AGU journal highlights - 9 June 2005

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., 2004GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

1. Underwater canyon protects California coast

New research suggests that underwater canyons may protect coasts from erosion by ocean waves. For four weeks in the fall of 2003, Thomson et al. observed the effect that the La Jolla submarine canyon had on ocean waves arriving at the California shoreline near San Diego. The researchers took underwater readings of wave pressure and velocity near the edge of the canyon, and from the recordings determined the percentages of the waves that arrived from the open ocean, were reflected by the canyon back to sea, traveled over the canyon to the shore, or ricocheted between the canyon walls. They found that as much as 60 percent of the energy carried by waves approaching the canyon was reflected back into the open ocean. Whether waves were reflected depended on their size, frequency, wavelength, and direction of travel. The authors recommend that reflection of waves by shallow-water features like the La Jolla submarine canyon should be included in future models of near-shore wave action. They also suggest that underwater features similar to the canyon could be used to protect shorelines from erosion and storms.

Title: Reflection and tunneling of ocean waves observed at a submarine canyon

Authors:
Jim Thomson, Steve Elgar, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA; T.H.C. Herbers, Naval Postgraduate School, Monterey, California, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022834, 2005

2. Catching and modeling elusive sprites over Brazil

Comparison of field research on thunderstorms and the predictions of a mathematical model promise to shed light on the elusive middle atmosphere phenomena known as sprites. In 1989, the first images were taken of sprites, large red flashes seen above some thunderstorms, often in conjunction with lightning. Many scientists believe that especially large lightning discharges drive electric fields in the middle atmosphere that generate these dim red flashes. During two large thunderstorms over southeast Brazil in 2002 and 2003, Thomas et al. took balloon-borne measurements of stratospheric electrical fields, magnetic fields correlated with lightning strokes. While clouds prevented imaging of sprites during the storms, the authors recorded two powerful lightning strokes typical of those associated with sprites during the 2002 storm. They compared the various observations from the storm with the results of a computer simulation of lightning-driven electric fields. They then fine-tuned their model to best fit the observed data. They posit that future research comparing measurements of electrical fields with the kind of model they used will further understanding of the causes of sprites.

Title: Predicting lightning-driven quasi-electrostatic fields at sprite altitudes using in situ measurements and a numerical model

Authors:
Jeremy N. Thomas, Robert H. Holzworth, and Michael P. McCarthy, University of Washington, Seattle, Washington, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022693, 2005

3. Searching for the origins of Martian methane

Methane has recently been found in Mars' atmosphere, but so far its source is unknown. The amount of methane in the atmosphere varies from location to location, suggesting that its source is concentrated in subsurface regions. Oze and Sharma propose that the methane may originate in chemical reactions involving water and olivine (an iron-magnesium silicate) in the Martian crust. The reaction releases hydrogen molecules, which can then combine with carbon dioxide to form methane. Olivine has been identified as a major mineral phase on Mars and is believed to be accessible to react with subsurface fluids at shallow depths. Methane currently observed in the atmosphere could be released from ancient reserves trapped in permafrost or from ongoing reactions of olivine and water. Isotopic comparisons between carbon in the atmosphere and in the permafrost may help clear up the mystery of the source of Martian methane.

Title: Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars

Authors:
Christopher Oze and Mukul Sharma, Dartmouth College, Hanover, New Hampshire, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022691, 2005

4. Tracking deep water through the Denmark Strait

Cold, dense water flows from the Greenland Sea into the northern Atlantic Ocean through the Denmark Strait, a long channel between Greenland and Iceland. This current, known as the Denmark Strait Overflow, helps drive ocean circulation and thus influences global climate. To track the movement of water through the strait, Tanhau et al. tracked 320 kilograms [710 pounds] of sulfur hexafluoride that was injected into waters of the Greenland Sea in 1996 at a depth of 300 meters [1,000 feet]. By sampling ocean sulfur hexafluoride concentrations from research ships at various locations in the North Atlantic and noting above-average concentrations, the authors tracked the movement of the sulfur hexafluoride-labeled water. By 2003, about four kilograms [nine pounds] of the introduced sulfur hexafluoride were transported across the Denmark Strait and had traveled as far as the Labrador Sea in the western North Atlantic. This confirmed that the dense water moved across the strait and gave some idea of its rate of travel.

Title: Spreading of overflow water from the Greenland to the Labrador Sea

Authors: Toste Tanhua, Klaus Bulsiewicz, and Monika Rhein, Leibniz Institute of Marine Sciences, Kiel, Germany

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022700, 2005

5. Climate and the rise and fall of the Nile River

People have been measuring water levels in the Nile River since the time of the Pharaohs. Though some gaps exist, records of water level at the Nile's Rodah Island, near Cairo, are continuous from A.D. 622 to 1922. Past analysis of these historical records indicated that water levels fluctuated in cycles repeating from every couple of years to as long as every 250 years. These patterns are related to a variety of periodic natural phenomena, such as Indian monsoons, the El Nino Southern Oscillation, and Earth's motion around the Sun. Using a new mathematical algorithm, Kondrashov et al. filled in water level trends for the missing time periods and searched the reconstructed data set for repeating patterns. They found a particularly strong pattern repeating about every seven years that appeared related to climate in the North Atlantic Ocean, a previously undocumented influence on East African climate. This periodicity could be related to Joseph's biblical prediction of seven years of plenty alternating with seven years of famine. The authors suggest that dramatic changes in the river level cycles in the region over the last 1500 years support concerns about the possibility of sudden climate change in the near future.

Title: Oscillatory modes of extended Nile River records (A.D. 622–1922) Authors:
D. Kondrashov, Y. Feliks, and M. Ghil, University of California, Los Angeles, California, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022156, 2005

6. Probing internal wave "hotspots" in the Japan/East Sea

The best-known ocean waves are those that travel across the surface of the water. But some waves, known as internal waves, travel below the ocean surface. Park and Watts investigated the relationship between ocean circulation and certain internal waves traveling through the southwestern Japan/East Sea. By mapping the direction and velocity of ocean currents using an array of sound velocity sensors moored to the seafloor to measure the bottom-to-surface acoustic echo time, they observed the effects of circulation on the propagation and distribution of internal wave energy. In a region known as the Ulleung Warm Eddy and near the southern and eastern edges of the study area, energy of internal waves was highly concentrated. Comparing the location of these energy hotspots with time-varying ocean circulation patterns, the authors found that the hotspots corresponded with currents circulating clockwise. The authors also found evidence that the internal waves moved at a downward angle towards the ocean bottom in the direction of the equator. They believe the study demonstrates the utility of using active sound sensors to investigate the effects that ocean circulation has on the refraction and trapping of internal waves.

Title: Near-inertial oscillations interacting with mesoscale circulation in the southwestern Japan/East Sea

Authors:
D Jae-Hun Park and D. Randolph Watts, University of Rhode Island, Narragansett, Rhode Island, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022936, 2005

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7. Earthquake causes distant tremors off Japan coast

Waves from a large earthquake travel long distances through the Earth's crust and can trigger tremors far from the quake's epicenter. In Japan, a new nationwide network of over 690 underground sensing stations allows researchers to search for these follow-on earthquakes. Using recordings from the sensor network, Miyazawa and Mori looked for distant tremors resulting from the Tokachi-oki earthquake that occurred off the coast of the Japanese island of Hokkaido on 25 September 2003. As far as 1,400 kilometers [870 miles] south of the earthquake's epicenter, they recorded a substantial number of small tremors near the Nankai subduction zone, where the Pacific tectonic plate moves underneath the Eurasian plate. In particular, there were a large number of tremors southwest of the island of Shikoku in the Bungo strait. The tremors appeared to be deep in the Earth and triggered by the waves traveling across the planet's surface. While these waves were probably too weak to cause earthquakes by themselves, they can change the flow of underground water, which the authors suggest may have triggered the Nankai tremors.

Title: Detection of triggered deep low-frequency events from the 2003 Tokachi-oki earthquake

Authors:
Masatoshi Miyazawa and Jim Mori, Kyoto University, Kyoto, Japan.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022539, 2005

8. Canadian rivers carry less water to northern oceans

In recent decades Canadian rivers carried less freshwater to the North Atlantic and Arctic oceans. Freshwater from roughly three-quarters of Canada's land mass flows through rivers to these seas, widely affecting ocean circulation, the atmosphere, ice formation and melting, and biology. Dery and Wood compiled and analyzed recorded river discharge rates for 64 Canadian rivers that delivered fresh water to the oceans at high latitudes. They found a 10 percent decrease (22 millimeters [0.87 inch] per year) in annual river discharge into the northern oceans from 1964 to 2000. This decline corresponded with a similar reduction in precipitation in the region, which is attributed to large-scale climate cycles such as El Nino and the Arctic Oscillation. The authors conclude that the reduced river flows are the result of the decline in precipitation. The reduction in freshwater has made the North Atlantic and Arctic oceans more salty, which may affect large-scale ocean circulation and, in turn, global climate.

Title: Decreasing river discharge in northern Canada

Authors:
Stephen J. Dery and E. F. Wood, Princeton University, Princeton, New Jersey, USA. Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022845, 2005

9. Panamanian Gateway closure chilled the North Pacific

For most of the last 64 million years, a channel called the Panamanian Gateway linked the Atlantic and Pacific oceans, but about 16 million years ago, the channel began to become shallower until it closed completely about 3 million years ago. Motoi et al. used a computer simulation to investigate the influence that the Panamanian Gateway closure had on ocean circulation and climate in the North Pacific. They ran two experiments, one simulating the gateway open and another with it closed. When the gateway was open, salty surface water traveled from the Atlantic into the Pacific, where surface waters sank into the deep oceans in a mixing process known as deep convection. The resulting large-scale ocean currents brought air from the south into the North Pacific, warming the region's climate. When the gateway closed, the mixing of ocean waters was absent and the climate was substantially colder due to halocline [ocean layer in which rate of salinity variation with depth is greater than in layers just above or below it] formation, similar to current conditions. According to the authors, these simulations are consistent with previous field research to determine the impact of the Panamanian Gateway closure.

Title: North Pacific halocline and cold climate induced by Panamanian Gateway closure in a coupled ocean-atmosphere GCM

Authors:
Tatsuo Motoi, Wing-Le Chan, Shoshiro Minobe, and Hiroshi Sumata, Meteorological Research Institute, Tsukuba, Japan.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022844, 2005

II. Ordering information for science writers

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)."
5. On the next screen, click on "To log-in to your AGU member services or personal subscription, click here."
6. On the next screen, click on "Purchase This Article."
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