News Release

AGU journal highlights: Nov. 28, 2008

Peer-Reviewed Publication

American Geophysical Union

The following highlights summarize research papers that have been published or are "in press" (accepted, but not yet published) in Geophysical Research Letters (GRL) or the Journal of Geophysical Research - Atmospheres (JGR-D).

Anyone may read the scientific abstract for any already-published paper (not papers "in press") by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2008GL035042. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions, who are registered with AGU, also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at http://www.agu.org/jinstructions.shtml .Please note that papers not yet published (i.e. "in press") are available only to journalists and public information officers.


1. Reduced Saharan dust due to rain boosts ocean heating

Between 1980 and 2006, the tropical North Atlantic was characterized by a significant increase in sea surface temperatures, a transition from a negative to a positive phase of the Atlantic multidecadal oscillation, and a significant increase in rainfall across Africa's Sahel region. This rainfall served to decrease dustiness across western Africa and the tropical North Atlantic Ocean. Using data from satellites and previously conducted field campaigns, Foltz and McPhaden not only confirm previously seen trends, but also quantify the increase in surface solar radiation associated with the decreasing trend in dust. Using a simple one-dimensional model, they further show that in the absence of a damping mechanism, the additional influx of solar radiation would have led to a 3 degrees Celsius (5.4 degrees Fahrenheit) increase in tropical North Atlantic sea surface temperatures. The authors suggest that coupled models significantly underestimate the amplitude of the Atlantic multidecadal oscillation in the tropical North Atlantic possibly because they do not account for changes in Saharan dust concentrations.

Title: Trends in Saharan dust and tropical Atlantic climate during 1980- 2006

Authors: Gregory R. Foltz: Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington, U.S.A.;

Michael J. McPhaden: Pacific Marine Environmental Laboratory, NOAA, Seattle Washington, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035042, 2008; http://dx.doi.org/10.1029/2008GL035042


2. Coastal Southern Ocean acts as powerful carbon sink

Of the 7 to 9 Petagrams (1 Petagram = 10^15 grams) of carbon released into the atmosphere by humans each year as carbon dioxide, about 25-35 percent is taken up by the oceans through physical and biological processes. Efforts to assess the role of the ocean as a carbon dioxide sink, through tracers and models, indicate that the largest sinks exist in the North Atlantic and the Southern Ocean. However, scientists disagree on the magnitude of these sinks, particularly in the Southern Ocean where historical estimates span a wide range. To help resolve this disagreement, Arrigo et al. ran a three-dimensional biogeochemical model of the Ross Sea and find that Antarctic shelf waters are a strong sink for carbon dioxide due to high biological productivity, intense winds, high ventilation rates, and extensive winter sea ice cover. Surprisingly, although the Ross Sea continental shelf comprises only 0.36 percent of the open water area in the Southern Ocean, the total atmospheric carbon dioxide sink on the Ross Sea shelf is about 27 percent of the most recent estimates of carbon dioxide sinks for the entire Southern Ocean. Thus, the authors stress that the highly productive waters around Antarctica need to be included in future budgets of anthropogenic carbon dioxide sinks.

Title: Coastal Southern Ocean: A strong anthropogenic carbon dioxide sink

Authors: Kevin R. Arrigo, Gert van Dijken, and Matthew Long: Department of Environmental Earth System Science, Stanford University, Stanford, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035624, 2008; http://dx.doi.org/10.1029/2008GL035624


3. Ionosphere fluctuations linked to sudden stratospheric warming

A significant portion of day-to-day variations in ionospheric parameters cannot be explained by major and relatively well understood drivers such as solar ionizing flux and geomagnetic activity. These unexplained variations constitute about 20 percent of the variability seen in the ionosphere's F region, which is where most of the atmosphere's plasma resides and acts as a dependable reflector for radio signals. Noting that some investigations have proposed that lower atmospheric processes account for this ionospheric variability, Goncharenko and Zhang study an episode of sudden stratospheric warming, which occurred in late January 2008 and lasted for about a week, and compare results with concurrent temperature fluctuations in the ionosphere and thermosphere as recorded by a ground-based radar. The authors find that ionospheric variations that could not be explained through the seasonal trends, solar flux, and geomagnetic activity are instead correlated with fluctuating temperatures in the stratosphere, demonstrating a link between the lower atmosphere and the ionosphere that has been previously unobserved. The authors conclude that studies of space weather should consider ionospheric variability in conjunction with stratospheric changes.

Title: Ionospheric signatures of sudden stratospheric warming: Ion temperature at middle latitude

Authors: Larisa Goncharenko and Shun-Rong Zhang: Haystack Observatory, Massachusetts Institute of Technology, Westford, Massachusetts, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035684, 2008; http://dx.doi.org/10.1029/2008GL035684


4. Earthquake probability models tested against 2000-year record

Is the probability of a large earthquake rumbling on major faults dependent on time, or is it constant? Under a time-dependent process, some period is required to recharge stresses released in the previous earthquake. Further, the probability of the next earthquake is lowest just after a large earthquake, but rises with time. Although accepted as important to earthquake forecasting, time-dependence probabilities have been difficult to prove from observations. Thus scientists commonly report the time-independent earthquake probabilities, which increase overall uncertainty. To help refine estimates, Parsons analyzes a record of earthquakes spanning the past 2000 years from California's south Hayward fault to determine its consistency with time-dependent and time-independent earthquake recurrence models. Using statistical tests, he finds that time- dependent distributions with recurrence intervals of 210 years, when melded with other parameters, reproduce the event series on the fault 5 times more often than time-independent models. Thus large earthquakes on the Hayward fault are quasi-periodic and are most consistent with stress renewal processes.

Title: Earthquake recurrence on the south Hayward fault is most consistent with a time dependent, renewal process

Authors: Tom Parsons: U.S. Geological Survey, Menlo Park, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035887, 2008; http://dx.doi.org/10.1029/2008GL035887


5. Melting ice likely formed Martian gullies

Small gullies observed on Mars, particularly near Mars's Russell crater, could have been formed by groundwater seepage from an underground aquifer or may have resulted from the melting of near- surface ground ice, both of which would have occurred during periods when water was prevalent on Mars. To test these different hypotheses, Vedie et al. conducted cold room-based laboratory experiments in which debris flows were simulated on sand dune slopes at a range of angles, different grain sizes, and varying permafrost conditions. Preliminary results suggested that the typical morphology of gullies currently observed on Mars can best be reproduced by the formation of linear debris flows related to the melting of near-surface ground ice with silty materials. From this, the authors conclude that the depth of the thawed layer, together with the permafrost table, significantly controls the characteristic morphology of the Martian linear gullies observed in Mars's Russell crater. Such an interpretation has important implications for the evolution of Martian climate.

Title: Laboratory simulations of Martian gullies on sand dunes

Authors: E. Vedie, M. Font and J. L. Lagarde: Laboratoire M2C UMR6143, Universite de Caen-Basse Normandie, INSU, CNRS, Caen, France;

F. Costard: Interactions et Dynamique des Environnements de Surface, UMR8148, Universite Paris-Sud 11, CNRS, Orsay, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035638, 2008; http://dx.doi.org/10.1029/2008GL035638


6. Topography tweaks solar tides

The daily cycle of solar radiation has important effects on atmospheric circulation. On a local scale, one such effect is how diurnal heating over topographic slopes produces mountain-valley breezes, formed from the rough balance between radiatively driven horizontal pressure gradients and small-scale turbulent friction. These pressure gradients, balanced by Coriolis and acceleration terms, produce a global-scale solar tide, which is a large Sun-following wave that can propagate vertically upward and/or downward from its forcing altitudes. Using a global general circulation model, Hamilton et al. examine solar tidal variation at very fine resolution. They find that the model successfully simulates tidal pressure oscillations, although simulated amplitudes of semidiurnal tides are significantly enhanced over those observed. Nonetheless, the authors find that high topography reduces semidiurnal pressure amplitudes as surface elevation rises and that weak semidiurnal pressure amplitudes exist to the west of very high and steep topography. The latter finding is attributed to a shadowing effect of topography on the global-scale westward propagating tide.

Title: Topographic effects on the solar semidiurnal surface tide simulated in a very fine resolution general circulation model

Authors: Kevin Hamilton: International Pacific Research Center, University of Hawaii, Honolulu, Hawaii, U.S.A.;

Steven C. Ryan: Mauna Loa Observatory, Global Monitoring Division, Earth System Research Laboratory, NOAA, Hilo, Hawaii, U.S.A.;

Wataru Ohfuchi: Earth Simulator Center, Japan Agency for Marine- Earth Science and Technology, Yokohama, Japan.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD010115, 2008; http://dx.doi.org/10.1029/2008JD010115


7. Clouds stimulate transport of air from troposphere to stratosphere

The transport of air from the troposphere to the stratosphere occurs through large-scale upwelling in the tropical upper troposphere. However, the vertical mass transport associated with clear-sky radiative heating is too slow to support tracers, forcing scientists to rely on models to define transport of air to the stratosphere. As a result, scientists have found many different mechanisms to explain troposphere-to-stratosphere transport, ranging from gradual processes to fountain-like deep convection. To learn more, Huang and Su study the radiative impact of cirrus clouds as a possible mechanism for increasing the rate of mass transport from the troposphere to the stratosphere. Using an atmospheric circulation model, the authors examine tropical cirrus layers at the top of the troposphere and find that the strongest upward motion in the model through this layer is generally driven by dynamics instead of radiation. However, the occurrence frequency of such strong ascent is much smaller than that of moderate ascent related to the radiative effect of these cirrus clouds. Further, cloudy skies rather than clear skies foster more vertical transport to the stratosphere.

Title: Cloud radiative effect on tropical troposphere to stratosphere transport represented in a large-scale model

Authors: Xianglei Huang: Department of Atmospheric, Oceanic, an Space Sciences, University of Michigan, Ann Arbor, Michigan, U.S.A.;

Hui Su: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035673, 2008; http://dx.doi.org/10.1029/2008GL035673


8. Using cosmic-ray neutrons to measure soil moisture

Because of its long residence time, soil moisture moderates regional climate much like the ocean does, making quantifications of soil moisture critical for weather and short-term climate forecasting. However, measuring soil moisture is difficult because point measurements must be extrapolated to larger areas, introducing significant error, and because satellite remote sensors have difficulty penetrating surface cover. To move past these difficulties, Zreda et al. develop a new, noninvasive technique that infers soil moisture content from measurements of low-energy cosmic ray neutrons that are generated within soil, moderated mainly by hydrogen atoms, and diffused back to the atmosphere. These neutrons are sensitive to water content changes but do not fluctuate with changes in soil chemistry. Further, their intensity above the surface is inversely correlated with hydrogen content in the soil. By placing a portable neutron detector a few meters above the ground, the authors generate high-resolution, long-term records of undisturbed soil moisture conditions over an area about 700 meters (2,300 feet) in diameter. This method will be valuable for calibrating satellites, studying plant/soil interaction and atmosphere/soil exchange, and initializing soil moisture conditions for short-term weather and climate forecasts.

Title: Measuring soil moisture content non-invasively at intermediate spatial scale using cosmic-ray neutrons

Authors: Marek Zreda, Darin Desilets, and T. P. A. Ferre: Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona, U.S.A.;

Russell L. Scott: Southwest Watershed Research Center, Agricultural Research Service, U.S. Department of Agriculture, Tucson, Arizona, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035655, 2008; http://dx.doi.org/10.1029/2008GL035655


9. Isotopes illuminate atmospheric convection

Cumulus convection constitutes a key process in the control of tropical precipitation and vertical transport of water vapor. To better understand the influence of convective processes on the isotopic composition of precipitation and water vapor, Bony et al. introduce two stable isotopes of water into a single-column model of the tropical atmosphere. They find that deep convective atmospheres are associated with characteristic isotopic features, including that the isotopic composition of air a few kilometers below the stratosphere (at 12-13 kilometers (7.5-8.1 miles)) is close to the typical values observed in the lowest stratosphere, a finding that can help scientists understand the mechanisms through which water is transported from the upper troposphere to the lower stratosphere. They also find that changes in large-scale atmospheric motions constitute the primary driver of isotopic changes in the precipitation. However, a global warming or cooling of the tropics can further change the isotopic composition. Understanding dynamical and thermodynamical influences on isotopic variations in precipitation may help scientists use ice cores collected from tropical mountain glaciers to reconstruct past climate variations.

Title: Influence of convective processes on the isotopic composition (delta oxygen 18 and delta D) of precipitation and water vapor in the tropics: 1. Radiative-convective equilibrium and Tropical Ocean- Global Atmosphere-Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) simulations

Authors: Sandrine Bony and Camille Risi: Laboratoire de Meteorologie Dynamique, IPSL, UPMC, CNRS, Paris, France;

Francoise Vimeux: IRD-UR Great Ice, LSCE, IPSL, CEA, UVSQ, CNRS, Gif-sur-Yvette, France.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD009942, 2008; http://dx.doi.org/10.1029/2008JD009942


10. Explaining isotope composition of tropical rains

In the tropics, the proportion of heavier water isotopes in precipitation varies inversely with the precipitation amount. To better understand the physical processes underlying this "amount effect," Risi et al. use a single-column model of the tropical atmosphere including two water stable isotopes, together with a representation of convection, and proposed a methodology to quantify the relative contributions to this effect. Focusing on convection, the authors find that two processes predominantly explain the amount effect: the reevaporation of falling rain and diffusive exchanges with surrounding vapor, as well as the recycling of the subcloud layer vapor feeding the convective system by convective fluxes. These findings highlight the importance of a detailed representation of rain evaporation processes to simulate accurately the isotopic composition of the tropical precipitation. In turn, the isotopic composition of the precipitation may be used to evaluate the representation of such processes in climate models. Regarding timescales, the authors also show that the isotopic composition of the tropical precipitation integrates the convective activity over several days.

Title: Influence of convective processes on the isotopic composition (delta oxygen 18 and delta D) of precipitation and water vapor in the tropics: 2. Physical interpretation of the Amount Effect

Authors: Camille Risi and Sandrine Bony: Laboratoire de Meteorologie Dynamique, IPSL, UPMC, CNRS, Paris, France;

Francoise Vimeux: IRD-UR Great Ice, LSCE, IPSL, CEA, UVSQ, CNRS, Gif-sur-Yvette, France.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD009943, 2008; http://dx.doi.org/10.1029/2008JD009943

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