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American Geophysical Union

AGU journal highlights -- Dec. 31, 2008

1. Surprise drop in carbon dioxide absorbed by East/Japan Sea

The East/Japan Sea in the western North Pacific is ventilated from the surface to the bottom of the ocean over decades. Such short overturning circulation indicates that carbon dioxide (CO2) from human emissions is able to pervade the East/Japan Sea on similarly short timescales. Three surveys of the East/Japan Sea (conducted in 1992, 1999, and 2007, respectively) have allowed scientists to measure changes in the sea accumulation rate of CO2 emitted by humans in response to changes in surface conditions. Park et al. analyze data from these surveys and find that the average uptake rate of anthropogenic CO2 by the East/Japan Sea from 1999 to 2007 was half of what it was for the period between 1992 and 1999. Further, anthropogenic CO2 absorbed by the water more recently was confined to waters less than 300 meters (984 feet) in depth. Because emissions have in fact accelerated over the past 10 years, the authors conclude that overturning circulation is weakening, slowing down the transport of anthropogenic CO2 from the surface to the interior of the East/Japan Sea.

Title: Sudden, considerable reduction in recent uptake of anthropogenic CO2 by the East/Japan Sea

Authors: Geun-Ha Park and Kitack Lee: School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, Korea;

Pavel Tishchenko: Pacific Oceanological Institute, Far East Division, Russian Academy of Sciences, Vladivostok, Russia.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL036118, 2008; http://dx.doi.org/10.1029/2008GL036118


2. Simulations suggest big raindrops favor tornado formation

One of the largest sources of uncertainty in weather prediction involves how microscale structures influence larger-scale phenomena. For instance, previous studies have demonstrated that the structure, dynamics, and evolution of thunderstorms are very sensitive to cloud microphysical parameters. However, those studies used resolutions too coarse to resolve tornadoes or tornado-like circulations and were therefore not able to study the sensitivity of tornadogenesis to microphysics. Snook and Xue have now conducted simulations of severe tornadic thunderstorms using a grid of 100-meter (328-feet) spacing. They find that when the sizes of rain and hail drops are large, weaker cold pools due to reduced evaporative cooling/melting over smaller geographic regions result. Such weak cold pools are found to produce conditions that enhance low-level rotation. The authors' simulations show that strong, sustained vertical updrafts are positioned near and above the low-level circulation centers, providing strong dynamic lifting and vertical stretching to the air at the lower levels, which favors the creation of tornadoes.

Title: Effects of microphysical drop size distribution on tornadogenesis in supercell thunderstorms.

Authors: Nathan Snook and Ming Xue: School of Meteorology and Center for Analysis and Prediction of Storms, University of Oklahoma, Norman, Oklahoma, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035866, 2008; http://dx.doi.org/10.1029/2008GL035866


3. Speediest sand dunes clocked from space

Monitoring the speeds of migrating dunes and the volumes of sand transported over time is important to understanding how arid landscapes respond to wind-driven changes. Traditionally, scientists have monitored such changes through detailed field surveys or long-term surveillance of stakes planted in dune fields. Vermeesch and Drake have developed a new and more convenient approach to monitoring the speed and sand flux of migrating dunes. By using pairs of high-resolution optical satellite images taken at differing times, the authors monitored dune migration in the Bodélé depression of northern Chad over time intervals of 1 month to 6.5 years. The displacement maps generated from each pair of satellite images were then used to automatically distinguish dunes from interdunes. By interpolating a surface between the interdune areas and subtracting it from the surface observed by the satellite images, the authors obtain dune heights and volumes over fine spatial and temporal scales. From this, pixel-by-pixel estimates of sand flux were generated, allowing the authors to confirm that the Bodélé contains some of the world's fastest moving dunes.

Title: Remotely sensed dune celerity and sand flux measurements of the world's fastest barchans (Bodélé, Chad)

Authors: Pieter Vermeesch: School or Earth Sciences, Birkbeck College, University of London, London, U.K.;

Nick Drake: Department of Geography, King's College, University of London, London, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035921, 2008; http://dx.doi.org/10.1029/2008GL035921


4. Odd-looking Martian craters indicate hidden ice

Surface features common in the northern and southern midlatitudes of Mars and known as lobate debris aprons and lineated valley fill are believed to have formed either as debris flows mobilized by pore ice or as debris-covered glaciers. To learn more, Kress and Head define and analyze ring mold craters, which are abundant on debris aprons and lineated valley fill but not seen in surrounding terrain. Ring mold craters are concentric crater forms named for their similarity to the cooking implement, in contrast to the bowl-shaped craters that are common at such small sizes (hundreds of meters (hundreds to thousands of feet) in diameter). On the basis of similarities in shape of ring mold craters to laboratory impact craters in ice and of the physics of impact cratering into pure ice, the authors interpret ring mold craters to result from projectiles hitting relatively pure ice below a thin debris layer. These results support the hypothesis that lobate debris aprons and lineated valley fill are debris-covered glaciers and that many hundreds of meters of ice remain in these deposits today on Mars.

Title: Ring-mold craters in lineated valley fill and lobate debris aprons on Mars: Evidence for subsurface glacial ice

Authors: Ailish M. Kress and James W. Head: Department of Geological Sciences, Brown University Providence Rhode Island, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035501, 2008, 2008; http://dx.doi.org/10.1029/2008GL035501


5. Explaining the scope of Earth's tropical air flows

The Hadley cells form the planetary overturning circulation in the tropical atmosphere. In a Hadley cell, air rises from near the equator at the surface to the upper troposphere, moves poleward until about 30 degrees in latitude, then descends into the subtropics before moving equatorward along the Earth's surface. Recent observations and simulations with climate models suggest that the north-south extent of the Hadley cells may be increasing, which may dramatically alter climate on regional scales. Korty and Schneider analyze dynamical mechanisms responsible for terminating the Hadley cells in the subtropics. They suggest that Hadley cells terminate where eddies generated in the extratropics become so deep that they reach the upper troposphere. At the latitude where this occurs, the direction of the momentum flux carried by the eddies changes sign; the tropical Hadley cells give way to the extratropical Ferrel cells. The authors show how the atmospheric thermal structure determines this latitude in dry atmospheres. It remains to be investigated how latent heat release in phase changes of water influences the Hadley cell termination.

Title: Extent of Hadley circulations in dry atmospheres

Authors: Robert L. Korty: Department of Atmospheric Sciences, Texas A&M University, College Station, Texas, U.S.A.;

Tapio Schneider: California Institute of Technology, Pasadena, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035847, 2008; http://dx.doi.org/10.1029/2008GL035847


6. Cause of glacial earthquakes in Greenland clarified

Satellite observations during the past decade have shown dramatic changes in flow speed on year-to-year timescales at Greenland's outlet glaciers. Seismic events traced back to glaciers during the same time period have been interpreted to have resulted from calving events at the glacier terminus or surging events lubricated by subglacial meltwater. To learn more, Nettles et al. conducted geodetic studies at Helheim Glacier, one of Greenland's largest outlet glaciers, during summer 2007. They observed several large and sudden increases in flow speed along the length of the glacier. These accelerations coincided with glacial earthquakes and major iceberg calving events. No offset in the position of the glacier surface was observed during these events. Instead, modest tsunamis associated with the glacial earthquakes implicate glacier calving as the generator of seismic events, putting to rest the idea that lurching glaciers are responsible for glacial earthquakes at outlet glaciers like Helheim, and demonstrating a link between ice loss and glacier acceleration.

Title: Step-wise changes in glacier flow speed coincide with calving and glacial earthquakes at Helheim Glacier, Greenland

Authors: M. Nettles and G. Ekström: Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, U.S.A.;

T. B. Larsen, A. P. Ahlstrøm, and M. L. Andersen: Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark;

P. Elósegui and J. de Juan: Institute for Space Sciences, CSIC/IEEC, Barcelona, Spain;

G. S. Hamilton and L. A. Stearns, Climate Change Institute, University of Maine, Orono, Maine, U.S.A.;

J. L. Davis: Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, U.S.A.;

S. A. Khan, L. Stenseng, and R. Forsberg: Danish National Space Center, Copenhagen, Denmark.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL036127, 2008; http://dx.doi.org/10.1029/2008GL036127


7. Sea rise over continental shelves significantly affected past global carbon cycle

Since the Last Glacial Maximum (LGM; about 21,000 years ago) sea level has risen by 130 meters (430 feet), resulting in continental shelf submergence and a massive expansion of the surface area of shelf seas. Although shelf seas only account for 7 percent of the oceanic surface area, recent observations demonstrate that they host significant fluxes of carbon dioxide (CO2) between the ocean and atmosphere. Further, dissolved and particulate carbon are thought to be transported from shelf areas into sinks in the deep ocean through a mechanism called the "continental shelf pump." Through reconstructions of shelf geography stretching to the LGM, Rippeth et al. analyze the effect of sea level rise and consequent flooding of continental shelves on the growth of the continental shelf pump. Combining these reconstructions with contemporary estimates of carbon flux between the ocean and atmosphere allows the authors to conclude that expanding shelf seas have significantly influenced the global carbon cycle via the continental shelf pump, with weaker pumping during times when shallower shelf seas were present.

Title: Impact of sea-level rise over the last deglacial transition on the strength of the continental shelf CO2 pump

Authors: Tom P. Rippeth, James D. Scourse, and Stephanie McKeown: School of Ocean Sciences, College of Natural Sciences, Bangor University, Anglesey, U.K.;

Katsuto Uehara: Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035880, 2008; http://dx.doi.org/10.1029/2008GL035880


8. Martian avalanches analyzed

The High Resolution Imaging Science Experiment (HiRISE), on NASA's Mars Reconnaissance Orbiter, recently discovered new and dramatic examples that Mars's carbon dioxide-dominated atmospheric cycles are not limited to quiet deposition and sublimation of frost. Earlier in 2008, during early northern Martian spring, HiRISE captured several cases of carbon dioxide frost and dust cascading down a steep polar scarp near the north pole in discrete clouds. Russell et al. analyze these events and find them to be similar to terrestrial powder avalanches, sluffs, and falls of loose, dry snow. The authors discuss material sources and initiating mechanisms for these cascades in the context of the Martian polar spring environment and of the active, wind-driven erosion observed above the scarp. They find that the small landslides may trigger deeper landslides within the fractured layers of the scarp's basal unit, and may also indirectly influence the retreat rate of the polar scarp in various ways.

Title: Seasonally active frost-dust avalanches on a north polar scarp of Mars captured by HiRISE

Authors: Patrick Russell and Nicolas Thomas: Department of Space Research and Planetary Sciences, Physikalisches Institut, University of Bern, Bern, Switzerland;

Shane Byrne: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, U.S.A.;

Kenneth Herkenhoff, Chris Okubo, and Moses Milazzo: U.S. Geological Survey, Flagstaff, Arizona, U.S.A.;

Kathryn Fishbaugh: Center for Earth and Planetary Studies, Smithsonian Air and Space Museum, Washington, D.C., U.S.A.;

Nathan Bridges and Candice Hansen: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.;

Ingrid Daubar and Alfred McEwen: HiRISE Operations Center, Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035790, 2008; http://dx.doi.org/10.1029/2008GL035790


9. Influence of biomass burning and mineral dust found in West Africa

The African Monsoon Multidisciplinary Analysis (AMMA) is a major international campaign investigating far-reaching aspects of the African monsoon, climate, and hydrological cycle. During the dry season of January and February 2006, a special observing period was conducted (SOP0) involving several ground-based measurement sites across West Africa. These were augmented by aircraft measurements as part of the Dust and Biomass-Burning Experiment (DABEX) and modeling efforts using a range of global and mesoscale models. Through analyzing data collected from AMMA SOP0 and DABEX campaigns, Haywood et al. determine the aerosol transport pathways over West Africa, finding biomass burning aerosols lofted above undercutting dust-laden air. Aerosols from biomass burning appear to absorb significantly more radiation than those measured in other areas. The influence of both mineral dust and aerosols from biomass burning on the radiation budget was significant throughout the period, implying that meteorological models should include aerosol radiative effects for accurate weather and climate forecasts.

Title: Overview of the Dust and Biomass-burning Experiment and African Monsoon Multidisciplinary Analysis Special Observing Period-0

Authors: J. M. Haywood, M. Brooks, G. Greed, M. Harrison, B. Johnson, S. Milton, and S. R. Osborne: Met Office, Exeter, U.K.;

J. Pelon: Service de Aéronomie, IPSL, Université Pierre et Marie Curie, Paris, France;

P. Formenti, C. Chou, K. Desboeufs, and B. Marticorena: LISA, CNRS, France;

N. Bharmal: ESSC, University of Reading, Reading, U.K.;

G. Capes and H. Coe: School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, U.K.;

P. Chazette and M. Schulz: CEA, DSM, LSCE, Gif sur Yvette, France;

S. Christopher: Earth System Science Center, NSSTC, University of Alabama, Huntsville, Alabama, U.S.A.;

J. Cuesta: Laboratoire de Métérologie Dynamique, Institute Pierre Simon Laplace, Ecole Polytechnique, Palaiseau, France;

Y. Derimian and D. Tanré: Laboratoire d'Optique Atmosphérique, Université des Sciences et Technologies de Lille, CNRS, Villeneuve d'Ascq, France;

B. Heese: Leibniz Institute for Tropospheric Research, Leipzig, Germany

E. J. Highwood: Department of Meteorology, University of Reading, Reading, U.K.;

M. Mallet: Laboratoire d/Aerologie, UPS, CNRS, UMR-5560 Toulouse, France;

J. Marsham and D. J. Parker: Institute for Climate and Atmospheric Science Environment, School of Earth and Environment, University of Leeds, Leeds, U.K.;

G. Myhre: Center for Interantional Climate and Environmental Research, Oslo, Norway;

J.-L Rajot: IRD UR 176, Niamey, Niger;

A. Slingo: ESSC, University of Reading, Reading, U.K.; now deceased;

P. Tulet: CNRM, GAME, Meteo-France, Toulouse, France.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD010077, 2008; http://dx.doi.org/10.1029/2008JD010077


10. Dust plays conflicting roles in West African rainfall

Recent studies over West Africa suggest that dust affects weather patterns, although no clear consensus exists on whether atmospheric feedbacks associated with dust are more likely to increase or decrease precipitation. To help answer this question, Solmon et al. study the influence of Saharan dust on the West African monsoon using a regional climate model interactively coupled with a dust model. They find that for the decade between 1996 and 2006, two competing dust effects were expressed. First, a reduction of monsoon intensity in the lower troposphere induced by the dust surface cooling can cause a reduction of precipitation. Second, an "elevated heat pump" effect in the higher troposphere induced by warming dust caused an increase of precipitation. In standard models, the net impact of these effects is a reduction of precipitation over most of the Sahel except over the border between the northern Sahel and southern Sahara. The authors expect that including dust in regional climate models will help models better match observations.

Title: Dust aerosol impact on regional precipitation over western Africa, mechanisms and sensitivity to absorption properties

Authors: Fabien Solmon, Marc Mallet, and Nellie Elguindi: Laboratoire d'Aérologie, Université de Toulouse, Toulouse, France;

Filippo Giorgi and Ashraf Zakey: Earth System Physics, Abdus Salam International Center for Theoretical Physics, Trieste, Italy;

Abdourahamane Konaré: Laboratoire de Physique de l'Atmosphére, Université de Cocody, Abidjan, Côte d'Ivoire

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035900, 2008; http://dx.doi.org/10.1029/2008GL035900

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