pinecone/core-2020-05-10-deduplication huggingface dataset:
Estimating changes in ocean ventilation from early 1990s CFC-12 and late 2000s SF6 measurements

Transient tracer measurements can constrain the rates and pathways of ocean ventilation and act as proxies for biogeochemically relevant gases such as CO2 and oxygen. Various techniques have deduced changes in ocean ventilation over decadal timescales using transient tracer measurements made on repeat sections, but these require a priori assumptions about mixing in the ocean interior. Here, we introduce a simple, direct observational method that takes advantage of the similar atmospheric increase rates of chlorofluorocarbon-12 and sulfur hexafluoride, but with a time lag (offset) of 1415 years. Such repeat measurements can be directly compared without prior assumptions about mixing. A difference larger than similar to 2 years between modern sulfur hexafluoride and historical chlorofluorocarbon-12 tracer ages implies a change in ventilation, although lack of difference does not necessarily imply no change. Several tracer data sets are presented, which suggest changes in ventilation in the South Pacific and North Atlantic Oceans
Dense bottom layers in the Scotia Sea, Southern Ocean: Creation, lifespan, and destruction

The lower limb of the Atlantic overturning circulation is\ud
renewed by dense waters from the Southern Ocean, a\ud
substantial portion of which flow through the Scotia Sea.\ud
We report dense bottom layers here, with gradients in\ud
temperature and salinity comparable to those seen near the\ud
surface of the Southern Ocean. These are overlain by layers\ud
with much weaker stratification, and are caused by episodic\ud
overflows of dense waters across the South Scotia Ridge,\ud
and topographic trapping within deep trenches. One such\ud
layer was found to be at least 3–4 years older than the water immediately above. The estimated vertical diffusivity to which this layer was subject is substantially less than the strong basin-average deep mixing reported previously. We\ud
conjecture that (a) vertical mixing in the Scotia Sea is\ud
strongly spatially inhomogeneous, and (b) the flushing of\ud
these layers, like their formation, is related to overflow\ud
events, and hence also strongly episodic
Southern Ocean bottom water characteristics in CMIP5 models

Southern Ocean deep water properties and formation processes in climate models are indicative of their capability to simulate future climate, heat and carbon uptake, and sea level rise. Southern Ocean temperature and density averaged over 1986–2005 from 15 CMIP5 (Coupled Model Intercomparison Project Phase 5) climate models are compared with an observed climatology, focusing on bottom water. Bottom properties are reasonably accurate for half the models. Ten models create dense water on the Antarctic shelf, but it mixes with lighter water and is not exported as bottom water as in reality. Instead, most models create deep water by open ocean deep convection, a process occurring rarely in reality. Models with extensive deep convection are those with strong seasonality in sea ice. Optimum bottom properties occur in models with deep convection in the Weddell and Ross Gyres. Bottom Water formation processes are poorly represented in ocean models and are a key challenge for improving climate predictions
A novel method to test for significant trends in extreme values in serially dependent time series

We propose a novel method to investigate the statistical significance of trends of extreme values in serially correlated time series based on quantile regression and surrogate data. This method has the advantage over traditional extreme value methods that it takes into account all data points from the time series. We test this method on a temperature time series from the Antarctic Peninsula (Faraday/Vernadsky station), which is highly non-Gaussian and serially correlated. We find evidence for a significant upward nonlinear trend in the extreme cold temperatures (95th percentile) and that most of the observed warming at Faraday/Vernadsky is due to a reduction in cold extremes. Quantile regression can also be used for multivariate regression with external factors. This multivariate regression analysis suggests that CO 2 emissions play a large role in the observed trend at Faraday/Vernadsky while also the ozone hole and solar fluctuations play some role
Climate impacts of a large-scale biofuels expansion

A global biofuels program will potentially lead to intense pressures on land supply and cause widespread transformations in land use. These transformations can alter the Earth climate system by increasing greenhouse gas (GHG) emissions from land use changes and by changing the reflective and energy exchange characteristics of land ecosystems. Using an integrated assessment model that links an economic model with climate, terrestrial biogeochemistry, and biogeophysics models, we examined the biogeochemical and biogeophysical effects of possible land use changes from an expanded global second-generation bioenergy program on surface temperatures over the first half of the 21st century. Our integrated assessment model shows that land clearing, especially forest clearing, has two concurrent effects—increased GHG emissions, resulting in surface air warming; and large changes in the land's reflective and energy exchange characteristics, resulting in surface air warming in the tropics but cooling in temperate and polar regions. Overall, these biogeochemical and biogeophysical effects will only have a small impact on global mean surface temperature. However, the model projects regional patterns of enhanced surface air warming in the Amazon Basin and the eastern part of the Congo Basin. Therefore, global land use strategies that protect tropical forests could dramatically reduce air warming projected in these regions.United States. Dept. of Energ
Potential of Equatorial Atlantic Variability to Enhance El Nino Prediction

Extraordinarily strong El Niño events, such as those of 1982/83 and 1997/98, have been poorly predicted by operational seasonal forecasts made before boreal spring, despite significant advances in understanding, improved models, and enhanced observational networks. The Equatorial Atlantic Zonal Mode – a phenomenon similar to El Niño but much weaker and peaking in boreal summer – impacts winds over the Pacific, and hence affects El Niño, and also potentially its predictability. Here we use a climate model to perform a suite of seasonal predictions with and without SST in the Atlantic restored to observations. We show for the first time that knowledge of Equatorial Atlantic sea surface temperature (SST) significantly improves the prediction across boreal spring of major El Niño events and also weaker variability. This is because Atlantic SST acts to modulate El Niño variability, rather than triggering events. Our results suggest that better prediction of major El Niño events might be achieved through model improvement in the Equatorial Atlantic
Sustained retreat of the Pine Island Glacier

We use satellite observations to show that, between 1992 and 2011, the Pine Island Glacier hinge line retreated at a rate of 0.95 ± 0.09 km yr−1 despite a progressive steepening and shoaling of the glacier surface and bedrock slopes, respectively, which ought to impede retreat. The retreat has remained constant because the glacier terminus has thinned at an accelerating rate of 0.53 ± 0.15 m yr−2, with comparable changes upstream. This acceleration is consistent with an intensification of ocean-driven melting in the cavity beneath the floating section of the glacier. The pattern of hinge-line retreat meanders and is concentrated in isolated regions until ice becomes locally buoyant. Because the glacier-ocean system does not appear to have reached a position of relative stability, the lower limit of sea level projections may be too conservative
The response of atmospheric nitrous oxide to climate variations during the last glacial period

Detailed insight into natural variations of the greenhouse gas nitrous oxide (N2O) in response to changes in the Earth's climate system is provided by new measurements along the ice core of the North Greenland Ice Core Project (NGRIP). The presented record reaches from the early Holocene back into the previous interglacial with a mean time resolution of about 75 years. Between 11 and 120 kyr BP, atmospheric N2O concentrations react substantially to the last glacial-interglacial transition (Termination 1) and millennial time scale climate variations of the last glacial period. For long-lasting Dansgaard/Oeschger (DO) events, the N2O increase precedes Greenland temperature change by several hundred years with an increase rate of about 0.8-1.3 ppbv/century, which accelerates to about 3.8-10.7 ppbv/century at the time of the rapid warming in Greenland. Within each bundle of DO events, the new record further reveals particularly low N2O concentrations at the approximate time of Heinrich events. This suggests that the response of marine and/or terrestrial N2O emissions on a global scale are different for stadials with and without Heinrich events
Prevalence of strong bottom currents in the greater Agulhas system

Deep current meter data and output from two high-resolution global ocean circulation models are used to determine the prevalence and location of strong bottom currents in the greater Agulhas Current system. The two models and current meter data are remarkably consistent, showing that benthic storms, with bottom currents greater than 0.2 m s(-1), occur throughout the Agulhas retroflection region south of Africa more than 20% of the time. Furthermore, beneath the mean Agulhas Current core and the retroflection front, bottom currents exceed 0.2 m s(-1) more than 50% of the time, while away from strong surface currents, bottom currents rarely exceed 0.2 m s(-1). Implications for sediment transport are discussed and the results are compared to atmospheric storms. Benthic storms of this strength (0.2 m s(-1)) are comparable to a 9 m s(-1) (Beaufort 5) windstorm, but scaling shows that benthic storms may be less effective at lifting and transporting sediment than dust storms
Sprites in low-frequency radio noise

Low-frequency radio noise is the electromagnetic background radiation which is compared here to the luminosity of 39 sprites recorded with a low-light video camera. It is found that the sprite luminosities coincide with ∼10-30 ms long sudden enhancements of the electromagnetic background radiation ∼6-8 μV mHz(∼6-9 dB) with a relative maximum near ∼125 kHz as measured with a wideband (∼1-400 kHz) digital radio receiver. The sprites cluster in 10 groups of 2-5 consecutive sprites which are paralleled by up to ∼1 s long slowly varying enhancements of the electromagnetic background radiation ∼4-5 μV mHz (∼2-4 dB). The observed electric field strengths place an upper bound on the low-frequency radiation from the electron multiplication associated with the exponential growth and branching sprite streamers predicted by Qin et al. [2012a]. This upper bound corresponds to a maximum of ∼300-5000 sprite streamers at ∼40 km height above thunderclouds. Some part of the observed electromagnetic background radiation might result from the superposition of low-frequency radiation emanating from the quick succession of numerous horizontal lightning strokes and/or stepped leaders inside thunderclouds which would constitute a fundamentally novel quasi-static discharge process inside thunderclouds radiating slowly varying low frequency radio noise
The International Bathymetric Chart of the Southern Ocean (IBCSO) version 1.0 - a new bathymetric compilation covering circum-Antarctic waters

The International Bathymetric Chart of the Southern Ocean (IBCSO) Version 1.0 is a new digital bathymetric model (DBM) portraying the seafloor of the circum-Antarctic waters south of 60° S. IBCSO is a regional mapping project of the General Bathymetric Chart of the Oceans (GEBCO). IBCSO Version 1.0 DBM has been compiled from all available bathymetric data collectively gathered by more than 30 institutions from 15 countries. These data include multibeam and single beam echo soundings, digitized depths from nautical charts, regional bathymetric gridded compilations, and predicted bathymetry. Specific gridding techniques were applied to compile the DBM from the bathymetric data of different origin, spatial distribution, resolution, and quality. The IBCSO Version 1.0 DBM has a resolution of 500 x 500 m, based on a polar stereographic projection, and is publicly available together with a digital chart for printing from the project website (www.ibcso.org) and at http://dx.doi.org/10.1594/PANGAEA.805736
Effects of increased isopycnal diffusivity mimicking the unresolved equatorial intermediate current system in an earth system climate model

Earth system climate models generally underestimate dissolved oxygen concentrations in the deep eastern equatorial Pacific. This problem is associated with the "nutrient trapping" problem, described by Najjar et al. [1992], and is, at least partially, caused by a deficient representation of the Equatorial Intermediate Current System (EICS). Here we emulate the unresolved EICS in the UVic earth system climate model by locally increasing the zonal isopycnal diffusivity. An anisotropic diffusivity of ∼50,000 m 2 s-1 yields an improved global representation of temperature, salinity and oxygen. In addition, it (1) resolves most of the local "nutrient trapping" and associated oxygen deficit in the eastern equatorial Pacific and (2) reduces spurious zonal temperature gradients on isopycnals without affecting other physical metrics such as meridional overturning or air-sea heat fluxes. Finally, climate projections of low-oxygenated waters and associated denitrification change sign and apparently become more plausibl
Pleistocene loess in the humid subtropical forest zone of East Asia

Loess deposits in Asia have been used as indicators of palaeoclimate, because they are usually found bordering deserts. This paper reports extensive and thick deposits of loess in tropical southwest China, between latitudes 18 and 23°30′N, which is 1300 km south of known, and extensively researched loess deposits in north China. The present climate of the reported loess areas is hot and humid, with mean annual rainfall of 1000-2000 mm, and vegetation of subtropical evergreen broadleaf forest. This compares with less than 400 mm rainfall and vegetation of semi-desert steppe, in areas of current loess accumulation on desert margins in north China. The source area of the loess, which is dated by optical luminescence to the late Pleistocene, from 90-222 ka, is thought to be the exposed East Asian Shelf, which was up to 140 m below present sea level during Quaternary arid phases. Recent research on the nature of the shelf environment, and the relatively large particle size of the loess, suggests a local origin. The reported loess is not interbedded with palaeosols, and small amounts of soil cover the loess, compared with well developed soils in loess in semiarid regions in north China. This is explained by elimination of the supply source by sea level rise following each arid phase, as continued dust supply appears necessary for soil to form. This preliminary report of loess in southwest China conflicts with palynological evidence, and suggests that recent reconstructions of Pleistocene aridity in east Asia may be conservative.Department of Land Surveying and Geo-Informatics2012-2013 > Academic research: refereed > Publication in refereed journalpublished_fina
Linking permeability to crack density evolution in thermally stressed rocks under cyclic loading

Additional supporting information may be found in the online version of\ud
this article.To improve our understanding of the complex coupling between circulating fluids and the development of crack damage, we performed flow-through tests on samples of Etna basalt and Westerly granite that were cyclically loaded by deviatoric stresses. The basalt was naturally microfractured, while the relatively crack-free Westerly granite was thermally pretreated to 500°C and 800°C to generate microcrack damage. Samples were repeatedly loaded and then unloaded under deviatoric stress paths and ultimately to failure. Permeability and water volume content were measured throughout the loading history together with the differential stress. Permeability decreases at low differential stresses and increases at intermediate differential stresses up to a steady value at failure. We use water volume content as a proxy for fluid storage and show that both permeability and storage evolve with damage and evolution of crack density. We use crack models to represent the evolution of permeability as a function of loading state and are able to independently link it to the observed evolution of deformability, used as an independent measure of crack density.Funded by\ud
U.S. Department of Energy. Grant Number: DOE-DE-FG36-04GO14289\ud
National Science Foundation. Grant Numbers: EAR-0510182, EAR-0746192, OCE-0648331Peer-reviewedPublisher Versio
Heat transport through diffusive interfaces

The article of record as published may be found at http://dx.doi.org/10.1002/grl.50440We perform a series of 3-D Direct Numerical
Simulations (DNS) to assess the vertical heat transport
through thermohaline staircases in the Arctic Ocean. The
diagnostics of DNS, performed for the first time in the
realistic parameter range, result in vertical fluxes
exceeding those of extant “four-thirds flux laws” by as
much as a factor of 2 and suggest that the 4/3 exponent
may require downward revision. Through a series of
equivalent 2-D DNS, we show that they are consistent with
their more resource-intensive 3-D counterparts for
sufficiently large density ratio (Rr) but underestimate heat
transport for low Rr. Finally, we examine the role of
boundary conditions in controlling the vertical heat transport.
Rigid boundaries—a necessary ingredient in laboratoryderived
flux-laws—are shown to reduce the estimates of heat
fluxes relative to the corresponding periodic boundary
conditions
Why isolated streamer discharges hardly exist above the breakdown field
  in atmospheric air

We investigate streamer formation in the troposphere, in electric fields
above the breakdown threshold. With fully three-dimensional particle
simulations, we study the combined effect of natural background ionization and
of photoionization on the discharge morphology. In previous investigations
based on deterministic fluid models without background ionization, so-called
double-headed streamers emerged. But in our improved model, many electron
avalanches start to grow at different locations. Eventually the avalanches
collectively screen the electric field in the interior of the discharge. This
happens after what we call the `ionization screening time', for which we give
an analytical estimate. As this time is comparable to the streamer formation
time, we conclude that isolated streamers are unlikely to exist in fields well
above breakdown in atmospheric air.Comment: Changed citation information. 6 pages, 4 figures, Geophysical
  Research Letters, Vol. 40, 2417-2422, 201
A mechanism for sandbar straightening by oblique wave incidence

Breaker bars in the surf zone of sandy beaches generally evolve between straight bars parallel to the shore and meandering crescentic bars associated with intense (dangerous) currents flowing seaward through rip channels. Understanding the behavior of such systems is fundamental as they control the entire surf zone dynamics, the shape of the coastline, and the exchange of floating material with the shoreface. Although the mechanisms behind the meandering of an originally straight bar have been studied extensively, a clear physical explanation on the crescentic bar straightening was missing. Recent field observations have highlighted that this morphological reset can be due to wave obliquity. By using a two-dimensional horizontal morphological model, we show that the bar straightening by oblique waves occurs because the rip current is both weakened in intensity and shifted downdrift from the channel deepest section. The technique employed is useful for the study of other types of bed forms.Postprint (published version
Seismoelectric effects due to mesoscopic heterogeneities

While the seismic effects of wave-induced fluid flow due to mesoscopic
heterogeneities have been studied for several decades, the role played by these
types of heterogeneities on seismoelectric phenomena is largely unexplored. To
address this issue, we have developed a novel methodological framework which
allows for the coupling of wave-induced fluid flow, as inferred through
numerical oscillatory compressibility tests, with the pertinent seismoelectric
conversion mechanisms. Simulating the corresponding response of a
water-saturated sandstone sample containing mesoscopic fractures, we
demonstrate for the first time that these kinds of heterogeneities can produce
measurable seismoelectric signals under typical laboratory conditions. Given
that this phenomenon is sensitive to key hydraulic and mechanical properties,
we expect that the results of this pilot study will stimulate further
exploration on this topic in several domains of the Earth, environmental, and
engineering sciences.Comment: 14 pages, 3 figure
Rapid loss of firn pore space accelerates 21st century Greenland mass loss

Mass loss from the two major ice sheets and their contribution to global sea level rise is accelerating. In Antarctica, mass loss is dominated by increased flow velocities of outlet glaciers, following the thinning or disintegration of coastal ice shelves into which they flow. In contrast, ∼55% of post‒1992 Greenland ice sheet (GrIS) mass loss is accounted for by surface processes, notably increased meltwater runoff. A subtle process in the surface mass balance of the GrIS is the retention and refreezing of meltwater, currently preventing ∼40% of the meltwater to reach the ocean. Here we force a high‒resolution atmosphere/snow model with a mid‒range warming scenario (RCP4.5, 1970–2100), to show that rapid loss of firn pore space, by >50% at the end of the 21st century, quickly reduces this refreezing buffer. As a result, GrIS surface mass loss accelerates throughout the 21st century and its contribution to global sea level rise increases to 1.7 ±0.5 mm yr−1, more than four times the current value.Peer reviewe
Observations of core-mantle boundary Stoneley modes

Core-mantle boundary (CMB) Stoneley modes represent a unique class of normal modes with extremely strong sensitivity to wave speed and density variations in the D″ region. We measure splitting functions of 8 CMB Stoneley modes using modal spectra from 93 events with Mw > 7.4 between 1976–2011. The obtained splitting function maps correlate well with the predicted splitting calculated for S20RTS+Crust5.1 structure and the distribution of Sdiff and Pdiff travel time anomalies, suggesting that they are robust. We illustrate how our new CMB Stoneley mode splitting functions can be used to estimate density variations in the Earth's lowermost mantle