AGU Journal Highlights - 8 September 2004 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.
1. Connection between El Nino and Antarctic cooling
A new study has found that climate changes from the El Nino Southern Oscillation are largely responsible for the observed cooling in the western Ross Sea near Antarctica. Bertler et al. present an analysis of isotope records from snow in Antarctica that may explain the connection between the warming of the Pacific sector of the Southern Ocean and the current cooling of the Ross Sea region. Their study confirms previous findings that the warming is linked to El Nino and provides new evidence that the terrestrial cooling is caused by a simultaneous El Nino-driven change in the atmospheric circulation from the Amundsen Sea and West Antarctica. The researchers report on the relationship between Antarctic low-level atmospheric circulation and the surrounding ocean, and theynote that the temperature change does not reflect a regional cooling, but instead represents a change in the atmospheric circulation that results in a regional cooling.
Title: El Nino suppresses Antarctic warming
Nancy A. N. Bertler, Antarctic Research Centre, Victoria University, Wellington, New Zealand, and Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand;
Peter J. Barrett, Antarctic Research Centre, Victoria University, Wellington, New Zealand,
Paul A. Mayewski, Karl J. Kreutz, Climate Change Institute, University of Maine, and Edward T. Bryand Global Science Center, Orono, Maine, USA;
Ryan L. Fogt, Byrd Polar Research Center, Ohio State University, Columbus, Ohio, USA;
James Shulmeister, University of Canterbury, Christchurch, New Zealand.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020749, 2004
2. How deep-sea islands feed the ocean
An analysis of water flow around a Japanese island provides new details to explain how islands stir and fertilize the upper ocean. Hasegawa et al. examined the water flow around an isolated deep-sea island in the North Pacific and report that horizontal and vertical shear likely drives the mixing and produces what is known as the island mass effect. The researchers produced a three-dimensional survey of the island's wake after observing a seven-fold increase in surface nitrate concentration that they attributed to a mixing of cold, nutrient-rich deep waters with the warm, nutrient-depleted surface layer. They note that evidence of upwelling, including anomalously cold water with increased organic carbon, is often seen on the current-protected side of islands and report that the addition of sunlight also contributes to a large increase in island productivity. While such behavior is readily explained for shallow waters, the authors state that deep-sea islands typically do not share the same conditions.
Title: How islands stir and fertilize the upper ocean
Daisuke Hasegawa, H. Yamazaki, Tokyo University of Marine Science and Technology, Tokyo, Japan;
R. G. Lueck, Center for Earth and Ocean Research, University of Victoria, Victoria, British Columbia, Canada;
L. Seuront, Lille University of Science and Technology, Wimereux, France.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020143, 2004
3. San Andreas likely a "weak" fault
An analysis of data from the 2.2-kilometer [1.4 mile] borehole near the San Andreas Fault finds evidence in support of the hypothesis that the fault is much weaker than the surrounding crust. Hickman and Zoback present measurements of the maximum horizontal compressive stress direction and magnitude of the fault, based on observations of borehole failure in the pilot hole. They show that the direction of maximum horizontal compressive stress rotates in a clockwise manner with depth, reaching an orientation at the bottom of the hole that is inclined at about 70 degrees to the fault, which is consistent with previous observations made at much greater distances from the fault. Their findings are compatible with the proposal that the San Andreas is a weak fault embedded within an otherwise strong crust and weak upper mantle. Such a "weak" fault has a strength of less than 20 megapascals averaged over the active portion of the fault, while laboratory friction estimates propose that a "strong" San Andreas could hold between 50-100 megapascals.
Title: Stress orientations and magnitudes in the SAFOD pilot hole
Stephen Hickman, U.S. Geological Survey, Menlo Park, California, USA;
Mark Zoback, Stanford University, Stanford, California, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020043, 2004
4. Long-distance pollution can affect regional health
Siberian forest fires during summer 2003 produced potentially hazardous surface ozone levels in western North America and led to air quality warnings in the Pacific Northwest that year. Jaffe et al. tracked the path of smoke from the extensive Russian fires using an aerosol analysis and prediction system and confirmed the transport of the pollutant particles with aircraft and surface observations. The authors found that the fires increased the summer background of carbon monoxide and ground-level ozone in North America by nearly 30 and 10 parts per billion by volume, respectively. The researchers suggest that the airborne pollutants may also explain interannual variations in background ozone and pollutant levels throughout North America. Their results show that regional air quality and health are linked to global processes, including climate, forest fires and the long-range transport of pollutants.
Title: Long-range transport of Siberian biomass burning emissions and impact on surface ozone in western North America
Dan Jaffe, Isaac Bertschi, University of Washington, Bothell, Washington, USA;
Lyatt Jaegle, University of Washington, Seattle, Washington, USA;
Paul Novelli, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA;
Jeffrey S. Reid, Douglas L. Westphal, Naval Research Laboratory, Washington, D.C., USA;
Hiroshi Tanimoto, National Institute for Environmental Studies, Tsukuba, Japan;
Roxanne Vingarzan, Pacific and Yukon Region, Environment Canada, Vancouver, British Columbia, Canada.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020093, 2004
5. Modeling rock behavior beneath the mid-ocean ridge
Inspired by new observations of a ubiquitous asymmetry in the depth of the axis of the global mid-ocean ridge system, researchers have developed a numerical model of underground rock flow and melting associated with the divergence of two tectonic plates. Katz et al. suggest that disturbances in the creeping flow of the shallow mantle beneath the ocean ridges caused by ridge migration can change melt production rates and lead to an imbalance in axial depth across discontinuities in the ridges. The authors' model also provides an improved estimate for magma movement beneath ridges and shows that plate-induced mantle dynamics may explain the morphological changes observed along the mid-ocean ridge. The quantitative predictions produced by their model are consistent with observations that initiated their research.
Title: Ridge migration, asthenospheric flow and the origin of magmatic segmentation in the global mid-ocean ridge system
Richard F. Katz, Marc Speigelman, Suzanne M. Carbotte, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020388, 2004
6. Linking solar storms and atmospheric chemical content
The strong solar storms of late 2003 likely caused the unusually high levels of nitrogen oxides (NOx) in the upper stratosphere observed in April 2004, providing evidence that such atmospheric chemical perturbations may have implications on high-latitude ozone variability. Natarajan et al. report the results from the Halogen Occultation Experiment (HALOE) that measured the chemical content of the upper atmosphere in early 2004. The authors suggest that the solar flares and energetic particle precipitation associated with the unprecedented solar storms may have enhanced the nitrogen oxide levels in the high-latitude mesosphere [approximately 50-80 kilometers or 30-50 miles altitude] that subsequently descended into lower atmospheric layers. The researchers note that the results from HALOE agree with photochemical model predictions, indicating that the elevated nitrogen oxide levels significantly reduced ozone levels and that the stratospheric composition is affected by the infusion of nitrogen oxides transported from higher altitudes.
Title: Anomalously high levels of NOx in the polar upper stratosphere during April, 2004: Photochemical consistency of HALOE observations
Murali Natarajan, Ellis E. Remsberg, Lance E. Deaver, NASA Langley Research Center, Hampton, Virginia, USA;
James M. Russell III, Hampton University, Hampton, Virginia, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020566, 2004
7. Refining estimates of Saturn-Titan interactions
A three-dimensional simulation of the plasma interactions between Saturn and its moon Titan can help researchers hone their estimates of the planet's magnetic field before the arrival of new data from the Cassini spacecraft. Smith et al. report that a cloud of energetic neutral nitrogen particles likely transfers charged particles from Titan toward Saturn when the moon moves inside Saturn's magnetosphere. The authors' simulation produced predictions of the cloud's morphology, including estimates for the solar ultraviolet radiation and magnetospheric plasma. Such information can provide a baseline for interpreting data from the Cassini-Huygens mission, which arrived at Saturn earlier this year and contains a plasma instrument that will provide continuous data during its four-year orbit around the planet. The researchers suggest that their model, combined with Cassini measurements, will provide an assessment of the plasma interaction between the two bodies and may allow them to determine the result of plasma interactions with Titan's atmosphere.
Title: Titan's atomic and molecular nitrogen tori
Howard Todd Smith, R. E. Johnson, University of Virginia, Charlottesville, Virginia, USA;
V. I. Shematovich, Institute of Astronomy, Russian Academy of Sciences, Moscow, Russia.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020580, 2004
8. Drainage basins on Mars formed differently than on Earth
A new comparison of drainage basins on Mars and Earth reveals fundamental differences in the ways that valleys were formed in the different environments. Stepinski and Coradetti observe that, unlike on Earth, where drainage flow typically conforms to the region's topographic features, drainage on Mars frequently appears to be independent of its surroundings. The authors used image analysis and data mining methods to quantify their observations and argue that the morphologies of Martian basins are incompatible with runoff from sustained, homogeneous rainfall. Instead, the researchers suggest that Martian valleys formed in a desert-like environment subjected to localized and intermittent fluvial activity, conditions somewhat similar to that found in Chile's Atacama Desert. They also report that a previous method using erosion diagnosis and surface elevation levels lacks the sensitivity to predict the origin of the runoff.
Title: Comparing morphologies of drainage basins on Mars and Earth using integral-geometry and neural maps
Thomas F. Stepinski, Lunar and Planetary Institute, Houston, Texas, USA;
S. Caradetti, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020359, 2004
9. Silent earthquakes in Cascades
Continuous Global Positioning System (GPS) and seismic data from northern California show that slow earthquakes periodically rupture the Gorda-North America plate interface within southern Cascadia. Szeliga et al. report evidence from GPS data and five seismometers indicating that slow faulting has occurred along the plate interface deep beneath the Klamath Mountains nearly every 11 months since at least 1998. The authors point out that such events, also known as silent earthquakes, along converging tectonic plate boundaries are known to release energy that may initiate additional earthquakes. Such slow faulting, accompanied by episodic low-intensity tremors may also modulate seismic rupture and limit the size of a future earthquake. Their findings demonstrate that the slow earthquakes occur throughout Cascadia and may also be prevalent in other subduction zones worldwide. The new study provides evidence for the role of fluid migration as the controlling trigger in other slow-slip faults and can also be used to refine physics estimates of slow crustal movement.
Title: Southern Cascadia episodic slow earthquakes
Walter Szeliga, Timothy I. Melbourne, M. Meghan Miller, V. Marcelo Santillan, Central Washington University, Ellensburg, Washington, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020824, 2004
10. Ocean release may extend chemical lifetimes
Airborne methyl chloroform may be coming from the high-latitude oceans rather than lasting longer in the atmosphere. Wennberg et al. suggest that the ozone-destroying compound is likely degassing from the oceans, contrary to assumptions that it showed a trend for a longer atmospheric lifetime than had been predicted. The authors examined the ocean-atmosphere interactions of methyl chloroform in a global ocean model and report that the oceans can provide an outlet for the compound into the troposphere. They note that the vast reduction in the production and use of the chemical solvent in the early 1990s greatly reduced atmospheric levels of the compound worldwide; however, scientists were unable to explain its lingering effects in the atmosphere. Their findings, if further confirmed, would alter the estimates of change in the atmospheric oxidation rate of hydrocarbons like methane and could provide guidance on changes in the ocean-atmosphere interactions of carbon dioxide levels in the future.
Title: Recent changes in the air-sea gas exchange of methyl chloroform
Paul O. Wennberg, James T. Randerson, California Institute of Technology, Pasadena, California, USA;
Synte Peacock, University of Chicago, Chicago, Illinois, USA; Rainer Bleck, Los Alamos National Laboratory, Los Alamos, New Mexico, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020476, 2004
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