bluuebunny/crossref_metadata_2025 huggingface dataset:
Crustal velocity structure of the Omineca Belt, southeastern Canadian Cordillera
Authors: Kanasewich E. R. , Burianyk M. J. A. , Ellis R. M. , Clowes R. M. , White D. J. , Côté T. , Forsyth D. A. , Luetgert J. H. , Spence G. D.
Published: February 2004

Travel time inversion and amplitude modeling of a 350‐km Lithoprobe seismic refraction/wide‐angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the Omineca Belt of the Canadian Cordillera. The upper crust to 12–18 km depth has velocities from 5.6 to 6.2 km s−1, and two shear zones, the Monashee Décollement and Gwillim Creek Shear Zone, are imaged by the wide‐angle reflections and velocity trends. Minor velocity differences on either side of the Monashee Décollement may be related to separate rock origins. Prominent reflections define the boundaries of a low‐velocity midcrustal layer from 10–15 km to 20–25 km depth with velocities less than 6.1 km s−1. The low velocities of the midcrust, associated with high electrical conductivities and high heat flow, may be considered as support for the hypothesis of fluids in the Cordilleran crust, though other possibilities, such as the effect of high temperatures on rock velocities are possible. In the lower crust velocities range from 6.4–6.5 km s−1 at the top of the lower crust to 6.6–6.8 km s−1 at its base. The Moho is very clearly defined by the refraction/wide‐angle reflection data and has a gentle southerly dip. Crustal thicknesses are 35–37 km. A thin crust‐mantle transition zone of 1–2 km thickness in which velocities vary between 7.6 and 7.7 km s−1 is consistent with coincident reflection data. Upper mantle velocities range from 7.9 to 8.1 km s−1 with indications from the data of upper mantle layering. In comparison with neighboring regions, the Omineca Belt has an anomalously thin crust, low crustal velocities, and a low‐velocity upper mantle, similar only to the Basin and Range province. The velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method. The characteristics of a thin crust and lithosphere, along with low velocities from midcrust to mantle suggests that both the Basin and Range and the southern Canadian Cordillera are currently being heated from a source within the mantle.
Accuracy of the GEM‐T2 geopotential from Geosat and ERS 1 crossover altimetry
Authors: Wagner C. A. , Klokočník J.
Published: February 2004

Extensive analyses of altimetrically determined sea height differences at crossovers have been used to assess the accuracy of the GEM‐T2 geopotential. The orbits used were determined with GEM‐T2 for Geosat in its 17‐day Exact Repeat Mission (ERM) in 1986–1989 and ERS 1 in both its 3‐day ERM in 1991–1992 and its 35‐day ERM in 1992. The data examined are completely independent of the data used in GEM‐T2's development though GEM‐T2 had considerable use of Doppler tracking information on Geosat. The test of the radial accuracy of the ERS 1 orbit (98.5° inclination) is especially significant because it is not “close” to any other orbit well represented in GEM‐T2. The assessment consists of a comparison of observed mean height differences at thousands of distinct geographic locations with error projections from the GEM‐T2 covariance matrix which was estimated from other data sources. This first comprehensive, independent test of the purely radial accuracy of an orbit‐geopotential model clearly shows that the covariant predictions for GEM‐T2 are broadly reliable for this purpose. Thus, the agreement of crossover predictions and observations suggests that the total radial errors for these ERMs, due only to GEM‐T2 (but excluding the effects of initial state error) are about 23 cm for Geosat and 115 cm (rms) for ERS 1. However, there is little detailed agreement of measurements and predictions for ERS 1 and only partial agreement in detail for Geosat. Our 30,000 mean crossover discrepancies for Geosat (derived from ERM cycles 1–44) are also shown to reduce substantially the crossover height differences in cycles 45–61, almost exactly as predicted if these are the true GEM‐T2 errors for this orbit.
Structure and tectonics in the region of flat subduction beneath central Peru: Crust and uppermost mantle
Authors: James David E. , Snoke J. Arthur
Published: February 2004

Seismic waveform modeling of boundary interaction phases is used to determine the discontinuity structure of the crust in the Subandean and foreland basin regions overlying the zone of flat subduction beneath east‐central Peru. The data analyzed are from intermediate‐depth earthquakes (110 to 155 km) recorded on an array of three‐component short‐period (1 Hz) digital seismographs deployed in the epicentral region. Full use is made of both P‐to‐S and S‐to‐P converted phases in the modeling. Results from the determination of crustal structure in the Subandean and foreland basin region of east‐central Peru confirm the presence of vast depositional basins comprised of low velocity sediments up to at least 8 km thick which flank the Andean orogen to the east and correlate with a substantially thickened crust atop the Brazilian shield. Crustal thickness in the foreland basin varies from about 35 km or less where sedimentary cover is minimal to 44 km in regions of maximum sedimentary deposition. There is some evidence that the crust thins slightly on the western side of the foreland basin (in the gap between basin and Subandean fold and thrust belt), but it rapidly thickens to 45–50 km beneath the Subandes proper, and to more than 50 km in the southern part of the Subandean belt. The results are consistent with, but do not require, a thick‐skinned model of foreland crustal deformation similar to that found for the block faulted terrane in Argentina above the zone of flat subduction there. At least some basin formation appears to be due to block faulting, where faults may penetrate into the mantle.
A spectral formalism for computing three‐dimensional deformations due to surface loads: 1. Theory
Authors: Mitrovica J. X. , Davis J. L. , Shapiro I. I.
Published: February 2004

We outline a complete spectral formalism for computing high spatial resolution three‐dimensional deformations arising from the surface mass loading of a spherically symmetric planet. The main advantages of the formalism are that all surface mass loads are always described using a consistent mathematical representation (expansions using spherical harmonic basis functions) and that calculations of deformation fields for various spatial resolutions can be performed by simply altering the spherical harmonic degree truncation level of the procedure. The latter may be important when incorporating improved observational constraints on a particular surface mass load, when considering potential errors in the computed field associated with mass loading having a spatial scale unresolved by the observational constraints, or when treating a number of global surface mass loads constrained with different spatial resolutions. These advantages do not extend to traditional “Green's function” approaches which involve surface element discretizations of the global mass loads. In the practical application of the Green's function approach the discretization is subjective and dependent on the particular surface mass load being considered. Furthermore, treatment of mass loads with higher spatial resolutions can require tedious rediscretization of the surface elements. Another advantage of the spectral formalism, over the Green's function approach, is that a posteriori analyses of the computed deformation fields, such as degree correlations, power spectra, and filtering analyses, are easily performed. In developing the spectral formalism, we consider specific cases where the Earth's mantle is assumed to respond as an elastic, slightly anelastic, or linear viscoelastic medium. In the case of an elastic or slightly anelastic mantle rheology the spectral response equations incorporate frequency dependent Love numbers. The formalism can therefore be used, for example, to compute the potentially resonant deformational response associated with the free core nutation and Chandler wobble eigenfunctions. For completeness, the spectral response equations include both body forces, as arise from the gravitational attraction of the Sun and the Moon, and surface mass loads. In either case, and for both elastic and anelastic mantle rheologies, we outline a pseudo‐spectral technique for computing the ocean adjustment associated with the total gravitational perturbation induced by the external forcing. Three‐dimensional deformations computed using the usual Love number approach are generally referenced to an origin located at the center of mass of the undeformed planet. We derive a spectral technique for transforming the results to an origin located at the center of mass of the deformed planet.
An equation of state for liquid iron and implications for the Earth's core
Authors: Anderson William W. , Ahrens Thomas J.
Published: February 2004

An equation of state is presented for liquid iron based on published ultrasonic, thermal expansion, and enthalpy data at 1 bar and on pulse‐heating and shock wave compression and sound speed data up to 10 Mbar. The equation of state parameters, centered at 1 bar and 1811 K (the normal melting point of iron), are density, ρ0 = 7019 kg/m3, isentropic bulk modulus, KS0 = 109.7 GPa, and the first‐and second‐pressure derivatives of KS, K′S0 = 4.66 and K″S0 = −0.043 GPa−1. A parameterization of the Grüneisen parameter γ as a function of density ρ and specific internal energy E is γ = γ0 + γ′(ρ/ρ0)n(E ‐ E0) where γ0 = 1.735, γ′ = −0.130 kg/MJ, n = −1.87, and E0 is the internal energy of the liquid at 1 bar and 1811 K. The model gives the temperature dependence of γ at constant volume as (∂γ/∂T)v|1bar,1811K = −8.4 × 10−5 K−1. The constant volume specific heat of liquid Fe at core conditions is 4.0–4.5 R. The model gives excellent agreement with measured temperatures of Fe under shock compression. Comparison with a preliminary reference Earth model indicates that the light component of the core does not significantly affect the magnitude of the isentropic bulk modulus of liquid Fe but does decrease its pressure derivative by ∼10%. Pure liquid Fe is 3–6% more dense than the inner core, supporting the presence of several percent of light elements in the inner core.
Melting phase relations of an anhydrous mid‐ocean ridge basalt from 3 to 20 GPa: Implications for the behavior of subducted oceanic crust in the mantle
Authors: Yasuda Atsushi , Fujii Toshitsugu , Kurita Kei
Published: February 2004

High pressure melting experiments on an anhydrous abyssal tholeiite collected from the Mid‐Atlantic Ridge have been conducted over the pressure interval 3 to 20 GPa to explore the fate of subducted oceanic crust in the mantle. The composition of the mid‐ocean ridge basalt (MORB) is almost identical to the average basaltic layer of the oceanic lithosphere. The melting phase relations of the MORB are summarized as follows: (1) the liquidus temperature is about 1425°C at 3 GPa, and it rises almost linearly to above 1900°C at 10 GPa. The slope of the liquidus curve decreases slightly above 10 GPa. Nevertheless, it still has a positive slope with increasing pressure. At 20 GPa, the liquidus temperature is about 2200°C. (2) The liquidus phase changes from clinopyroxene to garnet above 3.5 GPa. (3) The solidus temperature rises almost linearly to 2100 °C at 20 GPa; consequently, the melting interval is slightly narrower at high pressures (<140°C at 5 GPa, <100°C at 20 GPa). (4) Silica minerals (coesite, stishovite) are stable near and below the solidus. (5) At shallow mantle conditions (2∼7 GPa), the liquidus temperature of the MORB is slightly lower than the solidus of the mantle material (peridotite, KLB‐1); however, the solidus temperature of the MORB nearly equals or exceeds the liquidus temperature of the mantle material at about 16 GPa. The solidus temperature of the mantle material is expected to exceed the liquidus temperature of the MORB again at higher pressures where the liquidus phase in the peridotite system is majorite and/or modified spinel rather than olivine. In the ordinary thermal structure of the present mantle, subducted MORB is difficult to melt in the absence of volatiles because its dry solidus is much higher than the estimated mantle geotherm. However, in the ancient Earth, when the mantle was supposedly much hotter than today, complete or partial melting of subducting MORB was quite probable and this melting process could have played a significant role in controlling the movements of subducted slab and the evolution of the mantle and continental crust.
Source spectrum and source time function of volcanic tremor determined with a dense seismic network near the summit crater of Izu‐Oshima Volcano, Japan
Authors: Oikawa Jun , Ida Yoshiaki , Yamaoka Koshun
Published: February 2004

Digital seismic records of episodic volcanic tremor, obtained with a dense seismic network near the summit crater of Izu‐Oshima volcano, were analyzed to determine source spectrum and source time function. Source spectrum and transfer function could be separated because the seismic records showed a systematic change with distance from the source. The source spectrum of velocity amplitude had a different frequency, ƒ, dependence above and below a corner frequency of 8 to 10 Hz. At high ranges, the spectrum was proportional to ƒ−2, while at low ranges, it was proportional to ƒ2. Inversion of this frequency‐dependent source spectrum yields a source time function that can be represented by an impulse that attenuates in about 0.1 s. Repeated impulses could explain observed volcanic tremor that persists for many minutes or longer and that have complicated phase spectra. The source spectrum gives an energy release rate of about 5.2×102 J/s, so that the total energy released is about 1.0×105 J during a tremor episode of about 3 min at Izu‐Oshima. Such energy release is comparable to the seismic energy released by an earthquake of magnitude 0.1.
The Palos Verdes terraces, California: Bathtub rings from a buried reverse fault
Authors: Ward Steven N. , Valensise Gianluca
Published: February 2004

Uplift of the Palos Verdes peninsula has long been associated with a northwest trending, southwest dipping, reverse fault. Unfortunately, the Palos Verdes Hills fault has no obvious surface displacement and little background seismicity to substantiate its dimension, orientation, or earthquake potential. In this paper we investigate the tectonic style and slip rate of the Palos Verdes Hills fault and the uplift history of the Palos Verdes Hills by analyzing the geometry of 13 marine terraces that encircle the peninsula in a bathtub ring configuration. Elevations of 211 terrace remnants constrain a fault model with 3.0 to 3.7 mm yr−1 of oblique, dextral/reverse slip on a fault dipping 67° at 6 to 12 km depth beneath the peninsula. If the rate was constant through time, fault inception would have occurred 2.4–3.0 Ma. We propose that the largest credible earthquakes on the fault have magnitude ≈6¾ and could revisit every 2000 years.
Three‐dimensional attenuation tomography at Loma Prieta: Inversion of t* for Q
Authors: Lees Jonathan M. , Lindley Grant T.
Published: February 2004

Three‐dimensional Q−1 variations in the aftershock region of Loma Prieta are derived by tomographic inversion. The data set consists of over 4000 aftershock recordings at 22 PASSCAL (Program for Array Seismic Studies of the Continental Lithosphere) stations deployed after the Loma Prieta mainshock of 1989. Estimates of attenuation are determined from nonlinear least squares best fits to the Fourier amplitude spectrum of P and S wave arrivals. The linear attenuation inversion is accomplished by using three‐dimensional velocity variations derived previously in nonlinear velocity inversions. Low Q is observed near the surface and Q generally increases with depth. The southwest side of the San Andreas fault exhibits lower Q than does the northeast side and this feature apparently extends to approximately 7 km depth. The fault zone, as determined by the dipping plane of aftershock activity, is characterized by slightly higher Qp and lower Qs, compared to regions immediately adjacent to the fault. These correlate with high‐velocity anomalies associated with seismicity at depth. The results are in agreement with earlier observations regarding the association of high‐velocity anomalies, seismicity, and fault zone asperities.
On the temporal variability of the transport between Amsterdam and Kerguelen islands
Authors: Vassie J. M. , Harrison A. J. , Woodworth P. L. , Harangozo S. A. , Smithson M. J. , Thompson S. R.
Published: February 2004

Data from over 3 years of bottom pressure recording together with Geosat altimetry and the Fine Resolution Antarctic Model numerical ocean model have been used to investigate the temporal variability of the Antarctic Circumpolar Current between Amsterdam and Kerguelen islands in the southern Indian Ocean. An upper limit to the standard deviation of the variability in transport derived from pressure differences has been calculated to be 10–15 Sv, subject to an assumption of barotropic flow dominating the variability and after the application of a 31‐day low‐pass filter. This is slightly larger than the variability obtained in a similar experiment at the Drake Passage a decade ago. Although the seasonal cycles in pressure difference and in sea surface height difference measured by altimetry are different each year, their means are consistent with the presence of a semiannual component with an amplitude in terms of (assumed barotropic) transport of the order of 5 Sv, which is comparable to the Drake Passage findings. In addition, the pressure difference data contain a large, quasistationary annual component of amplitude of approximately 5 mbar, a possible source of which is different seasonal steric fluctuations at the two sites. Insofar as the bottom recorder data can be considered ground truth for the altimetry, results from the different techniques show some features in common, although significant differences between data sets remain, for which possible explanations are offered. It is concluded that when they are used for monitoring transport fluctuations, bottom pressure recorders should be accompanied by inverted echo sounders and comprehensive hydrographic measurements.
Color banding on Georges Bank as viewed by coastal zone color scanner
Authors: Yentsch C. S. , Phinney D. A. , Campbell J. W.
Published: February 2004

Observations of Georges Bank by coastal zone color scanner (CZCS) show bandlike patterns that appear to be related to the large sand dunes and ridges which dominate the bottom topography there. Ship measurements of temperature and chlorophyll on Georges Bank in July 1979 also reflect the influence of the underwater dune fields. The cause of the banding in the CZCS data is unknown but is speculated to be the creation of alternating zones of divergence and convergence by tidal currents as the water flows over the underwater dune‐trough configuration. The banding observed by CZCS appears to be seasonal, following the sequence of primary production in the Gulf of Maine, and is believed to be important to biological processes as a site of new production and/or as an effective transport mechanism. Any future models that attempt to interrelate tidal mixing and primary production must consider bottom topography and secondary flows.
Seasonality of coastal zone color scanner phytoplankton pigment in the offshore oceans
Authors: Banse K. , English D. C.
Published: February 2004

The NASA Global Ocean Data Set of plant pigment concentrations in the upper euphotic zone is evaluated for discerning geographical and temporal patterns of seasonality in the open sea. Monthly medians of pigment concentrations for all available years are generated for fields of approximately 77,000 km2. For the climatological year, highest and lowest medians, month of occurrence of the highest median, ratio of highest to lowest medians, and absolute range between the highest and lowest medians are mapped ocean‐wide between 62.5°N and 62.5°S. Seasonal cycles are depicted for 48 sites. In much of the offshore ocean, seasonality of pigment is inferred to be driven almost equally by the interaction of the abiotic environment with phytoplankton physiology and the loss of cells from grazing. Special emphasis among natural domains or provinces is given to the Subantarctic water ring, with no seasonality in its low chlorophyll concentrations in spite of strong environmental forcing, and the narrow Transition Zones, a few degrees of latitude on the equatorial sides of the Subtropical Convergences of the southern hemisphere and their homologs in the northern hemisphere, which have late winter blooms caused by nutrient injection into the upper layers.
On the destabilization of a longshore current on a plane beach: Bottom shear stress, critical conditions, and onset of instability
Authors: Dodd Nicholas
Published: February 2004

The effect on the stability of a wave‐driven longshore current of varying the bottom shear stress is investigated. The shear stress is assumed to be quadratic and parameterized (as usual) through a bottom friction coefficient (cd). The bottom shear stress affects both the mean longshore current (which it opposes) and the onset (or otherwise) of a shear instability in the longshore current through its aforementioned effect on the mean current (and therefore the current shear) and by damping instabilities directly. Reducing cd can destabilize the longshore current by increasing its strength, thereby increasing the current shear so as to overcome the damping of bottom friction and thus leading to the development of instabilities. At the same time, this decrease in cd directly reduces the damping experienced by the instabilities that develop. It is also shown that while the primary effect of bottom friction is to damp instabilities, there are other effects, notably linked to the flow curvature, which act to destabilize the flow. Both the weak‐ and the strong‐longshore‐current approximations are examined. For the weak‐current approximation it is shown that curvature effects are particularly important in providing a destabilizing mechanism. It is also shown that for both approximations a “global” cutoff frequency is likely to exist, below which no linear instability will develop. Critical conditions are identified for each approximation (and therefore for two different longshore current profiles), beyond which instabilities will start to develop. Results suggest that a critical shear will exist for any plane beach and that for Leadbetter Beach, Santa Barbara (examined in the 1980 Nearshore Sediment Transport Study (NSTS)), the current will not become unstable until an offshore shear of about 0.03 s−1 is reached. This was apparently not achieved during NSTS.
Microstructure and electrical properties of marine ice and its relationship to meteoric ice and sea ice
Authors: Moore John C. , Reid Andrew P. , Kipfstuhl Josef
Published: February 2004

A 215‐m ice core has penetrated the central part of the Ronne Ice Shelf (76°59′S, 52°16′W). The core consisted of meteoric ice above 152.8‐m depth; below this the ice was bubble free, and of seawater origin. The salinity of the layer below 152.8‐m depth is less than 0.05 ppt, very much lower than typical sea ice. The ice represents an unusual source of ice intermediate in salinity and some physical properties between meteoric ice and sea ice. The structure of the ice from four different depths, where salinities vary by a factor of 4, has been investigated using a scanning electron microscope (SEM). The fabric and grain structures are unusual and seem to depend on the impurity concentration in the ice. Chemical impurity localization has also been investigated. The dielectric properties of the ice show that the ice has a transitional behavior from the linear dependence of conductivity on chloride concentration found in meteoric ice, to the less predictable sea ice behavior. The behavior may be interpreted in terms of the structure of the ice. The higher‐salinity samples show convoluted grain boundaries, small grain size, and brine inclusions. There are indications that the brine inclusions were liquid at temperatures between −10°C and −30°C. By contrast, the lower‐salinity samples possess larger grain sizes and show no evidence of brine inclusions. All samples show brine concentrated at triple grain junctions and also along two‐grain boundaries in higher‐salinity samples. The dielectric properties of the lower‐salinity samples are well described by a Jaccard mechanism with L defects created in proportion to salinity, that also describes the behavior of meteoric ice. Higher‐salinity samples exhibit lower conductivity than would be seen if L defects continued to be created in proportion to salinity. The dielectric and structural data are consistent with a solubility limit of about 300 μM for Cl in the ice lattice.
Precise mean sea level measurements using the Global Positioning System
Authors: Kelecy Thomas M. , Born George H. , Parke Michael E. , Rocken Christian
Published: February 2004

This paper describes the results of a sea level measurement test conducted off La Jolla, California, in November of 1991. The purpose of this test was to determine accurate sea level measurements using a Global Positioning System (GPS) equipped buoy. These measurements were intended to be used as the sea level component for calibration of the ERS 1 satellite altimeter. Measurements were collected on November 25 and 28 when the ERS 1 satellite overflew the calibration area. Two different types of buoys were used. A waverider design was used on November 25 and a spar design on November 28. This provided the opportunity to examine how dynamic effects of the measurement platform might affect the sea level accuracy. The two buoys were deployed at locations approximately 1.2 km apart and about 15 km west of a reference GPS receiver located on the rooftop of the Institute of Geophysics and Planetary Physics at the Scripps Institute of Oceanography. GPS solutions were computed for 45 minutes on each day and used to produce two sea level time series. An estimate of the mean sea level at both locations was computed by subtracting tide gage data collected at the Scripps Pier from the GPS‐determined sea level measurements and then filtering out the high‐frequency components due to waves and buoy dynamics. In both cases the GPS estimate differed from Rapp's mean altimetric surface by 0.06 m. Thus the gradient in the GPS measurements matched the gradient in Rapp's surface. These results suggest that accurate sea level can be determined using GPS on widely differing platforms as long as care is taken to determine the height of the GPS antenna phase center above water level. Application areas include measurement of absolute sea level, of temporal variations in sea level, and of sea level gradients (dominantly the geoid). Specific applications would include ocean altimeter calibration, monitoring of sea level in remote regions, and regional experiments requiring spatial and temporal resolution higher than that available from altimeter data.
Validation of empirical orbit error corrections using crossover difference differences
Authors: Jolly G. W. , Moore P.
Published: February 2004

Numerous techniques for empirical correction of the dominant once per revolution orbit error present in satellite altimeter data have been suggested along with various tests of their validity. These validations typically involve comparison of data from other sources, for example tide gauges, with altimetry, comparison of different orbit error models with which to approximate the radial error, or comparison of variabilities from collinear analyses with those from crossover differences. A new means of validating empirical orbit error correction is here presented through use of crossover difference differences (CDDs) to independently eliminate the dominant orbit error terms. A CDD,Cij, is defined to beCi,j=Xi + 1,j + 1−Xi,j, whereXijis the crossover difference derived from the intersection of ascending arc numberiwith descending arc numberj; the arcsiandi+ 1, for example, are separated in time by one orbital period. This provides a measure of the change in ocean height difference between two geographical locations on the same circle of latitude, separated by an angle χ, equal to the angle though which the Earth has rotated during the orbital period. Removal of residual gravity field modeling error and systematic solar radiation pressure orbit errors produce a result that is purely ocean signal differences plus residual noise contamination. CDDs have been applied with Geosat crossover difference data from the Southern Ocean to validate an empirical correction technique obtained by combining the methods suggested by Milbert et al. (1988) and Tai (1988). The comparison has shown that a maximum, nonsteric annual signal of less than 1.5 cm was absorbed by empirical correction. In addition, it has been shown that relative height time series can be generated χ due east or west of a known time series without any need for further orbit error reduction.
A three‐scale composite surface model for the ocean wave–radar modulation transfer function
Authors: Romeiser Roland , Schmidt Anke , Alpers Werner
Published: February 2004

An improved three‐scale composite surface model for the modulation of the radar backscatter from the ocean surface by long ocean waves is presented. The model is based on Bragg scattering theory. In the conventional two‐scale model, only the geometric modulation of the radar backscatter and the hydrodynamic modulation of the short Bragg waves by the long waves is considered. In the three‐scale model, the impact of intermediate‐scale waves (wavelengths between the length of the Bragg waves and the length of the long waves which are resolved by the radar) is also taken into account, which leads to a modified theoretical ocean wave‐radar modulation transfer function (MTF). For the first time the proposed model includes not only geometric effects associated with the intermediate‐scale waves but also the additional hydrodynamic modulation of the Bragg waves. The resulting theoretical expression for the measured “hydrodynamic” MTF depends on the radar polarization as well as on the azimuthal (upwave / downwave or upwind / downwind) radar look direction. Especially for HH polarization, the predicted “hydrodynamic” MTF becomes significantly larger than expected from conventional theory. We compare model results with tower‐based scatterometer measurements at L, C, and X band (1.0, 5.3, and 10.0 GHz, respectively), which were obtained during the Synthetic Aperture Radar and X Band Ocean Nonlinearities‐Forschungsplattform Nordsee (SAXON‐FPN) experiment. The measured magnitudes and phases of the MTF are better reproduced by the proposed three‐scale model than by the conventional two‐scale model. However, the large measured “hydrodynamic” MTFs for high microwave frequencies (C and X band) are still underestimated. The agreement between model predictions and measurements can be improved if, for example, an additional variation of the wind stress over the long waves is assumed. The required wind stress modulation depends on the long‐wave slope and appears to be coupled to the hydrodynamic modulation of the surface roughness by a positive feedback mechanism.
Implicit free‐surface method for the Bryan‐Cox‐Semtner ocean model
Authors: Dukowicz John K. , Smith Richard D.
Published: February 2004

The surface pressure reformulation of the Bryan‐Cox‐Semtner global ocean model, which we developed recently, has been further improved by eliminating the rigid‐lid approximation in favor of an implicit free‐surface method for the barotropic mode. This retains the advantages of the surface pressure formulation and in addition (1) substantially improves the computational efficiency of the barotropic mode; (2) computes surface height directly, allowing comparison with and assimilation of altimetry data (with a rigid‐lid case the surface height is accurately computed only in the limit of a steady state); (3) improves accuracy in the computation of long‐wavelength Rossby waves and includes the effects of long‐wavelength surface gravity waves; (4) provides robustness by selectively damping computational modes; (5) simplifies satisfying global energetic balances required for energetic consistency; and (6) alleviates “checkerboarding” in the surface height or pressure fields. A variable thickness top‐layer model is developed to account for the free surface, and a generalized form of implicit time discretization of the barotropic equations is introduced. This discretization is analyzed in detail from the point of view of accuracy, stability, and damping properties, and certain cases are selected for their advantageous properties. The performance of the method is demonstrated on the Connection Machine, CM‐200, and the results of numerical simulations are compared using the stream function, rigid‐lid surface pressure, and free‐surface formulations.
An Introduction to SAXON‐FPN
Authors: Plant William J. , Alpers Werner
Published: February 2004

The Synthetic Aperture Radar and X Band Ocean Nonlinear it ies (SAXON)‐Forschungsplatform Nordsee (FPN) program was a 3‐year effort to investigate radar backscatter from the ocean and synthetic aperture radar (SAR) imagery of the ocean. A secondary objective of the program was to explore the relationship between acoustic and microwave scattering from the ocean surface. The program was a joint effort between the United States and the Federal Republic of Germany. It consisted primarily of a major field experiment (phase I) in the North Sea on and around the German Forschungs‐platform Nordsee (FPN) during November 1990, a second, smaller field experiment (phase II) on the same platform in November 1991, and a series of four data analysis workshops.
Fluctuations of Gulf Stream frontal position between Cape Hatteras and the Straits of Florida
Authors: Miller Jerry L.
Published: February 2004

Low‐frequency variations in the onshore/offshore position of the Gulf Stream thermal front have been examined at 10 cross‐isobath transects equally spaced along the South Atlantic Bight between the Straits of Florida and Cape Hatteras. The data were obtained from the U.S. Navy's frontal analysis charts for the 12‐year period, 1976–1988. A time domain empirical orthogonal function (EOF) analysis revealed two modes of fluctuation about the mean Gulf Stream position, which together explain the well‐known “weakly and strongly deflected” frontal position states which occur on timescales of a few months in the vicinity of the Charleston Bump. Analyses of fluctuations at the annual period revealed a 3‐month phase lag between frontal position, as shown by the most significant time domain EOF mode, and volume transport through the Straits of Florida. In addition, frequency domain EOF and cross‐spectral analyses of frontal position revealed annual waves which appear to propagate to the south (north) in the region south (north) of the bump. For transect pairs having significant coherence and nonzero phase estimates, the phase speed and wavelength were about 10 cm/s southward (7 cm/s northward) and 3000 km (2000 km) in the southern (northern) region. This suggests an annual generation of a perturbation in the vicinity of the bump which radiates away as a long annual wave and which may be related to, or part of, the mechanism which produces the annual modulation of Gulf Stream transport.
Calculating tidal current profiles from vertically integrated models near the critical latitude in the Barents Sea
Authors: Nøst E.
Published: February 2004

The method of extracting current profiles from vertically integrated hydrodynamic models is extended to incorporate more general boundary conditions, and the consistency of the method is investigated. Using this method, the tidal current profiles for semidiurnal and diurnal constituents are studied in high latitudes near the critical latitude, where the Coriolis frequency equals the tidal frequency for the semidiurnal tide. The current profiles are computed at stations in the Barents Sea where current measurements exist at up to six different depths. The observation stations are located close to the critical latitude for the largest semidiurnal constituents, and the current profiles are therefore sensitive to variations of magnitude and vertical distribution of the eddy viscosity. At this latitude the current is strongly depth dependent, and in deep water the tidal ellipse changes direction of rotation near the seabed. At four of the stations there are under‐ice measurements, and the effects of an ice‐covered ocean upon the tidal current profiles are also investigated.
A season of heat, water vapor, total hydrocarbon, and ozone fluxes at a subarctic fen
Authors: Moore Kathleen E. , Fitzjarrald David R. , Wofsy Steven C. , Daube Bruce C. , Munger J. William , Bakwin Peter S. , Crill Patrick
Published: February 2004

High‐latitude environments are thought to play several critical roles in the global balance of radiatively active trace gases. Adequate documentation of the source and sink strengths for trace gases requires long time series of detailed measurements, including heat and moisture budgets. A fen near Schefferville, Quebec, was instrumented during the summer of 1990 for the measurement of the surface energy, radiation, and moisture balances as well as for eddy correlation estimates of ozone and methane flux. Despite the limited fetch at this site, analysis of the tower flux“footprint”indicates that at least 80% of the flux observed originates from sources within the fen. Sensible heat fluxes averaged 25% of the daytime net radiation at the site, while the latent heat flux, determined from the energy balance, was 63%; the Bowen ratio varied from 0.2 to 0.8 from day to day, without a seasonal trend to the variation. The competing effects of rooted macrophyte development (with concomitant effects on roughness and transpiration) and the normal shift in synoptic pattern around day 200 to warm, dry conditions results in a lack of net seasonal effect on the energy partitioning. Over the period from days 170 to 230, the evaporation (167 mm) was double the rainfall, while the decline in water level was 107 mm, leaving a net runoff of 0.44 mm/d. The total hydrocarbon flux was 75–120 mg m−2 d−1, following a diurnal pattern similar to heat or moisture flux, while the daytime ozone flux was about −1.11×1011 molecules cm−2 s−1. A period near the end of the experiment, during week 30, produced the strongest total hydrocarbon flux, associated with warmer deep (1 m) soil temperatures, lower fen water levels, and the late summer shift in wind direction at that time. An early summer“flush”of total hydrocarbon was not observed.
Timescales in energy balance climate models: 1. The limiting case solutions
Authors: Watts Robert G. , Morantine Michael C. , Achuta Rao Krishna
Published: February 2004

To demonstrate the difference between the transient responses of systems of varying complexity, a hierarchy of energy balance climate models is analyzed for abrupt and linearly increasing radiative forcings. Only the simplest case (a zero‐dimensional mixed‐layer ocean) exhibits the e‐folding behaviour that is a property of linear first‐order systems. In each case, considered, the natural time scales that occur in the systems are illustrated and physically interpreted. For the case of linearly increasing radiative forcing, the asymptotic solutions are derived by employing the Laplace transform approach. The time scales are related to the asympotic solutions as illustrated in the text. In a companion paper (Morantine and Watts, this issue), analytical solutions for the transient responses valid for all values of time are presented for the models considered in this paper.
On the distribution of cold air near the vortex edge in the lower stratosphere
Authors: Tao Xin , Tuck Adrian F.
Published: February 2004

The horizontal distributions of air in the temperature ranges −70° > T > −77°C, −77° > T > −85°C and T < −85° C have been examined during the Arctic winter of 1988/1989 and the Antarctic winter of 1987, using T106 European Centre for Medium‐range Weather Forecasts (ECMWF) analyses of standard meteorological variables and potential vorticity. ER‐2 airborne data were used to define, using water vapor and nitrous oxide observations, a potential vorticity (PV) contour corresponding to a chemically defined vortex edge. The jet stream axis was used to select a PV contour corresponding to the maximum winds, or a dynamically defined vortex edge. Air in the range −70° > T > −77° C was confined within the dynamically defined Arctic vortex, with only small fractions appearing outside the chemically defined vortex. In the Antarctic, up to 14% of this air was outside the chemically defined vortex but very little was outside the dynamically defined vortex. After late January (Arctic) and late August (Antarctic), respectively, the chemically defined vortices shrank, being 27% and 20% smaller in February and September. The dynamically defined vortices remained essentially constant in area. There were no occurrences of nominal type II polar stratospheric clouds (PSCs) outside either vortex, however defined. One implication of these results is that heterogeneous conversion of ClONO2 and HCl to reactive forms of chlorine is confined almost entirely to the Arctic vortex, while some such conversion may occur at the edge of the Antarctic vortex. However, small and infrequent areas of air in the range −77° > T > −85°C did occur immediately outside both vortices, so separating the effects of peel‐off, type I PSCs and sulfuric acid aerosol processing may be difficult on occasions. What is clear is that all three processes are intimately associated with the polar night jet stream.
Chemical composition of the atmospheric aerosol in the troposphere over the Hudson Bay lowlands and Quebec‐Labrador regions of Canada
Authors: Gorzelska K. , Talbot R. W. , Klemm K. , Lefer B. , Klemm O. , Gregory G. L. , Anderson B. , Barrie L. A.
Published: February 2004

Atmospheric aerosols were collected in the boundary layer and free troposphere over continental and coastal subarctic regions of Canada during the July–August 1990 joint U.S.‐Canadian Arctic Boundary Layer Expedition (ABLE) SB/Northern Wetlands Study (NOWES). The samples were analyzed for the following water soluble species: sulfate, nitrate, ammonium, potassium, sodium, chloride, oxalate, methylsulfonate, and total amine nitrogen. Ammonium and sulfate were the major water soluble components of these aerosols. The nearly neutral (overall) chemical composition of summertime aerosol particles contrasts their strongly acidic wintertime composition. Aerosol samples were separated into several air mass categories and characterized in terms of chemical composition, associated mixing ratios of gaseous compounds, and meteorological parameters. The fundamental category represented particles associated with“background”air masses. The summertime atmospheric aerosols in background air over the North American subarctic and Arctic regions were characterized by relatively small and spatially uniform mixing ratios of the measured species. These aerosol particles were aged to the extent that they had lost their primary source signature. The chemical profile of the background air aerosols was frequently modified by additions from biomass fire plumes, aged tropical marine air, and intrusions of upper tropospheric/lower stratospheric air. Aerosols in boundary layer background air over the boreal forested region of Quebec‐Labrador had significantly larger mixing ratios of ammonium and sulfate relative to the Hudson Bay region. This may reflect infiltration of anthropogenic pollution or be due to natural emissions from this region.
Factors influencing atmospheric composition over subarctic North America during summer
Authors: Wofsy Steven C. , Fan S.‐M. , Blake D. R. , Bradshaw J. D. , Sandholm S. T. , Singh H. B. , Sachse G. W. , Harriss R. C.
Published: February 2004

Elevated concentrations of hydrocarbons, CO, and nitrogen oxides were observed in extensive haze layers over northeastern Canada in the summer of 1990, during ABLE 3B. Halocarbon concentrations remained near background in most layers, indicating a source from biomass wildfires. Elevated concentrations of C2Cl4provided a sensitive indicator for pollution from urban/industrial sources. Detailed analysis of regional budgets for CO and hydrocarbons indicates that biomass fires accounted for ≈ 70% of the input to the subarctic for most hydrocarbons and for acetone and more than 50% for CO. Regional sources for many species (including CO) exceeded chemical sinks during summer, and the boreal region provided a net source to midlatitudes. Interannual variations and long‐term trends in atmospheric composition are sensitive to climatic change; a shift to warmer, drier conditions could increase the areas burned and thus the sources of many trace gases.
Intrusions into the lower stratospheric Arctic vortex during the winter of 1991–1992
Authors: Plumb R. A. , Waugh D. W. , Atkinson R. J. , Newman P. A. , Lait L. R. , Schoeberl M. R. , Browell E. V. , Simmons A. J. , Loewenstein M.
Published: February 2004

Investigations of the kinematics of the lower stratospheric Arctic vortex during the winter of 1991–1992 using the contour advection with surgery technique reveal three distinct events in which there was substantial intrusion of midlatitude air into the vortex, in apparent contradiction of the view that the polar vortex constitutes an isolated air mass. Two of these events, in late January and mid‐February, were well documented. They were predicted in high‐resolution forecasts by the European Centre for Medium‐Range Weather Forecasts, most clearly in experimental forecasts with reduced diffusion. Direct confirmation of the presence of the intrusions and of their calculated locations was provided by aerosol observations from the airborne differential absorption laser lidar aboard the NASA DC‐8, taken as part of the second Airborne Arctic Stratospheric Expedition campaign; aerosol‐rich air of midlatitude origin was seen in the expected position of the intrusions. The reality of the February event was also confirmed by in situ measurements from the NASA ER‐2. Such events may be significant for the chemical processes taking place within the winter vortex. The intrusions were evidently related to the meteorology of the northern stratosphereduring this winter and in particular to persistent tropospheric blocking over the northeastern Atlantic Ocean and western Europe and concomitant ridging into the lower stratospheric vortex in this region. Nevertheless, preliminary investigations have indicated that such events are not uncommon in other northern hemisphere winters, although no such events were found in the southern hemisphere during the Antarctic winter of 1987.