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The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Paleoceanography (PA), Journal of Geophysical Research-Biogeosciences (JGR-G), and Journal of Geophysical Research-Oceans (JGR-C).

In this release:

  • Moon's shadow, like a ship, creates waves
  • Groundwater depletion's contribution to increase in sea level rise
  • An atmospheric precursor to the recent Japan megaquake
  • Peatland images show change due to global warming
  • Shells of microorganisms record seasonal temperature changes
  • Using an artificial brain to interpret Adriatic surface currents

Anyone may read the scientific abstract for any already-published paper 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. 10.1029/2011GL048604. 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.

1. Moon's shadow, like a ship, creates waves

During a solar eclipse, the Moon's passage overhead blocks out the majority of the Sun's light and casts a wide swath of the Earth into darkness. The land under the Moon's shadow receives less incoming energy than the surrounding regions, causing it to cool. In the early 1970s, researches proposed that this temperature difference could set off slow-moving waves in the upper atmosphere. They hypothesized that the waves, moving more slowly than the travelling temperature disparity from which they spawned, would pile up along the leading edge of the Moon's path-like slow-moving waves breaking on a ship's bow. The dynamic was shown theoretically and in early computer simulations, but it was not until a total solar eclipse on 22 July 2009 that researchers were able to observe the behavior.

Using a dense network of ground-based global positioning satellite receivers, Liu et al. tracked the influence of the 2009 eclipse as it passed over Taiwan and Japan. The researchers looked for changes in the total electron content in the ionosphere and find acoustic waves with periods between 3 and 5 minutes traveling around 100 meters per second (328 feet per second) that originated from the leading and trailing edges of the shadow, analogous to bow waves and stern wake common in maritime activity. They find that there was a 30 minute time difference between the arrival of the bow and stern waves suggesting that, were the Moon's shadow a ship, it would be 1,712 kilometers (1,064 miles) long. The researchers indicate that this would correspond to the part of the Moon's shadow that produced at least an 80 percent obscuration of the Sun's light.

Source: Geophysical Research Letters, doi:10.1029/2011GL048805, 2011 http://dx.doi.org/10.1029/2011GL048805

Title: Bow and stern waves triggered by the Moon's shadow boat

Authors: J. Y. Liu: Institute of Space Science, National Central University, Chung-Li, Taiwan, Center for Space and Remote Sensing Research, Chung-Li, Taiwan, and National Space Program Origination, Hsin-Chu, Taiwan;

Y. Y. Sun: Institute of Space Science, National Central University, Chung-Li, Taiwan;

Y. Kakinami: Institute of Seismology and Volcanology, Hokkaido University, Sapporo, Japan;

C. H. Chen: Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, Japan;

C. H. Lin: Department of Earth Science, National Cheng Kung University, Tainan, Taiwan;

H. F. Tsai: Center Weather Bureau, Taipei, Taiwan.

2. Groundwater depletion's contribution to increase in sea level rise

Since the turn of the twentieth century, industrial-scale redistribution of water from landlocked aquifers to the ocean has driven up the global average sea level by over 12 centimeters (4.7 inches). Between 1900 and 2008, roughly 4,500 cubic kilometers (1,079 cubic miles) of water was drawn from the ground, largely to feed an agricultural system increasingly reliant on irrigation. Of that 4,500-cubic km total (nearly the volume of Lake Michigan), 1,100 cubic km (264 cubic mi) were pumped out between 2000 and 2008 alone. This early 21st-century groundwater depletion was responsible for raising global sea level at a rate of 0.4 millimeters per year (0.016 inches per year), an eighth of the observed total. These updated values, falling near the middle of the range of previous estimates, are the product of an investigation by Konikow that draws together a variety of volumetric measurements of groundwater storage.

The researcher combines the results from a number of previous studies that estimated groundwater storage using measurements of groundwater levels, satellite observations of aquifer water volumes, or models designed to track long-term changes in groundwater storage. Groundwater depletion can have a host of negative consequences, including land subsidence, loss of wetlands, reductions in river flow rates, and, of course, the eventual elimination of an important source of freshwater. With sea level rise already an important facet of the challenge of planning for future climate change, Konikow notes that it is important to constrain the historical contribution of groundwater depletion to sea level rise, such that the range of possible future rates of sea level rise can be reined in.

Source: Geophysical Research Letters, doi:10.1029/2011GL048604, 2011 http://dx.doi.org/10.1029/2011GL048604

Title: Contribution of global groundwater depletion since 1900 to sea-level rise

Authors: Leonard F. Konikow: U.S. Geological Survey, Reston, Virginia, USA.

3. An atmospheric precursor to the recent Japan megaquake

Most scientists believe that earthquakes are inherently unpredictable, and reports of various kinds of earthquake precursor signals have been difficult to verify. However, in a new study, Heki reports a possible ionospheric precursor to the devastating 11 March 2011 magnitude 9 Tohoku earthquake in Japan. Analyzing data from the Japanese GPS network, he detects an increase in the total electron content (TEC) in the ionosphere above the focal region of the earthquake beginning about 40 minutes before the quake. The TEC enhancement reached about 8 percent above the background electron content. The increase in TEC was greatest above the earthquake epicenter and diminished with distance from the epicenter. The researcher also analyzes GPS records from previous earthquakes and finds that similar ionospheric anomalies occurred before the 2010 magnitude 8.8 Chile earthquake, possibly the 2004 Sumatra magnitude 9.2 earthquake, and possibly the 1994 magnitude 8.3 Hokkaido earthquake, but TEC enhancements were not seen before smaller earthquakes.

Although previous studies have shown that earthquakes could trigger atmospheric waves that travel upward and disturb the ionosphere, it is unclear how an ionospheric disturbance could occur before an earthquake begins. In addition, the ionosphere is highly variable, and solar storms can trigger large TEC changes, so nonearthquake causes of any TEC enhancement need to be ruled out. The researcher states that, unlike previously suggested earthquake precursors, the TEC enhancement before the Tohoku quake had obvious spatial and temporal correlation between the quake and precursor signal as well as clear magnitude dependence. Further research is needed to verify that TEC enhancements can indeed be a precursor to large earthquakes.

Source: Geophysical Research Letters, doi:10.1029/2011GL047908, 2011 http://dx.doi.org/10.1029/2011GL047908

Title: Ionospheric electron enhancement preceding the 2011 Tohoku-Oki earthquake

Authors: Kosuke Heki: Department of Natural History Sciences, Hokkaido University, Sapporo, Japan.

4. Peatland images show change due to global warming

As global average temperatures rise, vast tracks of peatland currently encased in permafrost will be affected. As the ground thaws, peatlands will evolve in either of two directions. Along one path, land that was previously propped up by supportive permafrost subsides, forming a shallow basin that fills with water-a thermokarst lake. In the new lake, peat undergoes anaerobic bacterial decay, releasing methane to the environment. Alternatively, permafrost thawing can result in lake drainage. In the drained lake beds, fen vegetation and mosses can grow, drawing down atmospheric carbon dioxide levels. The prevalence of these two processes, and their relationship with changing temperatures, remains an important question in understanding the consequences of permafrost thaw on the global carbon cycle.

Using high-resolution satellite imagery and aerial photography stretching back to the 1950s, Sannel and Kuhry track the transformation of three permafrost peatlands: a Canadian and a Russian site with relatively cold ground temperatures, and a Swedish peatland with permafrost temperatures close to 0 degrees Celsius (32 degrees Fahrenheit). The authors find that as winter precipitation, average atmospheric temperatures, and average ground temperatures increased throughout the study period, the Canadian and Russian peatlands saw small changes in lake extent. However, the Swedish site had 7.6 percent of its lake area overgrown by vegetation per decade, along with the formation of some small new lakes. The authors suggest that there is a threshold air temperature, between -5 degrees Celsius and -3 degrees Celsius (23 degrees Fahrenheit and 26.7 degrees Fahrenheit), above which temperature and precipitation changes begin to significantly affect peatlands.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1029/2010JG001635, 2011 http://dx.doi.org/10.1029/2010JG001635

Title: Warming-induced destabilization of peat plateau/thermokarst lake complexes

Authors: A. B. K. Sannel and P. Kuhry: Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, Sweden.

5. Shells of microorganisms record seasonal temperature changes

When microorganisms in the ocean known as planktonic foraminifera form their shells, the magnesium to calcium (Mg/Ca) ratios in those shells are sensitive to water temperature. This has enabled paleoceanographers to use Mg/Ca ratios measured from multiple fossil foraminifera shells in sediments as a proxy for average water temperatures in the past.

Now Haarmann et al. show that some foraminifera Mg/Ca ratios can even be used to determine seasonal variations in temperature. The researchers suggest that because planktonic foraminifera calcify over a period of a few weeks to months, the Mg/Ca ratios in single specimens could capture seasonal temperature variations. They tested this in samples of three different foraminifera species collected off the coast of northwestern Africa, where there is strong seasonal sea surface temperature variability. The researchers find that one of the three species shows strong variations in Mg/Ca that tracked seasonal temperature changes and thus could potentially be used to reconstruct seasonality in the near and distant past.

Source: Paleoceanography,

doi:10.1029/2010PA002091, 2011

http://dx.doi.org/10.1029/2010PA002091

Title: Mg/Ca ratios of single planktonic foraminifer shells and the potential to reconstruct the thermal seasonality of the water column

Authors: Tim Haarmann: MARUM-Center for Marine Environmental Sciences, Bremen, Germany;

Ed C. Hathorne: IFM-GEOMAR, Leibniz Institute for Marine Sciences, University of Kiel, Kiel, Germany;

Mahyar Mohtadi: MARUM-Center for Marine Environmental Sciences, Bremen, Germany;

Jeroen Groeneveld: MARUM-Center for Marine Environmental Sciences, Bremen, Germany, and Alfred Wegener Institute, Bremerhaven, Germany;

Martin K├Âlling: Department of Geosciences, University of Bremen, Bremen, Germany;

Torsten Bickert: MARUM-Center for Marine Environmental Sciences, Bremen, Germany.

6. Using an artificial brain to interpret Adriatic surface currents

The Adriatic Sea is largely cut off from global-scale ocean circulation patterns - it lies between the Italian peninsula and the northwestern Balkan nations, and is separated from the Mediterranean Sea by the Strait of Otranto. As a whole, the Adriatic has a permanent counterclockwise circulation, but in the shallow northern reaches, surface currents vary dramatically over short spans of time, with potentially dangerous consequences for maritime activity. To understand the primary drivers of the fluctuating surface currents, Mihanovic et al. perform self- organizing map (SOM) analysis-an emergent computational technique in oceanographic research-on data provided by three high-frequency radar stations operating in the region.

As a neural network technique, SOM analysis uses complex mathematical algorithms to train computers to pull patterns from jumbles of data, reducing complex multidimensional observations into simple visual maps. The approach is meant to emulate the learning abilities of biological brains. The researchers' SOM analysis reveals 12 patterns that explain the majority of northern Adriatic surface currents. By comparing SOM analyses run using radar data against those performed using the radar data along with surface wind data derived from a high- resolution operational model, the researchers deduce that surface currents in the northern Adriatic are controlled largely by surface winds. They find that three of the 12 patterns are linked to regularly recurring, dry, northeasterly winds (bora), and three are tied to moist southeasterly winds (sirocco) that are equally prevalent in the region. The remaining six surface current patterns are associated with calm conditions or with what remains of the basin-wide thermohaline circulation's influence. The number of patterns associated with each force indicates its relative importance in driving surface currents. The researchers suggest that their SOM analysis-derived patterns potentially could be used within operational oceanography systems to provide real-time estimates and forecasts of surface currents for the northern Adriatic.

Source: Journal of Geophysical Research-Oceans, doi:10.1029/2011JC007104, 2011 http://dx.doi.org/10.1029/2011JC007104

Title: Surface current patterns in the northern Adriatic extracted from high-frequency radar data using self-organizing map analysis

Authors: Hrvoje Mihanovic: Hydrographic Institute of the Republic of Croatia, Split, Croatia;

Simone Cosoli and Miroslav Gacic: Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Sgonico, Italy;

Ivica Vilibic, Damir Ivankovic and Vlado Dadic; Institute of Oceanography and Fisheries, Split, Croatia.

SOURCE

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