AGU journal highlights -- Oct. 11, 2006

October 11, 2006

  1. Antarctic iceberg breakups possibly caused by storm-induced sea swells from far away

    Many abrupt, millennial-scale shifts in past global climate are known to be associated with sudden releases of icebergs into the global ocean. To study iceberg calving, drift, melting, and disintegration, MacAyeal et al. deployed seismometers on Antarctica's Ross Ice Shelf and on various icebergs adrift in the Ross Sea. They found that low-frequency energy ranges, typically associated with storm swells, dominated the signal. The authors hypothesized that storms in the northern and tropical Pacific Ocean influence calving and iceberg breakup. They focused on a strong storm in the Gulf of Alaska on 21 October 2005 and traced swells from it to a large Ross Sea iceberg immediately prior to, and during, its break-up on 27 October 2005. Based on these observations, the authors note that a connection may exist between the Antarctic ice sheet mass balance and weather systems worldwide, and that similar mechanisms might have triggered past climate shifts.

    Title: Transoceanic wave propagation links iceberg calving margins of Antarctica with storms in tropics and Northern Hemisphere


    Douglas R. MacAyeal, Kelly M. Brunt, L. Mac. Cathles, Young-Jin Kim, and Marianne H. Okal: Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, U.S.A.;

    Emile A. Okal, Department of Geological Sciences, Northwestern University, Evanston, Illinois, U.S.A.;

    Richard C. Aster: Geophysical Research Center and Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico, U.S.A.;

    Jeremy N. Bassis and Helen A. Fricker: Institute for Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, U.S.A.;

    Robert Drucker and Seelye Martin: School of Oceanography, University of Washington, Seattle, Washington, U.S.A.;

    Mark P. Sponsler: Stormsurf, Half Moon Bay, California, U.S.A.;

    Jonathan E. Thom: Antarctic Meteorological Research Centre, University of Wisconsin, Madison, Wisconsin, U.S.A.;

    Olga V. Sergienko: Hydrospheric and Biospheric Sciences Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.

    Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027235, 2006

  2. The Little Ice Age affected the tropics

    The cooling period between about 1400 and 1850 A.D., known as the Little Ice Age, was one of the most extreme events recorded in many regions since the Last Glacial Maximum, 20,000 years ago. Though Little Ice Age climatic changes have been reconstructed from a number of high latitude locations, few studies show definitive proof for Little Ice Age effects within the tropics. Newton et al, analyzed a sediment core collected from Makassar Strait, Indonesia, a location that underlies the Indo-Pacific Warm Pool, one of the warmest regions in the modern ocean. Through sea surface temperature and salinity reconstructions, based on magnesium/calcium and oxygen isotope ratios within microfossils, the authors found a significant cooling and freshening trend within the Indo-Pacific Warm Pool that matches the time period of the Little Ice Age. The authors hypothesize that the changes in the Indo-Pacific Warm Pool were caused by a southern displacement of the Inter-Tropical Convergent Zone, which delivered more rain to the tropical Pacific, freshening waters and decreasing temperatures.

    Title: Climate and hydrographic variability in the Indo-Pacific Warm Pool during the last millennium

    Authors: Alicia Newton and Robert Thunell: Department of Geological Sciences, University of South Carolina, Columbia, South Carolina, U.S.A.;

    Lowell Stott: Department of Earth Sciences, University of Southern California, Los Angeles, California, U.S.A.

    Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027234, 2006

  3. New technique for determining ionospheric electron temperature

    The ionosphere is formed when solar radiation partially ionizes the neutral atmosphere to form plasma. During daytime, the temperature of ambient electrons is elevated above that of ions, due to energy transferred by photoelectrons released during the ionization process. Nicolls et al. used incoherent scatter radar data from Arecibo, Puerto Rico, to study this phenomenon. incoherent scatter radars detect the scattering of electromagnetic waves by electrons in the ionospheric plasma. The spectrum of the received signal contains a low-frequency component associated with the motion of ions (ion line), and a high-frequency resonance associated with electrons (plasma line); the latter represents scattering processes where electrons act as if ions are absent. Using information from plasma lines, the authors developed a new technique for measuring ambient electrons in the daytime ionosphere. They then compared their results with independent assessments of ambient electrons based on the traditional ion line approach, and found that the two techniques yielded similar results. The authors note that their approach can be used to test hypotheses on plasma kinetics and to analyze ionospheric composition.

    High-resolution electron temperature measurements using the plasma-line asymmetry

    Authors: M. J. Nicolls: School of Electrical and Computer Engineering, Cornell University, Ithaca, New York, U.S.A.; currently at National Astronomy and Ionosphere Center, Arecibo Observatory, Arecibo, Puerto Rico, U.S.A.;

    M. P. Sulzer, N. Aponte, R. Seal, and S. A. González: National Astronomy and Ionosphere Center, Arecibo Observatory, Arecibo, Puerto Rico, U.S.A.;

    R. Nikoukar: Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Illinois, U.S.A.

    Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027222, 2006

  4. In select case studies, MODIS-Terra is better than MODIS-Aqua at measuring mineral dust aerosols

    Mineral dust aerosol, which affects climate by scattering and absorbing solar radiation, is among the most difficult aerosol species to measure quantitatively from space, because remote sensing platforms have difficulty to distinguish dust plumes from clouds. To evaluate the effectiveness of various remote sensing platforms that measure mineral dust aerosols, Redemann et al. conducted a suite of statistical tests to compare satellite retrievals of spectral aerosol optical depth at visible to near-infra-red wavelengths with suborbital data collected by the NASA Ames Airborne Tracking Sunphotometer. They found that the Moderate-resolution Imaging Spectroradiometer (MODIS) retrievals from the Terra satellite agreed better with the suborbital observations than did the MODIS-Aqua retrievals. They hypothesize that the cause of the differences between Terra and Aqua could be instrument calibration.

    Title: Assessment of MODIS-derived visible and near-IR aerosol optical properties and their spatial variability in the presence of mineral dust

    Authors: J. Redemann, Q. Zhang, and B. Schmid: BAER Institute, Sonoma, California, U.S.A.;

    P. Russell: NASA Ames Research Center, Moffett Field, California, U.S.A.;

    J. Livingston: SRI International, Menlo Park, California, U.S.A.;

    H. Jonsson: CIRPAS, Marina, California, U.S.A.;

    L. Remer: NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.

    Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026626, 2006

  5. Anomalous current system off California caused marine birds to abandon breeding colonies

    During their 2005 spring survey of the breeding success and food habits of Cassin's Auklet, a plankton-eating marine bird, Sydeman et al. documented that the auklets on Southeast Farallon Island, California, abandoned breeding colonies en masse between 10-20 May. This marks the first time in the 35-year observational history of this island that reproductive success was zero. The authors found similar patterns in auklet behavior at Triangle Island, British Columbia, where reproductive success was only 8 percent; at both locations, planktonic biomass during the time of abandonment was lower than normal. Further studies of marine birds revealed high summer and fall auklet abundances in the southern California Bight, (32 to 36 degrees north) during 2005, suggesting emigration of failed breeders from the north. The authors attribute this unusual bird behavior to anomalously delayed spring conditions of the northern California Current system, possibly caused by unusual atmospheric blocking in the Gulf of Alaska, which in turn delayed the transport of nutrients to coastal waters north of Point Conception, California (34 degrees north) and reduced the abundance of auklet prey.

    Title: Plaktivorous auklet Ptychoramphus aleuticus responses to ocean climate, 2005: Unusual atmospheric blocking?

    Authors: William J. Sydeman, Russel W. Bradley, Peter Warzybok, Christine L. Abraham, and Jamie Jahncke: Marine Ecology Division, PRBO Conservation Science, Petaluma, California, U.S.A.;

    K. David Hyrenbach: School of Aquatic and Fisheries Science (visiting scholar), University of Washington, Seattle, U.S.A.; also at Nicholas School for the Environment and Earth Sciences, Duke University, Durham, North Carolina, U.S.A.;

    Vernon E. Kousky: Climate Prediction Center, U.S. National Oceanic and Atmospheric Administration, Camp Springs, Maryland, U.S.A.;

    J. Mark Hipfner, Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada;

    Mark D. Ohman, Integrative Oceanography Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, U.S.A.

    Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026736, 2006

  6. 18-month signal found within the Indian Ocean Dipole

    After the discovery of a basin-wide anomalous pattern of sea surface temperatures in the equatorial Indian Ocean, called the Indian Ocean Dipole, several studies focused on the tropical Indian Ocean's interannual variability. Noting that many such studies considered signals with time scales of two years and longer, Sakova et al. sought to describe more fully a shorter, repetitive signal they had previously discovered, which cycles every 18 to 20 months. Using weekly averaged sea surface height observations from 1992 to 2004, collected by the ERS/Envisat//Topex/Jason-1 satellites, and temperature profiles from data collected by expendable bathythermograph instruments between 1989 and 2002 near the Java-Sumatra coast, the authors identified the dominant frequency bands in sea surface height variability over the entire Indian Ocean and near Indonesia. They found that in most regions, the Indian Ocean showed five cycles in sea surface height variability, including the 18 to 20 month signal. Further investigations revealed that this signal is related to the Indian Ocean Dipole, was particularly strong between 1994 and 1997, and develops near the Sumatra coast before propagating across the basin.

    Title: Interannual variability in the Indian Ocean using altimeter and XBT data--does the 18-month signal exist?

    Authors: Irina V. Sakova: CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia; and School of Geography and Environmental Studies, University of Tasmania, Hobart, Tasmania, Australia;

    Gary Meyers: CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia;

    Richard Coleman: CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia; School of Geography and Environmental Studies, University of Tasmania, Hobart, Tasmania, Australia; and Antarctic Climate and Ecosystems CRC, Hobart, Tasmania, Australia.

    Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027117, 2006

  7. Many earthquake swarms start not with a bang but a whimper

    Earthquakes often occur in staccato bursts, some with an obvious mainshock trailing aftershocks in its wake, and others occurring in swarms, defined by a more steady rate of earthquakes for hours or days. To better understand how one earthquake leads to another, Vidale et al. analyzed crustal earthquake bursts along Japan's convergent plate margin; they compared these bursts with previous analyses of bursts in California, to see whether differing tectonic regimes influence earthquake behavior. The authors found that bursts in Japan also range between dribbling "swarms" and those adhering to the classic mainshock-aftershock pattern, and that this collective behavior appears general rather than restricted to one type of geologic region. In addition, they found that swarms within 50 kilometers [30 miles] of Japan's volcanoes tended last to more than twice as long as those at greater distances. The authors hypothesize that swarm-like activity hints at underlying fluid pressure redistribution and/or gradual movement that did not generate significant seismic waves, while mainshock-aftershock bursts result simply from a cascade of elastic failures.

    Title: Crustal earthquake bursts in California and Japan; their patterns and relation to volcanoes

    Authors: John E. Vidale: Department of Earth and Space Sciences, University of Washington, Seattle, Washington, U.S.A.;

    Katie L. Boyle: Lawrence Livermore National Laboratory, Livermore, California, U.S.A.;

    Peter M. Shearer: Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, San Diego, California, U.S.A.

    Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027723, 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).

You may read the scientific abstract for any of these papers that have already been published by going to 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 a published research paper, see Part II.

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 [], 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 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.

American Geophysical Union

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