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AGU Research Spotlight (Apr 26-May 02, 2018)

2018-05-03 22:22:39

I. Climate Change

1. Interactive Online Maps Make Satellite Ocean Data Accessible

A new online resource from the National Oceanic and Atmospheric Administration provides an interactive view of global satellite ocean color and true-color imagery.

https://eos.org/project-updates/interactive-online-maps-make-satellite-ocean-data-accessible

2. Will Cape Town Escape Its Water-Starved Fate?

Despite ramped-up conservation efforts and hopes lifted by a few recent rainstorms, residents of the South African metropolis still face the possibility of a water doomsday.

https://eos.org/articles/will-cape-town-escape-its-water-starved-fate

II. Hazards & Disasters

1. Understanding the Effects of Anthropogenic Space Weather

A large plasma hole generated by the vertical launch of the Formosat-5 satellite created temporary navigating and positioning errors of up to 1 meter, according to a new study.

https://eos.org/research-spotlights/understanding-the-effects-of-anthropogenic-space-weather

2. How Space Storms Affect the Satellite Superhighway

A powerful numerical model reveals how space weather disturbs magnetic field at geosynchronous orbit.

https://eos.org/research-spotlights/how-space-storms-affect-the-satellite-superhighway

III. Biogeosciences

1. Getting Littoral with Lake Carbon Efflux

Next generation forced diffusion chambers reveal dynamic environment for lake carbon exchange with distance from shoreline.

https://eos.org/editor-highlights/getting-littoral-with-lake-carbon-efflux

2. Winter Wave Heights, Extreme Storms Rising in Western Europe

The findings are important for scientists and coastal managers looking to predict future wave heights and take measures to protect coastal communities across Western Europe.

https://eos.org/scientific-press/winter-wave-heights-extreme-storms-rising-in-western-europe

IV. Geology & Geophysics

1. Harnessing Remote Infrasound to Study Volcanic Eruptions

Data from the 2015 eruption of Chile’s Calbuco volcano suggest the international network built to monitor nuclear explosions may also be used to detect and characterize volcanic activity.

https://eos.org/research-spotlights/harnessing-remote-infrasound-to-study-volcanic-eruptions

2. A Complete Picture of Southern Ocean Surface Circulation

For the first time, researchers combine estimates of sea surface height and circulation patterns in both ice-covered and ice-free regions of the Southern Ocean.

https://eos.org/research-spotlights/a-complete-picture-of-southern-ocean-surface-circulation

3. New Simulation Supports Chicxulub Impact Scenario

Mountains ringing the center of Earth's most famous impact crater consist of porous rocks. Computer models of the impact can now predict those rocks’ microstructure.

https://eos.org/articles/new-simulation-supports-chicxulub-impact-scenario

4. New Program Enables Scientists to Be Voices for Science

The American Geophysical Union has created a network of 30 individuals from diverse disciplines who will advocate for science in their congressional districts.

https://eos.org/agu-news/new-program-enables-scientists-to-be-voices-for-science

V. Science Policy & Funding

1. Focusing on Leadership to Improve Diversity in the Geosciences

Sparks for Change Institute; Boulder, Colorado, 18–20 September 2017

https://eos.org/meeting-reports/focusing-on-leadership-to-improve-diversity-in-the-geosciences

VI. Geophysical Research Letters

1. On Estimating Turbulent Reynolds Stress in Wavy Aquatic Environment

Several methods have been developed for the estimation of the turbulent Reynolds stress in wavy aquatic environment. They are based on different physical assumptions and often give discrepant results. It is practically difficult to quantify the uncertainties in these estimations. Using high‐resolution velocity measurements of acoustic Doppler velocimeter (ADV) from a coastal benthic layer subject to moderate wave influence (the ratio of rms wave orbital velocity to current magnitude was 0.23–0.92), this study tests a Synchrosqueezed Wavelet Transform (SWT)‐based method and three existing methods (i.e., the Coherence, Cospectra, and Ensemble Empirical Mode Decomposition [EEMD] methods) for wave‐turbulence decomposition. In particular, we evaluate the performance of different methods for objective estimation of the turbulent Reynolds stress. Power spectra and cospectra analysis is conducted to quantify the uncertainties in the estimations. The results suggest that the Coherence method tends to overestimate the Reynolds stress due to incomplete removal of wave motions from the observed velocity records; the Cospectra method performs poorly because the empirical model does not fit the observed cospectra well; both the EEMD and SWT methods underestimate the Reynolds stress, as they tend to attribute turbulent fluctuations at frequencies in the vicinity of the wave frequencies to wave motions. In general, the SWT method performs best inducing lowest uncertainty in the Reynolds stress estimation. For the data set analyzed in this study, the estimations with the Coherence, Cospectra, EEMD, and SWT methods account for 70%, 50%, 51%, and 60% of the total covariance of horizontal and vertical velocities, respectively.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017JC013230

2. Generation of Near‐Inertial Currents on the Mid‐Atlantic Bight by Hurricane Arthur (2014)

Near‐inertial currents (NICs) were observed on the Mid‐Atlantic Bight (MAB) during the passage of Hurricane Arthur (2014). High‐frequency radars showed that the surface currents were weak near the coast but increased in the offshore direction. The NICs were damped out in 3–4 days in the southern MAB but persisted for up to 10 days in the northern MAB. A Slocum glider deployed on the shelf recorded two‐layer baroclinic currents oscillating at the inertial frequency. A numerical model was developed to interpret the observed spatial and temporal variabilities of the NICs and their vertical modal structure. Energy budget analysis showed that most of the differences in the NICs between the shelf and the deep ocean were determined by the spatial variations in wind energy input. In the southern MAB, energy dissipation quickly balanced the wind energy input, causing a rapid damping of the NICs.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2017JC013584

3. Calibrated Seismic Imaging of Eddy‐Dominated Warm‐Water Transport Across the Bellingshausen Sea, Southern Ocean

Seismic reflection images of thermohaline circulation from the Bellingshausen Sea, adjacent to the West Antarctica Peninsula, were acquired during February 2015. This survey shows that bright reflectivity occurs throughout the upper 300 m. By calibrating these seismic images with coeval hydrographic measurements, intrusion of warm water features onto the continental shelf at Marguerite and Belgica Troughs is identified and characterized. These features have distinctive lens‐shaped patterns of reflectivity with lengths of 0.75–11.00 km and thicknesses of 100–150 m, suggesting that they are small mesoscale to submesoscale eddies. Abundant eddies are observed along a transect that crosses Belgica Trough. Near Alexander Island Drift, a large, of order  urn:x-wiley:21699275:media:jgrc22803:jgrc22803-math-0001 km3, bowl‐like feature, that may represent an anticyclonic Taylor column, is imaged on a pair of orthogonal images. A modified iterative procedure is used to convert seismic imagery into maps of temperature that enable the number and size of eddies being transported onto the shelf to be quantified. Finally, analysis of prestack shot records suggests that these eddies are advecting southward at speeds of  urn:x-wiley:21699275:media:jgrc22803:jgrc22803-math-0002 m s?1, consistent with limited legacy hydrographic measurements. Concentration of observed eddies south of the Southern Antarctic Circumpolar Current Front implies they represent both a dominant, and a long‐lived, mechanism of warm‐water transport, especially across Belgica Trough. Our observations suggest that previous estimates of eddy frequency may have been underestimated by up to 1 order of magnitude, which has significant implications for calculations of ice mass loss on the shelf of the West Antarctic Peninsula.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JC013833

4. Gulf Stream Transport and Mixing Processes via Coherent Structure Dynamics

The Gulf Stream has been characterized as either a barrier or blender to fluid transfer, a duality relevant to gyre‐scale climate adjustment. However, previous characterization depended on relatively sparse, Lagrangian in situ observations. The finite‐time Lyapunov exponent (FTLE) is calculated from satellite altimetry to identify Lagrangian coherent structures (LCS) in the Gulf Stream region. These LCS provide dense sampling of flow and capture distinct regions associated with mixing. Independent observations of ocean color contain similar flow‐dependent structures, providing verification of the method and highlighting transport and mixing processes that influence sea surface temperature and chlorophyll, among other water properties. Diagnosed LCS support the existing Bower kinematic model of the Gulf Stream, but also highlight novel behavior of comparable importance. These include vortex pinch‐off and formation of spiral eddies, clearly identified by LCS and which may be explained by considering changes to flow topology and the dynamics of shear‐flow instability at both small and large Rossby number. Such processes, seen though LCS, may further enable validation of climate models. The spatial distribution of these intermittent processes is characterized in terms of the criticality of jet dynamics with respect to Rossby wave propagation, and whether the jet is in an unstable or wave‐maker regime. The generation and connectivity of hyperbolic trajectories in the flow appears to play an important role in governing large‐scale transport and mixing across the Gulf Stream.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017JC013390

5. Effects of the Non‐breaking Surface Wave‐induced Vertical Mixing on Winter Mixed Layer Depth in Subtropical Regions

Compared to observations, the simulated multi‐model mean surface oceanic mixed layer depth (MLD) during winter in the subtropical regions of both hemispheres shows deep bias from 45 CMIP5 climate models. Our results from two numerical experiments using one of CMIP5 models show that the non‐breaking surface wave‐induced vertical mixing can serve as a remedy. The enhanced vertical mixing increases the heat content of the upper ocean and reduces the oceanic potential density in winter which then stabilize the upper ocean and shallow the simulated winter MLD in subtropical regions. This heat content increase is not directly induced by air‐sea heat fluxes during winter, but is the legacy of temperature increase during summer, when the additional vertical mixing induces an enhanced surface heating. The simulation biases of the annually averaged water temperatures in the upper 400 m reduced by 43% and 28% in south and north latitude bands between 20°and 40°, respectively. The non‐breaking surface wave‐induced vertical mixing shallows both boreal and austral winter MLDs by 2–11 m (a change of 5–20%) in both northern and southern subtropical regions. These results indicate that the incorporation of vertical mixing induced by the non‐breaking surface waves in our experiments can improve the simulation of boreal and austral winter MLDs in northern and southern subtropical regions.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017JC013038

6. Assessment of Export Efficiency Equations in the Southern Ocean Applied to Satellite‐Based Net Primary Production

Carbon export efficiency (e‐ratio) is defined as the fraction of organic carbon fixed through net primary production (NPP) that is exported out of the surface productive layer of the ocean. Recent observations for the Southern Ocean suggest a negative e‐ratio versus NPP relationship, and a reduced dependency of export efficiency on temperature, different than in the global domain. In this study, we complement information from a passive satellite sensor with novel space‐based lidar observations of ocean particulate backscattering to infer NPP over the entire annual cycle, and estimate Southern Ocean export rates from five different empirical models of export efficiency. Inferred Southern Ocean NPP falls within the range of previous studies, with a mean estimate of 15.8 (± 3.9) Pg C yr?1 for the region south of 30  urn:x-wiley:21699275:media:jgrc22785:jgrc22785-math-0001S during the 2005–2016 period. We find that an export efficiency model that accounts for silica(Si)‐ballasting, which is constrained by observations with a negative e‐ratio versus NPP relationship, shows the best agreement with in situ‐based estimates of annual net community production (annual export of 2.7?±?0.6 Pg C yr?1 south of 30  urn:x-wiley:21699275:media:jgrc22785:jgrc22785-math-0002S). By contrast, models based on the analysis of global observations with a positive e‐ratio versus NPP relationship predict annually integrated export rates that are ~ 33% higher than the Si‐dependent model. Our results suggest that accounting for Si‐induced ballasting is important for the estimation of carbon export in the Southern Ocean.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2018JC013787

VII. AGU Blogs

1. Old data, new tricks: Fresh results from NASA’s Galileo spacecraft 20 Years on

Far across the solar system, from where Earth appears merely as a pale blue dot, NASA’s Galileo spacecraft spent eight years orbiting Jupiter. During that time, the hearty spacecraft — slightly larger than a full-grown giraffe — sent back spates of discoveries on the gas giant’s moons, including the observation of a magnetic environment around Ganymede that was distinct from Jupiter’s own magnetic field. The mission ended in 2003, but newly resurrected data from Galileo’s first flyby of Ganymede is yielding new insights about the moon’s environment — which is unlike any other in the solar system.

https://blogs.agu.org/geospace/2018/05/01/old-data-new-tricks-fresh-results-from-nasas-galileo-spacecraft-20-years-nn/

2. Caijiazhuang village: a cracking-sliding landslide that has killed nine people

Yesterday I highlighted recently published research into the diurnal cycle and its impacts on cracking-sliding landslides on the Loess Plateau in China.  Ironically, the same day an example of such a landslide occurred, on this occasion at Caijiazhuang village in Lishi District of Lyuliang City, in Shanxi Province.  Xinhua has an unusually brief report on the landslide, which notes that.

https://blogs.agu.org/landslideblog/2018/05/01/caijiazhuang-1/

3. Satellites could further help mitigate river floods

Now, a new study in Geophysical Research Letters, a journal of the American Geophysical Union, analyzes what it would take for river-observing satellites to become an even more useful tool to mitigate flood damage and improve reservoir management globally in near real-time.

https://blogs.agu.org/geospace/2018/04/26/satellites-could-further-help-mitigate-river-floods/

4. New study finds glacial evidence for abrupt climate change

Results from a new study indicate that the physical impact of abrupt climate change in Britain, Ireland and maritime Europe may be markedly different from previous perceptions of these events.

https://blogs.agu.org/geospace/2018/04/26/new-study-finds-glacial-evidence-for-abrupt-climate-change/


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