Public Release: 

AGU journal highlights - 6 January 2004

American Geophysical Union

The following highlights summarize research papers in Earth Interactions (EI), Geophysical Research Letters (GL), and Journal of Geophysical Research-Oceans (JC). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

1. Oceanic monitor may help predict fish population
A new method to monitor sea surface temperatures and height in the North Pacific may help in measuring wind patterns and ocean dynamics and could provide insight into better managing fisheries located in the mid-latitudes. Robin Tokmakian developed an algorithm to monitor changes in the sub-surface ocean's heat content from shifts in the Alaskan and California currents, as the water follows its circulation pattern over the central and eastern North Pacific. Such sea temperature and height changes have been shown to affect the marine ecosystem and fish populations by affecting the density and mixing of nutrient-rich waters. Tokmakian proposes a space-based system that can monitor the circulation patterns from changes in the sea surface height and predict the size of various fish stocks by observing the North Pacific heat content progression during its typical southwesterly shift.

Title: Monitoring North Pacific heat content variability: An indicator of fish quantity?
Author: Robin Tokmanian, Naval Postgraduate School, Monterey, California.
Source: Earth Interactions (EI) paper 10.1029/2002EI000063, 2003

2. Vegetation's affect on summer warming may not last
Surface temperatures in North America and Eurasia are balanced by atmospheric feedback from increased vegetation during summer, which lowers the temperature, and reduced snow cover during winter, which increases annual average temperatures. Kaufmann et al. analyzed the effect of vegetation on surface temperatures, using satellite measures of surface greenness in the summer and snow extent in the winter. Previous research had found that enhanced vegetation leads to cooler surface temperatures, which the authors confirmed with their finding that vegetation growth during warm summer months slowed the ongoing increase in summertime temperatures. They note, however, that this mechanism for slowing global climate change may not be effective for much longer, as a temperature increase by another 3-5 degrees Celsius [5-9 degrees Fahrenheit] may harm vegetation growth. The browning or loss of vegetation would then accelerate further climate warming.
Title: The effect of vegetation on surface temperature: A statistical analysis of NDVI and climate data
Authors: R. K. Kaufmann, R. B. Myneni, N. V. Shabanov, Boston University, Boston, Massachusetts; L. Zhou, Georgia Institute of Technology, Atlanta, Georgia; C. J. Tucker, D. Slayback, Jorge Pinzon, NASA Goddard Space Flight Center, Greenbelt, Maryland.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018251, 2003

3. New theory explains meteors' double plumes
A new analysis of double plumes observed behind falling meteors suggests a new explanation for the phenomenon. Kelley et al. report on lidar and camera views of the heretofore-mysterious "trains" cast behind meteors. The authors studied the persistent emissions commonly left behind by meteors from the 1998 and 1999 Leonid meteor showers and propose that one of the tails is left by gaseous vapor emissions, while the other is caused by dust particles. They note that the two layers are separated by gravitational properties of the dust that keeps it segregated, evidence that has been confirmed by rocket-based observations of dust remnants behind other meteors. The researchers dismiss previous speculation that the double tails were the result of a hollow cylinder because of inconsistencies between the theory and observed light emissions from the plumes. They conclude that their explanation may solve the long-standing problem of explaining the phenomenon.

Title: A new explanation of persistent double meteor trains
Authors: M. C. Kelley, C. Kruschwitz, L. Gelinas, S. Collins, Cornell University, Ithaca, New York; J. Drummond, Starfire Optical Range, Kirtland Air Force Base, New Mexico; C. Gardner, University of Illinois at Urbana-Champaign, Urbana, Illinois; J. Hecht, The Aerospace Corporation, Los Angeles, California; E. Murad, Hanscom Air Force Base, Massachusetts.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018312, 2003

4. Climate models should include carbon dioxide increases
The impact of land cover change in climate models may be incorrectly interpreted if the vegetation response to greater atmospheric carbon dioxide concentrations is not taken into account. Narisma et al. suggest that climate models should include a measure of vegetation response to natural and man-made carbon dioxide increases during the 20th century to accurately account for biospheric feedback. The researchers examined the specific impacts of elevated atmospheric carbon dioxide concentrations during the Australian summer and report that the plant response to increased carbon dioxide influences atmospheric temperatures and the climate in ways that are not currently captured by climate models. They conclude that simulations of land cover changes that exclude biospheric feedback may lead both regional and global models to overestimate the impact of land cover change on heat flux and temperatures.

Title: The role of biospheric feedbacks in the simulation of the impact of historical land cover change on the Australian January climate
Authors: Gerry T. Narisma, A. J. Pitman, J. Eastman, I. G. Watterson, R. Pielke Sr., A. Beltran-Przekurat, Macquarie University, North Ryde, New South Wales, Australia.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018261, 2003

5. Identifying source of infrasonic waves from Denali earthquake
The sudden motion of ground rupture during the strong 2002 Denali earthquake in Alaska likely initiated infrasonic signals that were recorded approximately 150 kilometers [90 miles]away. Olson et al. traced the source of acoustic signals associated with the 7.9 magnitude earthquake by estimating the speed of the waves as they moved eastward along the Denali fault. The authors report that the large local ground motions produced the unusual infrasonic signature after the November event. Infrasonic waves are often used to detect faraway sounds from underground nuclear explosions and storms. The researchers linked the ground motion from the Denali temblor with observed infrasound signals and show that the largest amplitude waves correspond to the regions along the fault line that showed the largest ground motion. Previous studies had not been able to identify the direct cause of infrasonic waves, which are made up of low frequency waves inaudible to humans.
Title: Infrasound associated with the 2002 Denali fault earthquake, Alaska
Authors: John V. Olson, Charles R. Wilson, Roger A. Hansen, Geophysical Institute, University of Alaska, Fairbanks, Alaska.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018568, 2003

6. Emissions from Martian dust storms can be remotely identified
Martian dust devils and dust storms can likely be detected remotely by their electromagnetic radiation signals, which can be easily read in Mars' thin atmosphere. Renno et al. present a theoretical study that suggests that Martian dust storms, like terrestrial dust squalls, produce strong electrical fields that can be observed from Earth. They based their analysis on anomalously strong microwave activity seen in regions known to have enhanced dust activity. The collisions of sand and dust particles produce a static electricity-like charge among the grains that generates radiation that can be measured by radio and microwave emissions in the ultra-low frequency range. Such triboelectric charging of dust is especially likely to discharge in dust particles because of the low atmospheric density on Mars. The authors note that Martian dust storms are much stronger, larger and more frequent than those on Earth and could thus lead to simple observations of the electromagnetic emissions on Mars.
Title: Electrical discharges and broadband radio emission by Martian dust devils and dust storms
Authors: Nilton O. Renno, Ah-San Wong, Sushil K. Atreya, University of Michigan, Ann Arbor, Michigan; Imke de Pater, University of California, Berkeley, California; Maarteen Roos-Serote, Lisbon Astronomical Observatory, Tapada da Ajuda, Lisbon, Portugal.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL017879, 2003

7. First proof that plants absorb atmospheric nitrogen
Researchers have proven for the first time that vegetation can directly absorb atmospheric organic nitrogen. Sparks et al. report that plant leaves can remove organic forms of nitrogen, which could have a significant effect on atmospheric carbon and nitrogen cycles. The authors measured the uptake rate in eight plant species representing vegetation from a range of global climates and found that the pores on the leaves' surface largely determine a plant's absorption characteristics. They estimate that plants could remove enough precursor nitrogen pollutants to cause a three percent reduction in harmful nitrogen oxides in the troposphere [lowest part of the atmosphere] worldwide. The study used a more realistic atmospheric model than previous studies that measured the nitrogen deposition only during nighttime, when plant absorption would be minimized. The researchers suggest that the extent of plant uptake can provide insights on the global and regional budgets of near-surface carbon and nitrogen.

Title: The uptake of gaseous organic nitrogen by leaves: A significant global nitrogen transfer process
Authors: Jed P. Sparks, Cornell University, Ithaca, New York, and Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, Colorado; James M. Roberts, Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, and Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, Colorado; Russell K. Monson, University of Colorado, Boulder, Colorado, and Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, Colorado.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018578, 2003

8. GRACE may provide unprecedented ocean current data
The initial gravity model of the recently launched GRACE [Gravity Recovery and Climate] mission has been combined with results from satellite altimeter missions to study ocean surface currents. The preliminary results from this effort provide a dramatic improvement over previous observations. Tapley et al. show that ocean topography data from the GRACE global gravity model closely correspond with surface observations and note that, for the first time, all major current systems can be clearly observed from space. Previous satellite data produced a higher error level that prevented the accurate resolution of global ocean circulation patterns. The authors note that little highly detailed information exists in remote ocean regions and that the new data will potentially allow oceanographers to accurately map surface and subsurface currents worldwide. The researchers note that while the model requires further validation, the GRACE mission will likely provide a minimum of a ten-fold increase in the accuracy of the Earth's gravity model.

Title: Large scale ocean circulation from the GRACE GGM01 Geoid
Authors: B. D. Tapley, D. P. Chambers, S. Bettadpur, J. C. Ries, Center for Space Research, The University of Texas at Austin, Austin, Texas.
Source: Geophysical Research Letters (GL) paper 10.1029/2003GL018622, 2003

9. Coastal mixing enhances mercury levels in coastal ecosystems

The mixing processes in estuaries may contribute to elevated concentrations of reactive mercury in coastal ecosystems. Rolfhus et al. collected multiple samples from the Connecticut River estuary and found that the reactive mercury content increased with increasing salinity. Mercury is a potent human and wildlife neurotoxin found in lakes, rivers, and oceans worldwide; numerous health agencies warn against the consumption of fish and seafood contaminated with the element. The authors note that mercury's reactive form may enhance the availability of mercury to the aquatic food chain, yet little is known about how it is transformed in the estuarine environment. Their study simulated the mixing processes between salt and freshwater, which they suggest dilutes and/or alters the chemical compounds that strongly bind with mercury. The researchers conclude that ions exchanged during saltwater mixing alter the organic metal content in water and help spread readily bioaccumulated mercury in coastal ecosystems.

Title: Evidence for enhanced mercury reactivity in response to estuarine mixing
Authors: Kristofer R. Rolfhus, University of Wisconsin-La Crosse, La Crosse, Wisconsin; Carl H. Lamborg, William F. Fitzgerald, Prentiss H. Balcom, University of Connecticut, Groton, Connecticut.
Source: Journal of Geophysical Research-Oceans (JC) paper 10.1029/2001JC001297, 2003

10. Effects of sea ice keels on polar surface waters
The keels of sea ice contribute to the heat flux of Arctic waterways by enhancing the underwater mixing of warmer and cooler waters. Skyllingstad et al. simulated the effects from varying ice keel depths on the upper ocean during summer and winter, when the changes in ice thickness affect the polar oceans' flow patterns and ice coverage. The authors note that shallow ice keels (one meter [three feet] or less) seen during summer generate a turbulent wake below the ice sheets that extend hundreds of meters [yards] downstream and help mix warmer fresh meltwater with cooler saltwater. They then analyzed the strong differences estimated to occur between the summertime ice keels and the deeper wintertime keels, which can reach up to 10 meters [30 feet] below the surface, and propose that the shallower keels generate stronger mixing closer to the ice and increased ice melting rates. Conversely, the deeper keels follow a similar pattern but mix the water further from the surface and force ice bottom melting during the winter when new ice is usually forming.

Title: Effects of keels on ice bottom turbulence exchange
Authors: Eric D. Skyllingstad, Clayton A. Paulson, W. Scott Pegau, Oregon State University, Corvallis, Oregon; Miles G. McPhee, McPhee Research Company, Naches, Washington; Timothy Stanton, Naval Postgraduate School, Monterey, California.
Source: Journal of Geophysical Research-Oceans (JC) paper 10.1029/2002JC001488, 2003


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