AGU journal highlights - 31 May 2006

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GRL). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

You may read the scientific abstract for any of these papers by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the portion of the doi (digital object identifier) following 10.1029/ (e.g., 2005GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

1. An overview of the hemispheric dichotomy of Mars

The heavily cratered highlands of the southern hemisphere and the smooth, sparsely cratered lowland plains of the northern hemisphere are among the most distinct features on Mars. The nature of this hemispheric dichotomy has intrigued scientists since the 1970s, when the first global image mosaics of Mars were returned by Mariner 9 and the Viking Orbiters. Specifically, data from the Mars Global Surveyor, Mars Odyssey, and Mars Express orbiting spacecraft, along with the Mars Exploration Rovers, are providing scientists with a wealth of imaging, geochemical, and geophysical data that are transforming current understanding of the red planet. Watters and McGovern provide an overview of current research about this dichotomy and the modification and evolution of the dichotomy boundary and the northern lowlands, as an introduction to a special section of Geophysical Research Letters. They summarize current hypotheses about the origin of the dichotomy and when the dichotomy formed; the erosion, deposition and glacial modification near at the dichotomy boundary; and lithospheric flexure, gravity anomalies, and edge-driven convection at and near the boundary.

Title: Introduction to special section: The Hemispheric Dichotomy of Mars

Authors: Thomas R. Watters: Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, D.C.; U.S.A.;

Patrick J. McGovern: Lunar and Planetary Institute, Houston, Texas, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025755, 2006

2. Recent severe Arctic sea ice reduction is linked to inflows from the Pacific Ocean

Variability in Arctic sea ice extent, thickness, and movement is linked to ocean circulation, which affects the frequency of storms entering the Arctic Ocean and the pathways and amount of sea ice that outflows the system. Arctic Sea ice reduction from this process is thought to be an indication of Arctic climate change; from 1996 to 1998, sea-ice concentration in the Canada Basin decreased by more than half, without any observable recovery. Shimada et al. studied the spatial pattern of ice reduction in the Arctic Ocean, and found that it is similar to the distribution of warm Pacific Summer Water (PSW) that interflows the southern Canada Basin. However, they found that increases in PSW temperature in the basin do not correlate with the temperature of PSW source water in the northeastern Bering Sea. Instead, the authors propose a feedback mechanism where the continual early-winter delay in sea ice formation, which began in 1997/1998, reduced internal ice stresses and rigidity, allowing storm winds to more efficiently mix the upper ocean. This increased the flux of warm Pacific Summer Water into the basin.

Title: Pacific Ocean inflow: influence on catastrophic reduction of sea ice cover in the Arctic Ocean

Authors: Koji Shimada, Takashi Kamoshida, Motoyo Itoh, and Shigeto Nishino: Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan;

Eddy Carmack, Fiona McLaughlin, and Sarah Zimmerman: Fisheries and Oceans Canada, Institute of Ocean Sciences, Sidney, British Columbia, Canada;

Andrey Proshutinsky: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025624, 2006

3. Decadal variability in the Indian Ocean

The meridional circulation in the Indian Ocean differs drastically from that in the Atlantic and Pacific Oceans, because it does not have a northern subpolar regime with which to create deep water. As a result, heat gained in the northern Indian Ocean is exported along the surface southward across the equator, mainly through the Cross-equatorial Cell (CEC). Because little is known about year-to-year and longer-term variability of this circulation, Schoenefeldt and Schott investigated the CEC, which upwells off Northeast Africa and sinks in the southeastern subtropics and is driven by summer monsoons. Using the Simple Ocean Data Assimilation (SODA) model, which forecasts ocean physics based on historical meteorology and ocean observations, the authors found that between 1950 and 2001, the mean circulation of the Cross-equatorial Cell with the SODA model agreed well with observations. Moreover, between 1950 and 1991, significant changes evolved on all parameters that participate in the SODA's Cross-equatorial Cell, causing the cell to slow down in its transport.

Title: Decadal variability of the Indian Ocean cross-equatorial exchange in SODA

Authors: Rena Schoenefeldt and Friedrich A. Schott: IFM-GEOMAR, Leibniz-Institute für Meereswissenschaften, Kiel, Germany

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025891, 2006

4. Positive climate feedbacks between global warming and CO2 concentrations may significantly increase future temperatures

Manmade emissions of carbon dioxide will increase global temperatures, which in turn may generate more carbon dioxide emissions through a range of natural processes such as ocean nutrient cycling and biomass decomposition. However, this feedback has not been included in most predictive models because its magnitude has been difficult to quantify. Scheffer et al. attempted to solve this problem by developing a method to estimate the magnitude of potential positive climate feedback, based on reconstructions of natural changes between 1200 and 1700 C.E., which includes a period known as the Little Ice Age. Linking this information with estimates of warming expected from manmade emissions of carbon dioxide, the authors predict that global temperatures may increase by an additional 15 to 78 percent in the next century. They note that their estimate may be conservative, because it does not include the effects of other greenhouse gases on atmospheric warming.

[Note: See also AGU Press Release 06-17: http://www.agu.org/sci_soc/prrl/prrl0617.html]

Title: Positive feedback between global warming and atmospheric CO2 concentration inferred from past climate change

Authors: Marten Scheffer: Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences, Wageningen University, Wageningen, the Netherlands;

Victor Brovkin: Potsdam Institute for Climate Impact Research (PIK), Potsdam, Germany;

Peter Cox: Centre for Ecology and Hydrology, Dorset, United Kingdom.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025044, 2006

5. Seasonal variation in the waters off northern Chile

In the equatorial Pacific, wind stress changes can alter local sea surface height; as Earth rotates, this causes a pulse of water to propagate eastward. This pulse, called a Kelvin wave, hits the South American continental margin and deflects due to Coriolis forces. Recognizing this, Ramos et al. sought to more fully understand oscillations observed in the Peru-Chile Undercurrent off the coast of South America. Through studies of ocean cruise data between 1981 and 1998, they found that main thermocline (the underwater surface that represents the point at which temperature changes the most dramatically) oscillates annually according to changes in local wind stresses and, secondarily, to an annual equatorial Kelvin wave. In contrast, the semiannual component observed in hydrographic data off Peru and Chile is mainly due to Kelvin waves and their products. One such product, called Rossby waves, spin off from Kelvin waves as the latter hit the shore, causing westward (seaward) propagation. These Rossby waves may modulate the thermocline depth several hundreds of kilometers [miles] off shore; since biological activity decreases significantly below the thermocline, this can influence local carbon budgets.

Title: Seasonal variability of the permanent thermocline off northern Chile

Authors: Marcel Ramos: Departamento de Oceanografía; Center for Oceanographic Research in the eastern South Pacific (COPAS), Universidad de Concepción, Concepción, Chile;

Oscar Pizarro: Center for Oceanographic Research in the eastern South Pacific (COPAS); Departamento de Geofísica; and Laboratorio PROFC; Universidad de Concepción, Concepción, Chile;

Luis Bravo: Center for Oceanographic Research in the eastern South Pacific (COPAS), Universidad de Concepción, Concepción, Chile;

Boris Dewitte: Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, Institut de Recherche pour le Développement, CNES, Toulouse, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025882, 2006

6. Different heliospheric structure during the declining phase of solar cycle 23

The joint NASA/ESA Ulysses spacecraft, now in its third roughly six-year polar orbit of the Sun, has produced data that demonstrates that the three-dimensional structure of the solar wind varies dramatically not just over the solar cycle, the 11-year period in which the magnetic field of the Sun reverses, but also from cycle to cycle. McComas et al. analyzed recent observations from the solar wind experiment (SWOOPS) on the Ulysses spacecraft and found that in contrast to observations over the same latitudes in the declining phase of the previous solar activity cycle, data show a much more complex heliospheric structure in this cycle. For example, at the start of 2003, a band of slow solar wind resided between 20 degrees and 25 degrees North latitude at all of the Sun's longitudes, in contrast to the well organized co-rotating stream structure observed in the previous solar cycle. The authors also show that the solar wind dynamic pressure had been relatively stable over 2005, indicating that the location of the heliopause, the innermost boundary where the heliosphere interacts with interstellar space should be reasonably stable over 2006.

Title: Ulysses observations of very different heliospheric structure during the declining phase of solar activity cycle 23

Authors: D. J. McComas and H. A. Elliott: Southwest Research Institute, San Antonio, Texas, U.S.A.;

J. T. Gosling: University of Colorado, Boulder, Colorado, U.S.A.; and Southwest Research Institute, San Antonio, Texas, U.S.A.;

R. M. Skoug: Los Alamos National Laboratory, Los Alamos, New Mexico, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025915, 2006

7. Changing fire regimes in the North Atlantic boreal regions

Fires in boreal forests and peatlands significantly affect tree recruitment and vegetation recovery, post-fire soil temperature and respiration, and long-term accumulation of carbon. To explore fire regime characteristics at ecozone scales across the entire North American boreal region, Kasischke and Turetsky examined Canadian and Alaskan historic records from 1959 to 1999 and found that between the 1960s/70s and the 1980s/90s, fire regimes were characterized by a doubling of annual burned area and a more than doubling of the frequency of larger fire years. Western ecozones experienced greater increases in larger fire years, compared to eastern ecozones. These findings were consistent with climate-warming trends in Canada, which the authors feel resulted in more frequent summer droughts. Though the proportion of total burned area from human-ignited fires decreased over the studied time period, and burning during the early- and late-growing seasons increased, the authors suspect that fire seasons may be expanding. Moreover, the authors expect that future changes in climate will continue to influence the North American boreal region fire regime.

Title: Recent changes in the fire regime cross the North American boreal region--Spatial and temporal patterns of burning across Canada and Alaska

Authors: Eric S. Kasischke: Department of Geography, University of Maryland, College Park, Maryland, U.S.A.;

Merrit R. Turetsky: Department of Plant Biology and Wildlife, Michigan State University, East Lansing, Michigan, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025677, 2006

8. Warm temperatures in the equatorial Atlantic Ocean are linked to cold conditions in the tropical Pacific Year-to-year variability in the equatorial Atlantic Ocean can be explained by local ocean-atmosphere interactions, maintained and/or triggered by random forcing or by remote forcing, originating mostly from the tropical Pacific Ocean through atmospheric equatorial teleconnections. To investigate how changes in equatorial atmospheric circulation influence the equatorial Atlantic climate, Illig et al. focused on an interannual warm event that occurred in this basin in the boreal spring-summer of 1996. Using observations and ensemble simulations from an intermediate coupled model developed partly by the authors, they analyzed the role of local coupled air-sea interactions versus tropical Pacific teleconnections. They found that persistent cold conditions in the tropical Pacific from 1995 to 1996 were favorable to the growth of local ocean-atmosphere feedbacks that led to the observed 1996 warming event in the equatorial Atlantic. The authors expect that further work will link other anomalous warm events in the equatorial Atlantic with cold conditions in the tropical Pacific.

Title: The 1996 Equatorial Atlantic Warm Event: Origin and Mechanisms

Authors: S. Illig, B. Dewitte, N. Ayoub, and Y. du Penhoat: Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, Toulouse, France;

D. Guchshina: Geographical Faculty, Moscow State University, Moscow, Russia.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025632, 2006

9. Landslide hazards on Stromboli volcano, Italy

Landslides, a common hazard facing volcanic regions, are known to cause great damage and loss of life if they occur within populated areas. Falsaperla et al. analyzed the landslide hazard of Stromboli volcano, an Italian volcano known for its persistent explosive activity and whose unstable slopes pose a significant hazard for the local population. Focusing on the unstable volcanic scarp, named Sciara del Fuoco, the authors merged geostructural observations, live-camera records in the visible and infrared bands, analysis of vertical aerial photographs, and seismic records, to assess landform and structural changes on Stromboli between 2002 and 2004. In particular, they found that the latent instability of the scarp was enhanced by a landslide that followed a lava eruption in December of 2002. Later lava flows stabilized portions of the Sciara del Fuoco; however, erosion in the areas left uncovered by lava caused rock fall and debris flows. The authors hypothesize that if the crater conduits were affected by the sliding, it could cause a change in the eruptive behavior at Stromboli.

Title: Multidisciplinary study of flank instability phenomena at Stromboli volcano, Italy

Authors: S. Falsaperla, M. Neri, E. Pecora, and S. Spampinato: Sezione de Catania, Istituto Nazionale di Geofisica e Vulcanologia, Catania, Italy.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025940, 2006

10. Comparison of borehole data and climate simulations

General circulation models (GCMs) and past climate records should ideally provide similar views of the recent climate evolution. One such record is temperature versus depth profiles logged within continental boreholes, which provide information for the past long term temperature evolution from an integrated history of surface heat dissipation through the crust. To conduct such a comparison, Beltrami et al. used three millennial simulations with the ECHO-g GCM: one simulation kept present conditions of climate forcing constant and two other simulations incorporated estimates of the evolution of some external forcing factors (solar variability, volcanic aerosols, and greenhouse gases). Using surface air temperatures generated by these models, the authors constructed simulated temperature versus depth profiles, which they then compared to existing borehole data from Canada. Within the boundaries of the forcing factors and the GCM under consideration, they found that these forced simulations are in better agreement with observations than those from the control simulation and suggest that further analysis will shed light on understanding the internal dynamics of climate and climate models.

Title: Subsurface temperatures during the last millennium: Model and observation

Authors: Hugo Beltrami and M. Bruce Stevens: Environmental Sciences Research Centre, St. Francis Xavier University, Nova Scotia, Canada;

J. F. Gonzáles-Rouco: Departmento de Astrofísica y CC. de la Atmósfera, Universidad Complutense de Madrid, Spain.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026050, 2006

II. Ordering information for science writers and general public

Journalists and public information officers of educational and scientific institutions (only) may receive one or more of the papers cited in the Highlights by sending a message to Jonathan Lifland [jlifland@agu.org], indicating which one(s). Include your name, the name of your publication, and your phone number. The papers will be e-mailed as pdf attachments.

Others may purchase a copy of the paper online for nine dollars:
1. Copy the portion of the digital object identifier (doi) of the paper following "10.1029/" (found under "Source" at the end of each Highlight).
2. Paste it into the second-from-left search box at http://www.agu.org/pubs/search_options.shtml and click "Go."
3. This will take you to the citation for the article, with a link marked "Abstract + Article."
4. Clicking on that link will take you to the paper's abstract, with a link to purchase the full text: "Print Version (Nonsubscribers may purchase for $9.00)."

The Highlights and the papers to which they refer are not under AGU embargo.

Contact:
Harvey Leifert
American Geophysical Union
2000 Florida Avenue, N.W.
Washington, DC 20009
U.S.A.

Phone (direct): +1 (202) 777-7507
Phone (toll-free in North America): (800) 966-2481 x507
Fax: +1 (202) 328-0566
Email: hleifert@agu.org