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AGU journal highlights -- July 15, 2011

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL) and Geochemistry, Geophysics, Geosystems (G-Cubed).

In this release:

  1. Model gives 3-month warning of fire-prone season in Amazon forest
  2. Episodic tremor triggers small earthquakes
  3. Melting glaciers can change Earth's gravity field
  4. Model determines likelihood of freak waves
  5. Tremor activity changes following two San Andreas Fault earthquakes
  6. Droughts in Amazon forest increasing
  7. Ocean floor faulting explains differences in Central American lavas
  8. Ozone hole might affect Great Ocean Conveyor
  9. Chlorine radicals measured in Eyjafjallajkull volcanic plume
  10. High-detail snapshots of rare gigantic jet lightning to the ionosphere

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2011GL047392. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions who are registered with AGU also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at

1. Model gives 3-month warning of fire-prone season in Amazon forest

In the summer of 2005 the Amazon rainforest suffered widespread drought, being heralded at the time as the drought of the century. Because of the dehydrated conditions, supplemented by slash and burn agricultural practices, the drought led to widespread forest fires throughout the western Amazon, a portion of the rainforest usually too lush to support spreading wildfires. Only 5 years later, the 2005 season was outdone by even more widespread drought, with fires decimating over 3000 square kilometers (115.83 square miles) of western Amazonian rainforest. Blame for the wildfires has been consistently laid on deforestation and agricultural practices, but a convincing climatological explanation exists as well.

Drawing on precipitation records stretching back to 1970 and forest fire observations from the Moderate Resolution Imaging Spectroradiometer satellite, Fernandes et al. find that the occurrence of forest fires can be largely explained by July, August, and September precipitation anomalies. Previous research indicating a strong link between western Amazonian precipitation and the temperature of the tropical North Atlantic led the authors to investigate whether this long-distance relationship might hold any predictive power. Using an atmosphere-ocean coupled general circulation model, the researchers found that sea surface temperature predictions made as early as April could be used to statistically forecast precipitation anomalies, and thus the likelihood of forest fires, for the western Amazon. The authors suggest that 3 months warning of a fire-prone dry season could give authorities the necessary time to alert farmers and ranchers about the elevated risk of fire-based land clearing practices.

Source: Geophysical Research Letters, doi:10.1029/2011GL047392, 2011

Title: North Tropical Atlantic influence on western Amazon fire season variability

Authors: Katia Fernandes, Walter Baethgen, David G. DeWitt, Lisa Goddard and Dong Eun Lee: International Research Institute for Climate and Society, Columbia University, Palisades, New York, USA;

Sergio Bernardes: Department of Geography, University of Georgia, Athens, Georgia, USA;

Ruth DeFries and Maria Uriarte: Department of Ecology, Evolution and Environmental Biology, Columbia University, New York, New York, USA;

Waldo Lavado: Servicio Nacional de Meteorologa e Hidrologa, Lima, Peru;

Christine Padoch: Institute of Economic Botany, New York Botanical Garden, New York, New York, USA;

Miguel Pinedo Vasquez: Center for Environmental Research and Conservation, Columbia University, New York, New York, USA.

2. Episodic tremor triggers small earthquakes

It has been suggested that episodic tremor and slip (ETS), the weak shaking not associated with measureable earthquakes, could trigger nearby earthquakes, however, this had not been confirmed until recently. Vidale et al. monitored seismicity in the 4 months around a 16-day period of episodic tremor and slip in March 2010 in the Cascadia region. The authors observed five small earthquakes within the subducting slab during the ETS episode. They find that the timing and locations of earthquakes near the tremor suggests that the tremor and earthquakes are related. Furthermore, they observe that the rate of earthquakes across the area was several times higher within two days of tremor activity than at other times, adding to evidence of a connection between tremor and earthquakes.

Source: Geochemistry, Geophysics, Geosystems, doi:10.1029/2011GC003559, 2011

Title: Tiny intraplate earthquakes triggered by nearby episodic tremor and slip in Cascadia

Authors: John E. Vidale, Alicia J. Hotovec, Abhijit Ghosh, and Kenneth C. Creager: Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA;

Joan Gomberg: U.S. Geological Survey, Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA.

3. Melting glaciers can change Earth's gravity field

The Earth's rotation causes mass from the ductile mantle to bulge at the equator, making the radius of the Earth about 21 kilometers (13 miles) larger at the equator than at the poles. Over the past 20,000 or so years, the Earth has been becoming more round as it adjusts to the withdrawal of vast continental glaciers after the last ice agewithout the weight of ice pressing down, land has rebounded to give the Earth a more spherical shape. This in turn means that the Earth's geoidthe average gravity field across the globealso became more round. However, in the early 1990s, scientists began to notice that postglacial rebound was starting to become offset by something else, causing the Earth's gravity field to cease changing shape.

To learn more about what could be causing this change, Nerem and Wahr study measurements of gravity variations from the Gravity Recovery and Climate Experiment (GRACE) satellite. The authors' observations show that water from ice melting in Greenland and Antarctica flows into the oceans, changing the Earth's gravity field as mass once concentrated at the poles becomes spread over the entire Earth. They suggest that this transfer of mass away from high latitudes, which causes a flattening effect on the geoid, balances postglacial rebound at the poles and is the reason why the Earth's gravity field is no longer becoming rounder.

Source: Geophysical Research Letters, doi:10.1029/2011GL047879, 2011

Title: Recent Changes in the Earth's Oblateness Driven by Greenland and Antarctic Ice Mass Loss

Authors: R.S. Nerem: Colorado Center for Astrodynamics Research, Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado; and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA;

J. Wahr: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder Colorado; and Department of Physics, University of Colorado, Boulder, Colorado, USA.

4. Model determines likelihood of freak waves

Freak waves ocean waves more than twice the average wave height can rise up without warning on otherwise calm seas. Because they can arise suddenly and be quite large, these waves, also known as rogue waves, present a danger to ships. To work toward better understanding of the conditions under which these monster waves form, Waseda et al. analyzed wind and wave records from the northern North Sea from 2003 to 2005 and classified days into "freakish" and "nonfreakish" days based on freak wave occurrence. The authors find that on freakish days, an atmospheric pressure condition known as the Icelandic low was enhanced. This atmospheric pressure difference may promote the formation of freak waves by encouraging nonlinear focusing of waves. Knowing the meteorological conditions under which potentially dangerous freak waves are more likely to occur could help ships navigate safely.

Source: Geophysical Research Letters, doi:10.1029/2011GL047779, 2011

Title: Enhanced freak wave occurrence with narrow directional spectrum in the North Sea

Authors: T. Waseda and K. Ozaki: Department of Ocean Technology Policy and Environment, Graduate School of Frontier Sciences, The University of Tokyo, Chiba, Japan;

M. Hallerstig: Department of Ocean Technology Policy and Environment, Graduate School of Frontier Sciences, The University of Tokyo, Chiba, Japan; and Geophysical Institute, University of Bergen, Bergen, Norway;

H. Tomita: Department of Physics, Sophia University, Tokyo, Japan.

5. Tremor activity changes following two San Andreas Fault earthquakes

Large earthquakes change the stress on faults. This can either accelerate or decelerate deep fault creep, as suggested by changes in tremor activity rates. Analyzing tremor data from the San Andreas Fault in central California, Shelly and Johnson find that the 2003 San Simeon earthquake, with a magnitude 6.5, created a "stress shadow" north of Parkfield, causing tremor to stop abruptly there for about a month, while tremor south of Parkfield continued. On the other hand, tremor increased sharply following the 2004 magnitude 6.0 Parkfield earthquake. The authors examine the spatial and temporal variability of post-earthquake effects on the San Andreas Fault and note a depth dependence of tremor recurrence patterns. The study provides new information on the properties of these sections of the fault, as well as new insight into how earthquakes affect stress on a fault and deep fault motion.

Source: Geophysical Research Letters, doi:10.1029/2011GL047863, 2011

Title: Tremor reveals stress shadowing, deep postseismic creep, and depth-dependent slip recurrence on the lower-crustal San Andreas fault near Parkfield

Authors: David R. Shelly: U.S. Geological Survey, Menlo Park, California, USA;

Kaj M. Johnson: Dept. of Geological Sciences, Indiana University, Bloomington, Indiana, USA.

6. Droughts in Amazon forest increasing

In 2005 a severe, once-in-a-century drought struck the Amazon rain forest. Just 5 years later, an even more severe and widespread drought struck the region. How do these extreme events fit into the context of historical droughts? Marengo et al. looked at historical rainfall and river data sets to find out.

The authors find that the 2010 drought was unique. It started in early summer during an El Nio event and intensified due to warming of the tropical North Atlantic, which was warmer than in any previous year in the records since 1903, including 2005. Their analysis indicates that warming in the tropical North Atlantic affects the hydrology of the Amazon region by promoting a longer dry season, which can be aggravated if there was low rainfall in the previous wet season. The researchers observe that there has been an increase in dry and very dry events in the whole Amazon region since the middle 1970s as well as an increase in the length of the dry season.

Source: Geophysical Research Letters, doi:10.1029/2011GL047436, 2011

Title: The drought of 2010 in the context of historical droughts in the Amazon region

Authors: Jose A. Marengo, Javier Tomasella, Lincoln M. Alves, Wagner R. Soares, and Daniel A. Rodriguez: CCST, INPE, Cachoeira Paulista, Brazil.

7. Ocean floor faulting explains differences in Central American lavas

Off the west coast of Central America lies the Cocos tectonic plate. The plate's steady eastward progression forces it under the Caribbean and North American plates, driving seismic and volcanic activity. Along the Central American coast, the chemical content of the lava produced by such volcanism varies significantly, an unusual finding given that the raw material for the magmathe Cocos platefeeds the entire region. For instance, magma in central Nicaragua has a much higher ratio of barium to lanthanum than the magma in Costa Rica, with the concentrations of these and other trace elements serving as an indicator that distinct processes were involved in generating the different magma.

To identify structural properties that vary along the Cocos plate that may explain these differences, Van Avendonk et al. conducted a 396-kilometer (246-mile) sweep of its eastern boundary, collecting measurements of seismic velocity (i.e., how fast seismic waves are able to travel through the various rock layers). Previous research describes a complicated history of the Cocos plate. The northern half, which lies off Nicaragua, was the product of deep-ocean spreading at the East Pacific Rise (EPR). The Cocos-Nazca (CN) spreading region, along with intraplate volcanism near the Galpagos Islands, generated the southern portion of the plate. The authors find that seismic velocities within the northern portion of the plate are anomalously low. They suggest that as the Cocos plate is forced down, forming the Middle America Trench, the rock stretches and breaks. This high degree of faulting within the crust would allow seawater to penetrate into the mantle and form hydrated rocks called serpentinites. As the northern portion of the Cocos plate is subducted, these high serpentinite concentrations are carried deeper into the mantle, releasing their trapped water and significantly affecting the chemical composition of the magma.

Source: Geochemistry, Geophysics, Geosystems, doi:10.1029/2011GC003592, 2011

Title: Structure and serpentinization of the subducting Cocos plate offshore Nicaragua and Costa Rica

Authors: H.J.A. Van Avendonk: University of Texas Institute for Geophysics, Jackson School of Geosciences, Austin, Texas, USA;

W.S. Holbrook: University of Wyoming, Department of Geology and Geophysics, Laramie, Wyoming, USA;

D. Lizarralde: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, Massachusetts, USA;

P. Denyer: Escuela Centroamericana de Geologa, University of Costa Rica, San Pedro, Costa Rica.

8. Ozone hole might affect Great Ocean Conveyor

Previous studies have suggested that key aspects of the Southern Ocean are affected by elevated atmospheric carbon dioxide levels. Increasing greenhouse gas (GHG) concentrations, which strengthen surface winds over much of the Southern Ocean, may increase flow rates in the Antarctic Circumpolar Current (ACC), induce a temperature disparity between the northern and central Southern Ocean, and affect the strength of the meridional ocean circulation (MOC), also known as Great Ocean Conveyor. But GHGs are not the only set of chemicals arising from human activity that can trigger these changes. Stratospheric ozone depletion due to ozone depleting substances (ODSs) such as chlorofluorocarbons, whose production and use were heavily regulated by the 1987 Montreal Protocol, results in similar changes.

Using an atmosphere-ocean coupled general circulation model that allows for detailed calculations of stratospheric chemistry, Sigmond et al. simulate past and future changes for the Southern Ocean due to both greenhouse gases and ozone depleting substances. Their model calculations suggest that ODSs, which peaked in concentration in 1995, will be the dominant driver of changes in the ACC until the second quarter of the 21st century, at which point the monotonically increasing GHG levels will take over. Further, the authors find that the peak impact of ODSs on the ACC will occur a few decades after their peak concentration. The authors suggest that future research needs to take into account the effects of ozone depletionsomething that is not ordinarily done in investigations of Southern Ocean behavior.

Source: Geophysical Research Letters, doi:10.1029/2011GL047120, 2011

Title: Drivers of past and future Southern Ocean change: Stratospheric ozone versus greenhouse gas impacts

Authors: M. Sigmond: Department of Physics, University of Toronto, Toronto, Ontario, Canada;

M. C. Reader: School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada;

J. C. Fyfe and N. P. Gillett: Canadian Centre for Climate Modelling and Analysis, Environment Canada, Victoria, British Columbia, Canada.

9. Chlorine radicals measured in Eyjafjallajkull volcanic plume

When the Icelandic volcano Eyjafjallajkull erupted in spring 2010, it disrupted commercial air travel, stranding passengers across Europe and beyond. In response to the lack of information on the volcanic ash load and dispersion, scientific instruments were deployed on a number of special flights to observe the composition and chemistry of the volcanic plume, which included three deployments aboard a Lufthansa aircraft of the Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC; see observational instrument package. Baker et al. report on the first observation-based estimates of chlorine radical concentrations in the volcanic plume. Previous studies had suggested that chlorine radicals could exist in volcanic plumes. This study, the first to identify chlorine radical chemistry and quantify chlorine radicals in a volcanic plume, will help researchers to more fully understand volcanic chemistry, particularly halogen chemistry, and its effects on the atmosphere.

Source: Geophysical Research Letters, doi:10.1029/2011GL047571, 2011

Title: Investigation of chlorine radical chemistry in the Eyjafjallajkull volcanic plume using observed depletions in non-methane hydrocarbons

Authors: Angela K. Baker, Armin Rauthe-Schch, Tanja J. Schuck, and Carl A. M. Brenninkmeijer: Atmospheric Chemistry Division, Max Planck Institute for Chemistry, Mainz, Germany;

Peter F. J. van Velthoven: Royal Netherlands Meteorological Institute, De Bilt, Netherlands;

Adam Wisher and David E. Oram: National Centre for Atmospheric Science, School of Environmental Sciences, University of East Anglia, Norwich, UK.

10. High-detail snapshots of rare gigantic jet lightning to the ionosphere

In the ionosphere, more than 80 kilometers (about 50 miles) above Earth's surface, incoming radiation reacts with the thin air to produce highly charged ions, inducing an electric potential between the ionosphere and the surface. This charge difference is dissipated by a slow leak from the ionosphere during calm weather and reinvigorated by a charge built up near the surface during a thunderstorm. In 2001, however, researchers discovered gigantic jets, powerful lightning that arcs from tropospheric clouds up to the ionosphere, suggesting there may be an alternate path by which charge is redistributed. Gigantic jets are transient species, and little is known about how much charge they can carry, how they form, or how common they are.

In a step toward answering these questions, Lu et al. report on two gigantic jets that occurred near very high frequency (VHF) lightning detection systems, which track the development of lightning in three spatial dimensions, giving an indication of the generation mechanism. The researchers also measured the charge transfer in the two GJs through remote sensing of magnetic fields. They find that both jets originated from the development of otherwise normal intracloud lightning. The dissipation of the cloud's positively charged upper layer allowed the negative lightning channel to break through and travel up out of the top of the cloud to the ionosphere. The first jet, which occurred off the coast of Florida, leapt up to 80 km (50 mi), depositing 110 Coulombs of negative charge in 370 milliseconds. The second jet, observed in Oklahoma, travelled up to 90 km (56 mi), raising only 10-20 C in 300 ms. Each new observation of gigantic jets such as these can provide valuable information toward understanding the novel atmospheric behavior.

Source: Geophysical Research Letters, doi:10.1029/2011GL047662, 2011

Title: Lightning development associated with two negative gigantic jets

Authors: Gaopeng Lu, Steven A. Cummer and Jingbo Li: Electrical and Computer Engineering Department, Duke University, Durham, North Carolina, USA;

Walter A. Lyons: FMA Research, Fort Collins, Colorado, USA;

Paul R. Krehbiel, William Rison, Ronald J. Thomas, Harald E. Edens and Mark A. Stanley: Langmuir Laboratory, New Mexico Institute of Mining and Technology, Socorro, New Mexico, USA;

William Beasley: School of Meteorology, University of Oklahoma, Norman, Oklahoma, USA;

Donald R. MacGorman: National Severe Storms Laboratory, Norman, Oklahoma, USA;

Oscar A. van der Velde: Electrical Engineering Department, Technical University of Catalonia, Terrassa, Spain;

Morris B. Cohen: STAR Laboratory, Stanford University, Stanford, California, USA;

Timothy J. Lang and Steven A. Rutledge: Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA.


Contact: Maria-Jos Vias
American Geophysical Union

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