bluuebunny/crossref_metadata_2025 huggingface dataset:
A dynamic model for generating small‐scale heterogeneities in ocean floor basalts
Authors: Bideau Daniel , Hékinian Roger
Published: February 2004

Small‐scale distribution of basaltic compositions along the mid‐ocean ridge crests suggests that the magmatic processes are sensitive to spatial and temporal fluctuations related to proximity of ridge hot spots, changes in spreading rate, thermal boundary effects associated with ridge offsets, off‐axis volcanoes and ridge propagators, and variations in magma supply through time. A detailed sampling of the East Pacific Rise (EPR) axis, near 13°N, revealed that complex basalt compositional variations unrelated to morphology and structure occur both on the ridge axis and on nearby off‐axis seamounts. This small‐scale heterogeneity is attributed to successive magmatic cycles separated by periods of quiescence (amagmatic phases) where each cycle involves several melting stages of a composite mantle source, interrupted by extraction and a rapid migration of the melt toward the upper levels of the lithosphere. For a given composition of the mantle source, this process will produce primitive melts which are progressively depleted in incompatible elements. The final contrast in composition between the various extruded magmas depends on the opposing effects of mixing and fractionation during transport toward the ocean floor. A multistage melt extraction model for trace elements, based on nonmodal near‐fractional melting (1% increments) with three steps of accumulation (<2.5–5%) and extraction above the melting region, gives reliable results for less than 8–13% total melting of a spinel‐lherzolite (olivine 49–55%, orthopyroxene 25–29%, clinopyroxene 18–21%, spinel 1–2%). This partial melting model is only partly constrained on the basis of available chemical and physical data, as well as laboratory experiments, and it has several implications for the dynamics of the upper mantle and the lower crust which are not taken into account by present‐day physical models. The major consequences of this model are the introduction of discontinuities in the melting regime and the cyclicity of magma production.
Microcrack‐induced elastic wave anisotropy of brittle rocks
Authors: Sayers C. M. , Kachanov M.
Published: February 2004

The failure of brittle rocks during compression is preceded by the formation, growth, and coalescence of microcracks. Elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock. In general, these microcracks are not randomly oriented, and the rock displays an elastic anisotropy. The elastic anisotropy due to cracks can be expressed in terms of a second‐rank and fourth‐rank crack density tensor. For open cracks the contribution of the fourth‐rank crack density tensor to the elastic wave velocities is small. These results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to Berea sandstone. Inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress, in agreement with current rock mechanics theory. A reasonable fit to the data is obtained using only the second‐rank crack density tensor even though, at high confining stress, the cracks are expected to be in partial contact along their length. This is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by White. However, measurements of off‐axis wave velocities are required to fully quantify the contribution of the fourth‐rank crack density tensor.
GOES weather satellite observations and measurements of the May 18, 1980, Mount St. Helens eruption
Authors: Holasek R. E. , Self S.
Published: February 2004

We demonstrate the use of Geostationary Operational Environmental Satellite (GOES) images of the May 18, 1980, Mount St. Helens volcanic plume in providing details of the dynamics and changing character of this major explosive eruption. Visible and thermal infrared (IR) data from a sequence of images at 30‐min intervals from 0850 to 1720 Local Time (LT) give information on dispersal and plume top temperature. Initial visible and IR images at 0850 show the top of a spreading co‐ignimbrite‐like umbrella plume and an overshooting column emerging from it, both rising off the ground‐hugging pyroclastic gravity flow generated by the opening directed blast. The overshooting column had a minimum temperature significantly colder than local ambient atmosphere, indicating substantial undercooling, and a maximum altitude of 31±2 km at 0920. This large plume system then formed a high‐velocity, radially spreading, gravitationally driven current before becoming advected in the wind field at an average downwind velocity of 29 m/s. Within 30 to 60 min the undercooled plume adjusted to ambient temperatures. After this initial event, the eruption column fluctuated over the next 8 hours between stable Plinian column rise and unstable conditions favoring column collapse, leading to pyroclastic flows and further development of co‐ignimbrite ash columns. Over this period of observation, profiles of plume top temperatures derived from IR data versus distance along the downwind axis have distinctive patterns, with steep slopes associated with co‐ignimbrite plumes and shallower slopes associated with Plinian plumes. Moreover, reflectance values from visible GOES data change from lower to higher during periods of transition from darker toned Plinian to lighter toned coignimbrite plumes indicating that in this case satellite data resolved changes in eruptive style from plumes with a coarser to a finer dominant particle size.
Surface seismic and electrical methods to detect fluids related to faulting
Authors: Eberhart‐Phillips Donna , Stanley William D. , Rodriguez Brian D. , Lutter William J.
Published: February 2004

In the absence of drilling, surface‐based geophysical methods are necessary to observe fault zones and fault zone physical properties at seismogenic depths. These in situ physical properties can then be used to infer the presence and distribution of fluids along faults, although such observations are by nature indirect and become less exact with greater depth. Multiple observations of a range of such geophysical properties as compressional and shear seismic velocity (Vp and Vs), Vp/V5 ratio (related to Poisson's ratio), resistivity and attenuation in and adjacent to fault zones offer the greatest hope of making inferences of the fault zone geometry, fluids in the fault zone, and fluid reservoirs in the surrounding crust. For simple geometries, fault zone guided waves can provide information on fault zone width and velocities for faults of the order of 200 m wide. To address the question of whether a narrow fault zone can be imaged well enough at depths of seismic rupture to infer the presence of anomalously high fluid/rock ratios, we present synthetic seismic tomography and magnetotelluric examples for an ideal case of a narrow fault zone with a simple geometry, large changes in material properties, and numerous earthquakes within the fault zone. A synthetic 0.5‐km wide fault zone with 20% velocity reduction is well imaged using local earthquake tomography. When sequential velocity inversions are done, the true fault width is found, even to 9 km depth, although the calculated amplitude of the velocity reduction is lower than the actual amplitude. Vp/Vs is as well determined as Vp. Magnetotelluric imaging of a synthetic fault zone shows that a conductive fault zone can be well imaged within the upper 10 km. Further, a narrow (1 km) very low resistivity (3 ohm m) fault core can be imaged within a broad (5 km) low resistivity (10 ohm m) fault zone, illustrating that regions of a fault containing large quantities of interconnected fluids within a broader, conductive fault zone should be detectable. Thus variations in fluid content and fluid pressure can be inferred from electrical and seismic methods but there will always be uncertainty in these inferences due to the trade‐off with other factors, such as intrinsic variations in porosity, mineralogy, and pore geometry. The best approach is combined modeling of varied seismic and electrical data.
Magmatism as an essential driving force for formation of active metamorphic core complexes in eastern Papua New Guinea
Authors: Hill E. June , Baldwin Suzanne L. , Lister Gordon S.
Published: February 2004

The D'Entrecasteaux Islands in eastern Papua New Guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation. The plutons do not appear to have been intruded by diapiric processes as previously suggested. Late, relatively undeformed plutons form flat‐lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones. Granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography. It is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes. The source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the Woodlark seafloor spreading center into continental crust. It is suggested that a localized heat source of this type, which can provide heat and magmatic material to the crust, is essential for the development of metamorphic core complexes.
Petrogenesis and tectonic significance of the calc‐alkaline, bimodal Aztec Wash pluton, Eldorado Mountains, Colorado River extensional corridor
Authors: Falkner Claudia M. , Miller Calvin F. , Wooden Joseph L. , Heizler Matthew T.
Published: February 2004

The 15.7 Ma Aztec Wash pluton is located in the central Eldorado Mountains of the Colorado River extensional corridor in southern Nevada, immediately south of the well‐known imbricated volcanic sequence that has been widely cited in studies of extensional tectonism (e.g., Anderson, 1971). It is a shallow level (≤5 km), essentially bimodal complex, primarily made up of granite (∼72 wt % SiO2) and diabase and diorite (∼54 wt % SiO2), with minor amounts of more mafic, intermediate, and highly evolved rocks. The mafic and felsic magmas mingled extensively but mixed only to a limited extent. Late synplutonic mafic and felsic dikes represent continuing injection of the same bimodal magmas. The mafic rocks have high incompatible element concentrations (e.g., K2O ∼ 3 wt %, Ba ∼ 1600 ppm, light rare earth elements 350 × chondrite) and enriched isotopic compositions (εNd −7.5, 87Sr/86Sr 0.708); generation in ancient, enriched mantle lithosphere with limited subsequent crustal contamination is inferred. The granite is more potassic (∼5 wt %) than the mafic rocks, but it has comparable or lower concentrations of most incompatible elements; its isotopic composition (εNd −10, 87Sr/86Sr 0.710) is intermediate between those of the mafic rocks and local ancient crust. The granites thus indicate hybridization of the crust by mafic magma, but it is unclear whether this hybridization occurred at deeper levels in this magmatic system, or during an earlier mid‐Tertiary or Mesozoic magmatic event. Emplacement of the Aztec Wash pluton preceded peak east west extension in the northern Eldorado Mountains (∼15.2 Ma (Gans et al., 1994)), but it coincided with at least modest extension as indicated by the uniform NS orientation of the late dikes and the mafic injections into the magma chamber. Total extension and tilting of the pluton after crystallization was minor, in contrast to the east tilted area to the north and west tilted area to the south. Timing and style of extension are thus consistent with the pluton's location within, and perhaps mechanical influence on, a major accommodation zone, as suggested by Faulds et al. (1990) and Faulds (1994).
Airborne expendable bathythermograph surveys of the eastern Mediterranean
Authors: Horton C. , Kerling J. , Athey G. , Schmitz J. , Clifford M.
Published: February 2004

Two extensive airborne expendable bathythermograph (AXBT) surveys of the eastern Mediterranean were conducted during December 1991 and July 1992 from a Naval Oceanographic Office RP‐3 aircraft. Larger areas were sampled over shorter periods of time than is possible with conventional surveys carried out by ships. The circulation of the eastern Mediterranean consists of a collection of subgyres and fronts which show surprising seasonal and interannual variability. These surveys help to describe further this variability which is only partially understood. In particular, the surveys provide additional evidence that a relatively strong anticyclone located adjacent to the southeast corner of Crete during the late summer and through fall is a consequence of strong north winds from the Aegean, the Etesians, being blocked by Crete. Also notable in both surveys was the absence of “a” strong Mersa‐Matruh gyre, an anticyclone or anticyclonic subgyre off the Egyptian coast generally considered to be permanent. In contrast to published earlier observations, the Mersa Matruh gyre system was more a collection of weaker anticyclones rather than a single, sometimes double centered, strong feature. In both surveys we saw a string of anticyclones off the African coast between about 24° and 32°E. In general, these eddies did not exhibit a significant surface thermal structure, but were most evident in the 200 m to 300 m‐depth depictions. It may be that the formation of a strong southeast Crete anticyclone inhibits the formation of a large, relatively strong Mersa‐Matruh subgyre by interacting with the mid‐Mediterranean jet. If so, then both the southeast Crete anticyclone and the Mersa‐Matruh anticyclone could both be recurrent but tend to occur at different times.
New appraisal of radium 226 as a large‐scale oceanic mixing tracer
Authors: Ku Teh‐Lung , Luo Shangde
Published: February 2004

Radium 226 and Ba data in the eastern and central Indian Ocean collected during the Geochemical Ocean Sections Study (GEOSECS) expeditions are reviewed and used to delineate the geochemical cycle of both species. The data show that although particulate uptake of 226Ra and Ba occurs in the surface ocean, their regeneration may largely take place near or below the sediment‐water interface. Therefore, over much of the deep ocean save close to the bottom, 226Ra can be regarded as being chemically conservative. Taking advantage of this geochemical aspect of 226Ra enables us to compute the vertical (z) and horizontal (meridional, x) eddy diffusivities (Kz and Kx) and advective velocities (Vz and Vx) and their variability in the eastern and central parts of the Indian Ocean. The absolute magnitudes of these mixing parameters are calibrated against the decay rate of 226Ra. The computation is done by numerically solving the mass continuity equations for three properties: 226Ra, total salt, and mass of seawater. The grid size used in the calculations, in which the advective transport is taken to be along the isopycnal surface, has dimensions of Δx ≈ 700 km and Δz ≈ 400 m. The model calculation shows that in the eastern and central Indian Ocean along the GEOSECS traverses, values of diapycnal diffusivity (Kz) of 1–10 cm2 s−1 are found near the surface and the bottom. Minima occur in the thermocline region where Kz falls to about 10−2 cm2 s−1. Below, Kz has values largely in the range 0.1–1 cm2 s−1 and increases downward. The horizontal (isopycnal) diffusivity varies between 104 and 108 cm2 s−1, mainly in the range 106–107 cm2 s−1. Upwelling velocities are mostly in the lower end of the range 1–10 m yr−2, whereas downwelling, unlike upwelling which tends to be localized, occurs with velocities of 0.1–1 m yr−2. The meridional component of subsurface currents is in the range of 0.01–0.1 cm s−1, mostly toward the south, and not restricted to a particular depth interval. The current field deduced in the study area provides a broad picture of the strength and pattern of return flows of the deep water introduced from the south and thought to be principally transported northward via the relatively narrow, intense currents along the western boundaries of the basins. The present study as well as recent developments in the high‐precision and high‐sensitivity mass spectrometric analysis of 226Ra suggests that this naturally occurring isotope warrants further exploitation as a rate tracer for large‐scale ocean circulation and mixing processes.
Interactive Kaiman filtering
Authors: Bürger Gerd , Cane Mark A.
Published: February 2004

Data assimilation via the extended Kaiman filter can become problematic when the assimilating model is strongly nonlinear, primarily in connection with sharp, “switchlike” changes between different regimes of the system. The filter seems too inert to follow those switches quickly enough, a fact that can lead to a complete failure when the switches occur often enough. In this paper we replace the key feature of the filter, the use of local linearity for the error model update, with a principle that uses a more global approach through the utilization of a set of preselected regimes. The method uses all regime error models simultaneously. Being mutually incompatible, a compromise between the different error models is found through the use of a weighting function that reflects the ‘closeness’ of the error model to the correct model. To test the interactive Kaiman filter a series of numerical experiments is performed using the double‐well system and the well‐known Lorenz system, and the results are compared to the extended Kaiman filter. It turns out that, depending on the set of preselected regimes, the performance is worse than, comparable to, or better than that of the extended Kaiman filter.
Backscatter characteristics of the winter ice cover in the Beaufort Sea
Authors: Kwok Ronald , Cunningham Glenn F.
Published: February 2004

The microwave backscatter statistics of the sea ice cover in the winter Beaufort Sea are characterized using C band synthetic aperture radar (SAR) data collected by the European Earth Resources Satellite 1. Sea ice backscatter signatures were sampled from SAR image data collected during the winter of 1991–1992 and winter of 1992–1993. The spatial and temporal variabilities of the backscatter signatures of different ice types are discussed. The results show considerable seasonal stability of the backscatter signature of multiyear ice as well as of first‐year ice. Small amplitude regional variations of the multiyear ice backscatter can be observed. Consistent contrast between multiyear ice and first‐year ice is maintained throughout the season. The highest radiometric variability is observed in sea ice in leads. On the basis of these observations the backscatter from the principal ice types (multiyear and first‐year) are consistent with scatterometer observations and can be easily identified under winter conditions. Correlations of the regional variations of multiyear ice signatures to physical processes are suggested.
Seasonal variability of gyre‐scale circulation in the northern Tyrrhenian Sea
Authors: Artale V. , Astraldi M. , Buffoni G. , Gasparini G. P.
Published: February 2004

The principal hydrographic structures and dynamic conditions of the northern Tyrrhenian Sea are studied through the comparison of in situ field data and a modeling simulation. The field observations are long‐term Eulerian and Lagrangian current measurements associated with two hydrographic campaigns conducted in late summer and in winter. The data indicate that a well‐marked cyclonic gyre affecting the water column from the surface to the bottom is present in the northern Tyrrhenian Sea in both periods. The gyre undergoes significant seasonal changes in both shape and size. Following the indications of past studies, a numerical quasi‐geostrophic model driven by the wind jet blowing from the Strait of Bonifacio was able to reproduce the principal gyre characteristics. However, the wind jet alone does not explain all the gyre features, especially its seasonal variation. A study of the model results and field data indicates that these variations are linked to the seasonal variability of the northward surface current flowing along the eastern boundary of the Tyrrhenian Sea.
Sediment transport in oscillatory boundary layers in cases of rippled beds and sheet flow
Authors: Ribberink Jan S. , Al‐Salem Abdullah A.
Published: February 2004

An experimental study was focused on the process of sediment transport in unsteady flow conditions due to wave action. Wave‐induced oscillatory flow conditions near the seabed were simulated at full scale (1:1) in a new large oscillating water tunnel. Two sets of experiments (series A and B) were carried out. During series A, bed forms and wave‐cycle averaged suspended sediment concentrations were measured under sinusoidal waves. Series B focused on measurement of the wave‐cycle averaged sediment transport rates under regular and random asymmetric oscillatory flows (upper shoreface conditions) and was aimed at the verification of quasi‐steady formulas for the description of cross‐shore sediment transport.

Bedform dimensions appeared to decrease considerably under the influence of wave asymmetry and wave randomness. Only the measured ripple dimensions under asymmetric waves (series B) showed good agreement with Nielsen's (1979) relations. For most of the series B experiments the bed was plane (sheet flow), the net sediment transport was directed “onshore” and the measured transport rates showed a strong correlation with the velocity moment < U3 >. An empirical quasi‐steady transport model is proposed which is based on the new tunnel data and other existing data sets. The limitations of the quasi‐steady model approach became clear in the rippled‐bed regime and through the presence of a consistent influence of the wave period in plane‐bed conditions. In rippled‐bed conditions the suspended concentration profiles followed a negative exponential distribution. For most of the experiments the measured concentration decay length showed a linear relation with the ripple height. In plane‐bed/sheet flow conditions the measured suspended concentration profiles followed a negative power function. The power or concentration decay parameter was constant (α ≃ 2.1) for a wide range of velocity conditions. It is suggested that the mobile bed (sheet flow layer) has a strong damping effect on the mixing of suspended sediments (turbulence damping).
Wave‐current interaction study in the Gulf of Alaska for detection of eddies by synthetic aperture radar
Authors: Liu Antony K. , Peng Chich Y. , Schumacher James D.
Published: February 2004

High resolution ERS‐I Synthetic Aperture Radar (SAR) images are used to detect a mesoscale eddy. Such features limit dispersal of pollock larvae and therefore likely influence recruitment of fish in the Gulf of Alaska. During high sea states and high winds, the direct surface signature of the eddy was not clearly visible, but the wave refraction in the eddy area was observed. The rays of the wave field are traced‐out directly from the SAR image. The ray pattern gives information on the refraction pattern and on the relative variation of the wave energy along a ray through wave‐current interaction. These observations are simulated by a ray‐tracing model which incorporates a surface current field associated with the eddy. The numerical results of the model show that the waves are refracted and diverge in the eddy field with energy density decreasing. The model‐data comparison for each ray shows the model predictions are in good agreement with the SAR data.
Vertical structure of mean cross‐shore currents across a barred surf zone
Authors: Haines John W. , Sallenger Asbury H.
Published: February 2004

Mean cross‐shore currents observed across a barred surf zone are compared to model predictions. The model is based on a simplified momentum balance with a turbulent boundary layer at the bed. Turbulent exchange is parameterized by an eddy viscosity formulation, with the eddy viscosity Aυ independent of time and the vertical coordinate. Mean currents result from gradients due to wave breaking and shoaling, and the presence of a mean setup of the free surface. Descriptions of the wave field are provided by the wave transformation model of Thornton and Guza [1983]. The wave transformation model adequately reproduces the observed wave heights across the surf zone. The mean current model successfully reproduces the observed cross‐shore flows. Both observations and predictions show predominantly offshore flow with onshore flow restricted to a relatively thin surface layer. Successful application of the mean flow model requires an eddy viscosity which varies horizontally across the surf zone. Attempts are made to parameterize this variation with some success. The data does not discriminate between alternative parameterizations proposed. The overall variability in eddy viscosity suggested by the model fitting should be resolvable by field measurements of the turbulent stresses. Consistent shortcomings of the parameterizations, and the overall modeling effort, suggest avenues for further development and data collection.
The production of North Atlantic Deep Water: Sources, rates, and pathways
Authors: Dickson Robert R. , Brown Juan
Published: February 2004

Updating an earlier account by Dickson et al., (1990), this paper reviews the initial development phase of North Atlantic Deep Water (NADW) production from the points where the dense inflows from Nordic seas cross the Greenland‐Scotland Ridge to the point off south Greenland where the buildup of new production appears almost complete. In particular, three long‐term current meter arrays totaling 91 instruments and set at ∼160 km intervals south from the Denmark Strait sill are used to validate earlier short‐term arrays by others and, in combination with these earlier arrays, to describe the downstream evolution of mean speed, depth and entrainment, the variability of the overflow current in space and time, and the likely contribution of the other three main constituents of NADW production at densities greater than σθ = 27.8. From the points of overflow (5.6 Sv) the transport within this range increases by entrainment and confluence with other contributory streams to around 13.3 Sv at Cape Farewell. While recirculating elements prevent us from determining the net southgoing transport, a NADW transport of this order appears consistent with recent estimates of net abyssal flow passing south through the North and South Atlantic.
Microbial community structure at the U.S.‐Joint Global Ocean Flux Study Station ALOHA: Inverse methods for estimating biochemical indicator ratios
Authors: Christian James R. , Karl David M.
Published: February 2004

Modeling biogeochemical fluxes in the marine plankton requires the application of factors for extrapolation of biomass indicators measured in the field (chlorophyll a, adenosine triphosphate, bacterial counts) to biomass carbon or nitrogen. These are often inferred from culture studies and are poorly constrained for natural populations. A least squares inverse method with a simple linear model constrains the values of several common indicator ratios, giving self‐consistent solutions that provide useful information about the structure of the microbial community at our North Pacific Ocean study site (Station ALOHA (A Long‐term Oligotrophic Habitat Assessment)). These results indicate that the fraction of the microbial biomass that is autotrophic (pigmented) is greater in the mixed layer than at the deep chlorophyll maximum layer and that heterotrophic bacteria are a significant but not necessarily predominant component of the microbial community in the euphotic zone.
A microwave technique for mapping thin sea ice
Authors: Cavalieri Donald J.
Published: February 2004

A technique is presented for mapping the distribution of new, young and first‐year sea ice in seasonal sea ice zones that utilizes microwave spectral and polarization information from the Defense Meteorological Satellite Program Special Sensor Microwave/Imager. The motivation for this work stems from the need for accurate estimates of open water and thin ice within the Arctic ice pack. The technique utilizes the microwave polarization and spectral characteristics of these three ice types through two microwave radiance ratios: the 19.4 GHz polarization and the spectral gradient ratio, which is a measure of the spectral difference between the 19.4‐GHz and the 37.0‐GHz vertically polarized radiance components. The combined use of the spectral gradient ratio and polarization reduces the low ice concentration bias generally associated with the presence of thin ice types. The microwave polarization, which is sensitive to changes in ice thickness and ice surface characteristics, is used to classify new, young, and first‐year ice types. The polarization varies from about 0.3 for calm ice‐free water, to about 0.15 for new ice, and to 0.03 for thick first‐year ice. On the basis of this parameterization, improved sea ice concentration maps and maps of new, young and first‐year ice type distributions are derived. Examples from the Bering Sea are presented and comparisons are made with analyzed NOAA AVHRR imagery.
The use of satellite scatterometer winds to drive a primitive equation model of the Indian Ocean: The impact of bandlike sampling
Authors: Barnier Bernard , Capella Jorge , O'Brien James J.
Published: February 2004

The aim of this study is to evaluate the impact of the bandlike sampling of spaceborne scatterometers on the ability of scatterometer winds to successfully force the mean flow and seasonal cycle of an ocean model in the context of equatorial and tropical dynamics. The equatorial ocean is simulated with a four‐layer, primitive equation, reduced gravity model of the Indian Ocean. The variable wind stress used in this study is derived from one year (1988) of 6‐hour analyses of the 10‐m wind vector over the Indian Ocean performed at the European Centre for Medium‐Range Weather Forecasts (ECMWF). It is applied as a forcing at every grid point of the model to drive a reference circulation. Scatterometer winds are simulated from ECMWF winds, using the nominal configurations and orbital parameters of the European Remote Sensing 1 (ERS‐1) and NASA Scatterometer (NSCAT) missions. The model is forced in real time under swaths with the raw scatterometer winds of ERS‐1 and NSCAT, with a persistence condition (i.e., the wind is kept constant until the next passage of the satellite provides a new value). The circulation obtained for each of the scatterometer experiments is compared with the reference circulation. The seasonal circulation of the Indian Ocean with NSCAT winds is very similar to the reference. The perturbations introduced by the bandlike sampling and the persistence condition have an impact similar to that of a small uncorrelated noise added to the reference forcing. The persistence condition for ERS‐1 does not give results which are as good as those obtained for NSCAT.
Embedding a mixed layer model into an ocean general circulation model of the Atlantic: The importance of surface mixing for heat flux and temperature
Authors: Sterl Andreas , Kattenberg Arie
Published: February 2004

A primitive equation ocean general circulation model (OGCM) of the Atlantic is coupled with an explicit mixed layer model (MLM). The coupled model is used to investigate the impact and possible improvement achieved by an MLM on the representation of temperature and heat flux. The MLM improves model performance in regions where mixed layer (ML) formation is governed by wind stirring, which is identified as a major missing process in the OGCM. It is shown that the widely used parameterization of Pacanowski and Philander for the vertical exchange coefficients is not able to properly represent this process. The deepening of the ML by wind stirring leads to temperature and heat flux being in closer agreement with climatology than is the case without MLM. Integrations with a coupled ocean‐atmosphere model are needed to show whether this improvement leads to a reduced climate drift in such models. Furthermore, momentum mixing improves the vertical profiles of velocity. The Ekman transport now occurs over the whole depth of the ML instead of just within the upper model layer. Convective adjustment as incorporated in the OGCM already tends to overestimate the depth of the mixed layer in buoyancy‐driven situations. Adding the MLM cannot remedy this failure. Thus under these circumstances there are only slight improvments of some model aspects while others become even worse. We conclude that the parameterization of convection has to be improved.
A note on the representation of steric sea level in models that conserve volume rather than mass
Authors: Greatbatch Richard J.
Published: February 2004

This note discusses the representation of steric sea level in ocean circulation models. Changes in steric sea level are caused when changes in the density of the water column imply an expansion or contraction of the column. Models usually make the Boussinesq approximation and conserve volume, rather than mass, and so do not properly represent expansion or contraction. This means that although expansion/contraction is included in the equation of state, it is not accounted for by the model dynamics. In this note, we examine the equation governing the time evolution of the sea level displacement. It is shown that requiring conservation of mass, rather than volume, introduces a new term to this equation. A simple example is used to show the relationship of the new term to the surface buoyancy flux. The equilibrium response to the new term has two parts. One part consists of the Goldsbrough and Stommel gyres, for which, in the ocean interior, vortex stretching due to the local expansion/contraction of the water column is balanced by changes in planetary vorticity. The other part corresponds to the “inverse barometer.” The effect is to adjust sea level by a globally uniform but timevarying factor, determined by the net expansion/contraction of the global ocean. Since this correction is globally uniform, it has no dynamical significance. Both the Goldsbrough/Stommel gyres and the inverse barometer solution are missing from models as currently formulated. This does not represent a serious error. However, if comparison is made with observations of sea level, model‐calculated sea level should be adjusted by a globally uniform, time‐varying factor, determined by the net expansion/contraction of the global ocean. This would be important for assessing the likely rise in sea level in response to global warming.
A nonstationary stochastic model for long‐term time series of significant wave height
Authors: Athanassoulis G. A. , Stefanakos Ch. N.
Published: February 2004

In this paper an attempt is initiated to analyze long‐term time series of wave data and to model them as a nonstationary stochastic process with yearly periodic mean value and standard deviation (periodically correlated or cyclostationary stochastic process). First, an analysis of annual mean values is performed in order to identify overyear trends. It turns out that it is very likely that an increasing trend is present in the examined hindcast data. The detrended time series Y(τ) is then decomposed, using an appropriate seasonal standardization procedure, to a periodic mean value μ(τ) and a residual time series W(τ) multiplied by a periodic standard deviation σ(τ) of Y(τ)=μ(τ)+σ(τ)W(τ). The periodic components μ(τ) and σ(τ) are estimated and represented by means of low‐order Fourier series, and the residual time series W(τ) is examined for stationarity. For this purpose, spectral densities of W(τ), obtained from different‐season segments, are calculated and compared with each other. It is shown that W(τ) can indeed be considered stationary, and thus Y(τ) can be considered periodically correlated. This analysis has been applied to hindcast wave data from five locations in the North Atlantic Ocean. It turns out that the spectrum of W(τ) is very weakly dependent on the site, a fact that might be useful for the geographic parameterization of wave climate. Finally, applications of this modeling to simulation and extreme‐value prediction are discussed.
Is the North Pacific in Sverdrup balance along 24°N?
Authors: Hautala Susan L. , Roemmich Dean H. , Schmilz William J.
Published: February 2004

Hydrographic data from a zonal section along approximately 24°N is used to demonstrate that basin‐scale baroclinic geostrophic transport agrees with the Sverdrup relation in the subtropical North Pacific. Moreover, profiles of vertical velocity, as derived from the linear vorticity equation, are consistent with constraints upon deeper flows. Sverdrup balance from the eastern boundary to about 137°E is in accord with most other elements of the general circulation to the extent that they are known. In the Philippine Sea, west of 137°E, the results suggest that a mean northward flow of 5–10 Sv occurs. In the northeast Pacific Basin, a significant subbasin‐scale deviation of about 7 Sv from Sverdrup balance is revealed; this deviation is distinguished by excess shallow southward flow near the water mass boundary between the subtropical gyre and the subpolar waters adjacent to the American coast.
A comparison between TOPEX microwave radiometer, ERS 1 microwave radiometer, and European Centre for Medium‐Range Weather Forecasting derived wet tropospheric corrections
Authors: Stum Jacques
Published: February 2004

A comparison study has been carried out between the two wet tropospheric corrections in the TOPEX/POSEIDON merged geophysical data records. It is shown that the TOPEX microwave radiometer (TMR) wet tropospheric correction monitors 100‐ to 1000‐km water vapor scales better than the European Centre for Medium‐Range Weather Forecasting (ECMWF) derived wet tropospheric correction. The standard deviation for both corrections is about 3 cm, with a tendency for the ECMWF model to give higher humidities than TMR in very wet atmospheres. The ECMWF correction is of fairly high quality and could be used if the TMR fails. The TOPEX and ERS 1 microwave radiometer wet tropospheric corrections are also compared in the vicinity of TOPEX/ERS 1 crossovers. The standard deviation at crossovers is about 1 cm, with no bias. Along‐track comparison shows that the two radiometers perform similarly, except in high winds or rain. The small discrepancies may be due to differences in the antenna beams or in the geophysical transfer functions.
The contribution of Alaskan, Siberian, and Canadian coastal polynyas to the cold halocline layer of the Arctic Ocean
Authors: Cavalieri Donald J. , Martin Seelye
Published: February 2004

Numerous Arctic Ocean circulation and geochemical studies suggest that ice growth in polynyas over the Alaskan, Siberian, and Canadian continental shelves is a source of cold, saline water which contributes to the maintenance of the Arctic Ocean halocline. The purpose of this study is to estimate for the 1978–1987 winters the contributions of Arctic coastal polynyas to the cold halocline layer of the Arctic Ocean. The study uses a combination of satellite, oceanographic, and weather data to calculate the brine fluxes from the polynyas; then an oceanic box model is used to calculate their contributions to the cold halocline layer of the Arctic Ocean. This study complements and corrects a previous study of dense water production by coastal polynyas in the Barents, Kara, and Laptev Seas. Recurrent polynyas form on the Canadian and Alaskan coasts from Banks Island to the Bering Strait and on the Siberian coast from the Bering Strait to the New Siberian Islands. In the Bering Sea, polynyas form in Norton Sound, south and west of St. Lawrence Island, and in the Gulf of Anadyr. Two regions that account for almost 50% of the total dense water production are the Siberian coastal polynyas in the adjacent regions of the Gulf of Anadyr and Anadyr Strait and the Alaskan coastal polynyas which occur along the coast from Cape Lisburne to Point Barrow. For all of the western Arctic coastal regions examined, the mean annual total brine flux is 0.5±0.2 Sv. Combination of this flux with the contribution from the Barents, Kara, and Laptev Seas, which is recalculated from data in the earlier study, shows that over the entire Arctic, coastal polynyas generate about 0.7–1.2 Sv of dense water. This compares well with the theoretical estimates of 1–1.5 Sv. Because an unknown fraction of the Barents, Kara, and Laptev brine flux must go to the Eurasian Basin deep water, the coastal polynyas alone cannot renew the halocline layer. Other potential brine generation mechanisms include overall freezing on the shelves and the response of the ice to infrequent violent storms. For example, during February 1982 an intense storm generated a large region of low ice concentration in the eastern Chukchi Sea over Barrow Canyon. The refreezing of the region was followed by the flow of a dense plume down Barrow Canyon. Although the ocean dynamical response to this refreezing needs to be established, the possible response of a Barrow Canyon flow to this refreezing event suggests that the overall refreezing in response to infrequent violent storms may be a potential source of the additional brine needed to maintain the Arctic Ocean halocline.
Surface characteristics of lead ice
Authors: Perovich Donald K. , Richter‐Menge Jacqueline A.
Published: February 2004

Under cold conditions, ice growth in a lead occurs rapidly, giving young sea ice a high bulk salinity. The surface characteristics of this ice type are dominated by a thin, highly saline surface skim and often by the formation of frost flowers. These surface characteristics are of particular interest because of their significant temporal variability and large impact on the electromagnetic properties of sea ice. As part of the Lead Experiment held during March and April of 1991 and 1992 in the Alaskan Beaufort Sea, ice properties and surface conditions typical of springtime leads were monitored at three sites during the initial few days of growth. Observations indicate that once the ice thickness reached approximately 2 cm, a thin (∼1 mm), highly saline (∼100 parts per thousand (ppt)) skim of brine formed on the surface. After only a few hours of growth the initially smooth surface of the sea ice developed some small‐scale roughness. Frost flowers, the result of ice grown from the vapor phase, quickly formed on the surface of the sea ice and continued to develop during the observations. Depending on the temperature, the frost flowers were composed of various ice crystal types, including clumps, stellar dendrites, and needles. The initially fresh frost flowers quickly became salty, reaching salinities as high as 100 ppt. The salinity of both the frost flowers and the surface skim decreased at night as temperatures dropped and additional hoarfrost accumulated on the surface. These decreases were also due in part to the accumulation of snow on the surface of the leads. Combining these observations with simple calculations, possible mechanisms for the development and evolution of the surface skim and frost flowers are discussed. They include the hypothesis that the source of the surface skim is brine expelled upward from the sea ice as it cools, that the surface protrusions serve as nucleation sites for the frost flowers, and that the source of the excess water vapor necessary for frost flower growth is the wet surface skim.
Effects of the bottom friction formulation on the energy balance for gravity waves in shallow water
Authors: Luo W. , Monbaliu J.
Published: February 2004

The energy transfer equation for wind‐generated ocean waves is solved numerically for fetch‐limited conditions in waters of limited depth. The resulting growth curves for the total energy and the peak frequency are obtained for five different bottom friction dissipation formulations: an empirical Joint North Sea Wave Project (JONSWAP) expression (Hasselmann et al., 1973), three expressions based on the drag law friction model (Hasselmann and Collins, 1968; Collins, 1972; Madsen et al., 1988a) and a form based on the eddy viscosity friction model (Weber, 1991a). The effects of different bottom friction dissipation formulations on the energy balance are intercompared and evaluated. The results are also compared with the Coastal Engineering Research Center growth curves. Numerical simulation and mathematical analysis show that only the drag law formulations with a fixed drag coefficient scale in terms of the air friction velocity. The empirical JONSWAP, the drag law formulation with dynamically changing drag coefficient, and the eddy viscosity model do not scale. It is possible to tune the dissipation coefficients for one particular wind friction velocity but different water depths so that all five formulations give almost the same growth curves for the total energy and the peak frequency. This can be explained by the fact that the five tuned bottom friction dissipation models have the same effect on the energy balance when integrated over the frequency and angle space.
Gravity model development for TOPEX/POSEIDON: Joint Gravity Models 1 and 2
Authors: Nerem R. S. , Lerch F. J. , Marshall J. A. , Pavlis E. C. , Putney B. H. , Tapley B. D. , Eanes R. J. , Ries J. C. , Schutz B. E. , Shum C. K. , Watkins M. M. , Klosko S. M. , Chan J. C. , Luthcke S. B. , Patel G. B. , Pavlis N. K. , Williamson R. G. , Rapp R. H. , Biancale R. , Nouel F.
Published: February 2004

The TOPEX/POSEIDON (T/P) prelaunch Joint Gravity Model‐1 (JGM‐I) and the postlaunch JGM‐2 Earth gravitational models have been developed to support precision orbit determination for T/P. Each of these models is complete to degree 70 in spherical harmonics and was computed from a combination of satellite tracking data, satellite altimetry, and surface gravimetry. While improved orbit determination accuracies for T/P have driven the improvements in the models, the models are general in application and also provide an improved geoid for oceanographic computations. The postlaunch model, JGM‐2, which includes T/P satellite laser ranging (SLR) and Doppler orbitography and radiopositioning integrated by satellite (DORIS) tracking data, introduces radial orbit errors for T/P that are only 2 cm RMS with the commission errors of the marine geoid for terms to degree 70 being ±25 cm. Errors in modeling the nonconservative forces acting on T/P increase the total radial errors to only 3–4 cm RMS, a result much better than premission goals. While the orbit accuracy goal for T/P has been far surpassed, geoid errors still prevent the absolute determination of the ocean dynamic topography for wavelengths shorter than about 2500 km. Only a dedicated gravitational field satellite mission will likely provide the necessary improvement in the geoid.