AGU journal highlights -- Dec. 22, 2011

December 22, 2011

The following highlights summarize research papers that have been recently published in Journal of Geophysical Research-Atmospheres (JGR-D), Geophysical Research Letters (GRL), Journal of Geophysical Research- Planets (JGR-E), Journal of Geophysical Research-Earth Surface (JGR-F), and Geochemistry, Geophysics, Geosystems.

1. When will Antarctic ozone begin to recover?

Emissions of ozone-depleting substances have declined over recent decades, but it takes time for the ozone layer to recover. Regular measurements of ozone levels above the South Pole now stretch back 25 years. Hassler et al. analyze this recorded ozone data to assess changes in ozone loss rates. Consistent with previous studies, they find that ozone loss rates have been stable over the past 15 years, neither increasing nor decreasing. However, they predict that, assuming future atmospheric dynamics are similar to today's, ozone loss rates will begin to decline noticeably between 2017 and 2021.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1029/2011JD016353, 2011

Title: An assessment of changing ozone loss rates at South Pole: Twenty-five years of ozonesonde measurements

Authors: B. Hassler: Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado, USA and Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

J. S. Daniel: Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

B. J. Johnson: Global Monitoring Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

S. Solomon: Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA, and Department of Atmospheric and Oceanic Science, University of Colorado at Boulder, Boulder, Colorado, USA;

S. J. Oltmans: Global Monitoring Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA.

2. Preventing coral bleaching, one hurricane at a time

In recent decades, sea surface temperatures and the occurrence of heat stress in coral communities have soared. High surface water temperatures lead coral populations to evict their symbiotic, and colorful, algal residents. The photosynthesizing algae are what feed the coral, and the process-known as bleaching-can eventually kill it, leaving parched white exoskeletons in place of formerly vibrant reefs. However, not all coral reefs seem equally affected by mass bleaching at the hands of global warming. Some processes, like deep water upwelling, are known to offset rising temperatures locally, but Carrigan and Puotinen investigate a novel mechanism that they suggest may be responsible for protecting some susceptible populations.

Tropical cyclones (TCs) induce ocean mixing. Their strong winds whisk heat away from the sea surface, cooling surface temperatures by up to 6 degrees Celsius (10.8 degrees Fahrenheit) in an area typically spanning hundreds of kilometers from the eye of the storm. Though the strong waves associated with TCs are known to damage coral reefs, the extent of the cooling effect far exceeds the localized damage. Using historical TC storm tracks and the National Oceanic and Atmospheric Administration's Coral Reef Watch's records of thermal stress from 1985 to 2009, the authors analyze whether or not the cooling effect of TCs could temporarily alleviate escalating sea surface temperatures, staving off coral bleaching. At the basin scale, they find that TCs play a significant role in mitigating thermal stress for coral reefs in the North Atlantic. Further, their analysis suggests that TCs are likely important for the Great Barrier Reef, along with coral communities in western Australia, Japan, and the southwest Indian Ocean, though the spatial and temporal resolution of their model is not detailed enough to make a definitive statement. The authors note that their investigation only considered the effect of TCs on reef ecosystems that were already experiencing thermal stress. They raise the possibility that cyclones could play a preventative role, cooling the ocean waters before the corals' heat threshold is exceeded.

Source: Geophysical Research Letters, doi: 10.1029/2011GL049722, 2011

Title: Assessing the potential for tropical cyclone induced sea surface cooling to reduce thermal stress on the world's coral reefs

Authors: A. D. Carrigan and M. L. Puotinen: Institute for Conservation Biology and Environmental Management and School of Earth and Environmental Sciences, University of Wollongong, New South Wales, Australia and School of Earth Sciences, Ohio State University, Ohio, USA.

3. Gravity's effect on landslides: A strike against Martian water

A pile of sand, gravel, or other granular material takes on a familiar conical shape, with the slope of the pile's walls coming to rest at the static angle of repose. If the material exceeds this angle, it will trigger an avalanche, tumbling down until it comes to rest at the dynamic angle of repose. Static angles of repose for coarse, angular materials tend to be around 40° from the horizontal, while smooth grains are stable up to 20°. As largely a matter of geometry, grain properties, and internal friction, scientists have assumed these two angles of repose are fixed for a given substance. Observations of the angles of gully walls on Mars, found to be too shallow for the materials involved, have been used to argue that surface water must have played a part, either lubricating landslides or depositing the material directly. But research by Kleinhans et al., using the parabolic flight of an airplane to test the effect of gravity on angles of repose, demonstrates that water need not have been present.

As the plane followed its roller coaster style path, slowly rotating cylinders containing different materials experienced one tenth of Earth's gravity (0.1 g), Martian gravity (0.38 g) and the Earth's normal pull (1 g). The authors find that at 0.1 g, the static angle of repose for all materials increases by 5°, while the dynamic angle of repose decreases by 10°. They suggest weaker gravity would reduce internal friction for avalanching material and could explain the shallow gully walls on the Martian surface. Further, as angles of repose are commonly used as measures of material properties, this challenge to their presumed gravity independence will require a reassessment of many other surface processes at lower slopes.

Source: Journal of Geophysical Research-Planets, doi:10.1029/2011JE003865, 2011

Title: Static and dynamic angles of repose in loose granular materials under reduced gravity

Authors: M.G. Kleinhans and H. Markies: Faculty of Geosciences, Utrecht University, Utrecht, Netherlands;

S.J. de Vet: Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, Netherlands;

A.C. in 't Veld and F.N. Postema: Faculty of Aerospace Engineering, Delft University of Technology, Delft, Netherlands.

4. Estimating the destruction caused by remote rockslides

During the spring of 1991, near Randa, Switzerland, two subsequent landslides dropped 30,000,000 cubic meters (39,000,000 cubic yards) of debris on the town below. The rocks dammed the Vispa River, a temporary reservoir that would have failed catastrophically had the army not carved a channel through it. Many rock slides occur in remote alpine locations, so it can sometimes take days or weeks before they are detected, a delay that could have cost the town of Randa. Rock slides can range from deadly, to disruptive, to simple scientific curiosities, and Dammeier et al. have developed a method to remotely estimate their volume, location, and runout distances that can potentially be used in real time.

The authors analyzed the seismic readings of 20 rockslides that occurred in the Swiss Alps during the past 15 years and were recorded on Switzerland's operational seismic detection network. They assessed the seismograms' peak velocities, durations, and other properties and compared them against records of the rockslides' volumes, runout distances, potential energies, and failure mechanisms. From the analysis the authors developed an algorithm that they used to accurately estimate the properties of four separate rockslides. They find that by using the seismic readings they could pinpoint the location of a rockslide to within 11 kilometers (6.8 miles) and could reliably peg the order of magnitude for volume, energy, and other parameters. The authors note that their algorithm is likely location specific but that with a decent set of reference observations it could be trained for use in other rockslide susceptible regions.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1029/2011JF002037, 2011

Title: Characterization of alpine rockslides using statistical analysis of seismic signals

Authors: Franziska Dammeier, Jeffrey R. Moore, and Simon Loew: Department of Earth Sciences, Swiss Federal Institute of Technology, Zurich, Switzerland;

Florian Haslinger: Swiss Seismological Service, Swiss Federal Institute of Technology, Zurich, Switzerland.

5. Continued volcanic activity causes ground uplift in Oregon Cascades

As early as the summer of 1996, a 20 x 30 kilometer (12.4 x 18.6 mile) patch of Earth lying just west of the South Sister Volcano started to rise. Joined by the North and Middle Sisters, the Three Sisters volcanoes are the most prominent peaks in the Central Oregon stretch of the Cascade Mountains, a landscape littered with the remnants of volcanic activity. Though there has not been an eruption in the region in at least 1,200 years, the detected deformations presented a cause for concern and the region was put under continuous monitoring. Riddick and Schmidt, continuing the work initiated by other researchers, report on 14 years of satellite-based monitoring, describing the variable rate of the ground's movements and the likely cause of the activity-a sizeable magma intrusion lying 5-7 km (3-4 miles) underground.

Drawing data from the European Space Agency's European Remote Sensing (ERS) and Envisat radar satellites, the authors find that the terrain deformation went through three distinct phases since its onset. From 1996 to 1998, the ground rose by 1 centimeter per year (0.4 inches per year). The uplift rate rose to 3-4 cm per year (1.2-1.6 inches per year) between 1998 and 2004, then declined to only a few millimeters per year for the rest of the decade, resulting in a total of 25 cm (9.8 inches) of uplift to date. Analyzing the topographic changes led the researchers to suggest that the previously hypothesized magmatic intrusion had a volume between 50 million and 70 million cubic meters (65 million and 92 million cubic yards). Whether the uplift is the indication of an imminent eruption depends on whether it is a stand-alone event or part of a series of similar intrusions, a question that can only be answered through continued monitoring, the researchers say.

Source: Geochemistry, Geophysics, Geosystems, doi:10.1029/2011GC003826, 2011

Title: Time-dependent changes in volcanic inflation rate near Three Sisters, Oregon, revealed by InSAR

Authors: S. N. Riddick and D. A. Schmidt: Department of Geological Sciences, University of Oregon, Oregon, USA.

6. Microseismicity could provide clues to sea ice

When an ocean wave swells, the sudden change in water column mass sends a pressure wave down to the ocean bottom. If the wave lasts long enough to strike the shore, its kinetic energy is transferred to the rock. Both processes induce microseismicity-low-powered seismic waves with periods between 1 and 20 seconds-which is picked up by seismic monitoring stations as regular background motions of the Earth. The observed amplitude of the microseismic signal varies with the distance from the source and shows pronounced seasonality, because big winter storms kick up larger waves. Since microseismicity depends on the formation and propagation of surface ocean waves, Tsai and McNamara wondered if sea surface ice may have an appreciable effect on the signal.

To find out, the authors built a simple computer model and pulled together twice- weekly observations of sea ice extent, along with the hourly readings of three seismic stations, for the Bering Sea. Of the three stations, two were surrounded by the seasonal encroach of sea ice, while the third lay too far south and served as a control. The authors found that changes in sea ice concentration could explain between 75 percent and 90 percent of the variability in the amplitude of the microseismicity for the two northern stations. The authors hope their model, whose analyses should be less costly and more frequent than current satellite-based techniques, will eventually allow for remote estimations of the concentration and strength of sea ice. Such information would have important applications in the shipping industry and could be valuable for climate change research.

Source: Geophysical Research Letters, doi:10.1029/2011GL049791, 2011

Title: Quantifying the influence of sea ice on ocean microseism using observations from the Bering Sea, Alaska

Authors: Victor C. Tsai: Seismological Laboratory, California Institute of Technology, Pasadena, California, USA and Geologic Hazards Science Center, U.S. Geological Survey, Golden, Colorado, USA;

Daniel E. McNamara: Geologic Hazards Science Center, U.S. Geological Survey, Golden, Colorado, USA.
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