature-depth conditions of observed tremor activity, at approximately 500°C and 20 km depth, several mineralogical transitions may occur in cooling greywacke and mafic rocks, leading to localised, significant removal of free water and an associated volume decrease. This indicates that, contrary to subduction-related tremor, tremor on the San Andreas Fault is not linked to prograde, crustal metamorphic fluid production within the fault zone; rather it might be related to mantle-derived fluids from below the tremor zone, and/or fault zone weakening that occurs as phyllosilicates replace more competent and granular mineral phases.
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    Reconciling the changes in atmospheric methane sources and sinks between the Last Glacial Maximum and the pre-industrial era (2011)
    Wiley
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    Publication Date: 2011-12-08
    Description: We know from the ice record that the concentration of atmospheric methane, [CH4], at the Last Glacial Maximum (LGM) was roughly half that in the pre-industrial era (PI), but how much of the difference was source-driven, and how much was sink-driven, remains uncertain. Recent developments include: a higher estimate of the LGM-PI change in methane emissions from wetlands―the dominant, natural methane source; and the possible recycling of OH consumed in isoprene oxidation―the principal methane sink. Here, in view of these developments, we use an atmospheric chemistry-transport model to re-examine the main factors affecting OH during this period: changes in air temperature and emissions of non-methane volatile organic compounds from vegetation. We find that their net effect was negligible (with and without an OH recycling mechanism), implying the change in [CH4] was almost entirely source driven―a conclusion that, though subject to significant uncertainties, can be reconciled with recent methane source estimates.
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    Extracting waves and vortices from Lagrangian trajectories (2011)
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    Publication Date: 2011-12-07
    Description: A method for extracting time-varying oscillatory motions from time series records is applied to Lagrangian trajectories from a numerical model of eddies generated by an unstable equivalent barotropic jet on a beta plane. An oscillation in a Lagrangian trajectory is represented mathematically as the signal traced out as a particle orbits a time-varying ellipse, a model which captures wavelike motions as well as the displacement signal of a particle trapped in an evolving vortex. Such oscillatory features can be separated from the turbulent background flow through an analysis founded upon a complex-valued wavelet transform of the trajectory. Application of the method to a set of one hundred modeled trajectories shows that the oscillatory motions of Lagrangian particles orbiting vortex cores appear to be extracted very well by the method, which depends upon only a handful of free parameters and which requires no operator intervention. Furthermore, vortex motions are clearly distinguished from wavelike meandering of the jet—the former are high frequency, nearly circular signals, while the latter are linear in polarization and at much lower frequencies. This suggests that the proposed method can be useful for identifying and studying vortex and wave properties in large Lagrangian datasets. In particular, the eccentricity of the oscillatory displacement signals, a quantity which is not normally considered in Lagrangian studies, emerges as an informative diagnostic for characterizing qualitatively different types of motion.
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    Observed decreases in oxygen content of the global ocean (2011)
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    Publication Date: 2011-12-02
    Description: Comparing the high-quality oxygen climatology from the World Ocean Circulation Experiment to earlier data we reveal near-global decreases in oxygen levels in the upper ocean between the 1970s and the 1990s. This globally averaged oxygen decrease is −0.93 ± 0.23 μmol l−1, which is equivalent to annual oxygen losses of −0.55 ± 0.13 × 1014 mol yr−1 (100–1000 m). The strongest decreases in oxygen occur in the mid-latitudes of both hemispheres, near regions where there is strong water renewal and exchange between the ocean interior and surface waters. Approximately 15% of global oxygen decrease can be explained by a warmer mixed-layer reducing the capacity of water to store oxygen, while the remainder is consistent with an overall decrease in the exchange between surface waters and the ocean interior. Here we suggest that this reduction in water mass renewal rates on a global scale is a consequence of increased stratification caused by warmer surface waters. These observations support climate model simulations of oxygen change under global warming scenarios.
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    Relationship between aerosol and cloud fraction over Australia (2011)
    Wiley
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    Publication Date: 2011-12-02
    Description: We study the relationships between aerosols, clouds, and large scale dynamics over a north coastal Australia (NCA) region and a southeast Australia (SEA) region during the period 2002–2009 to evaluate the applicability of the aerosol microphysics-radiation-effect (MRE) theory proposed by Koren et al. (2008) in a low aerosol environment. We use aerosol optical depth (τa), fire counts, and cloud fraction (fc) from Aqua-MODIS, and NCEP Reanalysis vertical velocities at 500 mb (ω500) as a proxy for dynamic regime. In the NCA we find a monotonic increase fc (35%, absolute fc) as a function of increasing τa. In the SEA, we find that fc initially increases by 25% with increasing τa, followed by a slow systematic decrease (∼18%) with higher τa. We show that the MRE theory proposed by Koren et al. (2008) adequately represents the variation of fc with τa in both the NCA and SEA. By conditionally sorting data by ω500 we investigate the role dynamics plays in controlling the τa-fc relationship and the rate at which fc changes with τa. We find that the MRE theory can be used to empirically fit both −ω500 and +ω500 observations. By analyzing meteorological parameters from the NCEP Reanalysis, we find that variations in local meteorology are not likely the cause of the observed relationships of τa and fc during biomass burning seasons. However, additional factors such as aerosol type and cloud type may play a role.
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    Assessing the potential for tropical cyclone induced sea surface cooling to reduce thermal stress on the world's coral reefs (2011)
    Wiley
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    Publication Date: 2011-12-03
    Description: Coral reefs face an uncertain future as rising sea surface temperature (SST) continues to lead to increasingly frequent and intense mass bleaching. At broad spatial scales, tropical cyclone (TC) induced cooling of the upper ocean (SST drops up to 6° C persisting for weeks) reduces thermal stress and accelerates recovery of bleached corals - yet the global prevalence and spatial distribution of this effect remains undocumented and unquantified. A global dataset (1985–2009) of TC wind exposure was constructed and examined against existing thermal stress data to address this. Significant correlations were found between TC activity and the severity of thermal stress at various spatial scales, particularly for Caribbean reefs. From this, it is apparent that TCs play a role in bleaching dynamics at a global scale. However, the prevalence and distribution of this interaction varies by region and requires further examination at finer spatial and temporal scales using actual SST data.
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    Along-dip variation of teleseismic short-period radiation from the 11 March 2011 Tohoku earthquake (Mw 9.0) (2011)
    Wiley
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    Publication Date: 2011-11-16
    Description: Locations of coherent short-period seismic wave radiation from the 11 March 2011 Tohoku earthquake (Mw 9.0) are imaged by back-projecting teleseismic P waves recorded across North America for a series of narrow, overlapping passbands centered at 8s, 4s, 2s, 1s, and 0.5s. Initially the energy release for all five passbands migrates slowly down-dip, however over time the two longer-period passbands show coherent energy release systematically shifted up-dip of the shorter-period source regions. Back-projection images of P waves from ten (point-source-like) aftershocks do not show a frequency-dependent trend, implying that the frequency dependence observed for the main shock is not an artifact created by 3D earth structure, depth phase interference, or some other deficiency. We conclude that the unstable sliding properties along the megathrust are segmented, with faster moment rate variations in the down-dip region and relatively smooth sliding further up-dip.
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    A new source of water in seismogenic subduction zones (2011)
    Wiley
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    Publication Date: 2011-11-18
    Description: Seismogenic plate-boundary faults at accretionary margins (e.g., the Nankai margin, southwest Japan) may occur where the uppermost part of subducting oceanic crust, composed of basaltic rocks, is in contact with the overriding plate of a lithified accretionary prism. The plate-boundary faults in ancient accretionary complexes typically record high-velocity slip under fluid-rich conditions. Although previous studies have emphasized the mechanical significance of fluids in terms of dynamic slip-weakening, the source of fluid in seismogenic subduction zones remains poorly constrained. In this work, we focus on the hydrous smectite in the uppermost oceanic crust, an alteration product of intact basalt before arrival at the trench axis. A comparison between (1) new mineralogical data on basalt drillcore recovered by Integrated Ocean Drilling Program (IODP) Expedition 322 at site C0012, a reference site for subduction input to the Nankai Trough, and (2) mineralogical data on basalt within ancient oceanic crust embedded in a fossil accretionary complex of the Shimanto Belt, southwest Japan, suggests that progressive smectite–chlorite conversion would liberate bound fluids at a rate of 0.34 to 0.65 × 10−14 s−1 along the plate interface. This rate of fluid production appears to be more than an order of magnitude greater than that from other possible sources, including from overlying sediments via smectite–illite conversion and the expulsion of pore fluids, and may facilitate seismic slip along plate-boundary faults.
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    Solitary wave behavior in sand dunes observed from space (2011)
    Wiley
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    Publication Date: 2011-11-17
    Description: Although the dynamics of individual barchan dunes are well understood, their interactions are the subject of ongoing scientific interest and debate. Numerical and analog model predictions of shape-preserving binary dune collisions have been hard to test due to the long timescales over which such processes typically occur. This paper documents ten binary dune collisions in a 45-year time sequence of satellite images from the Bodélé Depression in Chad. The observations confirm that when two barchan dunes collide, a transfer of mass occurs so that one dune appears to travel through the other unscathed, like a solitary wave.
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    Limitations of nitrogen and phosphorous on the terrestrial carbon uptake in the 20th century (2011)
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    Publication Date: 2011-11-17
    Description: A climate model, coupled to a sophisticated land model, is used to explore the impact of nitrogen and phosphorous limitations on carbon uptake under increasing atmospheric carbon dioxide concentration, or [CO2], from 1870 to 2009. Adding nitrogen limitation strongly reduces the capacity of land CO2 uptake under increasing [CO2]. The further limitation by phosphorous has a smaller impact on the global uptake of CO2. However, phosphorous limitation has a strong impact on regional carbon uptake: increasing CO2 sinks over North America and Eurasia and decreasing sinks over China and Australia. Thus, while the global carbon balance can be resolved with just nitrogen limitation, simulations of continental-scale carbon sinks will need to include the additional limitation of phosphorous through the 20th century.
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    Skillful predictions of decadal trends in global mean surface temperature (2011)
    Wiley
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    Publication Date: 2011-11-17
    Description: We compare observed decadal trends in global mean surface temperature with those predicted using a modelling system that encompasses observed initial condition information, externally forced response (due to anthropogenic greenhouse gases and aerosol precursors), and internally generated variability. We consider retrospective decadal forecasts for nine cases, initiated at five year intervals, with the first beginning in 1961 and the last in 2001. Forecast ensembles of size thirty are generated from differing but similar initial conditions. We concentrate on the trends that remain after removing the following natural signals in observations and hindcasts: dynamically induced atmospheric variability, El Niño-Southern Oscillation (ENSO), and the effects of explosive volcanic eruptions. We show that ensemble mean errors in the decadal trend hindcasts are smaller than in a parallel set of uninitialized free running climate simulations. The ENSO signal, which is skillfully predicted out to a year or so, has little impact on our decadal trend predictions, and our modelling system possesses skill, independent of ENSO, in predicting decadal trends in global mean surface temperature.
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    Cold months in a warming climate (2011)
    Wiley
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    Publication Date: 2011-11-19
    Description: The frequency of cold months in the 21st century is studied using the CMIP3 ensemble of climate model simulations, using month-, location- and model-specific threshold temperatures derived from the simulated 20th century climate. Unsurprisingly, cold months are projected to become less common, but not non-existent, under continued global warming. As a multi-model mean over the global land area excluding Antarctica and under the SRES A1B scenario, 14% of the months during the years 2011–2050 are simulated to be colder than the 20th century median for the same month, 1.3% colder than the 10th percentile, and 0.1% record cold. The geographic and seasonal variations in the frequency of cold months are strongly modulated by variations in the magnitude of interannual variability. Thus, for example, cold months are most infrequently simulated over the tropical oceans where the variability is smallest, not over the Arctic where the warming is largest.
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    The persistence of solar activity indicators and the descent of the Sun into Maunder Minimum conditions (2011)
    Wiley
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    Publication Date: 2011-11-30
    Description: The recent low and prolonged minimum of the solar cycle, along with the slow growth in activity of the new cycle, has led to suggestions that the Sun is entering a Grand Solar Minimum (GSMi), potentially as deep as the Maunder Minimum (MM). This raises questions about the persistence and predictability of solar activity. We study the autocorrelation functions and predictability R2L(t) of solar indices, particularly group sunspot number RG and heliospheric modulation potential Φ for which we have data during the descent into the MM. For RG and Φ, R2L(t) 〉 0.5 for times into the future of t ≈ 4 and ≈ 3 solar cycles, respectively: sufficient to allow prediction of a GSMi onset. The lower predictability of sunspot number RZ is discussed. The current declines in peak and mean RG are the largest since the onset of the MM and exceed those around 1800 which failed to initiate a GSMi.
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    Fast slip-rate along the northern end of the Karakorum fault system, western Tibet (2011)
    Wiley
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    Publication Date: 2011-11-30
    Description: The exact location of the northern Karakorum fault (KF) in western Tibet is unclear and its current activity is debated. Here, we investigate the possible northern extension of the KF, the Muji fault, located in the Chinese Pamir, which belongs to the Kongur Shan extensional system, and provide the first quantitative estimate of its Holocene slip-rate. The fault cuts and offsets a series of 6 fluvial terraces, yielding a minimum slip-rate of 4.5 ± 0.2 mm/yr, by matching the largest terrace riser offset with its upper surface age (10Be, n = 24). Field evidences of right-lateral movement along the Kongur Shan fault, as well as geometry and kinematic similarities with the southern half of the KF attest that the Muji fault belongs to the KF system. Therefore, its fast slip-rate combined with the slow slip-rates along minor splays of the northern KF (maybe up to 4 mm/yr) southwest of the Tashkorgan basin agrees with the late Pleistocene southern KF slip-rate (〉8 mm/yr).
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    Ballistics and ash plumes discriminated by Doppler radar (2011)
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    Publication Date: 2011-11-16
    Description: Small scale eruptive ash plumes at Arenal volcano (Costa Rica) were recorded using a ground-based Doppler radar (VOLDORAD). The time-velocity distribution of the mass load (i.e., Doppler radargrams) exhibits two contrasted dynamics recorded simultaneously, evidenced by distinctive velocities, life spans, and transit speeds through the radar beam. Synthetic Doppler radargrams computed with a simple ballistic model indicate that the short-lived signal is consistent with the instantaneous projection of ballistics blocks accompanying the ash plume emission. The mass of centimeter- to decimeter-sized ballistics is confidently estimated at 0.5–7 tons, whereas the ash plume mass is loosely constrained at 5.8 × 102 tons, assuming a particle diameter of 2 mm close to the vent. These quantitative estimates of the mass proportion either falling on the slopes of the volcano or ejected into the atmosphere could help in the modeling and monitoring of tephra dispersal.
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    Satellite observations of Schumann resonances in the Earth's ionosphere (2011)
    Wiley
    Details
    Publication Date: 2011-11-16
    Description: Using electric field measurements gathered on the C/NOFS satellite, we report, Schumann resonance signatures detected in space, well beyond the upper boundary of the resonant cavity formed by the earth's surface and the lower edge of the ionosphere. The resonances are routinely observed in the satellite ELF data during nighttime conditions within the altitude region of 400–850 km sampled by the satellite. They exhibit the distinctive frequency patterns predicted for Schumann resonances and are consistent with the corresponding frequency characteristics of ground-based observations of this phenomenon. The observations of Schumann resonances in space support a leaky cavity interpretation of the ionosphere and call for revisions of models of extremely low frequency wave propagation in the ionosphere. They suggest new remote sensing capabilities for investigating atmospheric electricity on Earth and other planets.
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    Widespread low rates of Antarctic glacial isostatic adjustment revealed by GPS observations (2011)
    Wiley
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    Publication Date: 2011-11-16
    Description: Bedrock uplift in Antarctica is dominated by a combination of glacial isostatic adjustment (GIA) and elastic response to contemporary mass change. Here, we present spatially extensive GPS observations of Antarctic bedrock uplift, using 52% more stations than previous studies, giving enhanced coverage, and with improved precision. We observe rapid elastic uplift in the northern Antarctic Peninsula. After considering elastic rebound, the GPS data suggests that modeled or empirical GIA uplift signals are often over-estimated, particularly the magnitudes of the signal maxima. Our observation that GIA uplift is misrepresented by modeling (weighted root-mean-squares of observation-model differences: 4.9–5.0 mm/yr) suggests that, apart from a few regions where large ice mass loss is occurring, the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too high.
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    Sensitivity of the northeast Asian summer monsoon to tropical sea surface temperatures (2011)
    Wiley
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    Publication Date: 2011-11-18
    Description: Observations indicate increasing trends of summer precipitation amount, intensity, and frequency of extremes over northeast Asia since the 1960s. Climate models are generally able to simulate such increases of precipitation over northeast Asia over the 2nd half of the 20th century, and project continuations of these trends in response to the projected warming of the tropical Indo-Pacific Warm Pool, especially around the Philippines and the South China Sea. The principal basis for confidence in these projections is the simplicity and robustness of the mechanisms involved. In essence, the warming of these waters enhances the northward moisture transport from the tropics to northeast Asia, leading to an increase of the northeast Asian precipitation.
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    Plasma pressure in Mercury's equatorial magnetosphere derived from MESSENGER Magnetometer observations (2011)
    Wiley
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    Publication Date: 2011-11-17
    Description: Since insertion of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft into orbit around Mercury on 18 March 2011, the probe's Magnetometer has routinely observed localized reductions of the magnetic field magnitude below the level predicted by a planetary dipole model corrected for magnetospheric magnetic fields. These magnetic depressions are observed on almost every orbit, and the latitude at which they are observed is local-time dependent. The depression signatures are indicators of the presence of enhanced plasma pressures, which inflate the magnetic field locally to maintain pressure balance, thus lowering the magnetic flux density. Mapping the magnetic depressions in local time and latitude provides insight into the plasma distribution near the planet, which complements that provided by MESSENGER's Fast Imaging Plasma Spectrometer. The spatial distribution shows that magnetic depressions are concentrated in two distinct regions, one near the equator on the nightside and another at high latitudes principally on the dayside. Here we focus on the nightside, equatorial pressure signatures, which we attribute to the magnetotail plasma sheet. The plasma-sheet pressures extend from dusk to dawn and are offset northward from the planetary geographic equator by about 10° in latitude, commensurate with the offset of the planetary dipole. The pressures associated with the plasma-sheet depressions range from 0.1 to 3 nPa and are systematically higher at dawn than at dusk. Proton gradient-curvature and convection drift in Mercury's dipole magnetic field with a dawn-to-dusk electric field result in low drift velocities near dawn, leading to systematically higher densities and pressures at dawn than at dusk, consistent with the observations.
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    Scattering function for energetic neutral hydrogen atoms off the lunar surface (2011)
    Wiley
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    Publication Date: 2011-11-17
    Description: The Sub-keV Atom Reflecting Analyzer instrument on board the lunar orbiter Chandrayaan-1 provided a large number of measurements of lunar energetic neutral atoms (ENAs). These ENAs were formerly solar wind ions, which were neutralized and backscattered from the lunar surface. The angles under which the ENAs are scattered strongly depend on the solar wind ions' incidence angle, which corresponds to the solar zenith angle (SZA). Our large dataset provides us with a complete coverage of the SZA and almost complete coverage of the scattering angles. When combining all available measurements, four distinct features are discernible with SZA increase: amplitude decrease, less azimuthal uniformity, bigger ratio of sunward versus anti-sunward flux and shallower scattering. We analyzed more than 290′000 measurements and derived a mathematical description of the features and their dependencies on the SZA.
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    Dolomite III: A new candidate lower mantle carbonate (2011)
    Wiley
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    Publication Date: 2011-11-17
    Description: Dolomite is a major constituent of subducted carbonates; therefore evaluation of its phase stability and equation of state at high pressures and temperatures is important for understanding the deep Earth carbon cycle. X-ray diffraction experiments in the diamond anvil cell show that Ca0.988Mg0.918Fe0.078Mn0.016(CO3)2 dolomite transforms to dolomite-II at ∼17 GPa and 300 K and then upon laser-heating transforms to a new monoclinic phase (dolomite-III), that is observed between 36 and 83 GPa. Both high-pressure polymorphs are stable up to 1500 K, indicating that addition of minor Fe stabilizes dolomite to Earth's deep-mantle conditions.
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    Past and present-day ice mass variation on Svalbard revealed by superconducting gravimeter and GPS measurements (2011)
    Wiley
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    Publication Date: 2011-11-17
    Description: We use Superconducting Gravimeter (SG) and Global Positioning System (GPS) measurements from Ny-Ålesund, Svalbard, to infer changes in ice mass loss between September 1999 and September 2010. We find that during this period, the gravity rate and vertical crustal velocities are changing with time, adding to evidence about varying rates of ice mass loss. The gravity rate varies through 10 years of observation; −0.23 μGal/yr in 2000–2002, −3.22 μGal/yr in 2002–2005 and −1.10 μGal/yr in 2005–2010. The gravity changes agree well with the observed uplift rates measured by GPS, which are 4.4, 11.3 and 7.4 mm/yr, over the same periods. In addition, we generate model predictions which account for past and present-day ice mass variation. We find that the models under predict both the observed uplift rates and gravity changes.
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    Mesospheric temperature trends at mid-latitudes in summer (2011)
    Wiley
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    Publication Date: 2011-11-19
    Description: The Leibniz-Institute Middle Atmosphere Model LIMA is used to study mesospheric temperature trends in summer during the last 5 decades (1961–2009). In order to account for realistic atmospheric conditions LIMA adapts several observational data sets, namely a) tropospheric and stratospheric temperatures and winds from ECMWF at heights 0–35 km, b) daily Lyman-α fluxes, c) monthly carbon dioxide concentrations since 1961, and d) annual total ozone from ground-based data for 1964–1978 and monthly ozone profiles up to 0.60 hPa from satellites since 1979. This paper presents a comparison of simulated temperature trends with a) ground-based observations of lidar temperatures at 44°N, b) phase height measurements at mid-latitudes (51°N), and c) temperature trends derived from satellite data. In general there is excellent agreement between trends from LIMA and observations. Cooling in the mesosphere is on the order of 2–4 K/decade. The magnitude of the mesospheric temperature trend varies during the last five decades. In particular, the period from 1979–1997 shows large mesospheric cooling of 3–5 K/decade. This large cooling is primarily caused by long-term changes of ozone in the upper stratosphere in combination with a CO2 increase. For the first time, modeling of mesospheric temperature trends confirm the extraordinarily large trends from observations.
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    On the surface wind speed probability density function over complex terrain (2011)
    Wiley
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    Publication Date: 2011-11-19
    Description: The physical mechanisms determining the shape of the surface wind speed probability density function over complex terrain are investigated using observations from a dense mesoscale network and a high spatial resolution mesoscale simulation for the 1992 to 2005 period. Results indicate that the atmospheric stability plays a major role in controlling the shape of the wind speed distribution but its effects are strongly modulated by the contribution of different atmospheric scales of motion such as the mesoscale or synoptic scale. The local topographic features further modulate the relative contribution of each mechanism. As a consequence of the complicated interaction of these atmospheric processes the surface wind speed distribution can present a complicated shape that is not always expected to fit a unimodal Weibull type distribution over complex terrain regions.
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    Quantifying the influence of sea ice on ocean microseism using observations from the Bering Sea, Alaska (2011)
    Wiley
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    Publication Date: 2011-11-19
    Description: Microseism is potentially affected by all processes that alter ocean wave heights. Because strong sea ice prevents large ocean waves from forming, sea ice can therefore significantly affect microseism amplitudes. Here we show that this link between sea ice and microseism is not only a robust one but can be quantified. In particular, we show that 75–90% of the variability in microseism power in the Bering Sea can be predicted using a fairly crude model of microseism damping by sea ice. The success of this simple parameterization suggests that an even stronger link can be established between the mechanical strength of sea ice and microseism power, and that microseism can eventually be used to monitor the strength of sea ice, a quantity that is not as easily observed through other means.
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    Large internal waves advection in very weakly stratified deep Mediterranean waters (2011)
    Wiley
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    Publication Date: 2011-11-29
    Description: The Eastern Mediterranean Sea contains relatively small trenches O(1–10 km) horizontal width that go deeper than 4000 m. At a first glance, these deep waters are homogeneous, with weak currents
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    Observed and modelled record ozone decline over the Arctic during winter/spring 2011 (2011)
    Wiley
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    Publication Date: 2011-12-01
    Description: This work describes observational and modelling results of the ozone depletion which took place during the winter/spring of 2011 in the Arctic stratosphere. Assimilated total ozone data from GOME-2 were used to estimate the integrated ozone mass deficit at polar latitudes and the Oslo CTM2 model calculated low winter/spring ozone values over the Arctic, which compare well with the satellite observations. Model runs with and without chemistry in the Arctic during the winter/spring of 2011 show that the very low Arctic stratospheric air temperatures led to significant chemical ozone loss. The calculated winter/spring ozone mass deficit (O3MD) reached extreme high values in 2011 (2700 Mt) and the seasonal zonal mean total ozone extreme low values of 333DU. Dynamics have set up the conditions for cold temperatures in the lower stratosphere in winter/spring of 2011. Comparison of ozone columns with the previous 13 years shows record low ozone column values during winter/spring in the Arctic in 2011. A comparison is also given with similar model studies for the overall warmer winter/spring of 2010 which show higher ozone column values and significantly less chemical ozone loss. The interannual variability of column ozone over the northern polar region is, as expected, highly correlated with the corresponding year-to-year variability of the seasonally-averaged temperatures in the lower stratosphere.
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    Is horizontal convection really “non-turbulent?” (2011)
    Wiley
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    Publication Date: 2011-11-11
    Description: Within the geophysical community Horizontal Convection (HC) has been considered irrelevant or nearly so in driving large scale overturning flows, based primarily on an inference based on a century old experiment by Sandström (1908), and on a theoretical argument that would prevent HC to sustain a true turbulent flow, the latter deemed necessary to achieve mixing. We revisit Paparella and Young's (2002) argument with the aid of DNS of HC at Rayleigh number up to 1010. We argue that the criterion used by these authors is overly restrictive. On the contrary, geometrical statistics show that HC possesses the characteristic of turbulent flows. The surprising result is that HC can transport very large quantities of heat and sustain large amounts of diapycnal mixing with a surprisingly small amount of dissipation. Values of diapycnal mixing and dissipation in the ocean are shown to be consistent with a HC driven ocean provided the effect of wind-forcing are included.
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    Are secular correlations between sunspots, geomagnetic activity, and global temperature significant? (2011)
    Wiley
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    Publication Date: 2011-11-11
    Description: Recent studies have led to speculation that solar-terrestrial interaction, measured by sunspot number and geomagnetic activity, has played an important role in global temperature change over the past century or so. We treat this possibility as an hypothesis for testing. We examine the statistical significance of cross-correlations between sunspot number, geomagnetic activity, and global surface temperature for the years 1868–2008, solar cycles 11–23. The data contain substantial autocorrelation and nonstationarity, properties that are incompatible with standard measures of cross-correlational significance, but which can be largely removed by averaging over solar cycles and first-difference detrending. Treated data show an expected statistically-significant correlation between sunspot number and geomagnetic activity, Pearson p 〈 10−4, but correlations between global temperature and sunspot number (geomagnetic activity) are not significant, p = 0.9954, (p = 0.8171). In other words, straightforward analysis does not support widely-cited suggestions that these data record a prominent role for solar-terrestrial interaction in global climate change. With respect to the sunspot-number, geomagnetic-activity, and global-temperature data, three alternative hypotheses remain difficult to reject: (1) the role of solar-terrestrial interaction in recent climate change is contained wholly in long-term trends and not in any shorter-term secular variation, or, (2) an anthropogenic signal is hiding correlation between solar-terrestrial variables and global temperature, or, (3) the null hypothesis, recent climate change has not been influenced by solar-terrestrial interaction.
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    Whistler turbulence forward cascade: Three-dimensional particle-in-cell simulations (2011)
    Wiley
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    Publication Date: 2011-11-16
    Description: The first fully three-dimensional particle-in-cell (PIC) simulation of whistler turbulence in a magnetized, homogeneous, collisionless plasma has been carried out. An initial relatively isotropic spectrum of long-wavelength whistlers is imposed upon the system, with an initial electron β = 0.10. As in previous two-dimensional simulations of whistler turbulence, the three-dimensional system exhibits a forward cascade to shorter wavelengths and broadband, turbulent spectra with a wave vector anisotropy in the sense of stronger fluctuation energy at k$\perp$ than at comparable k$\parallel$ where the respective subscripts represent directions perpendicular and parallel to the background magnetic field Bo. However, the three-dimensional (3D) simulations display quantitative differences with comparable two-dimensional (2D) computations. In the 3D runs, turbulence develops a stronger anisotropic cascade more rapidly than in 2D runs. Furthermore, reduced magnetic fluctuation spectra in 3D runs are less steep functions of perpendicular wave numbers than those from 2D simulations. The much larger volume of perpendicular wave vector space in 3D appears to facilitate the transfer of fluctuation energy toward perpendicular directions.
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    Enhanced lunar semidiurnal equatorial vertical plasma drifts during sudden stratospheric warmings (2011)
    Wiley
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    Publication Date: 2011-11-16
    Description: Large scale electrodynamic and plasma density variations in the low latitude ionosphere have recently been associated with sudden stratospheric warming (SSW) events. We present average patterns of largely enhanced lunar semidiurnal equatorial vertical plasma drift perturbations during arctic winter low and high solar flux SSW events. These perturbations play a dominant role in the electrodynamic response of the low latitude ionosphere to SSWs. Our models indicate that the amplitudes of the enhanced lunar semidiurnal drifts are strongly local time and solar flux dependent, with largest values during early morning low solar flux SSW periods. These results suggest that ionospheric conductance strongly modulate low latitude ionospheric changes during SSWs. They also indicate that lunar semidiurnal effects need to be taken into account by global ionospheric models for their improved forecasting of the low latitude ionospheric response to SSW events, especially for low solar flux conditions.
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    The Mairan domes: Silicic volcanic constructs on the Moon (2011)
    Wiley
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    Publication Date: 2011-11-16
    Description: The Mairan domes are four features located in northern Oceanus Procellarum at ∼312.3E, 41.4N on the Moon. High resolution visible imagery, visible-to-mid-IR spectra, and Lunar Prospector Th abundance data all indicate that these four domes have a composition that is consistent with derivation from a Si-rich, highly evolved magma.
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    Jet front-driven mirror modes and shocklets in the near-Earth flow-braking region (2011)
    Wiley
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    Publication Date: 2011-11-18
    Description: We investigate the small-scale structure of jet fronts through a case study of multi-spacecraft Cluster observations in the near-Earth flow-braking region at ∼−10 RE. We find that the interaction between the earthward moving fast plasma jet and the high-β ambient plasma in the plasma sheet results in magnetic pileup and compression ahead of the jet and rarefaction trailing the jet. It is shown that mirror-mode structures of ion gyroradius scale develop within the pileup region due to the observed ion temperature anisotropy (Ti$\perp$ 〉 Ti$\parallel$). We suggest that the growth of these mirror modes is driven by the perpendicular total pressure perturbation (Δp$\perp$) generated by the braking jet. When Δp$\perp$ becomes too large, the mirror-mode structure cannot maintain pressure balance any longer, and consequently a shocklet is formed in the pileup region ahead of the jet front. We present the first evidence for such a kinetic shocklet in the flow-braking region.
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    The spatial distribution of solar radiation under a melting Arctic sea ice cover (2011)
    Wiley
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    Publication Date: 2011-11-18
    Description: The sea ice cover of the Chukchi and Beaufort Seas is currently undergoing a fundamental shift from multiyear ice to first-year ice. Field observations of sea ice physical and optical properties were collected in this region during June–July 2010, revealing unexpectedly complex spatial distributions of solar radiation under the melt-season ice cover. Based on our optical measurements of first-year ice, we found the under-ice light field in the upper ocean to be spatially heterogeneous and dependent on wavelength, ice thickness, and the areal and geometric distribution of melt ponded and bare ice surfaces. Much of the observed complexity in radiation fields arose because the transmission of light through ponded ice was generally an order of magnitude greater than through bare, unponded ice. Furthermore, while many sites exhibited a consistent, exponential decay in light transmission through both ponded and bare ice surfaces, light transmission under bare ice was also observed to increase with depth (reaching maximum values ∼5–10 m below the bottom of the ice). A simple geometric model shows these transmission peaks are a result of scattering in the ice and the interspersion of bare and ponded sea ice surfaces. These new observations of complex radiation fields beneath melt-season first-year sea ice have significant implications for biological production, biogeochemical processes, and the heat balance of sea ice and under-ice ocean waters and should be carefully considered when modeling these sea ice-related phenomena.
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    Observational changes and trends in northeast Pacific wave records (2011)
    Wiley
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    Publication Date: 2011-11-18
    Description: Routine wave observations from buoys in the northeast Pacific now extend up to 35 years. Several recent studies reported long-term trends extracted from these records. However, significant modifications of the wave measurement hardware as well as the analysis procedures since the start of the observations result in inhomogeneities of the records. We analyze significant wave heights from seven offshore wave records. Several step changes of the mean monthly significant wave height of a few decimetres are identified. These changes are induced by buoy modifications and poor data quality rather than changes in the wave climate. After adjusting the data for these step changes the wave heights show positive trends for some of the southern locations and negative trends at the northern buoys, however all trends are much smaller than reported in previous studies. Storm wave heights are extracted from the occurrence rate distributions of the adjusted significant wave heights. No statistically significant trends can be established for storm wave heights.
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    Frequency-dependent energy radiation and fault coupling for the 2010 Mw8.8 Maule, Chile, and 2011 Mw9.0 Tohoku, Japan, earthquakes (2011)
    Wiley
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    Publication Date: 2011-11-22
    Description: We carried out back-projections of teleseismic data filtered in different frequency bands for the 2010 Maule, Chile and the 2011 Tohoku, Japan earthquakes. For the Maule earthquake, there were differences along strike of the fault, with the high-frequency energy mainly originating from an area 200 km northeast of the epicenter, whereas low-frequency energy came from a location closer to the epicenter. The Tohoku earthquake shows strong frequency dependence in the dip direction. High-frequency sources were located about 100 km west of the epicenter, while low-frequency sources were around epicenter, near the Japan Trench. We compare the spatial distributions of energy with estimates of seismic coupling before the earthquakes. Areas of high-frequency radiation seem correlated with regions that were strongly coupled before the earthquakes. Areas of high coupling, may be associated with fault properties that are more heterogeneous and/or have overall higher stress, producing higher frequency seismic waves.
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    Oxidation of ambient biogenic secondary organic aerosol by hydroxyl radicals: Effects on cloud condensation nuclei activity (2011)
    Wiley
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    Publication Date: 2011-11-22
    Description: Changes in the hygroscopicity of ambient biogenic secondary organic aerosols (SOA) due to controlled OH oxidation were investigated at a remote forested site at Whistler Mountain, British Columbia during July of 2010. Coupled photo-oxidation and cloud condensation nuclei (CCN) experiments were conducted on: i) ambient particles exposed to high levels of gas-phase OH, and ii) the water-soluble fraction of ambient particles oxidized by aqueous-phase OH. An Aerodyne Aerosol Mass Spectrometer (AMS) monitored the changes in the chemical composition and degree of oxidation (O:C ratio) of the organic component of ambient aerosol due to OH oxidation. The CCN activity of size-selected particles was measured to determine the hygroscopicity parameter ($\kappa$org,CCN) for particles of various degrees of oxygenation. In both cases, the CCN activity of the oxidized material was higher than that of the ambient particles. In general, $\kappa$org,CCN of the aerosol increases with its O:C ratio, in agreement with previous laboratory measurements.
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    Amplification of hypoxic and acidic events by La Niña conditions on the continental shelf off California (2011)
    Wiley
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    Publication Date: 2011-11-23
    Description: Low-oxygen and low-pH events are an increasing concern and threat in the Eastern Pacific coastal waters, and can be lethal for benthic and demersal organisms on the continental shelf. The normal seasonal cycle includes uplifting of isopycnals during upwelling in spring, which brings low-oxygen and low-pH water onto the shelf. Five years of continuous observations of subsurface dissolved oxygen off Southern California, reveal large additional oxygen deficiencies relative to the seasonal cycle during the latest La Niña event. While some changes in oxygen related to the isopycnal depression/uplifting during El Niño/La Niña are not unexpected, the observed oxygen changes are 2–3 times larger than what can be explained by cross-shore exchanges. In late summer 2010, oxygen levels at mid-depth of the water column reached values of 2.5 ml/L, which is much lower than normal oxygen levels at this time of the seasons, 4–5 ml/L. The extra uplifting of isopycnals related to the La Niña event can explain oxygen reductions only to roughly 3.5 ml/L. We find that the additional oxygen decrease beyond that is strongly correlated with decreased subsurface primary production and strengthened poleward flows by the California Undercurrent. The combined actions of these three processes created a La Niña-caused oxygen decrease as large and as long as the normal seasonal minimum during upwelling period in spring, but later in the year. With a different timing of a La Niña, the seasonal oxygen minimum and the La Niña anomaly could overlap to potentially create hypoxic events of previously not observed magnitudes.
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    Numerical and experimental investigations of the effective thermal conductivity of snow (2011)
    Wiley
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    Publication Date: 2011-12-01
    Description: We carried out numerical simulations of the conductivity of snow using microtomographic images. The full tensor of the effective thermal conductivity (keff) was computed from 30 three-dimensional images of the snow microstructure, spanning all types of seasonal snow. Only conduction through ice and interstitial air were considered. The obtained values are strongly correlated to snow density. The main cause for the slight scatter around the regression curve to snow density is the anisotropy of keff: the vertical component of keff of facetted crystals and depth hoar samples is up to 1.5 times larger than the horizontal component, while rounded grains sampled deeply in the snowpack exhibit the inverse behavior. Results of simulations neglecting the conduction in the interstitial air indicate that this phase plays a vital role in heat conduction through snow. The computed effective thermal conductivity is found to increase with decreasing temperature, mostly following the temperature dependency of the thermal conductivity of ice. The results are compared to experimental data obtained either with the needle-probe technique or using combined measurements of the vertical heat flux and the corresponding temperature gradient. Needle-probe measurements are systematically significantly lower than those from the two other techniques. The observed discrepancies between the three methods are investigated and briefly discussed.
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  • 193
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    Ocean fronts trigger high latitude phytoplankton blooms (2011)
    Wiley
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    Publication Date: 2011-12-02
    Description: Density fronts are ubiquitous features of the upper ocean. Here, numerical simulations show that restratification at fronts inhibits vertical mixing, triggering phytoplankton blooms in low-light conditions. The stability of the water column at fronts is set by a competition between frontal instabilities, which restratify the upper ocean, and turbulent mixing, which acts to destroy this stratification. Recent studies have found that frontal instabilities can restratify the upper ocean, even in the presence of strong surface cooling and destabilizing winds. During winter at high latitudes, primary production by phytoplankton is generally limited by low ambient light levels and deep turbulent mixing. When the turbulent mixing, inhibited by frontal restratification, becomes smaller than a ‘critical turbulence’ threshold, a phytoplankton bloom can develop. The finding that fronts can trigger phytoplankton blooms by reducing mixing, provides an explanation for satellite observations of high chlorophyll concentrations at high latitude fronts.
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    Erosion of the continental lithosphere at the cusps of the Calabrian arc: Evidence from S receiver functions analysis (2011)
    Wiley
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    Publication Date: 2011-12-02
    Description: Mediterranean tectonics has been characterized by an irregular, complex temporal evolution with episodic rollback and retreat of the subducted plate followed by period of slow trench-migration. To provide insight into the geodynamics of the Calabrian arc, we image the characteristics and lithospheric structure of the convergent, Apulian and Hyblean forelands at the cusps of the arc. Specifically we investigate the crustal and lithospheric thicknesses using teleseismic S-to-p converted phases, applied to the Adria-Africa plate margin for the first time. We find that the Moho in the Apulian foreland is nearly flat at ∼30 km depth, consistent with previous P receiver functions results, and that the Hyblean crustal thickness is more complex, which can be understood in terms of the nature of the individual pieces of carbonate platform and pelagic sediments that make up the Hyblean platform. The lithospheric thicknesses range between 70–120 km beneath Apulia and 70–90 km beneath Sicily. The lithosphere of the forelands at each end of the Calabrian arc are continental in nature, buoyant compared to the subducting oceanic lithosphere and have previously been interpreted as mostly undeformed carbonate platforms. Our receiver function images also show evidence of lithospheric erosion and thinning close to Mt. Etna and Mt. Vulture, two volcanoes which have been associated with asthenospheric upwelling and mantle flow around of the sides the slab. We suggest that as the continental lithosphere resists being subducted it is being thermo-mechanically modified by toroidal flow around the edges of the subducting oceanic lithosphere of the Calabrian arc.
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    On seeding, large-scale wave structure, equatorial spread F, and scintillations over Vietnam (2011)
    Wiley
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    Publication Date: 2011-10-18
    Description: Understanding the day-to-day variability in occurrence of equatorial spread F (ESF) remains as a high-priority objective in space weather research. A major difficulty has been an inability to resolve the roles being played by large-scale wave structure (LSWS) and the post-sunset rise (PSSR) of the equatorial F layer, in the production of ESF. In this paper, we show conclusively that total electron content (TEC), measured as a function of latitude and longitude, provides clear, routine descriptions of LSWS. Then, together with ionosonde data, we show, for the first time, that while a seed for LSWS can occur in the late afternoon, its amplification takes place mostly during the PSSR. Implications of these findings are discussed in light of existing theories.
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  • 196
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    Direct pore-level observation of permeability increase in two-phase flow by shaking (2011)
    Wiley
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    Publication Date: 2011-10-18
    Description: Increases in permeability of natural reservoirs and aquifers by passing seismic waves have been well documented. If the physical causes of this phenomenon can be understood, technological applications would be possible for controlling the flow in hydrologic systems or enhancing production from oil reservoirs. The explanation of the dynamically increased mobility of underground fluids must lie at the pore level. The natural fluids can be viewed as two-phase systems, composed of water as the wetting phase and of dispersed non-wetting globules of gas or organic fluids, flowing through tortuous constricted channels. Capillary forces prevent free motion of the suspended non-wetting droplets, which tend to become immobilized in capillary constrictions. The capillary entrapment significantly reduces macroscopic permeability. In a controlled experiment with a constricted capillary channel, we immobilize the suspended ganglia and test the model of capillary entrapment: it agrees precisely with the experiment. We then demonstrate by direct optical pore-level observation that the vibrations applied to the wall of the channel liberate the trapped ganglia if a predictable critical acceleration is reached. When the droplet begins to progressively advance, the permeability is restored. The mobilizing acceleration in the elastic wave, needed to “unplug” an immobile flow, is theoretically calculated within a factor of 1–5 of the experimental value. Overcoming the capillary entrapment in porous channels is hypothesized to be one of the principal pore-scale mechanisms by which natural permeabilities are enhanced by the passage of elastic waves.
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    Inferring ecosystem parameters from observation of vegetation patterns (2011)
    Wiley
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    Publication Date: 2011-10-19
    Description: Spatial organization of vegetation into periodic, coherent patterns arises from the interaction of positive and negative ecological feedbacks. Naturally, the patterns reflect the characteristics of the ecological processes that underlie their formation. Direct inference of the parameters describing these ecological processes from observations of vegetation spatial patterns has not been attempted. If successful, such inference can facilitate the parameterization and predictive use of vegetation pattern models. An inference technique based on nonlinear filtering is proposed here and applied to estimate the parameters of a single-equation phenomenological model of vegetation biomass patterning. Results derived from modeled biomass data indicate that for sufficiently accurate biomass observations (signal-to-noise ratios 〉4), and spatial resolution of better than 10% of the pattern wavelength, nonlinear filtering techniques recovered model parameters with high fidelity. When applied to real-world imagery, reasonable parameters within the pattern-forming regime were inferred. The study demonstrates, for the first time, the feasibility of inferring quantitative ecological information from spatial observations of vegetation distributions.
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    Rupture process of the 2011 Tohoku-Oki mega-thrust earthquake (M9.0) inverted from strong-motion data (2011)
    Wiley
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    Publication Date: 2011-10-20
    Description: We investigate the rupture process of the M9.0 Tohoku-Oki mega-thrust earthquake using the relatively low-frequency strong-motion records (0.01–0.125 Hz) observed at 36 K-NET and KiK-net stations, the epicentral distances of which range from 120 km to 400 km. The fault model is a rectangular plane, the length and width of which are 510 km along the Japan Trench and 210 km along subducting direction of the Pacific Plate, respectively. We perform the multi-time-window inversion analysis with a 30 × 30 km2 subfault. The derived slip model has one large slip area. This area extends from the region around the hypocenter to the shallow part of the fault plane and further to the north and south along the trench axis, located far off southern Iwate, Miyagi, and northern Fukushima prefectures. The seismic moment is 4.42 × 1022 Nm (Mw 9.0) and the maximum slip is 48 m. The slips near the coast are relatively small, except off Miyagi prefecture, which experienced a slip greater than 5 m. The shallow large slip area, which continuously ruptured from 60 s to 100 s after the initial break, radiated seismic waves rich in very-low-frequency content (
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    Transport layer structure in intense bed-load (2011)
    Wiley
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    Publication Date: 2011-10-20
    Description: We report laboratory experiments on intense bed-load driven by turbulent open-channel flows. Using high-speed cameras and a laser light sheet, we measured detailed profiles of granular velocity and concentration near the sidewall. The profiles provide new information on transport layer structure and its relation to the applied Shields stress. Contrary to expectations, we find that intense bed-load layers respond to changes in flow conditions by adjusting their granular concentration at the base, slightly above the bed. Two mechanisms account for the resulting behavior: stresses generated by immersed granular collisions, and equilibration of the otherwise unstable shear layer by density stratification. Without parameter adjustment, the deduced constitutive relations capture the responses of velocity, concentration, and layer thickness to a ten-fold increase in Shields stress.
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    Active aeolian processes on Mars: A regional study in Arabia and Meridiani Terrae (2011)
    Wiley
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    Publication Date: 2011-10-22
    Description: We present evidence of widespread aeolian activity in the Arabia Terra/Meridiani region (Mars), where different kinds of aeolian modifications have been detected and classified. Passing from the regional to the local scale, we describe one particular dune field in Meridiani Planum, where two ripple populations are distinguished by means of different migration rates. Moreover, a consistent change in the ripple pattern is accompanied by significant dune advancement (between 0.4–1 meter in one Martian year) that is locally triggered by large avalanche features. This suggests that dune advancement may be common throughout the Martian tropics.
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