AGU Journal highlights - 23 February 2005

02/23/05

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GL), Journal of Geophysical Research-Atmospheres (JD), Journal of Geophysical Research-Earth Surface (JF), and Journal of Geophysical Research- Planets (JE). 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 following 10.1029/ (e.g., 2004GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

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1. Resolving clouds and climate

Climate simulations and numerical weather predictions rely on researchers' ability to model the atmosphere on a global scale. Such models need to keep track of global circulation while also accounting for the deep convection associated with cumulus clouds, particularly in the tropics. But large-scale atmospheric models lack the resolution to model relatively small-scale cumulus convection in detail. Instead atmospheric models rely on parameterizations, or broad estimates of cumulus convection that do not adequately portray the clouds. Kuang et al. resolve these problems by reducing the scale difference between large-scale atmospheric circulation and smaller scale convective circulation. To do this in their model, they shrink the Earth and speed up its rotation to reduce the scale of atmospheric circulation; they also speed up cumulus convection. This makes the scales of the two processes similar enough to ease calculations, but not so close that it alters the way they interact.

Title: A new approach for 3D cloud-resolving simulations of large-scale atmospheric circulation

Authors:
Zhiming Kuang, Peter N. Blossey, Christopher S. Bretherton, University of Washington, Seattle, Washington, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021024, 2005

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2. Magma shakes up earthquake locations

Volcanic activity is usually accompanied by small high-frequency earthquakes that result from the movement of magma. These earthquakes occur because as magma moves into a volcanic conduit, it puts pressure on the surrounding rock that, to a limited extent, slides out of the way. Since regional stresses influence magma movement, there may be relationships between the orientation of volcanotectonic faults and magma movement that could prove useful to volcanologists. Diana Roman employs numerical models to examine these relationships and finds that the direction of movement on these strike-slip faults should be opposite to that predicted on the basis of regional stresses. The results of this analysis do not, however, explain the location of volcanotectonic earthquakes during the 1992 eruption at Crater Peak, Alaska. Roman speculates that the locations of preexisting faults may be more important in influencing the location of these earthquakes.

Title: Numerical models of volcanotectonic earthquake triggering on non-ideally located faults

Author: Diana Roman, School of Earth and Environment, University of Leeds, Leeds, United Kingdom.

Source Geophysical Research Letters (GL) paper 10.1029/2004GL021549, 2005

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3. Explaining correlation between coral records and El Nino

Sea surface temperature increases in the Indian Ocean during the 1970s likely increased precipitation in the equatorial region and may have caused a shift in the upper ocean's oxygen content. Such a modification could explain the strong correlation seen between the El Nino Southern Oscillation (ENSO) cycle and isotopic oxygen found in the region's coral since then. Timm et al. analyzed the isotopic oxygen content in coral from an archipelago in the equatorial Indian Ocean from 1950-1994 and found that a change occurred in the 1970s that enhanced the relationship between the oxygen content and the ENSO cycle. The authors report that average sea surface temperatures in the region increased to nearly 28.5 degrees Celsius [83.3 degrees Fahrenheit] in the late 1970s, which is nearly the level at which sea surface temperature anomalies can trigger deep convection in the tropical atmosphere. They note that other interpretations for the increased oxygen exist, but suggest that the most likely cause is from greater precipitation.

Title: Nonstationary ENSO-precipitation teleconnection over the equatorial Indian Ocean documented in a coral from the Chagos Archipelago

Authors: Oliver Timm, International Pacific Research Center, University of Hawaii, Honolulu, Hawaii, USA;
Miriam Pfeiffer and Wolf-Christian Dullo, Leibniz-Institut fuer Meereswissenschaften an der Universitaet zu Kiel, Kiel, Germany.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021738, 2005

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4. Warming of the world ocean, 1955-2003

The warming trend of the world's oceans in the later half of the twentieth century may be modulated by periods of cooling, due to natural variability of the Earth system. In a new analysis of an expanded data set of ocean temperature profiles dating back to 1955, Levitus et al. confirmed the ocean warming trend, which was particularly pronounced in the Atlantic, and identified a decrease in ocean heat content beginning around 1980. The decrease, primarily in the Pacific Ocean, was seen in temperature records of both the uppermost 300 meters [1,000 feet] and the layer below that down to 700 meters [2,000 feet]. The decrease has since rebounded, and the overall warming trend continues in the new millennium. The authors point to the increase of greenhouse gases in the atmosphere as the cause of this long-term warming trend over the last 50 years. They attribute the large decrease in the early 1980s to internal variability of the Earth system on decadal time-scales.

Title: Warming of the world ocean, 1955-2003

Authors: Levitus, S.; Antonov, J.; Boyer, T., National Oceanographic Data Center, NOAA, Silver Spring, Maryland, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021592, 2005

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5. Sulfur dioxide cuts could increase fine particles in the summer air

When the number of fine particles in the air increases, respiratory problems also increase, so scientists want to identify the sources of these particles. Combustion processes are an obvious manmade source, but there are also natural sources, such as sea spray. Fine particles can also form, or nucleate, directly in the atmosphere. Researchers have assumed that this process was unimportant in and around large urban areas, because it would be overshadowed by high levels of urban particle pollution. But, Gaydos et al. report that nucleation events actually occur frequently in the urban eastern United States: one out of three days in Pittsburgh. By comparing air particle measurements taken between July 2001 and January 2002 with different formation models, the researchers show that reactions between sulphuric acid, water, and ammonia can explain the formation of these particles. Their modeling suggests that reducing ammonia emissions reduces particle development at any time of the year. In contrast, reductions in sulfur dioxide may reduce or increase particle formation in the summer depending on the size of the reduction.

Title: Modeling of in situ ultrafine atmospheric particle formation in the eastern United States

Authors: Timothy M. Gaydos, Charles O. Stanier, and Spyros N. Pandis, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA

Source: Journal of Geophysical Research-Atmospheres (JD) paper 10.1029/2004JD004683, 2005

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6. To predict wildfire erosion, find out how hot the soil got

Europe's 2003 summer heat wave fanned the flames of massive wild fires throughout the continent, from Sweden to Russia. After the fires, severely burned areas faced increased erosion due to the loss of vegetation and the creation of water-repellent soils that can cause flooding, vitiate streams and lakes with mud, and threaten people and property. The ability to predict erosion would greatly assist efforts to limit the long-term damage caused by fires. Knowing how hot soils get during a fire turns out to be a good way to predict erosion, according to Moody et al. They investigated how temperature affects critical shear stress: the force it takes to get soil moving. The scientists heated a variety of soils and measured the critical shear stress required to erode them. They found that soils heated to more than 275 degrees Celsius [527 degrees Fahrenheit] were most likely to erode and that the effect of soil temperature was more important than the composition of the soil. Burn severity maps based on low-resolution infrared photographs could be used to get a rough idea of soil temperatures. Maps based on high-resolution infrared and other remote-sensing methods might provide a much better picture of erosion potential, they say.

Title: Critical shear stress for erosion of cohesive soils subjected to temperatures typical of wildfires

Authors: John A. Moody and J. Dungan Smith, U. S. Geological Survey, Boulder, Colorado, USA;
B. W. Ragan, Boise State University, Boise, Idaho, USA.

Source: Journal of Geophysical Research-Earth Surface, paper 10.1029/2004JF000141, 2005

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7. Signs of a Martian ice age preserved in layers of ice and dust

The North Pole of Mars is covered with a thick cap of water ice, over 1000 kilometers [600 miles] across and up to three kilometers [two miles] deep, that is incised by a spiral pattern of numerous deep troughs. The walls of these canyons reveal many alternating layers of light and dark material, thought to be layers of water ice and dust. On Earth, changes in climate are the cause of such alternating layers that occur in thick layers of deep-sea sediment, and these cyclical climate changes are caused by cyclical changes in the Earth's orbit and rotation around the Sun. By analogy, planetary scientists believe that the light and dark layers in the Martian pole reflects changes in Martian climate and similar orbital variability. Thanks to the availability of remarkably clear and detailed images of these layers and detailed altimetry measurements, Milkovich and Head studied the planet using techniques developed by paleoclimatologists to read Earth's past ice ages and global warming events in deep sea sediment cores. They report that one particular pattern with a wavelength of about 30 meters [100 feet] can be traced across at least three-fourths of the ice cap and seems to reflect changes in the orbit of Mars that happened every 51,000 years. They also identify a unit that may relate to Mars' most recent ice age, which occurred between 500,000 and two million years ago.

Title: North polar cap of Mars: Polar layered deposit characterization and identification of a fundamental climate signal

Authors: Sarah M. Milkovich and James W. Head III, Brown University, Providence, Rhode Island, USA.

Source: Journal of Geophysical Research-Planets, paper 10.1029/2004JE002349, 2005

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