Public Release:  AGU journal highlights -- Aug. 6, 2009

American Geophysical Union

The following highlights summarize research papers that have been published in Geophysical Research Letters (GRL), the Journal of Geophysical Research - Atmospheres (JGR-D), or Geochemistry, Geophysics, Geosystems (G3).

In this release:

  1. Ozone depletion reduces ocean carbon uptake
  2. Study pinpoints sources of Earth's "hum"
  3. IPCC models might overestimate methane and nitrous oxide emissions from the ocean
  4. Martian dust storms generate lightning
  5. Scientists identify lake shorelines on Mars
  6. Land cover changes influence regional climate
  7. An overview of global dimming and brightening
  8. Can seismic variation in the mantle be explained by temperature alone?
  9. New model shows how short-lived chemicals get to the stratosphere
  10. How ocean mass flows around Antarctica
  11. A 3-D model of ocean dynamics
  12. Seasonal winds might drive current variability in the northern Indian Ocean
  13. Closely spaced fjords increase ocean mixing

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 http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2009GL038227. 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

1. Ozone depletion reduces ocean carbon uptake

The Southern Ocean plays an important role in mitigating climate change because it acts as a sink of atmospheric carbon dioxide. Most current models predict that the strength of the Southern Ocean carbon dioxide sink should increase as atmospheric carbon dioxide rises, but observations show that this has not been the case. To help resolve this discrepancy, Lenton et al. consider the effects of stratospheric ozone depletion, which most previous studies had not included. They compare coupled carbon-climate models with and without ozone depletion and find that including ozone depletion produced a significant reduction in Southern Ocean carbon uptake, in good agreement with observed trends. The simulations show that ozone depletion, combined with increased atmospheric greenhouse gas concentration, drives stronger winds above the Southern Ocean. These stronger winds bring more carbon-rich deep water to the surface, which reduces the ocean's ability to absorb more carbon dioxide from the atmosphere. The authors also find that ozone depletion increases ocean acidification. They suggest that future climate models should take stratospheric ozone into account.

Title: Stratospheric ozone depletion reduces ocean carbon uptake and enhances ocean acidification

Authors: Andrew Lenton and Nicolas Metzl: LOCEAN, IPSL, UPMC, MHNH, IRD, CNRS, Paris, France;

Francis Codron: LMD, IPSL, UPMC, X, ENS, CNRS, Paris, France;

Laurent Bopp, Patricia Cadule, and Alessandro Tagliabue: LSCE, IPSL, CEA, UVSQ, CNRS, Gif-sur-Yvette, France;

Julien Le Sommer: LEGI, UJF, INPG, CNRS, Grenoble, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038227, 2009; http://dx.doi.org/10.1029/2009GL038227


2. Study pinpoints sources of Earth's "hum"

The Earth emits a continuous low-frequency vibration, or "hum," that is not attributable to earthquakes. Previous studies had found that this hum is excited by infragravity waves, a type of ocean wave that originates in shallow water along coasts, but it was uncertain whether hum is generated primarily by infragravity waves in the deep ocean or along coastlines. To pinpoint the sources of Earth's hum, Bromirski and Gerstoft correlate hum intensity data from the EarthScope USArray transportable array with ocean wave height measurements and model simulations. Their results show that the hum is generated primarily along coasts, with no significant hum generation in the deep ocean. In particular, they find that the Pacific coasts of North America and Central America are important sources of the hum, and the west coast of Europe is a strong secondary source region, while no significant hum was detected from the Southern Hemisphere during the study period (November 2006 to June 2007). The study is the first to identify these specific source regions for Earth's hum.

Title: Dominant source regions of the Earth's "hum" are coastal

Authors: Peter D. Bromirski and Peter Gerstoft: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038903, 2009 http://dx.doi.org/10.1029/2009GL038903


3. IPCC models might overestimate methane and nitrous oxide emissions from the ocean

With a global warming potential about 20 times as large as carbon dioxide, methane is a strong greenhouse gas, as is nitrous oxide, which has about 300 times carbon dioxide's warming potential. Both are produced in the ocean by microbial activity. Although more than 30 years old, assessments of the rate of methane emitted by the ocean are included in the Intergovernmental Panel on Climate Change's (IPCC) recent reports and are used as inputs into climate forecasts. However, a new study finds that these older assessments might overestimate current oceanic emissions of methane. In 1998, Rhee et al. measured the concentrations of methane and nitrous oxide in the surface waters of the Atlantic Ocean, from 50 degrees North to 50 degrees South. By extrapolating these values, they recently found that their estimate of the rate of methane emitted by the global ocean is about 10 times less than the values used by the IPCC. Their nitrous oxide emission rates are about 3 times less. The authors' estimate for methane is similar to basin-wide studies of the Pacific Ocean, suggesting that IPCC overestimates were likely generated by an overly coarse sampling method.

Title: Methane and nitrous oxide emissions from the ocean: A reassessment using basin-wide observations in the Atlantic

Authors: T. S. Rhee: Center of Polar Climate Science, Korea Polar Research Institute, Incheon, Korea;

A. J. Kettle: Biogeochemistry Department, Max Plank Institute for Chemistry, Mainz, Germany; Now at Department of Earth Sciences, State University of New York at Oswego, Oswego, New York, USA;

M. O. Andreae: Biogeochemistry Department, Max Plank Institute for Chemistry, Mainz, Germany.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD011662, 2009; http://dx.doi.org/10.1029/2008JD011662


4. Martian dust storms generate lightning

Dust storms on Earth build up an electric field as dust particles collide, and then emit lightning as the electric field discharges. Some previous evidence suggested that Martian dust storms might also generate lightning in this manner, but the phenomenon had not been directly observed. To observe evidence of lightning on Mars, Ruf et al. use a new detector that is able to distinguish nonthermal microwave radiation, which indicates a large electric discharge, from ordinary thermal radiation. The instrument, installed in the 34-meter (111.55-feet) radio telescope of the Deep Space Network, made measurements from 22 May to 16 June 2006. The authors detected nonthermal microwave emission in bursts several minutes long during a period of about 3 hours that coincided with a large dust storm on 8 June 2006. On the basis of the spectrum of this radiation, the authors conclude that the radiation was probably excited by lightning in the Martian dust storm. They note that the discovery of electrical activity in Martian dust storms has implications for atmospheric chemistry, habitability, and preparations for human exploration.

Title: Emission of non-thermal microwave radiation by a Martian dust storm

Authors: Christopher Ruf, Nilton O. Renno, Jasper F. Kok, Etienne Bandelier, and Steven Gross: Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA;

Michael J. Sander and Lyle Skjerve: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA;

Bruce Cantor: Malin Space Science Systems, Inc., San Diego, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038715, 2009; http://dx.doi.org/10.1029/2009GL038715


5. Scientists identify lake shorelines on Mars

Scientists generally believe that warm, wet conditions existed on Mars until only about 3.7 billion years ago. In recent years, however, remote sensing studies have hinted at the existence of Martian lakes during the Hesperian epoch (about 3.5 billion to 1.8 billion years ago). Using images from the High Resolution Imaging Science Experiment (HiRISE) camera on board NASA's Mars Reconnaissance Orbiter, Di Achille et al. report direct evidence of lake strandlines in Mars's Shalbatana Vallis. The sub-meter-scale images show clear, unambiguous evidence of shorelines of a lake more than 450 meters (1,476 feet) deep that formed about 3.4 billion years ago. The study indicates that conditions favorable for flowing water and lake formation may have existed for thousands of years on Mars during the Hesperian epoch, which has been thought to be a period during which surface conditions did not allow significant hydrological activity. The authors suggest that the sedimentary deposits associated with the lake in Shalbatana Vallis should be considered a priority for further study by future landed Mars missions.

Title: Positive identification of lake strandlines in Shalbatana Vallis, Mars

Authors: Gaetano Di Achille and Mindi L. Searls: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA;

Brian M. Hynek: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA and Department of Geological Sciences, University of Colorado, Boulder, Colorado, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038854, 2009; http://dx.doi.org/10.1029/2009GL038854


6. Land cover changes influence regional climate

Human-induced land cover changes (LCCs), such as the clearing of forests for farming, can affect climate. To study the regional and global effects of LCC, Pitman et al. analyze seven different climate models. Each model simulation is run several times, with prescribed land cover reflecting conditions in 1870 and in 1992. The authors find that in all models, LCC has a statistically significant regional effect on latent heat flux and near-surface temperature. Furthermore, they find that LCC affect temperature only in the region where the land cover change took place, not in remote regions. While all models show significant regional effects, these vary across models for several reasons arising from differences in the implementation of the LCC, different land surface models, and different ways of representing the landscape. The researchers conclude that it is essential to include LCC in future regional and global climate studies but that it is not feasible to impose them in a common way across multiple models for the next Intergovernmental Panel on Climate Change assessment.

Title: Uncertainties in climate responses to past land cover change: First results from the LUCID intercomparison study

Authors: A. J. Pitman: Climate Change Research Centre, University of New South Wales, Sydney, New South Wales, Australia, and collaborators (for complete list see online abstract at link below.)

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039076, 2009; http://dx.doi.org/10.1029/2009GL039076


7. An overview of global dimming and brightening

Radiation from the Sun fuels life on Earth and determines the climate of its inhabitants. The amount of radiation that reaches the Earth's surface helps to control evaporation rates, snow and glacier melt, carbon uptake through plant photosynthesis, the severity of seasons, and agricultural productivity. Thus changes in the amount of solar energy reaching the Earth can affect society, the environment, and the economy. A growing body of evidence supports that human interference has affected the amount of solar radiation that is able to penetrate the atmosphere--in general, aerosol pollutants have been shown to scatter or absorb incoming solar radiation and increase the reflectance and lifetime of clouds, thereby reducing the amount of solar radiation reaching the Earth's surface. Dubbed "global dimming," this phenomenon has been countered in the past few decades by "global brightening" as pollution control efforts have met with success. Wild reviews the evidence surrounding global dimming and brightening, addressing a number of frequently asked questions such as how and when dimming and brightening originated, how dimming and brightening affects other global environmental processes, whether current climate models successfully replicate observed patterns in dimming and brightening, and how dimming and brightening may evolve over the coming decades.

Title: Global dimming and brightening: A review

Authors: Martin Wild: Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD011470, 2009; http://dx.doi.org/10.1029/2008JD011470


8. Can seismic variation in the mantle be explained by temperature alone?

The structure and dynamics of Earth's mantle are of great importance because they determine the forces acting upon tectonic plates. Imaging the deep Earth is accomplished through seismic tomography, where variations in seismic wave travel times help to map out elastic heterogeneity at depth. However, interpreting the tomographic images in terms of composition and flow remains a challenging task. On the basis of comparisons of geodynamic Earth models, which include detailed mineralogical information on mantle elasticity, with tomography studies, Schuberth et al. show that stark lateral temperature variations occur deep in the mantle and that these are consistent with a variety of seismic observations. Through providing a new way to compare geodynamic and seismic Earth models quantitatively, the authors suggest that the mantle could be chemically homogeneous but thermally differentiated because of a large heat flux from the core. This implies that thermal history models of the Earth may have to be reevaluated. Most important, the good agreement between their geodynamic models and tomographic images argues for mantle flow that is driven not by chemistry but predominantly by temperature variations.

Title: Tomographic filtering of high-resolution mantle circulation models: Can seismic heterogeneity be explained by temperature alone?

Authors: B. S. A. Schuberth and H.-P. Bunge: Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Munich, Germany;

J. Ritsema: Department of Geological Science, University of Michigan, Ann Arbor, Michigan, USA.

Source: Geochemistry, Geophysics, Geosystems (G3) paper 10.1029/2009GC002401, 2009; http://dx.doi.org/10.1029/2009GC002401


9. New model shows how short-lived chemicals get to the stratosphere

The exchange of chemicals between the troposphere and the stratosphere can influence ozone production and longevity. For example, bromine compounds can turn into active forms that can efficiently deplete ozone if they are transported from the troposphere to the stratosphere through what is known as the tropical tropopause layer (TTL). To better understand how compounds are transported through the TTL, Gettelman et al. develop a model that simulates the effect of chemical reactions important to several short-lived compounds that pass through this region. Their model is one-dimensional, allowing complex processes to be reduced to a few simple relationships. The authors find that compounds that pass through the TTL with chemical lifetimes of 25 days or longer will have significant concentrations in the stratosphere and will be governed by vertical advection. By contrast, convection cycles govern the distribution of compounds with lifetimes of less than 25 days. This fundamental difference will help scientists understand the mechanics of how stratospheric ozone becomes depleted through interactions with these short-lived compounds.

Title: Process regulating short-lived species in the tropical tropopause layer

Authors: A. Gettleman and M. Park: Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA;

P. H. Lauritzen: Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado, USA;

J. E. Kay: Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado, USA; also at Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2009JD011785, 2009; http://dx.doi.org/10.1029/2009JD011785


10. How ocean mass flows around Antarctica

Understanding the dynamics of how ocean masses move in response to wind and other stresses is important for understanding the global ocean circulation and the climate system. To study mass flows in the Southern Ocean near Antarctica and how they relate to wind stress, Ponte and Quinn analyze data from the Gravity Recovery and Climate Experiment (GRACE) satellite mission (which measures Earth's gravity field and can sense mass movement) and estimates of winds and currents produced by the Estimating the Circulation and Climate of the Ocean (ECCO) project. The results indicate that mass exchange occurs primarily between the Southern Ocean and the Pacific. The authors show how near-surface meridional flows directly driven by zonal winds are balanced by deepwater return flows that are slightly lagged in time. They point out that the study helps clarify the dynamic mechanisms that relate wind stress to ocean mass flows in the Southern Ocean and demonstrates the usefulness of the GRACE satellites in monitoring ocean mass variations.

Title: Bottom pressure changes around Antarctica and wind-driven meridional flows

Authors: Rui M. Ponte and Katherine J. Quinn: Atmospheric and Environmental Research, Inc., Lexington, Massachusetts, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039060, 2009; http://dx.doi.org/10.1029/2009GL039060


11. A 3-D model of ocean dynamics

Proposed space missions could soon be able to measure sea surface height (SSH) with resolution as high as 10 kilometers (6.2 miles), a significant improvement over conventional radar altimetry. According to a study by Klein et al., such SSH measurements will make it possible for scientists to study not only the surface of the ocean but also the circulation as deep as 500 meters (1,640 feet) below the surface. The authors develop a simulation to show that it is possible to use high-resolution SSH measurements, combined with information on the large-scale vertical stratification, to reconstruct 3-D motions, in particular the vertical velocity, in the ocean down to 500 m. Their model simulates a realistic situation involving a turbulent eddy field and wind-driven motions in the upper layers of the ocean. The authors believe that the results show that improved SSH measurements made by planned missions will enable significant advances in the study of ocean dynamics.

Title: Diagnosis of vertical velocities in the upper ocean from high resolution sea surface height

Authors: P. Klein, G. Roullet, E. Danioux, and S. Le Gentil: Laboratoire de Physique des Océans, IFREMER, UBO, IRD, CNRS, Plouzané, France;

J. Isern-Fontanet: Laboratoire d'Océanographie spatiale, IFREMER, Plouzané, France and Institut Català de Ciències del Clima, Barcelona, Spain;

G. Lapeyre: Laboratoire de Météorologie Dynamique, IPSL, ENS, CNRS, Paris, France;

B. Chapron: Laboratoire d'Océanographie spatiale, IFREMER, Plouzané, France;

H. Sasaki: Earth Simulator Center, JAMSTEC, Yokohama, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038359, 2009; http://dx.doi.org/10.1029/2009GL038359


12. Seasonal winds might drive current variability in the northern Indian Ocean

Oceanographers and meteorologists need to understand the forces driving upper ocean currents and sea level variability. To study the dynamics of the response of the northern Indian Ocean to intraseasonal winds, Vialard et al. analyze satellite observations of sea level and wind stress as well as a new data set of currents recorded at 15 degrees North on the west coast of India. They find that while sea level shows a seasonal variability, the alongshore current shows no clear seasonal cycle but is dominated by intraseasonal (55-110 day) fluctuations. These current variations, the authors find, arise as a response of the northern Indian Ocean to intraseasonal winds associated with the Madden-Julian Oscillation. The authors use linear wave theory to explain these observations. Although the study focuses on the Indian Ocean, the authors believe that similar dynamics could drive coastal current variability in the Atlantic and Pacific oceans. The results could also have implications for coastal current monitoring, the authors suggest.

Title: Intraseasonal response of the northern Indian Ocean coastal waveguide to the Madden-Julian Oscillation

Authors: J. Vialard: Laboratoire d'Océanographie Expérimentation et Approches Numériques, IRD, Université Pierre et Marie Curie, Paris, France;

S. S. C. Shenoi: Indian National Centre for Ocean Information Services, Hyderabad, India;

J. P. McCreary: International Pacific Research Centre, University of Hawaii, Honolulu, Hawaii, USA;

D. Shankar, V. Fernando, and S. R. Shetye: Physical Oceanography Division, National Institute of Oceanography, CSIR, Goa, India;

F. Durand: Laboratoire d'Etudes en Géophysique et Oceanographie Spatiales, IRD, Toulouse, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038450, 2009; http://dx.doi.org/10.1029/2009GL038450


13. Closely spaced fjords increase ocean mixing

Ocean mixing generated in confined areas such as fjords can play an important role in the larger-scale ocean circulation. However, most ocean circulation studies have focused on open regions. To examine how waves below the surface, known as internal waves, are modified by closely spaced submerged ridges, or sills, Xing and Davies create an idealized cross-sectional model of a fjord. They examine a version with a single isolated sill as well as one with two sills separated by 1.5 kilometers (0.93 miles), a separation typical in fjords. The model shows that, in the case of two sills, internal waves become trapped between the two sills, leading to standing wave generation as well as increased mixing as energy is transferred from larger waves to shorter waves. In contrast, in the single-sill simulation, standing wave generation and increased mixing do not occur. The authors note that the results explain recent observations around fjords and could also be applied to lakes or other situations with closely spaced bottom topography.

Title: Influence of multiple sills upon internal wave generation and the implications for mixing

Authors: Jiuxing Xing and Alan M. Davies: Proudman Oceanographic Laboratory, Liverpool, UK.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL038181, 2009; http://dx.doi.org/10.1029/2009GL038181

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