bluuebunny/crossref_metadata_2025 huggingface dataset:
Convergence rates across a displacement transfer zone in the western Transverse Ranges, Ventura basin, California
Authors: Huftile Gary J. , Yeats Robert S.
Published: February 2004

Three cross sections are balanced and retrodeformed to the top of the Pleistocene Saugus Formation and to a horizon close to the base of the Jaramillo subchron to yield crustal shortening and shortening rates for the western Transverse Ranges of California. The cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault (Red Mountain fault) to a blind thrust and from that to a combination of both a surface reverse fault (San Cayetano fault) and the blind thrust. Using an age of 250±50 ka for the top of the Saugus Formation based on amino‐acid racemization of fossil mollusks, crustal shortening rates appear to have abruptly accelerated through time from 3+4/−3 to 5±4 mm/yr between 250 and 975 ka to 20±6 to 28±1 mm/yr since 250±50 ka. Using an age of 500 ka for the top of the Saugus Formation based on paleomagnetic stratigraphy, there is little increase in deformation rates through time, ranging from 5±5 to 8±6 mm/yr between 975–500 ka and from 10±3 to 14±1 mm/yr since 500 ka. Crustal convergence rates determined by Global Positioning System surveys, taken over 4.6 years, indicate a shortening rate of 7–10 mm/yr across the basin. This is consistent with the slower rates of deformation calculated using an age of 500 ka for the top of the Saugus Formation.
Electrical resistivity sounding of the East Antarctic Ice Sheet
Authors: Shabtaie Sion , Bentley Charles R.
Published: February 2004

Electrical resistivity soundings using a Schlumberger array have been carried out at Dome C, East Antarctica (74°39′S, 124°10′E, elevation 3400 m), to sound the entire 3500‐m depth of the ice sheet. Changes in density and temperature are largely separated in the ice at Dome C, so activation energies for both firn and ice could be determined: we find an activation energy of 0.25 eV in both solid ice and firn between −15°C and −58°C. A common value of the activation energy points to a single transport regime in which the charge carriers and conduction paths are the same in firn and ice. To evaluate the variation of resistivity with density, we have considered five dielectric mixture models that fit the available data on the high‐frequency dielectric constant of firn. Only Looyenga's equation fits the field data for dc resistivity. In the upper 900 m of the ice sheet, where impurity concentrations are known from core samples, we find no correlation between resistivities and the concentrations of salts or acids. Instead, we find it likely that resistivities are correlated with the crystal size, hence with the Holocene‐Wisconsin boundary in the ice column. A pronounced increase in resistivity, to a value comparable with that in temperate glacier ice, occurs deep within the ice sheet. We attribute this to a large increase in the size and irregularity of the ice crystals, which destroys the continuity of the impurity shells surrounding the ice crystals that we believe supply the conduction paths (Shabtaie and Bentley, 1994a). High resistivity does not imply removal of the impurities from the system; moderate concentrations of impurities can be accommodated by locating them in disconnected domains.
Evidence for a high‐velocity slab associated with the Hindu Kush seismic zone
Authors: Mellors R. J. , Pavlis G. L. , Hamburger M. W. , Al‐Shukri H. J. , Lukk A. A.
Published: February 2004

We used teleseismic travel time residuals to determine lateral velocity variations of the crust and upper mantle in the Pamir‐Hindu Kush region in Tadjikistan and Afghanistan. Data from 29 analog seismic stations in Tadjikistan and northern Afghanistan were used to determine travel time residuals for 210 teleseismic events ranging in distance from 28° to 87° and covering a broad range of azimuths. We inverted for velocity perturbations over a rectangular grid with a block size of 99 × 99 km. The model extended to a depth of 350 km with a 50‐km‐thick first layer and two 150‐km‐thick deeper layers. The results show a strong and well‐resolved zone of high velocities in the upper mantle at depths greater than 200 km, coincident with the location of the Hindu Kush seismic zone. No clear velocity perturbations are associated with the Pamir seismic zone. Above 200 km little correlation is observed with the seismic zone, but indications of thicker crust under the Pamir and thinner crust under the Tadjik Depression are seen. The high velocities are most likely caused by the presence of oceanic lithosphere at depth.
Gravity anomalies and crustal thickness variations along the Mid‐Atlantic Ridge between 33°N and 40°N
Authors: Detrick Robert S. , Needham H. D. , Renard V.
Published: February 2004

Multibeam bathymetry and gravity data have been obtained along an ∼800‐km‐long section of the Mid‐Atlantic Ridge from just south of the Hayes fracture zone at 33°N to the northern edge of the Azores Platform near 40°N. A three‐dimensional analysis of these gravity and topography data, combined with results from earlier seismic refraction studies in this area, reveal two different scales of crustal heterogeneity. A systematic, along‐axis, segment‐scale (λ<20–100 km) variation in crustal thickness is present with the thickest crust (>8–9 km) near the middle of spreading segments and the thinnest crust (<3–4 km) near segment offsets. The magnitude of this along‐axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset. There also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs, inferred to be thinner crust, beneath the “inside corner” highs adjacent to these offsets. This segment‐scale crustal heterogeneity is similar to that reported from the Kane‐to‐Atlantis section of the Mid‐Atlantic Ridge and from parts of the intermediate‐spreading southern Mid‐Atlantic Ridge. It is superimposed on a second, longer wavelength variation in gravity and crustal thickness associated with the Azores hot spot. The most pronounced effect of the Azores hot spot on the Mid‐Atlantic Ridge occurs between 38°N and 40°N where the ridge axis rapidly shoals by more than 1000 m, the crust thickens by over 2 km, and the rift valley largely disappears. The absence of a deep axial rift valley on the Azores Platform, and near the midpoints of some ridge segments along the Mid‐Atlantic Ridge to the south, agrees well with the predictions of recent ridge crest thermal and rheological models that suggest a dependence of axial morphology on both crustal thickness and spreading rate. The transition from a rift valley to an axial high morphology at these spreading rates (∼11 mm/yr half rate) occurs at a crustal thickness of about 9±1km.
Metastability of domain state in multidomain magnetite: Consequences for remanence acquisition
Authors: McClelland E. , Shcherbakov V. P.
Published: February 2004

Experimental studies on isothermal remanent magnetization (IRM) acquisition of synthetic specimens containing crushed natural magnetite in seven grain sizes from 5–10 μm to 100–150 μm show that the initial demagnetization state before IRM acquisition strongly affects the samples' ability to acquire IRM. It is easier to acquire IRM from an initially thermally demagnetized state than from an alternating field (AF) demagnetized state, and low‐temperature demagnetization of the thermally demagnetized state increases resistance to IRM acquisition. These data are interpreted as confirmation of the existence of a number of possible domain states (metastable states) which have variable capacity for remanence acquisition. Our results also demonstrate that activation of domain structure is much more effective by AF than by direct (dc) fields. We suggest that the continually changing external field experienced during AF demagnetization allows the magnetic structure to reorganize incrementally towards the demagnetized state, requiring a considerably lower peak field (about 3 times lower) to achieve the same demagnetization effect. Thermoremanent magnetization (TRM) in larger grain sizes is partially self‐reversed. A difference in grain size dependence is observed for a number of magnetic parameters. All parameters vary with grain size below about 50 μm. Above 50 μm, parameters associated with coercivity, AF demagnetization, or low‐temperature treatment (Hcr; saturation IRM (SIRM) memory; TRM memory; median acquisition field (MAF) of IRM after AF; median destructive fields (MDF) of SIRM) are independent of grain size; parameters associated with TRM or thermal demagnetization (MDF ofTRM; MAF of IRM after thermal demagnetization; MAF of IRM after thermal demagnetization followed by low temperature demagnetization; self‐reversal as percentage of TRM) show grain size dependency throughout the grain size range. Our interpretation of the data invokes the existence of subtle material inhomogeneities in both natural and synthetic magnetites, giving rise to a limited range of Curie temperatures below 580°C (perhaps down to 560°C). We suggest that their control over the local energy minima states available to the grain is more significant in the TRM or thermally demagnetized state, while shielding by “soft” domain walls is dominant in the AF demagnetized state where walls are in the simplest domain configuration, leading to differing grain size dependencies of these different states.
Spatial and temporal relationships between mid‐Tertiary magmatism and extension in southwestern Arizona
Authors: Spencer Jon E. , Richard Stephen M. , Reynolds Stephen J. , Miller Robert J. , Shafiqullah M. , Gilbert Wyatt G. , Grubensky Michael J.
Published: February 2004

Cenozoic magmatism in southwestern Arizona, which is within the Basin and Range tectonic province, occurred almost entirely between 15 and 25 Ma. Volcanic rocks typically consist, in ascending order, of (1) a thin sequence of mafic to intermediate lava flows, (2) voluminous felsic lava flows and pyroclastic rocks with minor to moderate amounts of intermediate to mafic lava flows, and (3) basalt and andesite. Volcanic rock sequences rest disconformably on pre‐Tertiary bedrock in most areas but locally overlie substantial coarse clastic debris that was deposited immediately before and during earliest magmatism. Prevolcanic clastic debris is interpreted as a consequence of local early normal faulting. In most regions, tilting related to extension began later and occurred during or after eruption of felsic volcanic rocks and before the end of younger mafic volcanism. Extension generally ended before about 17 Ma except in a northwest trending belt adjacent to the relatively unfaulted and topographically elevated Transition Zone tectonic province which is adjacent to the Colorado Plateau. Rapid cooling of metamorphic core complexes and tilting of young basalts and coarse clastic rocks continued in this belt until as recently as 11 Ma. Extension was extreme in this belt, whereas it was generally moderate to slight in other parts of southwestern Arizona. Large‐magnitude extension was not associated with areas of greatest igneous activity, and rapid cooling and exhumation of core complexes postdated local magmatism. These relationships are inconsistent with theories that relate genesis of metamorphic core complexes to magma intrusion in the upper crust. Except for young extension in this northwest trending belt, there are no apparent regional migration trends for either magmatism or extension within southwestern Arizona. Lack of substantial extension before magmatism and general lack of magmatism during youngest extension are inconsistent with the hypothesis that magmatism was the product of decompression melting during lithospheric extension. The long duration and large magnitude of extension adjacent to the Transition Zone tectonic province and within an area of earlier crustal thickening are consistent with the hypothesis that extension was driven by the gravitational potential energy of elevated land mass and crustal roots. Regional magmatic heating apparently weakened the lithosphere and triggered extension but did not control extension locally.
Crustal fissuring and its relationship to magmatic and hydrothermal processes on the East Pacific Rise crest (9°12′ to 54′N)
Authors: Wright Dawn J. , Haymon Rachel M. , Fornari Daniel J.
Published: February 2004

The deep‐towed Argo I optical/acoustical vehicle and a geographic information system (GIS) have been used to establish the abundance, widths, and spatial distribution of fissures, as well as the relative age distribution of lavas along the narrow (<500 m wide) axial zone of the East Pacific Rise (EPR) from 9°12′ to 9°54′N. On a second‐order scale (∼78 km long), wider but less numerous fissures are found in the northern portion of the survey area; this changes to narrower, more abundant fissures in the south. A profile of the cumulative width added by fissures to the axial zone exhibits minima in three areas along strike (near 9°49′, 9°35′, and 9°15′N), where the most recent eruptions have occurred above sites of magmatic injection from the upper mantle, filling and covering older fissures. On a fourth‐order scale (5–15 km long) the mean density of fissuring on a given segment is greater where relative axial lava age is greater. Fissure density also correlates with hydrothermal vent abundance and type. Increased cracking toward segment tips is observed at the second‐order scale, whereas fourth‐order segments tend to be more cracked in the middle. Cracking on a fourth‐order scale may be driven by the propagation of dikes, rather than by the far‐field plate stresses. The above relations constrain the model of Haymon et al. [1991] in which individual fourth‐order segments are in different phases of a volcanic‐hydrothermal‐tectonic cycle.
Coseismic fault slip associated with the 1992 Mw 6.1 Joshua Tree, California, earthquake: Implications for the Joshua Tree‐Landers earthquake sequence
Authors: Bennett Richard A. , Reilinger Robert E. , Rodi William , Li Yingping , Toksöz M. Nafi , Hudnut Ken
Published: February 2004

Coseismic surface deformation associated with the Mw 6.1, April 23, 1992, Joshua Tree earthquake is well represented by estimates of geodetic monument displacements at 20 locations independently derived from Global Positioning System and trilateration measurements. The rms signal to noise ratio for these inferred displacements is 1.8 with near‐fault displacement estimates exceeding 40 mm. In order to determine the long‐wavelength distribution of slip over the plane of rupture, a Tikhonov regularization operator is applied to these estimates which minimizes stress variability subject to purely right‐lateral slip and zero surface slip constraints. The resulting slip distribution yields a geodetic moment estimate of 1.7 × 1018 N m with corresponding maximum slip around 0.8 m and compares well with independent and complementary information including seismic moment and source time function estimates and main shock and aftershock locations. From empirical Green's function analyses, a rupture duration of 5 s is obtained which implies a rupture radius of 6–8 km. Most of the inferred slip lies to the north of the hypocenter, consistent with northward rupture propagation. Stress drop estimates are in the range of 2–4 MPa. In addition, predicted Coulomb stress increases correlate remarkably well with the distribution of aftershock hypocenters; most of the aftershocks occur in areas for which the mainshock rupture produced stress increases larger than about 0.1 MPa. In contrast, predicted stress changes are near zero at the hypocenter of the Mw 7.3, June 28, 1992, Landers earthquake which nucleated about 20 km beyond the northernmost edge of the Joshua Tree rupture. Based on aftershock migrations and the predicted static stress field, we speculate that redistribution of Joshua Tree‐induced stress perturbations played a role in the spatio‐temporal development of the earthquake sequence culminating in the Landers event.
A P wave velocity model of Earth's core
Authors: Song Xiaodong , Helmberger Don V.
Published: February 2004

Present Earth core models derived from the retrieval of global Earth structure are based on absolute travel times, mostly from the International Seismological Centre (ISC), and/or free‐oscillation eigenfrequencies. Many core phase data are left out of these constructions, e.g., PKP differential travel times, amplitude ratios, and waveforms. This study is an attempt to utilize this additional information to construct a model of core P wave velocity which is consistent with the different types of core phase data available. In conjunction with our waveform modeling we used 150 differential time measurements and 87 amplitude ratio measurements, which were the highest‐quality observations chosen from a large population of Global Digital Seismograph Network (GDSN) records. As a result of fitting these various data sets, a one‐dimensional P wave velocity model of the core, PREM2, is proposed. This model, modified from the Preliminary Reference Earth Model (PREM) (Dziewonski and Anderson, 1981), shows a better fit to the combined data set than any of the existing core models. Major features of the model include a sharp velocity discontinuity at the inner core boundary (ICB), with a large jump (0.78 km/s), and a low velocity gradient at the base of the fluid core. The velocity is nearly constant over the lower 100 km of the outer core. The model features a depth‐dependent Qα structure in the inner core such that a constant t* for the inner core fits the amplitude ratios and waveforms of short‐period waves moderately well. This means the top of the inner core is more attenuating than the deeper part of the inner core. In addition, the P velocity in the lowermost mantle is reduced from that of PREM as a baseline adjustment for the observed separations of the DF and AB branches of PKP at large distances.
A model for the formation of lava tubes by roofing over a channel
Authors: Dragoni Michele , Piombo Antonello , Tallarico Andrea
Published: February 2004

The formation of lava tubes is a common phenomenon on some basaltic volcanoes, such as Etna. A model for tube formation by roofing of a channel is proposed and involves first describing lava as a Bingham liquid flowing down a slope. It is further assumed that lava flows in a channel with rectangular cross section: as a result of heat loss into the atmosphere, a crust is gradually formed on the upper surface of the flow and this crust eventually welds to the channel levées. We assume that a lava tube is formed when such a crust is sufficiently thick to resist the drag of the underlying flow and to sustain itself under its own weight. The minimum thickness of the crust satisfying such conditions depends on the tensile strength and shear strength of the crust itself. Assuming that the growth of the crust produces a downflow linear increase of the shear stress at the interface between flowing lava and the crust, the distance is evaluated between the eruption vent and the point where the tube is formed. The model predicts that if the flow rate is constant, the thickness of the flow increases as the crust fragments grow and weld to each other, and the velocity of the crust decreases to zero. Once the lava tube is formed, the initial flow rate can be achieved by a flow thickness smaller than the vertical size of the tube, with the same viscous dissipation: this may explain why under steady state conditions, the lava level inside a tube is frequently lower than the roof of the tube itself.
The rigid lid's contribution to the ill‐conditioning of oceanic inverse problems
Authors: Thacker W. C. , Raghunath R.
Published: February 2004

Fitting oceanographic general circulation models to data is a difficult computational undertaking. This paper addresses difficulties stemming from the rigid lid approximation. Adjusting the barotropic stream function to achieve the fit, in effect, requires the solution of a hidden elliptical equation, the resolution‐dependent difficulty of the elliptical problem manifest in the optimization problem. Adjusting the independent (subsuface) baroclinic velocity variables also contributes to the ill‐conditioning of the least squares computation, but Lagrange multipliers can be used to adjust all baroclinic velocity variables (surface and subsurface) symmetrically. Similarly, Lagrange multipliers can be used to adjust the total velocity variables, thereby circumventing the use of barotropic stream function and baroclinic velocities; determining the multipliers at each optimization iteration requires the solution of an elliptical partial differential equation, the solution of which can be thought of as a preconditioning transformation.
Effects of the Fieberling seamount group upon flow and thermohaline structure in the spring of 1991
Authors: Roden Gunnar I.
Published: February 2004

The circulation and thermohaline structure around Fieberling, Fieberling II and Hoke seamounts are examined on the basis of two high‐resolution surveys in April and May 1991. The large‐scale flow approached the seamounts from the southeast, impinging first on Hoke, then splitting, with one branch meandering westward in jetlike fashion over the Fieberling II and Fieberling seamounts and the other turning to the northeast. The jets were typically 10–20 km wide, with core speeds of 0.2–0.5 m/s and were accompanied by pairs of cyclonic and anticyclonic eddies, with diameters of 10–30 km, located near the seamounts. While the large‐scale flow pattern persisted over a week, significant changes were observed in the shape of the meanders and the locations of the eddies over this period. Transient, bottom intensified upwelling cones, 20 km in diameter and 150–220 m high, were encountered on all three seamounts investigated, as were pronounced 60‐m‐deep mixed layers above the summits. Their persistence at a given location was less than a week. Geopotential height perturbations of the order of 0.8 J/kg (equivalent to 0.08 m in elevation) were observed in the vicinity of the seamounts. These perturbations are dominated by mesoscale ridges, troughs, peaks, and depressions, the positions of which vary with depth as well as time. In general, the surface geopotential topographies differ in shape from those observed at depths where the seamount peaks occur, which is attributed to the vanishing influence of the California current beneath 200 m.
Modeling rotating flows with neutral and unstable stratification
Authors: Hassid Samuel , Galperin Boris
Published: February 2004

We investigate the effect of rotation on the structure of turbulence in neutral and convective mixed layers and assess the applicability of the Reynolds stress turbulence closure models to studies of deep convection. Unlike in the case of stable stratification, where rotational contribution is limited (Galperin et al., 1989), in neutral and convective flows this contribution is significant. The rotational terms endow eddy viscosity and eddy diffusivity with tensorial properties and algebraic complexity. In the present study we thoroughly investigate the mathematical features of this tensorial eddy viscosity‐eddy diffusivity formulation. To ensure physical readability of the model, we found it necessary to impose upper bounds on the dimensionless velocity and temperature gradients in the expressions for eddy coefficients. It was also found absolutely crucial to include in the model a limitation on the magnitude of the turbulence macroscale due to background rotation. This limitation is similar to the one imposed by stable stratification. The self‐similar solutions derived from the model are compared with the experimental data by Fernando et al. (1991) and are shown to be in good agreement. Implications of the results for oceanographic modeling and for simulation of deep convection are discussed.
Understanding the relation between wind‐ and pressure‐driven sea level variability
Authors: Ponte Rui M.
Published: February 2004

Sea surface adjustment to combined wind and pressure forcing is examined using numerical solutions to the shallow water equations. The experiments use coastal geometry and bottom topography representative of the North Atlantic and are forced by realistic barometric pressure and wind stress fields. The response to pressure is essentially static or close to the inverted barometer solution at periods longer than a few days and dominates the sea level variability, with wind‐driven sea level signals being relatively small. With regard to the dynamic signals, wind‐driven fluctuations dominate at long periods, as expected from quasi‐geostrophic theory. Pressure becomes more important than wind stress as a source of dynamic signals only at periods shorter than approximately three days. Wind‐ and pressure‐driven sea level fluctuations are anticorrelated over most regions. Hence regressions of sea level on barometric pressure yield coefficients generally smaller than expected for the inverted barometer response known to be the case in the model. In the regions of significant wind‐pressure correlation effects, to infer the correct pressure response using statistical methods, input fields must include winds as well as pressure. Because of the nonlocal character of the wind response, multivariate statistical models with local wind driving as input are not very successful. Inclusion of nonlocal wind variability over extensive regions is necessary to extract the correct pressure response. Implications of these results to the interpretation of sea level observations are discussed.
Summertime synoptic variability of frontal systems in the northern Bering Sea
Authors: Gawarkiewicz Glen , Haney J. Christopher , Caruso Michael J.
Published: February 2004

Hydrographic observations in the northern Bering Sea during August and September 1987 indicate the presence of a front dividing relatively warm, fresh Bering Shelf Water from relatively cool, saline Anadyr Water along the western and northern coasts of St. Lawrence Island near Anadyr Strait. A buoyant layer 20 m thick with surface salinities as low as 29.5 practical salinity units and a maximum temperature of 10°C was present adjacent to the island. The surface outcrop of the front migrated 80 km north during the nine‐day time period of the hydrographic observations. Surface thermal patterns suggest that this front may extend the length of the northern coastline of St. Lawrence Island during the summer. The front veers north and passes through the Bering Strait, where temperature differences as large as 6°C exist across the strait. An examination of the water mass properties of the Bering Sea suggests that the buoyant water north of St. Lawrence Island is Bering Shelf Water which has been carried northward through Anadyr Strait. The baroclinic transport (assuming no flow at the bottom) associated with the front is 0.07 Sv, which is roughly a third of the seasonal increase in transport through the Bering Strait in the summer.
Coherence scales of wave‐induced suspended sand concentration fluctuations
Authors: Hay Alex E. , Bowen Anthony J.
Published: February 2004

Multifreqiiency acoustic backscatter measurements made in the nearshore zone are analyzed to determine the correlation structure of the instantaneous suspended sediment concentration field at horizontal separations of 20 cm. Large‐amplitude backscatter fluctuations, representing suspended sediment concentrations more than a factor of 10 greater than the mean and with vertical coherence scales many times greater than the logarithmic wave boundary layer thickness, are shown to be spatially correlated at horizontal separations less than the wave orbital amplitude over successive wave cycles. The time lag between the appearance of fluctuations in different acoustic beams is inversely proportional to the fluid velocity, showing that the fluctuations in backscatter amplitude contain real information on fluctuations in suspended sediment concentration, advected with the fluid. It is found that the instantaneous suspended sediment field in shore‐normal cross section is punctuated by spatially discrete clouds extending well above the (logarithmic) wave boundary layer. The fluctuations appear to be present even in the absence of small‐scale ripples. It is suggested that the sediment clouds may be due to vortex shedding from megaripples, other bed forms of intermediate scale, or coherent structures in combined wave‐current flows.
Decadal variability of the North Atlantic in an ocean general circulation model
Authors: Weisse Ralf , Mikolajewicz Uwe , Maier‐Reimer Ernst
Published: February 2004

Climatic fluctuations on a decadal timescale in the North Atlantic in a global ocean general circulation model were considered. The analysis was carried out for the 3800‐year stochastic forcing simulation of Mikolajewicz and Maier‐Reimer in which the Hamburg Large‐Scale Geostrophic ocean model was driven by monthly climatologies of wind stress, air temperature, and freshwater flux with superimposed white noise freshwater fluxes with an amplitude of about 16 mm/month. We applied a Principal Oscillation Pattern analysis to the vector time series of the upper level salinity fields, so that the examined fluctuations appear as estimated eigenmodes of the system. In addition to an oscillation with a period of 320 years as already described by Mikolajewicz and Maier‐Reimer, we found a broadband Principal Oscillation Pattern with a timescale of the order of 10 to 40 years. It describes the generation of salinity anomalies in the Labrador Sea and the following discharge into the North Atlantic. In sensitivity experiments we clarified that the source of the variability lies in the Labrador Sea and showed that the generation of the salinity anomalies is mainly due to an undisturbed local integration of the white noise freshwater fluxes.
Energy transfer between surface and internal waves in the North Pacific Ocean
Authors: Watson Kenneth M.
Published: February 2004

The energy transfer between internal and surface waves is formulated as a scattering process of surface waves by packets of internal waves. For weak internal wave fields, the Born approximation may be used, and this is shown to give the same transfer rate as the “modulation model” of the resonant triad theory for weak nonlinear interactions. For stronger internal wave fields, the eikonal theory of ray paths is used to calculate surface wave scattering. The theory is applied to historical Väisälä frequency profiles and wind data for the North Pacific ocean. Energy transfer rates from the internal wave to the surface wave field in the range of 10−4 W/m2 and higher are found, except during winter months at higher latitudes, where the rates are somewhat lower. For the weaker internal wave fields, the resonant triad theory (or, equivalently, the Born theory) must be used. For the strongest fields observed, the weak interaction theory is not valid and the interaction is described as ray path scattering. There is a region of overlap for which the two theoretical models give comparable results.
Evaporation and solar irradiance as regulators of sea surface temperature in annual and interannual changes
Authors: Liu W. Timothy , Zhang Anzhen , Bishop James K. B.
Published: February 2004

Seven years of net surface solar irradiance (S) derived from cloud information provided by the International Satellite Cloud Climatology Project and 4 years of surface latent heat flux (E) derived from observations of the special sensor microwave imager were used to examine the relation between surface heat fluxes and sea surface temperature (Ts) in their global geographical distribution, seasonal cycle, and interannual variation. The relations of seasonal changes imply that evaporation cooling is significant over most of the ocean and that solar heating is the main drive for the change of Ts away from the equatorial wave guide where ocean dynamics may be more important. However, Ts is not the most direct and significant factor in the seasonal changes of S and E over most of the ocean; the solar incident angle may be more important to S, and wind speed and air humidity are found to correlate better with E. Significant local correlations between anomalies of Ts and S and between anomalies of Ts and E are found in the central equatorial Pacific; both types of correlation are negative. In this area, organized deep convection overlies the warm ocean, forms high clouds, and reduces S, while the low wind speed and high humidity that result from surface convergence reduce E. The negative correlation is not present in the surrounding areas where equally warm water and strong Ts anomalies are found under a subsiding atmosphere without similarly strong S and E anomalies. Correlation between anomalies of temperature tendency and the fluxes is weak, indicating that other factors are more influential in changing upper ocean heat balance during El Niño. The result shows that the relations between Ts and the flux components, in annual and interannual timescales, are not universal and not consistent with the local negative feedback postulations which require that an increase in Ts would result in an increase in local evaporative cooling and a decrease in local solar heating of the ocean. Large‐scale atmospheric circulation changes clouds, winds, and humidity; they, in turn, influence the fluxes significantly. The influence of ocean dynamics in changing Ts in the tropical ocean can not be ignored.
A Doppler spectral method of identifying water and ice in marginal ice zone imagery
Authors: Rufenach Clifford L. , Shuchman Robert A.
Published: February 2004

Complex synthetic aperture radar imageries at C band and X band acquired during March 1989 in the Greenland Sea are specially processed. This processing yields Doppler spectra from two backscatter regions near the marginal ice zone: (1) newly forming ice inside a tongue‐shaped region called the Odden and (2) open water adjacent to the Odden. We observe a consistent broadening of the Doppler bandwidth in the water spectra compared to the ice spectra. Short ocean waves (wavelengths <∼10 m) are attenuated as they travel into the ice, whereas long ocean gravity waves (wavelengths ≫10 m) exhibit little attenuation. A model is derived describing the Doppler bandwidth in terms of the platform velocity, the short waves, and the long waves. The short subresolution waves and the long gravity waves contribute to the Doppler broadening in the ocean. However, for the gentle ocean swell observed here we show that the long‐wave contribution can be neglected. Therefore the differential broadening measured between the water and the ice is primarily due to the short random waves. The Doppler bandwidth broadening is
Hz at C band and
at X band, whereas the measured bandwidth due to the platform velocity (155 m/s) is approximately 75 Hz at both C band and X band. The broadening can also be given in terms of the scene correlation time; 2τc ≈ 22 ms and 16 ms at C band and X band, respectively, under certain assumptions.
Diagnostic and prognostic calculations of the North Atlantic circulation and sea level using a sigma coordinate ocean model
Authors: Ezer Tal , Mellor George L.
Published: February 2004

The North Atlantic circulation and sea surface height, determined from hydrographic and wind stress data are calculated with a free surface, primitive equation ocean model. The model grid, a vertical sigma ‐ coordinate and horizontal curvilinear orthogonal system, makes it possible to resolve coastal regions with complicated topography that were unresolved in previous calculations. When running in a diagnostic mode, in which the temperature and the salinity fields are fixed and equal to the Levitus annual mean fields, the vertically integrated North Atlantic circulation is very similar to though more detailed than that obtained from previous calculations using simpler diagnostic models. On the other hand, the meridional, zonally averaged flows and the poleward heat transport from the purely diagnostic calculations are noisy and unrealistic. However, short prognostic calculations of only 30 days following the diagnostic run allow intensification of the western boundary current and removal of noise due to inconsistencies between the hydrographic data and bottom topography and produce a more realistic meridional circulation and poleward heat transport, with a maximum value of 1.2×1015 W which is, comparable to estimates based on observations. The calculated sea level along the North America continent agrees with previous estimates but provides more spatial detail. Analysis of the dynamic adjustment process shows that this process is dominated by the effect of bottom topography through the action of the bottom pressure torque. This study is a first step in applying a realistic numerical model, previously used primarily for estuaries and coastal regions, to basin‐scale ocean problems.
Dissolved methane distributions, sources, and sinks in the western Bransfield Strait, Antarctica
Authors: Tilbrook Bronte D. , Karl David M.
Published: February 2004

Dissolved methane (CH4) concentrations were measured in the upper water column (0–200 m) of western Bransfield Strait and southwestern Drake Passage on four cruises between December 1986 and March 1987 during the Research on Antarctic Coastal Ecosystem Rates (RACER) experiment. Methane concentration profiles were similar on all four cruises and showed distinct geographic variability. In contrast to most oceanic surface waters, CH4 in Drake Passage was undersaturated relative to atmospheric equilibrium. The degree of the undersaturation varied from 0% to 15% in a seasonally warmed surface layer to 46% at 200 m depth. We suggest that the undersaturations result from the entrainment of CH4‐depleted warm deep water into the surface layer and from seasonal ice cover restricting air‐sea gas exchange in winter. South of Drake Passage, a shelf break frontal zone marked a distinct transition to coastal waters that were typically supersaturated with CH4. The highest CH4 concentrations were associated with the shelf waters surrounding Bransfield Strait and the South Shetland Islands. In the deeper waters of central Bransfield Strait, CH4 concentrations tended to decrease to near‐saturation values. The distribution of CH4 appears to be largely controlled by mixing, coupled with the addition of CH4 to waters flowing over the shallow shelves in the region. The calculated fluxes of CH4 across the air‐sea interface varied from −0.77 to + 5.86 μmol m−2 d−1, where negative fluxes indicate a net uptake of CH4 into the surface ocean and positive fluxes indicate a net release of oceanic CH4 to the atmosphere. The average net flux in the Drake Passage was −0.35 μmol m−2 d−1, and the average flux out of coastal waters to the atmosphere was +0.69 μmol m−2 d−1. On the basis of our results the net air‐sea CH4 fluxes in the Southern Ocean out to the edge of the seasonal ice zone are small and should not significantly alter current estimates of the oceanic source of CH4 to the atmosphere.
Evidence for enhanced boundary mixing in the Santa Monica Basin
Authors: Ledwell James R. , Hickey Barbara M.
Published: February 2004

Transients in the heat content in Santa Monica and San Pedro Basins imply a basin‐wide diapycnal eddy diffusivity greater than 1 cm2/s. This is significantly larger than the value for SF6 of 0.25±0.08 cm2/s determined for the interior of Santa Monica Basin for September 1985 to February 1986 by Ledwell and Watson (1991). However, the exodus of SF6 from Santa Monica Basin after February 1986, by which time the tracer had mixed to the boundaries of the basin, was fast enough to be consistent with a greatly enhanced vertical flux. Since the kinetic energy in the basin had not changed significantly, it is unlikely that a temporal increase in forcing resulted in enhanced fluxes in the interior of the basin. The most likely interpretation is that diapycnal fluxes in the basin are dominated by processes in the boundary regions. Temperature and SF6 profiles from near the edges of the basin do not give conclusive evidence either for or against such enhanced mixing.
Diapycnal mixing in western boundary currents
Authors: Pelegrí J. L. , Csanady G. T.
Published: February 2004

In a previous study (Pelegrí and Csanady, 1991) we found upward entrainment and two‐way exchange between the upper thermocline and surface layers of the Gulf Stream. Here we use isopycnic coordinates to analyze the mechanism of diapycnal shear‐induced mixing in western boundary currents. A hydrographic section across the Gulf Stream off the Mid‐Atlantic Bight is used for a case study. It shows that the upper thermocline layers of the Gulf Stream are characterized by relatively small (16 m averaged) gradient Richardson numbers, Ri. Analysis of the Jacobian, J = ∂z/∂p (with z the depth and p the potential density), total vorticity and potential vorticity fields shows that this region has low Jacobian values associated with high values of potential vorticity, as required theoretically for the case of dominant diapycnal mixing. The density tendency, wp = Dp/Dt, is calculated from the vertical gradient of the Reynolds density flux, the latter parameterized as density eddy diffusivity divided by the Jacobian. We use Munk and Anderson's (1948) expression for the eddy diffusivity coefficient, modified to conform to data of Ueda et al. (1981), maintaining a Ri−3/2 dependence for large Ri. From this we obtain an explicit expression for wp in terms of the Ri and J distributions. The distributions of wp, its diapycnal gradient, ∂wp/∂p, and the diapycnal velocity, wd = Jwp, are then calculated. Regions of high diapycnal convergence (large negative ∂wp/∂p values) coincide with the location of anomalously low Jacobians, in agreement with the idea that diapycnal mixing is the consequence of anomalies produced during frontogenesis in some phase of the meanders. We suggest that this process, taking place intermittently in space and time, is responsible for two‐way exchange in western boundary currents.
Upper layer temperature structure of the western tropical Atlantic
Authors: Molinari Robert L. , Johns Elizabeth
Published: February 2004

Mean monthly topographies of the 20°C and 10°C isothermal surfaces are used to describe the vertical displacements of the upper and lower thermocline in the western tropical Atlantic. The isotherm topographies are generated from expendable bathythermograph data collected between 1966 and 1993. The topographies confirm, and extend closer to the coast, earlier findings that demonstrate large spatial and temporal variability in the region. For example, the ridge and trough systems observed previously in the interior are shown, and their extension to the western boundary is described. In particular, it is shown that the ridge associated with the North Equatorial Countercurrent (NECC) extends from the interior northwestward along the western boundary, reaching farther north along the boundary in the upper thermocline than in the lower thermocline. South of the equator the northwestern corner of the countercurrent trough is apparent on the lower surface but not on the upper. The annual and semiannual harmonics of the vertical isotherm displacements account on the average for about 60% of total variance on both surfaces. The horizontal structure of the first harmonic amplitude is similar for both surfaces, showing maximum amplitude along the axis of the NECC ridge. Minimum amplitudes are observed to the north along the axis of the countercurrent trough. These distributions are similar to the pattern of the first‐harmonic amplitude of the wind stress curl, supporting earlier studies of curl forcing of near‐surface current features.
Near‐bottom sediment response to combined wave‐current conditions, Mobile Bay, Gulf of Mexico
Authors: Bedford Keith W. , Lee Jongkook
Published: February 2004

A two‐hour length of near‐bottom flow and sediment concentration data was collected in 6.05 m of water in the Gulf of Mexico near Mobile Bay, Alabama, United States. The objective of this article is to present a comparison of the measured near‐bed sediment concentration profiles with those predicted by the combined wave‐current model of Glenn and Grant (1987). Acoustically sampled profiles of concentration were measured in the 90, 1.16‐cm tall, bins composing the sampling volume. Measured currents were progressing toward the east and steadily increased to a value of 23 cm s−1 at the end of the deployment. Peak wave periods were 5.8s, with significant wave heights of 80 cm and decreasing. The resulting profile comparisons, assessed by a reliability index, showed poor results in the first two 10‐min frames and discouraging discrepancies for all frames above 10 cm above bottom. The near‐bottom calculations steadily improved as time proceeded. A flux analysis revealed that the major discrepancies occurred when the model was making predictions for observed data which resulted from flow physics not included in the model. The sensitivity study revealed that the predictions were especially susceptible to small changes in the grain size distribution but not so for the resuspension coefficient γ0, which has been frequently examined in other studies. (Acoustics, resuspension, fluxes, model.)
The influence of domain size on the response characteristics of a hurricane storm surge model
Authors: Blain C. A. , Westerink J. J. , Luettich R. A.
Published: February 2004

The influence of domain size on boundary condition specification and on computed storm surge response is investigated. Storm surge response along the Florida shelf in the Gulf of Mexico due to Hurricane Kate is examined over three domains using two different open ocean boundary forcing functions, a still water (or zero elevation) condition and an inverted barometer condition which accounts for the atmospheric pressure component of the meteorological forcing. The first domain is relatively small and is situated primarily on the continental shelf in the region of intense storm surge generation. A second domain includes the entire Gulf of Mexico basin. The final domain covers the Gulf of Mexico, contiguous basins, and extends out into the deep Atlantic Ocean. The computed storm surge response indicates that the small domain is inadequate, since cross‐shelf boundaries are in regions of significant storm surge generation where surge and therefore boundary conditions are not known a priori. Also, the behavior of resonant modes that are physically excited within the Gulf of Mexico due to the passage of the hurricane is unknown at the boundaries of this small domain. The domain that includes the entire Gulf of Mexico captures the primary storm surge well but may not correctly model resonant modes. In general, these resonant modes are difficult to accurately set up by boundary condition specification, since they may be dependent on interactions between the Gulf and contiguous basins. The primary storm surge response as well as resonant modes excited by the storm are best represented using a domain which encompasses the western North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. This domain with deep Atlantic Ocean boundaries facilitates simple boundary condition specification and minimizes the influence of boundary conditions on storm surge generation in coastal regions. Basin resonant modes and basin to basin interactions are also captured.