bluuebunny/crossref_metadata_2025 huggingface dataset:
A diagnosis of interpentadal circulation changes in the North Atlantic
Authors: Greatbatch Richard J. , Fanning Augustus F. , Goulding Allan D. , Levitus Sydney
Published: February 2008

Diagnostic calculations of the circulation in the North Atlantic are described. Three basic cases are considered: the climatological mean state and the circulation in the pentads 1955–1959 and 1970–1974. Density data from Levitus (1982, 1989) are used as input together with the annual mean wind stress field of Hellerman and Rosenstein (1983) for the climatological case and wind stress data derived from the Comprehensive Ocean‐Atmosphere Data Set for the 1950s and 1970s. The results suggest that the Gulf Stream was some 30 Sv weaker in the 1970–1974 pentad than in the pentad 1955–1959. About 20 Sv of this is due to a dramatic weakening of the circulation of the subtropical gyre. This is traced to a change in bottom pressure torque associated with the bottom topography on the western side of the Mid‐Atlantic Ridge. This same general area is also shown to be important for enhancing the transport of the climatological mean subtropical gyre above that predicted by the flat‐bottomed Sverdrup relation. The remaining 10 Sv is due to a weakening of the cyclonic gyre in the continental slope region south of Atlantic Canada and north of the Gulf Stream. This too is associated with a change in bottom pressure torque. We find that changes in the density field above 1500 m depth contribute about half of the transport change. It is not clear how reliable is the estimate of the remaining half. This is because it is dependent on changes in the analyzed density field at depths greater than 1500 m, and these could be a result of insufficient or unreliable data. No significant change in the total transport of the subpolar gyre is diagnosed by our calculations. In order to interpret the results we have split the joint effect of baroclinicity and relief (JEBAR) term into two parts: a part associated with bottom pressure torque and a part associated with compensation by the density stratification for the effect of variable bottom topography. This leads to a natural division of the volume transport stream function Ψ into three parts; Ψ = Ψ W + Ψ C + Ψ B. Ψ W is calculated using wind forcing alone and assumes a uniform density ocean. Ψ C is the difference between this and the stream function, Ψ S, calculated using the flat‐bottomed Sverdrup relation. It is driven by that part of JEBAR associated with density compensation. Finally, Ψ B is the difference between Ψ and Ψ S and is that part of Ψ driven by bottom pressure torque. (Ψ C + Ψ B) then gives the total contribution to Ψ from the JEBAR term. We find that for the climatological mean subpolar gyre, density compensation is particularly important, with bottom pressure torque displacing the gyre southward rather then enhancing its transport. For the subtropical gyre, density compensation plays less of a role. Almost all the difference between the two pentads occurs in the bottom pressure torque part.
Gulf Stream frontal eddy influence on productivity of the southeast U.S. continental shelf
Authors: Lee Thomas N. , Yoder James A. , Atkinson Larry P.
Published: February 2008

Weekly period meanders and eddies are persistent features of Gulf Stream frontal dynamics from Miami, Florida, to Cape Hatteras, North Carolina. Satellite imagery and moored current and temperature records reveal a spatial pattern of preferred regions for growth and decay of frontal disturbances. Growth regions occur off Miami, Cape Canaveral, and Cape Fear due to baroclinic instability, and decay occurs in the confines of the Straits of Florida between Miami and Palm Beach, between 30° and 32°N where the stream approaches the topographic feature known as the Charleston bump and between 33°N and Cape Hatteras. Eddy decay regions are associated with elongation of frontal features, offshore transport of momentum and heat, and onshore transport of nutrients. Onshore transport of new nitrogen from the nutrient‐bearing strata beneath the Gulf Stream indicates that frontal eddies serve as a “nutrient pump” for the shelf. New nitrogen flux to the shelf due to Gulf Stream input could support new production of 7.4×1012 g C yr−1 or about 8 million tons carbon per year if all nitrate were utilized. Calculations indicate that approximately 70% of this potential new production is realized, yielding an annual new production for the outer shelf of 4.3×1012 g C.
Seasonal changes in the tropical Atlantic circulation: Observation and simulation of the Guinea Dome
Authors: Siedler Gerold , Zangenberg Norbert , Onken Reiner , Morlière Alain
Published: February 2008

The Guinea Dome is a permanent, quasi‐stationary feature on the eastern side of the thermal ridge extending zonally across the tropical North Atlantic. The dome is a part of the large‐scale near‐surface flow fields associated with the North Equatorial Current, the North Equatorial Countercurrent and the North Equatorial Undercurrent. In the present study, historical and recently obtained hydrographic data are combined to investigate the thermohaline structure and geostrophic flow field in the vicinity of the dome. It is shown that the Guinea Dome exists throughout the year both in subthermocline and thermocline layers, that it has a corresponding cyclonic geostrophic flow, and that seasonal changes occur with respect to its vertical structure, horizontal extent, and position. The observational results are then compared with simulations from a general circulation model of the tropical Atlantic. A seven‐year simulation forced by observed monthly winds is run to compute a monthly climatology. The model adequately simulates the Guinea Dome with respect to its structure, flow field, and seasonal variability.
The effect of small‐wave modulation on the electromagnetic bias
Authors: Rodríguez Ernesto , Kim Yunjin , Martin Jan M.
Published: February 2008

The effect of the modulation of small ocean waves by large waves on the physical mechanism of the electromagnetic (EM) bias is examined by conducting a numerical scattering experiment which does not assume the applicability of geometric optics. The modulation effect of the large waves on the small waves is modeled using the principle of conservation of wave action and includes the modulation of gravity‐capillary waves. The frequency dependence and magnitude of the EM bias is examined for a simplified ocean spectral model as a function of wind speed. These calculations allow us to assess the validity of previous assumptions made in the theory of the EM bias, with respect to both scattering and hydrodynamic effects. We find that the geometric optics approximation is inadequate for predictions of the EM bias at typical radar altimeter frequencies, while the improved scattering calculations provide a frequency dependence of the EM bias which is in qualitative agreement with observation. We also find that, for typical wind speeds, the EM bias contribution due to small‐wave modulation is of the same order as that due to modulation by the nonlinearities of the large‐scale waves.
Separation of tidal and subtidal currents in ship‐mounted acoustic Doppler current profiler observations
Authors: Candela Julio , Beardsley Robert C. , Limeburner Richard
Published: February 2008

A simple method is developed to analyze current measurements obtained from a moving platform. The need for this is motivated by the now common use of the ship‐mounted acoustic Doppler current profiler (ADCP) to acquire absolute velocity data during an oceanographic survey of a given region. The full potential of this new measurement technique is severely hindered when the presence of high‐frequency phenomena (e.g., tidal or inertial motions) prevents a clear visualization of the lower‐frequency current structure of interest. Our analysis method is based on a spatial interpolation scheme, using arbitrary functions, that allows for the tidal current time variability, which then permits the tide‐induced motions to be subtracted from the ADCP data to yield the subtidal current field. The method also allows nearby moored and drifter current measurements (if available) to be combined with the shipboard ADCP observations in a single analysis to obtain the best description of the tidal and subtidal currents. To illustrate this method, we present results from the analysis of ADCP data taken during oceanographic surveys in two different continental shelf regions, the East China Sea and the Amazon shelf. A 5‐day conductivity‐temperature‐depth (CTD) and ADCP survey was made in the East China Sea near the mouth of the Yellow Sea during January 1986. There the currents were essentially barotropic and dominated by the semidiurnal tide. The ADCP‐derived cotidal chart for the M2 (12.42 hours) component agrees well with existing charts derived empirically from sea level observations or from regional numerical models. The ADCP‐derived steady flow is also consistent with the observed density field and indicates little flow in or out of the Yellow Sea and a transport of about 1 Sv toward the Tsushima Strait. Two CTD‐ADCP surveys lasting 21 and 23 days were conducted over the Amazon shelf during March and May, 1990. Simultaneously moored current observations were also obtained at three locations along a cross‐shelf array 200 km north of the Amazon River mouth, for a common period of at least 2 months (February 12 to April 13). Over the shelf, tidal and subtidal currents were comparable in magnitude. Our analysis indicates that the tidal currents were essentially barotropic, semidiurnal, and polarized in the cross‐shelf direction, increasing in magnitude toward the inner shelf where current values of more than 2 m/s are common. The ADCP‐derived steady currents were aligned in the along‐shelf direction and strongly influenced by the North Brazil Current (NBC). During both ADCP surveys, the northwestward flowing NBC was transporting more than 2 Sv over the shelf at depths shallower than 100 m.
Identification of sea ice types in spaceborne synthetic aperture radar data
Authors: Kwok Ronald , Rignot Eric , Holt Benjamin , Onstott R.
Published: February 2008

An approach for identification of sea ice types in spaceborne synthetic aperture radar (SAR) image data is presented. The unsupervised classification approach involves cluster analysis for segmentation of the image data followed by cluster labeling based on previously defined look‐up tables containing the expected backscatter signatures of different ice types measured by land‐based scatterometer. The particular look‐up table used for labeling a segmented image is selected based on the seasonal and meteorological conditions at the time of data acquisition. The extensive scatterometer observations and experience accumulated in field campaigns during the last 10 years were used to construct these look‐up tables. These tables are expected to evolve as sea ice observations from the European ERS‐1 SAR become available. This paper presents the classification approach, its expected performance, the dependence of this performance on radar system performance, and expected ice scattering characteristics. Results using both aircraft and simulated ERS‐1 SAR data are presented. The results are compared to limited field ice property measurements and coincident passive microwave imagery. An algorithm based on this experimental approach has been implemented in the geophysical processor system at the Alaska SAR Facility for classification of sea ice data in ERS‐1 C band SAR data. The importance of an integrated postlaunch program for validation and improvement of this approach is discussed.
Dependence of cloud amount on horizontal resolution in the National Center for Atmospheric Research community climate model
Authors: Kiehl J. T. , Williamson D. L.
Published: February 2008

The dependence of the cloud fraction on horizontal resolution is explored with the National Center for Atmospheric Research Community Climate Model (CCM1). Using R15, T42, T63 and T106 versions of the CCM, we find that the total cloud amount monotonically decreases with increased horizontal resolution. This decrease is particularly evident in the low‐level cloud associated with stable condensation in the tropics. Decreases in low‐level and upper‐level extratropical clouds with increased resolution are also apparent. Analysis of these model simulations shows that this decrease is due to increased advective drying of the lower atmosphere by stronger subsidence. Stronger upward motion which is directly associated with stronger heating in the convergence region by moist convective adjustment and stable condensation processes is also apparent with increased horizontal resolution. This stronger upward motion affects the upper tropical cloud amount. The upper‐level extra‐tropical cloud amount is shown to depend strongly on the correction applied to the spectral transport of moisture in the model, the so‐called “q fixer” which eliminates negative moisture. At higher resolution this spectral filler plays less of a role at the poles than at the lower resolutions. This study has important implications for the parameterization of cloud activity in large‐scale models, since a certain resolved large‐scale circulation is necessary for embedding these cloud parameterizations. It also raises questions relating to scale dependence of cloud and precipitation parameterizations.
Damping rates for acoustic gravity waves in the ionosphere
Authors: Miesen Rob H. M.
Published: February 2008

Damping of acoustic gravity waves due to the ionization of the ionosphere is studied, and damping rates are calculated under the assumption that the wave vector is real. The magnitude of the damping rates is estimated under ionospheric conditions. The present paper is in fact a generalization of the paper by Miesen et al. (1989), where the geomagnetic field was assumed to be parallel to the surface of the Earth. An error in this paper is corrected.
The sensitivity of a general circulation model climate to the moisture transport formulation
Authors: Rasch Philip J. , Williamson David L.
Published: February 2008

The sensitivity of the climate of the NCAR Community Climate Model to the numerical method used to transport water vapor is described. We compare two versions of the model, one of which uses a spectral method and the other a shape‐preserving semi‐Lagrangian method for the water vapor transport. These methods are very different in terms of their computational properties and produce very different climatologies in model simulations. These differences in climate result primarily from the redistribution of water vapor in the model. The redistribution carries with it differences in convection and cloud distributions and the associated radiative heating, which in turn affects the model climate. We interpret the differences using a sensitivity study perspective. Some of the changes produced by changing from spectral to semi‐Lagrangian transport are beneficial (that is, the model climate is closer to observations), and some are detrimental. We believe that many of the detrimental changes may be explained in terms of prior tuning of the physical parameterizations to the “spectral climatology” and may also be due to inadequate or missing physics. We examine many of the pertinent feedback loops and diagnose the processes associated with the changed climate.
The Ozone and Aerosol Fine Structure experiment: Observing the fine structure of ozone and aerosol distribution in the atmosphere from the Salyut 7 Orbiter: 2. Formation of the Earth's twilight limb coloration and radiance: Numerical calculations
Authors: Elansky N. F. , Plotkin M. E. , Postylyakov O. V. , Ukhinov S. A.
Published: February 2008

Monte Carlo calculations of the Earth's twilight limb spectral radiance and coloration in a model of the spherical atmosphere are carried out for layered vertical distributions of ozone and aerosol. The possibility of space monitoring of ozone and aerosol stratification based on the panoramic twilight limb observations is studied. It is shown that for background aerosol conditions, stratospheric ozone layers 2–3 km in thickness manifest themselves in the twilight limb in the form of blue bands with increased color saturation. In the case of greatly enhanced aerosol content (compared with background conditions) the layered twilight coloration in the tropospheric and low stratospheric region is caused by aerosol layers.
A numerical solution for the equations of tropospheric chemistry based on an analysis of sources and sinks of odd hydrogen
Authors: Sillman Sanford
Published: February 2008

A method is presented for solving the stiff differential equations for photochemical production and loss in the troposphere. The method is based on analysis of sources and sinks of odd‐hydrogen radicals. The implicit equations are reduced to a single equation for OH which is solved in terms of odd‐H production and loss. Individual odd‐H sources and sinks are shown to have a simple functional dependence on OH. The resulting iterative process decreases computation time by a factor of 7 with little loss of accuracy and is applicable to both urban and remote tropospheric conditions. A discussion the odd‐H budget and photochemical behavior in the troposphere is included.
Multilevel measurement of the electric field underneath a thundercloud: 1. A new system and the associated data processing
Authors: Chauzy Serge , Médale Jean‐Claude , Prieur Serge , Soula Serge
Published: February 2008

In order to study the formation and evolution of the charge layer created by a corona at the ground underneath thunderclouds, we have developed a new system of electric field detection. It is composed of five self‐contained sensors designed to measure the ambient electric field at various altitudes between the surface and a tethered balloon that suspends the entire system of equipment. Each sensor can detect ambient field intensities up to 150 kV/m with an accuracy of 1.5% and without any detectable influence of a local corona effect on the measurement. A standard shutter field mill detects the surface electric field. The data collected aloft are telemetered via a 400‐MHz frequency band. An adapted data processing system provides a real‐time display of the multilevel evolution of the electric field within the first hundreds of meters above the ground. The digitized data can be used to display the instantaneous vertical profile of field intensity on various time scales.
A diagnostic stratospheric aerosol size distribution inferred from SAGE II measurements
Authors: Thomason Larry W.
Published: February 2008

An aerosol size distribution model for the stratosphere is inferred based on 5 years of Stratospheric Aerosol and Gas Experiment (SAGE) II measurements of multispectral aerosol and water vapor extinction. The SAGE II aerosol and water vapor extinction data strongly suggest that there is a critical particle radius below which there is a relatively weak dependence of particle number density with size and above which there are few, if any, particles. A segmented power law model, as a simple representation of this dependence, is used in theoretical calculations and intercomparisons with a variety of aerosol measurements including dustsondes, longwave lidar, and wire impactors and shows a consistently good agreement.
A control network of Triton
Authors: Davies Merton E. , Rogers Patricia G. , Colvin Tim R.
Published: February 2008

A control network for Triton has been computed using a bundle‐type analytical triangulation program. The network contains 105 points that were measured on 57 Voyager 2 pictures. The adjustment contained 1010 observation equations and 382 normal equations and resulted in a standard measurement error of 13.36 μm. We determined coordinates of the control points, the camera orientation angles at the times when the pictures were taken, and Triton's mean radius. A separate statistical analysis confirmed Triton's radius to be 1352.6±2.4 km. Attempts to tie the control network around the satellite were unsuccessful because discontinuities exist in high‐resolution coverage between 66° and 289° longitude, north of 38° latitude, and south of 78° latitude.
Introduction to special section on laboratory research for planetary atmospheres
Authors: Fox Kenneth
Published: February 2008

This special section of JGR‐Planets comprises representative invited and contributed papers originally presented at the Second International Conference on Laboratory Research for Planetary Atmospheres (2nd ICLRPA), held at the University of Virginia in Charlottesville on October 21, 1990. The purposes of such conferences include the identifications and reports of laboratory studies, required or accomplished, in support of planetary sciences. These studies are necessary for the derivation of meaningful data from spacecraft and ground‐based measurements and for mission plans generally. These conferences highlight progress in laboratory planetary sciences and emphasize the interdependence of laboratory research and modeling of planetary atmospheres.
Studies of proton irradiated H2O + CO2 and H2O + CO ices and analysis of synthesized molecules
Authors: Moore M. H. , Khanna R. , Donn B.
Published: February 2008

Infrared spectra of H2O + CO2 and H2O + CO ices before and after proton irradiation showed that a major reaction in both mixtures was the interconversion of CO2 ⇌ CO. Radiation synthesized organic compounds such as carbonic acid were identified in the H2O + CO2 ice. Different chemical pathways dominate in the H2O + CO ice in which formaldehyde, methanol, ethanol, and methane were identified. Sublimed material was also analyzed using a mass spectrometer. Implications of these results are discussed in reference to comets.
Spectral‐chemical analysis of lunar surface materials
Authors: Jaumann Ralf
Published: February 2008

Samples of lunar rocks and soils that have been examined in the laboratory both spectroscopically and chemically were analyzed with the goal of identifying and quantifying correlations between reflectance spectra and compositional properties of lunar materials. The method used to quantify spectral variations within the set of spectral samples is principal component analysis. The correlations between the chemical analysis of lunar samples and their spectral quantities are determined by using multivariate analysis. The combination of both methods yields functional relationships between the principal axes of variation and the chemical constituents, in particular, FeO, TiO2, SiO2, Al2O3, MgO, CaO, and MnO, and with the content of amorphous materials (glass) in lunar samples. To first order, the nature of these relationships is found to be linear. The results define a spectral‐chemical model that is applicable to derive abundances of chemical constituents and amorphous components from individual spectra. The accuracy of the model is better than ±2.4 wt % and ±4% for chemical constituents and amorphous components, respectively, making the model also sensitive to remotely sensed spectra of the lunar surface.
Sr Isotopic Variation in Shallow Water Carbonate Sequences: Stratigraphic, Chronostratigraphic, and Eustatic Implications of the Record at Enewetak Atoll
Authors: Quinn Terrence M. , Lohmann K. C. , Halliday A. N.
Published: February 2008

Sr isotope data from two boreholes within the lagoon at Enewetak Atoll have been used to evaluate the use of such data to correlate, date, and monitor sea level change in shallow water carbonate sequences. Correlative stratigraphic intervals of relatively invariant δ87Sr, separated by abrupt transitions to lower δ87Sr with increasing depth, are recognized in both boreholes. Conversion of δ87Sr values to age via calibration with the seawater δ87Sr trend with age indicates that correlative and synchronous deposition of atoll sediments occurred at ∼ 0.4, 1.2, and 2.1 Ma. In contrast, a ∼5 m.y. hiatus is recognized in one borehole but not the other. Sr isotope stratigraphy (SIS) is a powerful stratigraphic and chronostratigraphic tool in shallow water carbonate sequences only when significant secular variation of δ87Sr occurs and retention of depositional δ87Sr values is demonstrated. The latter is best demonstrated when δ87Sr data, are integrated with δ18O, δ13C, Sr content data and petrographic observations. Several diagenetically altered intervals have greater δ87Sr values, low δ13C values, and low Sr/Ca ratios relative to adjacent intervals, a combination that is consistent with open‐system meteoric diagenesis. Calcite cements from these intervals have early Pleistocene (∼1.2 Ma) δ87Sr values despite their occurrence well within the late Pliocene (∼2.1 Ma) sequence. Thus local sedimentological and diagenetic processes have produced intralagoon variability in the SIS of the two boreholes, complicating subsurface stratigraphic correlations. The occurrence of anomalously young calcite cement relative to adjacent limestone is a direct response of the interaction of sea level change and meteoric phreatic diagenesis whereby overlying metastable carbonates, with greater δ87Sr values, are dissolved during periods of atoll emergence and sea level lowstand liberating Sr and soil‐gas CO2to the pore fluid, which is then incorporated into downflow meteoric phreatic diagenetic calcite. The stratigraphic relation between the sites of carbonate dissolution and diagenetic calcite precipitation suggest an early Pleistocene (∼1.2 Ma) sea level fall of between 34 and 64 m. Moreover, the stratigraphic position and inferred age of paleophreatic lens calcite when backtracked for subsidence suggest that early Pleistocene (∼1.2 Ma) sea level was ∼72–81 m (range of 60–100 m) below present sea level. The Enewetak record of early Pleistocene sea level lowstand elevation agrees reasonably well with proxy sea level records inferred from sequence stratigraphy and deep‐sea foraminiferal δ18O data. However, the Enewetak record of the amplitude of early Pleistocene sea level change is more consistent with the deep‐sea foraminiferal δ18O proxy than the sequence stratigraphy proxy.
Past CO2 Changes and Tropical Sea Surface Temperatures
Authors: Crowley Thomas J.
Published: February 2008

The relationship between past CO2 variations and tropical climate change on time scales of millions of years is reviewed. Explanations for Cretaceous and early Cenozoic climate change often involve higher atmospheric CO2 levels. Climate models predict that such increases should have been associated with warmer tropical sea surface temperatures (SSTs). However, sparse mid‐Cretaceous δ18O‐based SST estimates from low‐latitude calcareous plankton are ∼ 1.0–1.5‰ heavier than predicted. Initial evaluations indicate that changes in salinity may not alleviate model‐data discrepancies for calcareous δ18O and that interpretation of altered latitudinal δ18O gradients in terms of increased poleward heat transport due to warm, saline bottom water (WSBW) formation would require very large levels of WSBW production (at least 60 Sv). Calcareous δ18O records may not always record true SST patterns, as δ18O analyses from phosphatic groups, which are less sensitive to diagenesis, indicate SSTs ∼5°C higher. This latter conclusion is supported by a number of other fossil groups, which indicate temperatures warmer than those estimated from calcareous δ18O. In order to more rigorously test the paleo‐CO2 hypothesis, and indeed the models used to forecast future climates, it is desirable to intensify investigations of past climate change in low latitudes in order to determine whether these preliminary conclusions stand the test of further scrutiny.
The δ18O of atmospheric O2 from air inclusions in the Vostok Ice Core: Timing of CO2 and ice volume changes during the penultimate deglaciation
Authors: Sowers Todd , Bender Michael , Raynaud Dominique , Korotkevich Y. S. , Orchardo Joe
Published: February 2008

One important question about the CO2‐climate connection is how increasing atmospheric pCO2 levels affect climate and thereby the mass balance of continental ice sheets. A record of atmospheric CO2 variations over the last 160,000 years has recently been constructed by analyzing the trapped gas in the Vostok ice core [Barnola et al., 1987]. The relationship between changes in atmospheric CO2 and the size of the continental ice sheets has been difficult to ascertain because the CO2 record is obtained from ice cores while the ice volume record has been constructed from the stable isotopic composition of biogenic CaCO3 in deep‐sea sediment cores. In order to compare these two records in a more precise manner, we present a record of the isotopic composition (δ18O) of atmospheric O2 trapped in the Vostok ice core, and propose that it may be considered a proxy for the δ18O of seawater and hence ice volume. Having a record of atmospheric CO2 along with a continental ice volume proxy in trapped air in the same ice core allows us to compare the timing of changes in these two parameters with little uncertainty in the relative ages of important events. Our results suggest that, during the penultimate glacial termination, atmospheric CO2 began to increase at least 3 kyr before the initial introduction of meltwater to the oceans. Possible errors in the relative age‐depth curve of the Vostok ice core and uncertainties in the influence of biological and hydrologic processes on the δ18O of atmospheric O2 introduce some uncertainty into our conclusions. However, our results are in general agreement with the observed phase relationship between atmospheric CO2 and ice volume [Imbrie et al., 1984; Imbrie et al., 1989] (inferred from changes in the δ13C difference between contemporaneous planktonic and benthic forams deposited in deep‐sea cores and the SPECMAP ice volume record).
A General Circulation Model of the Indian Ocean at 9000 Years B.P.
Authors: Bigg G. R. , Jiang D. , Mitchell J. F. B.
Published: February 2008

A general circulation model of the Indian Ocean was run using monthly average wind and thermal forcing from an atmospheric model for conditions appropriate for today and 9000 years B.P. The present‐day forcing is poor during the equinoxes but representative of actual conditions during the summer and winter. We therefore concentrate on discussing the simulated ocean circulation during these seasons. It is shown that the upper ocean circulation of the entire northern Indian Ocean was strengthened by 15–30% at 9000 years B.P. relative to today, with the largest effect in the Arabian Sea and south of India. The surface equatorial flow is weaker in the boreal summer, but the stronger easterly winds through much of the year lead to a bigger thermocline slope along the equator. This, in turn, leads to a stronger undercurrent but weaker equatorial upwelling than at present. The sea surface temperatures during the boreal winter are found to be generally 1°–2°C cooler over the Indian Ocean but mostly 1°C warmer in the summer. The Arabian Sea was, however, cooler at 9000 years B.P. The simulation supports quantitatively the palaeoclimate record.
Impact of crustal evolution on changes of the seismic properties of the uppermost ocean crust
Authors: Jacobson R. S.
Published: February 2008

One of the most important observations of oceanic crustal evolution is that the uppermost part of the igneous crust, layer 2A, appears to thin and eventually disappear as its seismic velocity doubles from approximately 2.2 to 4.5 km s−1 in the first 40 m.y. The most likely cause is decreasing porosity in extruded basalts that compose the uppermost igneous crust by filling cracks with hydrothermally generated minerals, a by‐product of off‐axis hydrothermal circulation and alteration of basalts. New insights from two recent papers on crustal hydrothermal circulation patterns and modeling of the effects of crack geometry distributions upon seismic velocities provide guidance to relate filling of cracks within the basement to seismic velocity increases. A qualitative model of layer 2A crustal evolution is outlined that satisfies many geophysical and geochemical observations, some seemingly unrelated. Somewhat surprisingly, the crustal age at which layer 2A seismic velocities reach their maximum value is primarily controlled by the integrated permeability of the overlying sediment, which in turn, controls the hydrologic regime necessary for secondary mineral precipitation. Speculative predictions can be used to verify, refine, or invalidate this dynamic model of crustal evolution. Major scientific and technical obstacles must be overcome before a definitive model can be quantified. With such a rigorous model of crustal upper crustal evolution in hand, remote geophysical sensing tools can begin to fulfill the promise of interpreting the physical state, composition, and past history of oceanic crust.
The effect of variability of ground conductivity on the calculation of atmospheric noise using thunderstorm‐based models
Authors: Ferguson Jerry A.
Published: February 2008

Radio emissions from lightning discharges in thunderstorms are the primary source of atmospheric radio noise at very low and low frequencies (10–60 kHz). Attenuation of the lightning‐induced signal between a source in a thunderstorm and a receiver occurs mainly by absorption in the ionosphere and in poorly conducting ground. The attenuation in poorly conducting ground depends primarily on the conductivity so uncertainty in this conductivity contributes directly to uncertainty in calculating the level of atmospheric noise. A thunderstorm‐based computer model of atmospheric noise is used to evaluate the effect of this uncertainty in ground conductivity on calculation of atmospheric noise. It is found that this model produces only small variations in the level of atmospheric noise when the conductivity of the ground is varied. This result occurs because it employs large numbers of sources which tend to mask variations in the signal level on paths which cross low‐conductivity ground.
Horizontal fields generated by return strokes
Authors: Cooray Vernon
Published: February 2008

Horizontal fields generated by return strokes play an important role in the interaction of lightning‐generated electromagnetic fields with overhead power lines. In many of the recent investigations on the interaction of lightning electromagnetic fields with power lines, the horizontal field was calculated by employing the expression for the tilt of the electric field of a plane wave propagating over the finitely conducting Earth. In this paper we show that the horizontal field generated by return strokes over the finitely conducting ground can be obtained to a high accuracy by using the expression for the surface impedance of the finitely conducting Earth. The method is suitable to calculate horizontal fields generated by return strokes at distances as close as 200 m. At these close ranges the use of the wave tilt expression can cause large errors.
Sea echo in tropospheric ducting environments
Authors: Pappert R. A. , Paulus R. A. , Tappert F. D.
Published: February 2008

Path loss formulas for sea surface backscatter are developed for horizontally stratified tropospheric ducting environments. First‐order Bragg backscatter results coupled with ray and waveguide formalism serve as the basis for the development. Two features of composite model formalism, namely shadowing and tilting of the surface normal in the plane of incidence, are included in the development. Though developed for hh polarization, conventional composite model modifications for vertical polarization and depolarization can be incorporated into the echo models. Comparisons are made with previously generated parabolic equation results and with empirical model results generated by Engineers Refractive Effects Prediction System. These comparisons are for a frequency of 9.6 GHz for the standard atmosphere and for 14‐ and 28‐m evaporation ducts. Results apply to wind speeds of 10, 20, 30, and 40 knots (≈ 5, 10, 15, and 20 m/s).
Constraints on the late thermotectonic evolution of the western Alps: EVidence for episodic rapid uplift
Authors: Hurford Anthony J. , Hunziker Johannes C. , Stöckhert Bernhard
Published: February 2008

Forty‐two new apatite and zircon fission track ages are presented for samples from the Western Alps in southern Switzerland, northern Italy, and southeastern France. Measured ages plotted against assumed closure temperatures yield cooling patterns for the final cooling, uplift, and exhumation of the Western Alps. Similar fission track zircon ages in the Penninic Gran Paradiso massif, Dent Blanche nappe, Sesia‐Lanzo Zone, and Ivrea Zone indicate cooling of all four units to ∼225°C by 33 Ma. Differences in apatite ages reveal differential cooling of the four blocks between 33 Ma and the present. In the Sesia‐Lanzo Zone, similarity of apatite ages regardless of elevation, together with near‐volcanic confined fission track length patterns suggest rapid cooling and uplift at ∼25 Ma compared with slow cooling of other Western Alps units around 12 Ma. Uplift is thus not continuous but episodic, often over a short time interval beyond the resolution of other methods. Such episodes of uplift, as revealed here in the Sesia‐Lanzo Zone, may be the rule rather than the exception.
Mechanical strength of extended continental lithosphere: Constraints from the Western Rift System, East Africa
Authors: Ebinger Cynthia J. , Karner Garry D. , Weissel Jeffrey K.
Published: February 2008

Although regional isostasy generally is associated with continental lithospheric compression and foreland basin formation, local isostatic compensation commonly is assumed in models of extensional basin formation. The assumption of negligible lithospheric strength during rifting often is justified on the basis of: (1) high heat flow and temperatures produced by elevating the lithosphere ‐ asthenosphere boundary and (2) fracturing of the crust and lithosphere by normal faults. By modeling the development of rift basins within the Western rift system of East Africa and their associated free air gravity anomalies, we assess the role of basin‐producing normal faults in modifying the flexural strength of extended lithosphere. Heat flow and seismicity data from the East African plateau region indicate that the Western rift system located on the western side of the plateau developed in old, cold continental lithosphere. These relatively narrow (40–70 km wide), but deep, basins are bounded along one side by high‐angle border faults that penetrate to lower crustal levels, as indicated by seismicity data. Along the length of the Western rift system, depth to pre‐rift basement and rift flank topography vary between basins from 1 to 8 km and from 1 to 2 km respectively, with deeper basins generally correlating with higher flanks. Comparison of model predictions with topography and free air gravity profiles reveals that the basin depth and the flank height in the majority of the Western rift basins studied can be explained simply by small heaves (3–10 km) across the border fault and with significant flexural strength of the lithosphere maintained during extension. Where both observed basin depth and flank height could not be reproduced, basins were located adjacent to eruptive volcanic centers active in Miocene‐Recent times. In these areas, basin depth, rift flank elevation, and free air gravity anomaly may be modified by magmatic underplating of the crust. Estimates of effective elastic thickness Te obtained from our forward models vary between 17 and 38 km. These Te estimates are consistent with previous values obtained through the wavenumber domain correlation of Bouguer gravity and topography data. We find that the lithosphere beneath East Africa maintains significant flexural strength in extension despite localized, but intense, fracturing of the crust by normal faults that penetrate to lower crustal levels. Therefore the assumption of local isostasy (Te ∼ 0) appears invalid within the youthful Western rift system.
Permo‐Triassic tectonism in volcanic arc sequences of the western U.S. Cordillera and implications for the Sonoma orogeny
Authors: Wyld Sandra J.
Published: February 2008

The Permo‐Triassic Sonoma orogeny, western U.S. Cordillera, involved tight folding and imbricate thrust faulting of basinal strata of the Golconda allochthon. This event is generally interpreted in terms of accretion of upper Paleozoic volcanic arc rocks of the western U.S. Cordillera (“McCloud arc”) to the North American continent, either via back arc basin closure or arc‐continent collision. The structural history of the McCloud arc during the Sonoma orogeny is poorly understood, however, as are the implications of this history for tectonic models. This paper presents a synthesis of the Permian to Triassic stratigraphic and structural evolution of the McCloud arc, incorporating new data from the Pine Forest Range, northwest Nevada. The synthesis indicates that, on a regional scale, the arc was affected by widespread uplift but only local tilting, folding and faulting in the Late Permian to Early Triassic. The timing of Permo‐Triassic tectonic activity in the McCloud arc is thus similar to the middle Permian to Early Triassic age of deformation in the Golconda allochthon but the structures produced in the two groups of rocks differ markedly. The significance of this contrasting structural response to Sonoma age tectonism can be evaluated by comparison with other better understood examples of arc‐continent accretion (back arc basin closure in the southern Andes, “compressional” arc‐continent collision in New Guinea and “extensional” arc‐continent collision in the Mediterranean). This comparison indicates that the contrasting structural history of the McCloud arc and Golconda allochthon is entirely compatible with an arc‐continent accretion model for the Sonoma orogeny and cannot be used as a basis for concluding that the McCloud arc was not involved in the orogenic event. The comparison further indicates that either back arc basin closure or “compressional” arc‐continent collision are the most likely settings for the Sonoma orogeny. In contrast, an “extensional” Mediterranean‐style arc‐continent collision, as has been recently proposed for the earlier (middle Paleozoic) Antler orogeny, is probably not an appropriate analogue for the Sonoma orogeny. The weak structural response of the McCloud arc during Sonoman compression also provides new constraints on the specific processes involved in arc accretion, either via arc‐continent collision or back arc basin closure. These constraints suggest that there may be few fundamental differences between the two models during the final stages of arc accretion. Regardless of tectonic setting, Permo‐Triassic uplift and associated deformation in the McCloud arc should be considered as much a response to the Sonoma orogeny as is tight folding and imbricate faulting in the Golconda allochthon.
Tertiary extension in the Old Woman Mountains Area, California: Evidence from apatite fission track analysis
Authors: Foster David A. , Miller Donald S. , Miller Calvin F.
Published: February 2008

Apatite fission track ages and confined track length distributions from the Old Woman Mountains area, southeastern California, record evidence for periods of rapid cooling during the Late Cretaceous following Mesozoic orogenesis and during Miocene crustal extension. Apatites from the Clipper, Marble, and Turtle Mountains give fission track ages between ∼62 and ∼69 Ma as a result of cooling of these upper crustal rocks after regional Late Cretaceous plutonism and metamorphism. In the Piute Mountains, apatite fission track ages decrease from >70 Ma with mean track lengths of ∼13.5 μm to <20 Ma with mean track lengths of >14 μm from west to east. Intermediate aged samples from the Piute Mountains have mean track lengths between 11 and 13 μm due to their extended residence in the apatite annealing zone during the early Tertiary. This pattern is the result of 20°–30° of westward tilting of the Piute Mountains block as indicated by the dip of Tertiary strata. The apatite data indicate that tilting occurred after ∼18 Ma and caused rapid cooling of rocks exposed on the eastern side of the mountain range. Knowing the amount of tilting and the apparent paleotemperatures across the Piute Mountains allows us to estimate that the synextensional geothermal gradient was 50° ± 20°C/km. Apatites from the northeastern Old Woman Mountains yield fission track ages of ∼68 Ma with mean track lengths of 13.8 μm. These samples record cooling after batholith emplacement at 73 Ma. In the northwestern Old Woman Mountains, apatite fission track ages decrease to <20 Ma and have mean track lengths ∼15 μm indicating very rapid cooling. We interpret this age pattern to be the result of minor eastward tilting of the Old Woman Mountains block after 18–19 Ma or flexural unroofing of the northwestern Old Woman Mountains due to movement on the Ship Mountains fault. Extension in the Old Woman Mountains area occurred between 19 and 16 Ma, which is coincident with extension in the Colorado River Extensional Corridor and later than major extension in the Mojave Extensional Belt.
Tectonomagmatic controls on Gondwana break‐up models: Evidence from the Proto‐Pacific Margin of Antarctica
Authors: Storey Bryan C. , Alabaster Tony
Published: February 2008

Geochemical and isotopic data are presented that suggest the existence of a large, Middle Jurassic subduction‐related magmatic province common to both the Antarctic Peninsula and southern South America. We argue that during the initial stages of Gondwana breakup, Pacific margin magmas were derived from an enriched lithospheric mantle source similar to that for the contemporaneous within‐plate Ferrar‐Tasman suite. Enriched lithospheric initial‐rifting magmas were succeeded, in at least part of the Rocas Verdes basin, by transitional early drift magmas and then by entirely asthenospheric mid‐ocean ridge basalt (MORB) magmas representing lithospheric rupture and seafloor spreading. We propose a plate interaction model for the initial stages of Gondwana breakup relating the broad zone of lithospheric mantle melting to a reduction in plate boundary forces. The change from Gondwanide compression to lithospheric extension in the Jurassic is linked to a change from shallow to steeply dipping subduction and to a slowing of subduction rates caused possibly by a decreasing age of the subducting plate. Ridge‐trench interaction may have followed subduction of young, hot oceanic lithosphere, possibly causing a temporary cessation of subduction and a further reduction in plate boundary forces, thus facilitating breakup.
Thermal evolution of a portion of the Sevier Hinterland: The Northern Ruby Mountains‐East Humboldt Range and Wood Hills, northeastern Nevada
Authors: Hodges K. V. , Snoke A. W. , Hurlow H. A.
Published: February 2008

Conventional thermobarometry and Gibbs' Method modeling have been used to obtain new information pertaining to the Mesozoic‐Tertiary thermal evolution of the Ruby Mountains‐East Humboldt Range and Wood Hills metamorphic complexes, northeastern Nevada. Kyanite‐grade assemblages from Clover Hill, in the extreme northeastern part of the East Humboldt Range, were produced at temperatures of 780–810 K and depths in excess of 35 km; previously published geochronologic data suggest an Early Cretaceous or Late Jurassic age for this event. The Clover Hill area subsequently experienced a major unroofing interval prior to Late Cretaceous time, during which it was brought roughly 20 km closer to the surface. The lack of stratigraphic evidence for substantial Cretaceous erosion in the region and the topology of the pressure‐temperature paths for the Clover Hill samples suggest that the unroofing mechanism may have been tectonic denudation related to the relaxation of large topographic gradients produced by crustal thickening in the hinterland of the Sevier orogen. Metamorphic rocks at Clover Hill and within the Wood Hills core complex equilibrated at 820–900 K and 500–640 MPa (18–24 km) in Cretaceous time, prior to 115 Ma. Other parts of the Ruby Mountains‐East Humboldt Range metamorphic complex, presently structurally separated from the Clover Hill sequence, were intruded extensively by Late Cretaceous granitic magmas and attained substantially higher temperatures than the Clover Hill‐Wood Hills block. Sillimanite‐grade assemblages from these areas apparently underwent roughly 10 km of unroofing and 130 K of cooling prior to final equilibration at 820–920 K and 360–430 MPa (13–16 km) during Oligocene extension.