bluuebunny/crossref_metadata_2025 huggingface dataset:
Active zone size versus activity: A study of different seismicity patterns in the context of the prediction algorithm M8
Authors: Kossobokov V. G. , Carlson J. M.
Published: February 2004

A new precursory measure called active zone size (AZS), based on the development of spatial correlations, was recently introduced in the context of predicting large events in dynamical models of faults. The new measure counts the fraction of the total seismic area which has exhibited earthquakes recently. In each of the models tested, AZS was found to be more strongly correlated with an increase in the likelihood of a large event than the more conventional measure activity (N), which is defined to be the total number of earthquakes. In this paper we report the first application of AZS to seismicity catalogs. We compare measurements of AZS and N both individually and in the context of the intermediate‐term earthquake prediction algorithm M8 applied here specifically to data from the western United States. To test the performance we replace functions in M8 which are based on N with similar functions based on AZS. When this is done, at least on this limited data set, the performance of MS is somewhat improved: false alarm time is reduced, and the Landers earthquake is successfully predicted.
The crustal structure of the Rockall Trough: Differential stretching without underplating
Authors: Hauser Franz , O'Reilly Brian M. , Jacob A. W. Brian , Shannon Pat M. , Makris Jannis , Vogt Ulrike
Published: February 2004

The crustal structure along the axis of the Rockall Trough, in the North Atlantic, has been studied along a 600‐km refraction/wide‐angle reflection transect, containing three lines each 200 to 250 km long, using explosives and ocean bottom seismometers. One‐dimensional inversions of each section were made using the τ – p method and forward modeling of the observed travel times. In the next stage, travel times and amplitudes were modeled using ray tracing techniques through two‐dimensional heterogeneous structures. The results indicate that there are three sedimentary layers with velocities ranging from 2 km/s to 4.5 km/s. The whole sedimentary section is up to 6 km thick and interpreted as late Paleozoic to Tertiary in age. A two‐layer continental crust, 5 to 7 km thick, occurs along the length of the profile. The upper crust (6.0–6.3 km/s), is circa 2 km thick and the lower crust (6.6–6.9 km/s), is circa 3 km thick. A Moho transition zone, approximately 1 km thick, lies at the base of the crust. Velocities in this transition zone increase from 6.9 km/s up to 7.8 km/s along the profile. The underlying upper mantle has a laterally variable velocity between 7.6 and 7.8 km/s. Unstretched crust onshore in Ireland comprises a three‐layered crust, with each layer approximately 10 km thick, and a Moho transition zone, which is about 3 km thick. The two upper layers in the onshore region are interpreted as corresponding to the upper crust in the Rockall Trough and indicate a stretching factor (β) of 8–10. The velocity pattern in the lower crust in the Rockall Trough and under Ireland are similar, suggesting significantly less stretching (β = 2 ‐ 3). The differential stretching model is supported by the presence of the Moho transition zone which is stretched by a similar factor to the lower crust. The bulk stretching factor for the crust as a whole is in the range of 4–6. If this represents the lithospheric stretching factor, significant underplating would be expected. However, if the stretching factor for the lower crust in the differential stretching model is more representative of overall lithospheric stretching, little or no underplating is predicted. The velocity patterns observed in the Rockall Trough indicate the absence of any significant underplating at the base of the crust, such as that observed at the continental margin west of the Hatton Basin.
Uplift of the Kenai Peninsula, Alaska, since the 1964 Prince William Sound earthquake
Authors: Cohen Steven , Holdahl Sandford , Caprette Douglas , Hilla Stephen , Safford Robert , Schultz Donald
Published: February 2004

Using Global Positioning System (GPS) receivers, we reoccupied several leveling benchmarks on the Kenai Peninsula of Alaska which had been surveyed by conventional leveling immediately following the March 27, 1964, Prince William Sound earthquake (Mw = 9.3). By combining the two sets of measurements with a new, high‐resolution model of the geoid in the region, we were able to determine the cumulative 1993–1964 postseismic vertical displacement. We find uplift at all of our benchmarks, relative to Seward, Alaska, a point that is stable according to tide gauge data. The maximum uplift of about 1 m occurs near the middle of the peninsula. The region of maximum uplift appears to be shifted northwest relative to the point of maximum coseismic subsidence. If we use tide gauge data at Nikishka and Seward to constrain the vertical motion, then the observed uplift has a trenchward tilt (down to the southeast) as well as an arching component. To explain the observations, we use creep‐at‐depth models. Most acceptable models require a fault slip of about 2.75 m, although this result is not unique. If the slip has been continuous since the 1964 earthquake, then the average slip rate is nearly 100 mm/yr, twice the plate convergence rate. Comparing the net uplift achieved in 29 years with that observed over 11 years in an adjacent region southeast of Anchorage, Alaska, we conclude that the rate of uplift is decreasing. A further decrease in the uplift rate is expected as the 29‐year averaged displacement rate is about twice the plate convergence rate and therefore cannot be sustained over the entire earthquake cycle.
Kinking of shallow dip‐slip zones
Authors: Jeyakumaran Murugesu
Published: February 2004

A two‐dimensional crack model is used to investigate the effect of the free surface on the propagation of kink cracks that develop at the tip of dip‐slip shear zones. The slip zones undergo frictional sliding under tectonic loading with the presence of the gravitational force. Because the numerical scheme incorporates the exact asymptotic form of slip near the end of the shear zone, results for the stress intensity factors are accurate. The calculations show that crack opening occurs along a portion of the planar slip zone near the kink as well as at the kink itself. For a typical shallow reverse dip‐slip zone, the mode I stress intensity factor at the tip of the kink can be 60% greater than that for a full space. The effect of free surface is negligible if the ratio of the depth of the shallow end to the length of the slip zone is greater than two. The influence of the free surface makes it possible for the kink crack to grow to much greater length. Under compressive tectonic loading, the kink that develops at the shallow end of a shear zone does not move toward the free surface; instead, the kink curves away from the free surface, extends stably parallel to the free surface, and is then arrested at a finite length. The calculations suggest that rupture termination of a reverse dip‐slip earthquake may be due to extensive tensile cracking near the free surface and that the growth of long extensional fractures parallel to the free surface dissipate sufficient energy to stop the fault rupture. In addition, the present calculations demonstrate that the orientation and the length of observed vein sets are consistent with the extensional fractures developed near the tip of the dip‐slip zone.
The transport properties of networks of cracks and pores
Authors: Bernabé Yves
Published: February 2004

The heterogeneous nature of the pore space of rocks was numerically simulated by varying the proportions of empty and filled bonds and nodes on a square lattice. A filled bond could contain a tube or a crack or both, the dimensions of which were randomly chosen. For each of 100 network realizations, the permeability, k, electrical formation factor, F, connected porosity, ϕ, specific surface area, A, and their pressure dependences were calculated. This large simulated database was then used for testing the validity of several models from the literature. The main result of this work is that permeability was well predicted by the relation k = R2/(8F), where R is an appropriate length scale. This relation was better satisfied by using the Katz‐Thompson (KT) critical radius for R rather than the Schwartz‐Sen‐Johnson (SSJ) hydraulic radius. In particular, the SSJ model was found to be more sensitive to the pore shape heterogeneity than was the KT model. It yielded estimates of k of equal (if not better) quality than did the KT model when only one type of channel was considered (cracks or tubes), but more scattered results were obtained when it was applied to mixtures of cracks and tubes.
Connection between igneous activity and extension in the central Mojave metamorphic core complex, California
Authors: Walker J. Douglas , Fletcher John M. , Fillmore Robert P. , Martin Mark W. , Taylor Wanda J. , Glazner Allen F. , Bartley John M.
Published: February 2004

The development of metamorphic core complexes and associated low‐angle detachment faults commonly is intimately associated with synextensional igneous activity. In most areas studied to date, the relation of magmatism to extension is obscured by imprecise dating and by the overprint of later tectonic events. We present data from the early Miocene central Mojave metamorphic core complex (CMMCC) which indicate that extension was accompanied by igneous activity, as reflected by prekinematic, synkinematic, and postkinematic plutons and coeval volcanic rocks deposited in the associated extensional basins. The principal intrusion is an early Miocene granite pluton exposed in outcrops across an area greater than 400 km2. Dikes adjacent to the pluton are common in the Mitchel Range, at The Buttes, and at Fremont Peak. The overall orientation of the pluton and associated dikes is west‐northwest, roughly perpendicular to the extension direction. Results of U‐Pb analyses on zircon from two pluton and two dike samples yield ages of 20 to 23 Ma. Two other dike samples yield inconclusive results. Synextensional basins formed by detachment faulting during the core complex development. Rocks in these basins compose the Jackhammer and Pickhandle formations and filled an elongate, NW trending trough more than 50 km long. The 40Ar/39Ar ages for tuff beds are as old as 23.8±0.3 Ma near the base of the lower Pickhandle Formation and as young as 21.3±0.5 Ma in the uppermost lower Pickhandle. Hence volcanism and plutonism are coeval. The diversity of intrusive relations relative to the timing and development of the mylonitic fabric in the CMMCC precludes any simple cause‐and‐effect relationship between magmatism and extensional deformation. Rather, magmatism and extension may have been localized at a releasing bend in a transfer‐fault system which links extension in the CMMCC with extension in the Colorado River area to the east.
Relation of peralkaline magmatism to heterogeneous extension during the middle Miocene, southeastern Nevada
Authors: Scott Robert B. , Unruh Daniel M. , Snee Lawrence W. , Harding Anne E. , Nealey L. David , Blank H. Richard , Budahn James R. , Mehnert Harald H.
Published: February 2004

Volcanism migrated southward in the northern Basin and Range province in the Oligocene and early Miocene to produce voluminous calcalkaline silicic ash flow tuffs. Alkaline volcanism became dominant by middle Miocene (17–14 Ma) as smaller volumes of rhyolite‐trachyte‐basalt suites were erupted from the relatively small Kane Springs Wash caldera complex including the Narrow Canyon, Boulder Canyon, and Kane Springs Wash calderas in southeastern Nevada. Only minor extension affected the Kane Wash area before the end of calcalkaline activity, but extension expressed by rate of progressive stratal tilt peaked (15–13.5 Ma) with peralkaline magmatism (14.7–14.4 Ma). Variations in distribution, degree, style, and timing of deformation demonstrate heterogeneous extension in the Kane Wash area. Only minor extension and tilting persisted post‐middle Miocene (<12 Ma). All major eruptive sources overlap domains of rapid extension. Most of the eruptive volumes from the two oldest calderas of the complex apparently pooled within their calderas, creating outflow deficits. Denudation faulting associated with magmatic tumescence may have followed preexisting active extensional fault systems to unload magma chambers, thus triggering eruptions into structural depressions. Evolution of alkaline magmas is demonstrated by progressive increases in peralkalinity and high field strength elements such as Zr, Y, and Nb. Nd, Pb, and Sr isotopic compositions provide evidence that significantly less crustal interaction affected middle Miocene peralkaline magmas than pre‐middle Miocene calcalkaline magmas. εNd values are −5 to −7 for peralkaline magmas and −7 to −11 for calcalkaline magmas; 208Pb/204Pb ratios are 38.2–38.6 for peralkaline magmas and 38.5–38.9 for calcalkaline magmas. Regional cooling, short duration of magmatism, small volumes of magma, and local extension caused less crustal interaction in peralkaline Kane Wash magmas than in earlier magmas. North of the Kane Wash area, older more voluminous calcalkaline magmas intruded hotter crust for a longer period and thus interacted with the crust to a greater degree in spite of synvolcanic extension.
Relation of deformation and multiple intrusion in the Death Valley extended region, California, with implications for magma entrapment mechanism
Authors: Holm Daniel K.
Published: February 2004

The crystalline core of the Black Mountains crustal section, Death Valley, California, exposes a tremendous volume of Miocene plutonic rock intruded at a depth of 10–13 km. Few plutons were intruded above ∼10 km prior to unroofing by tectonic denudation. In the eastern part of the core, a brittle detachment fault separates predominantly Miocene hanging wall strata from the midcrustal, Miocene (11.6 and ∼8.7 Ma), plutonic complex and Early Proterozoic basement. In the west, mylonitic lineations and foliations, locally well‐developed within the 11.6 Ma intermediate‐mafic Willow Spring pluton, are cut by dikes which emanate from an ∼8.7 Ma silicic plutonic complex. The younger silicic complex throughout the crystalline core exhibits few ductile deformational structures. Published thermal and barometric studies from both plutonic bodies indicate similar midcrustal (10–13 km) emplacement depths at ambient temperatures just above 300°C. The significant difference in densities of these magmas argues against a density control for magma entrapment. Also, the country rock above and below the plutonic complex contains no apparent differences (from field observations) that would suggest a change in density. The depth of entrapment corresponds well with the expected depth for the crustal strength maximum determined from laboratory experiments. The similar emplacement depths but contrasting styles of deformation of the two plutonic bodies further suggests that entrapment may have been controlled by a high‐strength barrier represented by the brittle‐ductile transition. Late hypabyssal intrusions and associated volcanism are linked to diachronous rapid unroofing of the range block; all show a northwest progression paralleling the regional extension direction. Thus when migration of magma through the high‐strength barrier did occur, it was apparently related to increased strain rates which allowed magma ascent by fracture exploitation. Rheological stratification of the crust may have played an important, if not major, role in trapping magmas in the middle crust in this area.
Measured and calculated elastic wave speeds in partially equilibrated mafic granulite xenoliths: Implications for the properties of an underplated lower continental crust
Authors: Rudnick Roberta L. , Jackson Ian
Published: February 2004

Ultrasonic compressional wave velocities measured at 1.0 GPa and room temperature are compared with calculated velocities (based on single‐crystal data and modal mineralogy) for a suite of mafic granulite xenoliths from the Chudleigh volcanic province, north Queensland, Australia. The xenoliths have nearly constant major element compositions but widely variable modal mineralogy, reflecting recrystallization under variable pressure‐temperature conditions at depth in the continental crust (20–45 km). They thus provide an excellent opportunity to investigate velocity variation with depth in a mafic lower crust. Measured P wave velocities, corrected for the decompression‐induced breakdown of garnet, range from 6.9 to 7.6 km/sec and correlate with derivation depth. These velocities are 5–12% lower than the calculated velocities (7.5–8.0 km/sec), apparently as a result of grain boundary alteration as well as irreversible changes that occurred in the xenoliths during rapid decompression. Calculated P wave velocities are similar to those estimated by Furlong and Fountain (1986) and Sobolev and Babeyko (1989) for mafic granulites formed through basaltic underplating of the continental crust. Depending upon in situ temperature, P wave velocities in the deepest samples may be interpreted as crustal (e.g., 7.3–7.6 km/sec, if heat flow is high) or mantle (7.7–7.8 km/sec, in areas of low heat flow). The range of velocities in the xenolith suite is larger than predicted for a fully equilibrated underplated basaltic layer, highlighting the importance of kinetic effects in determining the ultimate velocity profile of magmatically underplated crust. Comparison of our results with seismic profiles illustrates that the lower crust rarely reaches such high velocities, suggesting quartz‐bearing rocks (country rocks?) are present within magmatically underplated layers of the deep crust.
Horizontal surface deformation due to dike emplacement in an elastic‐gravitational layer overlying a viscoelastic‐gravitational half‐space
Authors: Hofton M. A. , Rundle J. B. , Foulger G. R.
Published: February 2004

We extend a technique previously used to model surface displacements resulting from thrust faulting in an elastic‐gravitational layer over a viscoelastic‐gravitational half‐space to the case of dike emplacement. The method involves the calculation of the Green's functions for a dike point source contained in an elastic‐gravitational layer over an elastic‐gravitational half‐space. The correspondence principle is then applied to introduce time dependence. The resultant Green's functions are integrated over the source region to obtain the near‐field displacements. Several example calculations are presented involving 90°, 60°, and 30° dipping dikes, extending completely and partially through the elastic layer. We also illustrate the time dependent deformation due to buried dikes. Dikes extending completely through the elastic layer produce a larger‐amplitude long‐wavelength component than those extending partially through the elastic layer. Inflexion points are seen in the dike‐normal horizontal deformation profiles when the base of the dike intersects the top of the half‐space, providing a means of differentiating between vertical surface dikes extending completely and partially through the elastic layer. All results show that the use of a viscoelastic half‐space underlying an elastic layer introduces a long‐wavelength component into the deformation field that cannot be predicted by elastic half‐space models.
The 40Ar/39Ar ages and tectonic setting of the middle Eocene northeast Nevada volcanic field
Authors: Brooks William E. , Thorman Charles H. , Snee Lawrence W.
Published: February 2004

Widespread middle to late Eocene calc‐alkalic volcanism, which formed the Northeast Nevada Volcanic Field, marks the earliest Tertiary volcanism in the northern Basin and Range. The central part of this major field in northeast Nevada and adjacent Utah is herein defined by 23 40Ar/39Ar ages that range from 42.6 to 39.0 Ma, rock chemistry from 12 localities, stratigraphic position of the volcanic rocks above a regional middle Eocene unconformity, volcanic setting, and lithology. The type area is at Nanny Creek, in the northern Pequop Mountains, Nevada, where rhyolite ash flow tuffs are overlain by a thick section of intercalated andesitic to dacitic flows and flow breccias and rhyolite ash flow tuffs. The intermediate composition rocks are locally derived throughout the volcanic field, whereas the sources for rhyolite ash flow tuffs are unknown. The uniform and widespread occurrence of the andesitic and dacitic flows and flow breccias strongly suggests that the upper crust was perforated by intermediate composition magma across the entire region. In the central part of the field the middle Eocene volcanic rocks rest with depositional angular discordance on deformed middle Paleozoic to Triassic strata; ostracode‐bearing limestone, probably of early Eocene age, is locally present below the volcanic rocks. In the western and southeastern parts of the field these middle Eocene volcanic rocks rest with depositional angular discordance on lower Eocene lacustrine strata of the Elko and White Sage Formations, respectively. This angular discordance documents a middle Eocene deformational event previously unrecognized in the region.
Mean zonal momentum balance in the upper and central equatorial Pacific Ocean
Authors: Johnson Eric S. , Luther Douglas S.
Published: February 2004

We examine the mean zonal momentum balance in the tropical mid‐Pacific using a year of acoustic Doppler current profiler velocities and conductivity‐temperature‐depth profiler densities from the Hawaii‐to‐Tahiti Shuttle Experiment. All significant contributions from the mean, annual cycle, and higher‐frequency flow fields are determined with the exception of the vertical stresses. We find that even neglecting vertical stresses, the zonal momentum equation is in rough balance at 90–117‐m depth at all latitudes from 4°S to 10°N. While the formal error bars are large, this rough balance is reproducible over four to five independent latitudes and so is probably real. The balance at 90‐m depth is geostrophic to within 5° of the equator. Closer to the equator, meridional mean convergence and meridional eddy stresses contribute important forces to balance the mean pressure gradient. Nearer the surface, the zonal momentum equation is dominated by eastward pressure gradients near the equator and eastward Coriolis forces from a strong, northward Ekman flow poleward of 2°N. In the vertical integral these forces roughly balance the surface wind stress; thus vertical stresses suffice to close our momentum budget. We conclude that on average vertical stresses arising from the wind forcing do not penetrate deeper than 90 m into the tropical ocean. This contradicts an earlier study of the equatorial zonal momentum budget but is consistent with turbulent dissipation measurements on the equator. Previous findings of stronger, deeper dissipation on the equator are probably due to the stronger, deeper mean shear there rather than to a locally altered stress profile. Vertical turbulent viscosities derived from our observations agree with previous observations on the equator but contradict the conventional, Richardson number parameterization off the equator.
Hydrographic section aeross the Kuroshio near 35°N, 143°E
Authors: Teague W. J. , Shiller A. M. , Hallock Z. R.
Published: February 2004

A closely spaced conductivity‐temperature‐depth/hydrographic section was conducted off the east coast of Japan in July 1992. The southeastward section crossed the Japan Trench and the Kuroshio in the vicinity of the Kashima 1 seamount. Vertical sections of temperature, salinity, density, oxygen, and nutrients are discussed in conjunction with the movement and interleaving of water masses. Complicated vertical and horizontal mixings of water masses are inferred from the temperature and salinity relationships. Mixing processes are patchy and not continuous beneath the front. Warm, salty water found beneath the Kuroshio may result from upward mixing of water from intermediate depths. The main axis of the Kuroshio, indicated by the 14°C isotherm at 200 m, is at 35.7°N, 142.6°E, about 20 km from the north wall surface thermal front. Geostrophic speeds exceed 170 cm s−1 at the surface; volume transport through the section is 81×106 m3 s−1.
Radar surface signatures for the two‐dimensional tidal circulation over Phelps Bank, Nantucket shoals: A comparison between theory and experiment
Authors: Cooper A. L. , Chubb S. R. , Askari F. , Valenzuela G. R. , Bennett J. R. , Keller W. C.
Published: February 2004

A comparison is made between real aperture radar (RAR) measurements and simulations (based on modeled tidal currents) of radar cross section over a complicated tidal basin (in the vicinity of the Phelps Bank region of the Nantucket shoals) in order to more fully understand the origin of radar signatures that are observed at the ocean surface as a consequence of variations in the topography of the ocean bottom. The Phelps Bank region was mapped under two extreme wind speed conditions: in high winds, in excess of 15 m/s, and in low winds, of the order of 2–3 m/s. For the light‐wind case the measured radar cross section over the west side of the Phelps Bank was enhanced by as much as 20 dB relative to the clutter background. For the high‐wind case, no discernible bathymetric signature was found in the highclutter background. Numerical results for the two‐dimensional M2 (semidiurnal) tidal currents over the Phelps Bank (Greenberg et al., 1989), with ⅛ × ⅛ min of arc resolution, are used as input to the surface signature models: the Alpers and Hennings (1984) first‐order Bragg relaxation model; a generalized form of this relaxation model (in which wind directional effects are incorporated in an approximate manner); and the full‐spectrum model of Lyzenga and Bennett (1988). Comparisons between the models (which do not include wave breaking) and an extreme case of 2–3 m/s winds (where strong wave breaking could become important) reveal that although the models predict correlation between variations in bottom topography and surface signature, they significantly underpredict the magnitude of the observed effect. The model calculations also are very sensitive at low (<2 m/s) wind speeds to the functional form that is assumed for the wind‐wave forcing in the wave action equation. Prior visual observations and measurements of wave spectra (and wave shoaling) in the vicinity of Phelps Bank strongly suggest that the deficiencies of the modeled results that occur explicitly at light winds are due to wave breaking. A number of additional experiments and measurements are suggested for more normal environmental conditions for further theory assessments.
Deep water nutrient and oxygen gradients in a modern coastal upwelling zone and their paleoceanographic implications
Authors: Jewell Paul W.
Published: February 2004

Dissolved phosphate and oxygen data from the waters at >300 m depth beneath the Peru upwelling system imitate observed onshore/offshore surface productivity data. Positive onshore phosphate gradients and negative onshore oxygen gradients are greater in deeper water (up to 1000 m), reflecting the decreasing degree of horizontal mixing at depth. Mass balance reconstructions of surface productivity, nutrient remineralization, and onshore‐offshore chemical gradients suggest that the lack of wide continental shelves during glacial climates would lead to more pronounced dissolved nutrient and oxygen gradients in the deep water adjacent to continental margins. Horizontal δ13C gradients in sediments beneath highly productive coastal upwelling zones may have been as high as 0.5–1.0‰ per 100 km, suggesting caution in the interpretation of benthic δ13C samples adjacent to continental margins during low sea level stands.
Modes of ocean variability in the tropical Pacific as derived from Geosat altimetry
Authors: Zou Jiansheng , Latif Mojib
Published: February 2004

Satellite‐derived (Geosat altimetry) sea surface height anomalies for the period November 1986 to September 1989 were investigated in order to extract the dominant modes of climate variability in the tropical Pacific. We applied the technique of principal oscillation patterns and computed associated wind stress patterns for each mode. Four modes were identified. The first mode has a time scale of about 3 months and can be identified with the first baroclinic equatorial Kelvin wave mode, which is excited by intraseasonal wind variations over the western equatorial Pacific. The second mode has a time scale of about 6 months and describes the semiannual cycle in the tropical Pacific sea level. Equatorial wave dynamics appears to be crucial for this mode also. The third mode is the annual cycle which shows evidence of off‐equatorial Rossby wave propagation. The fourth mode is associated with the El Niño/Southern Oscillation (ENSO) phenomenon. The ENSO mode is found to be consistent with the “delayed action oscillator” scenario.
The number and size of jet drops produced by air bubbles bursting on a fresh water surface
Authors: Spiel Donald E.
Published: February 2004

The size distributions of the jet drops produced by individual air bubbles bursting on a fresh water surface are presented. The bubbles studied ranged in size from 349 to 1479 μm radius. The probability that a bubble of radius rb produces at least n drops, p(rb, n), is given for n up to 7. The underwater sound made by collapsing bubbles is discussed briefly.
The equatorial Kelvin wave in finite difference models
Authors: Ng Max K. F. , Hsieh William W.
Published: February 2004

Coarse resolution ocean models tend to poorly resolve many smaller‐scale phenomena, including the equatorial currents narrowly confined around the equator. We study the free equatorial Kelvin wave in inviscid finite difference models using the Arakawa A, B, C, and E grids. Exact analytic solutions with meridional velocity ν = 0 are found on the A, C, and E grids. As the assumption ν = 0 is not valid on the B grid, the solution is obtained numerically by a “shooting” method. In all cases, the wave remains nondispersive, and the phase speed is unchanged from that in the continuum except in the B grid, where it decreases with worsening resolution. The mean zonal heat transport by the Kelvin wave during an El Niño is compared on the various grids. In terms of the currents and sea level displacements, the B grid best models the equatorial Kelvin wave under coarse resolution, though in terms of zonal heat transport and phase velocity, the C grid appears superior. The A and E grids appear to have the most trouble. Our theoretical predictions are checked experimentally by generating equatorial Kelvin waves in linear shallow‐water equation models on the various grids. Additional effects of Rayleigh damping and Newtonian cooling are studied in the appendix.
Application of a transient reduced gravity plume model to the Denmark Strait Overflow
Authors: Jungclaus Johann H. , Backhaus Jan O.
Published: February 2004

A hydrostatic, reduced gravity, two‐dimensional primitive equation numerical model for the simulation of transient bottom arrested gravity plumes was developed by adopting an algorithm from a wadden sea model (a model that simulates the wetting and drying of mud flats, sand banks, etc.) with a movable lateral boundary. The vertically integrated nonlinear rotational model accounts for the dissipation of momentum due to bottom friction and for the entrainment of water mass properties from a spatially structured but stagnant ambient water body. It predicts the temporal and spatial evolution of the flow field and the water mass modification within a gravity plume that descends on an arbitrary topography. Previous model investigations considered a laterally integrated streamtube, whereas our model resolves the plume horizontally. In the streamtube approach it was assumed that the plume descends in a stationary, almost geostrophic balance modified only by bottom drag and entrainment. The results of this model demonstrate that this balance may be disturbed whenever the plume encounters topographic disturbances, which, for the initial intruding phase in particular, accounts for its highly transient character. The model results are in good agreement with theoretical and laboratory investigations and with observations, as is demonstrated by some basic test experiments. An application to the overflow of dense water through the Denmark Strait is presented.
Spin‐up and breakdown of source‐driven deep North Atlantic flow over realistic bottom topography
Authors: Karcher Michael , Lippert Angelika
Published: February 2004

In this study we examine the time dependent behaviour of the deep North Atlantic dynamics. We use a 1½‐layer, nonlinear, shallow water model with realistic topography and high‐spatial resolution. It is driven by an inflow of deep water from a northern source and by parameterized density exchange processes at the interface. The evolving circulation pattern is complex. The Mid‐Atlantic Ridge with the Charlie Gibbs Fracture Zone has a fundamental influence on the large‐scale flow field. A reduction of the inflow causes a temporary reversal of the interior currents. The perturbation travels along the geostrophic contours. The response times of the currents for the spin‐up and breakdown experiments are of the order of years. They differ for different areas of the North Atlantic Ocean. The area which is affected last by the perturbation is the northwesternmost interior corner, which is close to the position of the source.
Low‐frequency variability in Delaware's inland bays
Authors: Wong Kuo‐Chuin , Lu Xin
Published: February 2004

The low‐frequency variability in Delaware's inland bays is examined based on observational data derived from a combination of tide gauges, moored current meters, and free‐floating Woodhead drifters. The currents and surface elevations in the bays exhibit considerable fluctuations at subtidal timescales of 2–7 days. These subtidal fluctuations are forced primarily by the remote atmospheric effects through the imposition of coastal sea level setup or setdown at the entrance of the bays. To first order, the current and sea level fluctuations in the interior of the system respond to coastal forcing in a pumping mode fashion. With a coastal setup (or setdown), water is pumped into (or out of) the system from all open boundaries. This pumping mode response produces vigorous subtidal current fluctuations (of the order of 25 cm/s) in the Indian River Inlet. In addition to the subtidal fluctuations, the data also indicate the presence of significant residual currents of the order of 10 cm/s in the Indian River Inlet. The residual currents have a pattern of surface outflow and bottom inflow that is consistent with the density‐induced gravitational circulation.
Multifractal representation of breaking waves on the ocean surface
Authors: Kerman Bryan R. , Dernier Lucie
Published: February 2004

It is established for the first time that the spatial distribution of breaking waves on the ocean surface is a multifractal process. This result is based on an analysis of airborne visible and near‐infrared imagery of the ocean surface under a limited range of wind speed and fetch. A detailed study of the optical spectra of the images and the cumulative probability structure of the prevalent background shows that the lower‐intensity reflective areas follow a Rayleigh probability distribution. By contrast, the higher‐intensity pixels associated with the scattered light from foam and breaking waves demonstrate scaling characteristics in both the optical spectra and the cumulative probability distributions. It is demonstrated that the degree to which the whitecaps are singularities on a dark background is described by a Lipschitz exponent α, which uniquely tags each breaking wave. This identification process, called “fractal” or “singularity filtering”, leads to a critical condition αc = 1 tentatively associated with the crossover from active entraining whitecaps to passively dissipating foam. The multifractal representation associated with the degree of singularity is simply a restatement that the imagery is composed of a continuum of sets, where each set consists of those breaking waves at a particular phase in their existence. The fractal spectrum of the image above a threshold is shown to be representable by a fractal generator. Physically, the fractal generator models the energy exchange in a breaking wave field as a flux of energy input from the atmosphere to the wave field cascaded over scales of the order of a kilometer to meters. If the energy flux is further parameterized in terms of the receiving area, an assumption similar to closure techniques used in classical turbulence models, the empirical results symmetrically span Phillips's basic arguments for the energy flux terms controlling a wind‐driven sea.
Concurrent measurements of the directional spectra of microseismic energy and surface gravity waves
Authors: Nye T. , Yamamoto T.
Published: February 2004

This paper presents concurrent measurements of surface wave directional spectra and double‐frequency, long‐wave microseisms. Long‐wave energy rapidly develops during periods of shifting winds which create bidirectional sea states. Theoretically, nonlinear sum interactions of opposing wavenumber vectors of approximately the same frequency create long‐wave energy at twice the frequency, which is only slightly attenuated in shallow water. Bidirectional sea states have been found using a buried ocean bottom seismometer measuring system from which the long wave energy has been measured at double frequencies. This system was incorporated into the Office of Naval Research sponsored Sources of Ambient Microseismic Ocean Noise (SAMSON) experiment for 3 months off the Army Corps of Engineers' Field Research Facility near Duck, North Carolina in the fall of 1990. Four working sensors produced directional spectra results from nearly 22 gigabytes of recorded data, which was collected 2 km offshore of the FRF under 12–13 m of water and approximately 1 m of sediment. Because of the insignificant attenuation the measured energy levels of the double frequency microseisms at the seafloor are of the same order of magnitude as the single‐frequency, surface wave energy induced seafloor motion. Various data sets were analyzed that confirmed Longuet‐Higgins' theory, which proposes that the propagation direction of double‐frequency microseisms occurs in the direction of the vector sum of the opposing single‐frequency seas.
Circulation of Prince William Sound, Alaska
Authors: Niebauer H. J. , Royer Thomas C. , Weingartner Thomas J.
Published: February 2004

The circulation of Prince William Sound, Alaska, is described using hydrographic (1974–1989), current meter (1977–1979), and acoustic Doppler current profiler (1986–1990) observations from both the sound and the adjacent Gulf of Alaska. Ancillary data include data for winds, freshwater runoff, and satellite‐tracked drifters. Prince William Sound is a small inland sea in that it is wide enough to have appreciable horizontal cyclonic circulation. It is also a fjord in that it has basin depths to 700 m but is stilled at 180 m to the open ocean. The general circulation pattern is defined by a portion of the westward flowing Alaska Coastal Current on the Gulf of Alaska shelf that enters Prince William Sound through Hinchinbrook Entrance and transits the sound from east to west before exiting through Montague Strait and rejoining the coastal current. However, there is much variability in this circulation, especially in the transport through Hinchinbrook Entrance. In addition, some of the water entering the sound becomes involved in the cyclonic circulation in the northern sound and so has a longer residence time. The circulation is strongly mediated by seasonal and interannual variations in winds and freshwater runoff as well as by local topography both inside and outside the sound. In winter, the strong cyclonic winds over the Gulf of Alaska cause coastal downwelling and strong flow in the upper layers into Prince William Sound through Hinchinbrook Entrance and out through Montague Strait. In summer, the downwelling ceases, allowing subsurface denser water to rise above the sill and flow into the sound through the bottom layers of Hinchinbrook Entrance. We conclude that the best transport data came from Montague Strait, from which we estimate that ∼40% of the volume of Prince William Sound is flushed in summer (May–September). This estimated volume rises to about 200% in winter (October–April).
Degradation of ocean signals in satellite altimetry due to orbit error removal processes
Authors: Wagner C. A. , Tai C. K.
Published: February 2004

The accuracy of recovering sea level changes with a satellite altimeter depends critically on the error in the satellite's ephemeris and how it is removed from the height data. We examine two global ocean data sets of sea level changes:(1) climatologic monthly hydrographic variations and (2) a 1‐year wind‐driven general circulation model (GCM). We then assess their degradation after orbit error removal is applied to the two data sets. The simulations use along track data sampling for about a year of the Geosat exact repeat mission (ERM). The simulated orbit error removals are made from differences of overlapping (or collinear) passes of the simulated sea heights. Radial orbit correction algorithms include along‐track polynomials of up to one half revolution in length and global sinusoids of from one half revolution to about 4 days long with a 1 cycle/revolution (cpr) fundamental period. Orbit error removal first degrades the ocean signal along track, usually in a mild way depending on the pass length and the seasonal differences between the collinear data sets. However, the resulting distortion of yearly time series at fixed locations is generally more severe, especially when the actual signal is weak and at higher latitudes. Average degradation of both climatologic and GCM time series at fixed locations is significant in all cases and more severe with local pass polynomials than with global sinusoids. Thus we find that in even the least destructive along‐track error removal process, the global error of the derived climatologic time series (defined as rms simulated‐orbit‐error/rms signal) is greater than 0.30. However, the correlation of time series, derived versus original, and their power ratios (rms derived/rms signal) is generally high (greater than 0.78 globally) for all methods, though certain locations result in negative correlation (reversal of phase). Degradation of the more variable GCM changes (with mesoscale eddies) is generally not as severe as in climatologic series. Thus the GCM time series error distortion with long arc 1‐cpr sinusoid orbit error removal was found to be only 0.20, while with full pass bias and tilts the error was 0.53. The corresponding global errors found for climate time series were 0.31 and 0.54. Similar results should be expected for any satellite mission using collinear or crossover altimetry spanning appreciable oceanographic change.
Seasonal and interannual variability of coastal upwelling along northwest Africa and Portugal from 1981 to 1991
Authors: Nykjær Leo , Van Camp Lieve
Published: February 2004

The seasonal and interannual variability of coastal upwelling in the northwest African upwelling area for the period from 1981 to 1991 has been investigated using remote sensing data of sea surface temperature (SST) and wind data. Approximately 3400 individual SST images have been generated from National Oceanographic and Atmospheric Administration Global Area Coverage data and resampled to form a coherent cloud‐free data set. From the satellite data set an SST upwelling index defined as the zonal temperature difference has been derived as a function of latitude and time. Wind data from the European Centre for Medium‐Range Weather Forecasts have been used to determine a second upwelling index calculated as the fraction of the Ekman transport perpendicular to the coast. The two independent indices have been compared with previously published data of seasonal variability and have been examined for interannual variabilities. In general the two upwelling indices demonstrate the known seasonal variability which exists south of 20°N and north of 25°N and the almost permanent upwelling between 20°N and 25°N. Off Portugal and Cape Ghir, there are temporal shifts between the maximum winds and the maximum zonal temperature differences of 2 and 3 months, respectively, while at most other locations the indices are temporally well correlated. Interannual variabilities are greater where upwelling is seasonal compared to those regions where upwelling is dominant throughout the year. The strongest interannual variabilities are found south of 20°N where upwelling was particularly weak during the period from the end of 1982 to the beginning of 1984, coinciding with the exceptionally strong 1982/1983 El Niño in the Pacific Ocean.
An improved model of the ocean surface wave vector spectrum and its effects on radar backscatter
Authors: Apel John R.
Published: February 2004

Images of the ocean surface taken with active microwave sensors often contain much information on near‐surface and subsurface processes. However, their interpretations may depend on a detailed understanding of the physics of electromagnetic scatter. In scattering theory, the surface hydrodynamics enters the equations via (1) the probability distribution function for either wave heights or slopes, and (2) the two‐dimensional wave height/slope autocovariance or its Fourier transform, the wave vector spectrum. This paper advances an improved model for the ocean surface wave vector spectrum based on recent work by M. A. Donelan, by M. L. Banner, and by B. Jähne and their collaborators. The model addresses the range of surface wavelengths from fully developed wind waves to the gravity‐capillary region. For gravity‐capillary waves, the spectral equation satisfactorily represents the observational data of Jähne et al. taken in tanks at large fetches, in the range from approximately 50 to 1500 rad/m to within the accuracy of the data. From the spectrum, the two‐dimensional autocovariance of the sea surface is computed and correlation lengths and curvatures obtained. When used with a modification of Holliday's formulation of microwave radar backscatter from a Gaussian sea, it quantitatively reproduces observational cross section data taken at vertical polarization from aircraft and spacecraft over the open ocean, with differences from the field data having a mean of −0.2 dB and a standard deviation of 1.7 dB, The range of parameters for which satisfactory fits are obtained includes: wind speeds from 1.5 to 24 m/s; frequencies from approximately 5.5 to 35 GHz; and incidence angles from 0° to greater than 60°. For horizontal polarization, the scattering calculations fail rather badly for larger incidence angles, as do all theories based on the Kirchhoff approximation. Additionally, in spite of the incorporation of an anisotropic angular distribution of wave energy, the observed azimuthal variation of radar scatter is not captured, indicating that the source of that variation lies elsewhere.