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AGU journal highlights -- Jan. 14, 2013

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

In this release:

1. Seabird activity influences Arctic methane and nitrous oxide emissions

2. U.S. cities less susceptible to water scarcity than previously thought

3. First direct evidence that breaking waves cause horizontal eddies

4. Variability of transparent organic particles in Arctic floodplain lakes

5. Tropical cyclone waves detected with infrasound sensor array

6. Aftershocks to Philippine quake found within nearby megathrust fault

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 and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2012JG002130. 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.

**Please note** AGU has recently partnered with Wiley, which will now publish AGU's journals. Registered reporters should have received an email from Wiley the week of 7 January with a new login and password, which will allow them to access journal articles for free through the Wiley Online Library at If you are a reporter and have not yet registered for a complimentary press subscription, please fill out the form at

1. Seabird activity influences Arctic methane and nitrous oxide emissions

Seabird activity is contributing significantly to methane and nitrous oxide emissions in the Arctic tundra, a new study shows. Methane emissions, which play an important role in the global carbon cycle, and nitrous oxide fluxes, a key element in the nutrient cycle, are predicted to increase in the Arctic and contribute to Arctic warming in the near future.

To study the effects of seabird activity on variations in nitrous oxide and methane fluxes from the tundra to the atmosphere, Zhu et al. compared fluxes from a seabird sanctuary and two non-seabird colonies on Ny-lesund, Svalbard, Norway. They find that seabird activity was a major factor in these emissions. Mean fluxes of nitrous oxide were about 18 micrograms per square meter per hour at the seabird sites, compared with about 8 micrograms per square meter per hour at the non-seabird tundra sites. For methane, seabird activity actually changed the tundra from a methane sink to a source: mean fluxes of methane were about 53 micrograms per square meter per hour at the seabird sites and about -83 micrograms per square meter per hour at the non-seabird sites.

The researchers considered other factors that could influence methane and nitrous oxide emissions, including soil moisture and temperature. However, they find that seabird activity was the predominant factor in controlling the flux of these gases from the tundra to the atmosphere. They conclude that sites with high seabird activity are likely to be hotspots of methane and nitrous oxide emissions.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1029/2012JG002130, 2012

Title: Impact of seabird activity on nitrous oxide and methane fluxes from High Arctic tundra in Svalbard, Norway

Authors: Renbin Zhu: Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China, and Department of Geography, University of California, Berkeley, California, USA;

Qingqing Chen: Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China;

Wei Ding: Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China;

Hua Xu: State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, China.

2. U.S. cities less susceptible to water scarcity than previously thought

The past few years have seen powerful droughts across the U.S., with water shortages threatening crop production, shipping traffic, energy production, and groundwater stores. Water scarcity issues are particularly relevant for those living in cities, a demographic which now includes roughly 4 out of every 5 Americans. Previous research has tallied average daily water needs, estimated at 600 liters (about 160 gallons) per person per day, and the availability of natural renewable water resources. The results suggested that up to 47 percent of the U.S. population is vulnerable to water scarcity issues. In many cases, urban water managers cope with natural variability through the use of infrastructure designed to pump, import, or store freshwater. Nationwide water resource assessments, however, overlook such infrastructure-based approaches to water management, instead assessing only water derived from local streamflow, runoff, or groundwater storage.

To more accurately assess the vulnerability of U.S. urban areas to water shortages, Padowski and Jawitz compiled publicly available records of water management resources for 225 cities across the country those with populations of 100,000 or greater for which adequate records were available. When they included in their tallies both natural renewable water resources and the capacity to import, pump, and store water, the authors find that only 17 percent of the U.S. population not 47 percent is vulnerable to water scarcity issues. They find that when water management infrastructure resources were taken into consideration, every U.S. city studied could provide an annual mean of 600 liters (about 160 gallons) per person per day or greater, even in areas where the local availability of water is scarce.

The authors find, however, that some cities, such as Miami, Florida, appear more vulnerable to water scarcity under the new assessment. Though Miami receives a high volume of water, a lack of storage or import capacity suggests that it is less resilient to natural variability than cities with more robust water management infrastructures.

Source: Water Resources Research, doi: 10.1029/2012WR012335, 2012

Title: Water availability and vulnerability of 225 large cities in the United States

Authors: J.C. Padowski and J.W. Jawitz: Soil and Water Science Department, University of Florida, Gainesville, Florida, USA.

3. First direct evidence that breaking waves cause horizontal eddies

As ocean waves pass from deeper water into the shallow coastal regions, they begin to break, churning up the surf zone waters. At the edges of the crests of the breaking waves, horizontally-rotating eddies (vertical vortices) are generated, converting some of the waves' kinetic energy into turbulence. These horizontally-rotating eddies are an important mechanism for dispersing nutrients, larvae, bacteria, sediments, and other suspended objects along the coastline.

Using a 10-meter-diameter (33-foot-diameter) ring of submerged current sensors, Clark et al. directly measured for the first time the generation of horizontal eddies by breaking coastal waves in the water off North Carolina. In line with theoretical and numerical modeling efforts, the authors find that turbulent eddies are created at the edges of breaking waves, and that the eddies rotate in different directions depending on whether they are produced by left- or right-handed waves. They find the eddies decayed after 20 to 60 seconds. Further, they find the eddies were strongest at low tide and weakest at high tide, a finding they attribute either to the sensor array's changing position in the surf zone due to the shifting tide, or to an increase in the amount of energy dissipated by waves breaking in the shallower low-tide water. They suggest that such horizontal eddies are an important source of dispersive energy for the near-shore environment.

Source: Geophysical Research Letters, doi: 10.1029/2012GL054034, 2012

Title: Vorticity generation by short-crested wave breaking

Authors: David B. Clark, Steve Elgar and Britt Raubenheimer: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA.

4. Variability of transparent organic particles in Arctic floodplain lakes

In the North American Arctic the Mackenzie River courses into the Beaufort Sea, the outlet of a watershed that drains a vast swath of the western Canadian landscape. At the river's mouth, the Mackenzie Delta is a broad floodplain peppered with roughly 45,000 lakes carved into the permafrost. Depending on their connectivity to the river, these floodplain lakes have different mixtures of organic compounds, and such differences affect carbon cycling and sediment processes in the lakes.

Among the organic compounds prevalent in these Arctic lakes are transparent exopolymer particles (TEP), a gel-like substance formed by the spontaneous aggregation of sticky chains of organic molecules that are secreted by algae, bacteria, and other organisms. Previous research found that aggregates of TEP form habitats for microscopic life and reaction surfaces for aqueous chemicals and can help promote sedimentation. In the ocean, TEP is a key component of ocean "snow." The role and variability of TEP in Arctic lakes and other inland waters, however, has been relatively unstudied.

Collecting weekly water samples from three Mackenzie Delta lakes over a 2-month span in 2006, Chateauvert et al. measured how the concentrations of TEP and other organic compounds varied following the annual flooding of the Mackenzie River. The authors find that the concentration of TEP varied by up to two orders of magnitude over the course of the study period, peaking immediately following the June flood and declining steadily afterward. They also find that the lake best connected to the river had the highest TEP concentrations. These findings ran directly counter to the authors' initial hypothesis of how TEP concentrations should evolve in the lakes. The authors find that changes in the concentration of chromophoric dissolved organic mattera form of organic material derived almost exclusively from river water in the regioncould account for more than half of the measured TEP variability.

Source: Journal of Geophysical ResearchBiogeosciences, doi:10.1029/2012JG002132, 2012

Title: Abundance and patterns of transparent exopolymer particles (TEP) in Arctic floodplain lakes of the Mackenzie River Delta

Authors: C. Adam Chateauvert and Lance F. W. Lesack: Departments of Geography and Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada;

Max L. Bothwell: Pacific Biological Station, Environment Canada, Nanaimo, British Columbia, Canada.

5. Tropical cyclone waves detected with infrasound sensor array

The strong winds of a tropical cyclone whip up the sea surface, driving ocean waves a dozen meters (about 40 feet) high. When one such ocean wave runs into another wave that has an equal period but is traveling in the opposite direction, the interaction produces low-frequency sound waves that can be detected thousands of kilometers away. Known as microbarom, the infrasound signals produced by interacting ocean surface waves have typical frequencies around 0.2 hertz. Researchers previously determined that as a hurricane travels along its track, waves generated by the storm earlier in time will interact with those generated later on, producing a strong microbarom signal in the storm's wake. Researchers also found, however, that microbarom signals are produced by regular surface ocean behavior, including swell, surface waves, or non-tropical cyclone storms.

To identify how tropical cyclone-produced waves interact with ambient surface ocean waves, and to determine whether the tropical cyclone microbarom signal could be isolated from the background noise, Stopa et al. examined the infrasound signals detected by an International Monitoring System infrasound sensor array in Hawaii during the passage of Hurricanes Neki and Felicia in 2009.

The authors used modeled wind speeds to simulate the wave conditions during the hurricanes, then used these estimates to drive an acoustic model that enabled them to calculate the microbarom infrasound activity. They find that the microbarom signals observed by the Hawaiian sensor array aligned with their modeled estimates. The authors note that the infrasound signal of the cyclone-generated waves tended to swamp the detectors, drowning out the much weaker signals of the ambient wave interactions. They suggest that given further refinements, measuring the infrasound signal of microbarom waves could be a good way to detect and measure the wave conditions near a tropical cyclone.

Source: Journal of Geophysical Research - Oceans, doi: 10.1029/2012JC008257, 2012

Title: Atmospheric infrasound from nonlinear wave interactions during Hurricanes Felicia and Neki of 2009

Authors: Justin E. Stopa and Kwok Fai Cheung: Department of Ocean and Resources Engineering, University of Hawaii at Manoa, Honolulu, Hawaii, USA;

Milton A. Garcs and Nickles Badger: Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Kailua-Kona, Hawaii, USA.

6. Aftershocks to Philippine quake found within nearby megathrust fault

On 31 August 2012 a magnitude 7.6 earthquake ruptured deep beneath the sea floor of the Philippine Trench, a powerful intraplate earthquake centered seaward of the plate boundary. In the wake of the main shock, sensors detected a flurry of aftershocks, counting 110 in total. Drawing on seismic wave observations and rupture mechanisms calculated for the aftershocks, Ye et al. find that many were located near the epicenter of the main intraplate quake, but at shallower depth and all involving normal faulting. Some shallow thrusting aftershocks were located farther to the west, centered within the potentially dangerous megathrust fault formed by the subduction of the Philippine Sea plate beneath the Philippine Microplate, the piece of crust housing the Philippine Islands.

In the past century, the most powerful earthquakes have occurred within megathrust faults. The particular portion of the megathrust fault nearest to the 31 August intraplate earthquake, and the section that housed the shallow thrusting aftershocks, have not had a strong earthquake since at least 1600. That aftershocks from the main intraplate shock took place within the interplate boundary suggests that the two systems may be coupled. Previous research in other locales suggests that the rupture of a nearby megathrust fault sometimes follows on the heels of a large offshore intraplate thrust earthquake. Similarly, previous research has found that the stress accumulation that could lead to the rupture of an intraplate thrust earthquake can at least in part be explained by the build-up of stress in a nearby interplate boundary. The authors suggest that more work needs to be done to identify whether there is an accumulating slip deficit within the Philippine Trench megathrust fault.

Source: Geophysical Research Letters, doi: 10.1029/2012GL054164, 2012

Title: Intraplate and interplate faulting interactions during the August 31, 2012, Philippine Trench earthquake (Mw 7.6) sequence

Authors: Lingling Ye and Thorne Lay: Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA;

Hiroo Kanamori: Seismological Laboratory, California Institute of Technology, Pasadena, California, USA.


Contact: Kate Ramsayer
American Geophysical Union

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