AGU journal highlights -- 23 June 2006

American Geophysical Union AGU Journal Highlights - 23 June 2006

Contents
I. Highlights, including authors and their institutions
II. Ordering information for science writers and general public

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GL) and Paleoceanography (PA). 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., 2005GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

1. Particle flow inside coronal streamers

Coronal streamers are bright structures that form over sunspots and magnetic neutral lines. They trap electrically charged gases, are denser than the rest of the corona, and create an outflow of slow solar wind that forms the prominent rays seen in coronographs. Because charged particles carried by solar wind have the potential to create magnetic storms at Earth and disrupt satellite communications and power grids, researchers model the patterns and dynamics of coronal streamers, to gain more information on space weather hazards. Suess and Nerney used streamer density data generated by instruments aboard the Solar Heliospheric Observatory (SOHO) and the Yohkkoh Soft X-Ray Telescope to develop a model of particle flow and geometry near the base of the streamers, out to a few solar radii. Their model is consistent with there being no measurable outflow below 2.5 solar radii and outflow of approximately 100 kilometers [60 miles] per second at five solar radii. Their results show how the magnetic field affects the shape of streamers and regulates the outflow of the slow solar wind.

Title: Flow speed inside the brightness boundary of coronal streamers

Authors: S. T. Suess: National Space Science & Technology Center, NASA Marshall Space Flight Center, Huntsville, Alabama, USA. S. Nerney: Ohio University, Lancaster, Ohio, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2006GL026182, 2006

2. Turbulence within an oceanic mixed layer inhibits sedimentation

The residence time of planktonic particles in the oceanic surface mixed layer (SML) is an important factor in carbon budget models and studies of primary production rates. A fundamental question in the dynamics of the surface mixed layer involves whether or not turbulence inhibits the rate of sedimentation through the layer. Previous studies have shown that turbulence can both retard and accelerate particle settling. To resolve this, Oliver Ross used a one-dimensional Lagrangian random walk technique and found that the same turbulence model can yield conflicting results depending on the implementation of the SML. If the SML is modeled as a homogeneous layer with a constant turbulent intensity throughout, increasing turbulence will produce an increase in particle sedimentation. If, however, a more realistic representation of the surface mixed layer is used, in which the turbulent intensity is allowed to decrease toward the base of the SML, then an increase in turbulence will lead to an increase in the residence time of particles in the surface mixed layer.

Title: Particles in motion: How turbulence affects plankton sedimentation from an oceanic mixed layer

Author: Oliver N. Ross: Department of Biological Sciences, University of Essex, Colchester, United Kingdom.

Source: Geophysical Research Letters (GL) paper 10.1029/2006GL026352, 2006

3. Zonal currents in the western equatorial Pacific Ocean can flip direction

Since the 1980s, scientists have studied the three-dimensional circulation of the upper layers in the equatorial Pacific Ocean, but data on the currents below 500 meters [2,000 feet] are sparse. To observe the upper and intermediate circulation (0–1200 meters) [0-1,300 yards] in this region, Gouriou et al. examined the measurements of a Lowered Acoustic Doppler Current Profiler, collected during two cruises in October 1999 and April 2000. Their analysis revealed that the secondary South Subsurface Countercurrent, usually lying at 6 degrees South and 400 meters [1,000 feet] depth, had a deep extension at 165 degrees East and a maximum core velocity around 5 degrees South and 1000 meters [3,000 feet] in depth during both cruises. Furthermore, the Equatorial Intermediate Current (EIC) and the Lower EIC, which flowed westward between 165 degrees East and 180 degrees during the first cruise, were replaced by eastward flow along the equator during the second cruise. The authors note that the bathymetry surrounding equatorial islands could factor into the complexity of this system; they stress that variability in water transport must be studied in greater detail to fully understand the zonal mass balance of the equatorial Pacific Ocean.

Title: Upper and intermediate circulation in the western equatorial Pacific Ocean in October 1999 and April 2000

Authors: Y. Gouriou: Institut de Recherche pour le Développment, Plouzané, France;

T. Delcroix and G. Eldin: Institut de Recherche pour le Développment, Toulouse, France.

Source: Geophysical Research Letters (GL) paper 10.1029/2006GL025941, 2006

4. Over the 20th century, droughts became shorter and less frequent over much of the U.S.

Droughts cause adverse effects on water management and aquatic ecosystems, with estimated annual losses between $6-8 billion. Some climate models have predicted changes in the severity and duration of drought, though temperature and precipitation over the continental United States likely increased in the latter half of the 20th century. To examine trends in drought characteristics, Andreadis and Lettenmaier constructed a time series of soil moisture and runoff over the continental United States for the period between 1925 to 2003. Their results show a wetting trend and an increase in runoff over most of the U.S., consistent with observed precipitation increases and streamflow records from streams minimally affected by dams and other human activities. Over most of the U.S., droughts became shorter, less frequent, and covered a smaller portion of the country. The main exception was the Southwest and parts of the interior of the West, where increased temperature led to longer and more severe droughts during the studied time period.

Title: Trends in 20th century drought over the continental United States

Authors: Konstantinos M. Andreadis and Dennis P. Lettenmaier: Civil and Environmental Engineering, University of Washington, Seattle, Washington, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025711, 2006

5. Decadal climate variability in the tropical Atlantic during the mid-Holocene

Since many coupled ocean and atmospheric models of past tropical Atlantic climate rely on extrapolation from direct sea surface measurements during the 20th century, modeling tropical Atlantic climate change has proven difficult. Greer and Swart sought to overcome this lack of data by studying the stable carbon and oxygen isotope variability of corals from the genera Montastraea and Siderastrea, collected from the Enriquillo Valley in the Dominican Republic. Noting that these isotope data can reflect fluctuations in precipitation during coral growth, they proposed that changes in precipitation seen during the mid-Holocene reflect decadal to multi-decadal variability in climate. These patterns may have been driven by changes in latitudinal migration of the Intertropical Convergence Zone or increased storm activity, both of which may have ultimately been driven by fluctuations in tropical Atlantic sea surface temperature. The authors caution that before current climate patterns of the tropical Atlantic are analyzed to look for any anomalous behavior possibly attributable to manmade global warming, the patterns of decadal climate variability must be firmly established for times past.

Title: Decadal cyclicity of regional mid-Holocene precipitation: Evidence from Dominican coral proxies

Authors: Lisa Greer: Washington and Lee University, Lexington, Virginia, USA;

Peter K. Swart: Rosenstiel School of Marine and Atmospheric Science, University of Miami Miami, Florida, USA.

Source: Paleoceanography (PA) paper 10.1029/2005PA001166, 2006

6. Water circulation fluctuated during the Mediterranean's last interglacial anoxic event

In records of ancient climates, marine organic-rich sediment layers, known as sapropels, mark periods of low-oxygen marine environments, which reflect the degree of regional surface water freshening. Rohling et al. present a multi-proxy analysis of a relatively recent, well-preserved Mediterranean organic-rich deposit known as sapropel S5, which formed 124 to 119 thousand years ago. They combined stable isotope and organic biomarker data with microfossil records of organisms known to have lived in surface ocean layers to study the conflicting evidence that deep-dwelling fauna called foraminifera coexisted with bacteria known to grow only when poor circulation prevents oxygen from reaching deep levels. They found that S5 started with a rapid freshwater-induced increase of density stratification that inhibited vertical mixing and deepwater ventilation. Then, over the next 900 years, these low-oxygen conditions persisted and extended from the bottom up to roughly 200 meters [700 feet] in water depth. For the next 1400 years, the region experienced fluctuations in the depth and intensity of this low-oxygen zone, allowing at times for deep sea fauna to exist in some areas. S5 deposition then terminated as the strength of circulation increased.

Title: Water column dynamics during the last interglacial anoxic event in the Mediterranean (sapropel S5)

Authors: E. J. Rohling: National Oceanography Centre, Southampton, United Kingdom;

E. C. Hopmans and J. S. Sinninghe Damsté: Royal Netherlands Institute for Sea Research (NIOZ), Den Burg, Texel, The Netherlands.

Source: Paleoceanography (PA) paper 10.1029/2005PA001237, 2006

II. Ordering information for science writers and general public

Journalists and public information officers of educational and scientific institutions (only) may receive one or more of the papers cited in the Highlights by sending a message to Jonathan Lifland [jlifland@agu.org], indicating which one(s). Include your name, the name of your publication, and your phone number. The papers will be e-mailed as pdf attachments.

Others may purchase a copy of the paper online for nine dollars:
1. Copy the portion of the digital object identifier (doi) of the paper following "10.1029/" (found under "Source" at the end of each Highlight).
2. Paste it into the second-from-left search box at http://www.agu.org/pubs/search_options.shtml and click "Go."
3. This will take you to the citation for the article, with a link marked "Abstract + Article."
4. Clicking on that link will take you to the paper's abstract, with a link to purchase the full text: "Print Version (Nonsubscribers may purchase for $9.00)."

The Highlights and the papers to which they refer are not under AGU embargo.

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