Journal of Geophysical Research

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ISSN / EISSN : 0148-0227 / 2156-2202
Published by: Wiley-Blackwell (10.1029)
Total articles ≅ 86,416
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, , , David D. Parrish, Sihua Lu, Ming Wang, Limin Zeng, , Yu Song, Jianbo Zhang, et al.
Published: 27 December 2012
Journal of Geophysical Research, Volume 117; https://doi.org/10.1029/2012jd018236

Abstract:
[1] Volatile organic compounds (VOCs) were measured online at an urban site in Beijing in August–September 2010. Diurnal variations of various VOC species indicate that VOCs concentrations were influenced by photochemical removal with OH radicals for reactive species and secondary formation for oxygenated VOCs (OVOCs). A photochemical age‐based parameterization method was applied to characterize VOCs chemistry. A large part of the variability in concentrations of both hydrocarbons and OVOCs was explained by this method. The determined emission ratios of hydrocarbons to acetylene agreed within a factor of two between 2005 and 2010 measurements. However, large differences were found for emission ratios of some alkanes and C8 aromatics between Beijing and northeastern United States secondary formation from anthropogenic VOCs generally contributed higher percentages to concentrations of reactive aldehydes than those of inert ketones and alcohols. Anthropogenic primary emissions accounted for the majority of ketones and alcohols concentrations. Positive matrix factorization (PMF) was also used to identify emission sources from this VOCs data set. The four resolved factors were three anthropogenic factors and a biogenic factor. However, the anthropogenic factors are attributed here to a common source at different stages of photochemical processing rather than three independent sources. Anthropogenic and biogenic sources of VOCs concentrations were not separated completely in PMF. This study indicates that photochemistry of VOCs in the atmosphere complicates the information about separated sources that can be extracted from PMF and the influence of photochemical processing must be carefully considered in the interpretation of source apportionment studies based upon PMF.
, Timothy J. McCoy, Larry R. Nittler, ,
Published: 22 December 2012
Journal of Geophysical Research, Volume 117; https://doi.org/10.1029/2012je004140

Abstract:
[1] We have conducted petrologic modeling of MESSENGER‐derived compositions and analog compositions to gain a better understanding of the petrogenesis of the crust of Mercury. Analog compositions included a 1425°C partial melt of the Indarch (EH4) meteorite and a range of Mg‐rich terrestrial rocks (magnesian basalt, basaltic komatiite, and peridotitic komatiite). All models were held at the iron‐wüstite buffer to simulate the reducing conditions that likely existed during Mercury's formation. We then compared modeled mineral compositions and abundances, liquidus temperatures, and viscosities to better constrain the characteristics of the lavas that erupted on Mercury's surface. Our results show that the surface composition of Mercury is most similar to that of a terrestrial magnesian basalt (with lowered FeO), composed mainly of Mg‐rich orthopyroxene and plagioclase. Because the model magmas are Mg‐rich, their counterparts on Mercury would have erupted at high temperatures and displayed low viscosities. Producing melts of these compositions would have required high temperatures at the mantle source regions on Mercury. The inferred low‐viscosity lavas would have erupted as thin, laterally extensive flows (depending upon their effusion rate) and would be expected to display surficial flow features that might be preserved to the present.
, Karen M. Fischer, Megan L. Anderson
Published: 22 December 2012
Journal of Geophysical Research, Volume 117; https://doi.org/10.1029/2012jb009538

Abstract:
[1] The goal of this study is to better constrain anisotropy and mantle flow above and below the Nazca slab from 28°S to 42°S through modeling of shear wave splitting in local S, SKS and SKKS (SK(K)S) phases. Comparisons of local S splitting times and path lengths in the slab, mantle wedge, and upper plate indicate that splitting times for arc and back‐arc stations are consistent with anisotropy in the mantle wedge, but long slab paths to fore‐arc stations imply that slab anisotropy is also significant. SK(K)S shear wave splitting observations and models for sub‐slab anisotropy show that significant anisotropy is present below the slab, and that the orientation of sub‐slab anisotropy sometimes differs from anisotropy above the slab. Anisotropy both above the slab and below the slab in the South American subduction zone is consistent with mantle flow that is driven by a combination of entrainment with downgoing slab motion and flow complexity related to variations in slab shape and slab rollback.
, Terrence M Joyce,
Published: 22 December 2012
Journal of Geophysical Research, Volume 117; https://doi.org/10.1029/2012jc008369

Abstract:
[1] Horizontal velocity, temperature and salinity measurements from the Line W array for the period 2004–2008 show large changes in the water mass structure and circulation of the Deep Western Boundary Current (DWBC). Fluctuations in the flow with periods from 10 to 60 days are bottom intensified: signals most likely associated with topographic Rossby waves (TRW). A fraction (∼15%) of the DWBC transport variability is caused by Gulf Stream rings and meanders. These flow anomalies are surface intensified and fluctuate at frequencies lower than the TRW. Interannual variability in the velocity field appears to be related to changes in the hydrographic properties. The dominant mode of variability is characterized by an overall freshening, cooling, a potential vorticity (PV) increase in the deep Labrador Sea Water (dLSW) and a PV decrease in the Overflow Water (OW). The variability in the flow associated with these property changes is not spatially homogeneous. Offshore (water depths larger than 3500 m) changes in the velocity are in phase with PV changes in the OW: a decrease in the OW PV is accompanied by an increase in the southward (negative) transport. Conversely, variations of the inshore flow are in phase with changes in the dLSW PV (increasing PV and decreasing transport). This trend, true for most of the record, reverses after the winter of 2007–2008. A sudden decrease of the dLSW PV is observed, with a corresponding intensification of the flow in the inner DWBC as well as a northward shift in the Gulf Stream axis.
, C. F. Larsen, S. O'neel, M. E. West
Published: 22 December 2012
Journal of Geophysical Research, Volume 117; https://doi.org/10.1029/2012jf002513

Abstract:
[1] Iceberg calving is known to release substantial seismic energy, but little is known about the specific mechanisms that produce calving icequakes. At Yahtse Glacier, a tidewater glacier on the Gulf of Alaska, we draw upon a local network of seismometers and focus on 80 hours of concurrent, direct observation of the terminus to show that calving is the dominant source of seismicity. To elucidate seismogenic mechanisms, we synchronized video and seismograms to reveal that the majority of seismic energy is produced during iceberg interactions with the sea surface. Icequake peak amplitudes coincide with the emergence of high velocity jets of water and ice from the fjord after the complete submergence of falling icebergs below sea level. These icequakes have dominant frequencies between 1 and 3 Hz. Detachment of an iceberg from the terminus produces comparatively weak seismic waves at frequencies between 5 and 20 Hz. Our observations allow us to suggest that the most powerful sources of calving icequakes at Yahtse Glacier include iceberg‐sea surface impact, deceleration under the influence of drag and buoyancy, and cavitation. Numerical simulations of seismogenesis during iceberg‐sea surface interactions support our observational evidence. Our new understanding of iceberg‐sea surface interactions allows us to reattribute the sources of calving seismicity identified in earlier studies and offer guidance for the future use of seismology in monitoring iceberg calving.
, Jianping Li
Published: 22 December 2012
Journal of Geophysical Research, Volume 117; https://doi.org/10.1029/2012jc008248

Abstract:
[1] Possible causes of the winter‐to‐winter recurrence (WWR) of atmospheric circulation anomalies in the central North Pacific (CNP) are investigated in the present study. Results show that tropical ENSO could not lead to the atmospheric WWR in the CNP because the persistence of ENSO itself does not show any recurrence regardless of the starting month. The effect of other external forcing, e.g., sea ice, is also not significant. These results suggest that the dominant source of the atmospheric WWR may come from internal atmospheric dynamics in the North Pacific. The Arctic Oscillation, the dominant pattern of sea level pressure variations north of 20°N, seems not to be the cause of atmospheric WWR in the CNP region. The effect of the local internal atmospheric dynamics on the atmospheric WWR may be more important in the CNP region. The CNP region was in the location of the storm track in the North Pacific. It was found that seasonal variability of storm track anomalies and associated synoptic transient eddy dynamics may be one of the causes for the atmospheric WWR. During the WWR years, transient eddy forcing on the mean flow is strong during the winter but very weak in the intervening summer, which leads to a quick transition of anomalous mean atmospheric circulation around March and the maintenance of the opposite sign anomalies for two to three seasons. But this characteristic of transient eddy forcing does not exist during the non‐WWR years.
, Christine Provost, , Nathalie Sennéchael, , , Jae Hak Lee
Published: 22 December 2012
Journal of Geophysical Research, Volume 117; https://doi.org/10.1029/2012jc008264

Abstract:
[1] The complex bathymetry of the Drake Passage and the meridional extent of the Shackleton Fracture Zone, in particular, force the Subantarctic Front (SAF) and the Polar Front (PF) to veer to the north, and the flow of the Antarctic Circumpolar Current concentrates in the Yaghan Basin. We have studied the circulation in the Yaghan Basin, using 3 years of velocity data (January 2006–March 2009) at five mooring sites and 18 years of satellite altimetry data. Mean velocities at our mooring sites show a dominant eastward component which decreases with depth, as expected, with a notable exception in the center of the Yaghan Basin, where mean velocities reveal a dominant westward component increasing with depth. The mooring data suggest the existence of a permanent, strong deep cyclonic circulation over the Yaghan seafloor depression in the northeastern part of the Yaghan Basin. The in situ data provide the first opportunity to compare altimetry‐derived velocities with high temporal resolution near‐surface current meter velocities in a large eddy kinetic energy environment at high latitudes. Globally, altimetry‐derived velocities compare rather well with the in situ velocities at 500 m depth both in strength and direction. Correlations are high between the in situ velocities and the surface velocities derived from satellite altimetric data. Mean sea level estimates lead to reasonable mean surface velocities with, however, a slight underestimation of the mean velocity at the mean location of the SAF on the continental slope and a more important underestimation of the westward current in the center of the Yaghan Basin. A dominant mode of velocity variations (23% of the variance) is observed both in the in situ and satellite data, corresponding to a strong southward meander of the SAF upstream of the mooring line and a northward meander of the PF downstream of the latter. The 18 yearlong altimetry time series shows that the mode is robust and has a strong semiannual component.
, Robert A. West, Jacques Gustin, Kristopher Larsen, A. Ian F. Stewart, Larry W. Esposito, William E. McClintock, Gregory M. Holsclaw, E. Todd Bradley
Published: 22 December 2012
Journal of Geophysical Research, Volume 117; https://doi.org/10.1029/2012ja017888

Abstract:
[1] In this paper we present the first nightside EUV and FUV airglow limb spectra of Titan showing molecular emissions. The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed photon emissions of Titan's day and night limb‐airglow and disk‐airglow on multiple occasions, including during an eclipse observation. The 71 airglow observations analyzed in this paper show EUV (600–1150 Å) and FUV (1150–1900 Å) atomic multiplet lines and band emissions arising from either photoelectron induced fluorescence and solar photo‐fragmentation of molecular nitrogen (N2) or excitation by magnetosphere plasma. The altitude of the peak UV emissions on the limb during daylight occurred inside the thermosphere at the altitude of the topside ionosphere (near 1000 km altitude). However, at night on the limb, a subset of emission features, much weaker in intensity, arise in the atmosphere with two different geometries. First, there is a twilight photoelectron‐excited glow that persists with solar depression angle up to 25–30 degrees past the terminator, until the solar XUV shadow height passes the altitude of the topside ionosphere (1000–1200 km). The UV twilight glow spectrum is similar to the dayglow but weaker in intensity. Second, beyond 120° solar zenith angle, when the upper atmosphere of Titan is in total XUV darkness, there is indication of weak and sporadic nightside UV airglow emissions excited by magnetosphere plasma collisions with ambient thermosphere gas, with similar N2 excited features as above in the daylight or twilight glow over an extended altitude range.
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