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AGU Research Spotlight (Jan 25-Jan 31, 2018)

2018-02-02 09:40:11

I. Climate Change

1. Critical Role of Grazing Animals in an Ecosystem

Scientists model the effect of horses, cattle, sheep, and goats on local environments and global climate.


2. Plotting the Pliocene Polar Front

Understanding changing conditions in the south polar oceans during the warm late Pliocene period may help predict the impact of contemporary warming.


II. Hazards & Disasters

1. Were Mexico’s September Quakes Chance or a Chain Reaction?

Last year, two major earthquakes—one 12 days after the first—shook Mexico. New analysis blames this very unlikely event on chance. But one of the pair may have triggered a third large nearby temblor.


2. Mapping Extreme-Value Geoelectric Fields

To help mitigate magnetic storm interference on electric power grid operations, extreme-value geoelectric fields have been mapped across the mid-Atlantic United States.


III. Science Policy

1. White House Announces Choice to Head U.S. Geological Survey

Observers said the nominee appears to have strong credentials but voiced uncertainty about his management qualifications and noted the challenge of maintaining the agency’s strengths.


2. Report Recommends Priorities for Earth Observations from Space

The new strategic plan, which builds on earlier road maps for observing the planet, identifies top targets for satellite scrutiny for the next decade.


IV. Global Change

1. New Estimates of Ozone Transport in Extratropical Cyclones

Cross-tropopause ozone transport in midlatitude cyclones, coincident with dry air intrusions, is derived from satellite and reanalysis data organized in cyclone-centric coordinates.


V. Geology & Geophysics

1. How Offshore Groundwater Shapes the Seafloor

The MARCAN project, launched last January, is working to fill a gap in our knowledge of how freshwater flowing underground shapes and alters the continental margins.


2. Dropping the Beat with Some Geoscience Data

Listen to the music of the ice, the turning of the planets, the ringing of rings, the rockin’ of quakes, and the mournful tones of global warming.


3. Computational Seismology Workshop Trains Early-Career Scientists

2017 CIG–LLNL Computational Seismology Workshop; Livermore, California, 18–22 September 2017


VI. Geophysical Research Letters

1. Prompt Disappearance and Emergence of Radiation Belt Magnetosonic Waves Induced by Solar Wind Dynamic Pressure Variations

Magnetosonic waves are highly oblique whistler mode emissions transferring energy from the ring current protons to the radiation belt electrons in the inner magnetosphere. Here we present the first report of prompt disappearance and emergence of magnetosonic waves induced by the solar wind dynamic pressure variations. The solar wind dynamic pressure reduction caused the magnetosphere expansion, adiabatically decelerated the ring current protons for the Bernstein mode instability, and produced the prompt disappearance of magnetosonic waves. On the contrary, because of the adiabatic acceleration of the ring current protons by the solar wind dynamic pressure enhancement, magnetosonic waves emerged suddenly. In the absence of impulsive injections of hot protons, magnetosonic waves were observable even only during the time period with the enhanced solar wind dynamic pressure. Our results demonstrate that the solar wind dynamic pressure is an essential parameter for modeling of magnetosonic waves and their effect on the radiation belt electrons.


2. Contributions of Greenhouse Gas Forcing and the Southern Annular Mode to Historical Southern Ocean Surface Temperature Trends

We examine the 1979–2014 Southern Ocean (SO) sea surface temperature (SST) trends simulated in an ensemble of coupled general circulation models and evaluate possible causes of the models' inability to reproduce the observed 1979–2014 SO cooling. For each model we estimate the response of SO SST to step changes in greenhouse gas (GHG) forcing and in the seasonal indices of the Southern Annular Mode (SAM). Using these step-response functions, we skillfully reconstruct the models' 1979–2014 SO SST trends. Consistent with the seasonal signature of the Antarctic ozone hole and the seasonality of SO stratification, the summer and fall SAM exert a large impact on the simulated SO SST trends. We further identify conditions that favor multidecadal SO cooling: (1) a weak SO warming response to GHG forcing, (2) a strong multidecadal SO cooling response to a positive SAM trend, and (3) a historical SAM trend as strong as in observations.


3. Large-Amplitude Extremely Low Frequency Hiss Waves in Plasmaspheric Plumes

Whistler-mode extremely low frequency hiss emissions commonly exist in the plasmasphere and the plasmaspheric plume and contribute to the precipitation loss of the radiation belt electrons. How these hiss waves are generated remains a critical unanswered question. Here we report the large-amplitude (up to 1.5 nT) hiss waves in the plasmaspheric plumes, nearly an order of magnitude stronger than previous observations. These waves are found to propagate toward higher latitudes, and the corresponding frequency dependence of wave power can be qualitatively (but not quantitatively) explained by the modeled linear instability of hot electrons near the equator. At the high-frequency end of hiss spectra, the discrete rising tones are shown to emerge, similar to the situation of whistler-mode chorus in the plasmatrough. These data and modeling suggest that these large-amplitude hiss waves were generated within the plasmaspheric plume probably through a combination of linear and nonlinear instabilities of hot electrons.


4. Rate of Mass Loss Across the Instability Threshold for Thwaites Glacier Determines Rate of Mass Loss for Entire Basin

Rapid change now underway on Thwaites Glacier (TG) raises concern that a threshold for unstoppable grounding line retreat has been or is about to be crossed. We use a high-resolution ice sheet model to examine the mechanics of TG self-sustained retreat by nudging the grounding line just past the point of instability. We find that by modifying surface slope in the region of the grounding line, the rate of the forcing dictates the rate of retreat, even after the external forcing is removed. Grounding line retreats that begin faster proceed more rapidly because the shorter time interval for the grounding line to erode into the grounded ice sheet means relatively thicker ice and larger driving stress upstream of the boundary. Retreat is sensitive to short-duration re-advances associated with reduced external forcing where the bathymetry allows regrounding, even when an instability is invoked.


5. Glacial Cycles Influence Marine Methane Hydrate Formation

Methane hydrates in fine-grained continental slope sediments often occupy isolated depth intervals surrounded by hydrate-free sediments. As

they are not connected to deep gas sources, these hydrate deposits have been interpreted as sourced by in situ microbial methane. We

investigate here the hypothesis that these isolated hydrate accumulations form preferentially in sediments deposited during Pleistocene glacial lowstands that contain relatively large amounts of labile particulate organic carbon, leading to enhanced microbial methanogenesis. To test this hypothesis, we apply an advection-diffusion-reaction model with a time-dependent organic carbon deposition controlled by glacioeustatic sea


variations. In the model, hydrate forms in sediments with greater organic carbon content deposited during the penultimate glacial cycle (~120–240 ka). The model predictions match hydrate-bearing intervals detected in three sites drilled on the northern Gulf of Mexico continental slope,

supporting the hypothesis of hydrate formation driven by enhanced organic carbon burial during glacial lowstands.


VII. AGU Blogs

1. Franz Josef Islands Separate due to Glacier Retreat

Hall Island (left) and Littrow Island (right) in 2002 and 2017 Landsat images.  The islands are connected by glacier in 2002 between the black

arrows.  The blue arrows indicate glacier flow.  In 2017 the glacier connection has failed and Nordenskjold Strait has formed.


2. Researchers find way to give advanced notice for hailstorms

A new study identifies a method for predicting the likelihood of damaging hailstorms in the United States—up to three weeks in advance.


3. New study reveals how icy surface ponds on Himalayan glaciers influence water flow

The flow of water that supports hydro-electric and irrigation infrastructure in the mountain regions of Nepal and India is regulated by hundreds

of large icy ponds on the surface of some of the world’s highest glaciers, scientists have revealed.



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