AGU journal highlights -- Feb. 25, 2009

The following highlights summarize research papers that have been published in Geophysical Research Letters (GRL).

Anyone may read the scientific abstract for these papers by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2008GL036309. The doi is found at the end of each Highlight below.

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Glaciers and ice caps can be split into regions where snow is accumulated and regions where snow and ice melt—if more snow accumulates than melts, the glacier will advance and grow larger. Currently, accumulation areas for mountain glaciers are very small; melting rates are surpassing accumulation rates, leading to glacier thinning and retreat. By analyzing mass balance data from 86 mountain glaciers and ice caps from around the world, Bahr et al. find that given current accumulation areas and climate regimes, glaciers will lose about 27 percent of their volume before attaining equilibrium, a state where accumulation equals loss. As a result, at least 184 mm (7.2 inches) of sea level rise will occur in the next 100 years through melting of the world's mountain glaciers and ice caps even if climate does not continue to warm. However, if the climate continues to warm along current trends, at least 373 mm (14.6 inches) of sea level rise over the same period is expected as glaciers and ice caps lose at least 55 percent of their volume.

Title: Sea-level rise from glaciers and ice caps: A lower bound

Authors: David B. Bahr: Department of Physics and Computational Sciences, Regis University, Denver, Colorado, U.S.A.;

Mark Dyurgerov and Mark F. Meier: Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/ 2008GL036309, 2009; http://dx.doi.org/10.1029/2008GL036309

2. Human environmental effects widen tropics

Previous studies have shown that the width of the tropical belt has been increasing since at least the late 1970s, based on a variety of indicators. Lu et al. use one such indicator, the frequent occurrence at high altitudes of the boundary between the troposphere and the stratosphere (tropopause), to show that the observed widening of the tropics can be accurately replicated by an atmospheric general circulation model forced by the observed evolution since 1958 of global sea surface temperatures and sea ice distributions along with the direct radiative effects from natural and human-generated sources. The authors then contrast this simulation with one forced only by sea surface temperatures and sea ice distributions and find that the widening trend of the tropics can be attributed entirely to direct radiative forcing, in particular those related to greenhouse gases and stratospheric ozone depletion. In fact, modifying sea surface temperatures actually causes no significant change in the width of the tropics and in some simulated seasons leads to a tropical belt contraction.

Title: Cause of the widening of the tropical belt since 1958

Authors: Jian Lu: Advanced Study Program, National Center for Atmospheric Research, Boulder, Colorado, U.S.A.; also at Center for Ocean-Land-Atmosphere studies, Calverton, Maryland, U.S.A.;

Clara Deser: National Center for Atmospheric Research, Boulder, Colorado, U.S.A.;

Thomas Reichler: Department of Meteorology, University of Utah, Salt Lake City, Utah, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL036076, 2009; http://dx.doi.org/10.1029/2008GL036076

3. Ozone abundance underestimated in troposphere

A key process that generates ozone in the lower stratosphere is the dissociation of molecular oxygen by specific ultraviolet wavelengths ranging from 195 to 215 nanometers. A small fraction of sunlight at these wavelengths also penetrates deeper, into the tropical troposphere, helping to generate ozone through oxygen dissociation. By analyzing tropospheric chemistry models that ignore this source of ozone production in the upper tropical troposphere, Prather finds that ozone abundance within the tropopause transition layer (TTL; above 14 kilometers, or 8.7 miles) is underestimated by between 5 and 20 parts per billion. Even if models include this process, uncertainty regarding oxygen cross sections leads to high uncertainty in estimates of ozone in the TTL. The author suggests that a better representation of ozone in the tropopause region may be important in calculating heating rates and climate forcing.

Title: Tropospheric ozone from photolysis of molecular oxygen

Authors: Michael J. Prather: Earth System Science Department, University of California, Irvine, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL036851, 2009; http://dx.doi.org/10.1029/2008GL036851

4. How extra robust, intense hurricanes form

Although hurricane tracks are now predicted well, anticipating hurricane intensity remains a challenge because it is difficult to observe and model fine details of the storm's inner core. Recent observations suggest that the formation of concentric eye walls allows particularly intense storms to transform into annular hurricanes. Annular hurricanes are highly circular, intense storms that are resistant to forces that typically weaken other hurricanes. Using the Weather Research and Forecasting (WRF) model, Zhou and Wang simulate the transformation of a hurricane into an annular storm. They find that the simulated hurricane experiences three distinct stages: the formation of a secondary eye wall, the replacement of the initial eye wall with the secondary eye wall, and the formation of an annular hurricane. These stages are accompanied by changes in storm intensity, with the total transformation occurring in less than 24 hours. The authors suggest that concentric eye wall replacement is an efficient route to the formation of an annular hurricane and that the WRF model has the potential to be used to predict eye wall cycles and associated intensity changes.

Title: From concentric eyewall to annular hurricane: A numerical study with the cloud-resolved WRF model

Authors: Xiaqiong Zhou and Bin Wang: Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawai'i at Manoa, Honolulu, Hawaii, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/ 2008GL036854, 2009; http://dx.doi.org/10.1029/2008GL036854

5. Sea level rise scars South Carolina marshes

Common theory holds that soil in coastal saltwater marshes will likely thicken as climate warms and sea levels rise because of enhanced plant productivity and higher rates of inorganic deposition associated with greater water depths. If organic production and inorganic accumulation cannot keep pace with the rate of sea level rise, the marsh platform will submerge. To learn more about how marshes respond to sea level rise, Hughes et al. study tidal creeks in Cape Romain, South Carolina (in the southern United States), which are extending rapidly into the established vegetated marsh platform. Through analyzing aerial photographs spanning the past 41 years, the authors find that the marsh platform is being progressively dissected and eroded by headward extending creeks, leading to a unique marsh morphology where land is replaced by channels that enlarge the drainage network. This rapid rate of creek extension suggests that the marsh platform is unable to keep pace with high local relative sea level rise through accretionary processes. The authors' research gives evidence for a new pattern of marsh response to sea level rise.

Title: Rapid headward erosion of marsh creeks in response to relative sea level rise

Authors: Zoe J. Hughes, Duncan M. FitzGerald, Carol A. Wilson, and Alama Mahadevan: Department of Earth Sciences, Boston University, Boston, Massachusetts, U.S.A.;

Steve C. Pennings and Kazimierz Więski: Department of Biology and Biochemistry, University of Houston, Houston, Texas, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL036000, 2009; http://dx.doi.org/10.1029/2008GL036000

6. Monitoring open-ocean deep convection from space

Open-ocean deep convection is a key process governing the global distribution of heat and nutrients throughout the world's oceans. As climate warms, monitoring the health of deep convection will be integral to assessing the effects of climate change. However, direct monitoring requires a huge investment and can only provide limited coverage in time and space. Herrmann et al. study the feasibility of monitoring the interannual variability and long-term evolution of deep convection from space using satellite altimetry. The authors focus on simulating circulation in the Mediterranean Sea between 1999 and 2007. Model results of interannual variability and sea surface elevations are in good agreement with altimetry data from that time period and indicate that winter sea surface elevations are depressed in areas where the model shows that deep waters are forming. The degree of depression varies from year to year, with lower sea surface elevations corresponding to locations where and times when the model predicts stronger deep convection. On the basis of this, the authors propose that annual characteristics of deep convection can be modeled using altimetry data.

Title: Monitoring open-ocean deep convection from space

Authors: Marine Herrmann: Centre National de Recherches Météoroligiques, Météo-France, CNRS, Toulouse, France;

Jérome Bouffard: Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, Université de Toulouse, CNRS, Toulouse, France;

Karine Béranger: Laboratoire d'Océanographie et du Climat: Expérimentations et Approches Numériques, Ecole Nationale Supérieure des Techniques Avancées: Université Paris 6, CNRS, Paris, France.

Source: Geophysical Research Letters (GRL)paper 10.1029/2008GL036422, 2009; http://dx.doi.org/10.1029/2008GL036422

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