AGU 期刊一周Research Spotlights (Sep 1~Sep 7, 2016)

发布时间:2016-9-8 15:48:21 点击次数:611

I. Atmospheric Sciences

1. Mysterious Anomaly Interrupts Stratospheric Wind Pattern

For the first time, scientists have observed a deviation from the typical alternating pattern of easterly and westerly winds in the equatorial stratosphere.

II. Geology & Geophysics

1. New AGU Congressional Science Fellows Take the Hill

This year's fellows hope to use their unique expertise to guide policy makers and shape legislation on pressing issues, such as climate warming and resource management.

2. Scientific Study Group Favors Recognizing Human-Influenced Epoch

A formal proposal could take 3–4 years to prepare and then would require evaluation and approval by other scientists.

3. New Digital Maps Depict Alaska in Unprecedented Detail

The Obama administration plans to release high-resolution terrain models in 2017 for the entire Arctic.

III.Hydrology, Cryosphere & Earth Surface

1. The Role of Seafloor Methane in Ancient Global Warming

New research suggests that release of methane from seafloor hydrates was much slower than hypothesized during a period of rapid global warming about 56 million years ago.

IV. Climate Change

1. Exploring Formal Recognition of the Anthropocene

Colin Waters of the Anthropocene Working Group, which has been exploring formal recognition of the Anthropocene as a unit in the geological time scale, discussed the group's recommendations at the IGC.

2. Sargassum Watch Warns of Incoming Seaweed

The Sargassum Watch System processes satellite data and feeds results to a Web portal, giving decision makers timely information on seaweed location and warnings for potential beaching events.

3. Subterranean Caverns Hold Clues to Past Droughts

Cave formations offer highly resolved paleoclimate data that scientists plan to use to reconstruct California's ancient patterns of drought.

V. Space & Planets

1. New Findings Suggest Dwarf Planet Ceres Is Geologically Active

Cryovolcanoes, landslides, and water ice all point to current activity, researchers found.

2. Delayed Launch Approved for Next Mars Mission

NASA has set a new 2018 launch date for a spacecraft to probe the Red Planet's interior, after instrument failure hobbled preparations for the mission.

VI. Earth and Space Science

1. A Tale of Atlin Ophiolite

This is the first in a series of dispatches from Rebecca Fowler, a science writer documenting the work of scientists conducting fieldwork at the Atlin ophiolite in British Columbia.

2. An Arctic fixation

Nitrogen fixation has always been thought to require warmth and a lot of light. But it turns out that’s not true. Surprisingly, nitrogen fixation is happening in the cold waters of the Chukchi Sea. Researchers found this to be the case during an Arctic research cruise in the summer of 2011. Is this a new phenomenon, or just newly noticed?

3. Sulfur, sulfur dioxide and graphitized carbon observed on asteroid for first time

The discoveries were made by comparing Ceres’ ultraviolet-visible spectra to laboratory measurements. The new HST observations are complementary to observations being made by instruments on the Dawn spacecraft in orbit at Ceres, covering additional wavelengths. The presence of graphitized carbon is consistent with weathering of carbonaceous material on the asteroid’s surface, caused by processes such as charged particle bombardment, according to the study authors.

4. Extreme-weather winters are becoming more common in U.S., research shows

The simultaneous occurrence of warm winters in the West and cold winters in the East has significantly increased in recent decades. The damaging and costly phenomenon is very likely attributable to human-caused climate change, according to a new study.

VII. Geophysical Research Letters

1. A physically based analytical model of flood frequency curves

Predicting magnitude and frequency of floods is a key issue in hydrology, with implications in many fields ranging from river science and geomorphology to the insurance industry. In this paper, a novel physically based approach is proposed to estimate the recurrence intervals of seasonal flow maxima. The method links the extremal distribution of streamflows to the stochastic dynamics of daily discharge, providing an analytical expression of the seasonal flood frequency curve. The parameters involved in the formulation embody climate and landscape attributes of the contributing catchment and can be estimated from daily rainfall and streamflow data. Only one parameter, which is linked to the antecedent wetness condition in the watershed, needs to be calibrated on the observed maxima. The performance of the method is discussed through a set of applications in four rivers featuring heterogeneous daily flow regimes. The model provides reliable estimates of seasonal maximum flows in different climatic settings and is able to capture diverse shapes of flood frequency curves emerging in erratic and persistent flow regimes. The proposed method exploits experimental information on the full range of discharges experienced by rivers. As a consequence, model performances do not deteriorate when the magnitude of events with return times longer than the available sample size is estimated. The approach provides a framework for the prediction of floods based on short data series of rainfall and daily streamflows that may be especially valuable in data scarce regions of the world.

2. Performance of the NCREE's on-site warning system during the 5 February 2016 Mw 6.53 Meinong earthquake

The National Center for Research on Earthquake Engineering in Taiwan has developed an on-site earthquake early warning system (NEEWS). The Meinong earthquake with a moment magnitude of 6.53 and a focal depth of 14.6 km occurred on 5 February 2016 in southern Taiwan. It caused 117 deaths, injured 551, caused the collapse of six buildings, and serious damage to 247 buildings. During the Meinong earthquake, the system performance of 16 NEEWS stations was recorded. Based on a preassigned peak ground acceleration (PGA) threshold to issue alarms at different stations, no false alarms or missed alarms were issued during the earthquake. About 4 s to 33 s of lead time were provided by the NEEWS depending on the epicenter distance. In addition, the directivity of the earthquake source characteristic and also possibly the site effects were observed in the diagram of the distribution of PGA difference between the predicted PGA and the measured PGA.

3. Impact of the 2008 Global Recession on air quality over the United States: Implications for surface ozone levels from changes in NOx emissions

Satellite and ground observations detected large variability in nitrogen oxides (NOx) during the 2008 economic recession, but the impact of the recession on air quality has not been quantified. This study combines observed NOx trends and a regional chemical transport model to quantify the impact of the recession on surface ozone (O3) levels over the continental United States. The impact is quantified by simulating O3 concentrations under two emission scenarios: business-as-usual (BAU) and recession. In the BAU case, the emission projection from the Cross-State Air Pollution Rule is used to estimate the “would-be” NOx emission level in 2011. In the recession case, the actual NO2 trends observed from Air Quality System ground monitors and the Ozone Monitoring Instrument on the Aura satellite are used to obtain “realistic” changes in NOx emissions. The model prediction with the recession effect agrees better with ground O3 observations over time and space than the prediction with the BAU emission. The results show that the recession caused a 1–2 ppbv decrease in surface O3 concentration over the eastern United States, a slight increase (0.5–1 ppbv) over the Rocky Mountain region, and mixed changes in the Pacific West. The gain in air quality benefits during the recession, however, could be quickly offset by the much slower emission reduction rate during the post-recession period.

4. A probabilistic study of the return of stratospheric ozone to 1960 levels

Anthropogenic emissions of greenhouse gases and ozone-depleting substances are expected to continue to affect concentrations of ozone in the stratosphere through the 21st century. While a range of estimates for when stratospheric ozone is expected to return to unperturbed levels is available in the literature, quantification of the spread in results is sparse. Here we present the first probabilistic study of latitudinally resolved years of return of stratospheric ozone to 1960 levels. Results from our 180-member ensemble, simulated with a newly developed simple climate model, suggest that the spread in return years of ozone is largest around 40°N/S and in the southern high latitudes and decreases with increasing greenhouse gas emissions. The spread in projections of ozone is larger for higher greenhouse gas scenarios and is larger in the polar regions than in the midlatitudes, while the spread in ozone radiative forcing is smallest in the polar regions.

5. Eddies reduce denitrification and compress habitats in the Arabian Sea

The combination of high biological production and weak oceanic ventilation in regions, such as the northern Indian Ocean and the eastern Pacific and Atlantic, cause large-scale oxygen minimum zones (OMZs) that profoundly affect marine habitats and alter key biogeochemical cycles. Here we investigate the effects of eddies on the Arabian Sea OMZ—the world's thickest—using a suite of regional model simulations with increasing horizontal resolution. We find that isopycnal eddy transport of oxygen to the OMZ region limits the extent of suboxia so reducing denitrification, increasing the supply of nitrate to the surface, and thereby enhancing biological production. That same enhanced production generates more organic matter in the water column, amplifying oxygen consumption below the euphotic zone, thus increasing the extent of hypoxia. Eddy-driven ventilation likely plays a similar role in other low-oxygen regions and thus may be crucial in shaping marine habitats and modulating the large-scale marine nitrogen cycle.

6. Observations of large-amplitude, parallel, electrostatic waves associated with the Kelvin-Helmholtz instability by the magnetospheric multiscale mission

On 8 September 2015, the four Magnetospheric Multiscale spacecraft encountered a Kelvin-Helmholtz unstable magnetopause near the dusk flank. The spacecraft observed periodic compressed current sheets, between which the plasma was turbulent. We present observations of large-amplitude (up to 100 mV/m) oscillations in the electric field. Because these oscillations are purely parallel to the background magnetic field, electrostatic, and below the ion plasma frequency, they are likely to be ion acoustic-like waves. These waves are observed in a turbulent plasma where multiple particle populations are intermittently mixed, including cold electrons with energies less than 10 eV. Stability analysis suggests a cold electron component is necessary for wave growth.

7. Unraveling the excitation mechanisms of highly oblique lower band chorus waves

Excitation mechanisms of highly oblique, quasi-electrostatic lower band chorus waves are investigated using Van Allen Probes observations near the equator of the Earth's magnetosphere. Linear growth rates are evaluated based on in situ, measured electron velocity distributions and plasma conditions and compared with simultaneously observed wave frequency spectra and wave normal angles. Accordingly, two distinct excitation mechanisms of highly oblique lower band chorus have been clearly identified for the first time. The first mechanism relies on cyclotron resonance with electrons possessing both a realistic temperature anisotropy at keV energies and a plateau at 100–500 eV in the parallel velocity distribution. The second mechanism corresponds to Landau resonance with a 100–500 eV beam. In both cases, a small low-energy beam-like component is necessary for suppressing an otherwise dominating Landau damping. Our new findings suggest that small variations in the electron distribution could have important impacts on energetic electron dynamics.