bluuebunny/crossref_metadata_2025 huggingface dataset:
Magmatic evolution of Quaternary mafic magmas at Long Valley Caldera and the Devils Postpile, California: Effects of crustal contamination on lithospheric mantle‐derived magmas
Authors: Cousens Brian L.
Published: February 2004

Isotopic variations in Cenozoic mafic volcanic rocks in the Basin and Range Province of the southwestern United States are considered by many to be due to differences in mantle sources, either subduction‐enriched subcontinental lithosphere or the asthenosphere. This is a detailed geochemical investigation of the magmatic evolution and mantle source(s) of Quaternary mafic volcanism at Long Valley Caldera and the Devils Postpile National Monument, Western Great Basin. The Quaternary mafic lavas are dominantly post‐Bishop Tuff (<0.7 Ma) in age, occur as isolated cinder cones and small‐volume lava flows within the caldera, and range in composition from alkali basalt to trachyandesite. Whereas fractional crystallization is the dominant process effecting the evolution of Long Valley lavas with MgO > 7%, assimilation of Sierra Nevada granitoids becomes increasingly important at MgO < 7%, such that SiO2, K/La, and 87Sr/86Sr increase as P/K, Zr/Ba, and 143Nd/144Nd decrease. Assimilation‐fractional crystallization calculations suggest that the most evolved trachyandesites include ∼30% of upper crustal component. In lavas with >7% MgO, 87Sr/86Sr and La/Sm decrease as 143Nd/144Nd, Nb/La, Zr/Ba and (to a lesser extent) Pb isotope ratios increase from the oldest to the youngest Quaternary lavas. The oldest lavas are chemically similar to other lavas in the western Basin and Range thought to have an enriched lithospheric mantle source. Whereas the shifts in Sr and Nd isotope ratios from oldest to youngest basalts might be consistent with a progressively increasing asthenospheric component in the lavas, the shifts in incompatible element ratios and lack of shifts in Pb isotope ratios with time are not. Instead, they indicate that with time the mafic lavas may have increasingly interacted with mafic crust, perhaps gabbroic/dioritic rocks at depth within the Sierra Nevada Batholith. Alternatively, a second, less enriched lithospheric mantle source is present beneath the Long Valley area that has only recently begun to undergo melt generation, and this source has made progressively larger contributions to the basaltic magmas erupted at the surface.
Fine‐scale crustal magnetization variations and segmentation of the East Pacific Rise, 9°10′–9°50′N
Authors: Lee Sang‐Mook , Solomon Sean C. , Tivey Maurice A.
Published: February 2004

Sea‐surface scalar magnetic field measurements collected over the East Pacific Rise near 9°30′N and within the Brunhes chron have been inverted for magnetization of the upper crust. A close track spacing (1–3 km) and careful corrections for sources of background noise make this data set particularly suitable for investigating fine‐scale crustal magnetization. A well‐defined axial magnetization high can be traced throughout the survey area but varies considerably along axis. Marked changes in the amplitude of axial magnetization are often accompanied by offsets in its along‐axis linearity (e.g., at 9°25′, 9°37′ and 9°46′N). On the basis of comparisons with seismic reflection data and geochemical measurements on basalts dredged from this rise area, we argue that the along‐axis variation in the axial magnetic high is the combined result of variations in the thickness of the magnetic source layer and in the magnetization of the layer caused by variations in the iron and titanium abundances in the extrusive upper crust. These two explanations are probably coupled, in that a thicker source layer arises from a deeper axial magma chamber, which is more prone to the production of evolved magmas rich in iron and titanium. Because crustal magnetization represents a time‐integrated signal averaged over a substantial volume of rock, the observation of offsets in the linearity of the axial magnetization high implies that such features, which we suggest mark boundaries between distinct magmatic systems within the axial crust, are stable on timescales of at least several tens of thousands of years. A distinct pattern of short‐wavelength (∼5 km) magnetization anomalies parallel to the rise axis to the north of 9°25′N correlates well with short reversed subchrons within the Brunhes chron. Although independent observations are needed to confirm this explanation, the correlation offers the potential that the determination of fine‐scale magnetization anomalies may prove useful for investigating temporal details of crustal evolution at fast spreading ridges.
Shear wave propagation in orthorhombic phenolic: A comparison of numerical and physical modeling
Authors: Rümpker Georg , Brown R. James , Thomson Colin J.
Published: February 2004

We analyze, both physically and numerically, certain effects of shear wave propagation in orthorhombic phenolic. This industrial laminate provides a physical model for the study of wave propagation in orthorhombic media and has been used in several laboratory experiments. Recently, we observed polarity reversals on seismogram traces along two profiles through a sphere of phenolic. The observations were attributed to the rapid variation of polarization in the neighborhood of slowness‐surface conical points (point singularities). We now present results of numerical modeling experiments that show amplitude variations similar to those observed in the physical modeling. For receiver positions along a symmetry plane of the anisotropic medium, these amplitude variations may indeed be attributed to rapid polarization changes due to conical points. For a profile crossing a symmetry plane, however, the numerical examples indicate that relatively smooth variations of the displacement can result in rapid amplitude variations (polarity reversals) on seismogram traces, depending on the particular source‐receiver configuration used. The computed seismograms also show characteristic Hilbert‐transform‐type wave forms due to wave front folding. This folding is a direct result of slowness‐surface conical points, and the related wave form characteristics may be used in future experiments to detect conical‐point effects. The detectability of these wave form variations depends strongly on the frequency range emitted by the source, i.e., the transmitting transducer.
Tidal calibration of borehole strain meters: Removing the effects of small‐scale inhomogeneity
Authors: Hart R. H. G. , Gladwin M. T. , Gwyther R. L. , Agnew D. C. , Wyatt F. K.
Published: February 2004

We investigate the estimation of Earth strain from borehole strain meter data in a study of tidal calibration of the Gladwin borehole tensor strain meter (BTSM) at Piñon Flat. Small‐scale geological inhomogeneity is one of several effects examined that cross couple remote areal/shear strain into measured areal/shear strain. A methodology is developed for incorporating cross coupling into the strain meter calibration. Using the measured strain tides from the colocated laser strain meter (LSM) as a reference, we show that calibration of the BTSM with cross coupling removes systematic errors of up to 30% in the borehole strain meter tides. This calibration accurately relates the BTSM measurements to strains at the scale length of the LSM, about 1 km. The calibration technique provides a solution to a major criticism of all short‐baseline strain measurements, namely, that tectonic strains are not representatively sampled due to small‐scale inhomogeneities. The technique removes errors potentially greater than 50% in observed strain offsets from fault slip, permitting improved constraint of slip mechanisms. We show that current theoretical estimates of strain tides in the instrument locality are not yet of sufficient accuracy for cross‐coupled calibration. Comparison of theoretical tides with measurements from the LSM suggest that at least half of the error is in the ocean load tide estimates.
Anisotropic migration‐velocity based on inversion of common azimuthal sections
Authors: Ikelle Luc T.
Published: February 2004

A knowledge of the background velocity model is crucial to achieve the accurate reservoir description now expected from three‐dimensional (3‐D) prestack imaging and inversion. Conventional methods for reconstructing the background velocity model, like migration‐velocity methods, often assume an isotropic subsurface and can yield inaccurate reservoir descriptions when the subsurface contains anisotropic rock formations. Here, we generalize the migration‐velocity concept by (1) replacing migration with linearized inversion and (2) permitting the background velocity to be anisotropic. The scheme consists of scanning different anisotropic velocity models using a linearized inversion in the ω‐k domain. As the anisotropic background velocity model is generally described by several elastic coefficients, it is important to adopt an efficient scanning procedure. We have chosen to work with common azimuthal sections. For a given common azimuthal section, we sequentially scan two parameters: normal move‐out velocity and the anisotropic parameter known as anellipticity. These two scans allow us to reconstruct an azimuthally isotropic velocity model. The procedure is then repeated for different common azimuthal sections; each common azimuthal section leads to a new azimuthally isotropic velocity model if the medium is azimuthally anisotropic. The number of common azimuthal sections, and therefore the number of azimuthally isotropic velocity models, needed to reconstruct an azimuthally anisotropic velocity model is dependent on the type of symmetries. For example, only three common azimuthal sections are needed for an orthorhombic medium.
Postrifting anelastic deformation around the spreading plate boundary, north Iceland: 1. Modeling of the 1987–1992 deformation field using a viscoelastic Earth structure
Authors: Hofton M. A. , Foulger G. R.
Published: February 2004

A third Global Positioning System (GPS) survey of a regional network surrounding the Krafla volcanic system, north Iceland, was conducted in 1992 following a major crustal spreading episode which began in this system in 1975. Differencing the 1992 results with those from 1987 and 1990 reveals a regional deformation field with a maximum, rift‐normal expansion rate of 4.5 cm/year near the rift, decreasing to 3 cm/year at large distances. The time‐averaged spreading rate in north Iceland, 1.8 cm/year, cannot account for this deformation. The vertical deformation field reveals regional uplift throughout the network area at its maximum closest to the rift and decreasing with distance. Three different models are applied to study the postdike injection ground deformation: (1) stress redistribution in an elastic layer over a viscoelastic half‐space, (2) stress redistribution in an elastic‐viscous layered medium, and (3) continued opening at depth on the dike plane in an elastic half‐space. Using model 1, the effects of historical episodes in the region are subtracted from the observed displacement fields, and the remaining motion is modeled as relaxation following the recent Krafla rifting episode. The best fit model involves a half‐space viscosity of 1.1 × 1018 Pa s, a relaxation time of 1.7 years, and an elastic layer thickness for northeast Iceland of 10 km. The vertical field indicates that the Krafla dike complex rifted the entire elastic layer. Using model 2, the motion 1987–1990 and 1990–1992 can be simulated adequately given the survey errors, but the 1987–1992 deformation is poorly fitted, suggesting that a more realistic geophysical model is required. Using model 3, a range of dikes will fit the deformation field.
Global Positioning System constraints on fault slip rates in southern California and northern Baja, Mexico
Authors: Bennett Richard A. , Rodi William , Reilinger Robert E.
Published: February 2004

We use Global Positioning System (GPS) estimates of horizontal site velocity to constrain slip rates on faults comprising the Pacific‐North America plate boundary in southern California and northern Mexico. We enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary. We estimate a Pacific‐North America relative plate motion rate of 49 ± 3 mm/yr (one standard deviation), consistent with NUVEL‐1A estimates. We are able to resolve robust slip rate estimates for the southernmost San Andreas, San Jacinto, and Elsinore faults (26 ± 2, 9 ± 2, and 6 ± 2 mm/yr, respectively) and for the Imperial and Cerro Prieto faults (35 ± 2 and 42 ± 1 mm/yr, respectively), accounting for about 86% of the total plate motion. The remaining 14% appears to be accommodated to the west of these fault systems, probably via slip along the San Clemente fault and/or the San Miguel, Vallecitos, Rose Canyon, and Newport‐Inglewood fault systems. These results are highly consistent with paleoseismic estimates for slip rates implying that off‐fault strain accumulation within the deforming zone of the plate boundary is largely elastic. We estimate that the seismically quiescent, southernmost San Andreas fault has incurred about 8.2 m of slip deficit over the last few hundred years, presumably to be recovered during a future large earthquake.
A deterministic and stochastic velocity model for the Salton Trough/Basin and Range transition zone and constraints on magmatism during rifting
Authors: Larkin Steven P. , Levander Alan , Okaya David , Goff John A.
Published: February 2004

As a high resolution addition to the 1992 Pacific to Arizona Crustal Experiment (PACE), a 45‐km‐long deep crustal seismic reflection profile was acquired across the Chocolate Mountains in southeastern California to illuminate crustal structure in the transition between the Salton Trough and the Basin and Range province. The complex seismic data are analyzed for both large‐scale (deterministic) and fine‐scale (stochastic) crustal features. A low‐fold near‐offset common‐midpoint (CMP) stacked section shows the northeastward lateral extent of a high‐velocity lower crustal body which is centered beneath the Salton Trough. Off‐end shots record a high‐amplitude diffraction from the point where the high velocity lower crust pinches out at the Moho. Above the high‐velocity lower crust, moderate‐amplitude reflections occur at midcrustal levels. These reflections display the coherency and frequency characteristics of reflections backscattered from a heterogeneous velocity field, which we model as horizontal intrusions with a von Kármán (fractal) distribution. The effects of upper crustal scattering are included by combining the mapped surface geology and laboratory measurements of exposed rocks within the Chocolate Mountains to reproduce the upper crustal velocity heterogeneity in our crustal velocity model. Viscoelastic finite difference simulations indicate that the volume of mafic material within the reflective zone necessary to produce the observed backscatter is about 5%. The presence of wavelength‐scale heterogeneity within the near‐surface, upper, and middle crust also produces a 0.5‐s‐thick zone of discontinuous reflections from a crust‐mantle interface which is actually a first‐order discontinuity.
Observations and analysis of self‐similar branching topology in glacier networks
Authors: Bahr David B. , Peckham Scott D.
Published: February 2004

Glaciers, like rivers, have a branching structure which can be characterized by topological trees or networks. Probability distributions of various topological quantities in the networks are shown to satisfy the criterion for self‐similarity, a symmetry structure which might be used to simplify future models of glacier dynamics. Two analytical methods of describing river networks, Shreve's random topology model and deterministic self‐similar trees, are applied to the six glaciers of south central Alaska studied in this analysis. Self‐similar trees capture the topological behavior observed for all of the glaciers, and most of the networks are also reasonably approximated by Shreve's theory.
Variability of crustal attenuation in the northeastern United States from Lg waves
Authors: Shi Jinghua , Kim Won‐Young , Richards Paul G.
Published: February 2004

High‐quality, digital seismograms from eight pairs of collocated earthquakes in the northeastern United States were analyzed to determine accurate source spectrum corner frequencies. This was accomplished by applying the empirical Green's function method to regional Pg and Lg (or Sg) phases recorded by vertical component seismographs of the U.S. National Seismographic Network (USNSN) and the Lamont‐Doherty Cooperative Seismographic Network (LCSN) stations. The frequency band used was 0.5–16 Hz for USNSN and 1–30 Hz for LCSN records. The source spectrum corner frequencies for the eight larger earthquakes of the event pairs (magnitudes between mb(Lg) = 2.5 – 4.1) range from about 4.3 to 16.3 Hz. Based on the comer frequencies obtained independently from the empirical Green's function analysis, Sg or Lg wave displacement amplitude spectra up to 30 Hz were used to determine the crustal average Q factors along 87 event‐station paths. These paths crossed diverse tectonic features in the northeastern United States and were in the epicentral distance range of 41 to 1394 km. We found that within the northeastern United States, the crustal average QLg we obtained was frequency dependent and showed spatial variability which correlated fairly well with the major tectonic features in the region. Our attenuation measurements indicated low Lg attenuation in the Adirondack Mountains with exposed Precambrian Grenville basement with QLg = 905 f0.40, high Lg attenuation in the central Appalachian Province with QLg = 561–586 f0.46–0.47, and an intermediate Lg attenuation in northern New England Appalachians with Q = 705 f0.41.
Quasi‐periodic wind signal as a possible excitation of Chandler wobble
Authors: Furuya Masato , Hamano Yozo , Naito Isao
Published: February 2004

The atmospheric contribution to the excitation of the Chandler wobble (CW) is studied for a period of about 11 years period beginning in September 1983, using the wobble data set (SPACE93). Two atmospheric angular momentum (AAM) functions are employed; one is based on Japan Meteorological Agency (JMA) data, and the other is based on U.S. National Meteorological Center (NMC) data. In the vicinity of the Chandler frequency, the excitation power for both AAM functions is comparable with that inferred from the observed wobble. If we separate the AAM excitation into wind and pressure effects, the wind contribution exceeds the pressure contribution around the Chandler frequency. In addition, the JMA wind AAM function reveals a spectral peak around the Chandler frequency. Significant coherence with the inferred excitation exceeding the 99% confidence level can be recognized for both JMA and NMC wind excitation. We propose that the wind contribution to the excitation of polar motion may be somewhat larger than at other frequencies because of the existence of quasi‐periodic atmospheric variations that have been found in other investigations. In a narrow‐band analysis of the excitation using least squares fit sinusoids, we found that both the amplitude and phase of the inferred excitation near the Chandler band indicate their strong dependence on the assumed Chandler period and that an assumed CW period of 431 days caused atmospheric and observed excitations to agree most closely.
Scales of spatial and temporal variability in the Southern Ocean
Authors: Gille Sarah T. , Kelly Kathryn A.
Published: February 2004

Spatial and temporal variability in Geosat altimeter data is used to consider whether the Antarctic Circumpolar Current (ACC) responds on the large scale to global changes in wind forcing or whether its variability is primarily mesoscale. The Geosat data indicate a spatial decorrelation scale of 85 km and a temporal e‐folding scale of 34 days. Using these scales to define autocovariance functions, the sea surface height variability is objectively mapped. The resulting maps indicate substantial evidence of mesoscale eddy activity. Over 17‐day time intervals, meanders of the Polar Front and Subantarctic Front appear to elongate, break off as rings, and propagate. Statistical analysis of ACC variability from altimeter data is conducted using empirical orthogonal functions (EOFs). The first‐mode EOF describes 16% of the variance in total sea surface height across the ACC; reducing the domain into basin scales does not significantly increase the variance represented by the first EOF, suggesting that the scales of motion are relatively short and may be determined by local instability mechanisms rather than larger basin‐scale processes. Likewise, neither complex nor extended EOFs indicate statistically significant traveling wave modes.
Response of microwave cross sections of the sea to wind fluctuations
Authors: Weissman David E. , Plant William J. , Stolte Siegfried
Published: February 2004

The response of microwave cross sections of the sea at four different microwave frequencies to fluctuations in the wind was measured during the Synthetic Aperture Radar and X Band Ocean Nonlinearities‐Forschungsplatform Nordsee experiment. Four separate CW microwave systems operating at L, S, X, and Ka bands and incidence angles near 45° measured the response of short surface waves with Bragg wavelengths of 14.1, 9.2, 2.1, and 0.6 cm, respectively, to near‐surface wind fluctuations. Cross sections from the four microwave systems were correlated with winds from a sonic anemometer, which was positioned near the sea surface areas illuminated by the microwave systems. We find that in the wind speed regime from 6.4 to 9.4 m/s, coherence functions relating the cross sections to fluctuations of the wind are appreciable only below 0.01 Hz and then only for the highest two microwave frequencies whose Bragg wavelengths correspond to gravity‐capillary waves. For all microwave frequencies, coherence functions are very low from 0.01 up to 0.05 Hz, the highest frequency to which our measurements are meaningful. We show that modulation transfer functions (MTFs) relating the cross sections to wind fluctuations should be nearly equal to the wind speed exponent of the cross section at low frequencies. These MTFs were impossible to determine accurately at L and S bands due to the low coherence, but they were between 1 and 2.5 at X and Ka bands, in line with measurements of the wind speed exponent. We infer that the low coherence at L and S bands is due to their low response to the mean wind. Since we expect microwave cross sections at these incidence angles to be nearly linearly dependent on Bragg wave spectral density, we infer that gravity‐capillary waves on the ocean respond directly to changing winds only on timescales longer than about 100 s and that gravity waves longer than about 9 cm respond to other factors more strongly than to the wind.
Impact of altimeter, thermistor, and expendable bathythermograph data on retrospective analyses of the tropical Pacific Ocean
Authors: Carton James A. , Giese Benjamin S. , Cao Xianhe , Miller Laury
Published: February 2004

This study explores the relative impact of three major components of the tropical Pacific Ocean observing system on data assimilation analyses. The extensive observing components that are available during the 2‐year period from October 1992 through September 1994 include sea level derived from TOPEX/POSEIDON altimetry, thermistor data from the Tropical Ocean/Global Atmosphere Tropical Atmosphere‐Ocean (TOGA TAO) mooring array, and expendable bathythermographs (XBTs) from the Volunteer Observing Ship program. In the first part of this study, methods are introduced to assimilate these data into a primitive equation model using optimal interpolation. The resulting velocity and thermal analyses are then compared with a numerical simulation that has no data assimilation. The most innovative aspect of the assimilation procedure is the development of a statistical model that uses satellite altimetry to update the subsurface thermal field. To determine the impact of individual components of the observing system, three additional experiments are conducted. In each experiment, one component is withheld from the assimilation. The resulting analysis is then compared with the missing data set. Our results show (1) To resolve major features of the seasonal cycle, it is necessary to have either the altimeter or the XBT data. In the absence of XBTs, the analysis appears to develop a significant temporal drift in its thermal structure. (2) Intraseasonal variability, such as tropical instability waves, is best resolved using altimetry because of that data set's superior spatial resolution. (3) Thermistor data from the TOGA TAO mooring array are helpful, but not as crucial, in resolving the seasonal cycle within ±8° latitude as the XBTs. The mooring array is too coarsely spaced to allow this analysis system to resolve tropical instability waves.
Interactions between thermohaline‐ and wind‐driven circulations and their relevance to the dynamics of the Antarctic Circumpolar Current, in a coarse‐resolution global ocean general circulation model
Authors: Cai Wenju , Baines Peter G.
Published: February 2004

We present an analysis of the interaction between wind‐driven and thermohaline‐driven circulations in a coarse‐resolution global ocean general circulation model of the Bryan‐Cox code. A series of experiments is described in which the flow is driven by wind forcing only, thermohaline forcing only, or both. In a global ocean with topography, the circulation driven by wind alone is strongly influenced by contours of constant potential vorticity, which limit the barotropic transport to relatively small values. With the same geometry, the flow driven by relaxing to observed surface temperature and salinity fields alone contains deep overturning circulation (producing North Atlantic Deep Water and Antarctic Bottom Water (AABW)) and a large barotropic Antarctic Circumpolar Current (ACC) generated by bottom form stress. The ACC is dependent on the overturning circulation, the deep density field and the bottom form stress and increases with ATV, the vertical mixing coefficient. If wind is added to this flow, the additional circulation is also dependent on the baroclinic structure but decreases with increasing ATV. This applies especially to the ACC, where wind‐induced additions are much larger than when the wind acts alone in a homogeneous ocean. An analysis of the dynamic balance of the ACC in the model shows that it is governed by lateral friction, bottom form stress, and wind. The mechanism for driving the ACC by deep convection and bottom topography is revealed in special experiments with simplified topography. In these runs, all topography is removed except for all or part of a submarine ridge across the Drake Passage; this topography alone causes an ACC with barotropic transport of 80 Sv, driven by a deep density difference and pressure gradient across the ridge. This ridge channels AABW formed in the Weddell Sea northward into the South Atlantic, and the ACC is driven by angular momentum conservation across these latitudes of the compensating southward flowing shallower flow (or, equivalently, by the Coriolis force acting on it) above the ridge. In most other parts of the model ocean, bottom form stress acts as a net drag on the zonal current.
Sea surface drag coefficients in the Risø Air Sea Experiment
Authors: Mahrt L. , Vickers Dean , Howell Jim , Højstrup Jørgen , Wilczak James M. , Edson Jim , Hare Jeffrey
Published: February 2004

This study examines the dependence of the computed drag coefficient on wind speed, stability, fetch, flux sampling problems, and method of calculation of the drag coefficient. The analysis is applied to data collected at a tower 2 km off the coast of Denmark during the Risø Air Sea Experiment (RASEX). Various flux sampling problems are evaluated to eliminate unreliable fluxes. Large drag coefficients are observed with weak large‐scale flow. However, the value of the computed drag coefficient at weak wind speeds is sensitive to flux sampling problems and the method of calculation of the drag coefficient, which might be a general characteristic of weak winds. The drag coefficient is significantly larger for short fetch conditions, particularly at strong wind speeds.
Northward abyssal transport through the Samoan passage and adjacent regions
Authors: Roemmich Dean , Hautala Susan , Rudnick Daniel
Published: February 2004

A conductivity‐temperature‐depth/hydrographic survey in January–February 1994 and a 17‐month deployment of current meter moorings from September 1992 to March 1994 were carried out to determine the volume transport, water mass characteristics, and diathermal fluxes of northward flowing abyssal waters in the Samoan Passage and adjacent regions of the South Pacific Ocean. Geostrophic calculations relative to 1.2°C potential temperature indicated northward transport of 7.8 Sv in the Samoan Passage, 1.1 Sv through a gap in Robbie Ridge, and 2.8 Sv along the eastern flank of the Manihiki Plateau. All of the total of 11.7 Sv of northward geostrophic transport was in waters colder than 1.1°C. The northward transport distribution was bimodal in temperature, with a cold mode of 3.6 Sv in the range 0.65°–0.70°C occurring entirely in the Samoan Passage and a warm mode of 3.0 Sv in the range of 0.80°–0.85°C occurring mainly along the Manihiki Plateau. Within the Samoan Passage, 7.1 Sv of the northward transport was below 4000 m where the geostrophic calculation was confirmed by an equal estimate of transport from current meters during the simultaneous 3‐day period. The 17‐month mean transport from the moored array was 6.0 Sv ± 0.5. By using the observed temporally varying flow within the Samoan Passage together with the hydrographic snapshot across the region, an estimate of the total mean northward transport of 10.6 Sv ± 1.7 was obtained. Estimates of the flow across near‐bottom potential temperature surfaces indicate extraordinarily high rates of mixing, with heating of the abyssal layer up to 20 W m−2, corresponding to diffusivities up to 10−1 m2 s−1.
Representation of convective plumes by vertical adjustment
Authors: Klinger Barry A. , Marshall John , Send Uwe
Published: February 2004

Open‐ocean deep‐water formation involves the interplay of two dynamical processes; plumes (≤1 km wide), driven by “upright” convection, and geostrophic eddies (≥5 km wide), driven by baroclinic instability. Numerical “twin” experiments are used to address two questions about the plumes: Can they be represented by a simple mixing process in large‐scale models? If so, is it important that the mixing occurs over a finite time tmix, or would instantaneous mixing produce the same effect on large‐scale properties? In numerical simulations which resolve the geostrophic eddies, we represent the plumes with a “slow” convective adjustment algorithm which is broadly equivalent to an enhanced vertical diffusivity of density in statically unstable regions. The diffusivity κ depends on tmix, the mixing timescale. The fidelity of the plume parameterization is then evaluated by comparison with plume‐resolving simulations of open‐ocean deep convection. Integral properties of the plumes, such as the temperature census of the convected water and the strength of the rim current that encircles the convecting region, are all accurately reproduced by the slow adjustment scheme. The importance of choosing an appropriate finite value for tmix is explored by setting tmix = 12 hours in some experiments, in accordance with scaling considerations, and tmix = 0 in others, corresponding to instantaneous adjustment, the conventional assumption. In the case of convection into a moderately or strongly stratified ocean the behavior does not significantly depend on tmix. However, in neutral conditions the slow adjustment does improve the parametric representation. Our experiments confirm the picture of plumes homogenizing the water column over a time tmix.
A study of currents south of Africa using Geosat satellite altimetry
Authors: Snaith Helen M. , Robinson Ian S.
Published: February 2004

Geosat altimeter data were used to study a region to the south and east of South Africa, which includes the Agulhas Current System and the Antarctic Circumpolar Current (ACC). The sea surface height variability was determined from 2 years of Geosat altimeter data by collinear techniques. Strong association was found between variability and changes in topography, even at scales of 100–200 km. The height anomaly profiles were used to determine geostrophic current speeds of the order of 20 cm s−1 for the ACC and up to 138 cm s−1 for the Agulhas Current System. Correlation length scales of the height anomalies were found to agree with the first internal Rossby radius of deformation. Differences appear to be related to physical mechanisms such as wind forcing and topographic stretching which alter the vorticity balance of the Agulhas Current System.
The subarctic frontal zone in the North Pacific: Characteristics of frontal structure from climatological data and synoptic surveys
Authors: Yuan Xiaojun , Talley Lynne D.
Published: February 2004

The subarctic front is a thermohaline structure across the North Pacific, separating colder, fresher water to the north from warmer, saltier water to the south. Levitus's [1982] data and 72 conductivity‐temperature‐depth/salinity‐temperature‐depth sections are used to show the spatial and seasonal variations of the climatological frontal zone and the characteristics of the frontal structure in synoptic surveys. The temperature gradient in the mean frontal zone is stronger in the western Pacific and decreases eastward, while the salinity gradient has less variation across the Pacific. The temperature gradient also has larger seasonal variation, with a maximum in spring, than the salinity gradient. The synoptic surveys show that the frontal zone is narrower and individual fronts tend to be stronger in the western Pacific than in the eastern Pacific. Density gradients tend to be more compensated at the strongest salinity fronts than at the strongest temperature fronts. A horizontal minimum of vertical stability is found south of the subarctic halocline outcrop. The northern boundary of the North Pacific Intermediate Water merges with the frontal zone west of 175°W and is north of the northern boundary of the subarctic frontal zone in the eastern Pacific. The shallow salinity minima start within the subarctic frontal zone in the eastern Pacific.
Convection in the Greenland Sea, 1982–1993
Authors: Rhein Monika
Published: February 2004

Convection in the Greenland Sea is studied, using chlorofluorocarbons (CFCs, components F11, F12) and hydrographic distributions taken in spring 1993. A comparison with CFC data in the Greenland Sea Deep Water taken in 1989 shows that the CFC concentrations increased ony slightly, that is, the deep convection activity below 2200 m depth remained on the very low level of the previous years 1982–1989. A simple convection model shows that in the depth range 550–2250 m the mean annual convection rate ceased significantly in the period from 1989 to 1993 compared to the previous years 1982–1989. For the upper 500 m, the modeled tracer concentrations deviate from the observations, most likely due to horizontal advection, neglected in the model.
A sink for atmospheric carbon dioxide in the northeast Indian Ocean
Authors: Kumar M. Dileep , Naqvi S. W. A. , George M. D. , Jayakumar D. A.
Published: February 2004

Intensive observations in the northeast Indian Ocean (Bay of Bengal) during the presouthwest and northeast monsoon seasons of 1991 reveal that freshwater discharge from rivers of the Indian subcontinent exerts the dominant control over total carbon dioxide (TCO2) and pCO2 distributions in surface waters. Low pCO2 levels occur within the low‐salinity zones, with a large area in the northwestern bay acting as a sink for atmospheric CO2. Only a part of the observed pCO2 variation can be accounted for by the effect of salinity, and biological production supported by external nutrient inputs in conjunction with strong thermohaline stratification may be more important in lowering surface water pCO2 by >100 μatm relative to that in the atmosphere. The pCO2 distribution is seasonally variable and appears to be controlled by the spreading of fresher waters by the prevailing surface circulation.
Interdecadal variability and oceanic thermohaline adjustment
Authors: Greatbatch Richard J. , Peterson K. Andrew
Published: February 2004

Changes in the strength of the thermohaline overturning circulation are associated, by geostrophy, with changes in the east‐west pressure difference across an ocean basin. The tropical‐polar density contrast and the east‐west pressure difference are connected by an adjustment process. In flat‐bottomed ocean models the adjustment is associated with viscous, baroclinic Kelvin wave propagation. Weak‐high latitude stratification leads to the adjustment having an interdecadal timescale. We reexamine model interdecadal oscillations in the context of the adjustment process, for both constant flux and mixed surface boundary conditions. Under constant surface flux, interdecadal oscillations are associated with the passage of a viscous Kelvin wave around the model domain. We carry out experiments suppressing wave propagation along each of the model boundaries. Suppressing wave propagation along either the tropical or eastern boundary does not eliminate the oscillation, but increases both its period and amplitude. Suppressing wave propagation along either the polar or the western boundary eliminates the oscillation. Our results suggest the oscillations can be self‐sustained by perturbations to the western boundary current arising from the southward boundary wave propagation. Mixed boundary condition oscillations are characterized by the eastward, cross‐basin movement of salinity‐dominated density anomalies, and the westward return of these anomalies along the northern boundary. We suggest the latter is associated with viscous Kelvin wave propagation. Under both types of boundary conditions, the strength of the thermohaline overturning and the tropical‐polar density contrast vary out of phase. We show how the phase relationship is related to the boundary wave propagation. Box models and zonally averaged models assume that the east‐west and north‐south pressure gradients vary in phase and are proportional to one another. We suggest this assumption is valid only on timescales long compared to the adjustment timescale. The importance of boundary regions indicates an urgent need to examine the robustness of interdecadal variability in models as the resolution is increased, and as the representation of the coastal, shelf/slope wave guide is improved.
Cycling of organic carbon and biogenic silica in the Southern Ocean: Estimates of water‐column and sedimentary fluxes on the Ross Sea continental shelf
Authors: Nelson David M. , DeMaster David J. , Dunbar Robert B. , Smith Walker O.
Published: February 2004

We examined the cycling of organic carbon and biogenic silica in the water column and upper sediments of the Ross Sea, seeking to understand the processes leading to the formation of opal‐rich, organic‐poor sediments over much of the Southern Ocean. Between January, 1990 and December, 1994 we conducted three cruises, performing tracer incubation studies (14C, 15N, 30Si, 32Si) to measure rates of primary production, nitrate‐based “new” production, biogenic silica production and biogenic silica dissolution in the upper 50 m over most of the Ross Sea shelf in spring, mid summer and late summer. We deployed sediment traps from January, 1990 to early March, 1992 to measure the mid‐water (250 m) and near‐bottom gravitational fluxes of particulate organic carbon, nitrogen and biogenic silica year‐round at three sites, and obtained sediment cores at 15 sites to assess the accumulation rates of organic carbon and biogenic silica in all known sediment regimes on the shelf. At 9 of those sites we also measured nutrient efflux from the sediments, enabling us to calculate benthic recycling fluxes of organic matter and opal. These data permit estimates of the annual production, near‐surface recycling, vertical sinking flux, delivery to the seabed, benthic regeneration and long‐term burial of both organic and siliceous material, integrated over a 3.3 × 105 km2 area that covers 75–80% of the Ross Sea shelf. The resulting annual budgets for carbon and silica indicate highly selective preservation of biogenic silica over organic carbon between 50 and 250 m in the water column, as well as in the upper seabed. Selective preservation of silica within the upper 50 m is not indicated, and both organic matter and silica are transported from 250 m to the sea floor with virtually 100% efficiency. The SiO2/C mass ratios for surface‐layer production, 250‐m sinking flux, delivery to the seabed and long‐term burial are approximately 0.85, 6.1, 6.2 and 27, respectively. This progressive enrichment in silica results in long‐term burial of 5.8% of the biogenic silica and 0.17% of the organic carbon produced by phytoplankton in the surface layer, a factor of 30 greater preservation efficiency for silica than for carbon. Nevertheless, the ratio of opal burial to opal production in the Ross Sea is only about twice the apparent global average of 3% and <1/3 of the estimated burial/production ratio for the Southern Ocean as a whole. It thus appears that both silica preservation and the decoupling between the cycles of silica and carbon must be even more effective in the waters overlying abyssal Southern Ocean sediments than they are over the Ross Sea shelf.
On the representation of sea ice in global ocean general circulation models
Authors: Stössel Achim , Oberhuber Josef M. , Maier‐Reimer Ernst
Published: February 2004

The performance of the sea ice component of two ocean general circulation models (OGCMs) is investigated under quasi‐identical forcing and boundary conditions and compared with the performance of a state‐of‐the‐art stand‐alone sea ice model. The latter reproduced realistic sea ice characteristics under the same external conditions. All three sea ice models employ a viscous‐plastic constitutive law to describe the variation in internal ice stress in the momentum balance. The individual thermodynamic formulations were unified to provide consistent reference versions for this investigation. The sea ice models are compared under various conditions to detect first‐order discrepancies. Finally, the treatment of the sea ice component in global OGCMs is discussed in a more general context, illustrating the effect of some simplifications commonly used in OGCMs. We focus on the Southern Ocean, where sea ice plays a critical role in bottom water formation. Our studies show that sea ice in present‐day global OGCMs can be formulated with the same quality as stand‐alone sea ice models designed for specific regional studies, without the sacrifice of notable extra computation time. A standardization of both the dynamic and the thermodynamic part of an OGCM sea ice component turns out to be necessary to prevent unrealistic upper boundary conditions in forced OGCM simulations and distorted sensitivities in coupled simulations.
On models of bomb 14C in the North Atlantic
Authors: Follows Michael J. , Marshall John C.
Published: February 2004

Coarse resolution general circulation ocean models show a tendency to accumulate an excess of bomb 14C on the eastern margin of the subtropical gyres, while observations suggest an excess is found in the west. Here simulations of the bomb 14C transient in the North Atlantic Ocean are made using coarse resolution (1.0° × 1.2°) and “eddy‐permitting” (1/3° × 2/5°) tracer transport models. The former employs large horizontal diffusivities to parameterize eddy transfer. Both models employ the same air‐sea exchange parameterization, with a specified tropospheric history of Δ14C. The coarse resolution model accumulates 14C evenly over the whole subtropical gyre, while the eddy‐permitting model has a pronounced maximum column inventory in the western part, consisent with the observed patterns. The prescence of explicit eddies enhances the ventilation of density surfaces in the recirculation zone on the western margin of the gyre, suggesting that the zonal gradient hinted at in the observations is intimately tied to properties of geostrophic eddy dynamics which are not captured parametrically in the coarse resolution model.
Modeling nutrient and plankton processes in the California coastal transition zone: 3. Lagrangian drifters
Authors: Moisan J. R. , Hofmann E. E.
Published: February 2004

Two types of numerical Lagrangian drifter experiments were conducted, using a set of increasingly complex and sophisticated models, to investigate the processes associated with the plankton distributions in the California coastal transition zone (CTZ). The first experiment used a one‐dimensional (1‐D; vertical) time‐dependent physical‐bio‐optical model, which contained a nine‐component food web. Vertical velocities, along the track of simulated Lagrangian drifters, derived from a three‐dimensional (3‐D), primitive equation circulation model developed to simulate the flow observed within the CTZ, were used to parameterize the upwelling and downwelling processes. The second experiment used 880 simulated Lagrangian drifters from a 3‐D primitive equation circulation model which was coupled to the same food web and bio‐optical model used in the first experiment. Parameterization of the biological processes in both experiments were based upon data obtained during the CTZ field experiments. Comparison of simulations with data provided insight into the role of the biological and physical processes in determining the development of the subsurface chlorophyll maximum and other related features. In both studies, the vertical velocities experienced by a simulated Lagrangian drifter as it was advected offshore while entrained within a filament played a major role in determining the depth to which the euphotic zone and the chlorophyll maximum developed. Also, as the drifters moved offshore, the food web changed from a coastal, neritic food web to an offshore, oligotrophic food web due to the decrease in nutrient availability. The temporal development of the food web constituents following the simulated drifters was dependent upon the environment to which the drifter was exposed. For example, the amount of time upwelled or downwelled and the initial location in the CTZ region greatly affected the development of the food web.
A recent change in the mean state of the Pacific basin climate: Observational evidence and atmospheric and oceanic responses
Authors: Kleeman Richard , Colman Robert A. , Smith Neville R. , Power Scott B.
Published: February 2004

The first half of the present decade has been characterized by anomalous conditions in a number of Pacific basin atmospheric and oceanic variables. The sea surface temperature (SST), in particular, has been warmer than normal over vast areas of the subtropics and the Gulf of Alaska. The SST anomaly pattern is shown to be part of a long‐term trend which began in the early 1970s. This trend is the dominant source of SST variability over significant portions of the basin. Experiments with atmospheric models demonstrate that the atmospheric anomalies of the 1990s are consistent with this change in SST and the mechanisms for this are analyzed. Finally, experiments with an ocean general circulation model are used to investigate which features of the atmospheric anomalies are responsible for the observed SST anomalies. It is determined that wind stress changes and changes in heat flux due to wind speed reductions are the most likely causes. Two potentially important mechanisms for positive ocean‐atmosphere feedback leading to the climate anomalies of the 1990s have thus been identified.