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AGU journal highlights -- Sept. 9, 2009

The following highlights summarize research papers that have been published in Geophysical Research Letters (GRL) or the Journal of Geophysical Research Atmospheres (JGR-D).

In this release:

  1. Dry years getting drier in Pacific Northwest
  2. Explaining the rainfall-humidity relationship
  3. Radioactive tracers illuminate ancient solar cycles
  4. New clues found in Saturn rotation mystery
  5. Flapping sheet affects Saturn's magnetism
  6. Balancing Earth's energy budget
  7. Ancient Antarctica had more room for ice than was thought

Anyone may read the scientific abstract for any of these papers by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2009GL039667. 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 .

1. Dry years getting drier in Pacific Northwest

The effects of climate change on the water cycle have important implications for water management as well as for aquatic and terrestrial ecology. Most recent studies of annual streamflow in the Pacific Northwest have looked at the mean or median streamflow and have found little or no change during recent decades. However, Luce and Holden point out that in addition to the average flow, it is important to have a clear picture of the severity of dry spells, which has not been studied as thoroughly. To test for trends in the driest years, they analyze annual runoff data from 43 stations in the Pacific Northwest covering the years 1948 to 2006. They find that while few stations had significant declines in median or mean streamflow, most stations had significant declines in streamflow in the driest 25 percent of years. In other words, dry years have been getting substantially drier. The authors conclude that water managers who must cope with water scarcity and its ecological consequences will face increasing challenges as these trends continue.

Title: Declining annual streamflow distributions in the Pacific Northwest United States, 1948-2006

Authors: C. H. Luce: US Forest Service, Boise, Idaho, USA; Z. A. Holden: US Forest Service, Missoula, Montana, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039407, 2009;

2. Explaining the rainfall-humidity relationship

It is not surprising that rainfall and humidity in the tropics are related, and observations have confirmed this connection, but details of the mechanisms underlying the relationship are not fully understood. To gain a physically motivated explanation for the relationship, Muller et al. create a simple two-layer model of the atmosphere that assumes that precipitation occurs when the amount of water vapor in the lower level exceeds a threshold value; this criterion is based on a stability argument. The amount of rainfall then depends on the total water vapor in both layers. The authors find that the model qualitatively reproduces the observed relationship between precipitation and water vapor, explains the precipitation-humidity relationship over a broad range of water vapor values, and may help explain the temperature dependence of the relationship. Because the humidity-rainfall relationship is a key component in theories for other tropical atmospheric phenomena, such as the Madden Julian Oscillation, the authors note that their model could also help improve scientists' understanding of those features.

Title: A model for the relationship between tropical precipitation and column water vapor

Authors: Caroline J. Muller, Larissa E. Back, Paul A. O'Gorman, and Kerry A. Emanuel: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039667, 2009;

3. Radioactive tracers illuminate ancient solar cycles

Past solar activity can be inferred using records of cosmogenic isotopes, including beryllium-10 and carbon-14, which are produced in cosmic ray showers at rates that vary with solar activity. To study solar variability and possible links with Earth's climate during the Holocene (the past approximately 11,700 years), Knudsen et al. analyze both beryllium-10 and carbon-14 data records. The authors find that both records showed that the periodic nature of the solar activity was especially pronounced 6000-4500 years ago and 3000-2000 years ago, but less pronounced during other times. The dominant periodic variations had cycles about 88, 150, 220, and 400 years long. Because beryllium-10 and carbon-14 have different geochemical properties, the fact that both records show the same periodic behavior indicates that they do reflect the behavior of the Sun, the authors suggest. They also find some indication that the Sun influenced Earth's climate when the 220- and 400-year solar cycles were particularly prominent.

Title: Taking the pulse of the Sun during the Holocene by joint analysis of 14C and 10Be

Authors: Mads Faurschou Knudsen, Bo Holm Jacobsen, and Marit-Solveig Seidenkrantz: Department of Earth Sciences, University of Aarhus, Aarhus, Denmark; Peter Riisager: Geological Survey of Denmark and Greenland, Copenhagen, Denmark; Raimund Muscheler and Ian Snowball: GeoBiosphere Science Centre, Department of Quaternary Geology, Lund University, Lund, Sweden.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039439, 2009;

4. New clues found in Saturn rotation mystery

Scientists have known for some time that Saturn emits intense kilometer-wavelength radio emission, known as Saturn kilometric radiation (SKR), that rotates with a period of 10.8 hours. However, scientists have been puzzled by more recent observations that found a component of this oscillation with a slightly different rotation period, about 10.6 hours. It had been thought that the motion of magnetospheric particles that emit SKR radiation was linked to motion of the planetary interior, but the discovery of the second component cast doubt on this interpretation. Further investigating the characteristics of the two SKR components, Gurnett et al. find that the 10.8-hour component originates from Saturn's southern auroral region, while the more recently discovered 10.6-hour component originates from the northern auroral region. They discuss several north-south asymmetries on Saturn that could be factors in explaining the asymmetry in SKR rotation rates. The authors believe that the study should help improve scientists' understanding of how angular momentum is transferred from the inner planet to Saturn's magnetosphere.

Title: Discovery of a north-south asymmetry in Saturn's radio rotation period

Authors: D. A. Gurnett, W. S. Kurth, A. M. Persoon, and J. B. Groene: Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA; A. Lecacheux and P. Zarka: LESIA, Observatoire de Paris, University Paris Diderot, Meudon, France; L. Lamy: Space and Atmospheric Physics, Imperial College London, London, UK; J. F. Carbary: Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039621, 2009;

5. Flapping sheet affects Saturn's magnetism

In recent years, NASA's Cassini satellite has found interesting activity in Saturn's magnetosphere that has attracted scientists' attention. Periodic motions in the north-south component of the magnetic field in Saturn's magnetotail have been observed, and some scientists have interpreted them as indicating the passage of large-scale plasma and magnetic field structures known as plasmoids. However, others have suggested that the motions may instead represent a regular wavering of Saturn's current sheet, which separates outward and inward pointing regions of Saturn's magnetic field. To clarify the issue, Jackman et al. analyze data from two intervals when Cassini was in Saturn's magnetotail. Their study indicates that the small periodic deflections of the magnetic field can be interpreted as due to normal periodic flapping of Saturn's current sheet. This is in contrast to the larger, rarer observations of plasmoids in Saturn's magnetotail. The authors believe that the analysis will help scientists better understand the dynamics of Saturn's magnetotail.

Title: Northward field excursions in Saturn's magnetotail and their relationship to magnetospheric periodicities

Authors: C. M. Jackman: Space and Atmospheric Physics Group, Imperial College London, London, UK; C. S. Arridge: Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, Dorking, UK and Centre for Planetary Sciences, University College London, London, UK; H. J. McAndrews, M. G. Henderson, and R. J. Wilson: Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039149, 2009;

6. Balancing Earth's energy budget

The Earth's surface can be considered a system with an energy budget, with energy being gained by solar radiation and by internal heating through volcanic eruptions, and energy being lost by reflection and diffusion into space. Recognizing that simple analyses can reveal bulk trends in the Earth system without needing to use global climate models, Murphy et al. examined the Earth's energy balance since 1950, focusing specifically on how this governs the warming Earth. Through measurements and radiative transfer models of ocean heat content, long-lived trace gases, and volcanic eruptions, the authors found that about 20 percent of the total energy available to warm the Earth from greenhouse gases and solar radiation since 1950 has been diffused back into space. By contrast, 10 percent has gone into heating the Earth, almost all into the oceans. An additional 20 percent has been balanced by cooling associated with volcanic eruptions. The remaining 50 percent has been balanced by anthropogenic aerosols. The data show that aerosol forcings are consistent with estimates of trends in global sulfate emissions.

See previous press release at:

Title: An observationally based energy balance for the Earth since 1950

Authors: D. M. Murphy, S. Solomon, R. W. Portmann, and K. H. Rosenlof: Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, U.S.A.; P. M. Forster: School of Earth and Environment, University of Leeds, Leeds, U.K.; T. Wong: NASA Langley Research Center, Hampton, Virginia, U.S.A.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2009JD012105, 2009;

7. Ancient Antarctica had more room for ice than was thought

About 34 million years ago, during the Eocene-Oligocene transition, Earth's climate shifted from warmer to cooler. Models for the growth of the Antarctic ice sheet during that transition show a lot of ice in East Antarctica but very little in West Antarctica. Other data, however, indicate that much more ice must have existed than those models predict, so climate scientists had trouble explaining where all the ice formed. To resolve the issue, Wilson and Luyendyk create a new model of the topography of Antarctica around 34 million years ago, taking into account several geologic factors that have affected topography since the Eocene-Oligocene transition but have not been considered in other models. Their reconstruction shows that West Antarctica had a higher elevation 34 million years ago than previously thought. This adds about 10-20 percent to the total land area above sea level, creating additional area that could have held ice during the formation of the Antarctic ice sheet. The authors believe the study will help improve understanding of the formation of Antarctic ice and will be useful for global climate models.

See previous press release at:

Title: West Antarctic paleotopography estimated at the Eocene-Oligocene climate transition

Authors: Douglas S. Wilson: Marine Science Institute, University of California, Santa Barbara, California, USA and Department of Earth Science, University of California, Santa Barbara, California, USA; Bruce P. Luyendyk: Department of Earth Science, University of California, Santa Barbara, California, USA and Institute for Crustal Studies, University of California, Santa Barbara, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039297, 2009;


Contact: Peter Weiss
American Geophysical Union

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