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AGU journal highlights -- Aug. 1, 2007

American Geophysical Union

1. Surface warming and the solar cycle

To accurately assess the effects of human-induced climate change, scientists must be able to quantify the contribution of natural variation in solar irradiance to temperature changes. The existence of a long-term trend in solar output is controversial, but its periodic change within an 11-year cycle has been measured by satellites. To assess how this less-controversial oscillatory forcing affects climate on Earth, Camp and Tung compare surface temperature measurements across the globe between years of solar maximum (with higher heat output) and years of solar minimum. They find that times of high solar activity are on average 0.2º C warmer than times of low solar activity, and that there is a polar amplification of the warming. This result is the first to document a statistically significant globally coherent temperature response to the solar cycle, the authors note.

Title: Surface warming by the solar cycle as revealed by the composite mean difference projection

Authors: Charles D. Camp and Ka Kit Tung: Department of Applied Mathematics, University of Washington, Seattle, Washington, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030207, 2007

2. Early humans in China 1 million years ago

Chronology and adaptability of early humans in different paleoclimatic and paleoenvironmental settings are important topics in the study of human evolution. China houses several early-human (Paleolithic) archaeological sites along the Nihewan Basin near Mongolia, some with artifacts that date back about 1 million years ago. Deng et al. analyze one specific locality in the Nihewan Basin, called the Feiliang Paleolithic Site, where several stone artifacts and mammalian bone fragments have been found buried in basin silts. By analyzing remnant magnetizations of basin silt layers and comparing these data with charts of known magnetic reversals, the authors identify that the artifact layer was deposited about 1.2 million years ago, just prior to a major climate transition that occurred during the mid-Pleistocene. The transition brought increased climate variability to the region. This finding, coupled with other studies, indicates a prominent early human presence in the high northern latitudes of East Asia. The authors indicate that further studies on the artifacts themselves could reveal the manner in which humans weathered these climate shifts.

Title: Magnetochronology of the Feiliang Paleolithic site in the Nihewan Basin and implications for early human adaptability to high northern latitudes in East Asia

Authors: Chenglong Deng, Caicai Liu, Hong Ao, Yongxin Pan and Rixiang Zhu: Paleomagnetism and Geochronology Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China;

Fei Xie: Hebei Province Institute of Cultural Relics, Shijiazhuang, China.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030335, 2007

3. An alternative mechanism for recent volcanism on Mars

High-resolution images of Martian volcanoes have revealed areas that show signs of recent lava flow. The youngest of these are about 2 million years old, as evidenced by the lack of impact craters on the flows. These flows are confined to Tharsis and Elysium, old volcanoes which exhibit a thickened crust compared to surrounding areas. Previous studies explain these recent volcanics by hypothesizing that strong mantle plumes arising from the core-mantle boundary fuel the melts, but the existence and sustainment of these plumes during the last few billion years is uncertain under Martian conditions. Schumacher and Breuer propose a different hypothesis whereby the crust insulates the mantle, causing the mantle to cool slower than previously expected. Through model simulations, they discover that a locally thickened crust with a reduced thermal conductivity and enriched in radioactive heat sources in comparison to the mantle could generate temperature variations in the upper mantle. These variations could form partial melts below this crustal lid, which, being more buoyant than surrounding material, could ooze out onto the Martian surface.

Title: An alternative mechanism for recent volcanism on Mars

Authors: Sandra Schumacher: Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Germany;

Doris Breuer: Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt, Berlin-Adlershof, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030083, 2007

4. Estimating tropical cyclone numbers in the North Atlantic before satellites

Trends in hurricane occurrence over the North Atlantic are difficult to determine because hurricanes likely went undetected before satellite observations were available. To estimate the occurrence of these undetected hurricanes, Chang and Guo examine ship track records before and during the satellite era. They map satellite-derived cyclone tracks from 1976 to 2005 against ship tracks from the same time period to determine the probability that ships recorded wind speeds high enough to detect a tropical cyclone. Then, they compute the probability that ships that sailed between 1900 and 1965 would have made observations of similar high wind speeds, if tropical cyclones from the satellite era had been present at the same rate during earlier years. From this, the authors find which storm tracks were too far away from ships to be detected. They determine that the number of tropical cyclones not making landfall over any continent or island likely was underestimated by 1 or less per year after World War I. Thus, their results suggest that the characteristics of North Atlantic tropical cyclone track statistics might have changed during recent decades.

Title: Is the number of North Atlantic tropical cyclones significantly underestimated prior to the availability of satellite observations"

Authors: Edmund K. M. Chang and Yanjuan Guo: Institute for Terrestrial and Planetary Atmospheres, Marine Sciences Research Center, Stony Brook University, Stony Brook, New York, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030169, 2007

5. Satellite gravity measurements detect deformation from Sumatra-Andaman earthquakes

Satellite gravity measurements, which have been used to estimate changes in groundwater storage, sea level, and polar ice sheet extent, can also provide a unique way to monitor deformation associated with major earthquakes, supplementing global positioning system measurements which are limited if earthquakes happen off shore. Chen et al. used data from the Gravity Recovery and Climate Experiment (GRACE) to observe and study the effects of seismic deformation due to ruptures from the magnitude 9.3 Sumatra-Andaman earthquake on 26 December 2004 and its companion Nias Earthquake (magnitude 8.7) on 28 March 2005. Through improved filtering methods and more refined processing of GRACE data, the authors find that gravity field disturbances caused by these earthquakes extend over 1800 kilometers (1100 miles) along the Andaman and Sunda subduction zones, and that these disturbances change with time following the earthquake. The authors expect that gravity changes are due to afterslip, viscous relaxation of the upper mantle, or other processes associated with upper mantle recovery after such great earthquakes.

Title: GRACE detects coseismic and postseismic deformation from the Sumatra-Andaman Earthquake

Authors: J. L. Chen and B. D. Tapley: Center for Space Research, University of Texas at Austin, Austin, Texas, U.S.A.;

C. R. Wilson: Center for Space Research, University of Texas at Austin, Austin, Texas, U.S.A.; Department of Geological Sciences, University of Texas at Austin, Austin, Texas, U.S.A.;

S. Grand: Department of Geological Sciences, University of Texas at Austin, Austin, Texas, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030356, 2007

6. Ocean supergyre in southern hemisphere

Mid-latitude oceanic circulation in the southern hemisphere is traditionally thought to consist of distinct, basin-wide, circular currents, or gyres, contained within the Indian, Pacific, and Atlantic oceans. However, model simulations suggest that these gyres are linked by a global-scale 'supergyre' that transfers water to all three ocean basins. From a thorough analysis of temperature and salinity profiles collected since 1950 between 60°S and the equator, Ridgway and Dunn locate this supergyre. Water flows at intermediate depths from the Pacific to the Indian Ocean through a pathway around Tasmania, an observation supported by models but not fully documented by previous field research until now. Consistent with previous observations, the authors find that in the upper layers, Pacific Ocean water principally flows through the Indonesian archipelago into the Indian Ocean. Water from the Indian Ocean then leaks into the Atlantic Ocean around the southern tip of Africa. A pathway on the northern edge of the Antarctic Circumpolar Current returns the gyre waters back to the Pacific. The supergyre is a central agent for distributing intermediate water around the global ocean.

Title: Observational evidence for a Southern Hemisphere oceanic 'supergyre'

Authors: K. R. Ridgway and J. R. Dunn: Marine & Atmospheric Research and Wealth from Oceans Flagship, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Hobart, Tasmania, Australia.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030392, 2007

7. Synchronized chaos: Mechanisms for major climate shifts

In the mid-1970s, a climate shift cooled sea surface temperatures in the central Pacific Ocean and warmed the coast of western North America, bringing long-range changes to the northern hemisphere. After this climate shift waned, an era of frequent El Ninos and rising global temperatures began. Understanding the mechanisms driving such climate variability is difficult because unraveling causal connections that lead to chaotic climate behavior is complicated. To simplify this, Tsonis et al. investigate the collective behavior of known climate cycles such as the Pacific Decadal Oscillation, the North Atlantic Oscillation, the El Nino/Southern Oscillation, and the North Pacific Oscillation. By studying the last 100 years of these cycles' patterns, they find that the systems synchronized several times. Further, in cases where the synchronous state was followed by an increase in the coupling strength among the cycles, the synchronous state was destroyed. Then. a new climate state emerged, associated with global temperature changes and El Nino/Southern Oscillation variability. The authors show that this mechanism explains all global temperature tendency changes and El Nino variability in the 20th century.

Title: A new dynamical mechanism for major climate shifts

Authors: Anastasios A. Tsonis, Kyle Swanson, and Sergey Kravtsov: Atmospheric Sciences Group, Department of Mathematical Sciences, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030288, 2007

8. A 30,000-year record of sea surface temperatures off South Australia

Continental glaciers originating at both poles reached their farthest extent about 20,000 years ago, marking a time known as the Last Glacial Maximum. Comparisons of ice cores from Greenland and Antarctica show that as these glaciers melted, warming occurred in asynchronous stages at the poles. While many northern hemisphere climate records match ice core records from Greenland, few southern hemisphere records exist to compare with ice core data from Antarctica. Calvo et al. analyze a marine core collected off South Australia and find that it contains detailed signatures of surface temperatures of waters that washed over it since the glacial maximum. Data from this core match well with Antarctic ice cores and paleoclimate records from the Australian continent, showing no signature of the Younger Dryas, a cooling event known to have occurred in the Northern Hemisphere about 13,000 years ago. The new core data also reveal a progressive drop in sea surface temperatures over the last 6,500 years, an observation not seen before for the Australian region.

Title: Antarctic deglacial pattern in a 30 kyr record of sea surface temperature offshore South Australia

Authors: Eva Calvo: Institut de Ciències den Mar, Consejo Superior de Investigaciones Científicas, Barcelona, Spain;

Carles Pelejero: Institució Catalana de Recerca i Estudis Avançats and Institut de Ciències den Mar, Consejo Superior de Investigaciones Científicas, Barcelona, Spain;

Patrick De Deckker: Department of Earth and Marine Sciences, The Australian National University, Canberra, Australia;

Graham A. Logan: Petroleum and Marine Division, Geoscience Australia, Canberra, Australia.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029937, 2007

9. Aseismic creep in Mexico

During an earthquake, friction between plates is released suddenly, causing the ground to rumble. However, during aseismic slow slip events, tension along faults is released gradually, causing plates to creep. First reported in the late 1990s, aseismic creep is of interest to scientists seeking to understand plate movement and seismic hazards. Through a network of GPS receivers, Larson et al. study an aseismic slow slip event that occurred from April to December 2006 in southern Mexico. This network includes a 275 kilometer (171 miles) transect extending from the coast to Mexico City and a swath that spans 75 kilometers (47 miles) along the coast parallel to the Middle America Trench, a subduction zone where the Cocos Plate is sinking under North America. The authors find that this event generated horizontal displacements of nearly 6 centimeters in the direction opposite to that recorded by earthquakes. Using this data, they model the aseismic creep and find that slip was concentrated on interior fault patches in the subduction zone. Energy released by this event had a minimum equivalent of magnitude 7.5.

Title: 2006 aseismic slow slip event in Guerrero, Mexico: New results from GPS

Authors: Kristine M. Larson: Department of Aerospace Engineering Sciences, University of Colorado Boulder, Boulder, Colorado, U.S.A.;

Vladimir Kostoglodov and Jose Antonio Santiago Santiago: Instituto de Geofisica, Universidad Nacional Autónoma de México, Mexico City, Mexico;

Shin'ichi Miyazaki: Earthquake Research Institute, University of Tokyo, Tokyo, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029912, 2007

10. Plasma-depleted flux tubes in Saturn's inner magnetosphere

Initial observations from the Cassini spacecraft have revealed evidence for interchanging magnetic flux tubes in Saturn's inner magnetosphere. These tubes interchange dense cold plasma with tenuous hot plasma originating from farther out in the magnetosphere. These tubes differ by their magnetic signatures, having a depressed or enhanced magnetic pressure relative to their surroundings. André et al. study interchanging flux tubes with enhanced magnetic pressure, which previous researchers hypothesize as having outward moving mass-loaded or inward moving plasma-depleted properties. Using data from several instruments aboard Cassini, the authors discriminate between the two previous and opposite interpretations of flux tubes with enhanced magnetic pressure. They find that all these flux tubes are in fact plasma-depleted, although they may possess different magnetic signatures. These signatures are associated with the latitude above which they form. Similar flux tubes have also been observed in Jupiter's magnetosphere.

Title: Magnetic signatures of plasma-depleted flux tubes in the Saturnian inner magnetosphere

Authors: N. André: Research and Scientific Support Department, European Space Agency, Noordwijk, The Netherlands;

A, M. Persoon, W. S. Kurth, and D. A. Gurnett: Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, U.S.A.;

J. Goldstein, J. L. Burch, F. J. Crary, and D. T. Young: Southwest Research Institute, San Antonio, Texas, U.S.A.;

P. Louarn: Centre d'Etude Spatiale des Rayonnements, Toulouse, France;

A. M. Rymer: Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland, U.S.A.;

G. R. Lewis and A. J. Coates: Mullard Space Science Laboratory, University College London, Dorking, U.K.;

E. C. Sittler, Jr.: NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.;

M. F. Thomsen: Los Alamos National Laboratory, Los Alamos, New Mexico, U.S.A.;

M. K. Dougherty: The Blackett Laboratory, Imperial College, London, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030374, 2007

11. Soil moisture and remote sensing

Soil moisture plays an important role in the exchange of water and heat between land and atmosphere. Although C-band (6-7 gigahertz) passive microwave frequencies have been used to map soil moisture from aircraft and satellites, many studies have suggested that L-band (1.4 gigahertz) is more useful for this application. Unlike other bands, L-band data are less obscured by the presence of vegetation. However, utilizing this frequency from space with conventional technologies requires an antenna size too large for most satellite missions. Problems with antenna size can be overcome with new interferometric technology. Ryu et al. analyze data from an airborne experiment that used this technology to collect L-band data over Alabama in 2003. Data is then compared with simultaneously collected C-band data and with surface soil measurements collected from the ground. The results demonstrate that L-band data provides soil moisture values more accurately than does C-band data. The new interferometric technology will be employed to monitor soil moisture using L-band for the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) satellite mission, scheduled for launch in 2008.

Title: L-band microwave observations over land surface using a two-dimensional synthetic aperture radiometer

Authors: Dongryeol Ryu, Thomas J. Jackson, and Rajat Bindlish: Agricultural Research Service, Hydrology and Remote Sensing Laboratory, U.S. Department of Agriculture, Beltsville, Maryland, U.S.A.;

David M. Le Vine: Instrument Science Branch, NASA Goddard Space Flight Center, Greenbelt, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030098, 2007

12. North Atlantic climate and deep-ocean flow speed

In the Greenland, Iceland and Norwegian Seas, density and temperature differences cause cold, salty water to sink. Part of these dense water masses flow across the submarine sill between Iceland and Scotland. They are called Iceland-Scotland Overflow Water (ISOW) and contribute to North Atlantic Deep Water, a current that hugs the seafloor as it traces a path around Africa before joining deep waters surrounding Antarctica and later upwelling in the Pacific Ocean. Water returns to the northern Atlantic through a series of surface currents; this global ocean conveyor regulates climate by redistributing heat across the world's oceans. Using a sediment core from the subpolar North Atlantic, Boessenkool et al. study variations in the flow speed of ISOW over the past 230 years. They find that deep flow speeds of ISOW are coupled with the North Atlantic Oscillation (NAO), a climate mode that shifts the pathway and strength of Atlantic storms and depressions across Europe, with more vigorous ISOW flow when these storm tracks were over southern Europe and less vigorous ISOW flow when storm tracks were over northern Europe.

Title: North Atlantic climate and deep-ocean flow speed changes during the last 230 years

Authors: K. P. Boessenkool and I. R Hall: School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff, U.K.;

H. Elderfield: Department of Earth Sciences, University of Cambridge, Cambridge, U.K.;

I. Yashayaev: Bedford Institute of Oceanography, Fisheries and Oceans Canada, Dartmouth, Nova Scotia, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030285, 2007

13. Comparing polar mesosphere summer echoes

Polar mesosphere summer echoes are strong signal power enhancements at very high radar frequencies that occur between about 80 kilometers (50 miles) and 95 kilometers (59 miles) in altitude at polar latitudes during summer. To determine whether the polar atmospheres between the Northern and Southern hemispheres are different, Latteck et al. compare continuous measurements of such echoes collected at Andenes, Norway, during the boreal summer of 2004 and Davis, Antarctica, during the austral summer of 2004-2005. They found that echoes observed at Davis are less reflective and reach maximum heights about 1 kilometer (0.6 miles) higher than those in the Northern Hemisphere. Further, polar mesosphere summer echoes occurred less frequently but with greater variability above Davis. Although both echo seasons started around the same time, the duration of the Davis season was about 9 days shorter than that at Andenes. Because such echoes are thought to provide information on middle atmosphere temperatures, the authors expect that continued research on this topic will help scientists understand climate patterns at the pole.

Title: Observation of polar mesosphere summer echoes with calibrated VHF radars at 69º in the Northern and Southern Hemisphere

Authors: R. Latteck and W. Singer: Leibniz-Institute of Atmospheric Physics, Kühlungsborn, Germany;

R. J. Morris and D. J. Murphy: Australian Antarctic Division, Kingston, Tasmania, Australia;

D. A. Holdsworth: Australian Antarctic Division, Kingston, Tasmania, Australia; and Atmospheric Radar Systems, Thebarton, South Australia, Australia.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030032, 2007


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