bluuebunny/crossref_metadata_2025 huggingface dataset:
Variable conductivity: Effects on the thermal structure of subducting slabs
Authors: Hauck Steven A. , Phillips Roger J. , Hofmeister Anne M.
Published: September 2002

Understanding the thermal structure of the lithosphere in subduction zone environments is a key element in the realization of the source of deep focus earthquakes and other geodynamic processes. Model predictions of the thermal structure of subducting slabs are dependent upon a priori knowledge of how thermal conductivity varies within the slab. Using recent results for the variation of thermal conductivity as a function of temperature, pressure, and gross mineralogy, we investigate how models that utilize a realistic model for variable conductivity compare with canonical models for constant conductivity. Results suggest that jumps in thermal conductivity across the phase boundaries that arise due to transformations to β and γ‐spinel phases lead to old, fast, steeply dipping slabs being considerably warmer than previously thought. Although these results do not rule out the possibility of a metastable wedge source of deep focus earthquakes, they do imply the likelihood of metastable olivine being found at depth is less than once thought.
Epicenter distribution and magnitude of earthquakes in fold‐thrust belts: Insights from Sandbox Models
Authors: Koyi Hemin. A. , Hessami Khaled , Teixell Antonio
Published: September 2002

Scaled analogue models are used to illustrate the effect of basal friction and erosion on fault activity and hence on epicentre distribution and magnitude of earthquakes in the sedimentary cover of active fold‐thrust belts. Model results suggest that in fold‐thrust belts shortened above low‐friction ductile decollements (rock salt or over‐pressured mudstone), low‐ to moderate‐magnitude earthquakes (Mw = 5.3–5.6), distributed over a wide area, occur along several long‐lived thrust faults. Conversely, in areas shortened above high‐friction decollements large‐magnitude earthquakes (Mw = 6.6–6.8), distributed over a narrow zone are likely to occur along few short‐lived thrust ramps. Calculated magnitude of earthquakes from models and their distribution are in agreement with recorded earthquake pattern from the Zagros mountain belt, which is partially shortened above a ductile decollement of Hormuz salt formation. Model results also showed that erosion reactivates older inactive thrusts and promotes formation of out‐of‐sequence thrusts.
The hydraulic conductance of a capillary with a sinusoidally varying cross‐section
Authors: Bernabé Yves , Olson John F.
Published: September 2002

Using lattice‐gas cellular automata we simulated fluid flow through a straight tube with a sinusoidally varying cross‐section, i.e., what we call a varicose tube. We found that the hydraulic conductance of such a constricted tube was bounded between the long‐wavelength values predicted by Reynolds' equation and the so‐called “stagnant limit” at short wavelengths. In particular, we conclude that pore space associated with roughness at wavelengths shorter than about 15% of the pore radius, should not contribute to hydraulic conductivity and need not be resolved for flow simulation purposes.
Does molecular HNO3 adsorb onto sulfuric acid droplet surfaces?
Authors: Donaldson D. J. , Anderson Darren
Published: September 2002

Room temperature surface tension measurements on the ternary system: nitric acid‐sulfuric acid‐water are reported. There is a dramatic decrease in the surface tension of sulfuric acid‐water solutions as nitric acid is added. This decrease is most pronounced in the stratospherically‐relevant range of sulfuric acid concentrations, between 50 and 75 wt% sulfuric acid. The surface tension decrease indicates a change in surface energy and surface composition and/or structure associated with partitioning of molecular HNO3 to the surface. The presence of HNO3 at the surface may be related to the reported changes in heterogeneous reactivity of sulfuric acid solutions when exposed to nitric acid.
Tilt signals derived from a GEOSCOPE VBB Station on the Piton de la Fournaise Volcano
Authors: Battaglia Jean , Aki Keiiti , Montagner J. ‐P.
Published: September 2002

We report some very long period transients observed on a very broadband seismic station of the GEOSCOPE network, which is situated 8 km away from the summit of the Piton de la Fournaise volcano. We transform the ground velocity measured by the seismic station into tilt and test the result by comparing with data from Blum‐type classical pendulum tiltmeters located in the same vault. The comparison validates the use of the horizontal seismometers for measuring tilt for periods shorter than 25 hours. The method is applied to the transients observed prior to several eruptions and magmatic intrusions since 1991. Because of the moderate distance between GEOSCOPE station and the summit of the volcano, the obtained tilt measurements provide information about eruptions and intrusions which complement the information on the geometry and location of the magmatic source provided by the summit tilt network.
Climate implications of GWP‐based reductions in greenhouse gas emissions
Authors: Fuglestvedt J. S. , Berntsen T. K. , Godal O. , Skodvin T.
Published: September 2002

Possible implications of a GWP‐based implementation of GHG reductions on future man‐made climate change are explored. Abatement scenarios are established in terms of “CO2 equivalents” using GWP100; one scenario reducing CO2 only, and one reducing short‐lived gases, mainly CH4. Future temperature changes are calculated by using a simple climate model. Equal emission scenarios in terms of CO2 equivalents do not imply equivalent climate responses and large differences in the magnitude and rate of temperature change are calculated.
A method for real‐time detection of PAN, PPN and MPAN in ambient air
Authors: Hansel Armin , Wisthaler Armin
Published: September 2002

A selected ion flow drift tube (SIFDT) apparatus and a proton transfer reaction mass spectrometer (PTR‐MS) were used to study the reactions of protonated water H3O+ and the proton bound water dimer (H2O)2H+ with PAN (peroxyacetic nitric anhydride, CH3C(O)OO NO2), PPN (peroxypropionic nitric anhydride, CH3CH2C (O)OONO2) and MPAN (peroxymethacrylic nitric anhydride, CH2C(CH3)C(O)OONO2). The protonated hydrates react rapidly with PAN, PPN and MPAN. Product ion peaks using ambient air as reactant gas could be unequivocally attributed to PAN, PPN and MPAN utilizing the thermal decomposition of these compounds on a hot surface. The results demonstrate that (H2O)nH+ (n=1,2) ions are excellent reagent ions for the real‐time detection of PAN and other organic nitrates in ambient air.
Cloud‐vegetation interaction: Use of normalized difference cloud index for estimation of cloud optical thickness
Authors: Marshak A. , Knyazikhin Y. , Davis A. B. , Wiscombe W. J. , Pilewskie P.
Published: September 2002

A new technique to retrieve cloud optical depth for broken clouds above green vegetation using ground‐based zenith radiance measurements is developed. By analogy with the Normalized Difference Vegetation Index (NDVI), the Normalized Difference Cloud Index (NDCI) is defined as a ratio between the difference and the sum of two zenith radiances measured for two narrow spectral bands in the visible and near‐IR regions. The very different spectral behavior of cloud liquid water drops and green vegetation is the key physics behind the NDCI. It provides extra tools to remove the radiative effects of the 3D cloud structure. Numerical calculations based on fractal clouds and real measurements of NDCI and cloud liquid water path confirm the improvements.
Absorption of P‐ and S‐waves above subduction zones
Authors: Temme Yorck‐Fabian , Weber Michael , Wicks Charles W.
Published: September 2002

We investigate local variations of absorption above the subduction zones of the northwest Pacific. Teleseismic broad‐band (BB) recordings at German arrays from 59 earthquakes in this area allow the determination of the quality factor QP and QS. The method to determine Q is the phase pair method. Here, for the first time, P‐wave BB‐recordings at dense arrays are used, extending the frequency range studied and increasing the reliability of the QP determination for the northwest Pacific. 1‐D Earth models have typical QP values of 200 to 800 (QS: 100 to 400) in the upper 200 km. QP under the Kuriles, the Kurile Basin and Japan is about 100, i.e. significantly lower than average. QS under Japan is also only about 1/3 of the value of 1‐D models. No frequency dependence of QP and QS could be detected for the broad frequency range studied [0.05 to 1 Hz].
Tropospheric wind and humidity derived from spaceborne radar intensity and phase observations
Authors: Hanssen Ramon , Weinreich Ilona , Lehner Susanne , Stoffelen Ad
Published: September 2002

Causal connections between mesoscale wind patterns and moisture distribution in the lower boundary layer can be studied using satellite synthetic aperture radar (SAR) observations. Wind information is retrieved from radar backscatter intensity over water areas, whereas the moisture distribution is reflected in the radar signal delay. The latter can only be observed over land areas, using the interferometric combination of two radar images. This paper reports on the relation between wind and moisture associated with boundary layer rolls and with a cold‐frontal rain band. The combination of both signals, originating from one sensor, is used to infer a quantitative description of the underlying dynamical processes.
Can we trust proxy‐based NAO index reconstructions?
Authors: Schmutz C. , Luterbacher J. , Gyalistras D. , Xoplaki E. , Wanner H.
Published: September 2002

Three existing cold season North Atlantic Oscillation (NAO) indices which were reconstructed from proxy data were assessed for their consistency and robustness at the inter annual and decadal timescales. All three indices were found to be inconsistent with an observed NAO index, whereas two other, mainly instrument ally based indices, showed good agreement with observations. The combination of the proxy‐based indices into one new index by means of multivariate regression yielded some improvement, such that the skill of the new index was comparable or higher than the individual indices. However, the new index still did not correlate significantly with the instrumentally based reference time series prior to 1850. Our results suggest non‐stationarities in the statistical relationships between the proxy‐based indices and the NAO. This has important implications with regard to the choice of verification periods and predictor data‐base for upcoming reconstruction attempts.
Seasonal trends in stratospheric water vapour
Authors: Smith Claire A. , Toumi Ralf , Haigh Joanna D.
Published: September 2002

Published analyses of HALOE water vapour data have shown an increase in stratospheric water vapour. Here the latest version of the data has been analysed for trends from Jan 1992 to April 1999. This shows an increase in stratospheric water vapour over this period although in the past few years the magnitude of the trend appears to have decreased. A seasonal analysis suggests that large increases in upper tropospheric water vapour in autumn are the source for the stratospheric increases. An analysis of trends in the seasonal amplitude supports this. It is suggested that an increase in the extremity of the Asian monsoon over Indonesia could be responsible.
Decadal variability in the lower‐tropospheric lapse rate
Authors: Brown Simon J. , Parker David E. , Folland Chris K. , Macadam Ian
Published: September 2002

Climate models predict that the surface and the troposphere will warm in response to increasing concentrations of greenhouse gases. Both have warmed since 1958, but radiosonde and satellite observations suggest that the atmosphere has not followed the strong surface warming over the last 20 years especially in the tropics. In the light of this, we investigate observed tropical lower‐tropospheric lapse rate variability on decadal and longer time scales using radiosonde data since 1958. We find evidence of interdecadal changes in the surface minus 700 hPa temperature difference with reduced values throughout the tropics in the late 1970s to mid 1980s followed by an increase to the present day. This ubiquitous signal implies that the lapse rate changes are real rather than artefacts of the radiosonde record. Thus, real variability may explain much of the discrepancy in tropical temperature trends.
Detection of a lightning influence on tropical tropospheric ozone
Authors: Martin Randall V. , Jacob Daniel J. , Logan Jennifer A. , Ziemke Jerry M. , Washington Richard
Published: September 2002

Empirical orthogonal functions (EOFs) are used to analyze a 14‐year record of tropical tropospheric ozone columns (TTOCs) determined from satellite measurements. The first EOF explains 54% of TTOC variance and represents biomass burning in Africa and South America. The second EOF explains 20% of the variance and has a distinct lightning signature.
Increased carbon sequestration by a boreal deciduous forest in years with a warm spring
Authors: Black T. A. , Chen W. J. , Barr A. G. , Arain M. A. , Chen Z. , Nesic Z. , Hogg E. H. , Neumann H. H. , Yang P. C.
Published: September 2002

A boreal deciduous forest in Saskatchewan, Canada, sequestered 144±65, 80±60, 116±35 and 290±50 g C m−2 y−1 in 1994, 1996, 1997 and 1998, respectively. The increased carbon sequestration was the result of a warmer spring and earlier leaf emergence, which significantly increased ecosystem photosynthesis, but had little effect on respiration. The high carbon sequestration in 1998 was coincident with one of the strongest El Niño events of this century, and is considered a significant and unexpected benefit.
High‐resolution reflection seismics applied to detection of groundwater resources in glacial deposits, Sweden
Authors: Juhlin Christopher , Palm Hans , Müllern Carl‐Fredrik , Wållberg Bo
Published: September 2002

A large proportion of Sweden's ground water resources is contained in sand/gravel glaciofluvial deposits that overlie Precambrian crystalline basement and are covered by clay. The depth to the sand/gravel deposits varies from near‐surface to about 100 m and their thickness is generally 10–20 m. Traditionally, due to logistics, short refraction seismic profiles and penetration tests have been used to localize the sand/gravel deposits below the clay and to determine their thickness. However, when the overlying clay deposits are thick or the glacial deposits show significant 2‐D structure, refracting waves may not penetrate into the sand/gravel deposits or into the crystalline basement. Reflection seismic methods provide an alternative mapping method and a test profile was acquired over glacial deposits where the structure is well known. Standard processing shows, in general, excellent agreement between the seismic image and the structure determined from penetration tests.
Relationship between column aerosol optical thickness and in situ ground based dust concentrations over Barbados
Authors: Smirnov A. , Holben B. N. , Savoie D. , Prospero J. M. , Kaufman Y. J. , Tanre D. , Eck T. F. , Slutsker I.
Published: September 2002

Aerosol optical depth measurements over Barbados acquired through the AERONET network are analyzed. Optical depth shows a pronounced seasonal pattern, with a maximum observed during summer. The temporal trends of the sunphotometry data were similar to those of ground‐based dust concentration measurements. A simple linear regression relationship has been established between mean monthly values of aerosol optical depth and dust concentrations (correlation coefficient r=0.93), which allows the estimation of optical depth for any specific time period when dust concentration measurements are available. The robustness of the regression was validated with an independent data set.
GEOTAIL observations of flow velocity and north‐south magnetic field variations in the near and mid‐distant tail associated with substorm onsets
Authors: Machida S. , Miyashita Y. , Ieda A. , Nishida A. , Mukai T. , Saito Y. , Kokubun S.
Published: September 2002

Temporal changes in the plasma flow and magnetic fields of the Earth's magnetotail in the region of −7.5 > X(RE) > −52.5 and −15 < Y(RE) < 15 in the GSM coordinates were studied by applying a superposed epoch analysis method to 263 substorm events observed by GEOTAIL spacecraft. The onset times of the substorms were determined with Pi‐2 pulsations at Kakioka station. It was found that the tailward flow associated with plasmoids started about 2 or 3 min before the Pi‐2 onsets, and the earthward flow became somewhat coherent 1 min before the onsets. The variation of the northward magnetic field component was investigated by obtaining the deviation of this component from that in the interval from 10.5 to 7.5 min before the Pi‐2 onsets. We found that the northward magnetic field increased at X ∼ −10RE and decreased at X ∼ −28RE simultaneously in the premidnight tail immediately after the onsets. These variations definetely correspond to the dipolarization and plasmoid formation, respectively.
Discrete Auroral Arcs and Nonlinear Dispersive Field Line Resonances
Authors: Rankin R. , Samson J. C. , Tikhonchuk V. T.
Published: September 2002

Dispersive effects in field line resonances (FLRs) are discussed in the context of potential structures, parallel currents, and auroral density cavities observed by the FAST satellite. Our model includes the Earth's dipole magnetic field, and accounts for electron inertia, electron thermal pressure, finite ion gyroradius effects, and field aligned variations of the plasma density and ambient electron and ion temperatures. For realistic backgound parameters, we show that finite plasma temperature effects determine the dynamics of FLRs and that solitary wave structures evolve out of the resonance region, producing deep density cavities above the polar ionospheres. Results are shown to be in reasonable agreement with ground and satellite observations, with the exception of the magnitude of low altitude electric fields.
Elves triggered by positive and negative lightning discharges
Authors: Barrington‐Leigh C. P. , Inan U. S.
Published: September 2002

Optical flashes in the lower ionosphere due to the transient heating caused by lightning electromagnetic pulses (EMP) are unambiguously identified with the Fly's Eye photometric array. Data from a thunderstorm over Mexico recorded at Langmuir Laboratory on August 27 1997 demonstrate that relatively common negative cloud‐to‐ground lightning is a previously unrecognized major cause of elves. The spatial extent of the transient heating is shown optically to be typically at least 200–700 km laterally, indicating the possibility for accumulation of ionization effects produced by successive flashes within large nighttime thunderstorm systems. One especially bright event suggests that temporal fine‐structure in the causative very low frequency EMP can manifest itself in the photometric record of elves.
Spectral Law and Polarization Properties of the Low‐Frequency Waves at the Magnetopause
Authors: Rezeau L. , Belmont G. , Cornilleau‐Wehrlin N. , Reberac F. , Briand C.
Published: September 2002

The magnetic fluctuations, at the magnetopause and in the adjacent magnetosheath, exhibit power law spectra which are very reminiscent of turbulent spectra. In prospect of future modelizations of such a turbulence, new information is brought about the experimental properties of these fluctuations. The power laws spectra previously obtained in the ULF range are shown to hold also in VLF, up to the lower hybrid frequency. Concerning the polarization, 1) the direction with respect to the static magnetic field is shown to be dominantly perpendicular at low frequencies, consistently with Shear Alfven modes in this range, and 2) no right‐hand sense of rotation can be evidenced at frequencies higher than the proton gyrofrequency, although one could expect the fast magnetosonic mode to be dominant in this range. The physical implications of this last observation for the non linear effects at work in the turbulence are briefly discussed.
Small mantle upwellings are pervasive on Venus and Earth
Authors: Herrick Robert R.
Published: September 2002

Observations of Venus and Africa, an analogous terrestrial plate, indicate that upwellings a few hundred kilometers across are pervasive in the ambient upper mantle of both Venus and Earth. Because of their small size, when they reach the lithosphere the upwellings affect the surface only under special circumstances. On Venus they form volcano‐tectonic structures called coronae where the lithosphere has been thinned by rifting. On Earth the small upwellings produce topographic swells where they impinge on a plate stationary with respect to the hot‐spot reference frame, as is Africa. The small upwellings' presence throughout the mantle explains both the corona paradox, in which coronae are associated with rifts on Venus but do not appear to drive rifting, and the basin and swell topography of the African plate. The morphology of coronae requires that the small upwellings have some diapiric properties. Their pervasiveness, transient nature, and generally small size make them fundamentally different from traditional Hawaiian‐style plumes, and they may be detached diapirs within a mantle convecting in the hard‐turbulence regime.
Cirrus detection using HALOE measurements
Authors: Hervig Mark , McHugh Martin
Published: September 2002

Cirrus effects in the Halogen Occultation Experiment (HALOE) are investigated and methods are developed to use HALOE multiwavelength particulate extinction measurements for cloud identification. Techniques were explored to identify cloud layers from the extinction spectrum, vertical gradient, and magnitude. Results from HALOE measurements are presented, including cirrus climatologies and comparisons of cloud top height with independent measurements.
Northern hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations
Authors: Mann Michael E. , Bradley Raymond S. , Hughes Malcolm K.
Published: September 2002

Building on recent studies, we attempt hemispheric temperature reconstructions with proxy data networks for the past millennium. We focus not just on the reconstructions, but the uncertainties therein, and important caveats. Though expanded uncertainties prevent decisive conclusions for the period prior to AD 1400, our results suggest that the latter 20th century is anomalous in the context of at least the past millennium. The 1990s was the warmest decade, and 1998 the warmest year, at moderately high levels of confidence. The 20th century warming counters a millennial‐scale cooling trend which is consistent with long‐term astronomical forcing.
Emissions of volatile organic compounds from cut grass and clover are enhanced during the drying process
Authors: de Gouw Joost A. , Howard Carleton J. , Custer Thomas G. , Fall Ray
Published: September 2002

The release of volatile organic compounds (VOCs) by drying grass and clover leaves and stems was studied in the laboratory using proton‐transfer chemical‐ionization mass spectrometry, which enables the simultaneous, on‐line monitoring of VOC concentrations. A burst of VOC emissions due to cutting the leaves and stems was followed by a second, more intense emission lasting for several hours when the vegetation was starting to dry out. In addition to (Z)‐3‐hexenal, (Z)‐3‐hexenol, and hexenyl acetate, that were emitted by the plant tissue in response to the wounding, enhanced emissions of methanol, acetaldehyde, acetone, butanone, and possibly formaldehyde were observed. These findings may have important implications for regional air quality in agricultural and urban areas.
Sigmoidal morphology and eruptive solar activity
Authors: Canfield Richard C. , Hudson Hugh S. , McKenzie David E.
Published: September 2002

Soft X‐ray images of solar active regions frequently show S‐ or inverse‐S (sigmoidal) morphology. We have studied the Yohkoh Soft X‐Ray Telescope video movie for 1993 and 1997. We have classified active regions according to morphology (sigmoidal or non‐sigmoidal) and nature of activity (eruptive or non‐eruptive). As well, we have used NOAA sunspot areas for each region as a measure of size. We find that regions are significantly more likely to be eruptive if they are either sigmoidal or large.
Plasma density enhancement and generation of spatially periodic irregularities in the ionospheric E‐region by powerful HF waves
Authors: Kuo S. P. , Koretzky E. , Lee M. C.
Published: September 2002

A new mechamism is investigated to generate spatially periodic temperature and density irregularities along the geomagnetic field in the ionospheric E‐region, using ground transmitted powerful HF (high frequency) waves. A thermal instability is first excited and then stablized by the nonlinear damping from the inelastic electron collisions with N2 and O2. Electron temperature enhanced by powerful HF waves reduces the recombination rates of electrons and NO+ and O2+ ions, and subsequently increases the plasma density. The thermal diffusion of electrons evolves the perturbation into spatially periodic density irregularities along the geomagnetic field. The spatial period between 400 m and 1.6 km of the irregularities varies with the HF wave power and frequency. The optimal conditions for exciting these irregularities are achievable at sites for ionospheric heating experiments. The investigated mechanism works effectively only in the presence of ionization sources, such as UV radiations during the daytime and precipitated energetic particles at night.
Intercomparison of ILAS and HALOE ozone at high latitudes
Authors: Lee K. ‐M. , McInerney J. M. , Sasano Y. , Park J. H. , Choi W. , Russell J. M.
Published: September 2002

Atmospheric O3 mixing ratios measured by ILAS and HALOE are compared for periods from March 25–31, 1997 in the NH and from November 20–24, 1996 and December 13–14, 1996 in the SH. Both instruments observe consistent vertical and horizontal structure in the highly variable polar vortex regions. In the ozone peak region, ILAS ozone values are smaller compared to HALOE values in both hemispheres, but the differences are less than 13% and 20% in 40–20 km altitude range, respectively, for the NH and the SH, which are both close to the sum of uncertainties of the two instruments. When validation is complete, ILAS data will be useful for the study of the chemistry and dynamics at high latitudes.
A subducting seamount beneath the Nankai Accretionary Prism off Shikoku, southwestern Japan
Authors: Park Jin‐Oh , Tsuru Tetsuro , Kaneda Yoshiyuki , Kono Yoshiteru , Kodaira Shuichi , Takahashi Narumi , Kinoshita Hajimu
Published: September 2002

A multi‐channel seismic (MCS) reflection survey was conducted to study the structure of the Nankai convergent margin off Shikoku Island in July 1997. Based on reflection characteristics, we could identify three major seismic reflection units, i.e., Units A, B, and C. The MCS data as well as swath‐bathymetric data reveal a buried circular seamount subducting beneath the Nankai accretionary prism. The subducting seamount is responsible for deformation of the accretionary wedge, resulting in a compressed uplifted sediment knoll and many steep escarpments with NE‐SW strike. A possible thrust fault was identified on the seaward flank of the seamount, indicating compressive deformation. A tectonic model for subduction of the seamount is presented based on the interpretation of the MCS data.
Rapid intensification and propagation of the dayside aurora: Large scale interplanetary pressure pulses (fast shocks)
Authors: Zhou Xiaoyan , Tsurutani Bruce T.
Published: September 2002

We present two cases of abrupt dayside auroral brightenings and very fast auroral propagation using the POLAR UV imaging data. The brightenings occur first at noon and then propagate along the auroral oval towards dawn and dusk. Ionospheric speeds of 6 to 11 km/s are determined. The auroral brightenings and motion are associated with the arrival and propagation of interplanetary shocks/pressure waves. The brightening at noon occurs within minutes of the shock compression of the noon‐time magnetopause. The speed of the auroral propagation in the ionosphere towards dawn and dusk corresponds extremely well to the solar wind downstream flow. Our model assumes that shocks/pressure waves compress the outer dayside magnetosphere, and plasma contained therein. This plasma compression leads to the loss cone instability, wave‐particle interactions, and concomitant particle loss into the ionosphere.
Distribution and inventory of 129I in the central Arctic Ocean
Authors: Buraglio Nadia , Aldahan Ala A. , Possnert Göran
Published: September 2002

The concentration of 129I was measured in water samples from three depth profiles collected in the central Arctic Ocean during 1996. The results indicate >7 × 108 atoms/liter in the cold, low salinity, 100‐m‐deep surface layer, and 0.06 × 108 atoms/liter at a depth below 3 km. The total inventory of 129I in the region is estimated at about 2.7 × 1027 atoms, representing 34% of the total discharge from Sellafield and La Hague until 1996. Marine input accounts for a major part (>90%) of the 129I inventory in the central Arctic Ocean while input from fresh water (ice melting and rivers), bomb tests and nuclear dumping contribute by about 3%. The inventory suggests a maximum marine transport time of 11 years for 129I from La Hague‐Sellafield to the central Arctic. The 129I concentration at a depth below 3 km is about one order of magnitude higher than the estimated pre‐anthropogenic global marine value, reflecting contribution from anthropogenic sources.
A linkage between decadal climate variations in the Labrador Sea and the tropical Atlantic Ocean
Authors: Yang Jiayan
Published: September 2002

According to observations, the Labrador Sea Water (LSW) thickness varies significantly on decadal time scales. Sea surface temperature (SST) in the tropical Atlantic also exhibits considerable decadal changes, including the so‐called tropical Atlantic SST dipole—a cross‐equatorial pattern. In this study, we propose that the SST dipole and variations of LSW thickness are linked through the Meridional Overturning Circulation (MOC). Southward transport of LSW along the deep western boundary must be compensated by northward flow in the upper ocean. Once the LSW pulse enters the tropics, it affects the cross‐equatorial heat transport and the upper ocean response generates a dipole pattern in the SST field. The correlation between observed LSW thickness and SST with a lag of 5 years is significant at 95% significance level. Results from an ocean model indicate that the time lag between LSW and tropical SST is set primarily by coastally‐trapped waves.
Mechanisms regulating sea‐surface temperatures and deep convection in the tropics
Authors: Sud Y. C. , Walker G. K. , Lau K. ‐M.
Published: September 2002

Scientific basis for the emergence of deep convection in the tropics at or above 28°C sea‐surface temperature (SST), and its proximity to the highest observed SST of about 30°C, is explained from first principles of moist convection and TOGA‐COARE data. Our calculations show that SST of 28–29°C is needed for charging the cloud‐base airmass with the required moist static energy for clouds to reach the upper troposphere (i.e., 200 hPa). Besides reducing solar irradiation by cloud‐cover, moist convection also produces cool and dry downdrafts, which promote oceanic cooling by increased sensible and latent heat fluxes at the surface. Consequently, the tropical ocean seesaws between the states of net energy absorber before, and net energy supplier after, the deep moist convection, which causes the SST to vacillate between 28° and 30°C. While dynamics of the large‐scale circulation embodying the easterly waves and Madden‐Julian Oscillations (MJOs) modulate moist convection, we show that the quasi‐stationary vertical profile of moist static energy of the tropics is the ultimate cause of the upper limit on tropical SSTs.
Depression of the 660 km discontinuity beneath the Tonga Slab determined from near‐vertical ScS reverberations
Authors: Roth Erich G. , Wiens Douglas A.
Published: September 2002

The occurrence of the March 9, 1994 (Mw 7.6) deep Tonga earthquake during a temporary deployment of broadband seismographs provides an excellent opportunity to study the seismic discontinuity structure in the vicinity of a subducting slab. Near vertical ScS reverberations from the 410, 520, and 660 discontinuities are visible at 10 stations in the South Pacific at frequencies between 0.003–0.033 Hz without stacking. We use a regional velocity model and ray theory to model the interval from ScS to ScS3. A grid search is then performed on the depth and impedance contrast at each of the seismic discontinuities to optimize the fit to both the underside and topside reflections. Stacked records indicate prominent discontinuities at 400 km and 665 km depth beneath and to the west of the slab, and 410 km, 500 km, and 640 km to the east of the slab. Analysis of individual records suggests a local depression of 30 km in the 660 and a broadening of the transition zone directly beneath and to the west of the Tonga slab, in a region of possible near horizontal slab. These results are consistent with a large cool region just to the west of the intersection of the Tonga slab with the 660.
Phytoplankton pigment distribution from SeaWiFS data in the subtropical convergence zone south of Africa: A wavelet analysis
Authors: Machu Eric , Ferret Bertrand , Garçon Véronique
Published: September 2002

The Agulhas system (Agulhas Current, Retroflection, Agulhas Return Current (ARC)), south of Africa, constitutes a complex energetic system subject to high frequency mesoscale and low frequency variability. Using chlorophyll pigment concentration data collected by the SeaWiFS ocean color sensor and sea surface height satellite data, we demonstrate the utility of wavelet analysis in jointly analyzing ocean color and altimetric signals. Two distinct dynamical scenarios of the Agulhas system (October 1997 and January 1998) yield different dominant wavelengths (620 and 500 km) for the ARC Rossby wave. They are simultaneously retrieved from the chlorophyll and sea level anomaly signals. Longitudinal extension of the power, resulting from the wavelet analysis, also depends on the dynamical regime.
El Niňo related meridional oscillation of global lightning activity
Authors: Sátori Gabriella , Zieger Bertalan
Published: September 2002

The self‐consistent behavior of the frequency level and semiannual intensity variations of the first three Schumann resonance (SR) modes observed at Nagycenk (47.6°N, 16.7°E) indicates a southward shift in the global position of the lightning activity in warm El Niňo years and a northward shift in cool La Niňa years, especially in the longitudinal range where Africa dominates the activity. The third SR mode has a crucial role in recognizing these meridional shifts due to the special angular distance of Nagycenk with respect to the African thunderstorm region. The diurnal zonal redistribution of the worldwide thunderstorm activity manifests in characteristic daily frequency patterns. The preserved daily frequency patterns with changing frequency levels in the same months of the different years imply that the meridional shifts of the lightning activity exist in worldwide sense on the ENSO time scale. The estimated shifts are about 4–8° in latitude.
Intermittency in the solar wind turbulence through probability distribution functions of fluctuations
Authors: Sorriso‐Valvo Luca , Carbone Vincenzo , Veltri Pierluigi , Consolini Giuseppe , Bruno Roberto
Published: September 2002

Intermittency in fluid turbulence can be emphasized through the analysis of Probability Distribution Functions (PDF) for velocity fluctuations, which display a strong non‐gaussian behavior at small scales. Castaing et al. (1990) have introduced the idea that this behavior can be represented, in the framework of a multiplicative cascade model, by a convolution of gaussians whose variances is distributed according to a log‐normal distribution. In this letter we have tried to test this conjecture on the MHD solar wind turbulence by performing a fit of the PDF of the bulk speed and magnetic field intensity fluctuations calculated in the solar wind, with the model. This fit allows us to calculate a parameter λ² depending on the scale, which represents the width of the log‐normal distribution of the variances of the gaussians. The physical implications of the obtained values of the parameter as well as of its scaling law are finally discussed.
July temperature during the second millennium reconstructed from Idaho tree rings
Authors: Biondi Franco , Perkins D. L. , Cayan D. R. , Hughes M. K.
Published: September 2002

An 858‐year proxy record of July temperature for east‐central Idaho shows multi‐decadal periods of extreme cooling centered around AD 1300, 1340, 1460, and after AD 1600. These cold intervals are interrupted by prolonged warm spells in the early 1400s, late 1500s, and in the 1930s. The spatial signature of the paleoclimate record is centered on the north‐central Rockies and central Great Plains, and expands over North America following a wave‐like pattern. Neither instrumental nor proxy data in Idaho northeast valleys show unusual warming during the twentieth century. Climate episodes over the last three centuries are in broad agreement with the Greenland borehole temperature history. Low‐frequency patterns are consistent with other northern hemisphere tree‐ring records for the late Holocene, and provide a chronology of warm and cold intervals during the Little Ice Age.
An assessment of aircraft as a source of particles to the upper troposphere
Authors: Anderson B. E. , Cofer W. R. , Crawford J. , Gregory G. L. , Vay S. A. , Brunke K. E. , Kondo Y. , Koike M. , Schlager H. , Baughcum S. L. , Jensen E. , Zhao Yongjing , Kita Kazuyuki
Published: September 2002

Condensation nuclei measurements are examined in conjunction with measurements of reactive nitrogen species (NOy) to identify aircraft plumes in primary air traffic corridors over the North Atlantic. Several hundred plumes exhibiting ≥100 pptv enhancements in NOy mixing ratio were observed. The plumes were typically a few hundred meters wide, exhibited high NO/NOy ratios, and ranged in age from ∼10 minutes to ∼10 hours. Assuming the sampled aircraft emitted ∼12 g NOx (as NO2) kg−1 fuel burned and that the loss of NOy to the particle phase was negligible, we calculate median aerosol emission indices in terms of number of particles kg−1 of fuel burned of ∼120×1015 for CN ≥8 nm in size; ∼50×1015 for CN ≥ 17 nm; and ∼3×1015 for the nonvolatile CN ≥ 17 nm. Using published fuel burn statistics, background aerosol concentrations, and a 10 day particle lifetime, we conclude that present aviation sources enhance global averaged upper‐tropospheric fine and nonvolatile aerosol number densities by ∼6% and ∼3%, respectively.
Mean seasonal and diurnal variations of PMSE and winds from 4 years of radar observations at ALOMAR
Authors: Hoffmann P. , Singer W. , Bremer J.
Published: September 2002

VHF radar observations of polar mesosphere summer echoes (PMSE) have been carried out at the ALOMAR facility near Andenes (69.3°N, 16.0°E) using the ALOMAR SOUSY radar during the four summer seasons from 1994 until 1997. First PMSE occur near the middle of May, and last PMSE are normally observed near the end of August. The mean seasonal variation of the occurrence rate of PMSE is characterized by a steep increase in late May/early June, a maximum some days after the summer solstice and a slow decrease starting during the last decade of July. Diurnal variations of PMSE strength as well as of the height of the maximum backscattered power are dominated by a pronounced semidiurnal variation. This behavior is markedly influenced by temperature changes induced by tidal waves in the mesospheric wind field.
Diamagnetic effect produced by the Fluxus‐1 and ‐2 artificial plasma jet
Authors: Gavrilov B. G. , Podgorny A. I. , Podgorny I. M. , Sobyanin D. B. , Zetzer J. I. , Erlandson R. E. , Meng C.‐I. , Stoyanov B. J.
Published: September 2002

The purpose of the Fluxus‐1 and ‐2 active experiments is to study plasma jets injected parallel to the magnetic field and to study the interaction of these jets with the magnetic field. The experiments were conducted using a shaped‐charge device, known as an explosive type generator (ETG), that produced an artificial aluminum plasma jet. In Fluxus‐1 and ‐2 the jet was injected nearly parallel to the geomagnetic field at an altitude of 140 km toward an instrumented diagnostic payload located about 100 m away from the ETG. A ∼50% reduction in the magnetic field was observed as the plasma jet passed by the diagnostic payload. Comparisons of 3‐dimensional simulation results with the observed magnetic field perturbations suggest that the Fluxus‐1 plasma jet was ∼30° from the magnetic field direction while the Fluxus‐2 plasma jet was directed nearly parallel to the magnetic field.
Invasion of the abyssal North Atlantic by modern anthropogenic lead
Authors: Alleman L. Y. , Véron A. J. , Church T. M. , Flegal A. R. , Hamelin B.
Published: September 2002

While anthropogenic emissions have dramatically elevated lead concentrations in the North Atlantic troposphere and surface waters by orders of magnitude above natural levels [Murozumi et al., 1969; Schaule and Patterson, 1983; Boyle et al., 1986], it has been assumed that the relatively low lead levels in North Atlantic abyssal waters are not yet contaminated [Schaule and Patterson, 1981; Flegal and Patterson, 1983]. That misperception is redressed by the following stable lead isotopic composition data which reveal the advective transport of industrial lead into those deep basin waters through the formation of North Atlantic Deep Water (NADW). Additionally, spatial gradients in the isotopic signatures of anthropogenic lead within the North Atlantic abyss appear to serve as transient tracers of contaminant penetration rates.
On the hemispheric symmetry in thermospheric nitric oxide
Authors: Crowley Geoff , Ridley Aaron , Winningham David , Frahm Rudy , Sharber Jim , Russell Jim
Published: September 2002

Middle atmosphere research has suggested the importance of charged particle precipitation for several minor species. To experimentally investigate these effects requires observations of both the minor species and particle precipitation. Particle precipitation is organized in a geomagnetic coordinate system, yet data are persistently presented in geographic coordinates. This has serious implications, as quantified here for nitric oxide observations, which could lead to erroneous conclusions regarding particle effects. In geographic coordinates UARS/HALOE measurements of high latitude NO at equinox show a 25% hemispheric asymmetry that disappears when the data are replotted in magnetic coordinates. This result is applicable to all high latitude particle effects and is important for understanding magnetospheric and solar influences on various minor species in the mesosphere and stratosphere.
Lunar influence on diurnal temperature range
Authors: Cerveny Randall S. , Balling Robert C.
Published: September 2002

While variations in the diurnal temperature range (DTR) have been linked to atmospheric enhancement of greenhouse gases, it is useful to identify all contributing factors potentially acting upon the DTR. In this study, an area‐weighted record of daily temperatures from the United States Historical Climate Network is used to examine the influence of lunar phase variations on the DTR. For the period from 1950 to 1995, a statistically significantly higher DTR occurs near the full moon (∼10.23°C) while a lower DTR occurs near the new moon (∼ 10.13°C). This synodic monthly difference appears to be primarily influenced by warmer maximum daily temperatures occurring near the full moon. This finding allows evaluation of two potential extraterrestrial factors influencing terrestrial temperatures as a function of lunar phase: a) the barycenter effect which is defined as variations in terrestrial and lunar position around their mutual gravitational centroid over the course of synodic month, and b) reflected solar/infrared emission off the lunar surface over the course of a synodic month. While both the barycenter effect and the reflected solar and infrared emission off the lunar surface would influence the minimum temperatures, we suggest that the barycenter effect is likely the dominating extraterrestrial mechanism in explaining lunar phase variations in the diurnal temperature range because its additional influence on daily maximum temperatures.
Water vapor of the polar middle atmosphere: Annual variation and summer mesosphere Conditions as observed by ground‐based microwave spectroscopy
Authors: Seele Cord , Hartogh Paul
Published: September 2002

We have been operating a ground‐based microwave radiometer for the 22.235GHz water vapor line at the ALOMAR observatory, Norway, in 1995/96 and continuously since July 1997. Its high latitude location (69°N, 16°E) provides a new and unique dataset of stratospheric and mesospheric water vapor observations. We briefly describe our instrument and measuring technique and present the data of our ongoing measurements. The water vapor mixing ratio shows a pronounced annual cycle that is stronger than what has been reported for mid‐latitudes. We also present an analysis of our data for the upper mesosphere for the relationships between mesospheric water vapor, temperature, polar mesosphere summer echoes (PMSE), and noctilucent clouds (NLC) during summer months. This investigation indicates that water vapor is already present near mesopause heights when the PMSE/NLC season starts and is still present long after it has declined.
Ozone destruction on ice
Authors: Langenberg Stefan , Schurath Ulrich
Published: September 2002

The destruction of ozone on ice surfaces was studied with an ice‐coated fused silica column used as a very long flow tube. Ozone destruction was found to be very slow. At 223 K and 258 K the measured reaction probabilities fall in the range 2 × 10−8 ‐ 4 × 10−10, decreasing with increasing ozone concentration (20–1000 nbar). The observed kinetics are explained by an at least two step mechanism. The low reaction rates observed are insignificant for ozone destruction in cirrus clouds.
Estimation of aerosol direct radiative effects over the mid‐latitude North Atlantic from satellite and in situ measurements
Authors: Bergstrom R. W. , Russell P. B.
Published: September 2002

We estimate solar radiative flux changes due to aerosols over the mid‐latitude North Atlantic by combining optical depths from AVHRR measurements with aerosol properties from the recent TARFOX program. Results show that, over the ocean, the aerosol decreases the net radiative flux at the tropopause and therefore has a cooling effect. Cloud‐free, 24‐hour average flux changes range from −9 W/m² near the eastern US coast in summer to −1 W/m² in the mid‐Atlantic during winter. Cloud‐free North Atlantic regional averages range from −5.1 W/m² in summer to −1.7 W/m² in winter, with an annual average of −3.5 W/m². Cloud effects, estimated from ISCCP data, reduce the regional annual average to −0.8 W/m². All values are for the moderately absorbing TARFOX aerosol (ω(0.55 µm) = 0.9); values for a nonabsorbing aerosol are ∼30% more negative. We compare our results to a variety of other calculations of aerosol radiative effects.