bluuebunny/crossref_metadata_2025 huggingface dataset:
The onset of the 1991–92 El Niño event in the tropical Pacific Ocean: The NECC subsurface pathway
Authors: Zhang Rong‐Hua , Rothstein Lewis M. , Busalacchi Antonio J. , Liang Xin‐Zhong
Published: September 2002

Based on the NCEP ocean re‐analysis data, the three‐dimensional co‐evolution of the tropical Pacific climate system is examined to explain the onset of the 1991–92 El Niño event. A logical sequence is discovered that links subsurface and surface temperature anomalies off and on the equator in the western and central Pacific. Along the North Equatorial Countercurrent (NECC) path, subsurface temperature anomalies propagated coherently eastward from the western boundary in mid‐1989 to the date line in mid‐1990, at a speed of about 10–20 cm−1. As the thermocline shoals eastward along the NECC, the subsurface anomalies outcropped in the regions near the date line, initiating warm sea surface temperature (SST) anomalies which further amplified while advectively extending into the equatorial wave guide. These subsurface‐induced SST anomalies could then trigger local coupled air‐sea interactions producing atmospheric‐oceanic anomalies that developed and evolved in 1991, thus setting onset conditions for the 1991–92 El Niño. These results differ markedly from the delayed oscillator physics in that a major role can be played by the eastward advection of off‐equatorial subsurface thermal anomalies and their outcropping along the NECC subsurface pathways, not necessarily involving wave reflections along the western boundary for triggering El Niño. This mechanism explains well the observed, otherwise unaccounted for, surface warming near the date line during late 1989 and early 1990.
Resonance in bidecadal and pentadecadal climate oscillations over the North Pacific: Role in climatic regime shifts
Authors: Minobe Shoshiro
Published: September 2002

The roles of interdecadal oscillations in climatic regime shifts, which are observed as rapid strength changes in the Aleutian low in winter and spring seasons, have been analyzed. A regime shift results from simultaneous phase reversals between pentadecadal and bidecadal variations, which synchronize with one another at a relative period of three. The pentadecadal variation, which is observed in both winter and spring seasons, provides the basic timescale of regime shifts, while the bidecadal variation, which is observed only in winter, characterizes the rapidity of the shifts. A Monte‐Carlo simulation has shown that the simultaneous phase reversals or resonance between the pentadecadal and bidecadal variations reflect a physical linkage between them and do not coincide accidentally. The role of this synchronization feature for assessing and predicting regime shifts is discussed.
Unsteady propagation of collisionless trans‐Alfvénic shocks at large timescales
Authors: Markovskii S. A.
Published: September 2002

In a collisional plasma, fast and slow shocks are time stationary. They evolve in such a way that the flow variations remain small under the action of a small perturbation. By contrast, in the case of a trans‐Alfvénic shock wave (TASW), at which the flow velocity passes over the Alfvén velocity, the small perturbation generates finite variation of the initial flow. As a result, the shock disintegrates or transforms into some other unsteady state. In the present paper, the problem of shock propagation is considered for a collisionless plasma with anisotropic pressure. Using a kinetic theory, it is shown that the transition of the flow velocity over the Alfvén velocity at a shock is crucial for its evolution. Namely, the small perturbation makes collisionless TASWs unsteady at timescales large enough compared to the ion gyroperiod. This conclusion is valid for both ordinary TASWs at which the Alfvén Mach number decreases and anomalous TASWs at which it increases. Possible manifestations of unsteady propagation of planetary bow shocks are discussed.
Observations of density fluctuations in Earth's magnetosheath with Geotail and Wind spacecraft
Authors: Seon J. , Park S. M. , Min K. W. , Frank L. A. , Paterson W. R. , Ogilvie K. W.
Published: September 2002

Large fluctuations of plasma densities in the earth's flank‐side magnetosheath (local times 1500–1700) are examined. The characteristics of the fluctuations are large amplitudes (ΔN/N ∼ 1), long oscillation periods (T ∼ 30 m) and anti‐ correlation between the plasma densities and magnetic field strengths. The variations of the solar wind plasma densities are negligible, whereas considerable fluctuations are found for the solar wind magnetic fields. Two possibilities are considered for the density fluctuations: 1) repeated traversals of the plasma depletion layer and the standing slow‐mode waves near the magnetopause and 2) convected slow‐mode waves generated at the earth's bow shock. Temperature anisotropy and density profile as expected from the repeated traversals are not confirmed with the observations. Based on the consideration that the present observations occur in the flanks where plasma flows are largely tangential to the magnetopause, we propose that the convected slow‐mode waves play a role in the generation of the density fluctuations.
The role of rheology in lithospheric thinning by mantle plumes
Authors: Moore William B. , Schubert Gerald , Tackley Paul J.
Published: September 2002

Lithospheric thinning by mantle plumes is an important planetary heat transfer process resulting in the broad topographic uplift that characterizes the volcanic rises on Venus, the Tharsis rise on Mars, and several hotspot swells on Earth. We present a suite of time‐dependent, three‐dimensional numerical calculations of a plume impinging upon the lithosphere in a temperature‐dependent viscosity mantle. Efficient lithospheric thinning is found to depend on the formation of convective instabilities in the plume‐lithosphere boundary layer. These instabilities are non‐axisymmetric, time‐dependent, and have horizontal scales of a few tens of kilometers. These instabilities depend on the temperature‐dependence of viscosity and occur when the plume's viscosity is about an order of magnitude less than the background mantle, as predicted by boundary‐layer theory. Thus, in planetary mantles, plumes with excess temperatures of 100 to 200 K will efficiently thin the lithosphere via small‐scale convective instabilities.
Large earthquakes and Earth rotation: The role of mantle relaxation
Authors: Soldati Gaia , Spada Giorgio
Published: September 2002

By means of a radially stratified spherical model and using the available moment tensor solutions, we evaluate the contribution of large earthquakes to polar drift in the last two decades. In contrast to previous analyses, which only considered the elastic response of the Earth to the release of seismic moment, we account for mantle rheology and thus for the time‐dependent postseismic inertia changes driven by global seismicity. We confirm that during the last twenty years the global seismic activity has not significantly altered the rotational parameters of the Earth. However, we find that for values of asthenospheric viscosity suggested by recent studies of postseismic stress diffusion, the delayed relaxation of the mantle amplifies the average rate of polar drift by a factor of 1.2 to 1.7 (depending on the width of the asthenosphere) even on a decade time‐scale.
GPS observation of crustal deformation in the Taiwan‐Luzon Region
Authors: Yu Shui‐Beih , Kuo Long‐Chen , Punongbayan Raymundo S. , Ramos Emmanuel G.
Published: September 2002

Fifteen GPS stations in the Batan‐Luzon area were surveyed three times during 1996 to 1998. In order to study crustal deformation in the Taiwan‐Luzon region, the data from these annual surveys are processed together with data from 20 permanent stations in the Taiwan area and IGS stations in the Asia‐Pacific region. We found that the Chinese continental margin is moving in directions of 102°–112° with rates of 11–12 mm/yr relative to stable Eurasia. There is a significant southward increase in crustal motion with respect to Eurasia along the N‐S trend of the Luzon arc. In the Coastal Range of Taiwan, the velocity is 53 mm/yr in the direction 313°. Further south at two volcanic islands, Lanhsu and Batan, the velocities increase to 68 mm/yr in the direction 306° and 81 mm/yr in the direction 299°, respectively. In northern Luzon the velocity reaches 86 mm/yr in the direction 301°. Remarkable fault slips of 17–31 mm/yr across the Philippine fault system are detected.
Observational evidence of quasi two‐day/gravity wave interaction using MF Radar
Authors: Herman Redina L. , Robinson Walter A. , Franke Steven J.
Published: September 2002

The gravity wave/quasi two‐day wave interaction is an example of gravity wave/large‐scale flow interaction, a subject of great interest but insufficient understanding. Here we present evidence, from MF radar data, for the quasi two‐day wave modulation of gravity wave activity in the summer mesosphere over Urbana, Illinois (40° 10′ 10″ N, 88° 09′ 36″ W). A composite analysis of two‐day wave events, identified using the meridional winds at 84 km, is performed on the four‐hour mean winds and four‐hour standard deviations about those means, the latter being representative of gravity wave activity. Our results show that quasi two‐day variations in the winds modulate gravity wave activity at higher altitudes.
The “normality” of El Niño
Authors: Burgers Gerrit , Stephenson David B.
Published: September 2002

The amplitude of the El Niño‐Southern Oscillation (ENSO) would be normally distributed if the coupled Pacific ocean‐atmosphere were a linear system forced by Gaussian weather noise. Moment estimates of skewness and kurtosis demonstrate that this is not the case for monthly mean anomalies in Pacific sea surface temperatures during 1950–97. The noted predominance of El Niño events compared to La Niña events is related to the high skewness in the eastern Pacific. Skewness and kurtosis both exhibit an intriguing geographical variation from positive in the eastern to negative in the western Pacific. We have also examined Niño‐3 indices generated by three climate models having widely different complexity. These exhibit a wide range of skewness and kurtosis values rather different from those found for the observations. Skewness and kurtosis can be used to diagnose non‐linear processes and provide powerful tools for validating models, and for testing observed sea‐surface temperatures for the presence of possible climate change.
Last‐Glacial to Post‐Glacial 10Be fluctuations in a sediment core from the Academician Ridge, Lake Baikal
Authors: Horiuchi Kazuho , Minoura Koji , Kobayashi Koichi , Nakamura Toshio , Hatori Satoshi , Matsuzaki Hiroyuki , Kawai Takayoshi
Published: September 2002

10Be concentrations, sediment composition and sediment accumulation rates were determined in a sediment core from the Academician Ridge of Lake Baikal. The 10Be concentrations vary from 5.07 × 108 atoms/g in the last glacial section to 11.25 × 108 atoms/g in the section representing the post‐glacial climate stage. 10Be fluxes, on the other hand, increase toward the last‐glacial stage coinciding with an increase of the sediment accumulation rate. These facts imply that climate‐related sedimentation is related to both flux and concentration of the nuclide in the sediment core. During the last‐glacial stage, arid conditions in the lake's watershed caused a marked increase in flux of clay particles with low 10Be content to the lake, resulting in an increased flux but lowered concentration of 10Be in the sediments. It is suggested that the 10Be record of the bottom sediments of Lake Baikal can be used as a proxy indicator of climate change in central Asia.
Indian Ocean Climate event brings floods to East Africa's lakes and the Sudd Marsh
Authors: Birkett Charon , Murtugudde Ragu , Allan Tony
Published: September 2002

During an El Niño, the expected rainfall increase over most of the Lake Victoria catchment area is ∼15–25%. However, due to anomalous warming of the western equatorial Indian Ocean during 1997, strong convection developed over parts of the Horn and eastern Africa. This resulted in a much larger 20–160% precipitation excess during the “short rainy” season. Satellite radar altimetry data reveals that not only did Lake Victoria rise by ∼1.7 m, but that the rainfall event similarly affected lakes Tanganyika, Malawi and Turkana. In addition, the seasonal level minima of the Sudd marshes and Lakes T'ana and Nasser continue to increase. Such a rainfall event will have severe, long‐term consequences for the natural surface flows and storages along the White Nile. Based on the hydrological impacts of the historic 1961 East Africa event, we can expect the current high levels of Lake Victoria to be maintained for the remainder of this decade. In addition, we anticipate a major expansion of the permanent swamp regions of the Sudd marshes over the forthcoming seasons. Blue Nile flows, further enhanced by the above‐average 1998 rainfall season, can also be expected to remain high, at least until early 1999.
Observation of gaseous and particulate products of monoterpene oxidation in forest atmospheres
Authors: Yu Jianzhen , Griffin Robert J. , Cocker David R. , Flagan Richard C. , Seinfeld John H. , Blanchard Pierrette
Published: September 2002

Atmospheric oxidation of biogenic hydrocarbons, such as monoterpenes, is estimated to be a significant source of global aerosol. Whereas laboratory studies have established that photochemical oxidation of monoterpenes leads to aerosol formation, there are limited field studies detecting such oxidation products in ambient aerosols. Drawing on prior results of monoterpene product analysis under controlled smog chamber conditions, we have identified organic aerosol components attributable to monoterpene oxidation in two forest atmospheres, Kejimkujik National Park, Nova Scotia, Canada, and Big Bear, San Bernardino National Forest, California, U.S.A. The major identified aerosol products derived from α‐pinene and β‐pinene oxidation include pinic acid, pinonic acid, norpinonic acid and its isomers, hydroxy pinonaldehydes, and pinonaldehyde, concentrations of which in the aerosol phase are in the sub ng m−3 range. Identification of oxidation products in atmospheric aerosol samples serves as direct evidence for aerosol formation from monoterpenes under ambient conditions.
Long‐term variability in the mesospheric tidal winds observed by MF Radar over Tirunelveli (8.7°N, 77.8°E)
Authors: Gurubaran S. , Rajaram R.
Published: September 2002

Observations of mesospheric tidal winds in the 84–98 km height range over a period of five years with the MF radar at Tirunelveli (8.7°N, 77.8°E), India, are presented in this study. The emphasis is on describing the long‐term variability in the tidal amplitudes. The results reported herein clearly indicate pronounced interannual variability in the tidal activity, and strengthen the earlier suggestion that this feature is linked to the stratospheric quasi‐biennial oscillation (QBO).
Experimental evidence for different strain regimes of crack populations in a clay model
Authors: Spyropoulos Chrysanthe , Griffith William J. , Scholz Christopher H. , Shaw Bruce E.
Published: September 2002

We report results from clay extension experiments used as a model for the evolution of fault populations due to stress interactions. At yielding cracks begin to appear and the brittle strain due to them quickly reaches a rate matching the applied stretching rate. The crack density (number of cracks per unit area) initially increases apace, then reaches a maximum at a critical strain, decreasing thereafter. At low strains, where the crack population is dilute, a power law length distribution is observed, which at high strain, gradually transitions to an exponential. This agrees with fault populations data observed in low and high strain settings. These results indicate that fault populations ranging from power law to exponential size‐frequency distributions reflect the population evolution with increased strain.
Modes in a continuous spectrum of atmospheric oscillations
Authors: Stepanov B. E.
Published: September 2002

Atmospheric observational studies reveal synoptic‐scale eastward‐traveling modes, but barotropic models of the atmosphere do not have free Rossby modes in the discrete spectrum that travel eastward. Nevertheless, we show that eastward‐traveling barotropic modes can be excited in the same way as discrete westward‐traveling normal‐mode Rossby waves. Within a linearized shallow water model, we have examined the response to forcing in individual hemispheres. This response includes eastward‐traveling modes that agree well with observations. The study also shows resonant low‐frequency responses in the continuous spectrum confined to a single hemisphere (“hemispheric Rossby modes”).
Toward the Autojuggie: Planting 72 geophones in 2 sec
Authors: Steeples Don W. , Baker G. S. , Schmeissner C.
Published: September 2002

Shallow seismic reflection surveys require dense spatial wave‐field sampling, contributing to their high cost. To assess the feasibility of planting geophones automatically, we planted 72 geophones in approximately 2 s in a test line, using an 11‐m‐wide farm tillage tool as a planting device. Geophones were attached rigidly, at 15 cm intervals, to five pieces of heavy‐duty channel iron bolted to the tillage‐tool frame. Conventional comparison‐line data collected about 75 cm away, parallel to the test line, were visually comparable with the seismic source 12 m distant. When the sources were placed 1 m from the geophones, a surface‐wave mode was excited by the channel iron and detected by geophones in both lines. This mode exhibited a different phase velocity than that of the desired seismic body‐waves and could be attenuated by frequency‐wavenumber filtering. These results suggest that automatic geophone placement is feasible and could decrease shallow seismic surveying costs.
Polar cap index as a proxy for hemispheric Joule heating
Authors: Chun Francis K. , Knipp Delores J. , McHarg Matthew G. , Lu Gang , Emery Barbara A. , Vennerstrøm Susanne , Troshichev Oleg A.
Published: September 2002

The polar cap (PC) index measures the level of geomagnetic activity in the polar cap based on magnetic perturbations from overhead ionospheric currents and distant field‐aligned currents on the poleward edge of the nightside auroral oval. Because PC essentially measures the main sources of energy input into the polar cap, we propose to use PC as a proxy for the hemispheric Joule heat production rate (JH). In this study, JH is estimated from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure. We fit hourly PC values to hourly averages of JH. Using a data base approximately three times larger than studies, we find a quadratic relationship between JH and PC, differentiated by season. A comparison during the November 1993 storm interval with earlier reported methods using the AE index and the cross polar cap potential, shows that the PC‐based Joule heating estimate is as equally accurate. Thus the single station PC index appears to provide a quick estimate of, and is an appropriate proxy for, the hemispheric Joule heating rate.
Possible solar influences on the dust profile of the GISP2 Ice Core from central Greenland
Authors: Ram Michael , Stolz Michael R.
Published: September 2002

The Greenland Ice Sheet Project 2 (GISP2) ice core dust profile in glacial (Wisconsinan) ice from Central Greenland has been measured using Laser‐Light Scattering (LLS) from ice. This data exhibits considerable high frequency variability. By applying appropriate running averages to the data, certain regularities emerge. Thus, for example, when the data was smoothed with rectangular 4 and 5 year running averages, ∼11 year dust modulations of considerable amplitude were revealed. In this paper, we show how further smoothing of the data causes very large, longer period dust modulations centered at ∼91 years in a Gaussian distribution to emerge. We believe that this corresponds to the 80–90 year modulation of the 11 year sunspot cycle first suggested by Gleissberg. We also show how adjacent 11 year dust periods can combine in a unique way to generate a 22 year (Hale) period of dust modulation. The way this occurs is similar to that by which a 22 year modulation of the measured terrestrial neutron flux is generated from two 11 year periods. Taken together with our observation of a ∼200 year (Suess) dust modulation period, these observations strengthen our claim that the dust modulations we measured in GISP2 ice are solar induced and suggest a mechanism for their production.
Crustal isostasy indicates anomalous densities beneath Iceland
Authors: Menke William
Published: September 2002

Recent seismological determinations of crustal thickness in Iceland, made mainly through identification of wide‐angle PmP and SmS reflections, indicate that the depth to the Moho deepens from 11 km beneath the low‐lying Reykjanes Penninsula to 39 km beneath the central highlands. By compiling all available crustal thickness measurements, we show that this trend holds generally for Iceland, with elevation (above m.s.l.) increasing at a rate of 35.54±6.4 (1 σ) meters for each kilometer of Moho depth (below m.s.l.). If one assumes that the topography and Moho are in local isostatic equilibrium with thickness changes in a lower crustal layer, while an upper crust maintains a constant thickness, with the part above sea level having a density of of 2500 kg/m³, then the rate of increase of depth with topographic elevation implies a density jump of 89±12 kg/m³ across the Moho. Petrological and seismological evidence indicate that the lower‐crustal densities probably do not exceed about 3060±50 kg/m³. Mantle densities must therefore be low, 3150±60 kg/m³. The combined effects of chemical depletion, thermal expansion and melt entrainment in the mantle can account for some, but not all, of the anomaly.
Simultaneous and common‐volume observations of noctilucent clouds and polar mesosphere summer echoes
Authors: von Zahn U. , Bremer J.
Published: September 2002

Joint operations of the ALOMAR Rayleigh/Mie/Raman (RMR) lidar and ALOMAR SOUSY radar (located at 69°N latitude) let us obtain a first series of simultaneous and common‐volume observations of noctilucent clouds (NLC) and polar mesosphere summer echoes (PMSE). We present the results of 22 examples of simultaneous NLC and PMSE observations made during the summers 1994 until 1997. These observations show a strong correlation of NLC with PMSE and they lead us to a classification (3 types) of the observed events. Furthermore, we conclude that, in all probability, there are common causes for the two types of layers.
Ramp nonstationarity and the generation of whistler waves upstream of a strong quasiperpendicular shock
Authors: Walker S. N. , Balikhin M. A. , Nozdrachev M. N.
Published: September 2002

A number of models have been proposed to explain the observation of low frequency (LF) whistler waves in the upstream region adjacent to the ramp of the quasiperpendicular part of the Earth's bow shock. One such model relates these waves to the nonlinear dynamics of the ramp itself. Two aspects of this model, the plasma frame spectrum of the turbulence and the occurrence of nonlinear waves propagating away from the shock in the upstream region have already been shown to be in agreement with observations. A third aspect, the interaction and energy transfer between these nonlinear waves and the whistler turbulence, is studied by applying high order spectral analysis to Interball magnetic field data. Evidence for the nonlinear coupling between these two types of waves and the possible transfer of energy are presented.
A case study of extreme tropospheric ozone contamination in the tropics using in‐situ, satellite and meteorological data
Authors: Randriambelo T. , Baray J. L. , Baldy S. , Bremaud P. , Cautenet S.
Published: September 2002

Since 1992, more than 150 radiosoundings have been performed from Reunion Island in the Indian Ocean in Southern tropics. On August 25, 1995, tropospheric ozone amounts of over 200 ppbv through the entire troposphere were encountered. Typical values during that period do not usually reach more than 80 ppbv. In order to explain these disturbances, we performed an analysis using satellite and ECMWF data. Results indicate that several mechanisms are working together, including photochemical effect of biomass burning associated with convection, and stratosphere‐troposphere exchange near the subtropical jet stream. Based on results shown in this paper, it is difficult to estimate the relative contribution of biomass burning and stratospheric intrusions. Quantitative estimates should stem from other chemical component measurements and from a more detailed dynamical analysis.
Observation of α‐stable noise induced millennial climate changes from an ice‐core record
Authors: Ditlevsen Peter D.
Published: September 2002

The last glacial period showed millennium scale climatic shifts between two different stable climate states. The state of thermohaline ocean circulation probably governs the climate, and the triggering mechanism for climate changes is random fluctuations of the atmospheric forcing on the ocean circulation. The high temporal resolution paleo‐climatic data from ice‐cores are consistent with this picture and a bi‐stable climate pseudo‐potential can be derived. It is found that the fast time scale noise forcing the climate contains a component with an α‐stable distribution. As a consequence the abrupt climatic changes observed could be triggered by single extreme events. These events are related to ocean‐atmosphere dynamics on annual or shorter time scales and could indicate a fundamental limitation in predictability of climate changes.
Can lower mantle slab‐like seismic anomalies be explained by thermal coupling between the upper and lower mantles?
Authors: Čížková Hana , Čadek Ondřej , van den Berg Arie P. , Vlaar Nicolaas J.
Published: September 2002

Below subduction zones, high resolution seismic tomographic models resolve fast anomalies that often extend into the deep lower mantle. These anomalies are generally interpreted as slabs penetrating through the 660‐km seismic discontinuity, evidence in support of whole‐mantle convection. However, thermal coupling between two flow systems separated by an impermeable interface might provide an alternative explanation of the tomographic results. We have tested this hypothesis within the context of an axisymmetric model of mantle convection in which an impermeable boundary is imposed at a depth of 660 km. When an increase in viscosity alone is imposed across the impermeable interface, our results demonstrate the dominant role of mechanical coupling between shells, producing lower mantle upwellings (downwellings) below upper mantle downwellings (upwellings). However, we find that the effect of mechanical coupling can be significantly weakened if a narrow low viscosity zone exists beneath the 660‐km discontinuity. In such a case, both thermally induced ‘slabs’ in the lower mantle and thermally activated plumes that rise from the upper/lower mantle boundary are observed even though mass transfer between the shells does not exist.
The kinetic to potential energy ratio and spectral separability from high‐resolution balloon soundings near the Andes Mountains
Authors: de la Torre A. , Alexander P. , Giraldez A.
Published: September 2002

The ratio R between the spectral kinetic and potential energies as a function of vertical wavenumber has been calculated from high‐resolution data obtained with open stratospheric balloons near the Andes Mountains. Two segments of altitude in the troposphere and stratosphere respectively were analyzed. The ratio values are larger in both the troposphere and stratosphere than those predicted from the separability of wavenumber and frequency spectra. A comparison was made with previous statistical results from soundings over flat terrain extending up to log m (cy/m) = −2.0. Our calculations prolong this interval to −2.0 ≤ log m ≤ −0.7. In the stratosphere, a remarkable similarity between that earlier work and ours is observed. This also happens in the troposphere, but only up to log m = −1.4. As suggested by the other authors, the enhanced R values might be explained by the propagation of inertial gravity waves generated in the mountain relief (this is supported by rotational spectra calculated here). Previous evidence in favor of spectral separability obtained by other authors has been extended here for larger wavenumbers by the observed constancy of the ratio between the temperature and vertical velocity spectra. In both the troposphere and the stratosphere, this ratio appears to be fairly uniform and similar to previous results obtained by other authors at lower resolution.
Upper mantle anisotropy in the New Zealand Region
Authors: Klosko E. R. , Wu F. T. , Anderson H. J. , Eberhart‐Phillips D. , McEvilly T. V. , Audoine E. , Savage M. K. , Gledhill K. R.
Published: September 2002

Shear‐wave splitting parameters of fast polarization direction (Φ) and delay time (δt) are determined using data from the Southern Alps Passive Seismic Experiment (SAPSE), on the South Island of New Zealand and in the surrounding region. Our results clearly show that Φ are subparallel to trends of the Alpine and Marlborough Faults, and to the Pacific‐Australian plate boundary. The δt values range from 0.6–2.2 s with an average value of 1.6 s; the largest values are from the central South Island. The main source of the observed shear‐wave splitting is an anisotropic region between 40–400 km. The width of the zone is approximately 200 km. We attribute the coincidence of surface structural trends with the measured Φ, and the large δt values, to significant shear deformation in a 200 km thick zone along the plate boundary extending from the surface to deep within the upper mantle.
Effects of sinking of salt rejected during formation of sea ice on results of an ocean‐atmosphere‐sea ice climate model
Authors: Duffy P. B. , Eby M. , Weaver A. J.
Published: September 2002

We show that results of an ocean‐atmosphere‐sea‐ice model are sensitive to the treatment of salt rejected during formation of sea ice. In our Control simulation, we place all rejected salt in the top ocean‐model level. In the Plume simulation, we instantaneously mix rejected salt into the subsurface ocean, to a maximum depth which depends on local density gradients. This mimics the effects of subgrid‐scale convection of rejected salt. The results of the Plume simulation are more realistic than those of the Control simulation: the spatial pattern of simulated salinities (especially in the Southern Ocean), deep‐ocean temperatures, simulated sea‐ice extents and surface air temperatures all agree better with observations. A similar pair of simulations using horizontal tracer diffusion instead of the Gent‐McWilliams eddy parameterization show similar changes due to instantaneous mixing of rejected salt.
Interannual variability of the sea surface temperature in the South Brazil Bight
Authors: Campos Edmo J. D. , Lentini Carlos A. D. , Miller Jerry L. , Piola Alberto R.
Published: September 2002

In the austral winter of 1993, during a COROAS hydrographic cruise in the South Brazil Bight (SBB), an unexpected mass of cold and fresh water was observed on the continental shelf near 23°S. Subsequent analyses of different data sets suggested that the origin of that water was probably the Argentine continental shelf, near the Rio de la Plata mouth. In this article, a 13‐year time series of AVHRR Sea Surface Temperature anomalies is analyzed to investigate the occurrence of this phenomenon in other periods. The results of these analyses and a good correlation with the Southern Oscillation Index suggest that the penetration of these waters into the SBB occurs in a frequency that may be associated with ENSO events.
Variations of the thermospheric nitric oxide mass mixing ratio as a function of Kp, altitude, and magnetic local time
Authors: Ridley A. J. , Crowley G. , Link R. , Frahm R. , Winningham J. D. , Sharber J. R. , Russell J.
Published: September 2002

In this letter, data from the Halogen Occultation Experiment (HALOE) and the Particle Environment Monitor on the Upper Atmospheric Research Satellite are presented. Five years of data from each instrument have been used to create high‐latitude maps of Nitric Oxide (NO) mass mixing ratio and electron impact and bremsstrahlung ionization rates as a function of altitude and Kp. We show that enhancements in NO at 100 km are coupled to the auroral oval and to increases in the local ionization rate, with both the peak in the ionization rate and NO mass mixing ratio occuring in the dawn magnetic local time sector between 60° and 70° magnetic latitude. Below this altitude, the NO is shown to be enhanced at lower latitudes and sunward of the dawn‐dusk meridian, most likely due to solar illumination. Above 100 km, the NO density enhancement stays in approximately the same MLT sector, while the peak ionization rate expands in MLT towards the night side. This is attributed to a build up of NO created by particle precipitation during the night, which is corotated towards the dawn region.
On the termination of El Niño
Authors: Harrison D. E. , Vecchi Gabriel A.
Published: September 2002

A feature of the end‐phase of recent El Niño periods is thermocline shallowing to normal depths, in the central and eastern equatorial Pacific, before sea surface temperature (SST) returns to normal. Also characteristic of El Niño periods is a late‐in‐the‐year shift of the El Niño equatorial westerly wind anomalies, from symmetric about the equator to centered south of the equator. We show that the directly forced ocean response to this shift produces thermocline shallowing like that observed. The wind shift may result from the normal seasonal evolution of Pacific waters warmer than 28°C. These shift from being relatively symmetric about the equator in October–November, to concentrated south of the equator in December–January. Thus, the seasonal cycle may be a central part of the mechanism of El Niño termination.
Investigating the fractal properties of geological fault systems: The Main Ethiopian Rift Case
Authors: Giaquinta A. , Boccaletti S. , Boccaletti M. , Piccardi L. , Arecchi F. T.
Published: September 2002

In this article we address the problem of distinguishing between two different geodynamical histories using a geometrical description of the fault system in the Ethiopian Rift. The regional distribution of structures has been characterized by the estimation of the generalized fractal dimensions for the whole system as well as for the subsets of faults oriented along two different directions, using the importance sampling approach. Results show that the two subsets are monofractal with different dimensions, providing an independent indication that different dynamical processes have been responsible for the fault generation of this area.
Climatic changes in the Aegean Sea influence the eastern Mediterranean thermohaline circulation (1986–1997)
Authors: Theocharis Alexander , Nittis Kostas , Kontoyiannis Harilaos , Papageorgiou Emanuel , Balopoulos Efstathios
Published: September 2002

Recent changes of water mass characteristics in the south Aegean Sea have considerably influenced the Eastern Mediterranean thermohaline circulation. A combination of salinity increase (1987–92) and temperature drop (1992–94) caused massive dense water formation and strong outflow towards the deep and bottom parts of the Eastern Mediterranean. This climatic shift, the most important since the existence of observations in the basin, is a combined effect of extreme meteorological events superimposed on large‐scale trends. It has initiated a series of modifications in the hydrology and dynamics of the entire Eastern Mediterranean, with possible influence on the Mediterranean outflow in the North Atlantic Ocean.
Evolution of aircraft‐generated volatile particles in the far wake regime: Potential contributions to ambient CCN/IN
Authors: Yu Fangqun , Turco Richard P.
Published: September 2002

We apply a detailed aerosol microphysics model to study the evolution of aerosols in the far wake of an aircraft plume. Constrained by in‐situ measurements, our simulations reveal that the largest volatile particles—those most likely to contribute to the background abundance of condensation nuclei—are dominated by ion‐mode aerosols formed on chemiions. By tracking the evolution of the aircraft‐generated particles from the near wake to the far wake, we estimate the properties of the aircraft particles that are effectively injected into the upper troposphere. The entrainment of ambient vapors and aerosols into the aging plume is taken into account, which significantly affects the outcome. The model calculations reveal that a substantial number of the larger ion‐mode volatile particles may survive long enough to act as potential cloud nuclei, thus perturbing the background CCN population. This perturbation is significant during the winter season, and in other locales with low background aerosol concentrations, but is likely to be negligible during the summer season or at locations with heavy aerosol loading. The implications and uncertainties of the results are discussed.
First mesopause temperature profiles from a fixed southern hemisphere site
Authors: Clemesha B. R. , Veselovskii I. , Batista P. P. , Jorge M. P. P. M. , Simonich D. M.
Published: September 2002

The INPE sodium lidar was recently modified so as to enable measurements to be made of the Doppler temperature of the Na atoms in the atmospheric sodium layer. Measurements have been made on a total of 15 nights from July to October, 1998, providing temperature profiles between heights of about 83 and 105 km. Almost all measurements showed a mesopause temperature structure strongly perturbed by oscillations which appear to be caused by gravity waves and/or tides, with peaks in sodium density occurring at almost the same height as the temperature maxima on the bottom side of the sodium layer. The lowest temperatures, between 170 and 200 K, typically occur above 100 km, and the average profile for all our measurements is similar to the winter profile seen at mid‐latitudes, with a mesopause temperature of 190 K at 103 km.
A mechanism for generating ENSO decadal variability
Authors: Kleeman Richard , McCreary Julian P. , Klinger Barry A.
Published: September 2002

A coupled ocean‐atmosphere model of the Pacific basin is used to illustrate a mechanism by which El Niño and the Southern Oscillation (ENSO) may be modulated on decadal time scales. For reasonable choices of model parameters, solutions exhibit two types of oscillation, an ENSO‐like interannual mode and a decadal one. The decadal mode affects the equatorial zone by means of an oceanic teleconnection that involves transport variations of the North Pacific Subtropical Cell. Since almost half of the cool, thermocline water that upwells in the eastern equatorial Pacific participates in this cell, these variations significantly alter the extent of the cold tongue, and hence provide an efficient mechanism for modulating ENSO.
Alkenones in the northwestern Mediterranean Sea: Interannual variability and vertical transfer
Authors: Sicre Marie‐Alexandrine , Ternois Yann , Miquel Juan‐Carlos , Marty Jean‐Claude
Published: September 2002

C37 alkenone fluxes were measured with sediment traps at 200 m depth over the years 1989/1990 and 1993/1994 to assess the interannual variability of the alkenone flux from the surface waters of the Mediterranean Sea. Fall and spring were identified as the high flux periods. SST estimates derived from the U37K′ index indicated 50 m and 30 m as major production depths in spring and fall, respectively. Althought interannual variation of alkenone fluxes was notable, the seasonality and depth of production appeared to be recurrent features of the coccolithophorid cycle of production. Alkenone fluxes at 1000 m measured over the year 1993/1994 were about 5 times lower than at 200 m and show no evidence of preferential preservation relative to the organic carbon between these depths. SST predicted at 200 m and 1000 m indicated a remarkably good transfer of the surface temperature signal to deeper layers.
A new mechanism of whistler wave generation by amplitude modulated HF waves in the polar electrojet
Authors: Kuo S. P. , Koretzky E. , Lee M. C.
Published: September 2002

It is shown that a spatially distributed mode current of whistler waves, oscillating at the modulation frequency of the ground‐transmitted powerful HF wave, can be induced in the E‐region of the high‐latitude ionosphere through the coupling between HF wave‐modulated electrojet current and induced density irregularities. As shown in Kuo et. al. [1999], these density irregularities generated by the HF wave via a thermal instability vary periodically along the geomagnetic field. This current produces whistler waves directly rather than through an antenna radiation process. This mechanism generates whistler waves in the VLF range (3–30 KHz) with reduced harmonic components. The frequencies of produced whistler waves depend on the polarization, frequency, power, and modulation scheme of the HF wave. It is predicted that whistler waves at frequencies around 25 KHz can be most favorably excited, awaiting the corroboration of future ionospheric heating experiments.
Radial evolution of density structure in the solar corona
Authors: Woo Richard , Habbal Shadia Rifai
Published: September 2002

White‐light measurements made by the SOHO coronagraphs (LASCO for Large Angle Spectrometric Coronagraph) and the HAO Mauna Loa Mk III K‐coronameter are combined to follow the evolution of path‐integrated density structure in the solar corona overlying both coronal hole and quiet Sun regions from 1.15 to 5.5 Ro. The global imaging provided by these two instruments confirms and strengthens earlier results discovered by spacecraft radio ranging measurements [Woo and Habbal, 1999], that the imprint of density structure at the Sun — as manifested in the background latitudinal density profile closest to the Sun at 1.15 Ro — is carried essentially radially into interplanetary space with the same density gradient. The only exceptions are coronal streamers that evolve into the heliospheric current sheet within a few solar radii of the Sun.
Incoherent Scatter radar study of the impact of the meteoric influx on nocturnal E‐region ionization
Authors: Zhou Q. H. , Mathews J. D. , Zhou Q. N.
Published: September 2002

We use the Arecibo incoherent scatter radar data collected from 1987 to 1995 to study meteoric impact on the nocturnal E‐region electron/ion content. The nighttime ionization is observed to increase after midnight, which we interpret to be due to increase in background meteoric input. The column incremental rate due to meteoric ionization between 0100 to 0600 LT is estimated to be 109/cm²/hr. Direct meteoric deposition is responsible for 60% of the total ionization in the morning hours in the altitude range of 97 to 114 km. Although there appears to be an increase in electron content during shower periods, there is not an one‐to‐one relation between meteor showers and E‐region ionization enhancement.
Four years of stratospheric aerosol measurements in the northern and southern hemispheres
Authors: Guzzi D. , Morandi M. , Santacesaria V. , Stefanutti L. , Agostini P. , Liley B. , Wolf J. P.
Published: September 2002

Lidar data, collected from stations located in both the Northern and Southern hemispheres using 532nm wavelength, were employed for monitoring the evolution of stratospheric aerosol loading from January 1994 up to December 1997. Lidar data covering the pre‐Pinatubo situation are available also for the 1989–1993 period only for the Antarctica station. These data are reported for comparison with the present situation. The analysis provided stratospheric aerosol loading measurements which show the return to a clean stratosphere as observed before the eruption of Mt. Pinatubo. During 1997, integrated backscattering values ranging between 8.2±3 10−5sr−1for Lauder and Florence/ Brasimone and 6.0±1.5 10−5sr−1for the polar stations of Sodankyla and Dumont d'Urville were measured. These values are smaller then the ones registered before the Pinatubo eruption: for example, at Dumont d'Urville in 1989 an integrated backscattering of 0.75± 0.12 10−4sr−1was measured. An evaluation of the decay time, i.e. the half time, for aerosols allowed a rough estimate of aerosol particle size. A comparison between the decay time for integrated backscattering in the 13 to 25‐km region obtained for the four sites did not show any strong variability between the northern and southern hemispheres: only small differences are observed between the Arctic and Antarctic regions. A time constant of about 3 years and half was retrieved for the 1994–1997 period.
From diffuse to localised damage through elastic interaction
Authors: Amitrano David , Grasso Jean‐Robert , Hantz Didier
Published: September 2002

Local damage processes that have been reported for ductile and brittle macroscopic behaviours are shown here to provide a possible link between these two contrasting behaviours. Using a local progressive damage law within a linear tensorial elastic interaction model, we reproduce experimentally observed macroscopic non‐linear behaviours that continuously range from ductility with diffuse damage to brittleness with localised damage. The model exhibits power law distributions of damage events in space and size domains. The diffuse‐localised and induced ductile‐brittle transition appear to be controlled by the internal friction angle which influences the local interaction geometry.
Evidence for surface rupture in 1868 on the Hayward Fault in North Oakland and major rupturing in prehistoric earthquakes
Authors: Lienkaemper James J. , Williams Patrick L.
Published: September 2002

WGCEP90 estimated the Hayward fault to have a high probability (0.45 in 30 yr) of producing a future M7 Bay Area earthquake. This was based on a generic recurrence time and an unverified segmentation model, because there were few direct observations for the southern fault and none for the northern Hayward fault. To better constrain recurrence and segmentation of the northern Hayward fault, we trenched in north Oakland. Unexpectedly, we observed evidence of surface rupture probably from the M7 1868 earthquake. This extends the limit of that surface rupture 13 km north of the segmentation boundary used in the WGCEP90 model and forces serious re‐evaluation of the current two‐segment paradigm. Although we found that major prehistoric ruptures have occurred here, we could not radiocarbon date them. However, the last major prehistoric event appears correlative with a recently recognized event 13 km to the north dated AD 1640–1776.
Wrinkle‐like Weertman pulse at the interface between two blocks of foam rubber with different velocities
Authors: Anooshehpoor Abdolrasool , Brune James N.
Published: September 2002

We verify the existence of self‐sustaining wrinkle‐like Weertman velocity pulses along a pre‐stressed interface between two dissimilar (ΔV = 60%) blocks of foam rubber. Properties of the observed velocity pulses are similar to the theoretical prediction of Weertman (1980), and the numerical calculations of Andrews and Ben‐Zion (1997). Particle displacement, in the direction perpendicular to the fault, is much larger in the slower medium than in the faster medium, resulting in a separation of the interface during the passage of the slip pulse. The rupture velocity is near the shear wave velocity in the slower material, and the direction of propagation is nearly always in the direction of the shear particle motion in the slower block. The pulse is self‐maintaining, that is, it does not die out from radiation damping, obtaining the necessary energy by releasing locally stored shear potential energy.