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AGU Journal Highlights -- Aug. 14, 2007

American Geophysical Union

1. Concurrent arrival of the 2004 Sumatra tsunami and storm-generated waves on North America's Atlantic coast

The Sumatra-Andaman earthquake generated a catastrophic tsunami that caused heavy damage and fatalities in coastal areas around the Indian Ocean. The tsunami, which struck on 26 December 2004, also propagated throughout the world's oceans, making it the first such event to be scrutinized with continuous observations of widespread oceanic monitoring networks. Thomson et al. analyze more than 100 tide gauge records from the Atlantic coast of North America and find that the tsunami was identified in most outer tide gauges from Florida to Nova Scotia. Maximum heights for northern regions were between 32 and 39 centimeters (1.0 and 1.3 feet), while southern regions experienced wave heights between 15 and 33 centimeters (0.49 and 1.1 feet). However, along the shores of Maine and Nova Scotia, the arrival of the tsunami coincided with the presence of tsunami-like waves generated by a major storm tracking northward along the U.S. eastern seaboard. The combined waves reached heights in excess of 1 meter (3.3 feet). The authors warn that, although the northern Atlantic Ocean has low tsunami hazards, tsunamis from distant seismic events could threaten coastal infrastructure and habitat when the waves coincide with winter storm waves.

Title: Double jeopardy: Concurrent arrival of the 2004 Sumatra tsunami and storm-generated waves on the Atlantic coast of the United States and Canada

Authors: Richard E. Thomson: Institute of Ocean Sciences, Sidney, British Columbia, Canada;

Alexander B. Rabinovich: Institute of Ocean Sciences, Sidney, British Columbia, Canada; also at P.P. Shirshov Institute of Oceanology, Moscow, Russia;

Maxim V. Krassovski: School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030685, 2007

2. Antarctic Bottom Water has warmed within recent decades

The western South Atlantic's coldest, deepest layer, called the Antarctic Bottom Water, originates in the waters surrounding Antarctica. In the subtropics, most of this current joins water in the Brazil Basin through the Vema Channel, a narrow gorge on the seafloor about 1000 kilometers (600 miles) southeast of Rio de Janeiro. Zenk and other researchers have made measurements of water temperatures and other parameters of the flow through the Vema Channel during the past 35 years. In this new analysis, he and Morozov find that temperatures in this channel were fairly level before 1992, but that the next 15 years were marked by a warming trend that raised temperatures about 0.0028ºC (0.005 ºF) each year. Although this trend is seen, the flow's properties through the Vema Channel were highly variable. Nonetheless, the authors use this long record to conjecture that the Antarctic Bottom Water also has undergone slight freshening. They note that additional long-term studies on deep circulation and water mass properties may help reveal whether abyssal oceans are warming at locations other than choke points such as the Vema Channel.

Title: Decadal warming of the coldest Antarctic Bottom Water flow through the Vema Channel

Authors: Walter Zenk: The Leibniz Institute of Marine Sciences at the University of Kiel (IFM-GEOMAR), Kiel, Germany;

Eugene Morozov: Shirshov Institute of Oceanology, Moscow, Russia.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030340, 2007

3. What caused widespread cooling 8,200 years ago?

Records from ice cores show that around 8,200 years ago the Northern Hemisphere's climate abruptly cooled. Many scientists link this event to the final drainage of Lake Agassiz, a large glacial lake covering much of central Canada that formed at the foot of North America's continental glaciers. This drainage is thought to have freshened waters in the northern Atlantic Ocean, slowing down the density-driven oceanic circulation that helps to distribute heat. Noting that an actual chronology of events must be established before scientists can speculate on causes of this cooling, Hillaire-Marcel et al. study oceanic records downstream from Lake Agassiz's flood discharge route. They find that the lake's drainage occurred between 8,500 and 8,350 years ago but that sea-surface and deep-current conditions, derived from oceanic sediment cores, lack significant concurrent changes in the northern Atlantic. Instead, the data shows that the 8,200-year-old cooling event was generated by several factors, including melting of North American continental glaciers and subsequent rapid sea level rise which induced a large-scale reorganization of broad oceanic circulation patterns.

Title: The ~8.4 ka Lake Agassiz drainage event in the northwest North Atlantic

Authors: C. Hillaire-Marcel and Anne de Vernal: Geochemistry and Geodynamics Research Centre, University of Quebec at Montréal and McGill University (GEOTOP-UQAM-McGill), Montreal, Quebec, Canada;

David J. W. Piper: Atlantic Division, Geological Survey of Canada, Dartmouth, Nova Scotia, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030396, 2007

4. Warmer surface temperatures in the North Atlantic may affect the mighty current that encircles Antarctica

In the northern Atlantic Ocean, cold salty water sinks, forming the North Atlantic Deep Water, a southward moving water mass centered around the depth of 2.5 kilometers (1.6 miles). This sunken water is replaced by water essentially originating in the Antarctic Circumpolar Current and flowing across the equator northward through surface currents such as the Gulf Stream and the North Atlantic Current. In a sensitivity study using a coarse-resolution ocean general circulation model in an idealized single-basin configuration with a circumpolar channel, Fučkar and Vallis find that deep water production diminishes as surface temperature increases in the north, affecting the basin's overturning circulation and stratification. This induces a change in water mass properties in the southern circumpolar region, causing a substantially higher volume transport around Antarctica. The authors note that significant variations in certain critical model parameters do not change this result. If their model holds true, a change of surface buoyancy in the Northern Hemisphere may significantly influence the stratification and transport of the Antarctic Circumpolar Current.

Title: Interhemispheric influence of surface buoyancy conditions on a circumpolar current

Authors: Neven S. Fučkar: Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, New Jersey, U.S.A.;

Geoffrey K. Vallis: Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, New Jersey, U.S.A.; also at Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, New Jersey, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030379, 2007

5. Estimating local tsunami wave height from great earthquakes

The massive 9.2-magnitude Sumatra-Andaman earthquake on 26 December 2004 generated a tsunami that propagated throughout the Indian Ocean, killing more than 250,000 people. By contrast, the nearby 8.7-magnitude Simeulue-Nias earthquake on 28 March 2005 generated a small tsunami that caused only a few casualties. Though these earthquakes occurred in similar tectonic settings, their tsunami were markedly different, highlighting the need for reliably determining tsunami hazards from earthquake geometry. Using geodetic and stress accumulation studies, McCloskey et al. model about 100 different complex earthquake ruptures in this area and calculate their sea-floor displacements and resulting tsunami wave heights. They find that, for locations close to the earthquake source, the timing of tsunami inundation is independent of the earthquake magnitude and slip distribution. However, the maximum tsunami wave height is directly proportional to the vertical displacement of the rupture. Because stress field studies indicate that the Sumatra-Andaman region is overdue for another great earthquake, the authors note that a single estimate of vertical displacement during an earthquake might provide a reliable short-term forecast of tsunami wave height.

Title: Near-field propagation of tsunamis from megathrust earthquakes

Authors: John McCloskey, Andrea Antonioli, Sandy Steacy, Suleyman S. Nalbant, JianDong Huang, and Paul Dunlop: Geophysics Research Group, School of Environmental Sciences, University of Ulster, Coleraine, Northern Ireland, U.K.;

Alessio Piatanesi, Massimo Cocco, and Carlo Giunchi: Seismology and Tectonophysics Department, Istituto Nazionale de Geofisica e Vulcanologia, Rome, Italy;

Kerry Sieh: Tectonics Observatory, California Institute of Technology, Pasadena, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030494, 2007

6. Soil moisture induces atmospheric circulation: Observations from Africa's Sahel

Theoretical studies and models have shown that contrasting land-surface properties can induce circulations in the atmosphere. Taylor et al. seek to observe this through monitoring Africa's Sahel region, an area where intense precipitation generates a varied spatial and temporal distribution of soil moisture. Using aircraft and satellite data, the authors find that the air just above wet soil was moister and up to 3ºC (5 ºF) cooler than nearby dry areas. In addition, winds near the ground were found to vary significantly over patches of wet soil as small as 10 kilometers (6 miles) across, consistent with the theory that soil moisture can drive atmospheric circulations. The authors expect that analysis of additional cloud data will reveal the role of these circulations in the generation of new storms, aiding forecasting in this notoriously unpredictable region and helping to refine models of tropical climate.

Title: An observational case study of mesoscale atmospheric circulations induced by soil moisture

Authors: Christopher M. Taylor and Phil P. Harris: Centre for Ecology and Hydrology, Wallingford, U.K.;

Douglas J. Parker: Institute for Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030572, 2007

7. Dehydration of water-bearing minerals helps to lubricate faults

Accurate seismic hazard assessments depend on deep knowledge of fault dynamics and motion during earthquakes. Scientists must understand the mechanical properties of faults, especially at the fast slip rates that occur in earthquakes. Although theoretical studies in major fault zones suggest that faults might be weak during large earthquakes, laboratory experiments and deep borehole measurements worldwide show that average stresses in the crust must be generally high. Noting that hydrous minerals such as serpentine are common in mature fault zones, Hirose and Bystricky conducted high-velocity friction experiments on simulated faults in serpentinite at earthquake slip conditions. They find that the fault strength dramatically decreases due to rapid heating produced by fast slip rates. They hypothesize that the rapid heating separates hydroxyl ions (OH-) from the serpentine mineral structure, forming water. This free water helps to lubricate the fault plane. The authors expect that this fault-weakening mechanism may explain the lack of pronounced heat flow in major faults such as the San Andreas.

Title: Extreme dynamic weakening of faults during dehydration by coseismic shear heating

Authors: Takehiro Hirose: Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto, Japan; currently at Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy;

Misha Bystricky: LMTG, Université de Toulouse, CNRS, IRD, Observatoire Midi-Pyrénées, Toulouse, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030049, 2007

8. Climate change and geoengineering: Is the cure worse than the sickness?

Global warming occurs when greenhouse gases, such as carbon dioxide from burning fossil fuels, build up in the atmosphere and alter outgoing longwave radiation. Some scientists have proposed that mitigating global warming could be accomplished by emulating a volcanic eruption because volcanic aerosols scatter incoming sunlight, reducing outgoing radiation. Trenberth and Dai caution against this mitigation proposal. They examine precipitation and streamflow records from 1950 to 2004 to document the effects of volcanic eruptions from Mexico's El Chichón (1982) and the Philippines' Pinatubo (1991). They find that, following the 1991 eruption of Mount Pinatubo, there was a substantial global decrease in precipitation over land, a record decrease in runoff and river discharge into the oceans, and widespread drying over land during the following year. Thus, the authors conclude that major adverse effects, including drought, could arise from geoengineering solutions to global warming.

Title: Effects of Mount Pinatubo volcanic eruption on the hydrological cycle as an analog of geoengineering

Authors: Kevin E. Trenberth and Aiguo Dai: National Center for Atmospheric Research, Boulder Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030524, 2007

9. Stomatal conductance and climate sensitivity to carbon dioxide

In order to fix carbon, plants transpire water and take up atmospheric carbon dioxide (CO2) through stomata, which are small openings in leaves. The density of stomata on a leaf indicates the abundance of carbon dioxide in the atmosphere--high densities mean that CO2 is scarce, while low densities mean that CO2 is plentiful. This effect has previously been used to reconstruct CO2 concentrations in the Earth's past. Now, by including the effect in a coupled vegetation-climate model, Kleidon finds that doubling the abundance of CO2 in the atmosphere could increase land temperatures by 2.7ºC to 3.2ºC, depending on whether stomata adapt optimally or not at all. The model incorporates the assumption that stomata function optimally to maximize growth. If such stomata functioning is not assumed, climate model predictions of the expected rise in CO2 will generally underestimate vegetative cover

Title: Optimized stomatal conductance and the climate sensitivity to carbon dioxide

Authors: A. Kleidon: Max-Plank-Institut für Biogeochemie, Jena, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030342, 2007

10. Indian Ocean response to anomalous conditions in 2006

The thermal structure of the equatorial Indian Ocean is characterized by warmer temperatures in the east and cooler temperatures in the west. During certain years, this pattern switches to anomalous conditions known as the Indian Ocean Dipole; one such dipole event occurred in 2006. From satellite observations over the oceans from that time, Vinayachandran et al. find that the eastern Indian Ocean not only exhibited colder sea surface temperatures, but also showed lower sea levels and higher chlorophyll content than normal. By contrast, the western Indian ocean was marked by warmer sea surface temperatures, higher sea level, and a steep, deeper thermocline. The authors model this event using an ocean general circulation model forced with satellite-derived wind data. Their reproductions match well with the actual event, and reveal that air-sea heat fluxes initiated the cold sea surface temperatures in the east, which were sustained by ocean dynamics. Similar fluxes fueled the warm surface temperatures in the west. The event reverted back to initial conditions in the fall of 2006.

Title: Indian Ocean response to anomalous conditions in 2006

Authors: P. N. Vinayachandran, Jaison Kurian, and C. P. Neema: Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030194, 2007

11. Coastal notches and seismic uplift rates in Greece

On rocky coastlines, abrasion, dissolution and biological activity can create an erosion notch where the ocean meets land. Given that sea level has generally risen since the end of the last glacial period 20,000 years ago, Cooper et al. hypothesize that notches stay at the coastline if sea level rise is matched by tectonic uplift. However, if sea level rise is outpaced by tectonic uplift, notches will be stranded above the coastline. Noting that stranded notches can reveal uplift rates in seismic areas, the authors study four known paleoshorelines along a stretch of coastline in Greece. Prior work indicates that marine fauna found in these four notches are 650, 1900, 3700 and 6500 years old, respectively, correlating to known periods of sea level stability. The new study's authors use the notches' elevations to calculate uplift rates. These values correspond well with known uplift rates during the last ice age, suggesting that notch sequences could be used in some locations to characterize long-term patterns of uplift, slip-rate, and seismic hazards on active faults.

Title: A comparison of 103 - 105 year uplift rates on the South Alkyonides Fault, central Greece: Holocene climate stability and the formation of coastal notches

Authors: Frances J. Cooper: University of Southern California, Los Angeles, California, U.S.A.;

Gerald P. Roberts and Charlie J. Underwood: Research School of Earth Sciences, Birkbeck College, University of London, London, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030673, 2007

12. Imaging the magma beneath New Zealand's Taupo Volcanic Zone

The Taupo Volcanic Zone is a region of intense volcanism associated with the subduction of the Pacific Plate beneath the continental crust of New Zealand's North Island. Heat output through hydrothermal venting in this region is high, but studies suggest that a much greater amount of magma is cooled within the crust. Because little is known about the properties and extent of this magma, Heise et al. map subsurface structures within the Taupo zone. Through magnetotelluric imaging methods, which plot spatial variations in electric currents induced by natural fluctuations in Earth's magnetic field, the authors generate a map of the zone's conductivity structure. They find a steep increase in conductivity at a depth of 10 kilometers (6 miles), corresponding to a region about three kilometers (1.9 miles) beneath the base of the seismogenic zone and eight kilometers (5 miles) above the base of the continental crust. The authors hypothesize that this increased conductivity marks the presence of molten material within the lower crust.

Title: Melt distribution beneath a young continental rift: The Taupo Volcanic Zone, New Zealand

Authors: Wiebke Heise, Hugh M. Bibby, T. Grand Caldwell, and Stephen C. Bannister: GNS Science, Lower Hutt, New Zealand;

Yasuo Ogawa: Volcanic Fluid Research Center, Tokyo Institute of Technology, Tokyo, Japan;

Shinichi Takakura and Toshihiro Uchida: Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029629, 2007

13. Modeling large-scale inundation on the Amazon River's seasonally flooded wetlands

Flood inundation dynamics for large remote flood plains are critical for understanding hydrological and biogeochemical processes in these important ecosystems. In particular, the Amazon River watershed, whose discharge comprises about 20 percent of total continental runoff, experiences seasonal heavy floods, effecting processes such as plant productivity, heavy metal accumulation, nutrient dynamics, and the carbon cycle. However, the remoteness of Amazon flood plains and wetlands make studying these locations difficult. Wilson et al. use topographic data from NASA's Shuttle Radar Topography Mission to model floodplain inundation on a section of the central Amazon. They then compare model results with sparsely-available flood gauge data and satellite-derived estimations of actual inundation extents. The authors find that their model is fairly accurate at high water levels, but that accuracy drops at low water levels due to initial model parameters and errors in the topographic data used. Nonetheless, they note that, with their method, predictions of floodplain dynamics can be used as quantitative inputs into biochemical and geomorphic studies requiring detailed hydrodynamic information.

Title: Modeling large-scale inundation of Amazonian seasonally flooded wetlands

Authors: Matthew Wilson: Department of Geography, University of Exeter, Exeter, U.K.;

Paul Bates: School of Geographical Sciences, University of Bristol, Bristol, U.K.;

Doug Alsdorf: School of Earth Sciences, Ohio State University, Columbus, Ohio, U.S.A.;

Bruce Fosberg: Instituto Nacional de Pesquisas da Amazonas, Manaus, Brazil;

Matthew Horritt: Halcrow Group Ltd., Wiltshire, U.K.;

John Melack: Bren School of Environmental Science and Management, University of California, Santa Barbara, California, U.S.A.;

Frederic Frappart, James Famiglietti: Department of Earth System Science, University of California, Irvine, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030156, 2007

14. Improving global vegetation models with satellite data on leaf area

To help monitor the effects of increased greenhouse gas emissions, models have been developed to quantify the spatial and temporal variations in carbon dioxide exchanges between the land surface and the atmosphere. However, many models show large differences in primary productivity and net carbon exchange, hindering scientists' ability to understand the current carbon cycle. To help resolve this, Demarty et al. study how global leaf area measurements from satellite data affect France's Dynamic Global Vegetation Model. They find that the added leaf-area data advances the model's onset and end of the growing season in the high northern latitudes by 20 days and 40 days, respectively. This results in lower estimates of primary productivity, with large variations from one vegetation biome to another. The authors also show how measurements of primary productivity from ground-based monitoring stations can help minimize model errors. They note that the further use of satellite products could be substantially enhanced if more ground-based data were used to independently check model results.

Title: Assimilation of global MODIS leaf area index retrievals within a terrestrial biosphere model

Authors: J. Demarty, F. Chevallier, A. D. Friend, N. Viovy, S. Piao, and P. Ciais: Laboratoire des Sciences du Climat et de l'Evnironnement, Centre National de la Recherche Scientifique/Commissariat à l'Énergie Atomique/Université de Versailles Saint-Quentin-en-Yvelines (CNRS/CEA/UVSQ), Gif sur Yvette, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030014, 2007

15. New insight into capillary pressure and groundwater properties

Capillary pressure helps regulate hydrocarbon recovery, groundwater interaction with the surface, and other processes that deal with buried gas or water. However, the difficulty of calculating capillary pressure impedes predicting distributions of multiple fluids in porous media. Plug et al. hypothesize that the ability of surrounding rock to transmit electrical current provides insight into the physical behavior of groundwater's capillary pressure. Through laboratory experiments on sand samples with varying degrees of water saturation, the researchers find that pressure perturbations as water drains are slow to take effect, whereas perturbations as sands soak up water are processed more quickly. Such results indicate that capillary pressure is a unique function of water saturation and electric permittivity. By measuring permittivity in buried sediments, scientists can gain insights into processes at the boundary between groundwater and overlying material.

Title: Capillary pressure as a unique function of electric permittivity and water saturation

Authors: Willem-Jan Plug, Evert Slob, and Johannes Bruining: Department of Geotechnology, Delft University of Technology, Delft, the Netherlands;

Jan van Turnhout: Faculty of Applied Sciences, Delft University of Technology, Delft, the Netherlands.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029674, 2007


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