AGU Journal highlights - 1 June 2005

I. 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.

1. Complex deformation found in subduction zone near Japan

Where tectonic plates collide, one plate moves down, or is subducted, under the other, and deforms due to high pressure and heat in the Earth's mantle. Anglin and Fouch used recordings of earthquake shockwaves to observe subduction processes in the Izu-Bonin subduction zone southeast of Japan. The orientation of rock crystals in the mantle affect the speed at which the shockwaves travel through the mantle rock, a property known as anisotropy. Recordings of the waves can be used to characterize the movement and composition of rock at subduction zones. The authors found a complex pattern of deformation in the Pacific plate as it moves under the Philippine Sea plate at Izu-Bonin. Mantle rock in Izu-Bonin appeared to be moving parallel to the line of convergence of the two plates and no longer tied to the downward moving slab, a feature found in several other subduction zones. The researchers propose that similar mantle rock dynamics will be found in other, as yet uninvestigated, subduction zones around the world.

Title: Seismic anisotropy in the Izu-Bonin subduction system

Authors:
D. Karen Anglin and Matthew J. Fouch, Arizona State University, Tempe, Arizona, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022714, 2005

2. Sea level rise and water storage on land

Climate change affects sea level in a variety of ways. From 1950 to 2000, rising temperatures expanded ocean water and melted mountain glaciers and polar ice sheets, raising sea level by an estimated 1.3 mm/year. Based on tidal gauge observations, however, global sea level rose at a rate of 1.8 mm/yr during that period. To investigate whether loss of water stored in soils and snowpack on land might also be contributing to sea level rise, Ngo-Duc et al. used climate data to model the relationship. The researchers found that water stored on land did not make any lasting contribution to sea level rise during the 50-year period, although strong variation in precipitation and subsequent runoff, particularly in the tropics, caused sea level to fluctuate every ten years or so. They also found that the thermal expansion of the oceans and storage of water on land may counteract each other. The warming that causes ocean waters to expand also increases evaporation, which results in more precipitation. More water is thus stored on land, causing sea level to drop.

Title: Effects of land water storage on global mean sea level over the past half century

Authors:
T. Ngo-Duc, K. Laval, J. Polcher, A. Lombard, and A. Cazenave, Universite Pierre et Marie Curie, Paris, France.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022719, 2005

3. Connecting outer space to the edge of Earth's atmosphere

When sunlight strikes gases in the outer layers of Earth's atmosphere, electrons break off from helium, oxygen, and other atoms, creating a plasma soup of charged particles known as the ionosphere. Very often, the concentration of the plasma in the ionosphere drops dramatically over the mid-latitudes. Previous research suggested that this depletion is related to vertical and horizontal motion of plasma in the ionosphere. Plasma located equatorward of the mid-latitude density depletion flows out into space, forming a region called the plasmasphere. Using data collected in early 2001 with GPS satellites and an ultraviolet imaging satellite, Yizengaw and Moldwin looked for a link between the two phenomena. Signals from the GPS satellites passed through the plasma layer before reaching receiving stations in Australia and Europe, allowing the researchers to find low plasma concentrations in the ionosphere. At the same time, the imaging satellite took profiles of the upper atmosphere, which showed plasma clouds bulging out into space. Comparison of the positions of the two features showed that they were on the same line in Earth's magnetic field and tilted towards the equator, the first direct observation of a link between the plasma rise and depletion.

Title: The altitude extension of the mid-latitude trough and its correlation with plasmapause position

Authors:
Endawoke Yizengaw and Mark B. Moldwin, University of California, Los Angeles, California, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022854, 2005

4. Siberian peatlands release copious carbon as they thaw

New research suggests that rising temperatures and thawing permafrost may release large amounts of carbon from West Siberian peatlands that could influence global climate. West Siberia's peatlands store the most carbon of any of the world's peatlands, and massive amounts of this carbon are transported by streams and rivers to the Arctic Ocean. Since the last ice age, northern peatlands have acted as a storehouse for carbon, but they may begin releasing it as a result of global warming. Frey and Smith measured the amount of dissolved organic carbon in streams and rivers in 96 watersheds in the region to find out how much carbon was being released from the peatlands. To the north, where peatlands remain frozen year round, watersheds released little carbon into streams. Measurements taken in warmer regions to the south, where peatlands are thawed completely, showed considerably higher concentrations of carbon. The authors say this suggests that peatlands will release substantially larger amounts of carbon over the next century due to rising Arctic air temperatures, which may have important consequences for the global carbon cycle.

Title: Amplified carbon release from vast West Siberian peatlands by 2100

Authors:
Karen E. Frey and Laurence C. Smith, University of California, Los Angeles, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2004GL022025, 2005

5. Arctic sea ice melting steadily since 1989

Twenty years of satellite observations show that arctic sea ice has steadily diminished since 1989. Using a neural network algorithm, Belchansky et al. searched for long-term patterns in measurements of arctic ice cover collected every January from 1979 to 2004. Ice cover remained stable throughout the 1980s, but beginning in 1989 perennial ice cover began to decline dramatically throughout the arctic. This decline corresponded with a shift in the Arctic Oscillation, a long-term climate cycle that alternates between high and low atmospheric pressure systems, and thus cooler and warmer conditions. Since 1989, the warm phase of the Arctic Oscillation become more prevalent, causing especially substantial losses of ice from the Arctic's Beaufort, Chukchi, and East Siberian seas. Although the climate oscillation has become less frequent in recent years, residual effects from past years may still linger, and arctic ice continues to decline. The authors suggest that increasing greenhouse gas concentrations may be keeping the Arctic Oscillation from returning to its cooler pre-1989 phase.

Title: Variations in the Arctic's multiyear sea ice cover: A neural network analysis of SMMR-SSM/I data, 1979-2004

Authors:
Gennady I. Belchansky, David C. Douglas, Vladimir A. Eremeev, and Nikita G. Platonov, Russian Academy of Sciences, Moscow, Russia.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022395, 2005

6. Saharan dust gives clues to weather patterns

Every year from late spring to early fall, a dry, warm mass of air known as the Saharan Air Layer (SAL) carries Saharan dust from Africa westward over the Atlantic Ocean. Prior research found that this dust-laden air layer suppressed tropical cyclone activity, possibly by preventing the upward movement (convection) of lower layers of air. Wong and Dessler investigated the influence of the SAL on convection over the Atlantic Ocean west of Africa using satellite measurements of atmospheric temperature and moisture and of dust concentrations in the SAL. Using the dust to track the SAL, they found that it did suppress convection over the ocean near Africa, by creating a barrier that prevents lower air masses from moving upward. Closer to the African continent, the relationship between the dust concentration and the suppression of convection was strong, but the relationship diminished further from the coast. The authors suggest that dust is a less effective proxy to track the SAL as it moves west and that other tracking techniques are required to track the air mass as it moves far from the continent.

Title: Suppression of deep convection over the tropical North Atlantic by the Saharan Air Layer

Authors:
Sun Wong and Andrew E. Dessler, University of Maryland, College Park, Maryland, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2004GL022295, 2005

7. A tsunami warning system for the Indian Ocean

Tidal gauge readings taken during the 2004 Indian Ocean tsunami may help in the development of a tsunami warning system for the Indian Ocean. On 26 December 2004, a magnitude 9.0 earthquake off the coast of Sumatra triggered a series of tsunami waves that devastated coastal areas of the Indian Ocean. Although no tsunami warning system was in place, tidal gauges throughout the region took measurements of sea level during the event. Compiling tidal gauge data from around the region, Merrifield et al. were able to reconstruct the height and speed of the waves and the time it took them to reach coastal regions. The authors point out that readings from the tidal gauges could have given advance warning to some areas hit by the tsunami, had a warning system been in place. They suggest that a tsunami warning system for the Indian Ocean region would need more tidal gauges taking real-time sea level measurements.

Title: Tide gauge observations of the Indian Ocean tsunami, December 26, 2004

Authors:
M. A. Merrifield, Y. L. Firing, T. Aarup, W. Agricole, G. Brundrit, D. Chang-Seng, R. Farre, B. Kilonsky, W. Knight, L. Kong, C. Magori, P. Manurung, C. McCreery, W. Mitchell, S. Pillay, F. Schindele, F. Shillington, L. Testut, E. M. S. Wijeratne, P. Caldwell, J. Jardin, S. Nakahara, F.-Y. Porter, and N. Turetsky, University of Hawaii at Manoa, Honolulu, Hawaii, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022610, 2005

8. Deep-sixing carbon dioxide from power plants

New research shows that climate change over the next 500 years may make the Atlantic Ocean a good place to put carbon dioxide generated by power plants. As evidence builds that carbon dioxide emitted by human activities could be an important factor in global warming, researchers are looking for ways to reduce the amount of carbon dioxide emitted into the atmosphere. One possibility is to inject carbon dioxide generated by power plants into the deep ocean, where it would be stored, maybe for centuries, before returning to the atmosphere. Climate change could, however, alter ocean circulation, and thus the ability of the ocean to store carbon for long periods. To explore this question, Jain and Cao simulated the injection and storage of carbon dioxide into the Atlantic, Pacific, and Indian Oceans over a 500-year period. They found that climate change increased the duration that injected carbon remained in the Atlantic, but had no influence in the Pacific and Indian oceans. The authors attribute this to a climate change-induced slowing of ocean circulation in the Atlantic.

Title: Assessing the effectiveness of direct injection for ocean carbon sequestration under the influence of climate change

Authors:
Atul K. Jain and Long Cao, University of Illinois, Urbana, Illinois, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022818, 2005

9. Cracks from a Japanese earthquake heal in two years

Large earthquakes damage rock around their rupture zone, causing small cracks to form in the Earth's crust. Previous laboratory experiments and field observations indicated that these cracks heal over time and the damaged rock recovers its original properties, factors that play an important role in whether another earthquake will occur at the same location. Hiramatsu et al. used 17 years of shockwave recordings to investigate crack healing after the large earthquake that hit the Tokai region of Japan on 16 March 1997. The recordings, taken at two stations in Japan from 1986 to 1996, were of shockwaves generated by other earthquakes, which passed through the crust region damaged by the 1997 Tokai quake. Comparing the observations taken before the earthquake to those taken afterwards, the researchers determined that the cracks healed in about two years, a finding similar to that of previous studies of crack repair. They emphasize that damage heals much quicker than the recurrence time between larger earthquakes, and suggest that water may speed recovery of cracks caused by earthquakes in the shallow crust.

Title: Seismological evidence on characteristic time of crack healing in the shallow crust

Authors: Yoshihiro Hiramatsu, Hiroshi Honma, Atsushi Saiga, Muneyoshi Furumoto, and Tooru Ooida, Kanazawa University, Kanazawa, Japan.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022657, 2005

10. Improving detection of atmospheric nuclear explosions

Explosions of nuclear bombs in the atmosphere generate low-frequency sound waves that can be detected by special infrasound receivers located throughout the world. The Comprehensive Test Ban Treaty requires a network of monitoring stations around the planet to detect and locate nuclear explosions. Detection of these sound waves is limited by ambient noise, low-frequency sound waves created primarily by wind and often confused for the signal created by atmospheric explosions. Bowman et al. inspected readings from 21 infrasound recording stations around the world, hoping to better understand how ambient noise varied depending on location and season. Ambient noise levels were different for each station and fluctuated substantially with the season and time of day. The authors suggest their results can be used by each station to develop baseline measurements of ambient noise that account for daily and seasonal variation. This will help in the detection of desired signals, such as those created by atmospheric explosions.

Title: Ambient infrasound noise

Authors:
J. Roger Bowman, G. Eli Baker, and Manochehr Bahavar, Science Applications International Corporation, San Diego, California, USA

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022486, 2005

11. El Nino events linked to rising global temperature

Recent analysis of climate data suggests that rising global temperature triggers greater numbers of El Nino events. El Nino Southern Oscillation events originate in the Southern Pacific, but affect weather patterns around the world, for instance, causing severe storms in some regions and calm periods in others. Tsonis et al. examined various ocean and atmospheric climate data for a better explanation of how El Nino events are triggered and the role that global climate change plays in El Nino Southern Oscillation cycles. Their analysis found that rising global temperatures tend to trigger El Nino events. Thus, in a warming climate, El Nino events will become more frequent than La Nina events. The authors suggest that models of global climate could be evaluated, based on how well they reproduce these climate patterns, and that prediction of global temperature change is crucial to forecasting weather patterns in the 21st century.

Title: Unfolding the relation between global temperature and ENSO

Authors:
A.A. Tsonis, J. B. Elsner, A. G. Hunt, and T. H. Jagger, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022875, 2005