bluuebunny/crossref_metadata_2025 huggingface dataset:
Titan's magnetic wake: Atmospheric or magnetospheric interaction
Authors: Kabin K. , Israelevich R. L. , Ershkovich A. I. , Neubauer F. M. , Gombosi T. I. , De Zeeuw D. L. , Powell K. G.
Published: September 2002

We compare the magnetic field topology in the Titan wake to an idealized picture of magnetic field lines draping about a conductive nonmagnetic obstacle. It is shown that in the inner part of the wake the magnetic field picture differs significantly from that expected for an idealized draping: The transverse magnetic field component rotates by 90° as compared with the direction of the upstream transverse magnetic field. Another difference is the existence of a deep magnetic field minima separating the inner and outer parts of the wake. Transverse magnetic field rotation can be explained neither by temporal changes in the upstream magnetic field nor by reconnection processes in the wake. We find that this behavior of the transverse magnetic field can be explained at least qualitatively if one assumes the existence of a small intrinsic magnetic field of Titan. An effective magnetic dipole of 1021 G cm3 can account for the observed topology of the Titan magnetic wake. The origin of this field may be related to a residual magnetization of Titan's crust or to induction in a conducting ionosphere of the satellite. We present results of MHD simulations which support the above theoretical conclusion.
Seismovolcanic signals at Deception Island volcano, Antarctica: Wave field analysis and source modeling
Authors: Ibáñez Jesús M. , Pezzo Edoardo Del , Almendros Javier , La Rocca Mario , Alguacil Gerardo , Ortiz Ramón , García Alicia
Published: September 2002

The seismovolcanic signals associated with the volcanic activity of Deception Island (Antarctica), recorded during three Antarctic summers (1994–1995, 1995–1996 and 1996–1997), are analyzed using a dense small‐aperture (500 m) seismic array. The visual and spectral classification of the seismic events shows the existence of long‐period and hybrid isolated seismic events, and of low‐frequency, quasi‐monochromatic and spasmodic continuous tremors. All spectra have the highest amplitudes in the frequency band between 1 and 4 Hz, while hybrids and spasmodic tremors have also significant amplitudes in the high‐frequency band (4–10 Hz). The array analysis indicates that almost all the well‐correlated low‐frequency signals share similar array parameters (slowness and back azimuth) and have the same source area, close to the array site. The polarization analysis shows that phases at high‐frequency are mostly composed of P waves, and those phases dominated by low frequencies can be interpreted as surface waves. No clear shear waves are evidenced. From the energy evaluation, we have found that the reduced displacement values for surface and body waves are confined in a narrow interval. Volcano‐tectonic seismicity is located close to the array, at a depth shallower than 1 km. The wave‐field properties of the seismovolcanic signals allow us to assume a unique source model, a shallow resonating fluid‐filled crack system at a depth of some hundreds of meters. All of the seismic activity is interpreted as the response of a reasonably stable stationary geothermal process. The differences observed in the back azimuth between low and high frequencies are a near‐field effect. A few episodes of the degassification process in an open conduit were observed and modeled with a simple organ pipe.
Earthquake locations in southern California obtained using source‐specific station terms
Authors: Richards‐Dinger K. B. , Shearer P. M.
Published: September 2002

We relocate 297,400 events recorded by the Southern California Seismic Network (SCSN) between 1975 and 1998 using spatially varying station terms to improve relative location accuracy. Our method uses existing SCSNPandSpicks, a smooth one‐dimensional velocity model, and an iterative grid search approach based on the L1 norm. We apply empirical corrections for three‐dimensional structure by computing station timing corrections that continuously vary as a function of source position. Station terms for each event are obtained by smoothing the residuals from nearby events using a natural neighbor (Delaunay) tessellation of the seismicity and then iterating until a stable set of locations and station terms is achieved. Our approach achieves relative location accuracy comparable locally to master event methods but can be applied uniformly over large regions. Median estimated standard errors for our final locations are 328 m in horizontal position and 741 m in depth. Our locations exhibit much less scatter, particularly in depth, than those of the SCSN catalog and a greater tendency to align into linear and planar features suggestive of fault structures. Our results appear comparable to, and in some cases better than, previous SCSN relocation studies using joint‐hypocenter‐velocity inversion techniques. Plots of daytime versus nighttime events permit discrimination between clusters of natural and artificial seismicity. We observe no simple relationship between the maximum depth of seismicity and surface geology.
Synthetic aperture radar interferometry of Okmok volcano, Alaska: Radar observations
Authors: Lu Zhong , Mann Dörte , Freymueller Jeffrey T. , Meyer David J.
Published: September 2002

ERS‐1/ERS‐2 synthetic aperture radar interferometry was used to study the 1997 eruption of Okmok volcano in Alaska. First, we derived an accurate digital elevation model (DEM) using a tandem ERS‐1/ERS‐2 image pair and the preexisting DEM. Second, by studying changes in interferometric coherence we found that the newly erupted lava lost radar coherence for 5–17 months after the eruption. This suggests changes in the surface backscattering characteristics and was probably related to cooling and compaction processes. Third, the atmospheric delay anomalies in the deformation interferograms were quantitatively assessed. Atmospheric delay anomalies in some of the interferograms were significant and consistently smaller than one to two fringes in magnitude. For this reason, repeat observations are important to confidently interpret small geophysical signals related to volcanic activities. Finally, using two‐pass differential interferometry, we analyzed the preemptive inflation, coeruptive deflation, and posteruptive inflation and confirmed the observations using independent image pairs. We observed more than 140 cm of subsidence associated with the 1997 eruption. This subsidence occurred between 16 months before the eruption and 5 months after the eruption, was preceded by ∼18 cm of uplift between 1992 and 1995 centered in the same location, and was followed by ∼10 cm of uplift between September 1997 and 1998. The best fitting model suggests the magma reservoir resided at 2.7 km depth beneath the center of the caldera, which was ∼5 km from the eruptive vent. We estimated the volume of the erupted material to be 0.055 km3 and the average thickness of the erupted lava to be ∼7.4 m.
Upper mantle seismic wave attenuation: Effects of realistic partial melt distribution
Authors: Hammond William C. , Humphreys Eugene D.
Published: September 2002

Frequency dependence of seismic velocity and attenuation resulting from viscoelastic relaxation of partially molten mantle is estimated. We consider the contribution of the melt squirt mechanism, through which pressure differences between disk‐shaped inclusions are equalized by melt passing through connecting tubes. The pressure differences arise as a result of shear strain compressing disk‐shaped pores differently on the basis of disk orientation with respect to the applied shear. The frequencies over which the transition from the unrelaxed to the relaxed states occurs are determined by representing the melt as a network of tubes connecting oblate ellipsoidal pores. The pressure equalization process is modeled by a system of first‐order linear differential equations, whose eigenvalues are the characteristic frequencies for melt squirt relaxation. It is shown that in this framework the set of frequencies is invariant to the absolute scale of the system but is sensitive to melt bulk modulus and viscosity, as well as distribution of melt inside pores and conduits. Use of realistic solid and melt physical properties and pore and conduit geometries demonstrates that it is the relaxed modulus that is most likely excited in the seismic band and that melt mobility has little effect on seismic attenuation. Some conceivable melt distributions, however, would result in detectable attenuation in the seismic band. In all cases investigated, attenuation increases with frequency, indicating that melt squirt is not responsible for global upper mantle Q observations.
An induced seismicity experiment across a creeping segment of the Philippine Fault
Authors: Prioul R. , Cornet F. H. , Dorbath C. , Dorbath L. , Ogena M. , Ramos E.
Published: September 2002

The location of seismicity induced by forced fluid flow provides information about domains of pore pressure variation, while changes in fluid content are identified through changes in seismic velocity. These effects have been investigated in the geothermal field of Tongonan, which lies on a creeping portion of the Philippine Fault on Ley te Island. Locally, the left‐lateral strike‐slip Philippine Fault branches out into three subparallel segments (Eastern, Central and Western Fault Lines). In June–July 1997, a water stimulation was undertaken in a well that intersects the Central Fault Line 1980 m below ground surface; 36,000 m3 were injected between the casing shoe at 1308 m and the well bottom at 2177 m. The seismicity was monitored with a surface station network of 18 stations. More than 400 events, induced by the injection experiment as well as by routine injections associated with the geothermal field exploitation, were recorded in the vicinity of the well. They have been located through a simultaneous three‐dimensional (3‐D) velocity‐hypocenter inversion procedure. None of the microearthquakes are located along the Central Fault Line, they all occurred below the casing shoe to the east of the fault line, i.e., within the geothermal reservoir and mostly below the bottom of the well. Results from the injection experiment and the 18 months of seismic monitoring along the Central and West Fault Lines suggest an aseismic behavior of this major continental fault at this location. The 3‐D velocity model, determined from the travel time inversion for seismic events observed during injections, is compared to that obtained from seismic monitoring conducted prior to any injection activities. An increase of P wave velocity is observed during the water injection. This velocity increase is localized within the seismicity cloud and is interpreted as an increase in liquid content within the initial liquid‐vapor multiphase part of the reservoir.
Whole Earth tomography from delay times of P, PcP, and PKP phases: Lateral heterogeneities in the outer core or radial anisotropy in the mantle?
Authors: Boschi Lapo , Dziewonski Adam M.
Published: September 2002

We perform tomographic inversions of compressional wave travel time measurements, in order to image the laterally varying radial anisotropy of P wave velocity in the entire mantle. In order to achieve an adequate sensitivity to both horizontal and vertical P velocity throughout the mantle, we include in our data set travel time measurements corresponding to ray paths of very different geometries. Specifically, we use data associated with the phases P, PcP (core‐reflected) and PKPbc, PKP df (core‐refracted), all extracted from the bulletins of the International Seismological Centre, 1964–1993. PcP, PKPbc, and PKPdf data are also sensitive to the topography of the core‐mantle boundary (CMB) and (PKP only) to the velocity structure of the Earth's core. These circumstances require that we also allow for CMB topographic anomalies as free parameters in our inversions and investigate the possibility of the existence of lateral heterogeneities in the outer core [e.g., Ritzwoller et al, 1986; Stevenson, 1987; Wahr and de Vries, 1989; Vasco and Johnson, 1998]. The solution models that we propose include alternatively a laterally heterogeneous outer core, a radially anisotropic mantle, or both, so that trade‐offs between those features can also be evaluated. We conclude that only minor, and very localized, radially anisotropic anomalies are present in the middle and lower mantle, the most prominent being located in the deepest 400 km, under the western Pacific. We find a discrepancy, particularly evident by a comparison between different models of the CMB topography, between the travel time measurements that are sensitive to the Earth's core and those that are not; we show that it is not possible to explain this discrepancy only in terms of radial anisotropy of the mantle and/or lateral heterogeneity of the outer core.
Three‐dimensional estimation of elastic thickness under the Louisville Ridge
Authors: Lyons Suzanne N. , Sandwell David T. , Smith Walter H. F.
Published: September 2002

A three‐dimensional approach to estimating elastic thickness is presented which uses dense satellite altimetry and sparse ship bathymetry. This technique is applied to the Louisville Ridge system to study the tectonic history of the region. The inversion is performed as both a first‐order approximation and a nonlinear relationship between gravity and topography based on Parker's [1973] equation. While the higher‐order effect on the gravity anomaly is nearly zero for most of the region, the magnitude is significant over the summits of the ridge. Nevertheless, the inclusion of the nonlinear terms has only a minor influence on the elastic thickness estimate within each region, lowering the value by ∼1–2 km compared with the linear result. The incorrect assumption of two dimensionality for circular features exhibits a marked effect on the gravitational anomaly, resulting in false sidelobe structure of nearly 20 mGal for large seamounts. Our elastic thickness estimates are compared with the contradictory values obtained in previous studies by Cazenave and Dominh [1984] and Watts et al.. [1988]. We find an increasing elastic thickness along the chain from southeast to northwest, with a discontinuity along the Wishbone scarp. The jump in elastic thickness values northwest of the scarp appears to be an indication of an age discontinuity caused by an extinct spreading center north of the ridge.
Paleomagnetism, rock magnetism, and fission track dating of the Kurihashi Granodiorite in northeast Japan: Evidence for a Cretaceous hydrothermal event and differential rotation along the eastern Eurasian margin
Authors: Itoh Y. , Amano K. , Danhara T.
Published: September 2002

Samples from the Late Cretaceous Kurihashi Granodiorite on the Pacific coast of northeast Japan are divided into two groups based on paleomagnetic data. One group (A) preserves stable single thermoremanent magnetizations (TRM) with large westerly deflected declinations. The other group (B) has two or three distinct components of natural remanent magnetization (NRM): components with high, middle, and low unblocking temperatures show westerly declinations (nearly identical with the stable TRM directions of group A), as well as northerly and easterly declinations. Rock magnetic experiments indicate that the dominant magnetic mineral of both groups is Ti‐poor titanomagnetite. Group A samples contain superparamagnetic (SP) titanomagnetite because they show a frequency dependence with the initial susceptibility measurements: Hysteresis parameters of group B samples are consistent with a mixture of single‐domain (SD) and multidomain (MD) grains. Electron microscopy confirms the presence of MD size magnetite in the group B samples. Fission track thermochronology indicates that the Kurihashi Granodiorite underwent a post cooling hydrothermal event during the Late Cretaceous; thus the northerly and easterly NRM components seem to have a thermoviscous origin. Many samples have anomalously high unblocking temperatures (up to 560°C) compared to predictions of SD theory and probably result from long thermal demagnetization “tails” of thermoviscous remanent magnetizations (TVRM) carried by MD magnetite. A strong TVRM was acquired during the hydrothermal event and is correlated with the long normal‐polarity superchron of the geomagnetic field. Because highly oblique (left‐lateral) subduction was active along the eastern Eurasian margin during the same period, the primary TRM and TVRM components record counterclockwise rotation (∼120°) of the Kurihashi Granodiorite and remarkable wrench deformation along the continental margin.
Surface vertical displacements and level plane changes in the front reservoir area caused by filling the Three Gorges Reservoir
Authors: Wang Hansheng
Published: September 2002

On the basis of the results of geological surveys and seismic prospecting in the Three Gorges area, two Earth models are constructed which are appropriate to the crustal structure in the front Three Gorges Reservoir area. Utilizing load‐induced deformation theory, we model the surface vertical displacements and level plane changes in the area caused by increased water loads of the reservoir. The two quantities are needed to determine height changes on the Earth's surface. The numerical results demonstrate that the heights will be significantly changed as the reservoir fills up. For example, when the reservoir water level reaches an elevation of 175 m, the crust subsides by 1.0∼48.3 mm, the level plane uplifts by 1.1∼7.5 mm, and the height falls by 2.1∼45.0 mm. Accordingly, our results provide the reliable corrected values for discriminating between the earthquake‐related crustal motion and the height changes that occur due to reservoir impoundment. Moreover, if the height data previously observed in the area need to be used for engineering surveys after the reservoir is filled, they must be corrected.
Dissolved hydrocarbon flux from natural marine seeps to the southern California Bight
Authors: Clark Jordan F. , Washburn Libe , Hornafius J. Scott , Luyendyk Bruce P.
Published: September 2002

Natural marine seepage near Coal Oil Point, Santa Barbara Channel, California, injects large quantities of hydrocarbons into the coastal ocean. The dispersal and source strength of the injected methane, ethane, and propane from this seep field was determined using a variety of oceanographic and geochemical techniques. The results show that hydrocarbons seep into stratified coastal waters creating plumes that extend for at least 12 km. The plume structure is complex because of the large geographical distribution of seep vents and because of the chaotic nature of advection and mixing near the seeps. At the time of the survey, hydrocarbons were injected onto density surfaces between σθ = 24.5–26.0 kg m−3. Earlier work has shown that subsurface methane maxima in the upper waters of the southern California Bight are typically found on these density surfaces. We estimate that the total flux of methane into the water column above the Coal Oil Point seeps is 2×1010 g yr−1 and is approximately equal to the total flux of dissolved methane to the atmosphere estimated for the entire southern California Bight. These observations strongly support the inference of others that coastal sources, which include some of the world's largest marine hydrocarbon seeps, maintain the methane maximum observed offshore California. Estimates of the global methane flux from coastal waters derived by extrapolating the flux from coastal California may be too large because of the anomalous amount of marine hydrocarbon seepage in these waters.
Dense saline plumes in Exuma Sound, Bahamas
Authors: Hickey B. M. , MacCready P. , Elliott E. , Kachel N. B.
Published: September 2002

Cold dense plumes have been associated with coral killoff on tropical shelves as well as with sediment movement from banks to the deep basins adjacent to shallow banks. This paper presents evidence that plumes of dense, salty water generated over shallow banks entrain ambient water rather than descending intact to a density compensation level (as has sometimes been assumed) and that plumes can spread laterally distances of tens of kilometers from their source. In Exuma Sound, dense, salty water appears to be formed year‐round by cooling or by evaporation over shallow banks. The water is forced by tidal currents through the channels between islands onto the narrow shelves of Exuma Sound where it likely moves as gravity currents. The gravity currents entrain basin water as they cross the shelf and cascade over the steep shelf edge into Exuma Sound, after which they are advected by ambient currents around the sound. A numerical streamtube model was used to explore early details of plume evolution. The model predicts that the plumes entrain 2.6–5.0 parts ambient water, particularly as they traverse the gently sloping shelf. The resulting loss of density contrast means that the model plumes reach their density level in Exuma Sound at relatively shallow depths, 50–95 m, just below the base of the mixed layer. The final depth of modeled plumes is relatively consistent with observed plume depths, which vary seasonally (∼75 m in early winter to ∼45 m in summer) depending on the depth and density characteristics of the seasonal mixed layer. Over the sound, observed plume thickness ranges from ∼40 m in winter to ∼20–30 m in summer. Final salinity values exceed 36.8 practical salinity units; final density ranges from sigmat ∼23.75 to ∼24.5. Plume depth, vertical structure, and longevity are shown to depend on upper ocean mixing processes such as convection and wind mixing.
Tide at the eastern section of the Strait of Gibraltar
Authors: Lafuente Jesús Garcia , Vargas Juan Miguel , Plaza Francisco , Sarhan Tarek , Candela Julio , Bascheck Burkard
Published: September 2002

From October 1995 to April 1996, three mooring lines were deployed at the eastern entrance of the Strait of Gibraltar. The spatial coverage of the mooring array allows for a good description of the tides. They exhibit a dominant semidiurnal nature and a noticeable baroclinic structure that matches the one of the mean exchange. Tidal currents in the upper layer are irregular and usually too weak to reverse the mean upper layer flow that keeps on flowing east. Lower layer flow reverses with semidiurnal periodicity because of the smallness of the mean flow and the appreciable amplitude of the regular semidiurnal oscillation of tidal currents in this layer. Tidal transports can be satisfactorily compared with previous estimates of Bryden et al. [1994] if we allow for strong internal divergences associated with the internal tide. No significant eddy flux of water transport (tidal rectification) is observed at the eastern section, contrary to the almost 50% of the total layer transport found by Bryden et al. [1994] in Camarinal Sill section. Time‐dependent hydraulic theory provides a good scenario for interpreting these two independent sets of observations despite the fact that the composite Froude number does not reach the critical values predicted in the hydraulic models most of the time.
Fractal properties of coastal profile evolution at Duck, North Carolina
Authors: Southgate H. N. , Möller I.
Published: September 2002

Observed variations in coastal morphology on monthly to decadal timescales are often difficult to explain in terms of physical processes operating on shorter timescales. One reason could be that the morphology evolves to high‐dimensional, self‐organized states. Circumstantial evidence for this would be indicated by fractal distributions of the dynamical variables. Fractal analysis techniques are applied to beach level time series collected at Duck, North Carolina, United States. After processing for this study, the data set consists of time series of monthly beach and nearshore levels at up to 86 locations, at 10 m spacing, along each of four shore‐normal profile lines, over a period of 10.5 years. Time series of shoreline positions are also analyzed. Several fractal analysis techniques are used based on the Hurst exponent H, and various sources of error are considered and quantified. Results indicate different fractal behavior in different cross‐shore zones. H values of around 0.5 are found in the nearshore bar zones and are thus consistent with a random Gaussian process, while values around 0.8 are found farther offshore and on upper parts of the beach, indicating long‐range persistency. Further analyses are done to determine time spans over which fractal behavior occurs. Generally, fractal responses are found at locations and timescales where the temporal variations of wave forcing are relatively weak. Suggestions are made to interpret these findings in terms of forced and self‐organized morphodynamic change in different cross‐shore zones and over different timescales. Some possible implications for predicting future coastal morphology are discussed.
Velocity moments in alongshore bottom stress parameterizations
Authors: Feddersen Falk , Guza R. T. , Elgar Steve , Herbers T. H. C.
Published: September 2002

The time‐averaged alongshore bottom stress is an important component of nearshore circulation models. In a widely accepted formulation the bottom stress is proportional to
, the time average of the product of the instantaneous velocity magnitude
and the instantaneous alongshore velocity component υ. Both mean and fluctuating (owing to random, directionally spread waves) velocities contribute to
. Direct estimation of
requires a more detailed specification of the velocity field than is usually available, so the term
is parameterized. Here direct estimates of
based on time series of near‐bottom currents observed between the shoreline and 8‐m water depth are used to test the accuracy of
parameterizations. Common
parameterizations that are linear in the mean alongshore current significantly underestimate
for moderately strong alongshore currents, resulting in overestimation of a drag coefficient determined by fitting modeled (with a linearized bottom stress) to observed alongshore currents. A parameterization based on a joint‐Gaussian velocity field with the observed velocity statistics gives excellent overall agreement with the directly estimated
and allows analytic investigation of the statistical properties of the velocity field that govern
. Except for the weakest flows,
depends strongly on the mean alongshore current and the total velocity variance but depends only weakly on the mean wave angle, wave directional spread, and mean cross‐shore current. Several other nonlinear parameterizations of
are shown to be more accurate than the linear parameterizations and are adequate for many modeling purposes.
A modeling study of the coastal eastern boundary current system off Iberia and Morocco
Authors: Batteen Mary L. , Martinez Johnny R. , Bryan Daniel W. , Buch Eric J.
Published: September 2002

To investigate the northern Canary Current system (NCCS), results from four numerical experiments of increasing complexity are examined. Experiment 1, which uses seasonal wind forcing only, shows that as expected, wind forcing is the key generative mechanism for the current, upwelling, meander, eddy, and filament structures. Experiments 2 and 3, which have the additional effects of irregular coastline geometry, show the following: capes are areas for enhanced upwelling, extensive filaments, maximum current velocities, and enhanced growth of cyclonic meanders and eddies; an embayment like the Gulf of Cadiz is a primary region for anticyclonic meander and eddy development. The results from the complex flow regime of Experiment 4, which has the additional effects of thermohaline gradients and Mediterranean Outflow, highlights the major characteristics and unique features (such as the generation of Meddies) of the NCCS with relatively close similarities to field observations.
Deep and bottom water of the Bransfield Strait eastern and central basins
Authors: Gordon Arnold L. , Mensch Manfred , Zhaoqian Dong , Smethie William M. , de Bettencourt Jose
Published: September 2002

Temperature, salinity, oxygen, and chlorofluorocarbon (CFC) data obtained in September 1997 define the water types ventilating the deep eastern and central basins of the Bransfield Strait. These water types are observed adjacent to Joinville Island and are clearly derived from the Weddell Sea. The eastern basin bottom water characteristics closely match that of the bottom water at 310 dbar depth ∼50 km east of Joinville Island. The eastern basin bottom water is a simple mixture of low‐salinity freezing point shelf water (65%) and relatively warm saline Weddell Deep Water (35%), with a CFC‐113: CFC‐Il ratio age of 8.5 years. The eastern basin bottom water shares a common origin with a weak salinity minimum observed at 1000 dbar within the central basin, though overflow from the central basin to the eastern basin floor may also occur. The bottom water within the central basin is colder and saltier with higher concentrations of oxygen and CFC‐Il than that of the eastern basin. The central basin bottom water with a CFC ratio age of 7.5 years is composed of same Weddell water types that form the eastern basin bottom water plus a large contribution of high‐salinity freezing point shelf water of the characteristics of water observed 10 km northeast of Joinville Island and a small amount of Pacific pycnocline water. The components (and their percentage) are Weddell Deep Water (11%), low‐salinity shelf (24%), high‐salinity shelf (60%), to Pacific pycnocline (5%).
On the importance of leads in sea ice to the energy balance and ice formation in the Weddell Sea
Authors: Eisen O. , Kottmeier C.
Published: September 2002

For a considerable coverage the energy balance of and ice formation by leads in sea ice in the Weddell Sea are evaluated on the basis of data obtained from drifting buoys for the winter periods from 1986 to 1994 and by using a kinematic‐thermodynamic sea ice model. The net heat flux is defined as the sum total of radiative and turbulent fluxes. For thin ice the net turbulent flux is 3–4 times the net radiative flux. The contribution of the net heat flux through open and refrozen leads to the total net heat flux through sea ice is twice as large as the area contribution of open and refrozen leads to the total area covered with sea ice. In the eastern and central parts of the Weddell Sea, leads contribute some 30% to the total energy flux from the ocean to the atmosphere. This flux increases from 10–15 W m−2 in the eastern and central Weddell Sea regions to 30 W m−2 in the western part of the Weddell Sea, where leads contribute more than 80% of the total net energy transfer. The increase is mainly due to the colder and windier atmosphere in connection with the higher variability of the ice motion in the diurnal and semidiurnal band in the western Weddell Sea. The contribution of leads in winter ice formation exceeds 50% in the whole of the Weddell Sea. Monthly area‐weighted ice growth is 10–15 cm in the east and up to 30 cm over the western continental shelf region. In the western part of the Weddell Sea, tidal and inertial motions in the diurnal and semidiurnal bands enhance lead formation, and thus contribute 7% to total net heat flux, 12% to ice formation, and 23% to the salt mass released during ice growth. The results are used to assess quantitatively the importance of leads to the interaction of the ocean, the sea ice, and the atmosphere.
UV‐visible absorption cross sections of nitrous acid
Authors: Stutz J. , Kim E. S. , Platt U. , Bruno P. , Perrino C. , Febo A.
Published: September 2002

Nitrous acid, HONO, is a source of OH radicals in the polluted atmosphere. Although the atmospheric chemistry of HONO is qualitatively understood, not much quantitative information exists. The magnitude of the OH production by HONO photolysis depends on the spectrum of its absorption cross sections; therefore the knowledge of σ'HONO(λ) is essential. The spectrum of the differential cross sections σ'HONO(λ) is needed to detect HONO in the atmosphere by differential optical absorption spectroscopy (DOAS). Here we present measurements of the HONO UV‐visible absorption cross sections with a spectral resolution better than 0.1 nm and a high signal‐to‐noise ratio. The maximum value of the absorption cross sections is σHONO (354 nm) = (5.19±0.26) × 10−19 cm2 and agrees well with literature data. Nevertheless, calculations based on data from this work and on literature data reveal that an uncertainty of ∼15% remains for the HONO photolysis rates. The new σHONO(λ) has been employed in DOAS measurements in Milan, Italy.
Extending the geochronometry of arctic lake cores beyond the radiocarbon limit by using thermoluminescence
Authors: Berger G. W. , Anderson P. M.
Published: September 2002

Reconstructing detailed paleoclimatic/paleoenvironmental histories from Pleistocene sediments in the Arctic is an ongoing challenge because of the difficulty of measuring accurate timescales beyond the range of radiocarbon (14C) dating. Important archives of such histories are arctic lakes that exist outside the known extent of the last major glaciations. To develop reliable geochronometry of sediment cores from arctic and other lakes, we applied thermoluminescence (TL) sediment‐dating methods to 24 samples (8 reported earlier but some reanalyzed here) selected from cores from Squirrel and Joe Lakes in northwestern Alaska. A sequence of 2314C ages from the upper parts of these cores permitted tests of the zeroing assumption of TL dating: that all fine‐silt grains were exposed to daylight before final burial. We performed multiple TL tests on most samples to isolate stable TL signals and measured TL emissions spectra to help characterize samples. TL ages generally agree with the ∼30 kyr age‐depth trend derived from14C dating and extend the timescale apparently to ∼180 ka for Squirrel Lake and to at least ∼60 ka for Joe Lake. An anomalously old TL result from Squirrel Lake is attributed to failure of the TL zeroing assumption within that sediment horizon. In the lower Joe Lake core we assign a timing of 45 ka to 50 ka for the beginning of an apparently climatic warming interval. These results identify and quantify the main TL sources of TL age inaccuracy and the preferred TL protocols for accurate TL sediment dating in northern high‐latitude lake deposits. They also provide the first direct chronology beyond the14C dating range for these lakes.
Chemical composition of aerosols collected over the tropical North Atlantic Ocean
Authors: Johansen Anne M. , Siefert Ronald L. , Hoffmann Michael R.
Published: September 2002

Ambient aerosol samples were collected over the tropical northern Atlantic Ocean during the month of April 1996 onboard the R/V Seward Johnson. Dichotomous high‐volume collector samples were analyzed for ferrous iron immediately after collection, while trace metals, anions, and cations were determined upon return to the laboratory. Data are analyzed with the aid of enrichment factor, principal component, and weighted multiple linear regression analyses. Average mineral aerosol concentrations amounted to 19.3±16.4 μg m−3 whereby the chemical characteristics and air mass back trajectories indicated the dust to be of a typical shale composition and Saharan origin. Calcite accounted for 3.0 and 7.9% of the mineral aerosol during the first and second halves of the cruise, respectively. Total iron concentrations (averaging 0.84±0.61 μg m−3) are crustally derived, of which 0.51±0.56% is readily released as Fe(II). Eighty‐six percent of this Fe(II) is present in the fine (<3 μm diameter) aerosol fraction and correlates with NSS‐SO42− and oxalate. Approximately 23% of the measured NSS‐SO42− in both size fractions appears to be biogenically derived, and the rest is of anthropogenic nature. Biogenic SO42− /methanesulfonic acid (MSA) ratios could not be easily extracted by employing a multiple linear regression analysis analogous to that of Johansen et al. [1999], possibly due to the varying characteristics of the aerosol chemistry and air temperature during the cruise. Because of the presence of anthropogenic SO42−, the non‐sea‐salt (NSS)‐ SO42−/MSA ratio, 37.4±6.4, is elevated over what would be expected if the NSS ‐ SO42− were purely biogenic. Cl− depletion is seen in all samples and averages 18.3±9.1%. The release of Cl from the aerosol phase appears to occur through acid displacement reactions with primarily HNO3 in the coarse and H2SO4 in the fine fraction.
Nitric and nitrous oxide emissions following fertilizer application to agricultural soil: Biotic and abiotic mechanisms and kinetics
Authors: Venterea Rodney T. , Rolston Dennis E.
Published: September 2002

Emissions of nitric and nitrous oxide (NO and N2O) from agricultural soils may have several consequences, including impacts on local tropospheric and global stratospheric chemistry. Elevated NO and N2O emissions following application of anhydrous ammonia to an agricultural field in California were driven by the biological generation of nitrite (NO2−) and subsequent abiotic decomposition of nitrous acid (HNO2). Maximum fluxes of > 1000 ng NO‐N cm−2h−1and > 400 ng N2O‐N cm−2h−1were observed, and emissions of > 100 ng NO‐N cm−2h−1and > 50 ng N2O‐N cm−2h−1persisted for >4 weeks. Laboratory experiments were performed to determine rate coefficients and activation energies for HNO2‐mediated NO and N2O production. Kinetic parameters describing the conversion of NO to N2O were measured and were found to vary with water‐filled pore space (WFPS). Regression models incorporating HNO2, WFPS, and temperature accounted for 75–77% of the variability in field fluxes. A previously developed NO emissions model was modified to incorporate a kinetic expression for HNO2‐ and temperature‐dependent production. The model tended to underestimate fluxes under low‐flux conditions and overestimate fluxes under high‐flux conditions. These data indicate that (1) control of acidity may be an effective means for minimizing gaseous N losses from fertilized soils and possibly for improving air quality in rural areas, (2) the transformation of HNO2‐derived NO may be an important mechanism of N2O production even under relatively aerobic conditions, and (3) mechanistic models which account for spatial heterogeneity and transient conditions may be required to better predict field NO fluxes.
Sensitivity of sulphate aerosol size distributions and CCN concentrations over North America to SOx emissions and H2O2 concentrations
Authors: von Salzen K. , Leighton H. G. , Ariya P. A. , Barrie L. A. , Gong S. L. , Blanchet J.‐P. , Spacek L. , Lohmann U. , Kleinman L. I.
Published: September 2002

To assess the influence of aerosols on climate, the Northern Aerosol Regional Climate Model (NARCM) is currently being developed. NARCM includes size‐segregated aerosols as prognostic and interactive constituents. In this paper, the model is being applied to sulphate aerosol over North America during time periods in July and December 1994. The results give evidence for considerable regional and seasonal variations in sulphate aerosol size distributions over North America. Comparisons of the results with different observations yield a reasonably good agreement in terms of meteorological and physicochemical parameters. Some of the differences in sulphate concentrations and wet deposition rates can be attributed to differences in cloud amounts and precipitation between model results and observations. Indirect tests of the simulated aerosol mass mean diameters are also encouraging. Additional simulations for hypothetical decreases in anthropogenic sulphur emissions and increases in hydrogen peroxide (H2O2) background concentrations are performed for the same time periods to study the responses of concentration, size distribution, and wet deposition of sulphate aerosol to these changes. Also, responses of cloud condensation nuclei (CCN) number concentrations are investigated. The simulation results show that sulphate aerosol concentrations respond almost linearly in both time periods to decreases in sulphur emissions but that CCN number concentrations respond nonlinearly due to decreases in sulphate mass mean diameters. Especially for the December period, increases in hydrogen peroxide background concentrations lead to increases in CCN number concentrations at critical diameters larger than about 0.07 μm. These results lead to the hypothesis that increased in‐cloud oxidation in convective clouds due to future increases in oxidant concentrations may produce larger CCN which eventually can be easily activated in subsequently forming stratiform clouds.
Temperature changes due to gravity wave saturation
Authors: Liu Han‐Li
Published: September 2002

Turbulence mixing due to gravity wave saturation may both induce dynamical cooling and produce dissipative heating. The net effect is studied here using the linear saturation theory by considering the wave heat flux divergence, turbulence dissipative heating, and turbulent diffusion. From the total heating rate it is shown that the net effect is critically dependent on the turbulence Prandtl number as well as on the localization of the turbulence region. The results from the linear analysis are consistent with previous observational and numerical results.
Ozone loss rates in the Arctic stratosphere in the winter 1994/1995: Model simulations underestimate results of the Match analysis
Authors: Becker Gaby , Müller Rolf , McKenna Daniel S. , Rex Markus , Carslaw Kenneth S. , Oelhaf Hermann
Published: September 2002

We present box model simulations of ozone loss rates in the Arctic lower stratosphere for the winter 1994/1995. The ozone loss was simulated along each of the trajectories of the Match data set for 1994/1995 to conduct a quantitative comparison with the Match results. The simulated ozone loss rates reach their maximum value of ≈ 4 ppb per sunlit hour at the end of January. For this period and for potential temperatures below 475 K the model results are in good agreement with the Match results, but for potential temperatures above 475 K the model underestimates the ozone loss rate by up to a factor of 2. This difference cannot be explained by known uncertainties of the model. Enhanced ozone loss has also been observed in March 1995 for potential temperatures below 475 K. These loss rates are also substantially underestimated by the model, but are within the range of the model uncertainties. In particular, the ozone loss rates simulated for March 1995 strongly depend on the extent of denitrification.
Impact of acetone on ozone production and OH in the upper troposphere at high NOx
Authors: Folkins Ian , Chatfield Robert
Published: September 2002

The impact of acetone (or any HOx source) on tropospheric photochemistry is largest in the high NOx regime. The fractional increases in OH and ozone production associated with acetone increase rapidly with NOx when NOx mixing ratios become larger than 300 parts per trillion by volume (pptv). This occurs in part because the HOx yield of acetone is larger at higher NOx mixing ratios, going from about 1 HOx at NOx ∼ 10 pptv to 3 HOx at NOx ∼ 1000 pptv. We also investigate the effect of acetone on the partitioning of the NOy family. Acetone increases the conversion of NO to NO2, HNO4, HNO3, and peroxyacetylnitrate (PAN). Conversion of NO to PAN dominates at low NOx, while conversion of NO to HNO3 dominates at high NOx. These NO decreases significantly diminish the increases in OH and ozone production one would otherwise anticipate from the increases in HO2. In particular, acetone can be expected to reduce ozone production for NOx<25 pptv. We also show that most of the changes in species concentrations arising from the introduction of a HOx source can be accurately predicted using simple expressions derived from linear perturbation theory.
CO2+O(1D) isotopic exchange: Laboratory and modeling studies
Authors: Johnston J. C. , Röckmann T. , Brenninkmeijer C. A. M.
Published: September 2002

Carbon dioxide in the middle atmosphere is mass independently enriched in the heavy oxygen isotopes relative to tropospheric values. That is, increasing with altitude, the 17O/16O ratio shows an additional enhancement over what is expected on the basis of the 18O/16O increase. As tropospheric CO2 has a mass‐dependent isotopic composition that varies by less than 3‰ in δ18O, isotopic measurements of middle atmospheric CO2, combined with a quantitative understanding of the enrichment mechanism, could provide valuable information regarding processes such as stratosphere‐troposphere exchange and the mean age of an air mass. It is known that the mass‐independent enrichment in stratospheric CO2 occurs when CO2 quenches an O(1D) atom formed by the photolysis of O3, but the details of this process remain uncertain. Here a series of laboratory and numerical experiments are presented which have been performed to study the time evolution and final equilibrium values of the CO2+O(1D) reaction in an effort to reach a better understanding of the CO2 enrichment mechanism. Results show that while the isotopic composition of the CO2 reservoir is qualitatively controlled by the isotopic composition of the O(1D) reservoir, there are a number of complicating factors. The simple mixing model discussed here consistently overpredicts the measured isotopic enrichment, thus indicating the CO2+O(1D) isotopic exchange is more complicated than has generally been recognized.
Interannual variability of the vertical descent rate in the Antarctic polar vortex
Authors: Kawamoto Nozomi , Shiotani Masato
Published: September 2002

To investigate a descent rate in the Antarctic polar vortex, we analyzed the long‐lived trace gas data derived from the Halogen Occultation Experiment on board the Upper Atmosphere Research Satellite during the 6‐year period from 1992 to 1997. By comparing the Antarctic fall (February and March) and spring (September and October) methane profiles, we estimated the middle stratospheric descent for each of the six winters. Large year‐to‐year variations are seen (1.2–1.8 km month−1 at 0.6 ppmv), which consist of a biennial oscillation and a decreasing trend for the period analyzed. The descent rate is larger in the even years (1992, 1994, and 1996) than in the odd years (1993, 1995, and 1997). Dynamical fields for the 6 years are also analyzed using the United Kingdom Meteorological Office assimilation data. The differences between the even and odd years are clear in the midwinter. In the even years the downward and poleward movement of the westerly jet occurs earlier. The thermal wind relation infers that this event is associated with the development of a “warm pool” around the Antarctic stratopause, resulting from adiabatic heating due to the downward motion of air. Planetary wave activity over the winter season is more vigorous in the even years than in the odd years, suggesting a close relationship between the mean flow and planetary waves.