AGU journal highlights -- Feb. 16, 2010

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

In this release:

1. Did tidal forces trigger Sumatra earthquakes?
2. First catalog of terrestrial gamma ray flashes
3. Direct observations of ocean-basin-wide acidification
4. Long-distance waves might trigger ice sheet collapse
5. How much climate data is needed for accurate predictions?

Anyone may read the scientific abstract for any of 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. 10.1029/2009GL041488. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions, who are registered with AGU, also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at http://www.agu.org/news/press/papers.shtml

Tidal forces may have contributed to triggering the devastating magnitude 9.0 Sumatra earthquake of 2004 and other large earthquakes in the region. Gravitational pull from the Sun and the Moon not only is responsible for ocean tides but also creates Earth tides, slight motions of the solid Earth that can lead to stress buildup along faults. Earth tides cause tiny variations in the stress on faults, but it isn't clear whether this stress could be enough to trigger an earthquake. Although the motion due to tidal forces is small, small stresses can accumulate on a fault. To determine whether earthquake occurrences off the coast of Sumatra correlate with an Earth tide phase, Tanaka carries out a simple statistical analysis. The phase of the tide determines whether the Earth moves in a direction that adds stress to a fault or relieves stress. The author finds a strong correlation between earthquakes and Earth tide phase in the decade before each megaquake, providing evidence that Earth tides can indeed induce an earthquake on a fault that is already critically stressed. She suggests that monitoring Earth tides might provide information that could improve earthquake forecasting.

Title: Tidal triggering of earthquakes precursory to the recent Sumatra megathrust earthquakes of 26 December 2004 (magnitude 9.0), 28 March 2005 (magnitude 8.6), and 12 September 2007 (magnitude 8.5)

Authors: Sachiko Tanaka: National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041581, 2009 http://dx.doi.org/10.1029/2009GL041581

2. First catalog of terrestrial gamma ray flashes

Gamma rays have been observed to come from astrophysical processes such as collapsing stars, so scientists in the early 1990s were surprised to observe intense, short (less than a millisecond) bursts of extremely high energy gamma rays from our own atmosphere, dubbed "terrestrial gamma ray flashes" (TGFs). These bursts are believed to result from collisions of electrons that have been accelerated to nearly the speed of light by high electric fields associated with lightning, but it remains largely a mystery exactly how, when, and where this "runaway" acceleration process takes place. Understanding TGF observations requires knowledge of the TGF location, but satellite detection can provide only a broad area over which the TGF may have occurred. To overcome this, Cohen et al. create the first catalog of precise locations of TGF sources. The authors use data from a network of ground-based radio receivers that detect impulses of lightning known as sferics at very low frequency, even half a world away. With multiple receivers, these observations pinpoint the location and provide a "fingerprint" of lightning discharges that generate TGFs. The researchers then compare the sferic data with gamma ray data from satellite instruments to associate TGFs with sferics and determine the correct location. The authors find that nearly every TGF had an associated lightning discharge within 1𔃀 ms. They also note that some TGFs are associated with a single powerful lightning discharge, while others occur with a train of smaller lightning impulses.

Title: Geolocation of terrestrial gamma-ray flash source lightning

Authors:

M. B. Cohen, R. K. Said: STAR Laboratory, Stanford University, Stanford, California, USA;

U. S. Inan: STAR Laboratory, Stanford University, Stanford, California, USA; and Koç University, Istanbul, Turkey;

T. Gjestland: Department of Physics and Technology, University of Bergen, Bergen, Norway.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041753, 2009 http://dx.doi.org/10.1029/2009GL041753

3. Direct observations of ocean-basin-wide acidification

Rising levels of atmospheric carbon dioxide are predicted to lead to increasing ocean acidification, which could have potentially harmful consequences for ocean life, including corals, plankton, and shellfish. Model calculations and direct measurements have shown that as atmospheric carbon dioxide levels rise, seawater absorbs the gas from the atmosphere and its acidity increases (its pH decreases), but large-scale, highly precise measurements of ocean pH have been limited. Byrne et al. report the first high-precision, basin-wide direct observations of recent ocean pH changes. The authors measure seawater acidity in the North Pacific Ocean in 1991 and again in 2006 along the longitude line 152 degrees West, between Oahu, Hawaii, and Kodiak, Alaska (22-56 degrees North). By analyzing about 2,100 samples collected from the sea surface to the ocean bottom, approximately 6,000 meters (3.72 miles) of depth, the scientists find that pH had decreased substantially in the ocean's upper 800 m (0.5 mi) during the 15 years between the two research cruises. These findings are consistent with other measurements in the Atlantic and Pacific. In the case of the recent North Pacific changes, human activity is responsible for almost all of the pH decrease seen in the surface mixed layer and about half the decrease seen below the mixed layer, according to the authors. Ocean acidification will likely continue to increase in the future as atmospheric carbon dioxide levels from human activities continue to rise

Title: Direct observations of basin-wide acidification of the North Pacific Ocean

Authors: Robert H. Byrne and Xuewu Liu: College of Marine Science, University of South Florida, Saint Petersburg, Florida, USA;

Sabine Mecking: Applied Physics Laboratory, University of Washington, Seattle, Washington, USA;

Richard A. Feely: Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL040999, 2009 http://dx.doi.org/10.1029/2009GL040999

4. Long-distance waves might trigger ice sheet collapse

Ocean waves generated by storms over the North Pacific affect Antarctic ice shelves and may trigger their collapse, according to a new study. Bromirski et al. propose that the recent Wilkins Ice Shelf collapse could have been triggered by infragravity waves, a type of long-period ocean wave generated when ocean swell strikes continental coastlines. This type of waves can travel far from their coastal source areas; those originating along the coasts of North and South America propagate southward toward Antarctica. When the waves encounter ice shelves, they transfer energy that could be enough to initiate collapse of an already weakened ice shelf. Furthermore, unlike shorter-period ocean waves, which have also been suggested as a possible trigger for ice shelf collapse, infragravity waves are not appreciably damped by sea ice, so they can affect the ice shelf year-round. The authors use seismic observations on the Ross Ice Shelf to determine how much an ice shelf flexes in response to infragravity wave impacts. They also develop a model to estimate the effects of infragravity waves on ice shelf stress. Antarctic Peninsula ice shelves have been observed to collapse in calm weather, with no obvious triggering event identified. Also, a major Wilkins Ice Shelf collapse event occurred during the austral winter, when warmer temperatures and melting ice cannot explain the breakup. All of the breakup events of the Wilkins Ice Shelf in 2008 coincide with the estimated arrival of infragravity waves, the study finds.

Title: Transoceanic infragravity waves impacting Antarctic ice shelves

Authors: Peter D. Bromirski: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA;

Olga V. Sergienko: AOS Program, Princeton University, Princeton, New Jersey, USA;

Douglas R. MacAyeal: Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041488, 2009 http://dx.doi.org/10.1029/2009GL041488

5. How much climate data is needed for accurate predictions?

To predict the climate 10 years from now, an ideal forecast would start with complete knowledge of the current climate. But in reality, scientists don't have perfect information about all possible variables that might influence the climate. Therefore, it becomes important to know how much and what type of input data are most important for models to include to get accurate forecasts. To help determine this, Dunstone and Smith conduct a set of model experiments using different amounts of ocean and atmosphere data to predict future climate on decade time scales. The authors find that sea surface temperature data alone were not enough to make accurate predictions, but that assimilating monthly average subsurface temperature and salinity data significantly improves the accuracy of forecasts.

Title: Impact of atmosphere and sub-surface ocean data on decadal climate prediction

Author: N. J. Dunstone and D. M. Smith: Met Office Hadley Centre, Exeter, UK.

Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL041609, 2009 http://dx.doi.org/10.1029/2009GL041609