Public Release: 

AGU journal highlights -- Dec. 31, 2007

American Geophysical Union

1. Loading by Hawaii's volcanoes: An explanation for the 15 October 2006 earthquakes

On 15 October 2006, two earthquakes shook the northwest coast of the island of Hawaii six minutes apart - the Kiholo Bay event (magnitude 6.7) and the shallower but still unexpectedly deep Mahukona event (magnitude 6.0). Their close proximity in space and time suggests a common origin, but sharp contrasts in mechanism and depth present an unusual fault-aftershock relationship. McGovern hypothesizes that these "fraternal twin" earthquakes are the divergent outcomes of a single process: downward flexing of the lithosphere in response to loading by the massive Hawaiian volcanoes. The initial Kiholo Bay event is readily explained through models of lithospheric flexure as brittle failure in a high-stress zone in the lower lithosphere. However, an explanation for the depth (also within the mantle) of the Mahukona event requires models that incorporate a strong stiffness contrast between crust and mantle in order to produce peak upper lithosphere stresses at the stiffer mantle's top. This compressional high-stress zone also traps magma near the base of the crust, thereby explaining the seismically observed thickening from below of Hawaiian volcanoes.

Title: Flexural stresses beneath Hawaii: Implications for the October 15, 2006, earthquakes and magma ascent

Author: Patrick J. McGovern: Lunar and Planetary Institute, Universities Space Research Association, Houston, Texas, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031305, 2007; http://dx.doi.org/10.1029/2007GL031305


2. Isolating the signal of global warming in the ocean

Identifying the degree to which global warming heats the ocean is challenging because changes in local ocean circulation patterns can influence local temperature readings. Although the Fourth Assessment of the Intergovernmental Panel on Climate Change reported that the ocean has heated about 0.21 watts per square meter per decade over the years between 1961 and 2003, panel scientists note that this value has considerable spatial, interannual, and interdecadal variability. To help resolve this variability, Palmer et al. present a new analysis of the historical ocean observational temperature record which is designed to filter out of each ocean profile the local dynamical changes that mask true readings of underlying warming. They found that all waters warmer that 14 degrees Celsius show large reductions in interannual to interdecadal variability and a more spatially uniform upper ocean warming trend of about 0.12 watts per square meter each decade for the last 40 years. The authors suggest that their analysis can help better evaluate coupled atmosphere-ocean climate models.

Title: Isolating the signal of ocean global warming

Authors: M. D. Palmer and T. J. Ansell: Hadley Centre for Climate Change, Met Office, Exeter, U.K.;

K. Haines: Environmental Systems Science Centre, Reading, U.K.;

S. F. B. Tett: Department of Earth Science, University of Edinburg, Edinburg, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031712, 2007; http://dx.doi.org/10.1029/2007GL031712


3. New insight into millennial-scale climate events: Comparisons between China and Brazil

A heavily debated topic among climate scientists involves whether millennial-scale climate fluctuations occur because of unusually strong El Nino events or because of changes in the high-latitude ocean circulation, the latter of which also effects the dynamics of tropical air circulation. To help resolve this debate, Wang et al. study speleothems (stalactites, stalagmites, and other cave formations), which provide valuable climate records. Ratios of the formations' light and heavy oxygen isotopes give insight into temperature and precipitation rates, while uranium isotope dating provides a chronological sequence of calcite deposition. Through comparisons of speleothems in China and Brazil, the authors find that rainfall patterns in China are out of phase with those from Brazil, suggesting that interhemispheric precipitation is anticorrelated. Further, speleothems from northern and southern Brazil correlate well with each other, a result not predicted if El Nino dominated precipitation. The authors conclude that the phase relationships among these records support the hypothesis that abrupt climate events during the last glacial period are triggered by oceanic circulation in the high latitudes and enhanced by tropical feedbacks.

Title: Millennial-scale precipitation changes in southern Brazil over the past 90,000 years

Authors: Xianfeng Wang, R. L. Edwards, Hai Cheng, Emi Ito, and Maniko Solheid: Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, U.S.A.;

Agusto S. Auler: Instituto do Carste, Belo Horizonte, Brazil;

Yongjin Wang and Xinggong Kong: College of Geography Science, Nanjing Normal University, Nanjing, China.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031149, 2007; http://dx.doi.org/10.1029/2007GL031149


4. Modulating ionospheric currents through high-frequency signals

The High Frequency Active Auroral Research Program (HAARP) seeks to understand, simulate, and control ionospheric processes that might alter the performance of communication and surveillance systems. At the HAARP field site located near Gakona, Alaska, scientists conducted a test in April 2003 where a high-frequency signal was delivered to a ground-based antenna array and then transmitted upward to the ionosphere. This signal produces extremely low and very low frequency disturbances that are then observed by sensitive instruments at the HAARP facility, helping scientists understand through signal retrieval patterns the dynamics of plasmas in the auroral electrojet. Payne et al. study these retrieved signal patterns and compare them to predictions made by computer models. The authors find good agreement between the ground electric field patterns observed following the HAARP experiment and those predicted by the model.

Title: HF modulated ionospheric currents

Authors: J. A. Payne, U. S. Inan, F. R. Foust, T. W. Chevalier, and T. F. Bell: Space, Telecommunications, and Radioscience Laboratory, Stanford University, Stanford, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031724, 2007; http://dx.doi.org/10.1029/2007GL031724


5. Effects of lower stratospheric ozone depletion

Antarctica's seasonal ozone hole, which reaches its maximum size in October and November, has been associated with observed decreases in surface temperatures about two months later. Past research has shown that surface temperatures are most sensitive to ozone loss at the boundary between the troposphere and the stratosphere, below an altitude of 15 kilometers (9 miles). This observation has caused some scientists to hypothesize that Antarctic surface cooling might be forced by ozone depletion in the lowermost stratosphere. Keeley et al. test whether this temperature response is driven by ozone depletion in the middle or lowermost stratosphere. Using an atmospheric model coupled to an idealized model of the ocean, the authors find that ozone loss in the middle stratosphere dominates temperature patterns in the troposphere. The authors also find that a tropospheric response in the fall/winter can be associated with ozone fluctuations in the middle stratosphere, a result that agrees well with observations. Nonetheless, the mechanisms underlying the tropospheric responses and the reasons for its lag compared with the stratospheric forcing have not been fully identified.

Title: Is Antarctic climate most sensitive to ozone depletion in the middle or lower stratosphere?

Authors: S. P. E. Keeley and N. P. Gillett: Climatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, U.K.;

D. W. J. Thompson: Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, U.S.A.;

S. Solomon: Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, U.S.A.;

P. M. Forster: School of Earth and Environment, University of Leeds, Leeds, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031238, 2007; http://dx.doi.org/10.1029/2007GL031238


6. Heavy negative ions on Titan

Saturn's moon Titan has a thick atmosphere, formed mostly of nitrogen with a small amount of methane. Observations from the Cassini spacecraft have shown that Titan's ionosphere, extending from 950 to 1200 km (590 to 750 miles) in altitude, contains a rich positive ion population and some negative ions. Coates et al. present a comprehensive view of these negative ions using Cassini data, showing that heavy ion chemistry takes place in Titan's upper atmosphere at altitudes much higher than had been predicted by models, and that throughout the ionosphere, a large population of negative ions exist. Because of this large population, negative ions might play an important role in ionospheric dynamics, particularly in the formation of organic-rich aerosols that eventually fall to the surface. Thus the abundance and behavior of negative ions on Titan may have long-term effects on the composition of the moon's surface.

Title: Discovery of heavy negative ions in Titan's ionosphere

Authors: A. J. Coates and G. R. Lewis: Mullard Space Science Laboratory, University College London, Dorking, U.K.; Center for Planetary Sciences, University College London, Dorking, U.K.

F. J. Crary, D. T. Young, and J. H. Waite Jr.: Space Science and Engineering Division, Southwest Research Institute, San Antonio, Texas, U.S.A.;

E. C. Sittler Jr.: NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL030978, 2007; http://dx.doi.org/10.1029/2007GL030978


7. Methane release from massive underwater seeps in California

Methane, a potent greenhouse gas, warms the Earth 23 times more than carbon dioxide when averaged over a century. Thus understanding regional and global environments can be aided by monitoring natural methane seeps from hydrocarbon deposits located beneath the ocean floor. Large quantities of methane are emitted from the seafloor into the ocean near Coal Oil Point, Santa Barbara Channel, California, and were observed down current at 79 surface stations in a 280-square-kilometers (110-square-miles) study area. Through analyzing this data, Mau et al. find that the methane plume spread over 70 square kilometers (27 square miles). Monthly sampling at 14 stations showed variable methane concentrations at the ocean's surface that correlated with medium-scale changes in surface currents. Only one percent of the dissolved methane originating from the Coal Oil Point seeps enters the atmosphere within the study area, leading the authors to hypothesize that most of the dissolved methane is transported below the surface away from the seep area and is oxidized by microbial activity.

Title: Dissolved methane distributions and air-sea flux in the plume of a massive seep field, Coal Oil Point, California

Authors: Susan Mau: Department of Earth Science, University of California, Santa Barbara, California, U.S.A.; Also at IFM-GEOMAR Leibniz-Institut fuer Meereswissenschaften, Kiel University, Kiel, Germany;

David L. Valentine, Jordan F. Clark, Justin Reed: Department of Earth Science, University of California, Santa Barbara, California, U.S.A.

Richard Camilli: Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.;

Libe Washburn: Institute for Computational Earth System and Department of Geography, University of California, Santa Barbara, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031344, 2007; http://dx.doi.org/10.1029/2007GL031344


8. Eruption forecasting and evacuation

Evacuating people from threatened areas during volcanic unrest is critical to reducing risk, yet the decision to evacuate is usually made with little quantitative decision support. Noting that minimizing the probability of false alarm is not optimal when many lives are at risk, Marzocchi and Woo propose a strategy to integrate eruption forecasting with cost-benefit analyses. Using Italy's Mount Vesuvius as a case study, the authors analyze a simulation carried out in 2006 where scientists provided a realistic preeruptive scenario that was disbursed through a series of bulletins. On the basis of this, the order to evacuate was issued after the sixth bulletin. Using the information from the bulletins, the authors conduct cost-benefits analyses where the decision to evacuate was weighed against the probability that 10 percent of evacuees would be saved by an evacuation call. According to their threshold, the order to evacuate should have been issued after the fourth bulletin. The authors suggest that governments should assess their tolerance of lives at risk prior to an eruption, and then use cost-benefit analyses to inform evacuation decisions.

Title: Probabilistic eruption forecasting and the call for an evacuation

Authors: Warner Marzocchi: Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy;

Gordon Woo: Risk Management Solutions, London, United Kingdom.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031922, 2007; http://dx.doi.org/10.1029/2007GL031922


9. El Niño events are triggered by surges of cold air from Asia

Two large-scale weather circulation patterns, the Northern Hemisphere Annular Mode/Arctic Oscillation (NAM) and the El Niño-Southern Oscillation (ENSO), play major roles in influencing midlatitude climate. For example, unusual amplifications of NAM result in unusual seasonal weather such as the heat wave Europe experienced in the summer of 2003. Past research has demonstrated that anomalies in NAM in the early spring can modulate westerly wind bursts in the western equatorial Pacific; these wind bursts are known to trigger El Niño outbreaks. To more concretely show how NAM causes El Niño events, Nakamura et al. hypothesize that an anomalous NAM influences a surge of cold air from Asia toward the tropics. Through statistical analyses of tropical atmospheric heat and moisture, the authors show that the surge of cold air strengthens atmospheric heating in the western tropical Pacific through increased air-sea interaction. This, in turn, induces the westerly wind bursts that excite El Niño events. Thus the cold surge acts as a messenger from NAM to ENSO.

Title: The importance of cold and dry surges in substantiating the NAM and ENSO relationship

Authors: Tetsu Nakamura and Haruhisa Shimoda: Graduate School of Earth and Environmental Science, Tokai University, Hiratsuka, Japan;

Yoshihiro Tachibana: Institute for Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology Center, Yokosuka, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031220, 2007; http://dx.doi.org/10.1029/2007GL031220


10. Land use changes in southeastern United States help to cool regional surface temperatures

Land cover changes in the southeastern United States are dominated by the conversion of abandoned agricultural land to forests. To quantify the impact of such conversion on surface temperature, Juang et al. study three adjacent ecosystems: a grass-covered old field, a planted pine forest, and a hardwood forest experiencing similar climatic and soil conditions. They find that changes in surface reflective properties can warm the surface by 0.9 degrees Celsius for the grassland to pine forest conversion, and 0.7 degrees Celsius for the grassland to hardwood forest conversion. However, changes in atmosphere-ecosystem energy exchange cool the surface by 2.9 degrees Celsius for conversion from grassland to pine forest and 2.1 degrees Celsius for conversion from grassland to hardwood forest. The resulting net cooling effect agrees well with models. Further, the authors note that the cooling pattern, as evidenced by other research at the site, is accompanied by increased carbon dioxide sequestration.

Title: Separating the effects of albedo from eco-physiological changes on surface temperature along a successional chronosequence in southeastern US

Authors: Jehn-Yih Juang, Kimberly Novick: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, North Carolina, U.S.A;

Gabriel Katul: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, North Carolina, U.S.A; also at Department of Civil and Environmental Engineering, Duke University Durham, North Carolina U.S.A.;

Mario Siqueira: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, North Carolina, U.S.A; also at Departmento de Engenharia Mecânica, Universidade de Brasília, Brazil;

Paul Stoy: Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031296, 2007; http://dx.doi.org/10.1029/2007GL031296


11. Salinity in the Mediterranean Sea is affected by inflow through Gibraltar

The Mediterranean Sea has been rising in salinity at a rate of 0.01 units each decade, an increase that has mostly been attributed to local environmental changes such as fluctuations in river runoff. Noting that prevailing theories assume an unchanged inflowing of Atlantic water to the Mediterranean, Millot studies data taken from a mooring on the Moroccan shelf in the Strait of Gibraltar. He finds that the salinity of water inflowing from the Atlantic displays a marked seasonal variability and a huge interannual variability, having continuously increased by about 0.05 units each year for the past four years. This yearly trend is dozens of times larger than the Mediterranean's decadal salinity trend. The author noted that reliable data analyses and numerical models dealing with Mediterranean hydrology must consider the interannual variability of Atlantic water inflow.

Title: Interannual salinification of the Mediterranean inflow

Authors: Claude Millot: Laboratoire d'Océanographie et de Biogéochimie, Antenne LOB-COM-CNRS, La Seyne-sur-mer, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031179, 2007; http://dx.doi.org/10.1029/2007GL031179


12. Past ice elevations in the West Antarctic Ice Sheet

Observations of dramatic loss of ice shelves and acceleration of outlet glaciers in Antarctica have caused scientists to worry about whether the West Antarctic Ice Sheet (WAIS) is susceptible to rapid collapse in response to warmer sea and air temperatures. Moreover, recent field results indicate that for about the past 8000 years, the WAIS has retreated without substantial sea level or climate forcing. This suggests that current ice loss trends could continue in the absence of further external forcing and that current estimates of sea level rise may be underestimated. Noting that a prerequisite to predicting the response of the WAIS to future climate changes is successfully simulating past WAIS history, Ackert et al. seek to better quantify benchmarks for these models, such as past ice elevations, in key areas of the WAIS. They find that cosmogenic exposure ages of glacial boulders show that about 11,000 years ago, the WAIS high-stand near the ice divide was about 125 meters (410 feet) above the present surface, considerably lower than elevations proposed by many models.

Title: Ice elevation near the West Antarctic Ice Sheet Divide during the last glaciation

Authors: Robert P. Ackert, Jr., and Sujoy Mukhopadhyay: Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A.;

Byron R. Parizek: Department of Physics, College of New Jersey, Ewing, New Jersey; also at Mathematics and Earth Sciences, Pennsylvania State University, DuBois, Pennsylvania, U.S.A.;

Harold W. Borns: Climate Change Institute, University of Maine, Orono, Maine, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031412, 2007; http://dx.doi.org/10.1029/2007GL031412


13. Large overtides in deep Atlantic Ocean

The Earth's principal lunar tide, termed by ocean physicists as M2, has a period of roughly 12 hours. A second harmonic of this tide, termed the M4 overtide, has a period of about 6.2 hours and is formed in shallow water by nonlinear processes. This overtide is generally no more than a few millimeters in amplitude in the deep ocean. Noting that such small periodic waves can now be mapped by analyzing altimetry observations from the long (15-year) time series of the Topex/Poseidon and Jason 1 satellites, Ray finds that in a few locations of the deep Atlantic Ocean, M4 amplitudes reach nearly 10 millimeters, confirming recent modeling efforts. The author proposes that these relatively large amplitudes represent a complex interference pattern of free waves radiating from a few shallow sources, especially the Patagonian Shelf. The tides also are seen to show standing wave resonance patterns in the Gulf of Guinea. The author expects that further work will reveal the decay scales of these free waves and the trade-off between radiation and dissipation.

Title: Propagation of the overtide M4 through the deep Atlantic Ocean

Authors: Richard D. Ray: NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031618, 2007; http://dx.doi.org/10.1029/2007GL031618


14. Analysis of Europe's anomalously high 2006 fall/winter temperatures

Unparalleled since the beginning of the twentieth century, persistent high daily minimal temperatures occurred in Europe during the fall and winter of 2006-2007, making this period the warmest on record. To see whether this anomalous warm period was the result of long-term fluctuations in weather regimes, Yiou et al. analyze about 100 years of data from ground-based stations that record surface temperature and weather patterns in Europe. They find that though the 2006 fall/winter atmospheric flow was favorable to warmth, weather alone cannot explain the exceptional anomaly, because the observed temperatures remained well above those found for analogue atmospheric circulations in other fall and winter seasons. The authors also find that the departure from average temperatures during this anomalously warm fall/winter was similar in scale to the heat wave that struck Europe during the summer of 2003. Such results suggest that the main drivers of recent European warming are not merely changes in regional atmospheric flow and weather regime frequencies but are a more global phenomenon.

Title: Inconsistency between atmospheric dynamics and temperatures during the exceptional 2006/2007 fall/winter and recent warming in Europe

Authors: P. Yiou, R. Vautard, and P. Naveau: Laboratoire des Sciences du Climat et de l'Environnement, Institut Pierre-Simon Laplace, UMR CEA-CNRS-UVSQ, Gif-sur-Yvette, France;

C. Cassou: Centre Europeen de Recherche et de Formation Avancée en Calcul Scientifique, CNRS URA 1875, Toulouse, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031981, 2007; http://dx.doi.org/10.1029/2007GL031981


15. A seasonal pattern of future climate change in Europe

Analyses of recent global climate change simulations show that Europe and the Mediterranean are particularly sensitive to global warming. In particular, these simulations reveal consistent patterns of 21st-century temperature and precipitation change over Europe, such as increased winter precipitation over central and northern Europe and decreased precipitation and large warming in summer over western and central Europe and the Mediterranean. Noting that these patterns are also found in regional climate simulations, Giorgi and Coppola hypothesize that Europe experiences a marked seasonal and latitudinal pattern related to future climate change. They term this pattern the European Climate-Change Oscillation (ECO). Using the same ensemble of models that supported the Intergovernmental Panel on Climate Change Fourth Assessment Report, the authors find signatures of ECO in temperature, precipitation, and air circulation patterns. They define two indices that can be used to measure ECO's strength based on latitudinal and seasonal climate change gradients.

Title: European climate-chance oscillation (ECO)

Authors: Filippo Giorgi and Erika Coppola: Physics of Weather and Climate Group, Abdus Salam International Centre for Theoretical Physics, Trieste, Italy.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031223, 2007; http://dx.doi.org/10.1029/2007GL031223


16. Titan's plasma wake

Titan, the largest moon of Saturn, is the only moon known to have a dense atmosphere. Titan's climate and weather patterns--along with other properties--have piqued the interest of scientists who see the moon as analogous to early Earth. When Voyager 1 passed through the wake of Titan in 1980, clear signatures of the presence of an induced magnetosphere were observed and scientists began to understand how the plasma in Saturn's magnetosphere interacts with the ionosphere of Titan. The Cassini spacecraft passed through the wake of Titan in late 2005, allowing comparison of observations with those taken from Voyager. Modolo et al. study the newly acquired Cassini data and find that there is a strong asymmetry in the wake of Titan's plasma as it orbits around Saturn. They find that the ionosphere was seven times denser on the Saturn-facing side of Titan than on the opposite side, showing that the Saturn-facing side is connected to Saturn's sunlit ionosphere. Further, the authors find that heavy and light ions dominate the Saturn and anti-Saturn side, respectively.

Title: The far plasma wake of Titan from the RPWS observations--a case study

Authors: R. Modolo, J.-E. Wahlund, and R. Boström: Swedish Institute of Space Physics, Uppsala, Sweden;

P. Canu: Centre d'Etudes des Environnements Terrestre et Planétaires, Institut Pierre Simon Laplace, Velizy, France;

W. S. Kurth and D. Gurnett: Department of Physics and Astronomy, University of Iowa, Iowa City, U.S.A.;

G. R. Lewis and A. J. Coates: Mullard Space Science Laboratory, University College London, Dorking, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030482, 2007; http://dx.doi.org/10.1029/2007GL030482

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