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AGU journal highlights -- June 4, 2008

American Geophysical Union

1. Human control of radiation belt

Charged particles within the Earth's radiation belts travel in spiral trajectories along geomagnetic field lines. The strength of the magnetic field increases as the particles approach the Earth; because charge flows perpendicular to magnetic field lines, the component of the particles' velocity parallel to the magnetic field decreases. Usually, this causes particles to reverse direction and spiral back along the field lines, continuing until they reach the opposite hemisphere, where they reverse again. These trapped particles can be deflected by certain electromagnetic waves (whistlers) into the Earth's dense atmosphere (below 100 kilometers (60 miles)), where they cannot escape back to the magnetosphere. Noting that whistlers can be generated by ground-based transmission signals in the very low frequency range (VLF, used for military communications), Sauvaud et al. use satellite observations to investigate how a VLF transmitter located in Australia (labeled NWC) affects the population of inner radiation belt electrons first above it and then along the electron drift path around the Earth. They find that about 300 times more high-energy electrons are lost by the inner radiation belt to the atmosphere during NWC transmission periods than during nontransmission periods, suggesting that human systems can control radiation belt dynamics.

Title: Radiation belt electron precipitation due to VLF transmitters: Satellite observations

Authors: J.-A. Sauvaud, R. Maggiolo, and C. Jacquey: CESR/University of Toulouse-CNRS, Toulouse, France;

M. Parrot: LPCE CNRS, Orléans, France;

J.-J. Berthelier: CETP CNRS, Saint Maur, France;

R. J. Gamble and Craig J. Rodger: Department of Physics, University of Otago, Dunedin, New Zealand.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033194, 2008;

2. Antarctic current roils deep ocean waters

The Antarctic Circumpolar Current (ACC), a prominent ocean current that flows around Antarctica, is an important component of global ocean circulation and climate. The current consists of a number of fronts. Observations indicate that turbulent mixing is enhanced in these fronts, penetrating through much of the water column. Noting that understanding the ACC is important to understanding regional and global ocean circulation, Saenko feeds a simple representation of mixing along the current's fronts into a global climate model to evaluate the mixing's potential impact on ocean's overturning circulation. For example, in the northern Atlantic Ocean, cold, dense water (called North Atlantic Deep Water, or NADW) sinks, travels south, and upwells around the latitudes of the ACC. Saenko finds that frontally intensified mixing in the ACC increases the transformation of NADW into Upper Circumpolar Deep Water (UCDW), a water mass that is a mixture of deep waters from the world's oceans and that contributes to upwelling in the Southern Ocean. Further, the frontally intensified dense water upwelling in the Southern Ocean diverts a fraction of Antarctic Bottom Water, which heads north from the ACC, back to the UCDW.

Title: Influence of the enhanced mixing within the Southern Ocean fronts on the overturning circulation

Author: Oleg A. Saenko: Canadian Centre for Climate Modelling and Analysis, Environment Canada, Victoria, British Columbia, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033565, 2008;

3. Did slow deformation mitigate Peru quake?

Between 1992 and 2007, nine magnitude 7 or larger earthquakes have rocked the subduction zone of southern Peru and northern Chile, including a magnitude 8.1 earthquake in August 2007 near Pisco, Peru. Pritchard and Fielding combine data from seismometers and interferometric synthetic aperture radar (InSAR) to determine the Pisco earthquake's heterogeneous slip distribution. The analysis includes some of the first data from the Japanese Advanced Land Observation Satellite (ALOS) and the European Envisat wide-swath beam. The seismic data indicate that the slip maximum occurred 60�� seconds after the main shock started, and the InSAR data constrain the main slip patch to be about 70 kilometers (43 miles)from the earthquake's origin. Combined, these factors suggest an extremely low rupture velocity or a long slip rise time. Historic data indicate that no large earthquake has occurred near Pisco since at least 1746. Though the Pisco earthquake was large, its expected magnitude would have been larger if all stress accumulated since the eighteenth century had been released. Thus, stress from subduction was likely released through aseismic deformation within this region.

Title: A study of the 2006 and 2007 earthquake sequence of Pisco, Peru, with InSAR and teleseismic data

Authors: M. E. Pritchard: Department of Earth and Atmospheric sciences, Cornell University, Ithaca, New York, U.S.A.;

E. J. Fielding: Jet Propulsion Laboratory, California Institute of Technology, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033374, 2008;

4. Australian dryness linked to sea surface temperatures

Since 1950, rainfall over southern and eastern Australia has decreased, significantly reducing inflows to the Murray and Darling rivers, Australia's longest river system. In particular, precipitation in late austral autumn has decreased by about 40 percent from the long-term seasonal average for Victoria, Australia's most densely populated state. To learn more about these rainfall changes, Cai and Cowan compare precipitation data from Australia with global data sets and find that the reduction occurs predominantly in May, with rainfall variations linked to two sources of sea surface temperature (SST) variability. First, higher SSTs in the waters off Indonesia, which occur as the Indo-Pacific system enters into La Niña-like conditions, is associated with a rainfall increase over northern Victoria; a reduction in La Niña events and an increase in El Niño conditions since 1950 likely contributed to the observed rainfall decline. Additionally, SST variation patterns in the Indian Ocean are linked to atmospheric wave trains, with a low-pressure center over Victoria that helps promote rainfall. However, these wave trains have weakened since 1950, also contributing to the observed rainfall decline.

Title: Dynamics of late autumn rainfall reduction over southeastern Australia

Authors: Wenju Cai and Tim Cowan: CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia; also at Wealth from Oceans National Research Flagship, CSIRO, North Ryde, New South Wales, Australia.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033727, 2008;

5. Warming and cooling from land cover changes

Land use practices influence land cover, which in turn affects Earth surface properties including the ratio of incoming solar radiation that is reflected by the Earth's surface back into space (albedo). Using recently available land cover/land use (LCLU) data from Landsat classification maps and snow-free albedo data from the Moderate Resolution Imaging Spectroradiometer, Barnes and Roy study the impact of LCLU change from 1973 to 2000 on surface albedo and net radiative flux from the troposphere to the stratosphere (radiative forcing) for 36 ecoregions covering 43 percent of the conterminous United States. They find that radiative forcing due to LCLU change may be disguised when spatially and temporally explicit data sets are not used. Further, radiative forcing due to recent LCLU albedo change varies geographically in sign and magnitude, with the most cooling due to forest loss and the most warming due to conversion of agriculture to other LCLU types. For the 36 ecoregions considered, the authors estimate that the surface is experiencing a small net warming.

Title: Radiative forcing over the conterminous United States due to contemporary land cover land use albedo change

Authors: Christopher A. Barnes and David P. Roy: Geographical Information Science Center of Excellence, South Dakota State University, Brookings, South Dakota, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033567, 2008;

6. Satellites give lowdown on polar lows

Polar lows are intense maritime polar cyclones of medium size. Knowledge about their phenomenology, origin, and evolution is limited, due mostly to their short lifetimes and a lack of meteorological data in the remote locations where polar lows originate. This complicates the representation of polar lows in numerical models. Noting that new information from satellites can help better characterize polar lows, Blechschmidt combines satellite-derived thermal infrared imagery with satellite-derived wind speeds and generates a new 2-year data set that documents the evolution and pathways of polar low events over the Nordic seas. She finds that a total of 90 polar lows occurred between 2004 and 2005, with a maximum of polar low activity occurring during winter months. In addition to documenting prime locations for cyclone generation, the author finds that interannual variability in polar low activity resulted mostly from more frequent cold air outbreaks in 2004. The new data set builds a basis for studying polar lows and validating numerical climate models that include polar atmospheric phenomena.

Title: A 2-year climatology of polar low events over the Nordic Seas from satellite remote sensing

Author: A.-M. Blechschmidt: Meteorological Institute, University of Hamburg, Hamburg, Germany

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033706, 2008;


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