bluuebunny/crossref_metadata_2025 huggingface dataset:
Forward and inverse three‐dimensional P wave velocity models of the southern California crust
Authors: Magistrale Harold , Kanamori Hiroo , Jones Craig
Published: February 2008

We construct a three‐dimensional P wave velocity model of the southern California crust by combining existing one‐dimensional models, each describing a region defined by surface geology, and calibrate the model with travel times from three explosions. The model is expressed as blocks, each of a given slowness. The variance of the P wave travel time residuals of ≈1000 earthquakes relocated in and near the Los Angeles basin, where the model is most detailed, is half that of the catalog locations in the standard one‐dimensional model for southern California. Starting from the forward model, we invert ≈21,000 P wave arrivals from earthquakes for hypocenters and block slownesses using the technique of Roecker (1981). The variance of these P wave travel time residuals decreases 47% during the inversion. Many of the blocks representing the upper crust and midcrust are well sampled and well resolved. The resulting model is useful both for locating earthquakes and for comparing the geologies of the different regions. For example, the velocity structure of the Los Angeles basin represents seismically slow sediments on top of basement rocks having velocities similar to the granitic rocks under the Peninsular Ranges. Moho is between 26 and 32 km depth. In contrast, the Ventura basin has mostly slower sediments above a deeper, higher‐velocity basement. Compared to catalog locations, relocations in the final three‐dimensional model of 98 ML ≥4 earthquakes throughout southern California tend to deepen below sediment filled valleys and basins, shallow in regions without sedimentary cover, and have a 44% lower P wave travel time residual variance.
Earthquake source parameters and the frequency dependence of attenuation at Coalinga, Mammoth Lakes, and the Santa Cruz Mountains, California
Authors: Lindley Grant T. , Archuleta Ralph J.
Published: February 2008

Least squares best fits were calculated to the Fourier amplitude spectra of P and S waves of locally recorded earthquakes. Attenuation, amplification, and earthquake source parameters were calculated from parameters of the fits. Approximately 8200 spectra were fit at 39 sites at Coalinga, Mammoth Lakes, and the Santa Cruz Mountains, California. Typically, the spectra were fit from 2 to 45 Hz for P waves and 2 to 20 Hz for S waves. The source spectral falloff of P waves was limited to be no greater than f−2.5 (f is frequency) by low attenuation sites in the Sierra Nevada. Attenuation was measured through t*, the travel time divided by Q. Attenuation was considered to be frequency dependent according to t* = to*f−α, which is equivalent to Q = Qofα. Negative values of α were found to minimize the sum of squared residuals at most of the sites studied. Attenuation and amplification varied from site to site systematically with site geology. Source parameters were determined for individual earthquakes by averaging fit parameters from between four and 18 spectra, depending on the earthquake. Seismic moments ranged from roughly 5×1012 to 1015 Nm at Coalinga and Mammoth Lakes. Moments ranged from 2×1011 to 3×1014 Nm in the Santa Cruz Mountains. When the fits were determined using negative values of a, the seismic moments trended approximately as fc−3(fc is the corner frequency). For α = −0.5, stress drops varied from roughly 0.2 to 50 MPa at Coalinga and Mammoth Lakes. Stress drops were lower in the Santa Cruz Mountains, ranging from approximately 0.05 to 8 MPa.
Strong motion and broadband teleseismic analysis of the 1991 Sierra Madre, California, earthquake
Authors: Wald David J.
Published: February 2008

Short period and broadband teleseismic waveform data and three‐component strong motion records were analyzed to obtain the source parameters of the 1991 Sierra Madre earthquake. Close‐in strong motion velocity records (analyzed from 5 s to 5 Hz) show two distinct pulses about 0.35 s apart, requiring some rupture complexity. The near‐field shear wave displacement pulse from this event has a relatively short duration (about 1 s) for the magnitude of the event, requiring a particularly high average stress drop. To further constrain the rupture process, the data were used in a finite fault source inversion to determine the temporal and spatial distribution of slip. We chose a fault plane orientation striking S 62°W and dipping 50° toward the northwest as required by the distribution of aftershocks, the first motion mechanism and the teleseismic bodywave point source inversion. In addition to the aftershock locations, depth constraints are provided by teleseismic short period and broadband recordings which require a centroid depth of 10–11 km. Our inverse modeling results indicate that both the teleseismic and strong motion data sets can be fit with a compact rupture area, about 12 km2, southwest and up‐dip from the hypocenter. The average slip is approximately 50 cm, and the maximum slip is 120 cm. The seismic moment obtained from either of the separate data sets or both sets combined is about 2.8 ± 0.3 × 1024 dyn cm and the potency is 0.01 km3. Using the area of significant slip estimated from the finite fault inversion, the resulting stress drop is on the order of 150–200 bars.
A model for large‐scale plastic yield of the Gorda Deformation Zone
Authors: Denlinger Roger P.
Published: February 2008

The lithosphere east of the Gorda ridge, often called the Gorda plate but referred to here as the Gorda deformation zone (Gorda DZ), is constricted as it spreads eastward to be subducted beneath the North American plate. For the plane strain conditions defined by the seismicity, the distortion of the Gorda DZ is analogous to the ironing of metal cups. Here the “punch” boundary is the Blanco fracture zone and the “die” that does the cutting is the Mendocino transform fault from the Gorda ridge to the Mendocino triple junction. This analogy is useful in analyzing this problem but is not identical due to differences in boundary constraints. The deformation of the Gorda DZ is determined for an elastoplastic lithosphere with a von Mises yield criterion. Plate motion is constrained to be parallel both to the Blanco fracture zone and to the Mendocino transform fault west of 1250W. The Gorda ridge is considered a free boundary (unlike the ironing analogy), and the lithosphere is pulled through the constriction from the trench. Any nonlinear feedback between distortion and ridge‐push forces is not considered. Traction due to slip along the Mendocino transform is varied to fit both seismicity and plate distortion. For these conditions, the amount of plate movement required before a constant resistance is achieved depends upon the ratio of elastic moduli to plastic yield stress and may vary from hundreds of meters to kilometers. The minimum slab pull for steady movement occurs when the transform is weak (“lubricated”) and is 60% of the average plate yield stress. The yield stress is determined using the temperature structure inferred from plate age and the laboratory constraints on brittle and ductile strength for olivine. For an average plate age of 3 m.y. the yield stress is about 550 MPa, so the pull required is 330 MPa. When the transform is about as strong as the lithosphere, the pull required is twice this value. The stress directions in the Gorda DZ can be inferred from both deformation and seismicity. The current seismic data are not precise enough to differentiate between a strong and weak transform, but the deformation data are sufficient. The constraints placed by distortion of magnetic isochrons on the modeling results indicate a mixed history. About 5 Ma, changes in the position of the pole of rotation of the Juan de Fuca system began to increase the convergence of the Gorda DZ with the Mendocino transform fault, increasing the effective normal stress and brittle shear stress across the fault. A strong fault is evident in the deformation data until about 2 Ma, and this corresponds to the time when convergence stabilized. With a stable configuration the analysis indicates that about a kilometer of relative slip reduced the apparent strength of the transform to negligible values compared to the strength of the plate.
Variations in oceanic crustal magnetization: Systematic changes in the last 160 million years
Authors: Johnson H. Paul , Pariso Janet E.
Published: February 2008

The amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age. Globally, the amplitudes decrease with age over the first 20 to 30 million years, a phenomenon that has been attributed to the low‐temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks. In oceanic crust older than 40 million years, however, the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years. In order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust, we compiled all of the existing rock magnetic data from relevant Deep Sea Drilling Project and Ocean Drilling Program sites that sampled extrusive rocks from “normal” ocean crust. This compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes, both in the decrease due to oxidation in the first 20 to 30 million years, and in the increase in magnetization in older (>80 million years) crust. This positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks. Further, the Curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years, and does not increase significantly beyond that point. Finally, the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust. Specifically, we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic FeTi oxides in the older crustal rocks. This large increase in abundance of the FeTi oxides may be related to a systematic increase in bulk FeTi content in older tholeiitic basalts, or, more likely, is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases.
Crustal structure of the Kapuskasing uplift from Lithoprobe Near‐vertical/wide‐angle seismic reflection data
Authors: Wu Jianjun , Mereu Robert F.
Published: February 2008

Reprocessing of one LITHOPROBE Kapuskasing deep seismic reflection profile discloses significant new information on the structure of the Kapuskasing Uplift (KU). The shallow structure of the Ivanhoe Lake fault zone (ILFZ), along which the high‐grade granulites of the KU were thrust to the surface, is conspicuously imaged on a seismic section as a series of prominent northwest‐dipping reflections with listric geometry. The new images show that the ILFZ is a steep fault (∼50°) near the surface which quickly flattens out at shallow depths. These reflections coincide with bright positive aeromagnetic anomalies over the fault zone. Direct correlation with geological observations indicates that the high reflectivity and high magnetism associated with the fault zone likely originate from mylonites. The reprocessing also reveals a pronounced midcrustal reflector within the Abitibi greenstone belt (AGB), dipping northwest and plunging under the KU. The existence of such a reflector is independently confirmed by wide‐angle reflection data acquired from a cross‐profile. This reflector is apparently also detected on two other reflection profiles crossing the ILFZ about 80 km to the southwest. Its concave‐down shape and broad lateral extent suggest that it represents underthrusting of the AGB beneath the KU. With these new results, a more complete structural cross‐section can be constructed. It shows that, on the northwest side, the KU was overthrust along the northwest‐dipping ILFZ and, on the southeast side, the AGB was underthrust northwesterly. It seems evident that the KU is a product of intraplate collision during the early Proterozoic and the underthrusting of the AGB is primarily responsible for the emplacement of the KU.
Quantitative comparison of seismic data sets with waveform inversion: Testing the age‐dependent evolution of crustal structure
Authors: Shaw Peter R.
Published: February 2008

Studies of seismic variability play a more important role in the understanding of processes that form oceanic crust than individual velocity‐depth solutions found in isolation. For such variability studies, solutions must be contrasted with one another; this is normally not straightforward owing to the lack of rigorous uncertainty estimates available for solutions. Here I present a method for the direct comparison of such solutions by hypothesis testing. A null hypothesis is proposed that a single velocity solution is consistent with two sets of data. The hypothesis is tested by seeking such a model using waveform inversion and comparing its misfit against the misfit from fully independent solutions. The degree of similarity of the velocity structures can be further quantified by solving for the pair of solutions that are as similar as possible while consistent with each set of data; the F test is used to evaluate the significance of each additional effective model parameter as it lends independence to the two solutions. The method possesses advantages of a hypothetical Monte Carlo search of all models because the complete forward problem is carried out at each step, yet it is tractable because the search direction is well defined. The method is applied to two data sets collected near the Mid‐Atlantic Ridge on 0 and 7 m.y. crust in order to quantify the differences between the solutions. The comparison finds quantifiable differences in both layers 2 and 3 between these two regions, suggesting that significant evolution of the entire crustal section has taken place in the first 7 m.y.
Crystalline lithology across the Kapuskasing Uplift determined using in situ Poisson's ratio from seismic tomography
Authors: White D. J. , Milkereit B. , Salisbury M. H. , Percival J. A.
Published: February 2008

Seismic refraction tomography has been used to image lateral velocity variations (both Vp and Vs) within the shallow crystalline basement using travel times from a high‐resolution reflection profile across the Kapuskasing uplift structure in northern Ontario. Comparison of these in situ velocities and the outcrop geology indicates a correlation of velocity with the major lithologic units: anorthosite, Vp = 6150–6350 m/s and Vs = 2700–3000 m/s; mafic gneiss, Vp = 6000–6250 m/s and Vs = 2775–3000 m/s; tonalite, Vp = 5850–6000 m/s and Vs = 2950–3025 m/s. Vp measurements on core samples from the main lithologic units confirm that tonalite can be be clearly distinguished from mafic gneiss and anorthosite based on Vp. The observed Vp values are attributed to mineralogy, based on modal analyses of core samples from the same region. Poisson's ratio calculated from in situ Vp and Vs helps to discriminate between velocity variations caused by porosity effects from those due to lithology. Values of Poisson's ratio obtained for the three lithologic units, within the upper 100 m of the basement, are anorthosite, 0.35–0.39; mafic gneiss, 0.34–0.38; and tonalite, 0.32–0.34. These relatively high values of Poisson's ratio indicate that the shallow basement is porous and water‐saturated. Poisson's ratio is a particularly effective discriminator in this instance because of its extremely low value in quartz (0.08) relative to other silicate minerals (0.21–0.32) and the greater quartz content of tonalite (27–36%) relative to anorthosite and mafic gneiss (0–23%). Reflection coefficients calculated for the contrasting lithologies range up to 0.065 for tonalite‐mafic gneiss contacts and up to 0.044 for tonalite‐anorthosite contacts, confirming that an interlayered sequence of tonalitic, anorthositic and mafic rocks mapped at the surface may be the cause of the shallow reflections observed in the coincident seismic reflection data.
Photoelectric charging of ultrafine volcanic aerosols: Detection of Cu(I) as a tracer of chlorides in magmatic gases
Authors: Ammann M. , Hauert R. , Burtscher H. , Siegmann H. C.
Published: February 2008

Volcanic gases contain ultrafine aerosol particles in the nanometer size range; typical concentrations are 105 to 106 cm−3. Photoelectric charging of particles (PCP) is an in situ method for the material specific detection of very small particles in a gas. Field studies at degassing lava flows of Mount Etna, Sicily, and Kilauea, Hawaii, show that the chemistry of the ultrafine aerosols depends strongly on the degassing state of the lava. Heating of a relatively undegassed lava sample in the laboratory reveals the chemical nature of the particles that form by nucleation and condensation in the cooling gas. In the initial stages of degassing, the particles are mainly NaCl and KCl nanocrystals that contain iron oxide and copper chloride. Cu is in the monovalent state, which is stable even in an oxidizing environment due to a redox mechanism with the Fe ions. The fraction of Cu(I) in the aerosols is determined by PCP. The evolution of the Cu(I) fraction is considered an effective tracer of chlorides in the magmatic gases and thereby of magma degassing. The PCP technique allows a fingerprint to be obtained of the magmatic gas by an aerosol measurement in the diluted plume.
Seismic diffraction tomography of array data
Authors: Ødegaard Erik , Doornbos Durk J.
Published: February 2008

A systematic analysis of short‐period time and amplitude variations across the NORSAR array reveals that the two are related and that the short‐ and long‐period time residuals are partly related. To utilize this information, we have experimented with a new formulation of diffraction tomography that combines the frequency dependent effects of diffraction with ordinary seismic tomography, alleviating some of the fundamental problems of both methods. Experiments with the inversion of synthetic data for particular lithospheric model structures demonstrate the improvement that can be obtained in comparison with results from ordinary tomography, especially when diffraction effects modify the observed waveforms and influence the measured relative travel times used for inversion. The method has been applied to a teleseismic data set from NORSAR (in its large‐aperture configuration). Short‐period data from 115 events and long‐period data from 31 events were used to model the velocity structure of the crust and lithosphere beneath the array down to a depth of 129 km. The final model (based on all available phase delay and amplitude data) reduces the variance of the short‐period phase delays by 90%, the short‐period amplitude anomalies by 45%, and the long‐period phase delays by 41%. Several of the prominent features are also recognized in the model produced by ordinary seismic tomography, but the structures are less pronounced. A comparison between different models is presented.
Short‐ and long‐term stability of the JILAG‐4 Absolute Gravimeter
Authors: Peter George , Klopping Fred J. , Sasagawa Glenn S. , Faller James E. , Niebauer Timothy M.
Published: February 2008

Variations of absolute gravity measured with the JILAG‐4 absolute gravimeter at intervals ranging from 2 hours to 5 years are reviewed to ascertain short‐ and long‐term instrument stability. We find that the standard deviation of the twenty‐four 2‐hourly drop set means taken during a given station occupation is 1–2 μGal when natural or man‐induced microseismic conditions are low and 3–5 μGal when the microseismic activity is high. The standard deviations of the station gravity values obtained by repeated occupations weeks or years apart are within these same ranges, with lower standard deviations found again at bedrock sites where the microseismic noise is low. Based on the repeatability of observations since the beginning of the measurement program in 1987, there is no indication for drift, gradual deterioration, or aging of the instrument. However, because of the degraded performance of the lasers used since 1990, the standard deviation of repeated station occupations increased from 2.27 to 2.87 μGal, and data had to be rejected at several sites. Individual station gravity values in excess of ±3 μGal from the station mean are found mostly at those sites where density variations between reoccupations are expected on the basis of geological conditions, usually due to groundwater table fluctuations and/or soil moisture changes.
Geocenter location and variations in earth orientation using Global Positioning System measurements
Authors: Malla R. P. , Wu S. C. , Lichten S. M.
Published: February 2008

We have studied the use of Global Positioning System (GPS) ground and flight tracking data to measure short‐period Earth orientation variations and changes in Earth's center of mass location (the geocenter). Ground‐based GPS estimates of daily and subdaily changes in Earth orientation presently show centimeter‐level precision. Comparisons between GPS‐estimated Earth rotation variations (UT1‐UTC) and those calculated from ocean tide models suggest that observed subdaily variations in Earth rotation are dominated by oceanic tidal effects. Our preliminary GPS estimates for geocenter location (from a 3‐week experiment) agree with an independent satellite laser ranging estimates to 10–15 cm. Covariance analysis predicts that temporal resolution of GPS estimates for Earth orientation and geocenter improves significantly when data collected from low Earth‐orbiting satellites as well as from ground sites are combined. The low‐Earth GPS tracking data enhance the accuracy and resolution for measuring high‐frequency global geodynamical signals over time scales less than 1 day. Low‐Earth orbiters such as Topex/Poseidon carrying GPS flight receivers will provide opportunities to demonstrate these enhancements.
Lattice‐Boltzmann studies of immiscible two‐phase flow through porous media
Authors: Gunstensen Andrew K. , Rothman Daniel H.
Published: February 2008

Using a recently introduced numerical technique known as a lattice‐Boltzmann method, we numerically investigate immiscible two‐phase flow in a three‐dimensional microscopic model of a porous medium and attempt to establish the form of the macroscopic flow law. We observe that the conventional linear description of the flow is applicable for high levels of forcing when the relative effects of capillary forces are small. However, at low levels of forcing capillary effects become important and the flow law becomes nonlinear. By constructing a two‐dimensional phase diagram in the parameter space of nonwetting saturation and dimensionless forcing, we delineate the various regions of linearity and nonlinearity and attempt to explain the underlying physical mechanisms that create these regions. In particular, we show that the appearance of percolated, throughgoing flow paths depends not only on the relative concentrations of the two fluids but also on a dimensionless number that represents the ratio of the applied force to the force necessary for nonwetting fluid to fully penetrate through the porous medium. Finally, we fit a linear model to our simulation data in the high‐forcing regions of the system and observe that an Onsager reciprocity holds for the viscous coupling of the two fluids.
Analysis of porous media heterogeneities using the diffusion of pressure waves
Authors: Rigord P. , Caristan Y. , Hulin J. P.
Published: February 2008

We present an experimental study and a model of the diffusion of sinusoidal pressure waves through porous media. We show that measurements of the hydraulic admittance A(ω) in the sine wave mode allow us to probe the structure of porous samples with an adjustable investigation depth depending on the frequency ω. The variations of A(ω) in heterogeneous media with a percolationlike geometry are modeled numerically on two‐dimensional percolation networks. One obtains a transition from normal diffusion at low frequencies (| A(ω)| ∝ ω½) to anomalous diffusion at higher frequencies (| A(ω)| ∝ ω0.7). At the transition, the penetration depth of the wave is of the order of the percolation correlation length. The hydraulic admittance and transmittance of 20% porosity pressed calcite have been investigated experimentally with sine wave excitations at pulsations ω between 2 × 10−4 and 0.42 rad/s. Both the modulus and the phase of the complex admittance A(ω) display normal diffusive variations as ω increases. Increasing the viscosity reduces the frequency above which the diffusive behavior is observed. The measured diffusion coefficient is 25% higher than that computed from permeability and compressibility values measured independently; this difference may be associated with nonconnected porosity.
A numerical treatment of geodynamic viscous flow problems involving the advection of material interfaces
Authors: Lenardic A. , Kaula W. M.
Published: February 2008

Effective numerical treatment of multicomponent viscous flow problems involving the advection of sharp interfaces between materials of differing physical properties requires correction techniques to prevent spurious diffusion and dispersion. We develop a particular algorithm, based on modern shock‐capture techniques, employing a two‐step nonlinear method. The first step involves the global application of a high‐order upwind scheme to a hyperbolic advection equation used to model the distribution of distinct material components in a flow field. The second step is corrective and involves the application of a global filter designed to remove dispersion errors that result from the advection of discontinuities (e.g., material interfaces) by high‐order, minimally dissipative schemes. The filter introduces no additional diffusion error. Nonuniform viscosity across a material interface is allowed for by the implementation of a compositionally weighted‐inverse interface viscosity scheme. The combined method approaches the optimal accuracy of modern shock‐capture techniques with a minimal increase in computational time and memory. A key advantage of this method is its simplicity to incorporate into preexisting codes be they finite difference, element, or volume of two or three dimensions.
Variation of the western equatorial Pacific Ocean, 1986–1988
Authors: Delcroix Thierry , Eldin Gérard , Radenac Marie‐Hélène , Toole John , Firing Eric
Published: February 2008

Twenty‐one oceanographic sections made along 165°E during 1984–1988 provide a unique picture of the 1986–1987 El Niño and the subsequent La Niña in the western equatorial Pacific. The mean of six cruises from January 1984 through June 1986, a relatively normal period, provides a reference with which the later sections are compared. The net warm water transport across 165°E within 10° of the equator was small in this mean reference section: 7 × 106 m3 s−1 to the east. In December 1986, strong westerly winds at and to the west of 165°E increased the net eastward transport of warm water to 88 × 106 m3 s−1, and the 1986–1987 El Niño was underway. During the following 2 years the net transport varied widely and rapidly; the extrema were 56 × 106 m3 s−1 to the east and 58 × 106 m3 s−1 to the west. Changes in the stratification along 165°E were correspondingly large, reflecting both the geostrophic balance of the strong zonal currents and the changes in the volume of warm water in the western equatorial Pacific. The anomaly of warm water volume corresponded closely to the time integral of the warm water transport across 165°E. Local wind forcing and remotely forced waves were both important causes of the transport fluctuations. Winds, precipitation, and currents were all important factors determining the depth of the surface mixed layer and the thickness of the underlying barrier layer. The way in which these factors interact is a strong function of latitude.
Floe pair interaction event rates in the marginal ice zone
Authors: Rottier Philip J.
Published: February 2008

Data from an accelerometer package are used to estimate the rates at which interaction events occur between adjacent ice floes in the marginal ice zone. Observations suggest that the events are principally the result of a modified collision process, where brash ice partially filling the gap between two floes acts as an intermediary in the transfer of momentum from one to the other. A model is presented from which the rate at which these interactions occur can be predicted. Comparison of the rates estimated from the data with those predicted by the model shows there to be some agreement, although there is considerable room for improvement. The predicted event rate depends strongly on the ratio of the root‐mean‐square wave height to the amount of open water between pairs of floes. The form of this dependence offers an explanation for grouping of events, a feature that may be readily observed in many extracts of data.
Reconstruction of oceanic flow characteristics from quasi‐Lagrangian data: 2. Characteristics of the large‐scale circulation in the Black Sea
Authors: Eremeev V. N. , Ivanov L. M. , Kirwan A. D. , Melnichenko O. V. , Kochergin S. V. , Stanichnaya R. R.
Published: February 2008

The circulation in the western portion of the Black Sea in 1987 is assessed from surface drifter trajectories, climatological data, and numerical modeling. These diverse data sources are combined by the paradigm reported in the companion article. The primary emphasis is on the low‐frequency and wave number circulation characteristics. The analysis suggests that in the Black Sea there may be a seasonal change in the direction of the large‐scale circulation.
Simultaneous measurements of spray and sea salt
Authors: Taylor Nancy J. , Wu Jin
Published: February 2008

Sizes and concentrations of spray droplets (radius r > 25 μm) were measured with an optical technique, while masses of sea salt were obtained through isokinetic sampling. Both sets of measurements were performed at the elevation of 8.2 m above the mean sea surface under relatively high wind velocities (U > 9 m s−1). From these simultaneously collected data, a complete spectrum of marine aerosols was constructed. Contributions of film, jet, and spume drops to marine aerosols are also discussed.
Wide‐scale validation of Geosat altimeter‐derived wind speed
Authors: Etcheto J. , Banege L.
Published: February 2008

A method for validating, on a worldwide scale, satellite measurements by comparison with ships of opportunity measurements has been derived. It has been applied to wind speed retrieved from the Geosat altimeter backscatter cross section for comparing the results obtained using either the smoothed Brown algorithm, the Chelton and Wentz algorithm, or the modified Chelton and Wentz algorithm. Among these three inversion algorithms, the Chelton and Wentz one best fits the ship reports at global scale, giving a bias less than 1 m s−1 in the whole wind speed range. In known windy regions, it retrieves wind speed values compatible with usually admitted values while the other algorithms give wind speeds lower by up to several m s−1.
Low‐frequency internal oceanic variability under seasonal forcing
Authors: Myers Paul G. , Weaver Andrew J.
Published: February 2008

A series of numerical experiments are conducted using the Bryan‐Cox ocean general circulation model to investigate the potential existence of low‐frequency variability of the thermohaline circulation under seasonal forcing. Experiments are performed with different combinations of a seasonal cycle being present or not on the restoring temperature, the surface freshwater flux fields (mixed boundary conditions), and the surface wind forcing. Despite the presence of forcing on the dominant seasonal time scale, it is found that the system may oscillate at the decadal period or longer. The decadal variability is excited by changes in the net surface density flux which are due to the advection of temperature and salinity anomalies in the model domain. The magnitude of the seasonal cycle also plays an important role in determining the time scale of variability. Violent overturning events may occur on the century time scale under seasonal forcing. The magnitudes of the flushes are reduced compared with those found in similar experiments without the presence of a seasonal cycle.
Prediction of the Gulf Stream path from upstream parameters
Authors: Cronin Meghan , Carter Everett , Watts D. Randolph
Published: February 2008

A linear Wiener (least squares optimal) filter is used to predict the path of the Gulf Stream in the region between Cape Hatteras and 70°W using upstream (“Inlet”) conditions as filter inputs. Although position, angle, curvature, geostrophic velocity, and curvature vorticity (κυ) measured at 73°W were tested as multiple Inlet parameters, it was found that Inlet position alone was the best predictor. With Inlet position as the predictor, this filter is equivalent to propagating meanders downstream with a typical phase speed and growth rate. Forecasts of the downstream path position are produced, more accurate than persistence, 12 days in advance at 140 km from the Inlet, 24 days in advance at 240 km from the Inlet, and 28 days in advance at 340 km from the Inlet.
Laboratory simulation of exchange through Fram Strait
Authors: Hunkins Kenneth , Whitehead J. A.
Published: February 2008

Laboratory experiments and theory were conducted to observe the flow patterns and transport in both buoyancy‐driven and wind‐driven rotating fluids. In “lock‐exchange” experiments, water with one density flows into a second basin after a sliding gate is removed. Water of a second density flows back into the first basin. The size and location of the currents for various values of density difference, rotation rate, and assorted sidewall geometries was recorded. Volume flux of the fluid was also measured and compared with a theory for lock‐exchange flow of a rotating fluid. In a separate group of experiments with a passive upper layer, easterly winds (like those in the Arctic Ocean) drive the upper level water into the Arctic Ocean and therefore oppose the buoyant exchange. Westerly winds would drive the water out of the Arctic Ocean. This indicates that the exchange between the Arctic Ocean and the Greenland‐Norwegian Sea is likely to be driven by buoyancy rather than by driven by wind. Crude estimates of the volumetric and fresh water exchange rate from the lock‐exchange formulas are compared with observed ocean fluxes, and approximate agreement is found.
Kinetic isotopic fractionation during air‐water gas transfer of O2, N2, CH4, and H2
Authors: Knox M. , Quay P. D. , Wilbur D.
Published: February 2008

We present experimental results that show that the kinetic isotopic fractionation during gas exchange is 0.9972 ± 0.0002 for oxygen, 0.9992 ± 0.0002 for methane, 0.9987 ± 0.0001 for nitrogen and 0.982 ± 0.002 for hydrogen, and that the equilibrium fractionation between water and gas phases is 1.037 for hydrogen. We show that the isotopic fractionation during gas transfer for these gases is not equal to the square root of their reduced mass in water, as would be predicted by an extension of the kinetic theory of ideal gases to dissolved gases. The use of isotopes as tracers of biogeochemical gases requires knowledge of the fractionation factor for air‐water gas transfer; there have been few direct measurements of these factors.
Sensitivities of a zonally averaged global ocean circulation model
Authors: Wright Daniel G. , Stocker Thomas F.
Published: February 2008

A global ocean circulation model is constructed in which the Pacific, Atlantic, Indian, and Southern oceans are separately resolved, each being represented by zonally averaged equations expressing conservation of momentum, mass, heat, and salt. Results are presented and compared with relevant zonally averaged observations. The sensitivity of the stream function and the meridional fluxes of heat and water are examined as functions of the horizontal and vertical diffusion coefficients, and as functions of a closure parameter introduced in averaging the equations. The results are sensitive to changes in the vertical diffusion coefficient and the closure parameter. The sensitivities to changes in the vertical diffusion coefficient are similar to those of a three‐dimensional ocean general circulation model. Results are relatively insensitive to the value of the horizontal diffusion coefficient provided it is of the order of 103 m2 s−1 or smaller. However, for larger values, a northward heat flux throughout the Atlantic basin, as observed, cannot be obtained. Wind stress significantly improves the comparison with observational estimates of the meridional heat and water fluxes, particularly near the equator, where there are large flux divergences associated with the Ekman transport. Results are further improved when horizontal diffusion is modified to include a contribution proportional to the local current speed. The effects of the Mediterranean and Red seas are examined and shown to be important in redistributing salt vertically. The model is then generalized to allow for depth‐integrated flow through the Bering Strait and the Indonesian passages. Both effects improve the comparison with observational estimates, but neither effect appears to be crucial in determining the global circulation or water mass properties in this model. The inclusion of a barotropic flow introduces the necessity to specify a reference temperature in order to calculate the heat flux directly from velocity‐temperature correlations. Simply neglecting the barotropic flow may contribute to large discrepancies between flux estimates based on integrated surface fluxes and “direct” correlation methods.
The structure and atmospheric forcing of the mesoscale velocity field in Fram Strait
Authors: Jónsson Steingrímur , Foldvik Arne , Aagaard Knut
Published: February 2008

We consider the connection between the wind field and the mesoscale circulation in the northern Greenland Sea, using over 50 record years of current observations to define the current structure in space and time. We find that east of the East Greenland Polar Front the motion is highly coherent in the vertical and that it scales in the horizontal as the internal Rossby radius. The structure depends very little on frequency. The East Greenland Current is dominated by smaller‐scale phenomena in both space and time, and the kinetic energy is low. The available energy in the wind field may be the largest in the world, especially over the Greenland and Iceland seas. The wind power shows a marked seasonal signal that east of the East Greenland Polar Front is paralleled by a similar signal in the fluctuating kinetic energy of the current, and the former leads the current fluctuations by 1–2 months. The mesoscale motions are probably dominated by nonlinear interactions, and the observed vertical and horizontal structure is in general agreement with predictions of the theory of quasi‐geostrophic turbulence forced by atmospheric fluctuations. We conclude that the majority of the mesoscale fluctuations in Fram Strait east of the East Greenland Polar Front are generated by the wind field. However, with few exceptions, the current field is not coherent with the curl of the wind stress, probably primarily owing to the isolating effect of the rough topography. The extent to which the wind field also generates mesoscale motions in the East Greenland Current is uncertain, although the eddy kinetic energy east of the East Greenland Polar Front is probably sufficient to account for conditions in the East Greenland Current by westward advection of eddies.
The influence of the Alaskan Gyre on the coastal circulation in the Gulf of Alaska
Authors: Heim Paul K. , Johnson Mark A. , O'Brien James J.
Published: February 2008

A wind‐driven, nonlinear, reduced gravity, primitive equation model is used to simulate the circulation of the northeast Pacific Ocean including the northeast Gulf of Alaska. The model includes the affects of bottom topography. Model geometry encompasses the entire Gulf of Alaska region from Vancouver Island to west of Kodiak Island. This region is traditionally beyond the latitude limits of the reduced gravity model imposed by a weakening thermocline away from the equator. The model resolution, 0.025° in latitude and longitude, is very fine. This is the first time that a model of this type and resolution has been successfully run at these latitudes. Although we expected that such resolution would allow the mesoscale eddy field to develop, it did not. One reason may be the coarse spatial and temporal winds. Twenty year of Comprehensive Ocean‐Atmosphere Data Set pseudo wind stress data used to drive the model are statistically analyzed to obtain a wind stress climatology. After model spin‐up, the monthly stresses for 1986–1989 are used to drive the model. The model reproduces many of the regional oceanographic features such as the Alaskan Gyre, the Alaska Current, the Alaska Coastal Current, the Sitka eddy, and an intense, cyclonic eddy (that has not yet been reported in the literature) northwest of the Queen Charlotte Island in the spring. In addition, the model displays seasonal and interannual variability. Model results are validated through comparisons with National Ocean Service coastal sea level records and a multilayer, oceanic basin scale model being run at the Institute for Naval Oceanography.
Sea surface temperature variability of the Iceland‐Faeroe front
Authors: Niiler Pearn P. , Piacsek Steve , Neuberg Lucas , Warn‐Varnas Alex
Published: February 2008

Several advanced very high resolution radiometer (AVHRR) images with spatial resolution of 1.1–3.3 km, together with several concurrent aircraft‐deployed expendable bathythermograph (AXBT) surveys and conductivity‐temperature‐depth (CTD) stations, from spring 1989 are used to describe the Iceland‐Faeroe sea surface temperature (SST) front. In the AVHRR images, SST fronts are located by maximizing |∇SST|. Single, large gradient segments of the SST front do exist, with some exceeding 100 km in length, indicating a multiple frontal structure. These single frontal lines are also segments where |∇2SST| is small, and they can be followed uniquely by a single isotherm eastward from Iceland for a distance of 300 km. With a 35‐km sampled AXBT survey, two small subsurface cold eddies were located south of the surface front in an area 170 km × 270 km east of Iceland. From a May 1987 AVHRR image on 1.1‐km resolution, a population of seven such cold eddies are found between Iceland and the Faroes. They appear to be generated along the surface expression of the Iceland Faroes front and populate the northern slope of the Iceland‐Faroes Ridge. Historical data from towed high‐resolution instruments suggest that the cold eddies are ∼30–50 km in size and uplift the main thermocline by 150 m.
Deriving inertial wave characteristics from surface drifter velocities: Frequency variability in the tropical Pacific
Authors: Poulain Pierre‐Marie , Luther Douglas S. , Patzert William C.
Published: February 2008

Two techniques have been developed for estimating statistics of inertial oscillations from satellite‐tracked drifters. These techniques overcome the difficulties inherent in estimating such statistics from data dependent upon space coordinates that are a function of time. Application of these techniques to tropical surface drifter data collected during the NORPAX, EPOCS, and TOGA programs reveals a latitude‐dependent, statistically significant “blue shift” of inertial wave frequency. The latitudinal dependence of the blue shift is similar to predictions based on “global” internal wave spectral models, with a superposition of frequency shifting due to modification of the effective local inertial frequency by the presence of strongly sheared zonal mean currents within 12° of the equator.
Reflection of interannual Rossby waves at the maritime western boundary of the tropical Pacific
Authors: White Warren B. , Tai Chang‐Kou
Published: February 2008

The reflection of interannual Rossby waves over a 2.7‐year period (i.e., November 1986 to August 1989) is examined in the western tropical Pacific utilizing altimetric sea level observations from the Geosat Exact Repeat Mission. In 1987 this reflection process generates upwelling Kelvin waves that tended to limit the growth of the 1986–1987 El Niño in the eastern equatorial Pacific. Two‐dimensional autocorrelation and spectral analyses verify the existence of westward traveling Rossby waves in both the equatorial and off‐equatorial domains, and eastward propagating Kelvin waves in the equatorial domain. Extended empirical orthogonal function (EEOF) analysis characterizes the time‐space evolutionary behavior of the Rossby wave reflection process, extending from the El Niño in 1986–1987 toward the anti‐El Niño in 1988–1989. Both the Philippines Archipelago in the northern hemisphere and the Solomon Archipelago/Bismarck Archipelago/New Guinea complex in the southern hemisphere appear to reflect incident Rossby waves, with maximum amplitude near 8°N and 8°S, generating Kelvin waves in the equatorial wave guide that conduct the anomalous signal eastward along the equator. This apparent reflection process is examined in terms of the linear theory, with sea level phase information in the northern hemisphere at the western boundary (130°W) expanded in terms of the Kelvin and symmetric Rossby meridional wave modes. These modes are formed as weighted sums of parabolic cylinder functions. This procedure, operating under the constraint that the reflection process occurred according to linear theory, determines the percent contribution that each incident Rossby wave mode amplitude has upon the amplitude of the reflected Kelvin wave. This percentage changed little over the evolution of the EEOF pattern. The amplitude of the first‐mode Rossby wave (i.e., mode 2) accounts for most of the Kelvin wave amplitude (i.e., 70–80%), with the higher‐mode Rossby wave amplitudes (i.e., 4, 6, and 8) contributing less (i.e., 20%, −10%, and 10%, respectively), interacting both constructively and destructively with the first.
The effect of severe storms on the ice cover of the northern Tatarskiy Strait
Authors: Martin Seelye , Munoz Esther , Drucker Robert
Published: February 2008

The Tatarskiy Strait is the northernmost region of the Japan Sea, with an ice‐covered area in winter of about 4 × 104 km2. This study uses the daily passive microwave images of the region from the special sensor microwave imager (SSM/I), which was launched in 1987, to estimate the production of ice and Japan Sea bottom water. In winter, the prevailing winds in the strait are northerly and cold; these conditions create a region of reduced ice concentration in the northern strait, which leads to an enhanced ice production throughout the season. In addition to these prevailing winds, each winter one or two severe storms generate very strong northerly winds and cold temperatures in the strait. These storms create a large transient polynya in the strait at a time simultaneous with the greatest heat flux out of the open water. Calculation of the ice growth from open water in the northern strait yields about 25 km3 of ice per season, of which severe storms provide about 25%. The total ice production is sufficient to form about 5–12 × 102 km3 of the Japan Sea bottom water, which is about 50–100% of the renewal rate required from 14C data. Because the oxygen bottom layer thickness decreased between 1969 and 1984, the study also investigates the frequency of the severe storms in the northern strait during 1966–1990 and finds that these storms are about 3 times as frequent at the beginning of this period than at the end. This suggests the storms may play a role in the generation of the Japan Sea bottom water.