AGU journal highlights - 27 April 2005

04/27/05

Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GL). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

You may read the scientific abstract for any of these papers by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the portion of the doi (digital object identifier) following 10.1029/ (e.g., 2004GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

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1. Computer simulation produces El Nino-like climate cycles

Results from a computer simulation of climate fluctuations in the Pacific Ocean may help climatologists better understand long-term cycles such as the El Nino-Southern Oscillation (ENSO). Tourre et al. modeled sea-surface temperatures and sea-level air pressure over a 200-year period to determine if cyclic patterns would emerge. During the simulation, the sea temperature and air pressure anomalies varied in two distinct cycles of unusual fluctuations. These fluctuations moved around the Pacific, one repeating every 8-12 years and another every 18-25 years. The researchers suggest that the shorter cycle can be viewed as El Nino oscillating more slowly than usual, and that the variations in sea-surface temperature in the longer cycle might influence the intensity and frequency of El Nino. With a few exceptions, the patterns seen in the simulation matched observations of actual climate variation related to El Nino cycles. The authors suggest that this is evidence that the type of model they used can accurately portray long-term climate cycles in the Pacific Ocean.

Title: Quasi-decadal and inter-decadal climate fluctuations in the Pacific Ocean from a CGCM

Authors:
Yves M. Tourre, Carole Cibot, Laurent Terray, Warren B. White, and Boris Dewitte, Centre National d'Etudes Spatiales, Toulouse, France.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022087, 2005

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2. Molten rock makes big earthquakes bigger

During an earthquake, the friction of rock plates grinding against each other creates high heat, which can melt rock at the interface. The larger the rock plates, the more heat that is generated. To simulate two plates sliding against each other, Spray used a modified friction welding device to spin one small cylinder of granite against another at high speed and pressure. He measured the velocity of the spinning granite cylinders, the force with which they were pushed together, and the temperature at their interface. When the temperature reached 1,150 degrees Celsius [2,150 degrees Fahrenheit]--the melting point of one of granite's principal components--the spinning cylinder accelerated, and the temperature dropped dramatically. Spray attributes the sudden change to lubrication by molten ground-up rock at the interface of the rock surfaces. He says the results contradict previous assumptions that viscous molten rock limits slipping of plates during an earthquake. He concludes that the high heat generated when large plates slip against each other melts rock which actually accelerates the slipping, thus resulting in more violent shaking than expected.

Title: Evidence for melt lubrication during large earthquakes

Authors:
John G. Spray, University of New Brunswick, Fredericton, New Brunswick, Canada.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022293, 2005

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3. New observations of Yellowstone volcanic activity

The volcanic activity in and around Yellowstone National Park results from the Earth's crust moving over a hotspot in the mantle. Characterizing the shape and dynamics of the partially molten rock that comprises the hotspot helps scientists understand historic and future volcanic activity in the region. Using a subsurface imaging technique, Yuan and Dueker observed a column of partially molten, upward-moving rock extending 500 kilometers [300 miles] below the Yellowstone volcanic caldera. The plume of hot rock was 100 kilometers [60 miles] in diameter and tilted 20 degrees to the northwest. The authors also discerned an adjacent area of relatively rapidly downward-moving rock that extended 250 kilometers [160 miles] below Wyoming's Wind River basin. They speculate that a "pond" of hot rock located 700 to 1,000 kilometers [400 to 600 miles] beneath the western United States might be the source of the plume they observed below Yellowstone. They speculate that the downward-moving rock under Wind River basin might be balancing the upward movement of rock under Yellowstone.

Title: Teleseismic P-wave tomogram of the Yellowstone plume

Authors:
Huaiyu Yuan and Ken Dueker, University of Wyoming, Laramie, Wyoming, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022056, 2005

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4. Breaking the mantle plume mold

The conventional image of a mushroom-shaped deep mantle plume may be too simplistic. Farnetani and Samuel investigate how plume shape and dynamics can be modified by chemically denser material at the base of Earth's mantle and by factors influencing the plume's journey to the surface. Mantle plumes modeled under complex conditions, such as the phase transition at 660 kilometers [410 miles] depth and the mantle "wind" exhibit a variety of shapes and sizes. These weird shapes are consistent with recent seismic images of plumes and superplumes in the deep mantle. In these simulations, the plume's arrival at the surface also presents some surprises: rather than the expected head-first, tail-last scenario, in some cases only a narrow plume tail reached the surface. In this way, a volcanic chain generated by a deep mantle plume may lack the initial flood volcanism associated with the large plume head. The authors also show that the internal structure of the plume tail is more complex than expected.

Title: Beyond the thermal plume paradigm

Authors:
C.G. Farnetani and H. Samuel, Institut de Physique du Globe, Paris, France.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022360, 2005

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5. Ocean cycling depends on small salinity differences

A computer simulation suggests that circulation of the world's oceans, known as the "global conveyor," may depend on slight differences in salt concentrations between the North Atlantic and the North Pacific oceans. In their experiments, Seidov and Haupt modeled the movement of ocean waters over 10,000 years. The only thing that varied between the five different simulations they ran was the movement of freshwater, through evaporation and precipitation, to and from the ocean surface waters. The simulation in which the surface waters of the Northern Pacific Ocean were set to have slightly higher salinity resulted in the best approximation of the world's current ocean circulation pattern. The authors conclude that salinity differences between the North Atlantic and North Pacific played a key role in the development of the global conveyor, and that movements of fresh water in the Southern Hemisphere have less influence on this process. They point out that if, as recent evidence suggests, the Atlantic Ocean is becoming less salty, it may affect future patterns of global ocean circulation.

Title: How to run a minimalist's global ocean conveyor

Authors:
Dan Seidov and Bernd J. Haupt, Pennsylvania State University, University Park, Pennsylvania, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022559, 2005

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6. Antarctic glaciers shrinking due to ice shelf collapse

New research shows that glaciers on the West Antarctic Peninsula are shrinking, following the collapse of the Wordie Ice Shelf. The study used mass and velocity measurements taken from 1995 to 2004 from satellites and airplanes to determine if the glaciers were growing or shrinking. Rignot et al. found that loss of glacier mass from melting and movement into the ocean exceeded snowfall accumulation for several glaciers on the peninsula. While decreased snowfall and increased melting probably play a part, the researchers believe that the collapse of the Wordie Ice Shelf, which used to block the glaciers' movement into Wordie Bay, is the most important factor in their decreasing size. The ice shelf weakened and broke up between 1974 and 1996, due to dramatic increases in air temperature in the region. Antarctic Peninsula glaciers were previously excluded from studies to determine the effects that melting glaciers have on sea level. The authors propose that as the region continues to warm, the Wordie Bay glaciers will melt more quickly, causing sea level to rise.

Title: Recent ice loss from the Fleming and other glaciers, Wordie Bay, West Antarctic Peninsula

Authors:
E. Rignot, G. Casassa, S. Gogineni, P. Kanagaratnam, W. Krabill, H. Pritchard, A. Rivera, R. Thomas, J. Turner, and D. Vaughan, California Institute of Technology, Pasadena, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2004GL021947, 2005

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7. Magnetic disconnection from the Sun

A survey of recent data from the Advanced Composition Explorer (ACE) spacecraft during a rare solar episode has allowed researchers to identify a magnetic reconnection exhaust event and develop better techniques to observe the features in the future. Gosling et al. interpreted the exhaust from evidence of accelerated plasma flow and increased proton density and temperature, combined with decreases in the magnetic field strength compared to the surrounding solar wind. Magnetic reconnection describes a process where pairs of magnetic field lines merge to produce changes in the field; the exhaust from such events comes from solar wind changes affected by the magnetic field shifts. The authors note that specific properties of charged particles within the solar wind can be used to trace the exhaust with relative certainty. Using their technique, the researchers identified 42 exhaust events in approximately seven years of data, though the incident they describe is the only time that produced magnetic field lines disconnected from the Sun.

Title: Magnetic disconnection from the Sun: Observations of a reconnection exhaust in the solar wind at the heliospheric current sheet

Authors:
Jack T. Gosling, R. M. Skoug, Los Alamos National Laboratory, Los Alamos, New Mexico, USA;
D. J. McComas, Southwest Research Institute, San Antonio, Texas, USA;
C. W. Smith, University of New Hampshire, Durham, New Hampshire, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022046, 2005.

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8. Resolving the motion of the Burmese arc

A new tectonic model for the Burmese arc region of the Indian tectonic plate may help resolve a longstanding discrepancy about whether previous earthquakes in the area were caused by inter-plate or intraplate motion. Rao and Kalpna analyzed data from 60 shallow quakes along the Indian lithospheric plate observed over approximately the past 30 years and determined the depth of the initial stress from the various tremors. The authors then characterized the quake movement, finding that the majority of the movement in the 0-90 kilometer [0-60 mile] depths was due to a strike-slip mechanism, while the events in the 90-140 kilometer [60-90 mile] range were exclusively caused by reverse faulting. The researchers suggest that the arc region is made up of distinct upper and lower parts, with the upper section governed by horizontal plate tectonic forces, while the lower portion is directed by tensile gravitational loading on the slab. They report that a model supporting their findings is confirmed by seismographic images from the region.

Title: Deformation of the subducted Indian lithospheric slab in the Burmese arc

Authors:
N. Purnachandra Rao, Kalpna, National Geophysical Research Institute, Hyderabad, India.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022034, 2005.

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