| 471. |
- Wynn, Jonathan, et al.
(författare)
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Processes of multibathyal aragonite undersaturation in the Arctic Ocean
- 2016
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Ingår i: Journal of Geophysical Research - Oceans. - 0148-0227 .- 2156-2202. ; 121:11, s. 8248-8267
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Tidskriftsartikel (refereegranskat)abstract
- During 3 years of study (2010–2012), the western Arctic Ocean was found to have unique aragonite saturation profiles with up to three distinct aragonite undersaturation zones. This complexity is produced as inflow of Atlantic-derived and Pacific-derived water masses mix with Arctic-derived waters, which are further modified by physiochemical and biological processes. The shallowest aragonite undersaturation zone, from the surface to ∼30 m depth is characterized by relatively low alkalinity and other dissolved ions. Besides local influence of biological processes on aragonite undersaturation of shallow coastal waters, the nature of this zone is consistent with dilution by sea-ice melt and invasion of anthropogenic CO2 from the atmosphere. A second undersaturated zone at ∼90–220 m depth (salinity ∼31.8–35.4) occurs within the Arctic Halocline and is characterized by elevated pCO2 and nutrients. The nature of this horizon is consistent with remineralization of organic matter on shallow continental shelves bordering the Canada Basin and the input of the nutrients and CO2 entrained by currents from the Pacific Inlet. Finally, the deepest aragonite undersaturation zone is at greater than 2000 m depth and is controlled by similar processes as deep aragonite saturation horizons in the Atlantic and Pacific Oceans. The comparatively shallow depth of this deepest aragonite saturation horizon in the Arctic is maintained by relatively low temperatures, and stable chemical composition. Understanding the mechanisms controlling the distribution of these aragonite undersaturation zones, and the time scales over which they operate will be crucial to refine predictive models.
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| 472. |
- Wåhlin, Anna, 1970, et al.
(författare)
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Influence of the coriolis force on the velocity structure of gravity currents in straight submarine channel systems
- 2011
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Ingår i: Journal of Geophysical Research C: Oceans. - 0148-0227. ; 115:11
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Tidskriftsartikel (refereegranskat)abstract
- Large-scale turbidity currents in submarine channels often show a significant asymmetry in the heights of their levee banks. In the Northern Hemisphere, there are many observations of the right-hand channel levee being noticeably higher than the left-hand levee, a phenomenon that is usually attributed to the effect of Coriolis forces upon turbidity currents. This article presents results from an analog model that documents the influence of Coriolis forces on the dynamics of gravity currents flowing in straight submarine channels. The observations of the transverse velocity structure, downstream velocity, and interface slope show good agreement with a theory that incorporates Ekman boundary layer dynamics. Coriolis forces will be important for most large-scale turbidity currents and need to be explicitly modeled when the Rossby number of these flows (defined as Ro = U/Wf, where U is the mean downstream velocity, W is the channel width, and f is the Coriolis parameter defined as f = 2Ω sin(θ), with Ω being the Earth's rotation rate and θ being the latitude) is less than order 1. When Ro ≪ 1, the flow is substantially slower than a nonrotating flow with the same density contrast. The secondary flow field consists of frictionally induced Ekman transports across the channel in the benthic and interfacial boundary layers and a return flow in the interior. The cross-channel velocities are of the order of 10% of the along-channel velocities. The sediment transport associated with such transverse flow patterns should influence the evolution of submarine channel levee systems.
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| 473. |
- Wästeby, N., et al.
(författare)
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Hydrochemical monitoring, petrological observation, and geochemical modeling of fault healing after an earthquake
- 2014
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Ingår i: Journal of Geophysical Research - Solid Earth. - 0148-0227 .- 2156-2202 .- 2169-9313 .- 2169-9356. ; 119:7, s. 5727-5740
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Tidskriftsartikel (refereegranskat)abstract
- Based on hydrochemical monitoring, petrological observations, and geochemical modeling, we identify a mechanism and estimate a time scale for fault healing after an earthquake. Hydrochemical monitoring of groundwater samples from an aquifer, which is at an approximate depth of 1200m, was conducted over a period of 10years. Groundwater samples have been taken from a borehole (HU-01) that crosses the Húsavík-Flatey Fault (HFF) near Húsavík town, northern Iceland. After 10weeks of sampling, on 16 September 2002, an M 5.8 earthquake occurred on the Grimsey Lineament, which is approximately parallel to the HFF. This earthquake caused rupturing of a hydrological barrier resulting in an influx of groundwater from a second aquifer, which was recorded by 15–20% concentration increases for some cations and anions. This was followed by hydrochemical recovery. Based on petrological observations of tectonically exhumed fault rocks, we conclude that hydrochemical recovery recorded fault healing by precipitation of secondary minerals along fractures. Because hydrochemical recovery accelerated with time, we conclude that the growth rate of these minerals was controlled by reaction rates at mineral-water interfaces. Geochemical modeling confirmed that the secondary minerals which formed along fractures were saturated in the sampled groundwater. Fault healing and therefore hydrochemical recovery was periodically interrupted by refracturing events. Supported by field and petrographic evidence, we conclude that these events were caused by changes of fluid pressure probably coupled with earthquakes. These events became successively smaller as groundwater flux decreased with time. Despite refracturing, hydrochemical recovery reached completion 8–10years after the earthquake.
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| 474. |
- Yamauchi, M, et al.
(författare)
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Dynamic response of the cusp morphology to the solar wind : A case study during passage of the solar wind plasma cloud on February 21, 1994
- 1996
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 101:A11, s. 24675-24687
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Tidskriftsartikel (refereegranskat)abstract
- On February 21, 1994, both Geotail and LMP 8 satellites detected an interplanetary plasma cloud with intense interplanetary magnetic field (IMF>50 nT) and high dynamic pressure (> 50 nPa). During this interval the Freja satellite detected intense cusp-like plasma injections in four out of six dayside traversals. The first two traversals are carefully studied, During the first traversal the overall morphology of the ion injection is characterized by a ''multiple-injection'' signature over a wide magnetic local time (MLT) range, whereas it is characterized by a ''single-injection'' signature with narrow injection region at 8 MLT in the second traversal, The solar wind conditions were also quite different between these two periods: while both dynamic and magnetic pressures stayed high during entire period, the dynamic beta was much higher during the first Freja traversal than during the second traversal. Between these two traversals, the cusp plasma injection is detected by the Sondre Stromfjord radar. The radar signature of the plasma injection is identified using the satellite particle data when the satellite and the radar were conjugate (the satellite's footprint was in the radar's field of view.) The cusp position and dynamics observed by the Sondre Stromfjord radar again show a very good correlation to the solar wind condition, especially to the dynamic pressure. The result indicates the following. (1) During southward IMF the cusp morphology differs for conditions of high or low solar wind dynamic pressure. High dynamic pressure widens the cusp (with multiple injections), whereas high magnetic pressure narrows it (with single injection), The effect of the IMF on the cusp locations and morphology becomes dominant only when the dynamic pressure is not very high, (2) Such a morphological difference reflects dynamic pressure more than dynamic beta during southward IMF at least during times of high solar wind dynamic pressure. (3) The cusp morphology responds very quickly to the changes in the solar wind conditions.
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| 475. |
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| 476. |
- Yang, Junhua, et al.
(författare)
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Modeling the origin of anthropogenic black carbon and its climatic effect over the Tibetan Plateau and surrounding regions
- 2018
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 123:2, s. 671-692
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Tidskriftsartikel (refereegranskat)abstract
- Black carbon (BC) in snow/ice induces enhanced snow and glacier melting. As over 60% of atmospheric BC is emitted from anthropogenic sources, which directly impact the distribution and concentration of BC in snow/ice, it is essential to assess the origin of anthropogenic BC transported to the Tibetan Plateau (TP) where there are few direct emissions attributable to local human activities. In this study, we used a regional climate-atmospheric chemistry model and a set of BC scenarios for quantitative evaluation of the impact of anthropogenic BC from various sources and its climate effects over the TP in 2013. The results showed that the model performed well in terms of climatology, aerosol optical properties, and near-surface concentrations, which indicates that this modeling framework is appropriate to characterize anthropogenic BC source-receptor relationships over the TP. The simulated surface concentration associated with the anthropogenic sources showed seasonal differences. In the monsoon season, the contribution of anthropogenic BC was less than in the non-monsoon season. In the non-monsoon season, westerly winds prevailed and transported BC from central Asia and north India to the western TP. In the monsoon season, BC aerosol was transported to the middle-upper troposphere over the Indo-Gangetic Plain and crossed the Himalayas via southwesterly winds. The majority of anthropogenic BC over the TP was transported from South Asia, which contributed to 40%-80% (mean of 61.3%) of surface BC in the non-monsoon season, and 10%-50% (mean of 19.4%) in the monsoon season. For the northeastern TP, anthropogenic BC from eastern China accounted for less than 10% of the total in the non-monsoon season, but can be up to 50% in the monsoon season. Averaged over the TP, the eastern China anthropogenic sources accounted for 6.2% and 8.4% of surface BC in the non-monsoon and monsoon seasons, respectively. The anthropogenic BC induced negative radiative forcing and cooling effects at the near surface over the TP.
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| 477. |
- Yoo, Kyungsoo, et al.
(författare)
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Rates of soil mixing and associated carbon fluxes in a forest vs. tilled agricultural field : Implications for modeling the soil carbon cycle
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116:G01014
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Tidskriftsartikel (refereegranskat)abstract
- In natural ecosystems, bioturbation is an essential component of soil formation, whereas tillage drives soil mixing in agricultural soils. Yet soil mixing is commonly neglected in modeling soil organic carbon (SOC) as it responds to land use changes. Here, in order to determine mixing-driven carbon fluxes, we combine a mass balance model with measurements of 210Pb activities and SOC contents. Soil mixing rates by tillage decrease from 3.4 ± 2.3 cm yr−1 at the surface to 0.8 ± 0.2 cm yr−1 at a depth of ∼20 cm, causing the SOC stored in the upper 25 cm of the soil to be physically turned over via mixing annually. In contrast, the bioturbation-driven soil mixing velocity at the forest increases from 0.6 ± 0.1 cm yr−1 at the surface to 2.7 ± 0.5 cm yr−1 at a depth of ∼10 cm, which results in physically turning over SOC in the A horizon via mixing on years to decadal time scales. Therefore, SOC fractions with different susceptibilities to decomposition may have significantly different physical trajectories within the soils over their lifespans, and thus the assumption of C-cycling models that all SOC fractions experience identical environmental conditions is unlikely to be realistic. Carbon sinks, excesses of plant carbon inputs over decomposition carbon losses, are found within the top portion of the A horizons. These carbon excesses are transferred, via mixing, to the lower portion of the A horizon, where they are decomposed. By quantifying mixing-derived SOC fluxes, this study shows a previously unrecognized complexity in understanding SOC dynamics associated with land use changes.
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| 478. |
- Yordanova, Emiliya, et al.
(författare)
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Reduced magnetic helicity behavior in different plasma regions of near-Earth space
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A07230-
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Tidskriftsartikel (refereegranskat)abstract
- We present a comparative study of the scaling laws of a signed measure calculated from the reduced magnetic helicity from Cluster observations in three different near-Earth plasma regions: solar wind, foreshock, and magnetosheath. The reduced magnetic helicity provides information about the handedness of the magnetic field, which can be related to the polarization (left or right) of the magnetic fluctuations, once the direction of propagation with respect to the mean magnetic field is known. We have found similar scaling properties of the reduced magnetic helicity in the solar wind and the magnetosheath; namely, there is no scale dependence at high frequency and cancellations between both polarizations continue beyond the ion cyclotron frequency. On other hand, in the foreshock we observe the prevalence of one sign of polarization, due to the presence of right-handed high-frequency waves in the plasma frame. We interpret the results in terms of the different variability in the plasma parameters and properties in the three plasma regions.
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| 479. |
- Yordanova, Emiliya, et al.
(författare)
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Turbulence and intermittency in the heliospheric magnetic field in fast and slow solar wind
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114:8, s. A08101-
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Tidskriftsartikel (refereegranskat)abstract
- We study the nonuniform solar wind turbulence using high-resolution Ulysses magnetic field data measured at different solar activity level, heliospheric latitudes, and distance. We define several types of solar wind dependent of the coronal region of origin and also of the dynamical behavior of the different streams, namely, "pure'' fast wind, fast streams, "pure'' slow wind, and slow streams. The turbulent properties of the solar wind types were investigated in terms of their scaling properties and spatial inhomogeneity. A clear trend in the power spectrum of the solar wind magnetic field magnitude is observed: the "pure'' fast wind has a slope similar to-1.33 (1/f-like), the fast streams similar to-1.48 (Kraichnan-like), the "pure'' slow wind similar to-1.67 (Kolmogorov-like), and the slow streams similar to-1.72. We find that the "pure'' fast wind in the polar heliolatitudes is less intermittent than the other types: "pure'' slow wind and both slow and fast streams, which is because of the absence of dynamical interactions between streams with different speeds. On the other hand, fast streams are more intermittent than the "pure'' fast wind, and slow streams are less intermittent than the "pure'' slow winds. A clear radial and latitudinal evolution of the intermittency is observed only for the "pure'' fast wind, while in the equatorial plane, the fast streams, the "pure'' slow wind, and the slow streams do not show evolution either in heliolatitude or in heliocentric distance.
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| 480. |
- Yurganov, L.N., et al.
(författare)
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A Quantitative Assessment of the 1998 Carbon Monoxide Emission Anomaly in the Northern Hemisphere Based on Total Column and Surface Concentration Measurements
- 2004
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 109:15, s. D15305-
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Tidskriftsartikel (refereegranskat)abstract
- Carbon monoxide abundances in the atmosphere have been measured between January 1996 and December 2001 in the high Northern Hemisphere (HNH) (30degrees-90degreesN) using two different approaches: total column amounts of CO retrieved from infrared solar spectra and CO mixing ratios measured in situ at ground-based stations. The data were averaged, and anomalies of the CO HNH burden ( deviations of the total tropospheric mass between 30degreesN and 90degreesN from the mean seasonal profile, determined as the 5 year average) were analyzed. The anomalies obtained from in situ and total column data agree well and both show two maxima, by far the largest in October 1998 and a lower one in August 1996. A noticeable decrease of the positive 1998 summer anomaly with increasing height was found. A box model was applied, and anomalies in source rates were obtained under the assumption of insignificant interannual sink variations. In August 1998 the HNH emission anomaly was estimated to be 38 Tg month(-1). The annual 1998 emission positive anomaly was 96 Tg yr(-1). Nearly all excess CO may be attributed to the emissions from boreal forest fires. According to available inventories, biomass burning emits around 52 Tg yr(-1) during the "normal'' years; therefore total biomass emissions in 1998 were as large as 148 Tg yr(-1). In August 1998, CO contribution from the biomass burning was twice as large as that from fossil fuel combustion. The results were compared to available emission inventories.
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