AGU journal highlights -- 29 June 2012

June 29th, 2012
The following highlights summarize research papers that have been recently published in Water Resources Research (WRR), Space Weather, Journal of Geophysical Research-Earth Surface (JGR-F), Journal of Geophysical Research-Atmospheres (JGR-D), Journal of Geophysical Research-Oceans (JGR-C), and Geophysical Research Letters (GRL).

In this release:

1. Section of Atlantic circulation driven by transient southern Africa current

2. Prediction system to protect astronauts from solar storms

3. Streamflow changes following the 2010 Chile earthquake

4. Reanalyses find rising humidity in the Arctic

5. Local factors important for water availability

Anyone may read the scientific abstract for any already-published paper 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. 10.1029/2011WR011586. 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/news/press/papers.shtml.


1. Section of Atlantic circulation driven by transient southern Africa current

The southward branch of the Atlantic Meridional Overturning Circulation (AMOC), the stretch that carries deep, cold water from the North Atlantic to the Southern Ocean, predominantly flows along the eastern shoreline of the Americas. This deep water transport, known as the Deep Western Boundary Current, hits a fork in the flow at 25 degrees South, off the coast of Brazil. Here the current splits in two with the majority continuing its southbound journey, and a smaller ribbon veering to the east. This eastbound water travels at depth across the South Atlantic, eventually passing around Africa's southern coast. In its path from Brazil to Africa the water must pass across lines of consistent vorticity—essentially travelling uphill. Though researchers have known of this current for decades, little is known of what drives it.

Using an ocean circulation model fed with data recorded from 1980 to 2006, van Sebille et al. find that the eastward flow is driven by Agulhas rings, a transient feature off the South African shoreline. The Agulhas current flows southbound along Africa's eastern shore. Where the Agulhas current meets the Southern Ocean most of the warm Agulhas water retroflects and turns back into the Indian Ocean. This sharp about-face causes the Agulhas current to shed counterclockwise rotating warm water eddies. These eddies travel westward along the African coast and break down in the South Atlantic. The authors used simulated floats to track three-dimensional ocean circulation, finding that the eastward deep water current traveled directly beneath the Agulhas rings. Further, they find that the Agulhas rings actually drive the eastward flow. They note that their finding implies a transitory nature for the eastward branch of AMOC deep water transport as a break in Agulhas ring occurrence would stymie the flow.

For a related story from the Australian Research Council's Centre of Excellence for Climate System Science, see bit.ly/Lldiyt. And for a related video, visit bit.ly/KHHLFq.

Source: Journal of Geophysical Research-Oceans, doi:10.1029/2011JC007684, 2012 http://dx.doi.org/10.1029/2011JC007684

Title: Does the vorticity flux from Agulhas rings control the zonal pathway of NADW across the South Atlantic?

Authors: Erik van Sebille: Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA, and Climate Change Research Centre, University of New South Wales, Sydney, New South Wales, Australia.

William E. Johns and Lisa M. Beal: Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA.


2. Prediction system to protect astronauts from solar storms

With the impending solar maximum expected to bring heightened rates of flares and coronal mass ejections (CMEs), putting at risk an ever-increasing human presence in space, Oh et al. designed and assessed a prediction system to keep astronauts safe from these solar storms. During a solar flare or CME, particles from the Sun can be accelerated to very high energies—in some cases travelling near the speed of light. Protons with energies surpassing 100 megaelectron volts essentially sandblast everything in their path. Though people on Earth are protected by the planet's magnetic field and thick atmosphere, astronauts in spacecraft beyond low-Earth orbit, or people at high altitudes near the poles, can be exposed to this increased radiation. This can potentially cause radiation sickness, with symptoms such as fever and vomiting.

The authors' prediction system uses two different types of neutron detectors installed at the geographic south pole to measure the intensity of the much faster gigaelectron volt neutrons also produced during a solar storm when protons interact with the atmosphere. By combining the observations of the two detectors—one located outside, and the other housed inside, the Amundsen-Scott South Pole Station—the authors calculated the energy spectrum of the arriving protons. They then extrapolated this spectrum to estimate the peak intensity and event-averaged flux (fluence) of the later-arriving megaelectron volt protons. The authors compared their predictions for 12 solar events against observations made by geosynchronous satellites, finding a good agreement for intensity and fluence predictions for protons with energies higher than 40 and 80 megaelectron volts, respectively. The system provides a warning time of up to 166 minutes, depending on the protons' energy, giving polar airplanes or astronauts ample time to reduce their altitude or seek out an armored area in their spacecraft.

Source: Space Weather, doi:10.1029/2012SW000795, 2012 http://dx.doi.org/10.1029/2012SW000795

Title: South Pole neutron monitor forecasting of solar proton radiation intensity

Authors: S. Y. Oh: Department of Astronomy and Space Science, Chungnam National University, Daejeon, South Korea and Bartol Research Institute, Department of Physics and Astronomy, University of Delaware, Newark, Delaware, USA;

J. W. Bieber, J. Clem, P. Evenson, and R. Pyle: Bartol Research Institute, Department of Physics and Astronomy, University of Delaware, Newark, Delaware, USA;

Y. Yi: Department of Astronomy and Space Science, Chungnam National University, Daejeon, South Korea.

Y.-K. Kim: Department of Nuclear Engineering, Hanyang University, Seoul, South Korea.


3. Streamflow changes following the 2010 Chile earthquake

Changes in streamflow and groundwater levels are known to occur following earthquakes. But the mechanisms for the changes in streamflow are not fully understood and vary from one location to another. Mohr et al. investigated streamflow response in small upland catchments in south central Chile following the magnitude 8.8 Chilean earthquake on 27 February 2010. Streamflow initially decreased after the quake, then increased to as much as 400 percent of pre-earthquake levels. The increases peaked hours to several days following the earthquake, after which flow gradually declined, though changes were not uniform across all the catchments the authors studied.

Where did the excess water come from? Several factors suggest to the authors that extra water came from the interface between the sandy saprolite layers and the bedrock. The earthquake main shock produced enough energy that the sandy layer could have acted as a liquid. Vertical permeability may also have increased, allowing a more efficient discharge of the water from the saprolite layer, which in turn enlarged the saturated zone and thereby enhanced streamflow. In addition, the extra released water elevated the ground water table, which enhanced plant transpiration.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1029/2011JF002138, 2012 http://dx.doi.org/10.1029/2011JF002138

Title: Streamflow response in small upland catchments in the Chilean coastal range to the MW 8.8 Maule earthquake on 27 February 2010

Authors: Christian H. Mohr: Institute of Earth and Environmental Science, University of Potsdam, Potsdam, Germany;

David R. Montgomery: Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA;

Anton Huber: Institute of Geosciences, Universidad Austral de Chile, Valdivia, Chile;

Axel Bronstert: Institute of Earth and Environmental Science, University of Potsdam, Potsdam, Germany;

Andrés Iroumé: Institute of Forest Management, Universidad Austral de Chile, Valdivia, Chile.


4. Reanalyses find rising humidity in the Arctic

Direct, reliable observations of atmospheric conditions stretch as far back as the mid seventeenth century, with otherwise consistent records being punctuated by periodic updates in methods, practitioners, and observational equipment. To bridge these shifts in technique and technology, scientists develop reanalysis models designed to tie together diverse observations into a coherent picture of the system's evolution. But, like all models or analytical techniques, reanalysis data sets can suffer from errors or biases. Identifying how the records produced by different reanalyses vary can be a difficult practice, but determining if a cluster of models consistently produces biased results can be even more difficult.

A number of reanalyses have recently been developed to track the rapidly changing Arctic atmosphere, and Serreze et al. compared them with one another and with the observational record. The authors focused on how the reanalyses represent the change in Arctic tropospheric water vapor from 1979 to 2010. They compared three of the most recent and complex reanalyses against meteorological measurements made using radiosondes at nine sites north of 70 degrees North. They find that the reanalyses consistently overestimate low-altitude temperatures and winter humidity. It is important to note that these positive biases caused the reanalyses to miss low-altitude wintertime temperature and humidity inversions identified by the radiosondes.

A finding shared by both reanalyses and radiosonde observations, however, is of an increasing availability of precipitable water in the low-altitude Arctic, which the authors suggest is associated with increasing air-sea surface temperatures, reduced sea ice extent, and other markers consistent with the polar amplification of global warming. Increasing Arctic humidity is a troubling result, as heightening atmospheric water vapor could further drive up regional temperatures.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1029/2011JD017421, 2012 http://dx.doi.org/10.1029/2011JD017421

Title: Recent changes in tropospheric water vapor over the Arctic as assessed from radiosondes and atmospheric reanalyses

Authors: Mark C. Serreze, Andrew P. Barrett, and Julienne Stroeve: National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA;


5. Local factors important for water availability

An important issue that has grabbed the attention of scientists and policy makers alike is the amount of freshwater that will be available to populations across different climate settings, especially as rain belts reorganize in response to warming temperatures over the 21st century. The amount of freshwater available on land, calculated from runoff, is a function of supply and demand, where annual rainfall determines the supply and the dryness determined by solar radiation largely controls the demand. Local factors, such as vegetation and soil types that are directly tied to regional climate, modulate the surface water supply and demand. However, there are no observations to quantify the effect of regional climate on surface water availability.

In a new study, Williams et al. investigate how such climate and vegetation factors modulate the regional surface water balance and associated freshwater supply. They incorporate new meteorological data from 167 FLUXNET sites across the globe. The researchers find that, consistent with previous studies, annual average solar radiation and rainfall control 62 percent of the surface water supply and demand. However, 13 percent of the supply and demand balance is controlled by vegetation type and other regional climatic factors.

Further, they find that in grasslands, evaporation rates are 9 percent higher than in forests in the same climate setting, contrary to common expectations. On the basis of their study, the researchers recommend that climate models investigating water availability should take into account local factors, regional climate, and even topography for more accurate prediction of future water resources.

Source: Water Resources Research, doi:10.1029/2011WR011586, 2012 http://dx.doi.org/10.1029/2011WR011586

Title: Climate and vegetation controls on the surface water balance: Synthesis of evapotranspiration measured across a global network of flux towers

Authors: Christopher A. Williams: Graduate School of Geography, Clark University, Worcester, Massachusetts, USA;

Markus Reichstein: Max Planck Institute for Biogeochemistry, Jena, Germany;

Nina Buchmann: ETH Zurich, Zurich, Switzerland;

Dennis Baldocchi: Environmental Science, Policy, and Management, University of California-Berkeley, Berkeley, California, USA;

Christian Beer: Max Planck Institute for Biogeochemistry, Jena, Germany;

Christopher Schwalm: Graduate School of Geography, Clark University, Worcester, Massachusetts, USA;

Georg Wohlfahrt: Institute of Ecology, University of Innsbruck, Innsbruck, Austria;

Natalia Hasler: Graduate School of Geography, Clark University, Worcester, Massachusetts, USA;

Christian Bernhofer: Institute of Hydrology and Meteorology, Technische Universität Dresden, Dresden, Germany;

Thomas Foken: Department of Micrometeorology, University of Bayreuth, Bayreuth, Germany;

Dario Papale: Department for Innovation in Biological, Agro-food and Forestry, University of Tuscia, Viterbo, Italy;

Stan Schymanski: Max Planck Institute for Biogeochemistry, Jena, Germany;

Kevin Schaefer: National Snow and Ice Data Center, University of Colorado, Boulder, Colorado, USA.


6. Peat-based climate reconstructions run into murky waters?

Peatlands are globally important ecosystems that serve as archives of past environmental change. Peatlands form over thousands of years from the accumulation of decaying plants and hold water, or in some cases purely rainwater. Hence, both external processes, such as climate, and internal processes, such as the rates of peat growth and decay, control the water table in peatlands. However, throughout the previous century and particularly over the past decade, paleoclimatologists have increasingly relied on reconstructions of the water table in rain-fed peatlands to infer changes in rainfall through the Holocene period (the past ~12,000 years), ignoring the potentially important role of internal processes.

But in a new study, Swindles et al. compare paleoecological data from a peatland in England with model simulations to show that the water table in the bogs may change independently of climate. Dynamics inherent in peatland development stabilize the internal environment of the bogs. As a result, the behavior of peatlands can become partially disconnected from external climate influences such as rainfall. The authors further show that water levels in peat bogs do not respond linearly to changes in rainfall. For example, a two-fold increase in rainfall does not result in a two-fold increase in height of water table in the bogs.

On the basis of these results, the authors caution against indiscriminate use of water table reconstructions in peatlands as indicators of past changes in rainfall. The authors suggest detailed investigation of internal dynamics of peatlands; they call for more studies that combine field observations, paleoenvironmental data, and model results to understand the relative importance of both climate change and internal processes in regulating water tables in peatlands.

Source: Geophysical Research Letters, doi:10.1029/2012GL051500, 2012 http://dx.doi.org/10.1029/2012GL051500

Title: Ecohydrological feedbacks confound peat-based climate reconstructions

Authors: Graeme T. Swindles: School of Geography, University of Leeds, Leeds, UK;

Paul J. Morris: Soil Research Centre, Department of Geography and Environmental Science, University of Reading, Reading, UK;

Andy J. Baird: School of Geography, University of Leeds, Leeds, UK;

Maarten Blaauw and Gill Plunkett: School of Geography, Archaeology and Palaeoecology, Queen's University Belfast, Belfast, UK.

Provided by American Geophysical Union

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