AGU Journal highlights — Nov. 4, 2010
The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Journal of Geophysical Research – Atmospheres (JGR-D), and Journal of Geophysical Research – Earth Surface (JGR-F).
1. Exploring climate patterns linking stratosphere, lower atmosphere
Roughly every 28 months, the zonal winds in the stratosphere at the equator cycle from easterly to westerly and then back to easterly. Known to atmospheric scientists as the quasi-biennial oscillation (QBO), these shifting wind patterns result when the energy from perturbations in the troposphere ripples through the stratosphere. However, QBO patterns are known not to be regular–the shift in wind patterns can be weak or strong and does not occur at precise intervals.
To better understand why the QBO is so variable, Taguchi compares records of the QBO derived from stratospheric data spanning the past 50 years with records of the strength and timing of shifts in the El Niño-Southern Oscillation (ENSO) cycle and annual shifts in the seasons. He finds that the phases of the QBO tend to be influenced by seasons. On broader time scales, comparisons of QBO to cold and warm ENSO conditions (La Niña and El Niño, respectively) reveal for the first time clear correlations between QBO and ENSO: The QBO signals exhibit weaker amplitudes and faster cycles during El Niño conditions. The QBO-ENSO connection may shed light on how the stratosphere influences tropical storms and vice versa.
Observed connection of the stratospheric quasi-biennial oscillation with El Niño-Southern Oscillation in radiosonde data
Masakazu Taguchi: Department of Earth Sciences, Aichi University of Education, Kariya, Japan.
Journal of Geophysical Research-Atmospheres, doi:10.1029/2010JD014325, 2010 http://dx.doi.org/10.1029/2010JD014325
2. Method separates pollution’s effects on clouds from sea spray’s
Aerosols suspended in the atmosphere over the ocean can typically be classified into two categories: particles from sea spray and particles from continents, the latter of which includes soot and other pollutants from anthropogenic emissions. Combined, these aerosols can influence the formation and behavior of clouds–water vapor tends to condense into droplets around these particles. However, detailed studies of how aerosols influence clouds is hampered by study regions needing to be broad to get the amount of data required for statistical analysis–cloud properties in the broad study regions tend to be heavily controlled by meteorological dynamics that mask the effects of aerosols on cloud formation.
Su et al. have developed a new method to get around these difficulties. By dividing satellite data retrievals over the ocean into one degree by one degree grid boxes, they can track atmospheric particles and thus can separate the aerosols of oceanic origin from those that originate from continents. Focusing on the atmosphere over the Atlantic Ocean just off the coast of western Africa, they find that when they analyze grid boxes that are at least 80 percent clouds, those with a prevalence of aerosols of continental origin have smaller cloud droplet effective radii than those associated with aerosols of oceanic origin. Further, cloud optical depth and the reflectivity of cloud tops in grids where continental aerosols dominate tend to be higher than grids where oceanic aerosols are abundant. These clear differences in how continental and oceanic aerosols influence clouds may help researchers better understand aerosol-cloud interactions and help modelers better represent aerosol-cloud interactions in global climate models.
An estimate of aerosol indirect effect from satellite measurements with concurrent meteorological analysis
Wenying Su and Zachary A. Eitzen: Science Systems and Applications Inc., Hampton, Virginia, USA;
Norman G. Loeb, Kuan-Man Xu, and Gregory L. Schuster: NASA Langley Research Center, Hampton, Virginia, USA.
Journal of Geophysical Research-Atmospheres, doi:10.1029/2010JD013948, 2010
3. Combined satellite data yields novel Earth gravity field model
Models of Earth’s gravity field show how the planet’s density varies in its interior and on its surface, depending on topography and geologic composition. For instance, dense mountains create an area of stronger gravitational pull. Pail et al. have developed a new model of Earth’s gravity field using measurements from both the Gravity Recovery and Climate Experiment (GRACE) satellites and the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite.
This is the first global satellite-only gravity model derived by combining data from these missions, without including any other gravity field prior information and providing a homogeneous view of the Earth’s gravity field. While large-scale features could be best derived from the precise measurement of distance changes between the GRACE twin satellites, the new technology of satellite gravity gradiometry as implemented by GOCE makes it possible to determine the detailed gravity field structure with unprecedented accuracy.
The authors validate the model by comparing it with complementary and independent gravity field information based on satellite and terrestrial gravity measurements. The new model is useful for studies of dynamic ocean topography and related mass and energy transport through the oceans and for geophysical models of the three-dimensional mass distribution and dynamic structure of Earth’s interior.
Combined satellite gravity field model GOCO01S derived from GOCE and GRACE
R. Pail, T. Fecher and T. Gruber: Institute of Astronomical and Physical Geodesy, Technische Universität München, Munich, Germany;
H. Goiginger and D. Rieser: Institute of Navigation and Satellite Geodesy, Graz University of Technology, Graz, Austria;
W.-D. Schuh, J. M. Brockmann, T. Mayer-Gürr and J. Kusche: Institute of Geodesy and Geoinformation, University of Bonn, Bonn, Germany;
E. Höck: Space Research Institute, Austrian Academy of Sciences, Graz, Austria;
A. Jäggi: Astronomical Institute, University of Bern, Bern, Switzerland.
Geophysical Research Letters, doi:10.1029/2010GL044906, 2010
4. Improving Earth surface displacement analysis
Advanced processing of spaceborne synthetic aperture radar (SAR) data allows improved measurements of Earth’s surface deformation over time. The common base of these advanced methods is the differential interferometric SAR (DInSAR) technique, which exploits temporally separated SAR image pairs of an area to provide measurements of ground deformation with centimeter to millimeter accuracy and large spatial coverage. DInSAR has been in use since the 1990s to study single Earth-deforming events such as earthquakes; in the past decade, advanced techniques have been developed to study Earth’s surface displacements over time through the analysis of SAR image sequences.
To illustrate the value of these advanced DInSAR techniques for the analysis of complex geophysical phenomena, Sansosti et al. present selected case studies focused on specific zones, including the earthquake-prone San Francisco Bay area and the region around Mount Etna volcano in Italy. In these cases, they apply the small baseline subset (SBAS) DInSAR technique to retrieve measurements of Earth’s surface deformation over the past 18 years by using the SAR data archives collected by European Space Agency satellites. They also present an analysis focused on the L’Aquila, Italy, area that was hit by an earthquake in April 2009. They analyze the surface deformation that occurred in this area by also considering data from the new-generation COSMO-SkyMed SAR constellation, operated by the Italian Space Agency, highlighting its capability to investigate phenomena with rather fast dynamics, such as postseismic deformation.
Space-borne radar interferometry techniques for the generation of deformation time series: An advanced tool for Earth’s surface displacement analysis
E. Sansosti, F. Casu, M. Manzo, and R. Lanari: IREA, CNR, Naples, Italy.
Geophysical Research Letters, doi:10.1029/2010GL044379, 2010
5. Sandbar migration difficult to predict
Nearshore sandbars migrate out toward the ocean and then can move back due to the interaction of sand and waves. Pape et al. studies several data-driven neural network models and a physically detailed model of sandbar migration to see how well they could predict sandbar migration over time scales of months to years. To investigate the effect of model complexity and the level of detail of the representation of sandbars and waves, they apply a range of different neural network architectures. They compare model results with more than 10 years of profile measurements collected daily along a 400-meter (1300 feet)-long pier in Japan.
They find that all models reproduced general features of sandbar behavior, including rapid offshore migration, slower onshore return, and net migration offshore. However, the simplest neural network model outperformed both the more complex neural network models and the physically detailed model. Furthermore, none of the models could accurately predict the sandbar location in the long term because errors accumulate over time. The study demonstrates the limitations of numerical modeling in predicting sandbar migration.
Models and scales for cross-shore sandbar migration
L. Pape: Department of Physical Geography, Faculty of Geosciences, Universiteit Utrecht, Utrecht, Netherlands; Now at Dalle Molle Institute for Artificial Intelligence, Manno-Lugano, Switzerland;
Y. Kuriyama: Marine Environment and Engineering Department, Port and Airport Research Institute, Yokosuka, Japan;
B. G. Ruessink: Department of Physical Geography, Faculty of Geosciences, Universiteit Utrecht, Utrecht, Netherlands.
Journal of Geophysical Research-Earth Surface, doi:10.1029/2009JF001644, 2010 http://dx.doi.org/10.1029/2009JF001644
6. Sulfate isotopes explain past Antarctic atmospheric conditions
The past 230 years have seen great changes in atmospheric conditions in the Northern Hemisphere, due to pollutants released from biomass and coal burning as societies became industrialized and the fossil fuel burning that began to dominate energy consumption at the turn of the twentieth century. The changes from low pollution levels to biomass burning to fossil fuel burning are recorded in Greenland ice cores–the burning of different fuels releases chemical species that influence the oxidation capacity (or reactivity) of the atmosphere. The oxidation capacity of the atmosphere influences atmospheric concentrations of pollutants and greenhouse gases. Sulfate isotopes record this oxidation chemistry in the atmosphere, and these isotopically distinct molecules then precipitate in the Arctic’s yearly snow cycles.
Kunasek et al. wondered whether and to what degree pollution burdens in the Northern Hemisphere migrated through atmospheric circulation into the Southern Hemisphere. By analyzing an ice core record in western Antarctica spanning the past 230 years, they found that while the industrial movement was occurring in the north, the sulfate aerosol abundances in the ice core–and thus in the atmosphere at those times–were episodic and showed isotopic signatures consistent with volcanic eruptions, including that of Mount Tambora in Indonesia in 1815. The twentieth century also demonstrates a lack of change in sulfate isotopes. The authors show that this is consistent with global models of atmospheric chemistry, which suggest changes in Southern Hemisphere oxidant concentrations that have opposing effects on the isotopes of sulfate. This demonstrates that pollution in the Northern Hemisphere indirectly influences the atmospheric chemistry of the Southern Hemisphere.
Sulfate sources and oxidation chemistry over the past 230 years from sulfur and oxygen isotopes of sulfate in a West Antarctic ice core
S. A. Kunasek, E. J. Steig, and D. J. Gleason: Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA;
B. Alexander and E. D. Sofen: Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA;
T. L. Jackson and M. H. Thiemens: Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California, USA;
J. R. McConnell: Desert Research Institute, Nevada System of Higher Education, Reno, Nevada, USA;
H. M. Amos: Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA; now at Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA.
Journal of Geophysical Research-Atmospheres, doi:10.1029/2010JD013846, 2010
7. Depletion of global groundwater deepens
Growing populations are increasing demand for water resources around the globe. In some regions where there is frequent water stress, groundwater is often used as a source of water in addition to surface water. When groundwater extraction consistently exceeds groundwater recharge through precipitation for large areas, groundwater becomes depleted. Wada et al. use a hydrological model to assess global groundwater recharge. They then subtract estimated groundwater extraction from groundwater recharge rates to provide a global overview of groundwater depletion. The researchers find that total global groundwater depletion has increased from 126 cubic kilometers per year (30 cubic miles per year) in 1960 to 283 cubic kilometers per year (68 cubic miles per year) in 2000. They also calculated that this groundwater depletion leads to continental runoff that currently contributes about 0.8 millimeters per year (0.032 inches) to global sea level rise, accounting for about 25 percent of the current rate of sea level rise.
See press release at:
Global depletion of groundwater resources
Yoshihide Wada and Ludovicus P. H. van Beek: Department of Physical Geography, Utrecht University,Utrecht, Netherlands;
Cheryl M. van Kempen, Josef W. T. M. Reckman, and Slavek Vasak: International Groundwater Resources Assessment Center, Deltares, Utrecht, Netherlands;
Marc F. P. Bierkens: Department of Physical Geography, Utrecht University,
Utrecht, Netherlands, and Unit Soil and Groundwater Systems, Deltares,
Geophysical Research Letters, doi:10.1029/2010GL044571, 2010, 2010