The following highlights summarize research papers that have beenpublished or are "in press" (accepted, but not yet published) inGeophysical Research Letters (GRL) or the Journal of GeophysicalResearch - Atmospheres (JGR-D).
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1. Reduced Saharan dust due to rain boosts ocean heating
Between 1980 and 2006, the tropical North Atlantic wascharacterized by a significant increase in sea surface temperatures, atransition from a negative to a positive phase of the Atlanticmultidecadal oscillation, and a significant increase in rainfall acrossAfrica's Sahel region. This rainfall served to decrease dustinessacross western Africa and the tropical North Atlantic Ocean. Usingdata from satellites and previously conducted field campaigns, Foltzand McPhaden not only confirm previously seen trends, but alsoquantify the increase in surface solar radiation associated with thedecreasing trend in dust. Using a simple one-dimensional model,they further show that in the absence of a damping mechanism, theadditional influx of solar radiation would have led to a 3 degreesCelsius (5.4 degrees Fahrenheit) increase in tropical North Atlanticsea surface temperatures. The authors suggest that coupled modelssignificantly underestimate the amplitude of the Atlanticmultidecadal oscillation in the tropical North Atlantic possiblybecause they do not account for changes in Saharan dustconcentrations.
Title:Trends in Saharan dust and tropical Atlantic climate during 1980-2006
Authors:Gregory R. Foltz: Joint Institute for the Study of the Atmosphereand Ocean, University of Washington, Seattle, Washington, U.S.A.;
Michael J. McPhaden: Pacific Marine Environmental Laboratory,NOAA, Seattle Washington, U.S.A.
Source:Geophysical Research Letters (GRL) paper 10.1029/2008GL035042,2008; http://dx.doi.org/10.1029/2008GL035042
2. Coastal Southern Ocean acts as powerful carbon sink
Of the 7 to 9 Petagrams (1 Petagram = 10^15 grams) of carbon releasedinto the atmosphere by humans each year as carbon dioxide, about25-35 percent is taken up by the oceans through physical andbiological processes. Efforts to assess the role of the ocean as acarbon dioxide sink, through tracers and models, indicate that thelargest sinks exist in the North Atlantic and the Southern Ocean.However, scientists disagree on the magnitude of these sinks,particularly in the Southern Ocean where historical estimates span awide range. To help resolve this disagreement, Arrigo et al. ran athree-dimensional biogeochemical model of the Ross Sea and findthat Antarctic shelf waters are a strong sink for carbon dioxide dueto high biological productivity, intense winds, high ventilation rates,and extensive winter sea ice cover. Surprisingly, although the RossSea continental shelf comprises only 0.36 percent of the open waterarea in the Southern Ocean, the total atmospheric carbon dioxidesink on the Ross Sea shelf is about 27 percent of the most recentestimates of carbon dioxide sinks for the entire Southern Ocean.Thus, the authors stress that the highly productive waters aroundAntarctica need to be included in future budgets of anthropogeniccarbon dioxide sinks.
Title:Coastal Southern Ocean: A strong anthropogenic carbon dioxide sink
Authors:Kevin R. Arrigo, Gert van Dijken, and Matthew Long: Departmentof Environmental Earth System Science, Stanford University,Stanford, California, U.S.A.
Source:Geophysical Research Letters (GRL) paper 10.1029/2008GL035624,2008; http://dx.doi.org/10.1029/2008GL035624
3. Ionosphere fluctuations linked to sudden stratosphericwarming
A significant portion of day-to-day variations in ionosphericparameters cannot be explained by major and relatively wellunderstood drivers such as solar ionizing flux and geomagneticactivity. These unexplained variations constitute about 20 percent ofthe variability seen in the ionosphere's F region, which is where mostof the atmosphere's plasma resides and acts as a dependable reflectorfor radio signals. Noting that some investigations have proposed thatlower atmospheric processes account for this ionospheric variability,Goncharenko and Zhang study an episode of sudden stratosphericwarming, which occurred in late January 2008 and lasted for about aweek, and compare results with concurrent temperature fluctuationsin the ionosphere and thermosphere as recorded by a ground-basedradar. The authors find that ionospheric variations that could not beexplained through the seasonal trends, solar flux, and geomagneticactivity are instead correlated with fluctuating temperatures in thestratosphere, demonstrating a link between the lower atmosphere andthe ionosphere that has been previously unobserved. The authorsconclude that studies of space weather should consider ionosphericvariability in conjunction with stratospheric changes.
Title:Ionospheric signatures of sudden stratospheric warming: Iontemperature at middle latitude
Authors:Larisa Goncharenko and Shun-Rong Zhang: Haystack Observatory,Massachusetts Institute of Technology, Westford, Massachusetts,U.S.A.
Source:Geophysical Research Letters (GRL) paper 10.1029/2008GL035684,2008; http://dx.doi.org/10.1029/2008GL035684
4. Earthquake probability models tested against 2000-year record
Is the probability of a large earthquake rumbling on major faultsdependent on time, or is it constant? Under a time-dependentprocess, some period is required to recharge stresses released in theprevious earthquake. Further, the probability of the next earthquakeis lowest just after a large earthquake, but rises with time. Althoughaccepted as important to earthquake forecasting, time-dependenceprobabilities have been difficult to prove from observations. Thusscientists commonly report the time-independent earthquakeprobabilities, which increase overall uncertainty. To help refineestimates, Parsons analyzes a record of earthquakes spanning the past2000 years from California's south Hayward fault to determine itsconsistency with time-dependent and time-independent earthquakerecurrence models. Using statistical tests, he finds that time-dependent distributions with recurrence intervals of 210 years, whenmelded with other parameters, reproduce the event series on the fault5 times more often than time-independent models. Thus largeearthquakes on the Hayward fault are quasi-periodic and are mostconsistent with stress renewal processes.
Title:Earthquake recurrence on the south Hayward fault is most consistentwith a time dependent, renewal process
Authors:Tom Parsons: U.S. Geological Survey, Menlo Park, California,U.S.A.
Source:Geophysical Research Letters (GRL) paper 10.1029/2008GL035887,2008; http://dx.doi.org/10.1029/2008GL035887
5. Melting ice likely formed Martian gullies
Small gullies observed on Mars, particularly near Mars's Russellcrater, could have been formed by groundwater seepage from anunderground aquifer or may have resulted from the melting of near-surface ground ice, both of which would have occurred duringperiods when water was prevalent on Mars. To test these differenthypotheses, Vedie et al. conducted cold room-based laboratoryexperiments in which debris flows were simulated on sand duneslopes at a range of angles, different grain sizes, and varyingpermafrost conditions. Preliminary results suggested that the typicalmorphology of gullies currently observed on Mars can best bereproduced by the formation of linear debris flows related to themelting of near-surface ground ice with silty materials. From this, theauthors conclude that the depth of the thawed layer, together withthe permafrost table, significantly controls the characteristicmorphology of the Martian linear gullies observed in Mars's Russellcrater. Such an interpretation has important implications for theevolution of Martian climate.
Title:Laboratory simulations of Martian gullies on sand dunes
Authors:E. Vedie, M. Font and J. L. Lagarde: Laboratoire M2C UMR6143,Universite de Caen-Basse Normandie, INSU, CNRS, Caen, France;
F. Costard: Interactions et Dynamique des Environnements deSurface, UMR8148, Universite Paris-Sud 11, CNRS, Orsay, France.
Source:Geophysical Research Letters (GRL) paper 10.1029/2008GL035638,2008; http://dx.doi.org/10.1029/2008GL035638
6. Topography tweaks solar tides
The daily cycle of solar radiation has important effects onatmospheric circulation. On a local scale, one such effect is howdiurnal heating over topographic slopes produces mountain-valleybreezes, formed from the rough balance between radiatively drivenhorizontal pressure gradients and small-scale turbulent friction. Thesepressure gradients, balanced by Coriolis and acceleration terms,produce a global-scale solar tide, which is a large Sun-followingwave that can propagate vertically upward and/or downward fromits forcing altitudes. Using a global general circulation model,Hamilton et al. examine solar tidal variation at very fine resolution.They find that the model successfully simulates tidal pressureoscillations, although simulated amplitudes of semidiurnal tides aresignificantly enhanced over those observed. Nonetheless, the authorsfind that high topography reduces semidiurnal pressure amplitudes assurface elevation rises and that weak semidiurnal pressure amplitudesexist to the west of very high and steep topography. The latterfinding is attributed to a shadowing effect of topography on theglobal-scale westward propagating tide.
Title:Topographic effects on the solar semidiurnal surface tide simulatedin a very fine resolution general circulation model
Authors:Kevin Hamilton: International Pacific Research Center, University ofHawaii, Honolulu, Hawaii, U.S.A.;
Steven C. Ryan: Mauna Loa Observatory, Global MonitoringDivision, Earth System Research Laboratory, NOAA, Hilo, Hawaii,U.S.A.;
Wataru Ohfuchi: Earth Simulator Center, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan.
Source:Journal of Geophysical Research-Atmospheres (JGR-D) paper10.1029/2008JD010115, 2008;http://dx.doi.org/10.1029/2008JD010115
7. Clouds stimulate transport of air from troposphere tostratosphere
The transport of air from the troposphere to the stratosphere occursthrough large-scale upwelling in the tropical upper troposphere.However, the vertical mass transport associated with clear-skyradiative heating is too slow to support tracers, forcing scientists torely on models to define transport of air to the stratosphere. As aresult, scientists have found many different mechanisms to explaintroposphere-to-stratosphere transport, ranging from gradual processesto fountain-like deep convection. To learn more, Huang and Su studythe radiative impact of cirrus clouds as a possible mechanism forincreasing the rate of mass transport from the troposphere to thestratosphere. Using an atmospheric circulation model, the authorsexamine tropical cirrus layers at the top of the troposphere and findthat the strongest upward motion in the model through this layer isgenerally driven by dynamics instead of radiation. However, theoccurrence frequency of such strong ascent is much smaller than thatof moderate ascent related to the radiative effect of these cirrusclouds. Further, cloudy skies rather than clear skies foster morevertical transport to the stratosphere.
Title:Cloud radiative effect on tropical troposphere to stratospheretransport represented in a large-scale model
Authors:Xianglei Huang: Department of Atmospheric, Oceanic, an SpaceSciences, University of Michigan, Ann Arbor, Michigan, U.S.A.;
Hui Su: Jet Propulsion Laboratory, California Institute ofTechnology, Pasadena, California, U.S.A.
Source:Geophysical Research Letters (GRL) paper 10.1029/2008GL035673,2008; http://dx.doi.org/10.1029/2008GL035673
8. Using cosmic-ray neutrons to measure soil moisture
Because of its long residence time, soil moisture moderates regionalclimate much like the ocean does, making quantifications of soilmoisture critical for weather and short-term climate forecasting.However, measuring soil moisture is difficult because pointmeasurements must be extrapolated to larger areas, introducingsignificant error, and because satellite remote sensors have difficultypenetrating surface cover. To move past these difficulties, Zreda etal. develop a new, noninvasive technique that infers soil moisturecontent from measurements of low-energy cosmic ray neutrons thatare generated within soil, moderated mainly by hydrogen atoms, anddiffused back to the atmosphere. These neutrons are sensitive towater content changes but do not fluctuate with changes in soilchemistry. Further, their intensity above the surface is inverselycorrelated with hydrogen content in the soil. By placing a portableneutron detector a few meters above the ground, the authorsgenerate high-resolution, long-term records of undisturbed soilmoisture conditions over an area about 700 meters (2,300 feet) indiameter. This method will be valuable for calibrating satellites,studying plant/soil interaction and atmosphere/soil exchange, andinitializing soil moisture conditions for short-term weather andclimate forecasts.
Title:Measuring soil moisture content non-invasively at intermediatespatial scale using cosmic-ray neutrons
Authors:Marek Zreda, Darin Desilets, and T. P. A. Ferre: Department ofHydrology and Water Resources, University of Arizona, Tucson,Arizona, U.S.A.;
Russell L. Scott: Southwest Watershed Research Center,Agricultural Research Service, U.S. Department of Agriculture,Tucson, Arizona, U.S.A.
Source:Geophysical Research Letters (GRL) paper 10.1029/2008GL035655,2008; http://dx.doi.org/10.1029/2008GL035655
9. Isotopes illuminate atmospheric convection
Cumulus convection constitutes a key process in the control oftropical precipitation and vertical transport of water vapor. To betterunderstand the influence of convective processes on the isotopiccomposition of precipitation and water vapor, Bony et al. introducetwo stable isotopes of water into a single-column model of thetropical atmosphere. They find that deep convective atmospheres areassociated with characteristic isotopic features, including that theisotopic composition of air a few kilometers below the stratosphere(at 12-13 kilometers (7.5-8.1 miles)) is close to the typical valuesobserved in the lowest stratosphere, a finding that can help scientistsunderstand the mechanisms through which water is transported fromthe upper troposphere to the lower stratosphere. They also find thatchanges in large-scale atmospheric motions constitute the primarydriver of isotopic changes in the precipitation. However, a globalwarming or cooling of the tropics can further change the isotopiccomposition. Understanding dynamical and thermodynamicalinfluences on isotopic variations in precipitation may help scientistsuse ice cores collected from tropical mountain glaciers to reconstructpast climate variations.
Title:Influence of convective processes on the isotopic composition (deltaoxygen 18 and delta D) of precipitation and water vapor in thetropics: 1. Radiative-convective equilibrium and Tropical Ocean-Global Atmosphere-Coupled Ocean-Atmosphere ResponseExperiment (TOGA-COARE) simulations
Authors:Sandrine Bony and Camille Risi: Laboratoire de MeteorologieDynamique, IPSL, UPMC, CNRS, Paris, France;
Francoise Vimeux: IRD-UR Great Ice, LSCE, IPSL, CEA, UVSQ,CNRS, Gif-sur-Yvette, France.
Source:Journal of Geophysical Research-Atmospheres (JGR-D) paper10.1029/2008JD009942, 2008;http://dx.doi.org/10.1029/2008JD009942
10. Explaining isotope composition of tropical rains
In the tropics, the proportion of heavier water isotopes inprecipitation varies inversely with the precipitation amount. To betterunderstand the physical processes underlying this "amount effect,"Risi et al. use a single-column model of the tropical atmosphereincluding two water stable isotopes, together with a representation ofconvection, and proposed a methodology to quantify the relativecontributions to this effect. Focusing on convection, the authors findthat two processes predominantly explain the amount effect: thereevaporation of falling rain and diffusive exchanges withsurrounding vapor, as well as the recycling of the subcloud layervapor feeding the convective system by convective fluxes. Thesefindings highlight the importance of a detailed representation of rainevaporation processes to simulate accurately the isotopic compositionof the tropical precipitation. In turn, the isotopic composition of theprecipitation may be used to evaluate the representation of suchprocesses in climate models. Regarding timescales, the authors alsoshow that the isotopic composition of the tropical precipitationintegrates the convective activity over several days.
Title:Influence of convective processes on the isotopic composition (deltaoxygen 18 and delta D) of precipitation and water vapor in thetropics: 2. Physical interpretation of the Amount Effect
Authors:Camille Risi and Sandrine Bony: Laboratoire de MeteorologieDynamique, IPSL, UPMC, CNRS, Paris, France;
Francoise Vimeux: IRD-UR Great Ice, LSCE, IPSL, CEA, UVSQ,CNRS, Gif-sur-Yvette, France.
Source:Journal of Geophysical Research-Atmospheres (JGR-D) paper10.1029/2008JD009943, 2008;http://dx.doi.org/10.1029/2008JD009943
Source: American Geophysical Union