AGU journal highlights -- Jan. 21, 2010

January 21, 2010

The following highlights summarize research papers that have recently been published in Geophysical Research Letters (GRL).

In this release:

  1. First study to show that seismic imaging detects ocean's internal tides
  2. Ice is "rotten" in the Beaufort Sea
  3. Global warming increases flood risk in mountain areas
  4. Worldwide nitrogen deficit constrains carbon dioxide uptake by plants
  5. Upper atmosphere influences weather near Earth's surface
  6. New finding on key element of Earth's lower mantle

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. 10.1029/2009GL038909. 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. First study to show that seismic imaging detects ocean's internal tides

Internal tides, waves below the ocean's surface that propagate at tidal frequencies, play an important role in ocean mixing but can be difficult to detect and study. Holbrook et al. find they can detect these waves with seismic imaging. Using seismic data acquired in the Norwegian Sea, the authors observe bands of reflections that indicate internal tides. The study is the first to show that seismic imaging can be used to detect internal tides and demonstrates that seismic oceanography could be an important tool in the study of ocean mixing.

Title: Images of internal tides near the Norwegian continental slope

Authors: W. Steven Holbrook: Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, USA; Ilker Fer: Geophysical Institute, University of Bergen, Bergen, Norway; Raymond W. Schmitt: Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038909, 2009

2. Ice is "rotten" in the Beaufort Sea

Recent observations show that Beaufort Sea ice was not as it appeared in the summer of 2009. Sea ice cover serves as an indication of climate and has implications for marine and terrestrial ecosystems. In early September 2009, satellite measurements implied that most of the ice in the Beaufort Sea either was thick ice that had been there for multiple years or was thick, first-year ice. However, in situ observations made in September 2009 by Barber et al. show that much of the ice was in fact "rotten" ice--ice that is thinner, heavily decayed, and structurally weak due to a uniform temperature throughout. The authors suggest that satellite measurements were confused because both types of ice exhibit similar temperature and salinity profiles near their surfaces and a similar amount of open water between flows. The authors note that while an increase in summer minimum ice extent in the past 2 years could give the impression that Arctic ice is recovering, these new results show that multiyear ice in fact is still declining. The results have implications for climate science and marine vessel transport in the Arctic.

Title: Perennial pack ice in the southern Beaufort Sea was not as it appeared in the summer of 2009

Authors: David G. Barber, Ryan Galley, Matthew G. Asplin, Kerri-Ann Warner and Mukesh Gupta: Centre for Earth Observation Science, Faculty of Environment, Earth and Resources, University of Manitoba, Winnipeg, Manitoba, Canada; Roger De Abreu: Canadian Ice Service, Environment Canada, Ottawa, Ontario, Canada; Monika Pućko: Centre for Earth Observation Science, Faculty of Environment, Earth and Resources, University of Manitoba, Winnipeg, Manitoba, Canada and Freshwater Institute, Fisheries and Oceans, Winnipeg, Manitoba, Canada; Simon Prinsenberg: Bedford Institute of Oceanography, Fisheries and Oceans, Halifax, Nova Scotia, Canada; Stéphane Julien: Laurentian Region, Canadian Coast Guard, Quebec, Quebec, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041434, 2009

3. Global warming increases flood risk in mountain areas

The world's mountainous regions are home to about 800 million people and the source of some of the world's major rivers. In these regions, runoff is strongly affected by temperature. This suggests that flooding could be quite sensitive to global warming, but there has been some lack of scientific consensus on the effects of temperature variations on floods. Allamano et al. show that global warming does increase flood risk significantly. The authors analyze runoff data recorded by 27 stations in the Swiss Alps and use a simple probabilistic model to study how flood risk varies with temperature, precipitation, and elevation in mountainous regions. The researchers find that large floods have occurred more frequently in recent years than in the past, and they predict that global warming will result in such floods occurring even more often in the future. In particular, they find that if global temperatures increase by 2 degrees Celsius (3.6 degrees Fahrenheit), then large floods that occurred about once every 100 years could occur up to 5 times more often.

Title: Global warming increases flood risk in mountainous areas

Authors: P. Allamano, P. Claps, and F. Laio: Dipartimento di Idraulica, Trasporti ed Infrastrutture Civili, Politecnico di Torino, Turin, Italy.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041395, 2009

4. Worldwide nitrogen deficit constrains carbon dioxide uptake by plants

Nitrogen is an essential nutrient for plants; limits on available nitrogen constrain how much plants can grow. This in turn affects the amount of carbon dioxide plants can absorb, which affects the global climate. Using a framework that considers interactions of carbon and nutrients, Wang and Houlton develop a new global estimate of nitrogen fixation rates. The authors consider the amount of nitrogen plants require to store additional carbon and found that a substantial deficit of nitrogen exists for plants in most areas of the world. They argue that most climate models that do not take into account nitrogen have overestimated carbon uptake and therefore underestimated predicted global warming. The authors suggest that it is important that the next Intergovernmental Panel on Climate Change consider interactions between the nitrogen and carbon cycles.

Title: Nitrogen constraints on terrestrial carbon uptake: Implications for the global carbon-climate feedback

Authors: Ying-Ping Wang: CSIRO Marine and Atmospheric Research and CAWCR, Aspendale, Victoria, Australia; Benjamin Z. Houlton: Department of Land, Air and Water Resources, University of California, Davis, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041009, 2009

5. Upper atmosphere influences weather near Earth's surface

To what extent does what's happening in the stratosphere, tens of kilometers above Earth, influence the weather in the troposphere, the layer of atmosphere that touches Earth? Gerber et al. perform a series of forecast experiments using a general circulation model to study the role of the stratosphere in influencing tropospheric weather following sudden stratospheric warming events. It is known that the troposphere influences the stratosphere in driving these events, but scientists were unsure of whether the stratosphere actively influenced the troposphere. The authors find that the stratosphere does indeed influence the troposphere, although this influence is weak and easily masked by tropospheric variability. The results indicate that improved resolution in stratospheric simulations would probably lead to better weather forecasts.

Title: Stratospheric influence on the tropospheric circulation revealed by idealized ensemble forecasts

Authors: E. P. Gerber: Center for Atmosphere Ocean Science, Courant Institute, New York University, New York, New York, USA; C. Orbe and L. M. Polvani: Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Science, Columbia University, New York, New York, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL040913, 2009

6. New finding on key element of Earth's lower mantle

The electronic spin state of iron in perovskite, a major component of Earth's lower mantle, can influence the physical and chemical properties of the mantle. However, previous studies have presented an unclear picture of the spin state of iron in perovskite, with some experiments contradicting computational results. In a new experiment designed to better control the conditions under which perovskite is synthesized, Grocholski et al. measure the spin state of iron in perovskite. They find the dominant spin state at pressures below 50 gigapascals (7.25 million pounds per square inch) to be high spin but observed a transition to a possible mixed spin state at higher pressures. These results indicate that perovskite is in a mixed or high spin state at least to 2,000 kilometers (1,242 miles) depth in the mantle, consistent with computational results.

Title: Spin and valence states of iron in perovskite

Authors: B. Grocholski and S. H. Shim: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA; W. Sturhahn and J. Zhao: Sector 3, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, USA; Y. Xiao and P. C. Chow: HPCAT, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041262, 2009

American Geophysical Union

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