bluuebunny/crossref_metadata_2025 huggingface dataset:
Variation in lithospheric stress along ridge‐transform plate boundaries
Authors: Blackman Donna K.
Published: February 2004

Three‐dimensional numerical models of asthenospheric flow and deformation in the oceanic lithosphere predict variability in the stress field that reflects the geometry of the ridge‐transform boundary. A series of 3‐D Boundary Element calculations shows how spreading rate, transform offset and segment length each influence the flow and stress fields that develop during plate‐driven asthenospheric flow beneath a ridge‐transform boundary. The predicted patterns of stress‐supported seafloor relief generally follow those observed: median valleys are predicted at slow‐spreading ridges vs no rift valley or, in some cases, a small axial high at fast spreading ridges; nodal deeps occur at ridge‐transform intersections for offsets greater than 25 km; longer segments have more along‐axis deepening than short segments which do not shoal much in their center. The predicted amplitude of the stress supported topography is up to 20% (across axis) and 40% (along axis) of that observed for an assumed asthenospheric viscosity of 5×1019 Pa s and a weak lithosphere (local compensation).
The fate of the outer plasmasphere
Authors: Elphic R. C. , Thomsen M. F. , Borovsky J. E.
Published: February 2004

Both the solar wind and the ionosphere contribute to Earth's magnetospheric plasma environment. However, it is not widely appreciated that the plasmasphere is a large reservoir of ionospheric ions that can be tapped to populate the plasma sheet. We employ empirical models of high‐latitude ionospheric convection and the geomagnetic field to describe the transport of outer plasmasphere flux tubes from the dayside, over the polar cap and into the magnetotail during the early phases of a geomagnetic storm. We calculate that this process can give rise to high densities of cold plasma in the magnetotail lobes and in the near‐Earth plasma sheet during times of enhanced geomagnetic activity, and especially during storms. This model can help explain both polar cap ionization patches and the presence of cold flowing ions downtail.
A new assessment of the abundance of serpentinite in the oceanic crust
Authors: Carlson Richard L. , Miller D. Jay
Published: February 2004

The ‘gabbro' model of the composition of the oceanic crust has prevailed for more than two decades, but serpentinites are common along much of the Mid‐Atlantic Ridge, and elsewhere, leading to renewed debate over the abundance of serpentinites in crust created at slow‐spreading ridges. Based on data from both oceanic and ophiolite samples, it has also been suggested that serpentinites are not distinguishable from meta‐gabbros and diabases by their seismic properties. We have made new statistical analyses of the relationship between Vsand Vpin oceanic gabbro and diabase samples and serpentinized peridotites, and compared them to in situ s‐wave and p‐wave velocities derived from Expanding Spread Profiles (ESPs) acquired in the Atlantic. Our findings demonstrate: (1) that there is some overlap of the properties of gabbroic samples taken from ophiolites with the properties of serpentinites, but (2) over the range of typical‐layer 3 seismic velocities, the seismic properties of oceanic gabbros and diabases are distinctly different from the properties of serpentinites, and (3) that in situ seismic velocities are in excellent agreement with the properties of the gabbros, while the shear velocities in serpentinites are significantly lower. Partially‐serpentinized peridotite is certainly present, but not abundant in ‘normal' oceanic crust.
Shallow structure of Mt. Vesuvius Volcano, Italy, from seismic array analysis
Authors: De Luca G. , Scarpa R. , Del Pezzo E. , Simini M.
Published: February 2004

Data from a portable dense seismic array deployed on Mt. Vesuvius in May 1994, during a 2D seismic tomography experiment, are analyzed in the present paper. The array consisted of two groups of short period geophones, 4.5 hz natural frequency, formed by 16 and 25 vertical components (plus two horizontal components), distributed along an arc like shape along the summit crater. Stacks of later arrivals, interpreted as reflected phases, provide a significant constraint to a boundary layer located in the depth range 1.5–2.2 km beneath the summit crater with average velocity V=1.8–2.2 km/s, interpreted as the top of the limestone basement. The correlation methods applied on microtremor records allowed to infer the shallow velocity structure, up to 400 m, beneath the crater rim.
Boundary layer formation in the magnetotail: Geotail observations and comparisons with a global MHD simulation
Authors: Raeder J. , Berchem J. , Ashour‐Abdalla M. , Frank L. A. , Paterson W. R. , Ackerson K. L. , Kokubun S. , Yamamoto T. , Slavin J. A.
Published: February 2004

We present Geotail plasma and field observations from the middle magnetotail near XGSE=−46 RE for the time period 1400 to 1800 UT on December 14, 1994. During that period, the Wind satellite monitored the solar wind plasma and interplanetary magnetic field (IMF) upstream of the bow shock. The IMF was northward and the plasma parameters near average. Geotail observed slow tailward flows and a northward field. The plasma and field parameters indicate that Geotail is either in the plasma sheet or in a boundary layer. We used the Wind solar wind plasma and IMF data as input for a global simulation of that time interval. Comparison of the simulation results with the observational data show very good overall agreement of the magnitudes of the plasma and field parameters. In particular, the simulation reproduces the slow tailward flows and northward field found at Geotail. Small scale temporal variations are less well reproduced. The simulation shows the formation of a broad boundary layer (which we call tail flank boundary layer, TFBL) that consists of closed flux which formed by magnetic reconnection of IMF and lobe field lines. The simulation results indicate that Geotail is located very close to the TFBL and may have entered the TFBL proper. We show that the TFBL plays an important role in energy transport from the solar wind into the magnetosphere during northward IMF conditions.
Chemical perturbation of the lowermost stratosphere through exchange with the troposphere
Authors: Lelieveld J. , Bregman B. , Arnold F. , Bürger V. , Crutzen P. J. , Fischer H. , Waibel A. , Siegmund P. , van Velthoven P. F. J.
Published: February 2004

In the troposphere, anthropogenic emissions of nitrogen oxides, hydrocarbons and carbon monoxide cause large‐scale photochemical build up of ozone. In the stratosphere breakdown of anthropogenic halocarbons damages the ozone layer. In the extratropics a transition region between these air layers occurs, the lowermost stratosphere (below 12–14 km', in which about half the current subsonic air traffic takes place. Here, we report aircraft measurements of HNO3, O3 and CO over western Europe in July 1994 (5 flights of several hours during a 10‐day period', at approximately 1–2 km above the tropopause. The HNO3 mixing ratios observed were highly variable (0.76–1.2 ppbv', while HNO3/O3 ratios seem relatively high (5.2–7.0 10−3'. Moreover, several times we observed very high levels of pollutant CO (up to ∼0.5 ppmv' that did not originate from aircraft exhausts. Instead, we pose that it had mixed‐in from the troposphere. Cross‐tropopause mixing also helps explaining the variable HNO3 and relatively high HNO3/O3 ratios. These measurements suggest that relatively short‐lived surface emitted pollutants can reach the lowermost stratosphere. We expect that this contributes to O3 formation.
State of stress before and after the 1994 Northridge Earthquake
Authors: Zhao Dapeng , Kanamori Hiroo , Wiens Douglas
Published: February 2004

The state of tectonic stress in the epicentral area of the 17 January 1994, Northridge earthquake (Mw 6.7' is investigated by applying a stress inversion method to P‐wave polarity data from earthquakes in Northridge from July 1981 to January 1994 and from the Northridge aftershocks during January 1994 to December 1995. A 3‐D crustal model is used to trace the rays taking off from the hypocenter, which reduced the effects of large structural heterogeneities on the determination of the stress tensor. We found significant temporal changes of stress orientations induced by the Northridge earthquake. The principal pressure (P' axis is oriented N32°E from 1981 to June 1992, and N30°E from 28 June 1992 to 16 January 1994, suggesting that the stress field in Northridge was not affected by the 1992 Landers earthquake. During two weeks following the Northridge mainshock, the P‐axis is oriented N13°E, which is a significant (17°' change from that before the earthquake (N30°E'. Between February 1994 and August 1995 the P‐axis orientation changes from N18°E to N26°E, and finally ends up at N34° by the end of 1995, which is close to that before the Northridge earthquake. These results suggest that the stresses rotated coseismically, then rotated more slowly back to their original orientation. The aftershocks caused by the mainshock changed the stress distribution in the crust, which showed up as a regional stress change. The stress recovery appears to have completed within two years after the mainshock, which is very short compared to the time scale of the earthquake cycle.
Non‐double‐couple earthquake mechanisms at the Hengill‐Grensdalur Volcanic Complex, Southwest Iceland
Authors: Julian B. R. , Miller A. D. , Foulger G. R.
Published: February 2004

The Hengill‐Grensdalur area in Iceland generates frequent small non‐double‐couple earthquakes with explosive volumetric components. We collected high quality three‐component digital recordings of 4,000 earthquakes on a purpose‐designed, 32‐station network in 1991, and determined focal mechanisms for 100 of the best‐recorded earthquakes by inverting amplitude ratios. Many of the mechanisms are consistent, within the errors, with simultaneous shear and tensile faulting, with tensile faults parallel to the local spreading ridge, and shear faulting similar to that in the South Iceland transform‐fault zone. Some events cannot be explained by this model, however, and require other processes, such as crack closing and partial compensation of tensile cracks by fluid flow.
Monitoring of total soil moisture on a scale of hectares using groundwater piezometers
Authors: van der Kamp G. , Schmidt R.
Published: February 2004

A method has been developed for continuous measurement of total soil moisture on a scale of hectares. The method is based on accurate measurement of groundwater pressures in thick clay formations and depends on the principle that changes of mechanical load result in changes of groundwater pressure. The resulting data are similar to those obtained by conventional weighing lysimeters, but on a much larger scale and with no significant hydrologic disturbance of the site. Several years of field testing show the method to be practicable.
Deformation of New Trident Volcano measured by ERS‐1 SAR interferometry, Katmai National Park, Alaska
Authors: Lu Z. , Fatland R. , Wyss M. , Li S. , Eichelberger J. , Dean K. , Freymueller J.
Published: February 2004

Using Synthetic Aperture Radar (SAR) interferometry, we detect several centimeters of uplift that accumulated during two years (1993–1995) around the vent of the New Trident volcano in Alaska's Katmai National Park. The areas of image coherence correspond to fresh, blocky lavas, while coherence is lost in ash‐covered areas. From the uplift gradient we estimate the depth of a pressure source under New Trident volcano to be approximately 0.8–2.0 km. Our results show that in spite of the difficult sub‐arctic environment of southern Alaska, strain build‐up can be monitored over a two‐year period, showing the potential for global monitoring of volcano deformation using SAR interferometry.
Estimated impact of Agung, El Chichon and Pinatubo volcanic eruptions on global and regional total ozone after adjustment for the QBO
Authors: Angell James K.
Published: February 2004

Dobson total‐ozone data are used to compare the impact on global and regional total ozone of Agung, El Chichon and Pinatubo volcanic eruptions taking into account the quasi‐biennial oscillation (QBO). This is done by comparing the total‐ozone variation following the 3 eruptions with the average quasi‐biennial variation in total ozone obtained by applying the superposed epoch method to the 13 QBO cycles between 1958 and 1995 which do not include these 3 eruptions. Confidence estimates are based on the 2‐standard‐deviation value of the 13 comparisons. The largest volcanic impact on total ozone is in the European region, with significant (at the 5% level) total‐ozone decreases of about 5% following Agung and El Chichon, and 8% following Pinatubo. The decreases in the north polar zone, Russia, North America and Asia are 5–6% following Pinatubo and 2–3 % following El Chichon. In the tropics the ozone decrease is less than 2% following all 3 eruptions, and there is little consistent impact on south temperate or south polar total ozone. Comparisons are made between these findings and those obtained by others using satellite data.
Model overestimates of NOy in the upper stratosphere
Authors: Nevison C. D. , Solomon S. , Garcia R. R.
Published: February 2004

Stratospheric NOy profiles from the Garcia‐Solomon two‐dimensional model are compared to observed profiles from the Upper Atmosphere Research Satellite (UARS) and the Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS). The model consistently overestimates NOy in the upper stratosphere beyond the range of uncertainty in the satellite data. The relatively short lifetime of NOy in this region suggests a largely photochemical explanation for the discrepancy. Uncertainties in the photochemical production of NOy from N2O strongly influence stratospheric NOy profiles, but this influence decreases with increasing height in the upper stratosphere. Photochemical loss of NOy is controlled by the NO photolysis rate and the reaction rates of N + NO and N + O2. All these rates are subject to substantial uncertainties that probably contribute to the discrepancy between model and observations.
Photolysis of ozone at 308 and 248 nm: Quantum yield of O(¹D) as a function of temperature
Authors: Talukdar Ranajit K. , Gilles Mary K. , Battin‐Leclerc Frederique , Ravishankara A. R. , Fracheboud Jean‐Marc , Orlando John J. , Tyndall Geoffrey S.
Published: February 2004

The quantum yield of O(¹D), Φ(T), in the 308 nm photolysis of ozone has been shown to decrease very slightly with temperature (T), from 0.79±0.10 at 298 K to 0.73±0.10 at 200 K. At 248 nm, Φ is (0.91±0.06) independent of temperature (200–320 K). The rate constant for the reaction of O(¹D) with O3 was measured to be (2.3±0.5) × 10−10 cm³ molecule−1 s−1, independent of temperature (200–350 K). The measured quantum yields suggest that the current recommendations over‐estimate the effect of temperature on O(¹D) quantum yields in O3 photolysis at 308 nm.
The 1995 Colima‐Jalisco, Mexico, Earthquake (Mw 8): A study of the rupture process
Authors: Courboulex Françoise , Singh Shri K. , Pacheco Javier F. , Ammon Charles J.
Published: February 2004

In this study we map rupture characteristics of the great, shallow, thrust earthquake of October 9, 1995 which caused extensive damage to the coastal towns of Colima and Jalisco. To isolate the earthquake rupture details, we deconvolve surface waves with two empirical Green's functions, the aftershock of October 12, 1995 (Mw 5.9) and the foreshock of October 6, 1995 (Mw 5.8), from the corresponding mainshock records. Specifically, we use a spectral water‐level deconvolution to obtain 80 Apparent Source Time Functions (ASTF) at 62 stations (Rayleigh and Love waves). Durations of the ASTF, as a function of azimuth indicate that the rupture propagated toward N70°W. The duration of the Source Time Function (STF) is around 62 s with a large pulse at 45 s. To map the main characteristics of the rupture, we use an inverse Radon transform of the ASTFs, assuming a ribbon fault‐model aligned in the direction of the rupture propagation. Our analysis indicates that the rupture initiated about 20 km offshore of Manzanillo and propagated almost unilaterally for 150 km towards N70°W, with an average rupture velocity of approximately 2.8 km/s. The earthquake was a composite of three significant subevents, the largest occurred 45 s after the initiation of the rupture and was located about 100 km away. This result is in good agreement with the inversion of deformation data, measured with GPS [Melbourne et al., 1997].
Antarctica Ice Sheet Curvature and its relation with ice flow and boundary conditions
Authors: Rémy Frédérique , Minster Jean‐François
Published: February 2004

The map of the Antarctica ice sheet surface curvature in the across‐slope direction shows a very coherent pattern, related to ice flow anomalies of each flowline with respect to adjacent ones. Near the coast, these anomalies are correlated to bedrock features which suggests that they result from outlet flow conditions that are transmitted from coast to domes. As a consequence, they should represent the drainage pattern, glacier positions and 3‐dimensional flowline directions. Thus a large part of the ice sheet flow pattern seems controlled by outflow boundary conditions which can be empirically estimated.
Evidence for short cooling time in the Io plasma torus
Authors: Volwerk M. , Brown M. E. , Dessler A. J. , Sandel B. R.
Published: February 2004

We present empirical evidence for a radiative cooling time for the Io plasma torus that is about a factor of ten less than presently accepted values. We show that brightness fluctuations of the torus in the extreme ultraviolet (EUV) at one ansa are uncorrelated with the brightness at the other ansa displaced in time by five hours, either later or earlier. Because the time for a volume of plasma to move from one ansa to the other is only five hours, the cooling time must be less than this transport time in order to wipe out memory of the temperatures between ansae. Most (∼80–85%) of the EUV emission comes from a narrow (presumably ribbon‐like) feature within the torus. The short cooling time we observe is compatible with theoretical estimates if the electron density in the ribbon is ∼104/cm³. The cooling time for the rest of the torus (which radiates the remaining 15–20% of the power) is presumably consistent with the previously derived 20‐hour values. A nearly‐continuous heating in both longitude and time is needed to maintain the EUV visibility of the torus ribbon—a requirement not satisfied by presently available theories.
Tracer signals of the intermediate layer of the Arabian Sea
Authors: Rhein Monika , Stramma Lothar , Plähn Olaf
Published: February 2004

In 1995, hydrographic and chlorofluorocarbon (CFCs, components F11, F12) measurements were carried out in the Gulf of Aden, in the Gulf of Oman, and in the Arabian Sea. In the Gulf of Oman, the F12 concentrations in the Persian Gulf outflow (PGW) at about 300m depth were significantly higher than in ambient surface water with saturations reaching 270%. These high values could not be caused by air‐sea gas exchange. The outflow was probably contaminated with oil, and the lipophilic character of the CFCs could then lead to the observed supersaturations. The intermediate F12 maximum decreased rapidly further east and south. At the Strait of Bab el Mandeb in the Gulf of Aden, the Red Sea outflow (RSW) was saturated with F12 to about 65% at 400m depth, and decreased to 50% while descending to 800m depth. The low saturation is not surprising, because the outflow contains deep and intermediate water masses from the Red Sea which were isolated from the surface for some time. The tracer contributions to the Arabian Sea for Indian Central Water (ICW) and PGW are about equal, while below 500m depth the RSW contribution greatly exceeds ICW. Modeling the CFC budget of the Arabian Sea, the inflow of ICW north of 12°N is estimated to be 1–6 Sv, depending mainly on the strength of the flow of Red Sea Water into the Arabian Sea.
Geodetic estimate of seismic hazard in the Gulf of Korinthos
Authors: Clarke P. J. , Davies R. R. , England P. C. , Parsons B. E. , Billiris H. , Paradissis D. , Veis G. , Denys P. H. , Cross P. A. , Ashkenazi V. , Bingley R.
Published: February 2004

The recent 15 June 1995, M0 = 6.0 × 1018 N m, Aigion earthquake in the western Gulf of Korinthos has focussed attention on the seismic hazard of the region. Although there have been few large earthquakes in the region during this century, the historical record suggests that there may have been many large earthquakes there in the interval 1750–1900. We present geodetic data that give estimates of the rate of extension of the Gulf of Korinthos during this century and which suggest that less than half of the elastic strain in the central and western Gulf of Korinthos has been released by earthquakes during this century. In contrast, the seismic and geodetic strains in the eastern Gulf of Korinthos are in agreement with each other. If the discrepancy between seismic and geodetic strains in the western Gulf of Korinthos that has accumulated during this century is removed in earthquakes, the moment release will be equivalent to several Ms > 6.5 earthquakes.
Observations of the Ekman Balance at 8°30′ N in the Arabian Sea during the 1995 Southwest Monsoon
Authors: Chereskin T. K. , Wilson W. D. , Bryden H. L. , Ffield A. , Morrison J.
Published: February 2004

The Ekman transport is estimated from two sets of hydrographic and shipboard acoustic Doppler current profiler (ADCP) velocity observations made during June and September 1995, during the southwest monsoon in the Arabian Sea. Both sets of measurements were made along latitude 8°30′ N, designated as World Ocean Circulation Experiment (WOCE) line I1W, from Somalia to Sri Lanka. The Ekman transport estimates calculated from ageostrophic velocity were southward: 17.6 ± 2.4 106 m³ s−1 in June and 7.9 ± 2.7 106 m³ s−1 in September. These direct estimates were in good agreement with those predicted by the Ekman balance using both shipboard and climatological winds. The vertical structure of the ageostrophic velocity and the stratification were quite different between the two occupations of the transect. The wind‐driven momentum was confined to a very shallow layer in June (about 50 m) and the surface layer was strongly stratified, with a maximum salinity layer at depths between 50 and 70 m. The ageostrophic velocity penetrated much deeper in September (to about 160 m) and the pycnocline was correspondingly deeper. In both cases, the Ekman transport penetrated beneath the mixed layer, to the top of the pycnocline.
Extreme inundation flows during the Hokkaido‐Nansei‐Oki Tsunami
Authors: Titov Vasily V. , Synolakis Costas Emmanuel
Published: February 2004

The tsunami generated by the July 12, 1993 Hokkaido‐Nansei‐Oki Mw=7.8 earthquake produced in Japan the worst local tsunami‐related death toll in fifty years, with estimated 10–18m/sec overland flow velocities and 30m runup. These extreme values are the largest recorded in Japan this century and are among the highest ever documented for non‐landslide generated tsunamis. We model this event to confirm the estimated overland flow velocities, and we find that, given reasonable ground deformation data, current state‐of‐the‐art shallow‐water wave models can predict tsunami inundation correctly including extreme runup, current velocities and overland flow. We find that even small local topographic structures affect the runup to first‐order, and that the resolution of the bathymetric data is more important than the grid resolution. Our results qualitatively suggest that—for this event—coastal inundation is more correlated with inundation velocities than with inundation heights, explaining also why threshold‐type modeling has substantially underpredicted coastal inundation for this and other recent events.
Ion cyclotron waves observed at Galileo's Io encounter: Implications for neutral cloud distribution and plasma composition
Authors: Warnecke J. , Kivelson M. G. , Khurana K. K. , Huddleston D. E. , Russell C. T.
Published: February 2004

We present observations of ion cyclotron waves measured by the magnetic field experiment onboard the Galileo spacecraft at its encounter with Jupiter's satellite Io on December 7, 1995. The waves exhibit highly coherent oscillations, with a period between 2 and 3 seconds. They are left‐handed, nearly circularly polarized, and their plane of polarization is nearly perpendicular to the background field. Their power spectra reveal peaks close to the gyrofrequencies of SO2+ and, sometimes, SO+. The waves are interpreted as L‐mode waves generated by a gyroresonance of pickup ions, whose source is neutral particles originating from Io. On the inbound leg of the flyby, in regions where the electron density is approximately constant, the observed decay of the wave power with distance from Io roughly follows the expected distribution of neutral clouds. The presence of wave power just above the SO2+ gyrofrequency suggests that there is only a small amount of SO2+ in the background plasma.
Improved determination of global mean sea level variations using TOPEX/POSEIDON altimeter data
Authors: Nerem R. S. , Haines B. J. , Hendricks J. , Minster J. F. , Mitchum G. T. , White W. B.
Published: February 2004

The TOPEX/POSEIDON satellite altimeter mission has measured sea level on a global basis over the last 4 years at 10 day intervals. After correcting for a recently discovered error in the measurements, the estimated rate of global mean sea level change over this time period is −0.2 mm/year. Comparisons to tide gauge sea levels measured in spatial and temporal proximity to the satellite measurements suggest there is a residual drift in the satellite measurement system of −2.3±1.2 mm/year, the origin of which is presently unknown. Application of this rate correction yields a “calibrated” estimate of +2.1±1.3 mm/year for the rate of sea level rise, which agrees statistically with tide gauge observations of sea level change over the last 50 years. Since the contribution of interannual and decadal mean sea level variations cannot presently be isolated, a longer time series is required before long‐term climate change signals can be detected. In addition, an improved understanding of the T/P measurement system performance with time is needed.
Morphology and simulation of solid state rounding process
Authors: Saiki Kazuto
Published: February 2004

A two dimensional rounding process model, including morphological analyses of grain form and calculation of flux of surface diffusion driven by difference of curvature, is developed and solved numerically. Results from simplified model runs, intended to illustrate the shape dependence of rounding rate, are presented. The method for evaluation of the morphological feature of grains during rounding and the method for estimation of actual flux from the morphological analysis of experimental products are discussed.
Signal strength and climate relationships in 13C/12C ratios of tree ring cellulose from oak in southwest Finland
Authors: Robertson I. , Rolfe J. , Switsur V. R. , Carter A. H. C. , Hall M. A. , Barker A. C. , Waterhouse J. S.
Published: February 2004

Annual oak (Quercus robur L.) latewood δ13C values are presented for the period 1895–1994 for two sites with different hydrological characteristics in southwest Finland. No appreciable anthropogenic 13C depletion was observed in the time‐series. δ13C indices record high‐frequency common forcing better than lower‐frequency forcing. High‐frequency fluctuations in the δ13C indices are highly correlated with climatic variables. Cross correlation analysis of tree ring δ13C indices and climate variables show that there is no significant difference (p < 0.01) in the climate response of tree ring δ13C indices between the two sites. This work demonstrates that significant climate signals are contained in isotopic measurements of trees whose ring‐widths contain little or no such information.
Sensitivity of simulated salinity in a three‐dimensional ocean model to upper ocean transport of salt from sea‐ice formation
Authors: Duffy P. B. , Caldeira K.
Published: February 2004

We show that explicit representation of sinking of salt rejected during sea‐ice formation dramatically improves simulated salinity in an ocean general circulation model (OGCM). In our “control” simulation, rejected salt goes into the top model layer, and simulated salinities are typical of OGCMs: the deep ocean is too fresh, and the intermediate‐depth salinity minimum associated with Antarctic Intermediate Water is absent. These problems are eliminated in our “test” simulation, in which we distribute rejected salt uniformly over the upper 160 m. Also, the strength of the Antarctic Circumpolar Current is more realistic in this simulation. These results show the need for, but do not provide, a better representation of sinking of rejected salt. The sensitivity of our model to sinking of rejected salt suggests that a similar sensitivity may exist in the real ocean, and that loss of Antarctic sea ice might have major effects.
Measurements of gravity wave activity within and around the Arctic stratospheric vortex
Authors: Whiteway J. A. , Duck T. J. , Donovan D. P. , Bird J. C. , Pal S. R. , Carswell A. I.
Published: February 2004

Lidar measurements of gravity wave activity have been conducted at Eureka in the High Arctic since 1993. The waves are detected by the fluctuations they induce in temperature. It has been found that the amount of wave energy in the upper stratosphere is related to the position of the stratospheric polar vortex. In each of the four winters reported here, the wave activity was a maximum within the westerly jet at the edge of the vortex, a minimum inside the vortex near its centre and intermediate outside the vortex. The spectra of wave induced fluctuations show that it is at the longest resolved vertical wavelengths (8 to 15 km) that wave energy is being influenced by the background meteorological conditions. These findings are interpreted in terms of the Doppler shifting and critical level filtering that is imposed by the background wind profile.
Forecasting of ionospheric critical frequency using neural networks
Authors: Altinay Orkun , Tulunay Ersin , Tulunay Yurdanur
Published: February 2004

Multilayer perception type neural networks (NN) are employed for forecasting ionospheric critical frequency (foF2) one hour in advance. The nonlinear black‐box modeling approach in system identification is used. The main contributions: 1. A flexible and easily accessible training database capable of handling extensive physical data is prepared, 2. Novel NN design and experimentation software is developed, 3. A training strategy is adopted in order to significantly enhance the generalization or extrapolation ability of NNs, 4. A method is developed for determining the relative significances (RS) of NN inputs in terms of mapping capability.
Comparison of observed and model magnetic fields at high altitudes above the polar cap: POLAR initial results
Authors: Zhou X.‐W. , Russell C. T. , Le G. , Tsyganenko N.
Published: February 2004

Data obtained from the high altitude, polar orbiting spacecraft, POLAR, are compared with the latest version of the data‐based magnetospheric magnetic field model. The data generally agree well with the model. The major directional discrepancies at high altitudes are near the dayside cusp and on the “open” field lines over the polar cap, especially close to the boundary of the polar cap. Near the cusp, the agreement is improved if a stronger solar wind dynamic pressure and more negative IMF By and Bz are used as the model input parameters, than was actually observed. The field measured in the vicinity of the polar cusps is generally weaker than predicted by the model. Close to noon the spacecraft enters a region of additional structured field depression that appears to be the polar cusp proper. Within the limited statistics presented here, the invariant latitude of the cusp appears to be controlled by the north‐south component of the IMF and the broad depression appears to be controlled by the tilt of the dipole.
Manganese in surface waters of the Atlantic Ocean
Authors: Shiller Alan M.
Published: February 2004

A map of the surface water distribution of manganese in the Atlantic Ocean has been created using 494 data points. Most of these data have been reported in the literature previously, though 93 new analyses are included. Despite this amount of data, many regions of the Atlantic have sparse coverage including much of the South Atlantic. In general the map conforms with previous conclusions regarding the input and removal of Mn in surface ocean waters. Highest concentrations are seen in coastal regions and areas affected by aeolian input; lowest concentrations are observed in high latitude regions. Although some advective features are seen, the lack of stronger advective signatures suggests that the Mn surface ocean residence time is likely close to the low end of the range of previous estimates (i.e., on the order of a few years).
Lower stratospheric chlorine partitioning during the decay of the Mt. Pinatubo aerosol cloud
Authors: Dessler A. E. , Considine D. B. , Rosenfield J. E. , Kawa S. R. , Douglass A. R. , Russell J. M.
Published: February 2004

We present measurements of lower stratospheric HCl between July 1992 and August 1996 made by instruments onboard the Upper Atmosphere Research Satellite (UARS). We find that at 46 hPa the fraction of total inorganic chlorine (Cly) in the form of HCl increased from 0.68 averaged between July 1992 and June 1993 to 0.78 averaged between July 1995 and June 1996. This increase of (16±9)% over three years is much smaller than the trend calculated from in situ measurements made by instruments onboard NASA's ER‐2 high‐altitude aircraft that has been used to suggest problems with our understanding of lower stratospheric chlorine partitioning. The trend determined from the UARS data is slightly larger than the trend predicted by a two‐dimensional model simulation of the atmospheric effects of the Mt. Pinatubo eruption.
Deep water exchange through the Owen Fracture Zone in the Arabian Sea
Authors: Quadfasel Detlef , Fischer Jürgen , Schott Friedrich , Stramma Lothar
Published: February 2004

From geostrophic calculations the exchange of deep water from the Somali into the Arabian Basin through the Owen Fracture Zone has been estimated to be about 2 Sv, with a seasonal modulation of the same magnitude. After leaving the Fracture Zone, the flow bifurcates into a northern and a southern branch, each closely following the slope of the Carlsberg Ridge. The weaker vertical gradients of the hydrographic properties in the deep Arabian Basin are consistent with enhanced vertical mixing at the rugged topography over the Carlsberg Ridge.
Unique products of the reaction of isoprene with atomic chlorine: Potential markers of chlorine atom chemistry
Authors: Nordmeyer Trent , Wang Weihong , Ragains Mark L. , Finlayson‐Pitts Barbara J. , Spicer Chet W. , Plastridge Robert A.
Published: February 2004

The contribution of atomic chlorine to the chemistry of marine regions as well as the Arctic at ground level at polar sunrise is the subject of a number of recent studies. However, identifying the specific chlorine atom precursors has proven difficult. One potential approach is the measurement of definitive products of chlorine atom reactions, for example with biogenic hydrocarbons. We report here product studies of the chlorine atom reaction with isoprene using ppm concentrations at one atmosphere air and 298 K in a NOx‐free system using atmospheric pressure ionization‐mass spectrometry (API‐MS) as well as GC‐MS. 1‐chloro‐3‐methyl‐3‐butene‐2‐one (CMBO) is identified as a unique product of this reaction, and there is evidence of the formation of three additional isomers of CMBO as well. Methyl vinyl ketone (MVK) is formed in small yields (9 ± 5 %), consistent with earlier studies of this reaction in which an upper yield of 13% was reported. The stable product expected from allylic hydrogen atom abstraction (measured in earlier kinetic studies to be 15% of the total reaction), 2‐methylene‐3‐butenal, is also tentatively identified using API‐MS. Assuming that similar chemistry occurs at the ppb‐ppt levels found in the atmosphere, identification of CMBO and/or its isomers in field studies could provide strong evidence of chlorine atom chemistry in low NOx environments where there are also sources of isoprene.
Excess temperature of mantle plumes: The role of chemical stratification across D″
Authors: Farnetani C. G.
Published: February 2004

The temperature increase across the thermal boundary layer at the base of the Earth's mantle (D″) may be as high as 1000–1300°C, while the excess temperature of mantle plumes, inferred from petrological studies, is ca. 200–300°C. These two estimates are generally considered separately, although this should not be the case if plumes do originate from D″. This paper investigates the evolution of plume excess temperature from the source region in the deep mantle to lithospheric depths. A finite element model in axisymmetric geometry is used to explore the effects of a number of model variables. The results show that if the source region is only a thermal boundary layer, the plume excess temperatures are unrealistically high, since entrainment of surrounding mantle does not cool significantly a large plume head rising through the mantle. I then investigate the role of a chemically denser layer at the base of D″, considering a range of values for its thickness and excess density. The presence of a 30 km thick layer, 5% chemically denser than the surrounding mantle, buffers the excess temperature of plumes to plausible values. The results suggest that chemical stratification across D″ may govern the excess temperature of mantle plumes.
Sea‐level fluctuations due to subduction: The role of mantle rheology
Authors: Piromallo C. , Spada G. , Sabadini R. , Ricard Y.
Published: February 2004

By means of a stratified viscoelastic Earth model we study the effect of sinking slabs on the dynamic topography, the non‐hydrostatic geoid and the long‐term sea level variations. Sea level fluctuations due to subduction are found to be sensitive to the nature of the 670 km seismic discontinuity and to the rheological layering of the mantle. The response of our model to both a single subduction and a realistic distribution of slabs is studied by a numerical simulation based on a simplified approach. Consistent with previous results, we find that an upper bound to relative sea level time variations associated with the initiation of a new subduction in the upper mantle is ∼ 0.1 mm/yr. Relative sea level changes driven by the dynamic readjustment of internal mass heterogeneities may thus be comparable with those attributed to other changes in the tectonic regime on a large scale. This confirms the relevance of subduction as an important contributor to long‐term sea level fluctuations.
Solitary waves in the western equatorial Pacific Ocean
Authors: Pinkel R. , Merrifield M. , McPhaden M. , Picaut J. , Rutledge S. , Siegel D. , Washburn L.
Published: February 2004

During the spring tides of early January and February 1993, groups of solitary internal waves were observed propagating through the Intensive Flux Array of the TOGA COARE experiment. The waves appear to originate near the islands of Nugarba (3°S 30′S–154° 30′E). They travel north‐eastward at 2.5–3 m/s, closely coupled with the semi‐diurnal baroclinic tide. Peak amplitudes exceed 60 m. Velocities are in excess of .8 m/s. Seasurface vertical displacements of order .3 m can be inferred directly from the lateral acceleration of surface waters. The Equatorial Undercurrent is displaced by soliton passage but apparently is unaffected otherwise. The intrinsic shear of the solitary crests is small compared to ambient equatorial shears. The crests, while not themselves unstable, are effective at triggering instabilities on the background flow. The motions potentially contribute 10–15 Watts/m² to the flux of heat into the mixed layer.
The apparent effects of the Mt. Pinatubo Eruption on the Greenland Ice Sheet melt extent
Authors: Abdalati Waleed , Steffen Konrad
Published: February 2004

The indirect climatological effects of major volcanic eruptions can be of great importance because of the complex feedbacks that exist in the global climate system. One such effect is the melt area reduction on the Greenland ice sheet. Using passive microwave satellite data for the period 1979–1995, the melt conditions of the ice sheet before and after the 1991 Mt. Pinatubo eruptions are examined. Statistical tests indicate that an observed drop following the eruptions has a 97% to 99% probability of being outside the limits of undisturbed variability. Furthermore, this anomaly diminishes as the time from the eruption increases. These characteristics, in conjunction with an observed increasing melt trend prior to the eruptions indicate that Mt. Pinatubo has had a significant effect on the melt conditions, and consequently the energy balance characteristics of the Greenland ice sheet.
Outflow of hydrogen ions from Ganymede
Authors: Frank L. A. , Paterson W. R. , Ackerson K. L. , Bolton S. J.
Published: February 2004

On 6 September 1996 plasma measurements were obtained in the vicinity of Ganymede as the Galileo spacecraft passed by this moon with a closest approach distance of 261 km. Near Ganymede a dense, cold plasma region was found to be embedded in Jupiter's hot plasma sheet. The cold plasmas are hydrogen ions flowing outwards from Ganymede at supersonic speeds. Temperatures and maximum number densities of these ions were about 4 × 104 K and 100 /cm³, respectively. Over Ganymede's polar caps there is strong plasma convection with speeds in the range of 50 km/s, i.e., the flow is not primarily directed parallel to the local magnetic fields. The corresponding potentials across a Ganymede diameter are in excess of 100 kV. The primary source of the hydrogen ions is believed to be the water ices on the surface of Ganymede. However, the anticipated oxygen ion outflow is not present, which implies that the oxygen is left on the moon's surface. The loss of hydrogen from Ganymede's surface is about 3 × 109 grams/year.
Stratospheric SO3: Upper limits inferred from ion composition measurements ‐ Implications for H2SO4 and aerosol formation
Authors: Reiner Thomas , Arnold Frank
Published: February 2004

Upper limits for the concentration of stratospheric gas phase SO3 have been inferred from a reanalysis of previous balloon‐borne ion composition measurements made at altitudes between 29 and 41 km. Assuming steady state conditions for the production and loss of gaseous SO3 lower limits for the first‐order loss rate coefficient of SO3 could be deduced. These lower limits are compared with calculated loss rate coefficients associated with potential loss processes for SO3 and implications for the formation of gaseous H2SO4 are discussed. In particular, the present analysis shows that no gas phase H2SO4 can be produced from the gas phase reaction of SO3 with H2O at altitudes above the stratospheric H2SO4/ H2O aerosol layer.
Stationary and nonstationary behaviour within the geomagnetic polarity time scale
Authors: Gallet Y. , Hulot G.
Published: February 2004

We analyse the geomagnetic polarity time scale (GPTS) since the Upper Jurassic by displaying the successive lengths of polarity intervals as a function of their order of occurrence. The sequence consists of three segments. Between the Upper Jurassic and the Lower Cretaceous, segment “A” comprises intervals of short duration, with a mean duration of about 0.29 My, and no clear long‐term evolution. Segment “B” begins around 130 Ma, displays a sudden increase of the duration of the magnetic intervals, an interval of maximum duration, the normal Cretaceous superchron, and a long and erratic sequence of intervals with decreasing average duration between 85 Ma and about 25 Ma. From 25 Ma to the present, segment “C” consists of intervals of short duration with a mean value of 0.23 My. This description suggests that the Earth's magnetic field could have experienced a fairly stationary regime until slightly before the onset of the Cretaceous superchron, when the regime has been rapidly and strongly perturbed before progressively returning to another stationary regime about 25 Ma ago. A geophysical explanation for this sequence of events could be that the geodynamo has been perturbed by the arrival of some cold material at the core mantle boundary. As this material would have heated up, the geodynamo would have been brought back to its stationary regime.
The role of ions in the formation and evolution of particles in aircraft plumes
Authors: Yu Fangqun , Turco Richard P.
Published: February 2004

We consider the effects on aircraft plume microphysics of ions generated by chemiionization processes within the engine combustors. Ions provide centers around which molecular clusters rapidly coalesce, thus promoting the formation of electrically charged sulfuric acid/water aerosols. The resulting charged micro‐particles exhibit enhanced growth due to condensation and coagulation aided by electrostatic effects. Simulations with a plume microphysics code show that volatile particles observed behind aircraft may be explained by such processes, as long as initial ion concentrations in the exhaust exceed ∼108/cm³. This analysis also suggests that the primary emissions of sulfuric acid (plus sulfur trioxide) should amount to at least 20–30% of the fuel sulfur to explain the observed number of volatile particles >9 nm in diameter. Ionized plume simulations reveal a distinct bimodal aerosol distribution, in which an “ion” mode constitutes the larger “activated” volatile sulfuric acid particles, while a smaller “neutral” mode comprises the residual slowly‐growing neutral molecular clusters formed in the highly supersaturated region of the plume.
Late Quaternary temperature changes seen in world‐wide continental heat flow measurements
Authors: Huang Shaopeng , Pollack Henry N. , Shen Po Yu
Published: February 2004

Analysis of more than six thousand continental heat flow measurements as a function of depth has yielded a reconstruction of a global average ground surface temperature history over the last 20,000 years. The early to mid‐Holocene appears as a relatively long warm interval some 0.2–0.6 K above present‐day temperatures, the culmination of the warming that followed the end of the last glaciation. Temperatures were also warmer than present 500–1,000 years ago, but then cooled to a minimum some 0.2–0.7 K below present about 200 years ago. Although temperature variations in this type of reconstruction are highly smoothed, the results clearly resemble the broad outlines of late Quaternary climate changes suggested by proxies.
Geocenter variations caused by atmosphere, ocean and surface ground water
Authors: Dong D. , Dickey J. O. , Chao Y. , Cheng M. K.
Published: February 2004

The geocenter variations reflect the global scale mass redistribution and the interaction between the solid Earth and the mass load. Determination of the geocenter variations due to surface mass load from various geophysical sources places constraints on the variations of the origin of terrestrial reference frame, and provides the expected range of variations for space‐geodesy. Our results suggest that on the time scale from 30 days to 10 years the primary variability of geocenter variations from atmosphere, ocean and surface ground water occurs on the annual and semiannual scales. The lumped sum of these surface mass load induced geocenter variations is within 1 cm level.