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AGU Journal Highlights -- May 16, 2007

American Geophysical Union

1. Three Gorges Dam shrinks Yangtze delta

River damming can damage downstream environments by retaining sediments and nutrients. Yang et al. quantify how China's Three Gorges Dam, the world's largest dam, influences downstream sediment delivery in the Yangtze River. The authors calculate supplies of water and sediment from ungauged areas and combine them with data sets from gauging stations. The Three Gorges Dam, which has regulated the waters of the Yangtze River since 2003, retains two-thirds of the upstream sediment each year, they find. Moreover, in response to this retention, significant erosion occurs in the riverbed downstream of the dam. Because the erosion does not offset the sediment lost in the reservoir, and because sediment flux to the Yangtze River mouth has decreased by 31 percent per year, the Yangtze delta is shrinking. Continued sediment retention at these rates, combined with more dams planned for the watershed, will severely affect the humans and ecosystems that reside on the Yangtze delta, the authors suggest.

Title: Influence of the Three Gorges Dam on downstream delivery of sediment and its environmental implications, Yangtze River

Authors: S. L. Yang, J. Zhang, and X. J. Xu: State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, China.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029472, 2007


2. Permafrost thaw" New exam yields healthier prognosis

A reexamination of projected melting of Arctic permafrost from global warming indicates that massive releases of methane from permafrost degradation are unlikely in this century. During the 20th century, humans' increasing greenhouse gas emissions have made Arctic regions warmer, causing permafrost to melt. Model projections indicate that, as the climate warms, permafrost will continue melting and methane bound in frozen sediments could escape to the atmosphere. Because methane is also a greenhouse gas, this would exacerbate global warming. One permafrost model, presented in late 2005, indicated that near-surface Arctic permafrost will completely degrade during the 21st century. Now, Delisle has critically reviewed this model, finding it to lack necessary initial parameters. He offers an alternative model designed to have a more complete mathematical formulation of the physical processes in permafrost. It projects that surface permafrost will persist in areas north of 70ºN latitude. Permafrost will also endure at depth between 60ºN and 70ºN. Delisle notes that ice-core analyses previously made by other scientists indicate minimal release of methane during warm periods in the last 9,000 years. Based on the new model and the ice-core findings, he concludes that scenarios calling for massive releases of methane in the near future from degrading permafrost are questionable.

Title: Near-surface permafrost degradation--how severe during the 21st century?

Authors: G. Delisle: Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029323, 2007


3. Mapping flood waves from space

In floods, interacting streams of varying depth, velocity, source, sediment concentration, and chemistry may threaten communities near rivers and restore or damage ecosystems. However, data taken from monitoring stations along rivers prone to flooding are geographically sparse and often are oversimplifications of floodplain water flux. To reflect the true complexity of floods, Alsdorf et al., use satellite data from synthetic aperture radars to study water fluctuations of several floods of the Amazon River in Brazil in the 1990s. Those radars have previously been used to measure centimeter-scale topographic displacements from earthquakes and advancing glaciers. Contrary to assumptions of many models that the water surface is horizontal and equal to the changing level of water in main river channels, the authors find sharp variations of the water surface during floods. Furthermore, they observe that hydraulic momentum of the rushing floodwaters can drive flow paths previously thought to be controlled by surface topography. Incorporating these observations into models will lead to improved identification of floodplain hazards, the authors suggest.

Title: Spatial and temporal complexity of the Amazon flood measured from space

Authors: Doug Alsdorf: School of Earth Sciences, Ohio State University, Columbus, Ohio, U.S.A.;
Paul Bates: School of Geographical Sciences, University of Bristol, Bristol, U.K.;
John Melack and Thomas Dunne: Bren School of Environmental Science and Management, University of California, Santa Barbara, Santa Barbara, California, U.S.A.;
Matt Wilson: Department of Geography, University of Exeter, Penryn, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029447, 2007


4. Imaging Earth's deep mantle near the core-mantle boundary

To learn more about the D" layer, the 200-kilometer (124-mile) thick layer above the core-mantle boundary, Kawai et al. used a new high-resolution method, called seismic waveform inversion. In this method, the best-fitting Earth model is obtained by objectively and quantitatively matching observed and computed seismograms. It was previously known that at the top of D" layer, perovskite, the dominant lower mantle mineral, transforms into a different crystal structure called post-perovskite, with a resulting jump in seismic wave velocities. The authors' method allowed them to study the details of how seismic velocities vary with depth within the D" layer, using data for the region beneath Central America. They found that in the lowermost 100 kilometers (62 miles) of D" layer, the velocities drop back to slower speeds that correspond to the perovskite crystal structure. These results tentatively provide independent confirmation of theoretical studies which suggest that post-perovskite should revert to perovskite in D" close to the core-mantle boundary.

Title: Possible evidence for a double crossing phase transition in D" beneath Central America from inversion of seismic waveforms

Authors: Kenji Kawai: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan;
Nozomu Takeuchi: Earthquake Research Institute, Tokyo University, Tokyo, Japan;
Robert J. Geller and Nobuaki Fuji: Department of Earth and Planetary Science, Graduate School of Sciences, Tokyo University, Tokyo Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029642, 2007


5. Tropical cloudiness and precipitation: Early results from CloudSat

Cloud convection in the tropics influences precipitation rates. Haynes and Stephens analyze the first three months of data from CloudSat, a satellite launched by NASA in April 2006, which measures water and ice content within clouds. Eighteen percent of clouds detected by CloudSat produced precipitation. Moreover, in all regions examined, precipitating clouds were related to deeper air convection cells than non-precipitating clouds, the team reports. Over tropical oceans, cloud tops centered around 2 or 12 kilometers (1.2 or 7.5 miles) in altitude. Precipitating clouds also followed this pattern, although some data indicated an additional cloud layer with tops at around 6 kilometers (3.7 miles). Mid-level precipitating clouds were most apparent in areas with convection throughout the air column. High precipitating clouds were more abundant over the Indian and Western Pacific oceans, than over the Eastern Pacific and Atlantic.

Title: Tropical oceanic cloudiness and the incidence of precipitation: Early results from CloudSat

Authors: John M. Haynes and Graeme L. Stephens: Department of Atmospheric Sciences, Colorado State University, Fort Collins, Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029335, 2007


6. Tropical instability waves in the Atlantic and Pacific oceans

Trade winds drive upwelling along the equator, bringing cold water to the ocean surface. In the Pacific and Atlantic oceans, temperature fronts oscillate between the cold equatorial water and warmer water nearer to the poles. These tropical instability waves (TIWs) propagate westward in both basins. Using satellite data, Wu and Bowman examine how these waves interact with year-to-year variations of sea-surface temperature. Based on eight years of data from the Tropical Rainfall Measuring Mission satellite, they find that the sea-surface-temperature effect of TIWs in the Pacific Ocean is weakest when El Nino warms eastern Pacific waters and strongest when the El Nino Southern Oscillation cycle cools those waters. Similar patterns occur in the Atlantic. Such variations in sea-surface temperature may produce corresponding variations in the regional atmosphere, the authors suggest.

Title: Interannual variations of tropical instability waves observed by the Tropical Rainfall Measuring Mission

Authors: Qiaoyan Wu and Kenneth P. Bowman: Department of Atmospheric Sciences, Texas A&M University, College Station, Texas, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029719, 2007


7. Improving land-surface model hydrology

The interaction between groundwater and the atmosphere has a potentially significant influence on regional climate variability. Researchers have focused increasing attention on improving representations of subsurface hydrology in land-surface models, which are computer programs that simulate exchanges of water and energy between land and atmosphere. Using the National Center for Atmospheric Research's Community Land Model over an area representing Illinois, Gulden et al. have examined how different representations of subsurface hydrology behave under three uncertainty scenarios. Those differ by the amount known about the system: nothing, limited knowledge, or everything. While the different hydrology representations can simulate known monthly values of Illinois' average terrestrial water storage, a more physically realistic representation of subsurface processes reduces the model's sensitivity to errors in parameters, they find. Moreover, approximate knowledge of parameters can't guarantee realistic model performance, because interactions among parameters exacerbate errors.

Title: Improving land-surface model hydrology: Is an explicit aquifer model better than a deeper soil profile?

Authors: Lindsey E. Gulden, Enrique Rosero, Zong-Liang Yang, and Guo-Yue Niu: Department of Geological Sciences, The University of Texas at Austin, Austin, Texas, U.S.A.;
Matthew Rodell: Hydrological Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.;
Charles S. Jackson: Institute for Geophysics, University of Texas at Austin, Austin, Texas, U.S.A.;
Pat J.-F. Yeh and James Famiglietti: Department of Earth System Science, University of California, Irvine, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029804, 2007

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