| 641. |
- Újvári, Gábor, et al.
(författare)
-
Two possible source regions for central Greenland last glacial dust
- 2015
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 42:23
-
Tidskriftsartikel (refereegranskat)abstract
- Dust in Greenland ice cores is used to reconstruct the activity of dust-emitting regions and atmospheric circulation. However, the source of dust material to Greenland over the last glacial period is the subject of considerable uncertainty. Here we use new clay mineral and < 10 mu m Sr-Nd isotopic data from a range of Northern Hemisphere loess deposits in possible source regions alongside existing isotopic data to show that these methods cannot discriminate between two competing hypothetical origins for Greenland dust: an East Asian and/or central European source. In contrast, Hf isotopes (< 10 mu m fraction) of loess samples show considerable differences between the potential source regions. We attribute this to a first-order clay mineralogy dependence of Hf isotopic signatures in the finest silt/clay fractions, due to absence of zircons. As zircons would also be absent in Greenland dust, this provides a new way to discriminate between hypotheses for Greenland dust sources.
|
|
| 642. |
- Ungstrup, E., et al.
(författare)
-
Rocket-borne particle, field, and plasma observations in the cleft region
- 1975
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 2:7, s. 345-348
-
Tidskriftsartikel (refereegranskat)abstract
- First results of comprehensive observations of magnetic and electric fields, and ambient and suprathermal plasmas above the dayside auroral oval with rocket-borne instrumentation which penetrated the cleft region are reported. Measurements were also obtained equatorward and poleward of the cleft. Convection velocities as inferred from electric field measurements were generally toward noon equatorward of the cleft and were antisunward over the polar cap. Our observations of electron temperatures, electric fields and low-frequency electrostatic noise provide strong evidence of a plasma instability (Farley-Buneman) in the E-layer, which is associated with the appearance of the ‘slant E condition’ identified in ground-acquired ionograms. The positions of these measurements relative to that of the cleft were firmly established via the determination of the plasma environment with an electrostatic analyzer.
|
|
| 643. |
- Uribe, Alejandro, 1984-, et al.
(författare)
-
Observed and CMIP6 Modeled Internal Variability Feedbacks and Their Relation to Forced Climate Feedbacks
- 2022
-
Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 49:24
-
Tidskriftsartikel (refereegranskat)abstract
- Inter model variations in global temperature response to increasing atmospheric carbon dioxide stem mostly from uncertainties in modeled climate feedbacks. To study potential reductions in model feedback uncertainties, we estimate observed feedbacks in response to internal variability using changes in Top Of the Atmosphere energy balance with temperature. We compare those observations with internal variability feedbacks from historical simulations of coupled and atmosphere-only experiments from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to identify that simulated feedbacks exhibit biases in the tropics, subtropics, and the Southern Ocean. Furthermore, we find a relation between simulated longwave and shortwave internal variability feedbacks and those where atmospheric carbon dioxide is abruptly quadrupled. In the model range of internal variability feedbacks, the observations are more consistent with moderately negative longwave feedback, and weak shortwave feedback, but the observations can't be used to rule out any models or their long-term feedback.
|
|
| 644. |
- Uribe, Alejandro, et al.
(författare)
-
Sensitivity of Tropical Extreme Precipitation to Surface Warming in Aquaplanet Experiments Using a Global Nonhydrostatic Model
- 2021
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 48:9
-
Tidskriftsartikel (refereegranskat)abstract
- Increases of atmospheric water vapor holding capacity with temperature (7% K−1–8% K−1, CC-rate) can lead to increasing extreme precipitation (EP). Observations show that tropical EP has increased during the last five decades with a rate higher than in the extratropics. Global climate models (GCM's) diverge in the magnitude of increase in the tropics, and cloud-resolving models (CRM's) indicate correlations between changes in tropical EP and organization of deep convection. We conducted global-scale aquaplanet experiments at a wide range of resolutions with explicit and parameterized convection to bridge the gap between GCM's and CRM's. We found increases of tropical EP beyond the CC rate, with similar magnitudes when using explicit convection and parametrized convection at the resolution it is tuned for. Those super-CC rates are produced due to strengthening updrafts where extreme precipitation occurs, and they do not exhibit relations with changes in convective organization.
|
|
| 645. |
- Vaivads, Andris, et al.
(författare)
-
Source of whistler emissions at the dayside magnetopause
- 2007
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 34:9, s. L09106-
-
Tidskriftsartikel (refereegranskat)abstract
- Observations of whistler emissions are common in the magnetosphere near the dayside magnetopause. We show that one of the major source regions for these emissions is magnetic field minima that form along magnetic flux tubes at high latitudes. Using multispacecraft Cluster observations we experimentally confirm for the first time the existence of the magnetic field minima at high latitudes and we show that whistler emissions propagate away from the magnetic field minima. The strongest whistler emissions are observed on the magnetospheric flux tubes that are newly opened due to the magnetic reconnection. These flux tubes still have a density of magnetospheric plasma, but part of the high energy magnetospheric electrons have already been lost from the flux tubes. The partial loss of high energy electrons most probably causes anisotropy in electron distributions at high energies which should be the source of whistler emissions. Whistler emissions on opened flux tubes disappear as soon as the plasma density of flux tubes increases due to the entering of magnetosheath ions. We speculate that whistler emissions can most probably be used to trace the dynamics of the first opened field lines and thus the dynamics of magnetic reconnection sites.
|
|
| 646. |
- Vaivads, A., et al.
(författare)
-
What high altitude observations tell us about the auroral acceleration : A cluster/DMSP conjunction
- 2003
-
Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 30:3
-
Tidskriftsartikel (refereegranskat)abstract
- [1] Magnetic conjugate observations by Cluster and DMSP F14 satellites are used to study the field lines of auroral arc. Cluster is well above the acceleration region and observes upward keV ion beams and bipolar electric structures. The integrated potential at Cluster altitudes shows a dip that is consistent with the keV electron acceleration energy at low altitude. The earthward Poynting flux at Cluster altitudes is comparable to the electron energy flux at low altitudes. Thus, for this event the auroral acceleration can be described as a quasi-stationary potential structure with equipotential lines reaching the Cluster altitudes. The arc forms at the outer edge of the plasma sheet at a density gradient. Multiple Cluster satellite measurements allow us to study the density increase associated with the development of the arc, and to estimate the velocity of the structure. The quasi-potential structure itself may be part of an Alfven wave.
|
|
| 647. |
- Valk, O., et al.
(författare)
-
Importance of Hydrothermal Vents in Scavenging Removal of Th-230 in the Nansen Basin
- 2018
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 45:19, s. 10539-10548
-
Tidskriftsartikel (refereegranskat)abstract
- In this study we present dissolved and particulate Th-230 and Th-232 results, as well as particulate Th-234 data, obtained as part of the GEOTRACES central Arctic Ocean sections GN04 (2015) and IPY11 (2007). Samples were analyzed following GEOTRACES methods and compared to previous results from 1991. We observe significant decreases in Th-230 concentrations in the deep waters of the Nansen Basin. We ascribe this nonsteady state removal process to a variable release and scavenging of trace metals near an ultraslow spreading ridge. This finding demonstrates that hydrothermal scavenging in the deep-sea may vary on annual time scales and highlights the importance of repeated GEOTRACES sections. Plain Language Summary This study presents new results of thorium isotopes from the central Arctic Ocean. Thorium-230 is produced continuously in seawater by radioactive decay of U-234 and subsequently removed by particle scavenging. We show that observed changes in Th-230 concentrations compared to earlier times are related to submarine volcanic eruptions. We use Th-230 data from three different expeditions conducted in 1991, 2007, and 2015. The Nansen Basin is part of the Eurasian Basin of the Arctic Ocean. It is divided from the Amundsen Basin by the Gakkel Ridge. The Gakkel Ridge is a region where the Eurasian and the North American plates spread apart, triggering volcanism. Submarine volcanos and hydrothermal vents release trace elements such as iron. Iron is known to be oxidized to particles that react with Th-230. Thus, when iron particles sink they remove Th-230 from the water column. In the Nansen Basin this process took place between 2007 and 2015, triggered by earthquake-induced volcanic eruptions in 2001. In this study, we present a conceptual hydrothermal scavenging process and plume dispersal by deep water circulation.
|
|
| 648. |
- Van Pelt, Ward J.J., et al.
(författare)
-
Dynamic response of a High Arctic glacier to melt and runoff variations
- 2018
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 45:10, s. 4917-4926
-
Tidskriftsartikel (refereegranskat)abstract
- The dynamic response of High Arctic glaciers to increased runoff in a warming climateremains poorly understood. We analyze a 10-year record of continuous velocity data collected atmultiple sites on Nordenskiöldbreen, Svalbard, and study the connec tion between ice flow andrunoff within and between seasons. During the melt season, the sensitivit y of ice motion to runoffat sites in the ablation and lower accumulation zone drops by a fac tor of 3 when cumulative runoff exceedsa local threshold, which is likely associated with a transition from inefficient (distributed) to efficient(channelized) drainage. Average summer (June–August) velocities are found to increase with summerablation, while subsequent fall (September–November) velocities decrease. Spring (March–May)velocities are largely insensitive to summer ablation, which suggests a short-lived impact of summermelt on ice flow during the cold season. The net impact of summer ablation on annual velocities is foundto be insignificant.
|
|
| 649. |
- Vanderkelen, I., et al.
(författare)
-
Global Heat Uptake by Inland Waters
- 2020
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 47:12
-
Tidskriftsartikel (refereegranskat)abstract
- Heat uptake is a key variable for understanding the Earth system response to greenhouse gas forcing. Despite the importance of this heat budget, heat uptake by inland waters has so far not been quantified. Here we use a unique combination of global‐scale lake models, global hydrological models and Earth system models to quantify global heat uptake by natural lakes, reservoirs, and rivers. The total net heat uptake by inland waters amounts to 2.6 ± 3.2 ×1020 J over the period 1900–2020, corresponding to 3.6% of the energy stored on land. The overall uptake is dominated by natural lakes (111.7%), followed by reservoir warming (2.3%). Rivers contribute negatively (‐14%) due to a decreasing water volume. The thermal energy of water stored in artificial reservoirs exceeds inland water heat uptake by a factor ∼10.4. This first quantification underlines that the heat uptake by inland waters is relatively small, but non‐negligible.
|
|
| 650. |
- Vasko, I. Y., et al.
(författare)
-
Solitary Waves Across Supercritical Quasi-Perpendicular Shocks
- 2018
-
Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 45:12, s. 5809-5817
-
Tidskriftsartikel (refereegranskat)abstract
- We consider intense electrostatic solitary waves (ESW) observed in a supercritical quasi-perpendicular Earth's bow shock crossing by the Magnetospheric Multiscale Mission. The ESW have spatial scales of a few tens of meters (a few Debye lengths) and propagate oblique to a local quasi-static magnetic field with velocities from a few tens to a few hundred kilometers per second in the spacecraft frame. Because the ESW spatial scales are comparable to the separation between voltage-sensitive probes, correction factors are used to compute the ESW electric fields. The ESW have electric fields with amplitudes exceeding 600mV/m (oriented oblique to the local magnetic field) and negative electrostatic potentials with amplitudes of a few tenths of the electron temperature. The negative electrostatic potentials indicate that the ESW are not electron phase space holes, while interpretation in terms of ions phase space holes is also questionable. Whatever is their nature, we show that due to the oblique electric field orientation the ESW are capable of efficient pitch-angle scattering and isotropization of thermal electrons. Due to the negative electrostatic potentials the ESW Fermi reflects a significant fraction of the thermal electrons streaming from upstream (downstream) back to upstream (downstream) region, thereby affecting the shock dynamics. The role of the ESW in electron heating is discussed. Plain Language Summary Processes governing electron thermalization across shock waves are not entirely understood. The high resolution particle and 3-D electric field measurements provided by the Magnetospheric Multiscale Mission make it possible to study the Earth's bow shock that is an excellent laboratory for addressing the electron thermalization across supercritical shock waves. Previous observations showed that electron heating across the bow shock is generally governed by macroscopic cross-shock electrostatic field. On the other hand, the role of the turbulence observed across the bow shock in the electron thermalization has remained unclear. In this letter we consider a particular bow shock crossing by the Magnetospheric Multiscale Mission and focus on the role of the high amplitude electrostatic solitary waves in the electron thermalization process. We accurately estimate the electrostatic solitary wave parameters and show that due to electric fields oriented oblique to a local DC magnetic field and negative electrostatic potentials with amplitudes of a few tenths of the electron temperature, these Debye-scale structures are capable of efficient pitch angle scattering, Fermi reflection, and isotropization of thermal electrons.
|
|
