bluuebunny/crossref_metadata_2025 huggingface dataset:
Models of recurrent strike‐slip earthquake cycles and the state of crustal stress
Authors: Lyzenga Gregory A. , Raefsky Arthur , Mulligan Stephanie G.
Published: February 2008

Numerical models of the strike‐slip earthquake cycle, assuming a viscoelastic asthenosphere coupling model, are examined. The time‐dependent simulations incorporate a stress‐driven fault, which leads to tectonic stress fields and earthquake recurrence histories that are mutually consistent. Single‐fault simulations with constant far‐field plate motion lead to a nearly periodic earthquake cycle and a distinctive spatial distribution of crustal shear stress. The predicted stress distribution includes a local minimum in stress at depths less than typical seismogenic depths. The width of this stress “trough” depends on the magnitude of crustal stress relative to asthenospheric drag stresses. The models further predict a local near‐fault stress maximum at greater depths, sustained by the cyclic transfer of strain from the elastic crust to the ductile asthenosphere. Models incorporating both low‐stress and high‐stress fault strength assumptions are examined, under Newtonian and non‐Newtonian rheology assumptions. Model results suggest a preference for low‐stress (a shear stress level of ∼;10 MPa) fault models, in agreement with previous estimates based on heat flow measurements and other stress indicators.
Gravity field improvement using global positioning system data from TOPEX/Poseidon: A covariance analysis
Authors: Bertiger Willy I. , Wu J. T. , Wu Sien C.
Published: February 2008

The TOPEX/Poseidon satellite (inclination 65°, period 113 min) is scheduled for launch in 1992 to study the world's ocean topography through precise altimetric measurements. It will also carry an experimental Global Positioning System (GPS) receiver to demonstrate precise orbit determination with GPS. Data for the GPS experiment will be collected by the on‐board receiver and a worldwide network of ground receivers. These data may also be used to improve the knowledge of the Earth's gravitational field. The GPS data are especially useful for improving the gravity field over the world's oceans, where the current tracking data are sparse. Using a realistic scenario for processing 10 days of GPS data, we perform a covariance analysis to obtain the expected improvement to the GEM‐T2 gravity field. The large amount of GPS data and the large number of parameters (1979 parameters for the gravity field, plus carrier phase biases, etc.) require special filtering techniques for efficient solution. We have used the gravity bin technique introduced earlier to compute the covariance matrix associated with the spherical harmonic gravity field. The covariance analysis shows that the GPS data from one 10‐day arc of TOPEX/Poseidon with no a priori constraints can resolve medium degree and order (3–26) parameters with sigmas (standard deviations) that are an order of magnitude smaller than the corresponding sigmas of GEM‐T2. When the information from GEM‐T2 is combined with the TOPEX/Poseidon GPS measurements, an order of magnitude improvement is observed in low‐ and medium‐degree terms with significant improvements spread over a wide range of degree and order.
Physical model of source region of subduction zone volcanics
Authors: Davies J. Huw , Stevenson D. J.
Published: February 2008

The thermal structure of a generic subduction zone is investigated to elucidate the source region of subduction zone volcanics. The steady state thermal field is evaluated for a model subduction zone where the plates are prescribed by kinematic boundary conditions, such that the subducting slab induces a flow in the mantle wedge. The resulting model suggests that the oceanic crust of the downgoing slab is not melted extensively, if at all, and hence is not the source of subduction zone magmatism (with the possible exception of the special case of very young oceanic crust). The temperature in the mantle wedge is high enough to produce melting at the amphibole‐buffered peridotite solidus. It is proposed that the combination of vertical motion of water as a free phase and the transport of hydrous phases (e.g., amphiboles) by the slab‐induced mantle wedge flow lead to the net transport of water being horizontal, across the mantle wedge from the slab. Provided the subducting oceanic crust enters the asthenosphere at a velocity > 6(±2) cm/yr, the mantle wedge will be hot enough at the limit of the lateral water transport mechanism to generate melting at the amphibole‐buffered solidus. The model was then extended to include the effect of localized sources of buoyancy (melt, residue, etc.) as a stationary body force, to investigate the possibility of reversing the slab‐induced flow. Best estimates of the buoyancy sources and appropriate viscosity in the wedge suggest that there is likely to be only a weak modulation of the slab‐induced flow unless the slab and wedge are locally decoupled, for instance by shear heating, the presence of water, or dehydration/hydration reactions. If there is decoupling, then it is possible for there to be an appreciable reversal of the slab‐induced flow. Such an appreciable reversal of flow, if it persists, leads to cooling of the mantle wedge. Hence flow reversal cannot be a steady state mechanism. Instead it would lead to a cycle in the melting with a period of O(1 m.y.). The time dependence of a model with appreciable flow reversal would be reinforced by the need to clear the wedge of infertile material.
Geochemical logging in the Cajon Pass Drill Hole and its application to a new, oxide, igneous rock classification scheme
Authors: Pratson Elizabeth Lewis , Anderson Roger N. , Dove Roy E. , Lyle Mitchell , Silver Leon T. , James Eric W. , Chappell Bruce W.
Published: February 2008

A new elemental oxide classification scheme for crystalline rocks is developed and applied to geochemical well logs from the Cajon Pass drill hole. This classification scheme takes advantage of measurements of elements taken by a geochemical logging tool string. It uses K2O versus SiO2/Al2O3 to distinguish between granites, granodiorites, tonalites, syenites, monzonites, diorites, and gabbros. Oxide measurements from cores are used to calibrate the elemental abundances determined from the well logs. From these logs, a detailed lithologic column of the core is generated. The lithologic column derived from the well log classification scheme is compared with a lithologic column constructed from core samples and well cuttings. In the upper 1295 m of the well, agreement between the two columns is good. Discrepancies occur from 1295 to 2073 m and are believed to be caused by the occurrence of rock types not distinguished by the classification scheme and/or the occurrence of secondary minerals. Despite these discrepancies, the well log‐based classification scheme helps to distinguish changes in rock type and shows potential as an aid to the construction of lithologic columns in boreholes of crystalline rocks.
Stress near geometrically complex strike‐slip faults: Application to the San Andreas Fault at Cajon Pass, southern California
Authors: Saucier Francois , Humphreys Eugene , Weldon Ray
Published: February 2008

Slip on an undulatory strike‐slip fault induces predictable residual stresses in the adjacent crust. Elastic analytic and finite element models are developed to quantify these stresses for arbitrary fault geometries. Across fault‐parallel planes, domains of reverse, normal, right‐lateral and left‐lateral residual stresses are induced near the bends in the fault. These residual stresses increase in magnitude from one slip event to the next, typically reaching failure level after several events. A two‐dimensional analytic elastic plate model for slip on a sinusoidal fault provides general results on the scales and patterns of slip‐induced stresses, and Fourier synthesis allows for the solution with arbitrary but small fault distortions. Maximum fault‐normal residual stress is proportional to the square of the wavenumber of the sinusoidal trace and decays exponentially away from the fault, reduced to 2/e times the maximum value at a distance equal to 1/wavenumber. This fault‐parallel residual shear stress oscillates between dextral and sinistral along the fault, with maximum magnitude adjacent to the maximum excursions in the fault trace. Fault‐parallel residual shear stress is maximum at a distance from the fault of 1/wavenumber, where its magnitude is 1/e times the maximum normal stress on the fault. Two‐ and three‐dimensional finite element analyses extend the analytic model; they account for depth‐decaying fault perturbation amplitude and slip deficit and are valid for large fault undulations. Application of the model to the San Andreas fault in the Cajon Pass region produces the complex distribution of fault‐parallel normal, reverse, right‐lateral and left‐lateral structures recognized near the main trace. Left‐lateral stresses on planes subparallel to the San Andreas fault at the Cajon Pass well are predicted by this model.
The chemical basis for the electrical stratigraphy of ice
Authors: Moore John C. , Wolff Eric W. , Clausen Henrik B. , Hammer Claus U.
Published: February 2008

Antarctic and Greenland ice core samples were studied using two different stratigraphic electrical techniques. The electrical conductivity measurement (ECM) technique is a dc method, while dielectric profiling (DEP) is an ac method. It was found that ECM responds only to acid, even in large excess of neutral salt concentrations. DEP responds to both acid and salt content of the ice. Acids may be giving rise to conduction through an increase in the number of ionisation defects, or through a network of liquid veins between ice grains. Salts on the other hand appear to give rise to Bjerrum defects, which are bound charges and cannot provide a dc current, but do produce a dielectric ac conductivity. The two methods can be used together to give a rapid prediction of both acid and salt content of cores. This may be of particular use in Wisconsin‐age ice from Greenland, but can generally be used to define parts of cores worthy of detailed chemical study.
A revised estimate for the temperature structure of the oceanic lithosphere
Authors: Denlinger Roger P.
Published: February 2008

Models for conductive cooling of the lithosphere provide the foundation for interpretation of oceanic bathymetry and heat flow and consequently provide the basis for the currently accepted temperature structure of the oceanic lithosphere. These models are based upon average values for the thermal properties of olivine and pyroxene and do not account for any variation in these properties with temperature. In this paper a solution of the heat flow equation is presented that incorporates the observed variation of thermal diffusivity with temperature for olivine and pyroxene. The surface heat flow and bathymetry can vary with the square root of lithosphere age with variable diffusivity just as they do for a constant diffusivity solution, but the temperature structure is significantly different. Specifically, the temperature gradients and temperatures at shallow depths (above the 800°C isotherm) obtained with the variable diffusivity model are less than those obtained with the averaged diffusivity model, whereas the temperature gradients at depths below the 800°C isotherm are greater. Temperatures at a given depth within the mechanically strong portion of the lithosphere (<800°C) are about 100°C less using a variable diffusivity model, making this portion of the lithosphere thicker and the boundary with the asthenosphere more sharply defined mechanically as well as thermally. These changes reduce the effective elastic moduli interpreted from flexure of the lithosphere.
Variation of seismic slip in the Gulf of California and the possible effect on geodetic measurements of Pacific‐North American Plate motion
Authors: Tajima Fumiko , Tralli David M.
Published: February 2008

The Gulf of California is a unique tectonic transition zone which progresses from an oceanic ridge‐transform system at the mouth of the gulf to a predominantly continental transform system in southern California. Slip rates estimated from seismic moment data amount to about 17 to 30% of the accumulated tectonic slip in the gulf predicted by the NUVEL‐1 plate motion model, even after accounting for the transition in crustal structure. The seismic to tectonic slip ratios also appear to increase slightly from south to north. This trend in the seismic moment release may reflect changes in the accommodation of Pacific‐North American plate motion along the gulf which can be examined geodetically. On the other hand, geodetic measurements of the relative plate motion can be disturbed by short‐term effects associated with episodic seismicity. To assess these effects, the surface displacements due to typical transform events in the gulf are estimated using a simple dislocation model. The results of this numerical calculation suggest that if a large transform event (M0 ∼ 1.5 × 1026 dyne cm) were to occur within 100 to 200 km of a geodetic baseline, for example one comprised in the GEOMEX Global Positioning System network across the southern gulf, the relative distance measurements could be affected by up to 15 mm. This is marginally at the error level of a few millimeters plus 2 parts in 108 of baseline length for GEOMEX measurements, which thus are sensitive only to the far‐field displacement along the plate boundary (over baselines of a few hundred kilometers in length).
Textural development of mixed, finely porphyritic silicic volcanic rocks, Inyo Domes, eastern California
Authors: Gibson Richard G. , Naney Michael T.
Published: February 2008

Magma mixing in silicic volcanic rocks of the Inyo chain is manifest by bimodal mineral chemistry and distinctive banding, defined by either variations in (1) glass color and chemistry or (2) microlite abundance within colorless glass. Bands composed of brown or microlite‐rich, colorless glass are characterized by more Mg‐rich mafic silicates and more calcic plagioclase than microlite‐poor, colorless glass domains. A rhyolitic and a dacitic mixing end‐member can be defined on the basis of this mineral distribution. Thermobarometry using the mineral assemblages unique to each end‐member indicates that prior to mixing, the dacite was more oxidized and water‐rich than the rhyolitic magma. Determined mineral crystallization conditions compare favorably with experimentally derived phase equilibria for melts of broadly similar composition. The spatial association of contrasting mineral assemblages with the flow banding in these rocks implies that the banding is a direct product of magma mixing. It formed as either (1) volumes of the comingled magmas were immediately vented to the surface and quenched, preserving domains of two distinct glass compositions, or (2) bands of dacitic magma precipitated microlites when comingled with the rhyolitic magma in an attempt for the two magmas to approach equilibrium.
Basin dewatering near salt domes and formation of brine plumes
Authors: Ranganathan Vishnu
Published: February 2008

A previous study noted the occurrence of a subsurface brine plume with more than 160 g/L sodium chloride perched above Welsh salt dome in southwestern Louisiana. Plumes of saline groundwaters have been reported above other Gulf Coast salt domes as well. The U.S. Geological Survey code SUTRA was used to model the formation of brine plumes around a generic salt dome by basin dewatering along the flanks of the dome, and also to study the gravitational instability of a brine plume initially perched above a dome, in the absence of an externally impressed driving force for upwelling. The simulations take into account the effect of salt concentration gradients on brine density. The first set of simulations showed that a brine plume initially perched above a salt dome sank rapidly in the absence of an overpressured section. Where observed, perched brine plumes above salt domes must therefore be either (1) young (< 2 Ma), if formed by basin dewatering and vertical fluid expulsion along the flanks of salt domes, or (2) formed by a continuous mechanism. The second set of simulations modeled the dewatering of a thick initially overpressured sedimentary section extending from a depth of 3 km to a depth of 10 km. The deflation rate of the overpressured section profoundly depended on whether or not a permeable conduit along the flank of the dome existed, which could collect water expelled radially inward to the flank of the dome from the overpressured sediments, and focus it upward along the flank of the dome. In most simulations the volume of compaction water was limited to only 10% of the total volume of overpressured sediment around the dome, and there was an ample amount of compaction water for a perched brine plume to form. In fact, when a perched brine plume formed, it formed very quickly (in < 0.01 m.y.), after a negligible degree of deflation of the overpressured section. It is conceivable that if cycles of conduit sealing and seal rupturing occurred, perched brine plumes could be formed several times during deflation. Conditions necessary for the formation of a perched brine plume are as follows: (1) waters must be focused laterally toward the dome, within in the overpressured section, (2) there must be a conduit of sediment with relatively high vertical permeability along the flank of the dome, and (3) sediments above the dome must be more permeable than sediments along the flank of the dome. For some simulations the average linear velocity of upward flow along the flanks of the dome was 3 m/yr, possibly of large enough magnitude to cause a thermal anomaly. If perched brine plumes form by basin dewatering, the thermal buoyancy effects associated with such a thermal anomaly may make the plumes rise to depths shallower than indicated in the simulations here. The inability of the code SUTRA to simultaneously model heat and solute transport did not permit an evaluation of this.
Geophysical constraints on the shear stress along the Marquesas Fracture Zone
Authors: Christeson Gail L. , McNutt Marcia K.
Published: February 2008

We examine bathymetry and gravity observations acquired by surface ships and deflection‐of‐the‐vertical data derived from the Geosat radar altimeter for the purpose of determining if the bathymetric relief along the Marquesas Fracture Zone, in the central Pacific Ocean, is caused by differential thermal subsidence across a locked fault. If the fracture zone can maintain between 20 and 40 MPa of shear stress, we would predict that a ridge and a parallel trough would develop to the south and north, respectively, of the fracture zone on account of flexural modification of the step in bathymetry locked into the plate along the fracture zone at the ridge‐transform intersection. We find that only 2 out of 32 deflection‐of‐the‐vertical profiles from the Geosat altimetric mission are consistent with this model, while none of the 27 bathymetric profiles or 9 shipboard gravity profiles fits the theoretical predictions. In cases where bathymetry and gravity can be modeled simultaneously, it appears that the gravity anomaly is simply not large enough in magnitude to be consistent with the Moho flexing conformably with the observed bathymetry, as required if the relief is produced by deeper thermal variations across the locked fracture zone. Rather, the potential field data allow shear stresses only 25% of those required to lock the fault and are more consistent with a model in which the relief on the Moho is opposite in sign to that of the surface topography, as would be produced by local or regional compensation of surface loads arising from volcanic activity along the fracture zone. Thus we conclude that the Marquesas Fracture Zone, in contrast to those other Pacific fracture zones which fit the predictions of the locked‐fault (high stress) model, may be unusually weak along almost its entire length.
The Santa Monica Basin tracer experiment: Comparison of release methods and performance of perfluorodecalin and sulfur hexafluoride
Authors: Watson Andrew J. , Ledwell James R. , Sutherland Stewart C.
Published: February 2008

We describe the methods used to prepare and inject sulfur hexafluoride (SF6) and perfluorodecalin (PFD) for the Santa Monica Basin tracer experiment. The use of two tracers gave quasi‐independent estimates of mixing rates in the basin, as well as a check on faults in the injection procedure. PFD was the easier substance to inject, but SF6 proved to be the most useful tracer for two reasons: the SF6 measurements had a higher signal‐to‐noise ratio, and PFD showed some tendency to adsorb onto the surfaces of the sampling gear, whereas SF6 was completely inert in this respect. This surface affinity suggests that PFD may be transported to some extent by settling particles. The PFD profiles obtained 6 months after injection show some enrichment towards the bottom relative to SF6, which might be due to particulate transport.
Organized structures in subsurface bubble clouds: Langmuir circulation in the open ocean
Authors: Zedel Len , Farmer David
Published: February 2008

Observations of the movement and distribution of subsurface bubbles reveal the structure of coherent motions near the ocean surface. The measurements were obtained in the open ocean from a freely drifting, self‐contained acoustical instrument equipped with side scan sonar, dual‐frequency echo sounders, and ambient sound recording systems. The bubble clouds are organized into long, narrow plumes aligned with the wind, consistent with windrows caused by Langmuir circulations. They have widths of about 3 to 5 m and lengths up to 100 m, with multiple scales coexisting. Their mean separation depends upon wind speed: below 5 m s−1, mean spacings were about 5 m, increasing to 10 m when winds exceeded 10 m s−1 The maximum depth to which the plumes were observed to penetrate depended to some extent on wind speed, with the greatest penetrations of 12 m occurring at the highest wind speeds (13 m s−1). However, the time‐averaged depth of observed bubble plume penetration bears only a weak dependence on wind speed. Whenever winds exceeded 5 m s−1, a mean penetration depth of 6 m was observed. Associated with these bubble plumes were maximum downward velocities of about 0.06 m s−1. These speeds were detected at 8 m depth in the center of the plumes; magnitudes decreased to zero near the surface and horizontally toward the boundaries of the plumes.
Low‐frequency, motionally induced electromagnetic fields in the ocean: 2. Electric field and Eulerian current comparison
Authors: Luther Douglas S. , Filloux Jean H. , Chave Alan D.
Published: February 2008

The theoretical relationship between the motional horizontal electric field (HEF) and the seawater conductivity‐weighted vertical average of horizontal water velocity is validated at subinertial frequencies with seafloor point measurements of HEF and moored measurements of horizontal water velocity collected in 1986–1987 in the central North Pacific during the Barotropic, Electromagnetic and Pressure Experiment (BEMPEX). The comparison is limited principally by inaccurate estimation of vertically averaged water velocity due to weak vertical coherences among the current meters and excessive rotor stalls at the deepest instruments. In the BEMPEX area, conductivity weighting results in only a trivial baroclinic contribution to HEF, so that HEF is an accurate measure of the vertically averaged water velocity (or, transport divided by depth) at periods greater than approximately 5 days. Furthermore, the actual transport divided by the depth is nearly identical to the barotropic (approximately depth‐independent) component of motion. The magnetic field at the seafloor is found to have no detectable relationship to horizontal water currents at subinertial frequencies.
Effects of the Gulf Stream on ocean waves
Authors: Holthuijsen L. H. , Tolman H. L.
Published: February 2008

In the present study a third‐generation numerical wave model is used to study effects of a straight Gulf Stream and a Gulf Stream ring on ocean waves in swell and storm conditions. The model accounts for all relevant processes of propagation, generation, and dissipation of the waves (including current effects) without imposing a priori restraints on the spectral development of the waves. The dominating mechanism affecting the waves appears to be current‐induced refraction even though the short‐crestedness of the incoming waves tends to mask its effects (also in swell conditions). Depending on wind and wave conditions, refraction may trap locally generated waves in the straight Gulf Stream or it may reflect wave energy back to the open ocean. In the Gulf Stream ring, refraction induces a considerable variation in significant wave height and short‐crestedness, but it hardly affects the mean wave direction. In storm conditions the processes of generation and dissipation are considerably enhanced in countercurrent situations and reduced following‐current situations.
The seasonal circulation of the upper ocean in the Bay of Bengal
Authors: Potemra James T. , Luther Mark E. , O'Brien James J.
Published: February 2008

Analysis of the results of a multilayer, adiabatic, numerical model of the upper Indian Ocean, driven by climatological monthly mean winds, shows that the simulated currents in the northeastern Indian Ocean are in general agreement with available observations and interpretations. The main features of the ocean currents include large anticyclonic flow in the Bay of Bengal surface waters during the northern hemisphere winter. This gyre decays into eddies in spring and then transitions into a weaker, cyclonic gyre by late summer. The western recirculation region of this flow is an intensified western boundary current which changes direction twice during the year. In the Andaman Sea, east of the Bay of Bengal, the oceanic flow changes direction twice during the year; it is cyclonic during the spring and early summer and anticyclonic the rest of the year. Flow in the equatorial region shows the North Equatorial Current (NEC) flowing west during winter. Further south is the eastward flowing Equatorial Counter Current (ECC) and the westward flowing South Equatorial Current. In summer, the NEC switches direction, joins the ECC, and forms the Indian Monsoon Current. Investigation of the second layer of the model (the upper 450 m of the ocean) shows that flow during much of the year is baroclinic (strong vertical shear). Model layer thickness reveals coastal Kelvin waves propagating along the coast, traveling the entire perimeter of the Andaman Sea and the Bay of Bengal. This wave excites westward propagating Rossby waves into the interior of the bay. Time series analysis of transport calculations yield significant peaks in the 20‐ to 30‐day range and 50‐ to 60‐day range which are not likely directly forced by the applied wind stress.
Basin‐scale estimates of oceanic primary production by remote sensing: The North Atlantic
Authors: Platt Trevor , Caverhill Carla , Sathyendranath Shubha
Published: February 2008

The estimation by remote sensing of annual primary production at ocean basin scales is illustrated for the Atlantic Ocean, using the monthly averaged Coastal Zone Color Scanner data for 1979. The principal supplementary data used were some 873 vertical profiles of chlorophyll and some 248 sets of parameters derived from photosynthesis‐light experiments. This information was used to parametrize the local algorithm for calculation of primary production in 12 subregions of the entire domain for each of the four seasons. Four different procedures were tested for calculation of primary production. These differed according to whether the autotrophic biomass distribution was uniform with depth and whether the irradiance was resolved with respect to wavelength: the spectral model with nonuniform biomass was considered as the benchmark for comparison against the other three models. At particular locations and times, the less complete models gave results that differed by as much as 50% from the benchmark. After integration to basin scale, vertically uniform models tended to underestimate primary production by about 20% compared to the nonuniform models. At large horizontal scale, the differences between spectral and nonspectral models were negligible, a result that was believed to follow from mutual compensation of underestimates and overestimates, according to the local biomass, in different parts of the domain. Calculation of primary production is highly sensitive to the algorithm used to retrieve the biomass. The linear correlation between biomass and estimated production was poor outside the tropics, suggesting caution against the indiscriminate use of biomass as a proxy variable for primary production. The annual primary production for the Atlantic between 20°S and 70°N was 9 ± 3 Gt yr−1, higher than previous estimates made without reference to remotely sensed data. It is argued that the remote‐sensing method is the method of choice for calculation of primary production at the ocean basin scale.
Global‐scale sea surface variability from combined altimetric and tide gauge measurements
Authors: Wunsch Carl
Published: February 2008

By correcting Geosat altimetric data for a set of line frequencies, and then doing a heavy average in space and time, a set of global maps of sea surface variability is produced over time intervals ranging from three months to two years. These maps are demonstrated to be consistent with the global network of tide gauges and sets of maps incorporating those data into the altimetric estimates are also produced. Tide gauge records too fragmentary for computation of a two‐year mean are employed by time differencing them. An estimate of the global difference of sea level change over one year, on very large space scales, shows the effects of the 1987 El Niño. The spherical harmonic coefficients employed in the computations are then used to estimate the full three‐dimensional frequency/wavenumber spectrum of surface circulation variability.
The physical structure of cold filaments near Point Arena, California, during June 1987
Authors: Ramp Steven R. , Jessen Paul F. , Brink Kenneth H. , Niiler Pearn P. , Daggett Francis L. , Best Jeffrey S.
Published: February 2008

Shipboard conductivity‐temperature‐depth, acoustic Doppler current profiler, and continuous temperature and salinity observations were made in the coastal transition zone off Point Arena, California (39°N, 124°W), during June 15–28, 1987, to describe the hydrographic structure and velocity fields associated with the cold filaments found there. An adaptive sampling plan was used to measure the properties of these filaments, guided in real time by satellite Advanced Very High Resolution Radiometer sea surface temperature imagery and feedback from the in situ sensors. The primary feature observed was a large, cool (12.0°–13.5°C), salty (32.7–33.0 psu) filament which extended over 200 km offshore from Point Arena and exceeded 500 m depth. This feature was bounded in the offshore direction by a continuous equatorward meander, with offshore velocities (60–87 cm s−1) on the northern edge of the cool filament and onshore velocities (69–92 cm s−1) along the southern edge, and persisted for at least 3 weeks. A second feature was advected into the study area from the north by an anticyclonic eddy offshore and later merged with the Point Arena filament. Smaller (30 km wide by 50–100 km long by 50–100 m deep) very cold (10.0°–12.0°C) high salinity (>33.0 practical salinity units (psu)) features were observed within the Point Arena filament, but persisted for only 6–10 days. The net volume transport of the larger feature was offshore at ∼3 × 106 m3 s−1 and suggests it was fed by an inflow to the region from the north. The smaller features were correlated with bursts of equatorward wind stress on an event by event basis but not with times of large‐scale wind stress convergence. Surface drifters deployed during the experiment closely followed the surface dynamic topography. Some of the drifters followed the path of the offshore meander, while others moved south inshore.
Population genetic responses of the planktonic copepod Metridia pacifica to a coastal eddy in the California Current
Authors: Bucklin Ann
Published: February 2008

Despite the intermixing that planktonic populations might be expected to experience in the ocean's fluid regime, zooplankton species may be subdivided across their range into genetically distinct populations. This subdivision, or population genetic structure, may be generated by the interplay of biological processes (reproduction, dispersal, differential mortality) and physical forces governing planktonic distributions. Significant population genetic structure in the planktonic copepod Metridia pacifica occurred during a period of upwelling in the coastal transition zone off the west coast of the United States during April and May 1987. During this period a coastal eddy of saline, recently upwelled water was bordered by a southward flowing current stream; offshore waters were nutrient poor and slower flowing. Metridia pacifica was the most abundant copepod in zooplankton samples collected in this domain; the species was sufficiently abundant in 16 samples to allow genetic analysis. Individual copepods were assayed for allozymic variability by polyacrylamide gel electrophoresis. The samples were genetically heterogeneous (by a contingency chi‐square analysis of allozymic frequencies). The genetic differentiation of the samples was characteristic of geographically separated conspecific populations. Cladistic (tree building) analyses were used to visualize the genetic relatedness of the 16 samples, based on the similarity of allozymic frequencies. This analysis resulted in two heterogeneous groups (of five and six samples each) and five anomalous samples that neither constituted a third group nor belonged to the two groups. Overlay of these groupings on the dynamic height topographies showed a concordance between the population genetic and oceanographic structures. Five of the six offshore samples belonged to one group; the five samples of the other group were found either in the eddy (two of three central eddy samples) or in the current jet. The remaining five samples were scattered in nearshore regions or in the frontal region between the eddy and offshore. The division of the samples into eddy and offshore groups may result from redistribution by currents of planktonic populations of different geographic origin and distinct genetic character. Thus the eddy may have entrained copepod populations originating from different source regions than the populations in offshore waters. Further genetic analysis on appropriate time and space scales will be required to determine the mechanisms generating structure in oceanic zooplankton.
Photosynthetic characteristics and estimated growth rates indicate grazing is the proximate control of primary production in the equatorial Pacific
Authors: Cullen John J. , Lewis Marlon R. , Davis Curtiss O. , Barber Richard T.
Published: February 2008

Macronutrients persist in the surface layer of the equatorial Pacific Ocean because the production of phytoplankton is limited; the nature of this limitation has yet to be resolved. Measurements of photosynthesis as a function of irradiance (P‐I) provide information on the control of primary productivity, a question of great biogeochemical importance. Accordingly, P‐I was measured in the equatorial Pacific along 150°W, during February‐March 1988. Diel variability of P‐I showed a pattern consistent with nocturnal vertical mixing in the upper 20 m followed by diurnal stratification, causing photoinhibition near the surface at midday. Otherwise, the distribution of photosynthetic parameters with depth and the stability of P‐I during simulated in situ incubations over 2 days demonstrated that photoadaptation was nearly complete at the time of sampling: photoadaptation had not been effectively countered by upwelling or vertical mixing. Measurements of P‐I and chlorophyll during manipulations of trace elements showed that simple precautions to minimize contamination were sufficient to obtain valid rate measurements and that the specific growth rates of phytoplankton were fairly high in situ, a minimum of 0.6 d−1. Diel variability of beam attenuation also indicated high specific growth rates of phytoplankton and a strong coupling of production with grazing. It appears that grazing is the proximate control on the standing crop of phytoplankton. Nonetheless, the supply of a trace nutrient such as iron might ultimately regulate productivity by influencing species composition and food‐web structure.
Field verification of acoustic Doppler surface gravity wave measurements
Authors: Herbers T. H. C. , Lowe R. L. , Guza R. T.
Published: February 2008

A compact acoustic Doppler current meter, designed for nearshore surface gravity wave measurements, was field tested by comparison to a colocated array of pressure transducers. Both measurement systems were bottom mounted in a water depth of 7 m. Each of four acoustic beams, inclined 45° from vertical, measures the alongbeam velocity at a single range (1 m) about 1.5 m above the seafloor. These four velocity beams are used to estimate low‐order moments of the frequency‐directional wave spectrum and are compared to pressure measurements on four occasions. Predictions of the (nondirectional) bottom pressure spectrum at sea and swell frequencies (0.04–0.30 Hz), based on the velocity measurements and linear theory, are in excellent agreement with directly measured pressure. The general level of agreement (gain errors less than 5%) is somewhat better than results reported from similar (but spanning a much wider range of conditions ) intercomparison studies using conventional in situ current meters. Observed cross spectra between colocated pressure and horizontal velocity components, frequently used to separate turbulence and wave orbital velocities (assuming that the coherence of wave velocity and pressure is equal to 1), are compared to predictions based on the pressure array data and linear wave theory. The observed and predicted pressure‐velocity cross spectra are in excellent agreement and show that large coherence reductions can occur in natural wind waves owing to wave directional spreading effects, despite relatively low turbulence energy levels. Wave radiation stresses, estimated from the velocity measurements, also agree well with estimates extracted from the pressure array data. Overall, the intercomparisons show that the present acoustic Doppler system has directional resolution comparable to a pitch‐and‐roll buoy, and they suggest that higher‐order directional information as well as weak nonlinear properties of natural wind waves may be examined with a slightly modified compact system.
Estimating new production in the equatorial Pacific Ocean at 150°W
Authors: Dugdale Richard C. , Wilkerson Frances P. , Barber Richard T. , Chavez Francisco P.
Published: February 2008

A major goal of the WEC88 cruise of the R/V Wecoma to the equatorial Pacific (made in February‐March 1988), was to establish rates of new production along a meridional section at 150°W and to compare these measured rates with the relatively high values for the equatorial Pacific that had been reported previously using indirect methods and models. New production values were obtained from the traditional approach (in the sense of Dugdale and Goering (1967)) using 15N labelled nitrate uptake, and by using 14C fixation values multiplied by ƒ (proportion of new production) from various sources: from 15N data, from a 14C fixation versus ƒ relationship (Eppley and Peterson, 1979) or from a nitrate versus ƒ relationship (Platt and Harrison, 1985). The ratios of directly measured nitrate and carbon uptake and the ratios of nitrate to nitrate plus ammonium uptake, i.e., values of ƒ, agree well; values of ƒ calculated from carbon uptake or from nitrate concentration are overestimates for the equatorial upwelling region. Carbon to nitrogen uptake ratios measured with 14C and 15N, respectively, approximate the Redfield molar ratio, 6.6 C:N. The overall mean value of ƒ (0.17) helps confirm the view that the low primary production in the enriched eastern equatorial Pacific is due to failure of the nitrate uptake system.
A statistical description of the microstructure of young sea ice
Authors: Perovich Donald K. , Gow Anthony J.
Published: February 2008

In order to fully exploit microwave models of sea ice, the standard ice characterization must be supplemented by a statistical description of the ice microstructure. For the strong fluctuation theory this statistical description takes the form of the mean and variance of the permittivity plus correlation lengths. We have computed these statistics for over 50 samples of young ice, including both columnar congelation ice and desalinated bubbly ice. Samples were taken from different vertical depths and horizontal positions within the ice sheet and encompassed a comprehensive range of ice temperatures and brine volumes. For each of these samples, horizontal thin sections were photographed, then digitized and analyzed on a personal computer‐based image processing system. Results indicate that correlation lengths correspond to the physical dimensions of the inclusions: the brine pocket size (typically tenths of a millimeter) for saline ice and the air bubble size (typically millimeters) for desalinated ice. For saline ice the shape of the correlation function was elliptical, similar to the shape of the brine pockets. In one case as the ice warmed from −20° to −1°C and the brine volume increased from 2 to 37‰, the correlation length increased from 0.14 to 0.22 mm.
The mesoscale structure of the Leeuwin Current: A comparison of laboratory models and satellite imagery
Authors: Pearce A. F. , Griffiths R. W.
Published: February 2008

Details of the mesoscale variability of the Leeuwin Current are determined from a study of NOAA satellite advanced very high resolution radiometer images over the period 1984–1985 for the area between Shark Bay and Cape Leeuwin on the west coast of Australia. At most times during the selected period there are large wavelike protrusions along the current. During at least three separate periods these appear as a set of three or four roughly equally spaced meanders, or wave crests. Each meander diverts the warm water far offshore of the shelf break, where the core of the southward current is otherwise normally found. Their amplitude generally grows in time, and the anticyclonic motions in the meanders are usually accompanied by cool cyclonic circulations between the meanders. Large meanders are more than 200 km across. Although the flow is unsteady and fronts sometimes translate at speeds of up to 0.8 m s−1, the meanders do not move along the coast and can be followed through the images for 30 to 60 days. These features represent very major diversions to the southward flow in the Leeuwin Current and result in large fluctuations in local current velocity and sea surface temperature. On this scale, the structure of the flow has many features in common with that observed in laboratory experiments with coastal currents driven by steady thermal forcing. Smaller billow structures are also apparent along the strongest temperature fronts in the satellite images.
Consequences of sea surface temperature variability on the validation and applications of satellite measurements
Authors: Minnett P. J.
Published: February 2008

Sea surface temperature (SST) measurements from both spaceborne and in situ sensors are analyzed to determine likely temperature differences that can arise between two measurements separated in space and time. This has implications both for satellite data validation, where an in situ measurement is compared with a remotely sensed one, and for the use of SST measurements in numerical upper ocean models, where an SST value is ascribed to or compared with one from a nearby grid point at the closest time step. It is shown that spatial separations of about 10 km and time intervals of about 2 hours can introduce rms differences of 0.2 K into the error budget of a satellite validation data set, this being an upper limit for the meaningful validation of current infrared radiometers. The length and time scales at which the autocorrelation functions decay to 0.5 is used as a criterion for the meaningful use of SST data with ocean models, and these are found to be about 10–20 km and about 6 hours. This time constraint is not very severe, but the spatial constraint poses a significant sampling problem. On these length scales there appears to be no spatial anisotropy with respect to the dominant surface current flow direction.
Response of sea ice drift to wind forcing on the northeastern Newfoundland Shelf
Authors: Fissel D. B. , Tang C. L.
Published: February 2008

Observations of sea ice drift obtained with satellite‐tracked ice beacons in March–April of 1988 and 1989 have been used to examine the response of sea ice drift to wind forcing over the northern Newfoundland continental shelf. The short‐term (5–20 days) response of sea ice drift ranges from 2.6 to 5.4% of the local wind over much of the inner continental shelf, which is comparable to that in the Bering Sea and the Antarctic but larger than that in Arctic. Sea ice drifts to the right of the local wind, at angles ranging from 10° to 63°. The response to wind forcing is largest near the ice edge, both over the middle portions of the shelf and along the southern margins of the seasonal ice zone and during strong and steady wind of several days' duration. The large wind‐driven response of ice drift observed in this study, in comparison with the Arctic, may result from (1) reduced levels of internal ice stress associated with the generally thin ice cover and lower areal concentration of sea ice, (2) large atmospheric drag coefficients associated with the small ice floes in areas of comparatively higher ice concentration, and (3) smooth ice bottom caused by melting. In nearshore areas the ice to wind coupling is reduced owing to larger internal ice stresses experienced locally due to ice pileup.
Water discharged from the Gulf Stream north of Cape Hatteras
Authors: Churchill James H. , Cornillon Peter C.
Published: February 2008

Satellite radiometer‐derived sea surface temperature images together with moored instrument and hydrographic survey data indicate that water ejected from the Gulf Stream often occupies the upper 200 m of the water column over the continental slope between Cape Hatteras and Hudson Canyon. At times this water resembles the energetic Gulf Stream frontal eddies commonly seen to the south of Cape Hatteras. However, many of the observed parcels of this water differ markedly from frontal eddies and Gulf Stream warm‐core rings and so appear to form a class of discharged Gulf Stream water distinct from any previously reported. These parcels generally cover a broad area (compared with frontal eddies), contain relatively weak currents (generally < 40 cm s−1 at 100 m), and are remarkably long‐lived (sometimes distinguishable for more than 2 months). Salinity anomaly distributions indicate that despite their persistence, these water masses contain intrusions of and mix with surrounding lower‐salinity water, particularly in the upper 50 m. Continuity of various tracers along σθ surfaces reveals that the discharged Gulf Stream water observed near the sea surface originated within the nutrient‐bearing stratum of the Gulf Stream, having upwelled hundreds of meters along density surfaces. As a result, it tends to enhance nutrient concentrations over the continental slope, to a significant extent at 100 m depth but only marginally at 50 m depth. By contrast, this water does not carry unusually large kinetic energy densities into the slope region, but apparently gives up a good deal of kinetic energy, on the order of 104 cm2 s−2 per unit mass, while leaving the Gulf Stream.