AGU Journal Highlights -- May 14, 2008

May 14, 2008

1. Did global sea level rise start centuries ago?

Global sea level rise, an important consequence of climate change, will likely affect the lifestyles of people living in coastal communities. Yet controversy and uncertainty cloud discussion of how fast sea level is rising, and why. To learn more, Jevrejeva et al. are the first to reconstruct global sea level since 1700 using tide gauge records from around the world. The authors then analyze the evolution of sea level changes for the past 300 years and present observational evidence that recent global sea level acceleration may have started at the end of the eighteenth century. They also find that sea level rose by 6 centimeters (2.4 inches) during the nineteenth century and 19 cm (7.5 in) during the twentieth century. If the conditions that established the acceleration continue, sea level will rise 34 cm (13 in) over the 21st century. The authors conclude that sea level acceleration will depend on the actual rate of temperature increase in the 21st century and that the latest Intergovernmental Panel on Climate Change estimates of sea level rise for the 21st century are probably too low.

Title: Recent global seal level acceleration started over 200 years ago"

Authors: S. Jevrejeva and P. L. Woodworth: Proudman Oceanographic Laboratory. Liverpool, U.K.;

J. C. Moore and A. Grinsted: Arctic Centre, University of Lapland, Rovaniemi, Finland; Moore also at Thule Institute, University of Oulu, Oulu, Finland.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033611, 2008; http://dx.doi.org/10.1029/2008GL033611


2. Clues suggest U.S. east coast subsided

At midmantle depths beneath eastern North America, the remnants of the subducted Farallon plate create a broad mantle downwelling. Spasojević et al. seek to investigate how such a mantle downwelling has influenced surface topography and regional sea level. Using mantle models and an analysis of paleo shorelines, they find that the U.S. east coast experienced dynamic subsidence since about 40 million years ago, concurrent with the global sea level fall. Superimposing dynamic subsidence and global sea level change (the latter having a larger magnitude than the former) explains how paleo shorelines along the U.S. east coast are lower than those predicted by global sea level changes, and why regional maximum sea level for New Jersey is significantly lower than global predictions.

Title: The case for dynamic subsidence of the U.S. east coast since the Eocene

Authors: Sonja Spasojević, Lijun Liu, and Michael Gurnis: Seismological Laboratory, California Institute of Technology, Pasadena, California, U.S.A.;

R. Dietmar Müller: School of Geosciences, University of Sydney, Sydney, Australia.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033511, 2008; http://dx.doi.org/10.1029/2008GL033511




3. Wind-launched ocean eddies

Oceanic eddies are common features that form in the wakes of islands around the world. Their motion is an important mechanism for distributing energy on local scales, which affects the supply of nutrients to different regions of the ocean. Prior studies on eddies suggest that they shed primarily through ocean flow instability formed by underwater topography. Pullen et al. hypothesize that wind forcing could also uniquely influence eddy detachment. Through studies of observational and modeled data, the authors identify monsoon surges that interacted with the island topography of the Philippines and triggered eddy formation in the South China Sea during January 2005. Further analysis of the modeled data reveal that wind jets and wakes in the lee of Mindoro and Luzon islands induced the generation and migration of a pair of counterrotating oceanic eddies, with propagation direction related to the orientation of winds during each of the surges. The authors suggest that monsoon surges likely represent a robust forcing mechanism for oceanic eddy formation and propagation in the South China Sea.

Title: Monsoon surges trigger oceanic eddy formation and propagation in the lee of the Philippine Islands

Authors: Julie Pullen and James D. Doyle: Naval Research Laboratory, Monterey, California, U.S.A.;

Paul May: Computer Sciences Corporation, Monterey, California, U.S.A.;

Cedric Chavanne and Pierre Flament: University of Hawaii at Manoa, Honolulu, Hawaii, U.S.A.;

Robert A. Arnone: Naval Research Laboratory, Stennis Space Center, Mississippi, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL033109, 2008; http://dx.doi.org/10.1029/2007GL033109




4. Ancient Antarctic dust reveals past wind patterns

Understanding the geographic origin of continental windblown dust reaching Antarctica today and under different climate conditions is essential for tracking past changes in atmospheric circulation regimes. Scientists study windblown dust in Antarctic ice cores by determining the dust's strontium and neodymium isotope ratios, which are distinct for each source region and do not change while dust is being transported. Analyses of ice cores in central East Antarctica have revealed source regions for dust spanning past 50,000 years. Noting that data from the Vostok and Dome-C ice cores span the past 800,000 years, Delmonte et al. analyze ancient dust from these ice cores and find that their isotopic signatures indicate that during glacial periods between 350,000 and 800,000 years ago, dust originated mainly from South America. Similar to what is seen today, the data show an overall westerly wind circulation pattern that allows for dust from South America to reach Antarctica's interior. However, small but significant dissimilarity exists between old and recent glacial ages, suggesting that dust contributions from Patagonia were slightly reduced during ancient glaciations.

Title: Aeolian dust in east Antarctica (EPICA-Dome C and Vostok): Provenance during glacial ages over the last 800 kyr

Authors: B. Delmonte and V. Maggi: DISAT-University Milano-Bicocca, Milano, Italy;

P. S. Andersson and H. Schöberg: Laboratory for Isotope Geology, Swedish Museum for Natural History, Stockholm, Sweden;

M. Hansson: Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, Sweden;

J. R. Petit: LGGE-CNRS-Université Joseph Fourier, St. Martin d'Hères, France;

I. Basile-Doelsch: CEREGE-Université Paule Cézanne, Europole Méditerannéen de l'Arbois, Aix-en-Provence, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033382, 2008; http://dx.doi.org/10.1029/2008GL033382




5. Lab tests show plate-slip progression

In shallow dipping subduction zones, a variety of disparate slip patterns have been revealed by seismic and geodetic monitoring. These include regular earthquakes, slow slip events, and silent quakes. Noting that the present understanding of these slip patterns is based upon numerical simulations, Voisin et al. investigate slip patterns using a friction lab-scale experiment. In particular, the authors are curious to see how cumulative slip changes frictional and acoustic patterns in a lab-scale subduction zone. They find that as the contact interface between plates ages, energy is released through a progression of shallow loud earthquakes, medium-depth slow earthquakes, deeper silent quakes, and the deepest steady state creep. This progression of seismic signals, from strong earthquake events to smaller-amplitude and longer-duration signals similar to those seen for nonvolcanic tremor, indicates that all motion appears as different aspects of a more holistic process that combines motion, aging, and displacement.

Title: Evolution of seismic signals and slip patterns along subduction zones: Insights from a friction lab scale experiment

Authors: Christope Voisin, Jean-Robert Grasso, and Eric Larose: Laboratoire de Géophysique Interne et Tectonophysique, CNRS, Université Joseph Fourier, Grenoble, France;

François Renard: Laboratoire de Géodynamique des Chaînes Alpines, CNRS, Université Joseph Fourier, Grenoble, France; Physics of Geological Processes, Oslo University, Oslo, Norway.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033356, 2008; http://dx.doi.org/10.1029/2008GL033356




6. Atlantic surface temperatures linked to south Asian monsoons

Coupled modeling studies have shown that a weakened Atlantic thermohaline circulation (THC), where deepwater formation slows down, generates cooling in the North Atlantic and a slight warming in the South Atlantic, leading to a southward shift of the Atlantic Intertropical Convergence Zone. This in turn affects the climate of the Pacific and changes El Niño-Southern Oscillation (ENSO) variability. Noting that ENSO influences variability in the south Asian summer monsoon, Lu et al. use a fully coupled atmosphere-ocean model to investigate how a weakened THC, caused by additional freshwater flux into the North Atlantic, trickles down to change south Asian summer monsoons. They find that weakening THC leads to an intensified ENSO-south Asian summer monsoon relationship and enhanced monsoon variability. This intensification in the ENSO-monsoon relationship is likely due to the enhanced ENSO variability induced by the weakened THC. Thus low-frequency fluctuations in Atlantic sea surface temperatures might be connected to south Asian summer monsoon interannual variability and ENSO interactions, and that the ENSO-monsoon relationship may be modulated by a nonlocal source.

Title: How does a weakened Atlantic thermohaline circulation lead to an intensification of the ENSO-south Asian summer monsoon interaction"

Authors: Riyu Lu and Wei Chen: State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, and Center for Monsoon Systems Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China;

Buwen Dong: Walker Institute for Climate System Research, University of Reading, and National Center for Atmospheric Science-Climate, Reading, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033394, 2008; http://dx.doi.org/10.1029/2008GL033394




7. Investigating Andean stress patterns

The Nazca plate subducts under the South American plate at a rapid rate, about 6.7 centimeters (2.6 inches) per year, forming the steeply dipping Peru-Chile Trench and the high Andes Mountains. However, scientists know that 10 million years ago, the convergence rate between the plates was about 10.3 cm (4.06 in) per year. Recent numerical models that couple global mantle circulation with lithosphere dynamics show that growth of the central Andes controls the 30 percent reduction in convergence rate. Noting that such growth defines the pattern of lithospheric stress in mountain ranges, Heidbach et al. use a numerical model that reproduces the Nazca-South America convergence history to predict how stress patterns in the central Andes have changed over the past 10 million years. Comparing models of current stress fields with field measurements reveals that the model fits observations well. Cycling their model back, the authors find that current stress orientations are very similar to orientations that existed about 3 million years ago. From this, they infer that stress rotations occurred between 10 and 3 million years ago, when topography of the Andes gained about 75 percent of the present height.

Title: Topography growth drives stress rotations in the central Andes: Observations and models

Authors: Oliver Heidbach: Geophysical Institute, Universität Karlsruhe, Karlsruhe, Germany;

Giampiero Iaffaldano: Section of Geophysics, Ludwig-Maximilian University, Munich, Germany; now at Department of Earth and Planetary Science, Harvard University, Cambridge, Massachusetts, U.S.A.;

Hans-Peter Bunge: Department of Earth and Planetary Science, Harvard University, Cambridge, Massachusetts, U.S.A

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032782, 2008; http://dx.doi.org/10.1029/2007GL032782




8 Climate models overheat Antarctica

Establishing multidecadal, continental-scale records of near-surface air temperature (NSAT) and snowfall accumulation in Antarctica is important for understanding regional climate variability and the role Antarctic ice sheets play in global sea level change. Although generating such records is challenging due to Antarctica's sparse observational network, several recent studies have enhanced the observational data on NSAT and snowfall primarily during the past 50 years. Monaghan et al. compare these enhanced data with global climate models (GCMs), particularly those that support the Intergovernmental Panel on Climate Change's (IPCC) Fourth Assessment Report. They find that annual Antarctic snowfall accumulation trends in GCMs agree with observations during 1960-1990, and the sensitivity of snowfall to NSAT fluctuations is similar to that observed. However, longer-term NSAT trends (spanning 1880-1999) in the GCMs are about 2.5-5 times larger than observed, possibly due to the radiative impact of unrealistic increases in water vapor. Until these issues are resolved, IPCC projections of 21st-century Antarctic temperatures and snowfall amounts should be treated with caution.

See press release at http://www.agu.org/sci_soc/prrl/2008-17.html .

Title: Twentieth century Antarctic air temperature and snowfall simulations by IPCC climate models

Authors: Andrew J. Monaghan: Polar Meteorology Group, Byrd Polar Research Center, Ohio State University, Columbus, Ohio, U.S.A.;

David H. Bromwich: Polar Meteorology Group, Byrd Polar Research Center, Ohio State University, Columbus, Ohio, U.S.A.; Also at Atmospheric Sciences Program, Department of Geography, Ohio State University, Columbus, Ohio, U.S.A.;

David P. Schneider: Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032630, 2008; http://dx.doi.org/10.1029/2007GL032630




9. Ozone-hole recovery may spur Antarctic warming

Over the past decades, the Southern Hemisphere's polar climate has undergone significant changes, in particular a strengthening of summertime westerly winds at the surface. Climate models link such changes to the depletion of stratospheric polar ozone and the increase of anthropogenic greenhouse gases, the former being the more dominant factor. Now that successful control of ozone-depleting substances has facilitated the recovery of the ozone hole, Perlwitz et al. use a chemistry climate model to simulate how this recovery will affect the Southern Hemisphere's climate. They find that if the ozone hole closes by 2100, surface wind patterns caused by ozone depletion between 1970 and 2000 will almost reverse despite increasing greenhouse gas concentrations. On the basis of these results and results from the Intergovernmental Panel on Climate Change's Fourth Assessment Report, the authors stress that including stratospheric processes in climate assessments will be necessary for accurate simulations of future climate change.

See press release at http://www.agu.org/sci_soc/prrl/2008-15.html .

Title: Impact of stratospheric ozone hole recovery on Antarctic climate

Authors: Judith Perlwitz: Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, Colorado, U.S.A.; also at Physical Sciences Division, Earth System Research Laboratory, U.S. National Oceanic and Atmospheric Administration, Boulder Colorado, U.S.A.;

Steven Pawson and Eric Nielsen: Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.;

Ryan L. Fogt and William D. Neff: Physical Sciences Division, Earth System Research Laboratory, U.S. National Oceanic and Atmospheric Administration, Boulder Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033317, 2008; http://dx.doi.org/10.1029/2008GL033317
-end-
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