AGU highlights: Nov. 22, 2010
The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Water Resources Research (WRR), and Journal of Geophysical Research – Earth Surface (JGR-F).
In this release:
- Changing winds can influence amounts of carbon dioxide the ocean holds
- Large methane release from ocean sediments during glacial periods?
- Magnetic island observed at Earth’s magnetopause
- Understanding particle movement improves models of stream erosion and deposition
- New method for assessing uncertainty in groundwater models
- Large errors in hydrological models can arise from computational techniques
- Split tectonic plate could explain earthquake activity in western Japan
- New interpretation of atmospheric bromine sources during Arctic spring
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 http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2010GL045261. 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 http://www.agu.org/news/press/papers.shtml.
1. Changing winds can influence amounts of carbon dioxide the ocean holds
The Southern Hemisphere Westerlies, the prevailing winds in the Southern Hemisphere, can strongly influence ocean circulation. D’Orgeville et al. use a climate model to study how changes in the Southern Hemisphere Westerlies affect atmospheric carbon dioxide through their influence on ocean carbon storage. They confirm earlier assumptions that an increase in the wind amplitude would have the effect of accelerating the deep overturning circulation, decreasing ocean carbon storage, and releasing carbon dioxide into the atmosphere.
However, the researchers find that a latitudinal shift of the Southern Hemisphere Westerlies would affect carbon storage in the upper and deep ocean oppositely, resulting in little effect on atmospheric carbon dioxide. The study aims to contribute to understanding of past climate changes and carbon dioxide variations as well as future changes in uptake of carbon dioxide by the oceans.
On the control of glacial-interglacial atmospheric CO2 variations by the Southern Hemisphere westerlies
M. d’Orgeville, W. P. Sijp, M. H. England, and K. J. Meissner: Climate Change Research Centre, University of New South Wales, Sydney, New South Wales, Australia.
Geophysical Research Letters, doi:10.1029/2010GL045261, 2010 http://dx.doi.org/10.1029/2010GL045261
2. Large methane release from ocean sediments during glacial periods?
Methane, a potent greenhouse gas, exists in large quantities in methane hydrates in sediments beneath the seafloor. In hydrates, methane molecules are trapped in cages of water molecules, but under some conditions these hydrates can become unstable and release methane into the ocean and atmosphere. A recent study shows that large amounts of methane may have been released from the seafloor during past peak glacial or glacial-interglacial transition periods.
Using multibeam swath bathymetry data, Davy et al. find many large seafloor depressions on the seafloor off the coast of New Zealand. The authors hypothesize that these features, up to 11 kilometers (about 7 miles) in diameter, were likely pockmarks formed during the sudden release of large amounts of methane derived mostly from melting methane hydrates. The hydrate dissociation leading to these gas escape events, the authors suggest, may have occurred at peak glacial periods due to depressurization accompanying sea level lowering. Ocean temperature variations may have reinforced the hydrate dissociation.
These features, which are more than twice the size of previously reported pockmarks, could have been a source of methane gas released into the ocean and perhaps the atmosphere at the peak of glaciation. In fact, the authors estimate that for the largest of these features, about 7 billion kilograms of methane would have been released, which is about 3 percent of the current annual global methane release into the atmosphere from natural sources. Such a large release of methane could have affected ocean chemistry and contributed to transitions from glacial to warmer interglacial conditions.
Gas escape features off New Zealand: Evidence of massive release of methane from hydrates
Bryan Davy, Ingo Pecher, and Ray Wood: Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand;
Lionel Carter: Antarctic Research Centre, Victoria University Wellington,
Wellington, New Zealand;
Karsten Gohl: Department of Geosciences, Alfred Wegener Institute for Polar Sciences, Bremerhaven, Germany.
Geophysical Research Letters, doi:10.1029/2010GL045184, 2010
3. Magnetic island observed at Earth’s magnetopause
Understanding processes in the Earth’s magnetosphere can help scientists understand and predict space weather and its effects on technology such as satellites and communications and navigation systems. Magnetic reconnection, in which magnetic field lines break, rearrange, and rejoin each other, is an important process that converts magnetic field energy into thermal and kinetic energy in the space plasma environment. Simulations and some observations have shown that when field lines reconnect, features known as magnetic islands can form. These magnetic islands play a role in controlling the reconnection rate and in accelerating electrons and ions to higher energies.
Teh et al. present observations from the THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission of a small-scale secondary magnetic island (about 100 kilometers, or 62 miles, wide and 200 kilometers, or 124 miles, long) within the ion diffusion region at the Earth’s magnetopause. They use a two-dimensional reconstruction method to recover the magnetic field line map of the island. The results demonstrate that secondary magnetic islands can be formed in antiparallel reconnection and can also help understanding of reconnection physics in the diffusion region.
THEMIS observations of a secondary magnetic island within the Hall electromagnetic field region at the magnetopause
W.-L. Teh and S. Eriksson: Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Boulder, Colorado, USA;
B. U. Ö. Sonnerup: Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Boulder, Colorado, USA and Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, USA;
R. Ergun: Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Boulder, Colorado, USA;
V. Angelopoulos: IGPP, University of California, Los Angeles, California, USA;
K.-H. Glassmeier: Institute for Geophysics and Extraterrestrial Physics, Technical University of Braunschweig, Braunschweig, Germany;
J. P. McFadden and J. W. Bonnell: Space Science Laboratory, University of California, Berkeley, California, USA.
Geophysical Research Letters, doi:10.1029/2010GL045056, 2010
4. Understanding particle movement improves models of stream erosion and deposition
Streams and rivers, through eroding banks and depositing sediment, are primary agents of change to Earth’s landscapes. At a fundamental level, such erosion and deposition are dependent on bed load transport?the motion of particles rolling, sliding, or traveling in a succession of slow jumps or “saltations” along the bed of a stream. Bed load transport governs processes such as river bed morphology and the rate at which a river incises relief. Yet despite the importance of bed load transport, little is known about the physics behind erosion and deposition at the grain scale.
Lajeunesse et al. developed an experimental apparatus to investigate how particles in a flat sediment bed of uniform grain size move under steady and spatially uniform turbulent flow. Using a high-speed video imaging system, they tracked grain by grain the trajectories, velocities, and surface densities of moving particles.
They find grains move intermittently, with periods of motion and periods of rest. Moreover, any given particle may switch between rolling and saltation episodes. As a whole, these observations demonstrate that the rate at which grains are eroded from the bed increases linearly with the stress exerted by the river. Such observations allow the authors to develop a model of bed load transport that may help scientists better understand the development of bed forms, ripples, and dunes.
Bed load transport in turbulent flow at the grain scale: Experiments and modeling
E. Lajeunesse and L. Malverti: Laboratoire de Dynamique des Fluides Geologiques, Institut de Physique du Globe de Paris, Paris, France;
F. Charru: Institut de Mécanique des Fluides de Toulouse, CNRS/ Université de Toulouse, Toulouse, France.
Journal of Geophysical Research-Earth Surface, doi:10.1029/2009JF001628, 2010 http://dx.doi.org/10.1029/2009JF001628
5. New method for assessing uncertainty in groundwater models
Groundwater models used to predict flow and transport through aquifers often have many parameters and take a significant amount of computational time. Because of this, it has been challenging to assess the uncertainty in these models. Keating et al. propose a method for analyzing the uncertainty of such models by selecting and analyzing a simpler surrogate model that has many of the characteristics of the process model to be evaluated.
They compare two different methods for estimating the uncertainty of predictions made by the model and found them to be consistent. The researchers demonstrate their method with a test case model of groundwater flow at Yucca Flat, Nevada, where underground nuclear tests were conducted. The researchers suggest that the method should be widely applicable for assessing uncertainty of models in hydrology and other fields.
Optimization and uncertainty assessment of strongly nonlinear groundwater models with high parameter dimensionality
Elizabeth H. Keating: Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico, USA;
John Doherty: National Centre for Groundwater Research and Training, Flinders University, Adelaide, South Australia, Australia and Watermark Numerical Computing, Brisbane, Queensland, Australia;
Jasper A. Vrugt: Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico, USA; Department of Civil and Environmental Engineering, University of California, Irvine, California, USA; and Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, Netherlands;
Qinjun Kang: Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico, USA.
Water Resources Research, doi:10.1029/2009WR008584, 2010
6. Large errors in hydrological models can arise from computational techniques
Hydrological models are frequently used in flood forecasting, water resources assessments, and other environmental management projects. They are also useful tools for advancing scientific understanding of hydrological processes. In many applications to date, especially those using conceptual models, data uncertainty and model conceptualization are tacitly assumed to be the main sources of modeling error. However, in a recent review of practical model robustness, Kavetski and Clark focus on errors arising from the computational technique used to approximate the time-dependent catchment dynamics and on how the lack of numerical error control can affect model behavior. They find that unless careful attention is paid to numerical calculations, troublesome artifacts arise and severely deform the response characteristics of hydrological models.
The artifacts result in biased inferences and compromise the predictive ability under a wide range of catchment and hydroclimatic conditions. Ultimately, this can result in erroneous and/or misleading advice to resource managers and other decision makers. These findings have serious implications for other fields of environmental modeling in which simplistic computational techniques are used. The authors call on the hydrological modeling community to adopt robust numerical computational techniques as a required standard in scientific and operational endeavors.
Ancient numerical daemons of conceptual hydrological modeling: 2. Impact of time stepping schemes on model analysis and prediction
Dmitri Kavetski: Environmental Engineering, University of Newcastle, Callaghan, New South Wales, Australia;
Martyn P. Clark: National Center for Atmospheric Research, Boulder, Colorado, USA.
Water Resources Research, doi:10.1029/2009WR008896, 2010
7. Split tectonic plate could explain earthquake activity in western Japan
Western Japan, a densely populated region, is prone to large earthquakes and volcanic eruptions, but the tectonics of the underlying region is not well understood. The shape of the Philippine Sea plate subducting beneath western Japan is a key factor in understanding the spatial distribution of earthquakes.
Ide et al. propose that the subducting Philippine Sea plate has a tear that formed when the plate split along a ridge due to an abrupt change in subduction direction that occurred between 2 million and 4 million years ago. This was followed by a deformation of the plate and an accumulation of stress near the ridge. They suggest that the location of the tear in the plate controls where fluids can ascend through the crust; ascending fluids can drive large earthquakes and volcanoes. The proposed plate shape could help explain active tectonics in western Japan in the past 2 million years and could be useful for hazard assessment.
Split Philippine Sea plate beneath Japan
Satoshi Ide: Department of Earth and Planetary Science, University of Tokyo,
Katsuhiko Shiomi: National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan;
Kimihiro Mochizuki and Takashi Tonegawa: Earthquake Research Institute, University of Tokyo, Tokyo, Japan;
Gaku Kimura: Department of Earth and Planetary Science, University of Tokyo,
Geophysical Research Letters, doi:10.1029/2010GL044585, 2010
8. New interpretation of atmospheric bromine sources during Arctic spring
Bromine, which destroys ozone, is emitted into the atmosphere during Arctic spring from inorganic sources including sea-salt aerosols, frost flowers, and cracks in sea ice. It had been believed that all additional atmospheric bromine observed from space at high latitude during spring originated from these sources at Earth’s surface. However, a new analysis by Salawitch et al. suggests that previous satellite measurements may have been misinterpreted.
Their analysis of satellite, aircraft, and ground-based observations during Arctic spring shows that significant contributions to geographic variations in atmospheric bromine column abundance come from the stratosphere as well as the troposphere. Stratospheric enhancements are associated with weather systems that cause sporadic, severe depressions in the height of the tropopause. The bromine responsible for some of the observed enhancements appears to be resident in the lower stratosphere and is produced by biological activity in the tropical oceans rather than by inorganic processes at high latitude. The authors suggest that prior studies may have overestimated the extent of elevated tropospheric bromine in the Arctic by associating all enhancements with high-latitude surface emission. Understanding the tropospheric and stratospheric contributions to atmospheric bromine as well as the strength of various sources is important for properly quantifying the effects of bromine on ozone.
A new interpretation of total column BrO during Arctic spring
R. J. Salawitch: Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland, USA; Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland, USA; Earth System Science Interdisciplinary Center, University of Maryland, College Park, Maryland, USA.
For the names of the 40 co-authors, please follow the link below.
Geophysical Research Letters, doi:10.1029/2010GL043798, 2010