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AGU journal highlights -- Sept. 6, 2007

1. Solving the mystery of booming sand dunes

Booming noises in deserts can sometimes be heard after a natural slumping event or during a sand avalanche generated by humans sliding down the slip face of a large dune. This sound is composed of one dominant audible frequency ranging between 70 and 105 hertz, and several higher harmonics. Previous studies have sought to explain this noise by suggesting that the frequency is a function of the average size of sand grains. Vriend et al. challenge this finding and instead demonstrate through field measurements that the booming frequency results from a natural waveguide, where sound waves channeled through the dune amplify as they constructively interfere. They also show that during slipping events, the surficial layer of dry, loose sand interacts with the air above it, behaving as a loudspeaker. With their observations, the authors were able to develop a mathematical relationship that predicts the frequency and harmonics of the booming emission of dunes. They determine that this frequency is dependent on compressional wave velocities and the depth of the surficial sand layer.

Solving the mystery of booming sand dunes

Nathalie M. Vriend, Melany L. Hunt, Christopher Earls Brennen, Katherine S. Brantley, and Angel Ruiz-Angulo: Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, U.S.A.;
Robert W. Clayton: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, U.S.A.

Geophysical Research Letters (GRL) paper 10.1029/2007GL030276, 2007,

2. Reductions in the northward incursion of the South Asian monsoon since AD 1400

Variations in the Asian monsoon, one of the Earth's largest seasonal weather patterns, can bring drought or torr

During solar flares and coronal mass ejections the Sun's magnetic field is carried by the solar wind to the rest of the solar system. The solar wind smashes into the Earths magnetosphere, causing field lines in both systems to break and rejoin at a current sheet. Whether this magnetic reconnection process is determined by dynamics at large or small scales is heatedly debated. Kuritsyn et al. study reconnection through laboratory experiments and find that the reconnection rate is a function of the plasma parameters at both large (system-wide) and small (local) scales. At the local scale the rate that charged particles collide with themselves governs the observed reconnection. When local collision rates are lowered, the current sheet is shortened and the effective dissipation is enhanced, both of which increase the reconnection rate. At the global scale, when collision rates are fixed, the current sheet length increases along with the global size, effectively lowering the reconnection rate.

Effects of global boundary and local collisionality on magnetic reconnection in a laboratory plasma

A, Kuritsyn: Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas, and Department of Physics, University of Wisconsin, Madison, Wisconsin, U.S.A;
H. Ji, S. P. Gerhardt, Y. Ren, and M. Yamada: Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas, and Princeton Plasma Physics Laboratory, Princeton, New Jersey, U.S.A.

Geophysical Research Letters (GRL) paper 10.1029/2007GL030796, 2007,

12. Evidence for interhemispheric coupling between the stratosphere and the mesosphere

Air circulation in the mesosphere, a region of the atmosphere between 50 and 90 kilometers (31 and 56 miles) above the Earth's surface, is driven by upwelling and cooling at the summer pole and downwelling and heating at the winter pole. This circulation creates extremely cold temperatures at the top of the mesosphere above the summer pole, often causing ice clouds called noctilucent clouds to form. Through data from the Swedish satellite Odin, Karlsson et al. study noctilucent clouds in order to estimate the average state of mesospheric circulation. They compare these conditions with the state of the stratosphere and find that an abundance of noctilucent clouds in the summer mesosphere coincides with cold winter stratospheric temperatures in the opposite hemisphere. This cross-hemispheric correlation operates such that fluctuations in the winter stratosphere lead the summer mesosphere response by several weeks. Although models have predicted such a correlation, the authors expect that the documentation of this phenomenon will help refine predictions of future atmospheric behavior.

Evidence for interhemispheric stratosphere-mesosphere coupling derived from noctilucent cloud properties

B. Karlsson, H. Krnich, and J. Gumbel: Department of Meteorology, Stockholm University, Stockholm, Sweden.

Geophysical Research Letters (GRL) paper 10.1029/2007GL030282, 2007,

ential rain to southern Asia. Noting that few detailed long-term records of monsoon variability exist, Kaspari et al. analyze data from a recently retrieved ice core from a glacier on the northeast ridge of Mt. Everest. A decrease in sea salt aerosols and isotopically heavy water and an increase in dust in the core's yearly ice layers indicate that the Himalayas experienced a decrease in marine and an increase in continental air masses. This is interpreted as a reduction in northward incursions of the summer South Asian monsoon since about AD 1400. Previously published records from south of the Himalayas indicate a strengthening of the monsoon since this time. The authors hypothesize that the difference reflects a southward shift in the average summer position of the monsoon trough since the 15th century. They note that the change in monsoon circulation seen here is synchronous with a reduction in solar irradiance and the onset of the Little Ice Age.

Reduction in northward incursions of the South Asian monsoon since ~1400 AD Inferred from a Mt. Everest ice core

S. Kaspari, S. Sneed, R. Hooke, K. Kreutz, D. Introne, M. Handley, and K. Maasch: Climate Change Institute and Department of Earth Sciences; University of Maine, Orono, Maine, U.S.A.;
P. Mayewski: Climate Change Institute and Department of Earth Sciences; University of Maine, Orono, Maine, U.S.A, and Joint Key Laboratory of Cryosphere and Environment, Lanzhou, China;
S. Kang, S. Hou, D. Qin, and J. Ren: Joint Key Laboratory of Cryosphere and Environment, Lanzhou, China.

Geophysical Research Letters (GRL) paper 10.1029/2007GL030440, 2007,

3. Ozone-destroying chemicals wane

Halogenated hydrocarbon pollutants, which are chemicals used in manufacturing that destroy ozone, break down in the atmosphere, forming hydrogen chloride. So, knowing the abundance of atmospheric hydrogen chloride is important to monitoring ozone depletion and ozone recovery following restrictions imposed by the Montreal Protocol, a treaty established in 1987 that sets timetables for phasing out chemicals that contribute to ozone destruction. Wallace and Livingston analyze data on atmospheric hydrogen chloride collected since 1971 by observers at the McMath-Pierce Solar Telescope on Arizona's Kitt Peak. Their data show the previously well established increase in hydrogen chloride abundance from 1971 to about 1993, thought to be due to unregulated use of chemicals that destroy ozone. These abundances plateau from about 1993 to about 1997. Since 1997, values have steadily decreased, the authors show. They attribute this decrease to the success of the Montreal Protocol.

The thirty-five year trend of hydrogen chloride amount above Kitt Peak, Arizona

L. Wallace: National Optical Astronomy Observatories, Tucson, Arizona, U.S.A.;
W. C. Livingston: National Solar Observatory, Tucson, Arizona, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030123, 2007,

4. Robotic probes improve global ocean analysis

Observations of the ocean are important to understanding global weather. Although satellites observe the open ocean, the Argo float network, a set of nearly 3000 oceanic robotic probes throughout the Earth's oceans that measure salinity and temperature at depths down to 2 kilometers (1.2 miles), has collected in situ data of the world's oceans since 2001. To analyze the importance of Argo in collecting accurate oceanic information, Balmaseda et al. use the European Centre for Medium-Range Weather Forecasts's operational ocean analysis system to replicate the past 5 years of global ocean behavior with and without Argo observations. After comparing results with actual ocean patterns seen since 2001, they find that the salinity data from Argo are instrumental in correcting the salinity estimations from the Centres ocean analyses on a basin-wide scale. Further, Argo temperature corrections help to better characterize the Indian, Atlantic, and Southern oceans, particularly when coupled with satellite altimeter data. Finally, using Argo data in the initialization of seasonal forecasts significantly improves the skill of forecasts of sea-surface temperatures, especially in the Indian Ocean and the western Pacific Ocean.

Impact of Argo on analysis of the global ocean

M. Balmaseda: European Centre for Medium-Range Weather Forecasts, Reading, U.K.;
D. Anderson: Bureau of Meteorology Research Centre, Melbourne, Australia;
A. Vidard: Institut National de Recherche en Informatique et en Automatique, Grenoble, France.

Geophysical Research Letters (GRL) paper 10.1029/2007GL030452, 2007,

5. A universal rule for the timing of repeating earthquakes?

In some seismic settings, groups of events rupture at the same patch of fault repeatedly with nearly identical seismic signatures. These repeating sequences are suggestive of a renewal process taking place on the fault patches. Determining what controls the recurrence time of these ruptures is important to understanding dynamic failure processes fundamental to earthquakes. Chen et al. analyze data from recurring earthquake systems in Taiwan, California, and Japan with the goal of understanding the physics responsible for their similarities and differences. They find that the ratio of recurrence interval to seismic energy released in each location varied widely. However, when each earthquake system was adjusted to account for differences in the geodetically derived slip rates, the scaling between recurrence interval and seismic energy released was remarkably consistent. This suggests that tectonic loading rates are likely the most important factors that control earthquake repeat times, and that a universal rule governing recurrence intervals of repeating earthquakes may possibly exist despite differences in tectonic settings.

Towards a universal rule in the recurrence interval scaling of repeating earthquakes?

Kate Huihsuan Chen and Ruey-Juin Rau: Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan;
Robert M. Nadeau: Berkeley Seismological Laboratory, University of California, Berkeley, California, U.S.A.

Geophysical Research Letters (GRL) paper 10.1029/2007GL030554, 2007,

6. Abrupt climate change clues in tropical African sediments

Tropical Africa, affected by both Indian and Atlantic monsoons, is responsible for much of the heat and water vapor that drives deep atmospheric convection. Noting that the relationship between this region's climate and global climate change is not well understood, Tierney et al. compare records of past climate events in tropical Africa with global paleoclimate records. The authors studied a sediment core from Lake Tanganyika, a lake on Tanzania's western border. In the Southern Hemisphere's winter, Indian Ocean monsoons interact with local winds, making southern lake habitats favorable for silica-bearing diatoms. During summer, these winds slacken as the low-pressure Intertropical Convergence Zone (ITCZ) moves southward, making lake habitats favorable for cyanobacteria. The authors analyze the concentrations of biogenic silica found in the core and notice that during known abrupt climate shifts in the Northern Hemisphere, the lake contained fewer diatoms, perhaps influenced by weakened monsoons and the convergence zones southward shift. Additional variations in biogenic silica of equal magnitude were also found, suggesting that abrupt climate change could originate from within tropical systems.

Abrupt climate change in southeast tropical Africa influenced by Indian monsoon variability and ITCZ migration

Jessica E. Tierney and James M. Russell: Department of Geological Sciences, Brown University, Providence, Rhode Island, U.S.A.

Geophysical Research Letters (GRL) paper 10.1029/2007GL029508, 2007,

7. Antarctic ice cores record ancient meteoritic events

Antarctica represents the best site to collect small meteoritic particles because windblown terrestrial dust is scarce and extreme environmental conditions prevent chemical weathering. Although many investigations have searched for micrometeorites deposited within modern times, few have looked for them within ancient ice. As part of a new micrometorite collection project launched at the permanent French-Italian station of Concordia, on the East Antarctic Plateau, Narcisi et al. study the Dome C ice core, a core collected by the European Project for Ice Coring in Antarctica. They find two distinct dust layers which, through comparisons with extraterrestrial debris found in deep-sea sediments and polar caps, they determined to be of meteoritic origin. Closer inspection revealed that these layers represented individual meteoritic events, with the first occurring about 481,000 years ago and the second occurring 434,000 years ago, as indicated by layers in the ice core. The authors note that, similar to ashfall from a known volcanic explosion, such meteoritic events have the potential to serve as time markers in other less detailed stratigraphic records.

First discovery of meteoritic events in deep Antarctic (EPICA-Dome C) ice cores

Biancamaria Narcisi: Ente per le Nuove tecnologie, l'Energia e l'Ambiente, Roma, Italy;
Jean Robert Petit: Laboratoire de Glaciologie et Gophysique de l'Environnement/Centre National de la Recherche Scientifique, Saint Martin d'H res, France;
Ccile Engrand: Centre de Spectrom trie Nuclaire et de Spectrom trie de Masse, Institut National de Physique Nuclaire et de Physique des Particules, Centre National de la Recherche Scientifique, Orsay, France.

Geophysical Research Letters (GRL) paper 10.1029/2007GL030801, 2007,

8. New angle on solar winds magnetic reconnections

Magnetic reconnection is a fundamental plasma process in which pairs of magnetic field lines merge to produce topological changes in the field. It is an important mechanism for converting magnetic energy to bulk flow energy and particle heating and often manifests itself in spectacular ways in space, solar, astrophysical and laboratory plasmas. One of the characteristic signatures of reconnection is the acceleration of plasma away from a reconnection site in a pair of oppositely directed plasma jets. Gosling et al. utilize 3-second observations of such plasma jets by the Wind spacecraft to show that, somewhat surprisingly, reconnection commonly occurs in the solar wind at times when the angle between the reconnecting field lines is considerably less than 90 degrees. The process thus often produces extremely narrow plasma jets that cannot be resolved by most plasma experiments presently operating in the solar wind. The authors find that near solar activity minimum reconnection preferentially occurs in the low-speed wind and at a rate of ~1.5 events per day, a factor approximately 36 times greater than previously found.

Prevalence of magnetic reconnection at small field shear angles in the solar wind

J. T. Gosling: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, U.S.A.;
T. D. Phan and R. P. Lin: Space Sciences Laboratory, University of California, Berkeley, California, U.S.A.;
A. Szabo: NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030706, 2007,

9. Understanding permeability in sea ice

Polar sea ice is an indicator and regulator of climate change; its thinning and retreat show the effects of climate warming, and its presence greatly reduces solar heating of the polar oceans. Sea ice also is a primary habitat for microbial communities, sustaining marine food webs. The permeability of sea ice and its ability to transport brine are important to many problems in geophysics and biology, yet remain poorly understood. Golden et al. generate a unified picture of sea ice permeability through analytical and numerical modeling, as well as through comparisons with field and laboratory measurements. The latter included X-ray computed tomography of sea ice pore microstructure as a function of temperature. Their study demonstrates that sea ice displays universal transport properties similar to fluid transport in some crustal rocks, though over a much narrower temperature range. The authors present permeability in terms of temperature and bulk salinity, preparing the way for more realistic representations of sea ice evolution in climate and biogeochemical models.

Thermal evolution of permeability and microstructure in sea ice

K. M. Golden, A. L. Heaton, and J. Zhu: Department of Mathematics, University of Utah, Salt Lake City, Utah, U.S.A.;
H. Eicken and J. Miner: Geophysical Institute, University of Alaska, Fairbanks, Alaska, U.S.A.;
D. Pringle: Arctic Region Supercomputing Center, University of Alaska, Fairbanks, Alaska, U.S.A., and Geophysical Institute, University of Alaska, Fairbanks, Alaska, U.S.A.

Geophysical Research Letters (GRL) paper 10.1029/2007GL030447, 2007,

10. Global observations of large oceanic eddies

Oceanic Rossby waves are medium-scale motions that transport energy westward in response to the Earth's rotational forces. By contrast, other motions called eddies can transport momentum, heat, and seawater, contributing to general circulation and affecting ocean biology. Distinguishing between Rossby waves and eddies is difficult because identifying both requires long-term and detailed monitoring over a broad spatial scale. Chelton et al. have analyzed 10-years of global sea-surface height fields using a new database that combines altimeter data from NASA and European Space Agency satellites. They find that the higher resolution of the merged dataset shows that nearly 60 percent of the variability over much of the world's oceans is due to eddies, with the majority of eddy motion occurring outside of the tropics. These eddies had sea surface heights of 525 centimeters (2-9.8 inches) and diameters of 100200 kilometers (60-120 miles). The eddies are thought to be generated by instabilities in background currents or by the instability of Rossby waves themselves.

Global observations of large oceanic eddies

Dudley B. Chelton, Michael G. Schlax, Roger M. Samelson, and Roland A. de Szoeke: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, U.S.A.

Geophysical Research Letters (GRL) paper 10.1029/2007GL030812, 2007,

11. Magnetic reconnection at large and small scales

Contact: Peter Weiss
American Geophysical Union

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