AGU journal highlights -- April 22, 2008

April 22, 2008

1. Cooling a climate disagreement

Aerosols from fossil fuel burning and other human activities cool the climate by scattering incoming solar radiation, but models and estimates appear to differ on the strength of this cooling's global effect. Previous analysis of data from the satellite-based Moderate Resolution Imaging Spectroradiometer (MODIS) suggested that human-made aerosols exert a global direct radiative forcing of -1.9 watts per square meter in cloud-free skies, while models predict -0.7 watts per square meter. The more negative the forcing, the more incoming radiation is scattered, causing greater cooling. Bellouin et al. use updated MODIS aerosol data to estimate a global cloud-free radiative forcing of -1.3 watts per square meter. Further analysis using the Hadley Centre climate model reveals that the MODIS-based estimate is biased high by 40-50 percent mainly because it does not account for preindustrial aerosols such as those released by slash-and-burn agricultural techniques. The researchers conclude that for cloud-free conditions over oceans, the difference between modeled and observation-based estimates falls within the range of uncertainty. Over land, however, data on aerosol optical depth require refinement to further reconcile both estimates.

Title: Updated estimate of aerosol direct radiative forcing from satellite observations and comparison against the Hadley Centre climate model.

Authors: Nicolas Bellouin, Andy Jones, and Jim M. Haywood: Met Office Hadley Centre, Exeter, U.K.;

Sundar A. Christopher: Department of Atmospheric Sciences, University of Alabama, Huntsville, Alabama, U.S.A.

Source: Journal of Geophysical Research - Atmospheres (JGR-D) paper 10.1029/2007JD009385, 2008; http://www.agu.org/journals/pip/jd/2007JD009385-pip.pdf . This paper is "in press".




2. Southern skies sensitive to ozone variation

Seasonal variation in stratospheric ozone concentration over Antarctica is large, with the greatest depletions occurring in the austral spring when the Sun returns to Antarctica. The resulting ozone hole, which is often displaced from the pole, cools the stratosphere because less ozone means less absorption of ultraviolet radiation. Although models suggest that the ozone hole also cools the troposphere over Antarctica, simulations are not entirely realistic because ozone concentrations used in models are usually averaged over latitude bands. Noting that ozone concentrations are more heterogeneous than this, Crook et al. prescribe a realistic three-dimensional distribution of ozone in a high-vertical-resolution atmospheric model and simulate the climate response to this ozone distribution. Comparing results with simulations containing averaged ozone concentrations reveals that the three-dimensional ozone yields cooler temperatures in the stratosphere and upper troposphere, with a magnitude of cooling comparable to that caused by ozone depletion itself. This result suggests that heterogeneous ozone concentrations influence Southern Hemisphere climate and that this influence will change in the future as the recovery of ozone decreases this heterogeneity.

Title: Sensitivity of Southern Hemisphere climate to zonal asymmetry in ozone

Authors: Julia A. Crook and Nathan P. Gillett: Climate Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, U.K.;

Sarah P. E. Keeley: Climate Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, U.K.; also at Department of Meteorology, University of Reading, Reading, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032698, 2008; http://dx.doi.org/10.1029/2007GL032698




3. Might surges trigger geomagnetic substorms?

Geomagnetic substorms are driven by variations on the Sun and can disrupt satellite systems, produce aurorae, and increase the radiation dose of astronauts and passengers on transpolar flights. Right before the onset of a geomagnetic substorm, a large upsurge in duskward ion flux occurs that some scientists believe excites cross-field current instabilities and possibly triggers substorm expansion. Saito et al. seek to discover whether this ballooning instability influences the magnetotail near Earth in the late-growth stage of substorms. Using data from the joint Japan/U.S. Geotail satellite, the authors determine that the ballooning instability deforms magnetic field lines in the magnetotail, producing magnetic field fluctuations. Further examination of a set of six substorms that were detected in the vicinity of the magnetic equator reveal that that ballooning mode was identified in cases where plasma flow velocities were high. From this knowledge of the plasma flow velocity, the scientists are able to estimate the wavelength of the instabilities. The authors expect that such studies will help scientists better predict geomagnetic substorms.

Title: Ballooning mode waves prior to substorm-associated dipolarizations: Geotail observations

Authors: M. H. Saito, Y Miyashita, M. Fujimoto, I. Shinohara, and Y. Saito: Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan;

K. Liou: Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, U.S.A.;

T. Mukai: Japan Aerospace Exploration Agency, Tokyo, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033269, 2008; http://dx.doi.org/10.1029/2008GL033269




4. Model warns early of Indonesia, Australia drought

The Indian Ocean Dipole (IOD) is an air-sea coupled climate oscillation. In the dipole's positive phase, greater-than-average sea surface temperatures and precipitation occur around the western Indian Ocean and southern India, and waters cool in the eastern Indian Ocean causing drought conditions in Indonesia and Australia. Opposite conditions occur in the negative phase. Although most research synchronizes dipole patterns with El Niño oscillations in the Pacific Ocean, in both 2006 and 2007 during fall in the Northern Hemisphere, two consecutive positive dipole events occurred. One took place during an El Niño condition and the other during a La Niña condition. Luo et al. find that forecasts using a coupled model could have predicted these dipole phases three to four seasons in advance, although the model did not predict the weaker 2007 positive dipole event very robustly. Further analysis reveals that seasonal climate anomalies in the Eastern Hemisphere associated with the two positive dipole events can be predicted one or two seasons ahead, implying that published dipole predictions could help societies better prepare for potential adverse weather.

Title: Successful prediction of the consecutive IOD in 2006 and 2007

Authors: Jing-Jia Luo, Swadhin Behera, and Hirofumi Sakuma: Frontier Research Center for Global Change, Japan Marine Science and Technology Center, Yokohama, Japan;

Yukio Masumoto and Toshio Yamagata: Frontier Research Center for Global Change, Japan Marine Science and Technology Center, Yokohama, Japan, also at Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032793, 2008; http://dx.doi.org/10.1029/2007GL032793




5. Corals reveal oceans' carbon reservoir age

Radiocarbon dating of corals requires correction for the carbon reservoir age of the ocean, a measure of the carbon-14 deficit in ocean surface waters relative to the atmosphere when the corals formed. From 1600 to 1900, the reservoir age is calculated to be about 400 years. In a study of fossil corals from Florida's Biscayne National Park, Druffel et al. determine the reservoir age for two periods during the Holocene (about 4930 and 3040 years ago) and find it to be significantly lower, about 292 years. They measured thorium concentrations in the corals to obtain their calendar age, and then measured their carbon-14 content. The difference between the carbon-14 content of the atmosphere at the time (from calibration curves based on tree ring data) and that of the coral reveals the oceans' carbon reservoir age. The authors suggest several mechanisms that could account for the lower reservoir age, including increased ocean stratification during the Holocene periods. The lower reservoir age has implications for radiocarbon dating other corals and shells in the North Atlantic.

Title: Low reservoir ages for the surface ocean from mid-Holocene Florida corals

Authors: Ellen R. M. Druffel, Sheila Griffin, and John R. Southon: Earth System Science Department, University of California-Irvine, Irvine, California, U.S.A.;

Laura F. Robinson: Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.;

Robert B. Halley: U.S. Geological Survey, St. Petersburg, Florida, U.S.A.;

Jess F. Adkins: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, U.S.A.

Source: Paleoceanography (PA) paper 10.1029/2007PA001527, 2008; http://www.agu.org/journals/pip/pa/2007PA001527-pip.pdf . This paper is "in press".




6. Unusual tremor jiggles Mexican zone

Nonvolcanic tremors are episodes of seismic radiation (mostly in the 1-10 Hertz range) likely associated with aseismic slow slip events within the transition zone between the seismogenic coupled and deep free-slipping segments of a fault. Such tremors have been observed on the San Andreas Fault and on some subduction thrust faults in Japan, the United States, and Costa Rica. To learn more about the phenomenon, Payero et al. study data from a transect across a subduction zone in central Mexico that is overdue for a major earthquake and has been well monitored with Global Positioning System (GPS) networks and broadband seismic stations from the Mesoamerican Subduction Experiment (MACE). GPS data show that two large slow slip events occurred in the Guerrero-Oaxaca region, one in 2001-2002 and the other in 2006. Seismological analyses revealed that nonvolcanic tremor activity was higher during those two slow slip events compared with that for the "quiet" period of 2003-2005. Models suggest that the tremors might be related to low-temperature metamorphic processes and dehydration of the subducted oceanic crust.

Title: Nonvolcanic tremor observed in the Mexican subduction zone

Authors: Juan S. Payero: Posgrado en Ciencias de la Tierra, Instituto de Geofísica, Universidad Nacional Autónoma de México, Mexico City, Mexico; also at Instituto Sismológico Universitario, Facultad de Ciencias, Universidad Autónoma de Santo Domingo, Santo Domingo, Dominican Republic;

Vladimir Kostoglodov, Takeshi Mikumo, Arturo Igelsias and Xyoli Pérez-Campos: Instituto de Geofísica, Universidad Nacional Autónoma de México, Mexico City, Mexico;

Nikolai Shapiro: Institut de Physique du Globe de Paris, Paris, France;

Robert W. Clayton: Seismological Laboratory, California Institute of Technology, Pasadena, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032877, 2008; http://dx.doi.org/10.1029/2007GL032877
-end-
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