AGU journal highlights - 6 October 2005

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I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GL). 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.

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1. Ocean height a good indicator of long-term climate cycles

Recording sea surface temperatures is one way of monitoring ocean conditions corresponding to long-term climate cycles such as El Niño and the Pacific Decadal Oscillation, the climate cycle thought to be the main component of sea surface temperature variation in the Northern Pacific. Cummins et al. surmised that as temperatures change during these cycles, so too would sea surface height, since water expands and contracts as it heats and cools. They compared satellite measurements of sea surface height in the northeast Pacific Ocean from 1993-2004 to recordings of sea surface temperature in the region during the same period. The sea surface height measurements proved to be as accurate as temperature measurements as indicators of ocean conditions resulting from long-term climate cycles. They were also more consistent. The data suggested the northeast Pacific entered the "cool" phase of the Pacific Decadal Oscillation in late 1998, but these conditions ended in early 2003 following development of a weak El Niño. The authors say an index of sea surface heights gathered by satellites could be a useful indicator of upper ocean climatic conditions.

Title: A regional index of northeast Pacific variability based on satellite altimeter data

Authors:
Patrick F. Cummins, Fisheries and Oceans Canada, Institute of Ocean Sciences, Sidney, British Columbia, Canada;
Gary S.E. Lagerloef, Earth and Space Research, Seattle, Washington, USA;
Gary Mitchum, University of South Florida, St. Petersburg, Florida ,USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023642, 2005

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2. Nuclear test detection system might warn of tsunamis

The devastating tsunami generated by the Great Sumatra Earthquake of 26 December 2004 was the first such event recorded globally with modern instrumentation. Hanson and Bowman analyzed recordings from hydrophones and seismic stations around the Indian Ocean. The hydrophones, located near the island of Diego Garcia and off Cape Leeuwin, Australia, are part of the international system for detecting nuclear tests. The hydrophones recorded the pressure fluctuations caused by the tsunami at frequencies orders of magnitude lower than their nominal acoustic recording band. The observations show a remarkably coherent, dispersive wave train that lasts up to 36 hours with frequencies spanning 1 to 25 millihertz--more than a factor of two higher than previously observed from earthquake tsunamis. These same signals are observed at Indian Ocean seismic stations such as at Cocos Island and Casey, Antarctica. In addition, coherent reflections are observed from prominent seamounts and plateaus in the Indian Ocean. The authors suggest that these and similar hydrophone stations in the world's oceans could be a useful part of an early warning system.

Title: Dispersive and reflected tsunami signals from the 2004 Indian Ocean Tsunami observed on hydrophones and seismic stations

Authors:
Jeffrey A. Hanson and J. Roger Bowman, Science Applications International Corporation, Monitoring Systems Division, San Diego, California, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023783, 2005

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3. Filling the gap between glacial runoff and sea level rise

Raper and Braithwaite published a paper in Geophysical Research Letters in 2005, which suggested that the lack of an inventory of world glaciers limits understanding of the relationship between glacier runoff and sea level rise. In this comment on that paper, Meier et al. question the methods and thus the results used by Raper and Braithwaite. Meier et al. say it was unrealistic to estimate glacier runoff using a spatial grid and that each glacier should instead be considered as a single mass. They also suggest the researchers oversimplified the relationship between the roughness of topography of a region and glacier size and coverage area in that region. They argue that the equation Raper and Braithwaite developed is useful but incomplete, and that their research neglected many small glaciers in Greenland and Antarctica that probably contribute to sea-level rise. They commend the researchers for bringing awareness to the lack of a comprehensive glacier inventory, but say that gap in knowledge has yet to be filled.

Title: Comment on ''The potential for sea level rise: New estimates from glacier and ice cap area and volume distribution'' by S. C. B. Raper and R. J. Braithwaite, INSTAAR, University of Colorado, Boulder, Colorado, USA

Authors:
Mark F. Meier, David B. Bahr, Mark B. Dyurgerov, and W. Tad Pfeffer, Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023319, 2005

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4. Arctic Ocean waters warm suddenly

New research on water flowing from the North Atlantic Ocean into the Arctic provides evidence that the Arctic Ocean is warming. Polyakov et al. measured the temperature, salinity, and velocity of the Atlantic Water, a warm, salty layer (150-900 meters [500-3,000 feet]) of ocean water that flows through the Norwegian Sea into the Arctic Ocean. The temperature of the Atlantic Water entering the Arctic Ocean increased dramatically in 2004, warming in two abrupt stages in February and August. The anomalously warm water is currently flowing along the basin margins toward the interior of the Arctic Ocean. The authors suggest that enhanced westerly winds in the North Atlantic pushed warm water into the Norwegian Sea, and from there it flowed into the Arctic Ocean. These winds are due to changes in atmospheric conditions, but the authors say more evidence is needed to determine if the associated warming is the result of long-term change or part of a recurrent climate cycle. The authors' observations do indicate, however, that the Arctic Ocean is currently warming, a trend that could reduce Arctic ice cover and influence ocean processes in more southerly regions.

Title: One more step toward a warmer Arctic

Authors:
Igor V. Polyakov, Igor A. Dmitrenko, Vladimir V. Ivanov, Harper L. Simmons, and John E. Walsh, International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, Alaska, USA;
Agnieszka Beszczynska, Eberhard Fahrbach, Rüdiger Gerdes, Ursula Schauer, and Frank Kauker, Alfred-Wegener Institut fur Polar- und Meeresforschung, Bremerhaven, Germany;
Eddy C. Carmack, Department of Fisheries and Oceans, Sidney, British Columbia, Canada;
Ivan E. Frolov, Vladimir T. Sokolov, and Leonid A. Timokhov, Arctic and Antarctic Research Institute, Saint Petersburg, Russia;
Edmond Hansen and Jürgen Holfort, Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska, USA;
Mark A. Johnson, O.A.Sys - Ocean Atmosphere Systems GbR, Hamburg, Germany;
James Morison and Michael Steele, Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington, USA; Kjell A. Orvik and Oystein Skagseth, Bjerknes Centre for Climate Research, GI/UB, Bergen, Norway;
David Walsh, Naval Research Laboratory, Washington, D.C., USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023740, 2005

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5. Mountain building and the propagation of tectonic waves

The Earth's lithosphere is comprised of the crust and attached portion of the upper mantle. Mountains are formed when separate sections of the lithosphere collide and one section is pushed upward as the other moves underneath it. Which plate will be pushed up to form mountains and which will be forced down into the mantle to become molten rock depends on variations in the stresses at the base of the plates and the buoyancy of the plates. Differences in buoyancy result from variations in the density of rock layers in the colliding sections of crust. Ricard and Husson propose a model of the mountain-building process that views the plates as thin viscous sheets, and takes into account variations in the composition and temperature of tectonic plates. Their model suggests that tectonic waves created by the collision of the plates can propagate according to the composition and thermal characteristics of the plates. The speed of the waves is typically on the order of one centimeter [0.4 inch] per year, which increases with the thickness of the crust and decreases as the lithosphere becomes more viscous.

Title: Propagation of tectonic waves

Authors:
Y. Ricard, Department of Geology and Geophysics, Yale University, New Haven, Connecticut, USA;
L. Husson, Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023690, 2005

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6. Odd water found in the ocean near southern Japan

Measurements of the ocean temperature and salinity near southern Japan revealed a rare occurrence of water from the North Pacific in subsurface eddy currents. Takikawa et al. recorded seawater temperature and salinity at a variety of depths in a region of ocean southeast of Okinawa. The measurements were taken from a Japan Meteorological Agency research ship between 1993 and 2003. In February 2002, they showed a 300-meter [1,000-foot] wide, 100-kilometer [60-mile] long mass of unusual water at a depth of about 300 dbar in one of the region's subsurface eddy currents. The temperature and salinity of the water suggested it was North Pacific Subtropical Mode Water, a layer of water that forms in the winter under cold surface waters. This was the only time during the ten years of observation that this water was found in the subsurface waters near southeastern Japan. The researchers say this finding will help oceanographers understand the movement of North Pacific Subtropical Mode Water and the dynamics of ocean currents in the region's giant, clockwise circulating current, known as the North Pacific subtropical gyre.

Title: Extraordinary subsurface mesoscale eddy detected in the southeast of Okinawa in February 2002

Authors:
Tetsutaro Takikawa, National Fisheries University, Shimonoseki, Japan;
Hiroshi Ichikawa and Kaoru Ichikawa, Institute of Observational Research for Global Change, Yokosuka, Japan;
Satoshi Kawae, Nagasaki Marine Observatory, Japan Meteorological Agency, Nagasaki, Japan.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023842, 2005

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7. Atmospheric oxygen distributions linked to ocean processes

Studying the distribution of atmospheric oxygen offers important insight into a range of ocean processes, including ocean productivity, currents, and upwelling. It is often difficult, however, to separate the effects that ocean processes have on oxygen distribution from seasonal variations due to photosynthesis and respiration by land plants. To avoid this confusion, Tohjima et al. studied the distribution of a combination of oxygen and carbon dioxide known as atmospheric potential oxygen, which is unaffected by land vegetation. They collected air samples from ships traveling between Japan and the United States and between Japan and Australia and New Zealand from December 2001 to August 2004. They found that the largest seasonal changes in atmospheric potential oxygen take place in the higher latitudes of both hemispheres and that the lowest seasonal changes take place about 10 degrees north of the equator. The largest annual mean atmospheric potential oxygen in the tropics was found in the Western Pacific. The authors say these results support the validity of recent ocean transport simulations and models of ocean biology. They also suggest the results support the upper ocean oxygen circulation patterns suggested in 2001 by Gruber et al.

Title: First measurements of the latitudinal atmospheric O2 and CO2 distributions across the Western Pacific

Authors:
Y. Tohjima, H. Mukai, T. Machida, and Y. Nojiri, National Institute for Environmental Studies, Tsukuba, Japan;
M. Gloor, Atmosphere & Ocean Science Program, Princeton University, Princeton, New Jersey, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023311, 2005

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8. African dust in the wind

Atmospheric aerosol concentrations affect the balance of solar radiation entering and leaving the atmosphere and are a major source of uncertainty in climate change research. The Bodele Depression, an ancient lake bed in central Chad, releases more dust into the atmosphere each year than any other location on Earth. Strong winds carry the dust into the air, substantially increasing aerosol concentrations. Washington and Todd explored the various factors that combine to make the Bodele Depression such a prolific source of dust. They used satellite imaging to determine atmospheric dust distribution during two time periods: from 1979-1992 and 2002-2004. Comparing this data to atmospheric circulation reconstructions for the same period, they found that a low level jet (a low-altitude, fast-moving ribbon of wind) sometimes blows over the region. The amount of dust produced by the jet was related to its size and strength, which varied from season to season. The jet is was strongest during the dustiest years. The authors suggest the jet plays a role in the generation of the dust as well as lifting it into the atmosphere.

Title: Atmospheric controls on mineral dust emission from the Bodele Depression, Chad: The role of the low level jet

Authors:
Richard Washington, Climatology Research Group, Oxford University Centre for the Environment, University of Oxford, Oxford, United Kingdom;
Martin C. Todd, Department of Geography, University College London, London, United Kingdom.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023597, 2005

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9. Storm tracks head for the poles in the 21st century

Storm tracks are regions where large-scale weather phenomena such as frontal storms often occur. Jeffrey Yin used the output from 15 climate simulations, each performed with a different general circulation model, to predict how the positions of storm tracks might change over the course of this century. The general circulation model results suggested that during the 21st century, storm tracks will move towards Earth's poles and climb to higher altitudes. In addition, the general circulation model results suggested that storms would intensify and that surface winds and precipitation would increase towards the poles. The shifting of the storms was also accompanied by warming high in the atmosphere over the tropics. Yin says these changes would have wide-ranging effects on the planet's climate. Winds, for example, are an important driver of ocean circulation, and changes in their strength and location might affect ocean and atmospheric processes, such as carbon dioxide storage. Yin says the significant influence storm tracks have on climate makes it crucial to understand how and why they move poleward.

Title: A Consistent Poleward Shift of the Storm Tracks in Simulations of 21st Century Climate

Author:
Jeffrey H. Yin, National Center for Atmospheric Research, Boulder, Colorado, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023684, 2005.

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10. Eddies act as energy chimneys in the Southern Ocean

Storm winds whip up the ocean surface, generating energy that propagates outward in the surface water and downward into the deep ocean waters, where it contributes to the mixing of ocean layers. Previous research suggested ocean eddies, or rotating currents, facilitate the movement of this energy downward into the ocean. Zhai et al. used a computer model of a 5000-meter [three mile] deep channel in the Southern Ocean that contained many eddy currents to investigate this phenomenon. They simulated a moving storm with winds blowing against the surface waters of the channel, and tracked the movement of the energy that the wind transferred to the waters. They found that eddies did somewhat facilitate the downward propagation of the energy into the deep layers of the ocean. But contrary to prior research findings, the authors' results suggested that counter-clockwise circulating eddies in particular act as "chimneys" to drain energy from the surface to the deep ocean. They say this could have important implications for ocean currents and mixing of water in the Southern Ocean.

Title: Enhanced vertical propagation of storm-induced near-inertial energy in an eddying ocean channel model

Authors:
Xiaoming Zhai, Richard J. Greatbatch, and Jun Zhao, Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023643, 2005

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II. Ordering information for the general public

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