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AGU journal highlights -- 31 July 2012

The following highlights summarize research papers that have been recently published in Water Resources Research (WRR),Geophysical Research-Atmospheres (JGR-D), Journal of Geophysical Research-Space Physics (JGR-A), Journal of Geophysical Research-Biogeosciences (JGR-G), and Journal of Geophysical Research-Oceans (JGR-C).

In this release:

1. The random walk of pollutants through river catchments

2. Atmospheric CO2 drove climate change during longest interglacial

3. Shear layers in solar winds affect Earth's magnetosphere

4. Dams impact carbon dynamics in U.S. rivers

5. Comparison with observations shows cloud simulations improving

6. Turbulent forces within river plumes affect spread

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 and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2011WR011367. 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. The random walk of pollutants through river catchments

River catchments play critical roles in regional economies and in the global economy. In addition, rivers carry large volumes of nutrients, pollutants, and several other forms of tracers into the ocean. An intricate system of pathways and channels, both on the surface and in the subsurface of catchments, allows rivers to carry large volumes of tracers. However, scientists do not yet fully understand how pollutants and other tracers travel through the intricate web of channels in the catchment areas of rivers.

In a new study, Cvetkovic et al. show that the travel path of tracers through channels can be modeled as a random walk, which is mathematically similar to the path an animal would trace when foraging. Previous studies have applied the random walk approach to understand the behavior of fluids flowing through aquifers and soils but not to model the transport mechanism of tracers that travel passively with water flowing through catchments.

The authors also show that the random walk behavior of tracers in river catchments depends on how the velocity of particles in the catchment has varied over time; the variability in velocity in turn depends on how the physical space of the channel itself has evolved over a long time. Their study provides a new approach to understanding transport of tracers, such as pollutants and nutrients, within a river catchment.

Source: Water Resources Research, doi:10.1029/2011WR011367, 2012

Title: Water and solute transport along hydrological pathways

Authors: Vladimir Cvetkovic and Christoffer Carstens: Department of Land and Water Resources Engineering, Royal Institute of Technology, Stockholm, Sweden;

Jan-Olof Selroos: Department of Geoscience and Safety, Swedish Nuclear Fuel and Waste Management Co., Stockholm, Sweden;

Georgia Destouni: Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, Sweden.

2. Atmospheric CO2 drove climate change during longest interglacial

Known as the marine isotope stage 11 (MIS 11), the interglacial period centered around 400,000 years ago was the longest and possibly the warmest interglacial in the past 0.5 million years. Because the orbital configurations, atmospheric greenhouse gas concentrations, climate, and faunal characteristics during MIS 11 closely resemble those of the past 5,000 years, paleoclimatologists use MIS 11 as a geological analogue of the present and the near future.

There exist several high-resolution records documenting almost all aspects of terrestrial and marine climate through MIS 11. However, there is neither a clear understanding about how climactic parameters such as atmospheric carbon dioxide (CO2), sea surface temperature, the isotopic makeup of carbon in marine and terrestrial reservoirs, and annual air temperature interact, nor a consensus regarding the major drivers of climate change during this interval.

Using 15 of the most robust proxy records of marine and terrestrial climate, Das Sharma et al. employ new statistical and mathematical techniques to quantify the interactions among climatic parameters and to investigate which of these parameters could be the primary drivers of climate change during MIS 11. The authors find that atmospheric CO2 concentration was indeed the primary driver of both terrestrial and marine climate: Sea surface temperature and the isotopic makeup of carbon in terrestrial and marine reservoirs responded "instantaneously" (i.e., within 1,000 years) to changes in atmospheric CO2 content.

They further report that MIS 11 had warm and cool phases that can be detected from sea surface temperature records alone. During the relatively cold phases, sea surface and air temperatures behave coherently and respond to atmospheric CO2 faster. However, during warmer intervals, ocean surface and air temperatures behave more independently of each other and atmospheric CO2. The authors suggest that over the course of the next century, air and sea surface temperatures are likely to change in ways that will be difficult to predict.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1029/2012JD017725, 2012

Title: Sea surface temperatures in cooler climate stages bear more similarity with atmospheric CO2 forcing

Authors: S. Das Sharma, D. S. Ramesh, C. Bapanayya, and P. A. Raju: National Geophysical Research Institute, Council of Scientific and Industrial Research, Hyderabad, India.

3. Shear layers in solar winds affect Earth's magnetosphere

Human society is increasingly reliant on technology that can be disrupted by space weather. For instance, geomagnetic storms can cause high-latitude air flights to be rerouted, costing as much as $100,000 per flight; induce errors of up to 46 meters (151 feet) in GPS systems; and affect satellites and the International Space Station. Space weather is determined by how the solar wind, a stream of hot plasma from the Sun, interacts with Earth's magnetic field. In studying space weather, scientists have largely neglected the fact that the solar wind contains layers of very strong velocity shear. Scientists understand very little about how these wind shears affect space weather.

Combining statistical analysis of solar wind data from the Advanced Composition Explorer satellite, which measures solar particles approaching Earth, with a series of magnetohydrodynamic simulations, used to model the behavior of the Earth's magnetosphere, Borovsky characterizes the properties of the shear layers that travel past the Earth and the reaction of the Earth to those passing layers.

The author finds that as many as 60 of these shear zones can pass by Earth each day at velocities above 50 kilometers per second (31 miles per second). Passage of a shear layer perturbs the entire magnetosphere and ionosphere, which could produce a comet-like disconnection of the Earth's magnetotail (the tail-like extension of Earth's magnetic field on the side facing away from the Sun). Although the velocity shears will not cause a geomagnetic storm, they may determine how such a storm works. Hence, the author recommends several follow-up studies of the reaction of Earth to sudden wind shear.

Source: Journal of Geophysical Research-Space Physics, doi:10.1029/2012JA017623, 2012

Title: The effect of sudden wind shear on the Earth's magnetosphere: Statistics of wind shear events and CCMC simulations of magnetotail disconnections

Author: Joseph E. Borovsky: Space Science Institute, Boulder, Colorado, USA, Also at Department Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA.

4. Dams impact carbon dynamics in U.S. rivers

Dissolved organic carbon (DOC)which leaches into freshwater systems from plants, soils, and sediments, and from other detritus present in the water itselfis the major food supplement for microorganisms and plays an important role in several environmental processes and in the global carbon cycle. In some aquatic systems such as estuaries the optically measurable colored component of dissolved organic matter (CDOM) is often proportional to the concentration of DOC.

CDOM forms when light-absorbing compounds are released into the water by decaying organic material and through photochemical degradation of certain organic compounds. Hence, CDOM reflects not just the environment and ecosystem, which is the source of the detritus, but also processes that deliver the organic matter into aquatic systems. Human activities, such as logging, agriculture, and waste water treatment, also affect CDOM levels in aquatic systems. It is relatively easy and inexpensive to measure the CDOM content in small volumes of water.

To examine the circumstances under which CDOM reflects DOC concentration, Spencer et al. measured CDOM and DOC concentrations in water collected from 30 rivers across the United States; the rivers represent a wide range of climate, watershed environments, ecosystems, and anthropogenic influence. Overall, the authors find that the CDOM level reflects the DOC concentration in the river water, except in four large rivers, namely, the Colorado, Columbia, Rio Grande, and St. Lawrence rivers.

These four rivers either drain from the Great Lakes or have significant restrictions within their watersheds such as dam building and other similar modifications. These activities result in long residence times of water, which may increase phytoplankton production, the relative contribution from human sources, or degradation of land-derived material by photochemical processes. As a result, there may have been a decoupling of CDOM from DOC, i.e., the amount of CDOM in these four rivers may have decreased without a concomitant decrease in DOC content. On the basis of their findings, the authors suggest that CDOM measurements in rivers are a useful way to investigate water quality and to monitor delivery of DOC into coastal regions as ecosystems respond to human activity and changes in climate in the near future.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1029/2011JG001928, 2012

Title: Dissolved organic carbon and chromophoric dissolved organic matter properties of rivers in the USA

Authors: Robert G. M. Spencer: Global Rivers Group, Woods Hole Research Center, Falmouth, Massachusetts, USA;

Kenna D. Butler and George R. Aiken: United States Geological Survey, Boulder, Colorado, USA.

5. Comparison with observations shows cloud simulations improving

Climate projections, such as those used by the Intergovernmental Panel on Climate Change, rely on models that simulate physical properties that affect climate, including clouds and water vapor content. Clouds and water vapor are difficult to simulate in global climate models because they are affected by small-scale physical processes, and cloud feedback on climate is therefore a large source of uncertainty in climate predictions.

A new study finds that model simulations of vertically averaged cloud water amount have improved in recent years. Jiang et al. develop a quantitative scoring method to evaluate the accuracy of 19 climate models at various vertical heights between the surface and the tropopause (16 to 18 kilometers (10 to 11 miles) in altitude) over the tropical oceans (30 degrees North to 30 degrees South). They compare the models' simulated multiyear mean of cloud water content and water vapor with observations made using several NASA satellites.

Many of the new models, which were submitted to phase 5 of the Coupled Model Intercomparison Project (CMIP5), have attempted to improve representation of clouds using finer-scale simulations. The authors find that more than half of the models did show improvement over previous models from CMIP3 in simulating the amount and distribution of clouds and water vapor over the tropical oceans. In addition, they find that the models simulated boundary layer water vapor amounts accurately. However, there are large differences among the models and between the models and observations at high altitudes in the upper troposphere.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1029/2011JD017237, 2012

Title: Evaluation of cloud and water vapor simulations in CMIP5 climate models using NASA "A-Train" satellite observations

Authors: Jonathan H. Jiang, Hui Su, Chengxing Zhai, and Vincent S. Perun: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA;

Anthony Del Genio and Larissa S. Nazarenko: Goddard Institute for Space Studies, New York, New York, USA;

Leo J. Donner, Larry Horowitz, and Charles Seman: Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA;

Jason Cole: Canadian Centre for Climate Modeling and Analysis, Environment Canada, Toronto, Ontario, Canada;

Andrew Gettelman: National Center for Atmospheric Research, Boulder, Colorado, USA;

Mark A. Ringer: Met Office Hadley Centre, Exeter, UK;

Leon Rotstayn: Commonwealth Scientific and Industrial Research Organisation, Clayton South, Victoria, Australia;

Stephen Jeffrey: Queensland Climate Change Centre of Excellence, Dutton Park, Queensland, Australia;

Tongwen Wu: Beijing Climate Center, China Meteorological Administration, Beijing, China;

Florent Brient and Jean-Louis Dufresne: Laboratoire de Mtorologie Dynamique, Institute Pierre Simon Laplace, Paris, France;

Hideaki Kawai and Tsuyoshi Koshiro: Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Japan;

Masahiro Watanabe: Model for Interdisciplinary Research on Climate, Atmospheric and Ocean Research Institute, University of Tokyo, Chiba, Japan;

Tristan S. Lcuyer: University of Wisconsin-Madison, Madison, Wisconsin, USA;

Evgeny M. Volodin: Institute for Numerical Mathematics, Russian Academy of Sciences, Moscow, Russia;

Trond Iversen: Norwegian Climate Centre, Meteorologisk Institutt, Oslo, Norway;

Helge Drange and Michel D. S. Mesquita: Bjerknes Centre for Climate Research, Uni Research, Bergen, Norway;

William G. Read, Joe W. Waters, Baijun Tian, Joao Teixeira, and Graeme L. Stephens: JPL, California Institute of Technology, Pasadena, California, USA.

6. Turbulent forces within river plumes affect spread

When rivers drain into oceans through narrow mouths, hydraulic forces squeeze the river water into buoyant plumes that are clearly visible in satellite images. Worldwide, river plumes not only disperse freshwater, sediments, and nutrients but also spread pollutants and organisms from estuaries into the open ocean. In the United States the Columbia River, the largest river by volume draining into the Pacific Ocean from North America, generates a plume at its mouth that transports juvenile salmon and other fish into the ocean. Clearly, the behavior and spread of river plumes, such as the Columbia River plume, affect the nation's fishing industry as well as the global economy.

A delicate balance between density and velocity controls turbulent mixing within the plume and how far river plumes extend into the deep ocean. On the other hand, coastal winds and currents affect the shape and orientation of the plumes. However, current understanding of momentum changes along a river plume is limited by poorly constrained numerical models and sparse remote sensing data.

In a new study, Kilcher et al. document velocity, density, and turbulence along the centerline of the Columbia River plume at a high resolution for 10 tidal cycles. The authors find that turbulence varied by 2 to 3 orders of magnitude within a single tidal cycle as well as between cycles. The authors also show that turbulence, which is most vigorous at the beginning of the strongest ebbs, dramatically reduces the velocity of water at the river mouth. The quickly decelerating plumes tend to spread less into the deep ocean. Their observations supplement satellite data and could help to fine-tune numerical models that predict the behavior and spread of river plumes as they drain into coastal waters.

Source: Journal of Geophysical Research-Oceans, doi:10.1029/2011JC007398, 2012

Title: The role of turbulence stress divergence in decelerating a river plume

Authors: Levi F. Kilcher, Jonathan D. Nash, and James N. Moum: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA.


Contact: Kate Ramsayer
American Geophysical Union

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