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AGU Journal highlights -- Jan. 10, 2008

American Geophysical Union

1. Pulses in Saturn's rings

In 2005, the Cassini spacecraft began a series of experiments to profile ring structure and measure the size distribution of particles in Saturn's rings. During these experiments, Cassini flew behind the ring plane and transmitted radio waves through ring particles to Earth. Scientists on Earth analyzed the signals' diffraction patterns to help determine properties of the rings. Thomson et al. study these data and find that in limited regions of rings A and B (A and B lie close to the outside of Saturn's ring system), the diffraction pattern reveals the presence of fine-scale structures that are characterized by periodic radial variation in optical depth. They define specific periods of variation in optical depth for distinct regions of rings A and B. Past research suggests that the dynamic interplay of gravitational and collisional forces leads to the formation of viscous oscillations and gravity wakes in Saturn's rings. The authors speculate that the number density of ring particles contracts and relaxes to form periodic structures that affect the radio signals seen on Earth.

Title: Periodic microstructure in Saturn's rings A and B


Fraser S. Thomson and G. Leonard Tyler: Center for Radar Astronomy, Stanford University, Stanford, California, U.S.A.;
Essam A. Marouf: Department of Electrical Engineering, San Jose State University, San Jose, California, U.S.A.;
Richard G. French: Department of Astronomy, Wellesley College, Wellesley, Massachusetts, U.S.A.;
Nicole J. Rappoport: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.


Geophysical Research Letters (GRL) paper 10.1029/2007GL032526, 2007;

2. Flow velocity controls timing and magnitude of daily fluctuations in streamflow

Studies of streamflow generation and routing typically focus on monitoring single events such as rainfall impulses to characterize whole-watershed response. Wondzell et al. instead analyze daily fluctuations in summertime streamflow from a small watershed in the Cascade Mountains of Oregon. They show that seasonal changes in daily fluctuations in flow volume at the watershed's mouth can be explained by transport of evaporation and transpiration (ET) generated signals down the stream network. When streamflow velocity is high, ET-generated signals transported down the stream network tend to reach the stream gauge in phase, which reinforces the signals and produces strong daily fluctuations. As flow velocity slows over the summer, ET-generated signals become increasingly out of phase, which masks discharge fluctuations. The authors conclude that the pattern of naturally produced fluctuations in discharge could be used to analyze the ecology and hydrology of whole watersheds.


Flow velocity and the hydrologic behavior of streams during baseflow


Steven M. Wondzell: Olympia Forestry Sciences Laboratory, Pacific Northwest Research Station, Forest Service, U.S. Department of Agriculture, Olympia, Washington, U.S.A.;
Michael N. Gooseff: Department of Civil and Environmental Engineering, Pennsylvania State University, State College, Pennsylvania, U.S.A.;
Brian L. McGlynn: Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, Montana, U.S.A.


Geophysical Research Letters (GRL) paper 10.1029/2007GL031256, 2007;

3. Internal waves across the Pacific Ocean

When ocean tidal currents encounter undersea topography, waves called internal tides are generated. These waves propagate into the ocean interior and can contribute significantly to oceanic mixing when they break, influencing how nutrients are distributed and how energy is transported throughout the ocean. Understanding where this breaking occurs in the ocean is thus central to understanding the global climate system. Prior models showed that a particular breaking mechanism known as "parametric subharmonic instability" (PSI) could in principle remove a large amount of energy from the internal tides at a "critical latitude" of 28.8 degrees North. To test this notion, Alford et al. heavily instrumented a 1400-km (870-mile)-long line beginning at French Frigate Shoals, a major generation site at the Hawaiian Ridge, with the intention of tracking the internal tide's northward progress past the critical latitude. They found strong evidence that PSI does occur, leading to intense alternating bands of clockwise-rotating velocity, but that the process appears not to substantially attenuate the internal tide (whose fate remains uncertain). However, PSI does appear to strongly affect the latitudinal distribution of internal wave energy.


Internal waves across the Pacific


M. H. Alford: Applied Physics Laboratory, University of Washington, Seattle, Washington, U.S.A.; also at School of Oceanography, University of Washington, Seattle, Washington, U.S.A.;
J. A. MacKinnon and Rob Pinkel: Scripps Institution of Oceanography, La Jolla, California, U.S.A.;
Zhongxiang Zhao: Applied Physics Laboratory, University of Washington, Seattle, Washington, U.S.A.;
Jody Klymak: School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada;
Thomas Peacock: Mechanical Engineering, Massachusetts Institute of Technology, Cambridge Massachusetts, U.S.A.


Geophysical Research Letters (GRL) paper 10.1029/2007GL031566, 2007;

4. Understanding ultraslow mid-ocean ridges

The structure, shape, and properties of mid-ocean ridges depend on the rate at which plates separate. As spreading rates decrease, magma cools and the lithosphere thickens along the ridge axis. At ultraslow spreading rates, the ridge axis becomes sufficiently cold that magma crystallizes and volcanism is limited to localized centers widely spaced along the ridge. Noting that some slow spreading ridges adopt ultraslow characteristics when their axes are oblique to the spreading direction, Montési and Behn analyze oblique ridges worldwide. Through comparing observations with theoretical calculations, the authors verify that the thermal structure and crustal thickness beneath an oblique ridge are controlled by the ridge's effective spreading rate, defined as the contribution to spreading that is perpendicular to the oblique axis. Through these calculations, the authors define the threshold effective spreading rate at which spreading shifts from slow to ultraslow, and conclude that this transition is not related to a change in melt productivity, but rather to the efficiency with which magma is able to breach the surfaces.


Mantle flow and melting underneath oblique and ultraslow mid-ocean ridges


Laurent G. J. Montési: Department of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.; Now at Department of Geology, University of Maryland at College Park, Maryland, U.S.A;
Mark D. Behn: Department of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.


Geophysical Research Letters (GRL) paper 10.1029/2007GL031067, 2007;

5. A younger, thinner Arctic ice cover: Increased potential for rapid, extensive sea ice loss

Over the past two decades, reductions in the amount of Arctic sea ice that survives the summer melt have resulted in more newly formed ice (first-year ice) and less of the thick, old ice that forms perennial ice cover. Though past studies have described the extent of multiyear ice, defined as ice that survives at least one melt season, little is known about changes within the multiyear ice cover itself. Using satellite-derived estimates of sea ice age and thickness, Maslanik et al. constructed a thickness record for 1982 to the present. The authors find that 58 percent of multiyear ice now consists of relatively young and thin 2- to 3-year-old ice, compared with 35 percent in the mid-1980s, with nearly complete loss of the oldest, thickest ice. The authors expect that this decline helps to expose more open water, which in turn increases the absorption rate of solar energy. Not only do such feedbacks help explain recent large ice loss trends, but they also increase the potential of the current younger, thinner Arctic ice cover to rapidly melt.


A younger, thinner Arctic ice cover: Increased potential for rapid extensive sea-ice loss


J. A. Maslanik, C. Fowler, S. Drobot, and W. Emery: Colorado Center for Astrodynamics Research, University of Colorado, Boulder, Colorado, U.S.A.;
J. Stroeve: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, U.S.A.;
J. Zwally and D. Yi: NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.


Geophysical Research Letters (GRL) paper 10.1029/2007GL032043, 2007;

6. Windy Mars revealed through camera on the Mars Reconnaissance Orbiter

The surface of Mars is heavily influenced by wind-driven processes. The High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter provides new views of Mars's wind-driven geology because of its ability to see features less than half a meter in size. Bridges et al. study images from HiRISE to observe current sand movement on the surface of Mars. The authors see evidence of recent bedform development, and find that dunes and ripples exhibit complex surfaces down to the limits of imaging resolution. Wind-scoured ridges, called yardangs, have diverse textures, some with noticeable horizontal and cross-cutting layers and others containing blocky material. At high elevations, the surface exhibits "reticulate" textures, characterized by a fine-scale polygonal morphology. The authors expect that future images will give insight into wind-driven processes currently on Mars and potentially provide information on past Martian environments.


Windy Mars: A dynamic planet as seen by the HiRISE camera


N. T. Bridges and B. J. Thomson: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.;
P. E. Geissler, K. E. Herkenhoff, and L. P. Keszthelyi: U.S. Geological Survey, Flagstaff, Arizona, U.S.A.;
A. S. McEwen: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, U.S.A.;
F. C. Chuang: Planetary Science Institute, Tucson, Arizona, U.S.A.;
S. Martinez-Alonso: Department of Geological Science, University of Colorado, Boulder, Colorado, U.S.A.


Geophysical Research Letters (GRL) paper 10.1029/2007GL031445, 2007;

7. Human-generated aerosols affect atmospheric circulation trends in the Southern Hemisphere

Though most human-generated aerosols reside in the Northern Hemisphere, a recent model study shows that in response to rising aerosol concentrations, the Southern Hemisphere's ocean circulation, including retroflecting eddies and the entire subtropical gyre, intensifies and shifts poleward. Cai and Cowen seek to determine if these ocean responses also manifest in the Southern Hemisphere's atmospheric circulation. Through analyzing data from two sets of 20th century simulations in a coupled ocean-atmosphere global climate model, the authors find that, as a result of the poleward shift in oceanic circulation, maximum sea surface temperatures, midlatitude storms, and the westerly jet shift southward. This shift in air circulation intensifies the trend of the Southern Annular Mode, a low-frequency pattern of atmospheric variability near Antarctica, causing a poleward shift and intensification in zonal wind and vertical velocities generated from the atmosphere-ocean interface to the middle of the troposphere. These atmospheric circulation responses, in turn, reinforce the ocean circulation changes. Thus, the authors suggest a contribution by human-generated aerosols to the observed trend of the Southern Annular Mode.


Impacts of increasing anthropogenic aerosols on the atmospheric circulation trends of the Southern Hemisphere: An air-sea positive feedback


Wenju Cai and Tim Cowan: Marine and Atmospheric Research, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Aspendale, Victoria, Australia; also at Wealth from Oceans National Research Flagship, CSIRO, North Ride, New South Wales, Australia.


Geophysical Research Letters (GRL) paper 10.1029/2007GL031706, 2007;

8. Effects of global change on Canada's Mackenzie River Delta

River delta regions along the Arctic coast are poorly understood ecosystems that are expected to change rapidly as the climate warms, sea levels rise, and seasonal river ice jams become less frequent. In northern Canada's Mackenzie River Delta, flood pulses driven by ice breakup control the degree to which river water moves off-channel to replenish nearby lakes, which only exist because of such river dynamics. Lesack and Marsh analyze more than 30 years of data on the Mackenzie Delta and find that the duration of river-to-lake connection has lengthened on average more than 30 days since the 1970s. Further, the duration of river-to-lake connection has shortened in the highest elevation lakes, likely owing to the declining effects of river-ice breakup. The authors conclude that not only are the higher elevation lakes at risk of drying up from declining water level peaks, but lower elevation lakes now contain more water than can be accounted for through sea level rise, suggesting that increasing storm surge intensity, permafrost melting, or backwater flow might play an unexpected role.


Lengthening plus shortening of river-to-lake connection times in the Mackenzie River Delta respectively via two global change mechanisms along the arctic coast

Authors: Lance F. W. Lesack: Departments of Geography and Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada;

Philip Marsh: National Water Research Institute, Saskatoon, Saskatchewan, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031656, 2007;


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