AGU Journal highlights -- May 12, 2011

(Press-News.org) The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Water Resources Research (WRR), the Journal of Geophysical Research-Oceans (JGR-C), and the Journal of Geophysical Research-Solid Earth (JGR-B).

In this release: New freshwater source for Antarctic coastal waters Phytoplankton affect clouds and precipitation River model enhanced by floodplain dynamics Weighing natural variability in projected precipitation change Study suggests no slowing of Atlantic 'conveyor belt' current Warming, salinity effects spur half of Southern Hemisphere sea-level rise Carbon may be the element lightening up Earth's core Lightning not detected on Titan

1. New freshwater source for Antarctic coastal waters Over the past several decades, research into submarine groundwater discharge (SGD), predominantly regarding its prevalence as a source of freshwater and nutrients to coastal ecosystems, has grown in prominence. Using a newly developed groundwater discharge sensor specifically designed for use in the cold polar ocean, Uemura et al. measured the flows of freshwater streaming through the Antarctic subsurface and into the surrounding coastal waters.

The researchers find that SGD rates measured in Lützow-Holm Bay in eastern Antarctica, south of Madagascar, showed important differences from SGD rates measured elsewhere on Earth. At midlatitudes, discharge rates drop with increasing ocean depth, while the Antarctic flows were relatively consistent despite differences in depth between the seven survey sites scattered throughout the bay. Also, the measured average flow rates, ranging from 0.85 to 0.95 micrometers per second (0.00000028 to 0.0000031 feet per second), were 10 to 100 times higher than flow rates at similar depths made at midlatitudes. The authors also find that SDG rates oscillated with a period of 12.8 hours, peaking at low tide. Further, the discharge rates roughly tracked the size of the tide, having higher peaks in spring, when tides were strongest. The researchers propose that the most likely source of the freshwater flow is meltwater formed beneath the massive glaciers surrounding the bay.

Source:
Geophysical Research Letters, doi:10.1029/2010GL046394, 2011
http://dx.doi.org/10.1029/2010GL046394

Title: Submarine groundwater discharge in Lützow‐Holm Bay, Antarctica

Authors: Takeshi Uemura: Department of Polar Science, Graduate University for Advanced Studies (Sokendai), Tachikawa, Japan;

Makoto Taniguchi: Research Institute for Humanity and Nature, Kyoto, Japan;

Kazuo Shibuya: Department of Polar Science, Graduate University for Advanced Studies (Sokendai), Tachikawa, Japan; and National Institute of Polar Research, Tachikawa, Japan.

2. Phytoplankton affect clouds and precipitation Phytoplankton in the ocean produce the gases dimethyl sulfide and isoprene, which can enter the atmosphere. To find out how this affects cloud properties and precipitation, Krüger and Graßl analyze a variety of satellite observations over the Southern Ocean from 45 degrees south to 65 degrees south. They find that phytoplankton emission of these gases leads to an increase in cloud albedo—the clouds reflect more incoming sunlight back upward—as well as to reduced cloud droplet radius and increased cloud optical thickness. Furthermore, they find that less precipitation occurred over the Antarctic Polar Front zone during strong plankton blooms.

Source:
Geophysical Research Letters, doi:10.1029/2011GL047116, 2011
http://dx.doi.org/10.1029/2011GL047116

Title: Southern Ocean phytoplankton increases cloud albedo and reduces precipitation

Authors: Olaf Krüger: Ludwig Maximilian University of Munich, Munich, Germany; and Brockmann Consult, Helmholtz Centre Geesthacht, Geesthacht, Germany;

Hartmut Graßl: Max-Planck-Institute for Meteorology, Hamburg, Germany.

3. River model enhanced by floodplain dynamics Surface water flow is important to the climate system and to water resources management. Floodplain inundation dynamics are a key part of surface water flow, but most current river routing models do not include realistic representation of floodplain dynamics. A new global river routing model created by Yamazaki et al. does include these dynamics. The researchers test the model by comparing results of their model simulation with in situ and satellite observations and find that the model improved simulation of river discharge in most of the world's major river basins.

Source:
Water Resources Research, doi:10.1029/2010WR009726, 2011
http://dx.doi.org/10.1029/2010WR009726

Title: A physically based description of floodplain inundation dynamics in a global river routing model

Authors: Dai Yamazaki: Institute of Industrial Science, University of Tokyo, Tokyo, Japan;

Shinjiro Kanae: Department of Mechanical and Environmental Informatics, Tokyo Institute of Technology, Tokyo, Japan;

Hyungjun Kim: UC Center for Hydrologic Modeling, University of California, Irvine, California, USA; Institute of Industrial Science, University of Tokyo, Tokyo, Japan;

Taikan Oki: Institute of Industrial Science, University of Tokyo, Tokyo, Japan.

4. Weighing natural variability in projected precipitation change The majority of projections made using general circulation models (GCMs) are conducted to help tease out the effects on a region, or on the climate system as a whole, of changing climate dynamics. Sun et al., however, use model runs from 20 different coupled atmosphere-ocean GCMs to try to understand different aspects of climate projections, namely, how bias correction, model selection, and other statistical techniques might affect the estimated outcomes. As a case study, the authors focus on predicting the potential change in precipitation for the Murray-Darling Basin (MDB), a 1 million square kilometer (386,100 square mile) area in southeastern Australia that suffered a recent decade of drought that left many wondering about the potential impacts of climate change on this important agricultural region.

The authors first compare the precipitation predictions made by the models with 107 years of observations, and they then make bias corrections to adjust the model projections to have the same statistical properties as the observations. They find that while the spread of the projected values was reduced, the average precipitation projection for the end of the 21st century barely changed. Further, the authors determine that interannual variations in precipitation for the MDB could be explained by random chance, where the precipitation in a given year was independent of that in previous years. By comparing the projected change in precipitation against the variance in historical observation and simulations, the authors are able to determine which models were misbehaving and were not restricting themselves to this expectation of randomness. In all, the authors determine that if future changes in precipitation for the MDB were driven purely by random chance, the average annual precipitation could change by up to 11 percent between 30-year periods. Any projected change that was smaller than this could not be distinguished from natural variability. The authors note that the large year-to-year variability of regional precipitation has made it difficult to identify trends. By incorporating the variance, they suggest that the new approach makes it possible to fully account for natural variability when compiling regional-scale projections of precipitation change.

Source:
Water Resources Research, doi:10.1029/2010WR009829, 2011
http://dx.doi.org/10.1029/2010WR009829

Title: Hydroclimatic projections for the Murray‐Darling Basin based on an ensemble derived from Intergovernmental Panel on Climate Change AR4 climate models

Authors: Fubao Sun, Wee Ho Lim, and Graham D. Farquhar: Research School of Biology, Australian National University, Canberra, ACT, Australia;

Michael L. Roderick: Research School of Biology, Australian National University, Canberra, ACT, Australia; and Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia.

5. Study suggests no slowing of Atlantic 'conveyor belt' current The Atlantic meridional overturning circulation (AMOC), which carries warm water to high northern latitudes near the surface and returns cold water in the deep ocean to the Southern Hemisphere, affects and is affected by global climate change. There has been debate as to whether the AMOC has begun slowing down due to global warming, but research on AMOC variability based on instrumental records is limited. One possible indicator of change in the AMOC is the North Brazil Current (NBC), a strong current that flows northward in the tropical South Atlantic, connecting the North and South Atlantic oceans, and plays an important role as a major pathway for surface return flow in the AMOC.

Zhang et al. calculate how the NBC varies on multidecadal time scales based on a record of 50 years of observations off the coast of Brazil. They find that NBC transport changes are correlated with Labrador Sea deep convection (important for deepwater formation) and with a broad pattern of sea surface temperature anomalies in the Atlantic (sometimes referred to as the Atlantic Multidecadal Oscillation or Atlantic Multidecadal Variability), both of which have previously been linked to AMOC fluctuations. The researchers therefore suggest that observed NBC variability is a useful indicator of AMOC variations. They confirm this using a climate model simulation. Furthermore, the authors note that although some studies have suggested that the AMOC is slowing down due to global warming, the NBC shows multidecadal variability but no significant slowing trend over the past 50 years.

Source:
Journal of Geophysical Research-Oceans, doi:10.1029/2010JC006812, 2011
http://dx.doi.org/10.1029/2010JC006812

Title: Multidecadal variability of the North Brazil Current and its connection to the Atlantic meridional overturning circulation

Authors: Dongxiao Zhang: Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington, USA; NOAA Pacific Marine Environmental Laboratory, Seattle, Washington, USA;

Rym Msadek: AOS Program, Princeton University, Princeton, New Jersey, USA; NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA;

Michael J. McPhaden: NOAA Pacific Marine Environmental Laboratory, Seattle, Washington, USA;

Tom Delworth: NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA.

6. Warming, salinity effects spur half of Southern Hemisphere sea-level rise Sea surface height has increased by 3 millimeters/year (0.12 inches/year), globally averaged, since 1993. Some fraction of sea surface height change is due to added water, for instance, from melting glaciers, and some is due to increasing heat and salinity changes (steric effects). Focusing on the Southern Hemisphere, Sutton and Roemmich analyze temperature and salinity data from the Argo float array in relation to World Ocean Circulation Experiment (WOCE) data from about a decade earlier to estimate the steric changes. These are compared with the total sea surface height changes over the same period seen in satellite altimetric data. The authors find that on decadal time scales, about half of the rise in sea surface height in the Southern Ocean is due to steric effects, with the proportion increasing southward. The accompanying increase in ocean heat content south of 30 degrees south can account for most of the global heat content change during this period.

Source: Geophysical Research Letters, doi:10.1029/2011GL046802, 2011 http://dx.doi.org/10.1029/2011GL046802

Title: Decadal steric and sea surface height changes in the Southern Hemisphere

Authors: P. Sutton: National Institute of Water and Atmospheric Research, Wellington, New Zealand;

D. Roemmich: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA.

7. Carbon may be the element lightening up Earth's core The Earth's dense core consists primarily of iron, but seismological data and mineral physics results show that the inner core is lighter than pure iron by 3 percent to 7 percent and the outer core is lighter than pure iron by 5 percent to 10 percent, so the core must also contain some lighter element. One possible light element is carbon, which readily forms alloys with iron and is abundant in the universe. Geochemical studies also suggest that carbon is present, but it is uncertain how much carbon is in the core or what form it is in.

Recent studies have shown that iron carbide is a good candidate. Mookherjee et al. use theoretical and experimental studies to gain insight into the properties of iron carbide at the high pressures in the inner core. They also explore the magnetic structure, equations of state, and elasticity of iron carbide at inner core pressures. They find, based on the density of iron carbide at inner core conditions, that the maximum possible carbon content of the inner core is about 1.5 percent by weight.

Source:
Journal of Geophysical Research-Solid Earth, doi:10.1029/2010JB007819, 2011
http://dx.doi.org/10.1029/2010JB007819

Title: High-pressure behavior of iron carbide (Fe7C3) at inner core conditions

Authors: Mainak Mookherjee, Yoichi Nakajima, Gerd Steinle-Neumann, Konstantin Glazyrin, Leonid Dubrovinsky and Catherine McCammon: Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany;

Xiang Wu: Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany; School of Earth and Space Sciences, Peking University, Beijing, China;

Aleksandr Chumakov: ID18 Nuclear Resonance Group, European Synchrotron Radiation Facility, Grenoble Cedex, France.

8. Lightning not detected on Titan Scientists have speculated that lightning on Saturn's moon Titan could produce changes in atmospheric chemistry and could even spark production of organic compounds that could be precursors for evolution of life, but so far there has been no conclusive detection of lightning on Titan. Extending previous searches for lightning on Titan, Fischer and Gurnett analyze radio data up to the 72nd close flyby of Titan by the Cassini spacecraft. They find no evidence of lightning and conclude that if lightning occurs at all on Titan, it is probably a very rare event.

Source:
Geophysical Research Letters, doi:10.1029/2011GL047316, 2011
http://dx.doi.org/10.1029/2011GL047316

Title: The search for Titan lightning radio emissions

Authors: G. Fischer: Space Research Institute, Austrian Academy of Sciences, Graz, Austria;

D. A. Gurnett: Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA.

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