bluuebunny/crossref_metadata_2025 huggingface dataset:
The Joint Gravity Model 3
Authors: Tapley B. D. , Watkins M. M. , Ries J. C. , Davis G. W. , Eanes R. J. , Poole S. R. , Rim H. J. , Schutz B. E. , Shum C. K. , Nerem R. S. , Lerch F. J. , Marshall J. A. , Klosko S. M. , Pavlis N. K. , Williamson R. G.
Published: February 2004

An improved Earth geopotential model, complete to spherical harmonic degree and order 70, has been determined by combining the Joint Gravity Model 1 (JGM 1) geopotential coefficients, and their associated error covariance, with new information from SLR, DORIS, and GPS tracking of TOPEX/Poseidon, laser tracking of LAGEOS 1, LAGEOS 2, and Stella, and additional DORIS tracking of SPOT 2. The resulting field, JGM 3, which has been adopted for the TOPEX/Poseidon altimeter data rerelease, yields improved orbit accuracies as demonstrated by better fits to withheld tracking data and substantially reduced geographically correlated orbit error. Methods for analyzing the performance of the gravity field using high‐precision tracking station positioning were applied. Geodetic results, including station coordinates and Earth orientation parameters, are significantly improved with the JGM 3 model. Sea surface topography solutions from TOPEX/Poseidon altimetry indicate that the ocean geoid has been improved. Subset solutions performed by withholding either the GPS data or the SLR/DORIS data were computed to demonstrate the effect of these particular data sets on the gravity model used for TOPEX/Poseidon orbit determination.
Steady state dislocation densities in experimentally deformed calcite materials: Single crystals versus polycrystals
Authors: De Bresser J. H. P.
Published: February 2004

The theoretical relationship between dislocation density ρ and applied stress σ of the type σ ∝ ρ0.5 is an essential part of many microphysical models for dislocation dynamics and creep. Thus, application of these microphysical models requires demonstration of this relation. In an attempt to determine the relation between ρ and σ for calcite, uniaxial compression tests were carried out on optical quality single crystals, at temperatures of 550–800°C and strain rates of 3 × 10−4–3 × 10−8 s−1. The tests were performed with the compression direction parallel to [4041] (i.e., parallel to the intersection of two cleavage rhombs). Earlier data on crystals from the same batch already showed that, at the imposed conditions, steady state flow occurs by slip on the r‐ and f‐glide systems, hence well within the dislocation creep regime. Dislocation densities measured by transmission electron microscopy achieved steady state values by ∼2% strain. The steady state dislocation density can be related to the flow stress according to the empirical relation σ = 10−6.21(±0.86) ρ0.62(±0.07) (σ in MPa, ρ in m−2) in close agreement with the theoretically expected relation. Dislocation densities measured in experimentally deformed calcite polycrystals (previous and present studies) match the single‐crystal data at high stress (>40 MPa) but deviate from them toward low stress. The deviation appears to be more pronounced if the polycrystal has a smaller grain size. This can be explained using a theory of nonhomogeneous deformation in which strain incompatibility at grain boundaries is accommodated by “geometrically necessary dislocations”. The contribution of these dislocations to the internal stress of the material (hence to the applied stress required for deformation at a given rate) is significant if the grain size is small. Consequently, a comparison of dislocation creep behavior of calcite materials with microphysical models based on dislocation dynamics should take into account the possible influence of grain size if stress levels are relatively low.
Global lateral variations of shear wave attenuation in the upper mantle
Authors: Bhattacharyya Joydeep , Masters Guy , Shearer Peter
Published: February 2004

We analyze several thousand high‐quality, globally recorded SS – S differential waveforms to constrain the lateral variation of shear wave attenuation (Qβ) in the upper mantle. We use a multitaper frequency domain technique to measure attenuation, parameterized by a t* operator, and implement a robust estimation technique to compute t* and its variance. The differential waveform technique minimizes the effect of factors such as finite source duration and structural complexity near the source and receiver so the differential SS – S waveforms are mainly sensitive to the shear attenuation in the upper mantle under the SS bounce point. We use seismograms recorded at ranges of 45° to 100° and compute the SS – S differential t* from the broadening of the SS waveform relative to the Hilbert transform of the S waveform. A careful choice of fitting windows allows us to reduce the biasing effects of interfering phases which can affect t* by up to 0.5 s. The t* residuals (with respect to preliminary reference Earth model (PREM)) vary by ±1.5 s with an average of ≈0.24 s. Our study suggests an average Qβ value of 112 (most of the lateral variations of Qβ are within 30% of this value) in the top 400 km of the mantle, slightly lower than the PREM value of 128. There is a qualitative correlation of t* residual with tectonic region with distinctly higher attenuation observed under young oceans compared to platforms and shields. Also, the lateral variations of the residuals are similar in trend to those observed in studies of the attenuation of SCS multiples. At long wavelengths, the Qβ map shows a modest correlation with shear wave attenuation maps computed from surface wave analyses and with the patterns of lateral variations of shear velocities at certain upper‐mantle depths predicted by the model S16B30. The correlation with the velocity model is highest at 300–500 km depth indicating that there may be a contribution to long‐wavelength attenuation from relatively deep regions. Formal inversion for an upper mantle Qβ model shows that while lateral resolution is quite good, depth resolution is poor as might be expected. Better depth resolution must await combined body wave and surface wave inversions.
Rectangular harmonic analysis revisited
Authors: Malin S. R. C. , Düzgit Z. , Baydemir N.
Published: February 2004

Alldredge's method of rectangular harmonic analysis has been reexamined. After correction of errors, it is found to give improbable values between the data points and wild values outside them. A much more realistic model has been obtained by (1) determining only the most significant coefficients (those that exceed their standard deviations, obtained by an iterative process), (2) introducing new parameters to allow for a linear trend across the region, and (3) increasing the scaling factors so that the sinusoids start and finish outside the region. Modification 1 is the most important for improving the interpolative qualities of the model. Modifications 2 and 3 reduce, but do not entirely eliminate, the wild values near the edges.
Seismic analysis of the axial magma chamber reflector along the southern East Pacific Rise from conventional reflection profiling
Authors: Hussenoeder Stefan A. , Collins John A. , Kent Graham M. , Detrick Robert S.
Published: February 2004

The thickness and internal properties of the magma sill located at the top of the axial magma chamber (AMC) along the southern East Pacific Rise (EPR) have been investigated through a combination of waveform modeling the near‐vertical incidence reflections from this body and analysis of reflection amplitude variation as a function of source‐receiver offset (or slowness). Our results show that the AMC reflector observed along the southern EPR is best modeled by a thin (<100 m thick) sill of partial melt (Vs ≠ 0 km/s) sandwiched between higher‐velocity material, and that the thickest sills are generally associated with the lowest P and S wave velocities. The comparatively high P wave velocities and nonzero shear wave velocities inferred for this sill indicate that it is filled with partially molten magma which in some locations has a high crystal content. This may have important implications for eruption mechanisms and along‐axis mixing of magma at the EPR. There is no simple relationship between morphologic indicators of magma supply (e.g., axial depth or volume) and sill thickness, depth, or velocity. Magma sill properties may be closely tied to the eruption and replenishment cycle of the AMC and thus may vary on a much shorter spatial and temporal scale than axial morphology, which reflects longer‐term variations in magma supply to the ridge.
Quantitative investigations of the Missouri gravity low: A possible expression of a large, Late Precambrian batholith intersecting the New Madrid seismic zone
Authors: Hildenbrand Thomas G. , Griscom Andrew , Van Schmus W. R. , Stuart W. D.
Published: February 2004

Analysis of gravity and magnetic anomaly data helps characterize the geometry and physical properties of the source of the Missouri gravity low, an important cratonic feature of substantial width (about 125 km) and length (>600 km). Filtered anomaly maps show that this prominent feature extends NW from the Reelfoot rift to the Midcontinent Rift System. Geologic reasoning and the simultaneous inversion of the gravity and magnetic data lead to an interpretation that the gravity anomaly reflects an upper crustal, 11‐km‐thick batholith with either near vertical or outward dipping boundaries. Considering the modeled characteristics of the batholith, structural fabric of Missouri, and relations of the batholith with plutons and regions of alteration, a tectonic model for the formation of the batholith is proposed. The model includes a mantle plume that heated the crust during Late Precambrian and melted portions of lower and middle crust, from which the low‐density granitic rocks forming the batholith were partly derived. The batholith, called the Missouri batholith, may be currently related to the release of seismic energy in the New Madrid seismic zone (earthquake concentrations occur at the intersection of the Missouri batholith and the New Madrid seismic zone). Three qualitative mechanical models are suggested to explain this relationship with seismicity.
Steady tetrahedral and cubic patterns of spherical shell convection with temperature‐dependent viscosity
Authors: Ratcliff James Todd , Schubert Gerald , Zebib Abdelfattah
Published: February 2004

Steady thermal convection of an infinite Prandtl number, Boussinesq fluid with temperature‐dependent viscosity is systematically examined in a three‐dimensional, basally heated spherical shell with isothermal and stress‐free boundaries. Convective flows exhibiting cubic (𝓁 2, 𝓂 = {0, 4}) and tetrahedral (𝓁 = 3, 𝓂 = 2) symmetry are generated with a finite‐volume numerical model for various combinations of Rayleigh number Ra (defined with viscosity based on the average of the boundary temperatures) and viscosity contrast 𝓇μ (ratio of maximum to minimum viscosities). The range of Ra for which these symmetric flows in spherical geometry can be maintained in steady state is sharply reduced by even mild viscosity variations (𝓇μ ≤ 30), in contrast with analogous calculations in Cartesian geometry in which relatively simple, three‐dimensional convective planforms remain steady for 𝓇μ ≈ 104. The mild viscosity contrasts employed place some solutions marginally in the sluggish‐lid transition regime in Ra‐𝓇μ parameter space. Global heat transfer, given by the Nusselt number N𝓊, is found to obey a single power law relation with Ra when Ra is scaled by its critical value. A power law of the form N𝓊 ∼ (Ra/Racrit)1/4 (Racrit is the minimum critical value of Ra for the onset of convection) is obtained, in agreement with previous results for isoviscous spherical shell convection with cubic and tetrahedral symmetry. The calculations of this paper demonstrate that temperature‐dependent viscosity exerts a strong control on the nature of three‐dimensional convection in spherical geometry, an effect that is likely to be even more important at Rayleigh numbers and viscosity contrasts more representative of the mantles of terrestrial planets. The robustness of the N𝓊‐Ra relation, when scaled by Racrit, is important for studies of planetary thermal history that rely on parameterizations of convective heat transport and account for temperature dependence of mantle viscosity.
Noncharacteristic normal faulting surface ruptures from the Gulf of Corinth, Greece
Authors: Roberts Gerald P.
Published: February 2004

The February 24–25th, 1981, Gulf of Corinth earthquakes, ruptured only the western end of the South Alkyonides Fault Segment (SAFS) whose length has been confirmed by study of spatial variations in fault throw, footwall uplift and hangingwall subsidence, and fault slip directions: this contrasts with what we would expect of the surface ruptures from characteristic earthquakes. Specifically, the eastern end of the 1981 ruptures, where the smallest coseismic throws and northwest plunging coseismic slip vectors were reported, coincides with the position where the greatest values for uplift/subsidence and fault throws exist along the SAFS, and where fault slip data record northwest, north and northeast plunging slip vectors from previous earthquakes. The SAFS appears to have grown by recurrence of noncharacteristic earthquakes. A new model of earthquake recurrence is proposed which accounts for ruptures that are shorter than the host fault segments, spatial variations in cumulative throw and fault slip directions along a fault segment, and temporal variation in the coseismic slip direction for successive earthquakes. The model implies that recurrence intervals vary both along single fault segments and with time for single localities. Thus palaeoseismological data from one site may not constrain earthquake recurrence at another along the same fault segment or fault slip rates over time periods containing numerous earthquake cycles.
Seismic velocity structure across the central Washington Cascade Range from refraction interpretation with earthquake sources
Authors: Schultz Andreas P. , Crosson Robert S.
Published: February 2004

A two‐dimensional seismic P wave velocity model, extending to a depth of ∼50 km, is interpreted for a 425 km long profile extending from Hood Canal in western Washington across the central Cascade Range to Walla Walla in eastern Washington. Existing Pacific Northwest Seismograph Network stations and earthquake sources are employed in a refraction/wide‐angle reflection interpretation. In the preferred model, crustal velocity decreases slightly from west to east in the depth range 10–25 km beneath the Cascade Range. The continental Moho is estimated to dip 2.7° to the west in eastern Washington and 4.4° to the east beneath Puget Sound forming a distinct crustal root for the Cascade Range. Crustal thickness ranges from ∼35.5 km on the west end beneath Puget Sound and 34 km on the east end near Walla Walla to 47 km under the high Cascades. Seismic velocity in the interpreted crustal root suggests rock of mafic composition which is consistent with a process of underplating resulting from dehydration of the subducting Juan de Fuca plate. Simplified analysis also suggests that the present topography of the Cascades in the vicinity of our profile is supported by the isostatic response of this root zone.
Rupture process of the 1990 Luzon, Philippines (Mw = 7.7), earthquake
Authors: Velasco Aaron A. , Ammon Charles J. , Lay Thorne , Hagerty Michael
Published: February 2004

The rupture process of the destructive July 16, 1990, Luzon, Philippines, earthquake is analyzed using a long‐period surface wave spectral inversion technique to estimate the average source properties and a broadband body wave and surface wave empirical Green function technique to investigate the slip distribution. The average source model has a seismic moment of 4.2 ± 0.1 × 1020 N m (Mw = 7.7), a total duration of 30 to 35 s, a left lateral strike‐slip focal mechanism with strike, ϕ = 153°, dip, δ = 89°, and rake, λ = 16°, and a north‐northwestward directivity. These results are consistent with previous work on the Luzon earthquake. However, the broadband analysis indicates that, for periods longer than about 20 s, the teleseismic source time function is characterized by a single, relatively smooth pulse of energy release with azimuthal variations in duration of 24 to 60 s. This contrasts with previous body wave results that inferred two discrete pulses of energy release. The simpler source process is inferred as a result of using empirical Green functions rather than standard theoretical Green functions for a layered crustal model, and by considering only the seismic energy with periods longer than 20 s. The Luzon earthquake began rupturing bilaterally, but evolved into a predominantly northwest directed rupture that extended for 75 to 100 km. The largest slip (10–15 m) occurred about 25 km northwest of the epicenter, beyond which the slip decreased gradually as the rupture propagated into a restraining bend. The peak slip at depth appears to exceed the surface rupture by a factor of 2 to 3. There is little evidence for significant energy release later than 50 s after the rupture initiated.
Short slip duration in dynamic rupture in the presence of heterogeneous fault properties
Authors: Beroza Gregory C. , Mikumo Takeshi
Published: February 2004

Recent studies of strong motion data consistently show that the risetime (duration of slip at particular locations on the fault) is significantly shorter than the overall rupture duration. The physical explanation for this observation and its implications have become central issues in earthquake source studies. Two classes of mechanisms have been proposed to explain short risetimes. One explanation is that velocity‐weakening frictional behavior on the fault surface causes the fault to self‐heal. This possibility is suggested by rate‐dependent friction observed in laboratory experiments and by some two‐dimensional dynamic numerical simulations of earthquake rupture. It has recently been demonstrated, however, that the velocity dependence of friction observed in the laboratory is too weak to cause faults to self‐heal. An alternative explanation for short risetimes is that spatially heterogeneous fault strength (e.g., barriers) limit the slip duration. In this paper we investigate this second explanation for short risetimes by constructing a three‐dimensional dynamic rupture model for the 1984 Morgan Hill, California earthquake (Mw= 6.2) using a kinematic model previously obtained from waveform inversion of strong motion data. We assume velocity‐independent friction and a critical stress fracture criterion and derive a dynamic model specified by the spatial distribution of dynamic stress drop and strength excess that reproduces the slip and rupture time of the kinematic model. The slip velocity time functions calculated from this dynamic model are then used in a subsequent inversion to fit the strong motion data. By alternating between dynamic and kinematic modeling, we obtain a dynamic model that provides an acceptable fit to the recorded waveforms. In this dynamic model the risetime is short over most of the fault, which is attributable entirely to the short scale‐length slip/stress drop heterogeneity required by the strong motion data. A self‐healing mechanism, such as strongly velocity‐dependent friction, is not required to explain the short risetimes observed in this earthquake.
Slow, unconfined spreading of a mudflow
Authors: Coussot Philippe , Proust Sébastien
Published: February 2004

Mudflows are natural, highly concentrated water‐clay‐grain mixtures that flow in mountain streams after long or intense rainy periods and may cause considerable damage if they overflow on the alluvial fan. The possibility of predicting the extent of these flows on the basis of material and flow parameters is examined. The simplest realistic case of a yield stress mudflow moving through a narrow open channel followed by a wide, long plane is considered. It is demonstrated that the unconfined flow of a yield stress fluid over an inclined plane cannot be uniform; even in steady state the flow width should increase continuously from the channel exit. A complete treatment of the flow equation on the basis of the long‐wave approximation, including an appropriate three‐dimensional expression for the constitutive equation, makes it possible to establish a system of equations from which flow characteristics at any point (longitudinal and lateral mean velocities and fluid depth) can be deduced. In particular, for a Herschel‐Bulkley fluid with a flow index of 1/3 it is found that the lateral extent will increase proportionally to the distance from the channel exit to the power 9/20 and that, in the sheared part, the fluid depth in a cross section will have a parabolic distribution. Experiments have been carried out with fine mud suspensions (at different solid fractions) whose rheological behavior is similar to that of natural mudflows. The theory is in fair agreement with experimental data concerning fluid depth distribution but systematically overestimates lateral extent (by 30%). This is certainly due to the fact that the assumption of lateral extent much smaller than flow length is not respected in our tests.
The ocean‐continent boundary off the western continental margin of Iberia: Crustal structure west of Galicia Bank
Authors: Whitmarsh Robert B. , White Robert S. , Horsefield Susan J. , Sibuet Jean‐Claude , Recq Maurice , Louvel Véronique
Published: February 2004

A seismic refraction transect across the Galicia Bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin‐parallel peridotite ridge (PR). Immediately west of the PR, oceanic crust is only 2.5–3.5 km thick, but farther west (oceanward) it thickens to 7 km. The PR caps a ∼60‐km‐wide lens‐shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust. When superimposed on a reflection time version of the velocity model, the S reflector is clearly intracrustal at its east end. Westward, S cuts down to lower crustal levels, eventually coinciding with the top of the serpentinized peridotite lens (original crust‐mantle boundary). These observations render almost impossible the seafloor exposure of the PR by S acting as a top‐to‐the‐west detachment fault. Numerical models of melting and borehole subsidence information constrain our rifting model. The easternmost continental crust experienced a total stretching factor of 4.3 (most likely in two stages); it probably occurred over ∼25 m.y., with the highest rate of stretching at the beginning of the main earlier rift phase (Valanginian; 141–135 Ma). The 3 (4.7) km thick continental crust (depending on whether serpentinized peridotite is assigned to crust or mantle), which may include melt products, requires stretching factors of more than 11 (7) and a rift duration of more than 25 (13) m.y. The thin oceanic crust immediately west of the PR is explained by conductive cooling of the mantle during the long prebreakup stretching phase, which temporarily caused reduced melting immediately after breakup.
Seismic triggering by rectified diffusion in geothermal systems
Authors: Sturtevant Bradford , Kanamori Hiroo , Brodsky Emily E.
Published: February 2004

Widespread seismicity was triggered by the June 28, 1992, Landers California, earthquake at a rate which was maximum immediately after passage of the exciting seismic waves. Rectified diffusion of vapor from hydrothermal liquids and magma into bubbles oscillating in an earthquake can increase the local pore pressure to seismically significant levels within the duration of the earthquake. In a hydrothermal system modeled as a two‐component H2O‐CO2 fluid in porous rock the pressure initially increases linearly with time. The rate of pressure buildup depends sensitively on the mean bubble radius and is large for small bubbles. The diffusion‐induced pressure is relaxed by percolation and resorption of vapor into the liquid solution. The induced seismicity itself also relieves stress. Values of parameters used in the present calculations give results consistent with observations of triggered seismicity at Long Valley caldera after the Landers earthquake. For one representative condition, at 250°C and 5.6 km depth, oscillating strain acting on 10‐μm‐diameter bubbles increases pore pressure at the rate of 151 Pa/s resulting in a pressure increase of 12 kPa in the 80‐s duration of the Landers earthquake. The elevated pressure induced by a single 26‐m‐diameter cloud of bubbles in saturated rock relaxes by percolation through soil of 0.2‐mdarcy permeability in 53.6 hours. Observations of earthquake swarms at other locations suggest that self‐induced buildup of pore pressure by rectified diffusion can provide a positive feedback mechanism for amplifying seismicity.
Acoustic waves in pressurized boreholes: A finite difference formulation
Authors: Sinha Bikash K. , Liu Qing‐Huo , Kostek Sergio
Published: February 2004

A velocity‐stress, finite difference formulation of acoustic waves in a fluid‐filled, pressurized borehole yields synthetic waveforms for monopole or dipole sources before and after borehole pressurization. Processing of these waveforms using a variation of Prony's procedure isolates dominant arrivals in the full wave field. Differences between the slownesses of individual arrivals before and after pressurization provide stress‐induced changes in propagation characteristics that are of importance in estimating mechanical properties of the formation. The borehole pressurization in an isotropic formation produces insignificant changes in the compressional head wave slownesses; and small changes in the shear head wave slownesses. The most significant changes occur in the Stoneley and flexural slownesses at relatively higher frequencies in the range of 5–10 kHz for a borehole of diameter 20.32 cm (8 inches). These differences in the Stoneley and flexural slowness dispersions for a known increase in the borehole pressure can be used to calculate the acoustoelastic coefficients of the formation. These coefficients are measures of nonlinear elastic parameters of the formation that are generally larger for poorly consolidated slow formations than those of tightly consolidated fast formations.
Tidal currents in the Indonesian Seas and their effect on transport and mixing
Authors: Hatayama Takaki , Awaji Toshiyuki , Akitomo Kazunori
Published: February 2004

We have investigated the characteristics of tides and tidal currents in the Indonesian Seas, with particular emphasis on the predominant constituents, the barotropic M2 and K1 components. The calculated harmonic constants of the M2 and K1 tides agree well with those obtained from extensive tide gauge observations: the mean root‐mean‐square errors between models and observations for M2 and K1 over the entire Indonesian Seas are 11.2 and 8.65 cm, respectively, smaller than previous studies. The features of the tidal current field are qualitatively similar to those reported by Wyrtki [1961]. Relatively strong tidal currents are found in the Java Sea and in the vicinities of the narrow straits, such as the Lombok and Malacca Straits. Tracking of numerous labeled particles in the calculated velocity field clearly shows that the tidal currents play essential roles in the transport and mixing processes in the Indonesian Seas. For example, in the eastern Indonesian Seas a significant clockwise residual circulation appears around Buru Island, due primarily to the tidal rectification over variable bottom topography, with a minor impact from the nonlinear effects around narrow straits. The tide‐induced residual transport from the Seram Sea to the eastern Banda Sea attains about 3.8 Sv (1 Sv = 106 m3 s−1), comparable to that of the Indo‐Pacific throughflow along the eastern route. In the western Indonesian Seas (the Java Sea and the Makassar Strait) the residual transport is much smaller than the throughflow transport along the western route. Our result suggests, however, that the properties of water flowing through this route could be modified by strong tidal mixing around the shelf of the southern Makassar Strait and the Lombok Strait.
Hydrological and chlorofluoromethane measurements of the Indonesian throughflow entering the Indian Ocean
Authors: Fieux M. , Andrié C. , Charriaud E. , Ilahude A. G. , Metzl N. , Molcard R. , Swallow J. C.
Published: February 2004

The Java Australia Dynamic Experiment high‐resolution February–March 1992 conductivity‐temperature‐depth and chlorofluoromethane section obtained between Australia and Bali and on the sills between Flores, Sumba, Sawu, Roti, and the Australian continental shelf allows detailed examination of the water masses distribution and their inferred circulation. A sharp hydrological front between the Indonesian waters and the southern Indian Ocean waters is found between 13°S and 14°S in both seasons (February–March 1992 and August 1989). It separates the high‐salinity surface waters to the south from the lower‐salinity surface waters derived from the Indonesian Seas to the north. It reaches the surface in February 1992, whereas it was capped by a particularly low salinity surface layer in August 1989. Near Bali, the NW monsoon of February–March produces large intrusions of low‐salinity water from the Java Sea, through Lombok Strait in the upper 100 m. At depth, the North Indian Intermediate Water, flowing along the Indonesian coast, brings salty, low‐oxygen and low‐chlorofluorocarbon water. It enters the Sawu Sea through Sumba Strait toward the east, while it undergoes strong mixing with the Indonesian Seas water. The primary pathway of the Indonesian waters is found north of the front and south of the North Indian Intermediate Water, between 13°S and 9°30′S, and the associated salinity minimum can be followed all across the Indian Ocean.
Mean sea surface topography in the Baltic Sea and its transition area to the North Sea: A geodetic solution and comparisons with oceanographic models
Authors: Ekman Martin , Mäkinen Jaakko
Published: February 2004

A consistent height system designed for comparisons between geodesy and oceanography is presented for the Baltic Sea area. Mean sea surface topography is computed geodetically in this height system at 42 reliable long‐term sea level stations, connected by high‐precision levelings, along the coasts of the Baltic Sea, the Kattegat, the Skagerrak, and the adjacent part of the North Sea. Based on this, a map is drawn of the mean sea surface topography in the Baltic Sea and its transition area to the North Sea. The general agreement between our geodetic solution and recent oceanographic models turns out to be excellent; the discrepancies rarely exceed 2–3 cm. Hence the mean sea surface topography in the Baltic Sea area to a dominating extent is caused by the distribution of salinity. There are two main characteristics of the sea surface topography found here. First, there is a continuous increase of the sea surface height from the North Sea into the Baltic Sea, the height difference between the inner part of the Gulf of Bothnia and the Skagerrak amounting to 35–40 cm. The main reason behind this is the considerable difference in salinity, close to the maximum possible one. Second, there is a steep sea level gradient in the border zone between the Kattegat and the Skagerrak, reaching 2 cm per 10 km. This reflects the salinity front there, separating the brackish Baltic Sea water from the saline North Sea water, and the associated Baltic current. A local maximum in the sea surface can be seen in the Oslo Fiord, reflecting an accumulation of low‐salinity water there. An area where there is still no oceanographic model agreeing sufficiently with the geodetic solution is the Belt Sea/southwestern Kattegat.
Dynamics of the East India Coastal Current: 1. Analytic solutions forced by interior Ekman pumping and local alongshore winds
Authors: Shankar D. , McCreary J. P. , Han W. , Shetye S. R.
Published: February 2004

A linear, continuously stratified model is used to investigate how forcing by interior Ekman pumping and local alongshore winds affects the East India Coastal Current (EICC). Solutions are found analytically to an approximate version of the equations of motion. They are obtained in a basin that resembles the actual Bay of Bengal north of 6°N and are forced by Hellerman and Rosenstein [1983] winds. The mathematical structure of the solution clearly illustrates the model physics. In response to interior Ekman pumping, baroclinic Rossby waves are excited in the interior of the Bay; they propagate to the western boundary, where they generate a northward, coastal current if the interior circulation is anticyclonic and vice versa. In contrast, the response forced by the local alongshore winds is a coastal current that is trapped within the surface mixed layer. Consistent with the observed surface current, the model EICC flows northward along the Indian coast from March to September and equatorward along the Indian and Sri Lankan coasts from October to January. In contrast to the observations, however, the onset of northward flow along the Indian coast occurs 1–2 months late, the Sri Lankan coastal current does not reverse to flow southward during the summer, and the maximum northward transport of the model EICC is 5 Sv in May, only about half the transport estimated from hydrographic data. We conclude that, although interior Ekman pumping and local alongshore winds have a significant impact on the EICC, other driving mechanisms must be taken into account in order to simulate the observed variability adequately.
Persistent westward flow in the Azores Current as seen from altimetry and hydrography
Authors: Cromwell David , Challenor Peter G. , New Adrian L. , Pingree Robin D.
Published: February 2004

Temporal and spatial variability of the eastward flowing, jet‐like Azores Current is investigated by applying a new method for combining altimetry with hydrography in order to produce absolute surface geostrophic currents [Challenor et al., 1996]. The altimeter surface measurements contain information about both the geoid and the geostrophic ocean circulation: the geoid signal dominates and is generally not sufficiently well known at the small length scales required to identify the mesoscale ocean signal. In this study, hydrography is used to distinguish the mean ocean signal from the geoid along an ERS 1 satellite track south of the Azores during both 3‐day repeat phases and thereby determine cross‐stream absolute dynamic topography of the Azores Current at high temporal resolution. Hydrographic data collected near 28°W in March 1992 is combined with ERS 1 altimeter data from January–March 1992 and January to early April 1994. Time series of 3‐day repeat profiles of absolute surface geostrophic velocity perpendicular to the altimeter/cruise track are examined. The Azores Current absolute velocity profile along this track is rather coherent: the current variability is less than the mean. Surprisingly, a persistent westward flow of ∼25 cm s−1 is observed near 35°N, 28°W. This flow appears to be a “retroflection” from the main eastward flowing current. An isopycnal analysis reveals the westward flow to be cooler (by 0.5°C) and fresher (by 0.15 practical salinity units) than the main body of the Azores Current. The flow appears to persist throughout each of the above 3‐month periods in late winter/early spring, but the question of whether this is a seasonal feature or more permanent remains open. It is suggested that the westward flow is topographically controlled.
Effects of topographic stress on Caribbean Sea circulation
Authors: Sou Tessa , Holloway Greg , Eby Michael
Published: February 2004

A numerical model of the Caribbean Sea is modified by a parameterization of eddy‐topographic forcing. Inclusion of the parameterization strengthens deep cyclonic flow, which is consistent with observations and inverse model inferences.
Doppler radar measurements of wave groups and breaking waves
Authors: Smith M. J. , Poulter E. M. , McGregor J. A.
Published: February 2004

A 3‐GHz Doppler radar has been used to study wave dynamics and backscatter from the sea surface at low grazing angles. Vertical polarization results are dominated by Bragg scatter even at low (∼8°) grazing angles. Horizontal polarization results, however, show a strong upwind‐downwind asymmetry with additional, high‐velocity intermittent scatter in the upwind direction associated with steep or breaking waves. These characteristics have been exploited to distinguish spilling breaking events from the background Bragg scatter. While these “spikes” at a single range may appear random in time, the combined range and time information reveals a well‐determined propagation pattern. It is shown that for a developing sea in deep water, group behavior modulates the occurrence of wave breaking. The frequency‐wavenumber spectrum shows a clear separation between the linear dispersion curve and nonlinear effects related to breaking. The most important nonlinear feature is a line near the dominant wave group velocity which is identified with the spectrum of breaking intermittency. The slope of this line suggests that the wave components which are most likely to break lie at frequencies significantly above the dominant wave frequency.
Empirical orthogonal function analysis of global TOPEX/POSEIDON altimeter data and implications for detection of global sea level rise
Authors: Hendricks James R. , Leben Robert R. , Born George H. , Koblinsky C. J.
Published: February 2004

Two years of TOPEX/POSEIDON altimeter data are examined to determine the dominant spatial features and timescales of sea surface height variability in the global oceans and to estimate the rate of global sea level rise. Empirical orthogonal function (EOF) decomposition of 69 cycles of TOPEX altimeter data into the significant modes of variability reveals dominant annual and interannual timescales. The annual modes include the hemispheric‐scale changes in steric height due to seasonal heating variations, changes in the strength of the major current systems in the equatorial Pacific, and the reversing monsoonal circulation in the Indian Ocean. The interannual modes capture oscillations in the tropical Pacific characteristic of recent El Niño events. A 2‐year history of the change in mean sea level derived from TOPEX altimeter data reveals a rise of 5.2 mm/yr. By analyzing the contribution of each EOF mode to global mean sea level variations, we find that 82% of the rise in mean sea level is caused by a single interannual mode of variability. Altimeter data spanning only 2 years, however, are insufficient to resolve a complete El Niño‐Southern Oscillation (ENSO) cycle which dominates the interannual EOF modes. Thus most of the rise in mean sea level derived from TOPEX altimetry is an artifact of incomplete temporal sampling of interannual variability. When a longer time series of TOPEX altimeter data is obtained and a complete ENSO cycle is observed, a significant reduction in the rate of global mean sea level rise estimated from TOPEX altimetry is expected. Most of the remaining rise in global mean sea level is explained by the annual EOF modes, suggesting a possible connection between sea level rise and changes in the steric component of sea surface height.
Fine structure and microstructure characteristics across the northwest Atlantic Subtropical Front
Authors: Polzin K. L. , Oakey N. S. , Toole J. M. , Schmitt R. W.
Published: February 2004

Vertical profiles of temperature, salinity, horizontal velocity, rate of dissipation of turbulent kinetic energy, and thermal variance are used to examine the parameterization of turbulent mixing associated with internal waves in an upper ocean front. Previous attempts to quantify the rate of turbulent dissipation [e.g., Gregg, 1989; Polzin et al., 1995] are based upon dynamical models of wave‐wave interactions using the Garrett and Munk [1979] (GM) spectrum. Non‐GM conditions (vertical and horizontal anisotropy, deviations in vertical wavenumber and frequency spectral shapes, and the presence of a background flow) may affect the character of wave interactions, altering the upwavenumber energy flux, which is equated with the rate of turbulent dissipation. Non‐GM conditions in the data are documented, and revisions to the wave‐wave interaction models are discussed. Additionally, the strength of wave‐wave interactions is directly compared with the strength of wave‐mean flow interactions. These data indicate that the dissipation rate is relatively insensitive to the presence of a background flow and anisotropic wave conditions, in agreement with theoretical expectations based upon wave‐wave interaction models and a model of wave‐mean flow interaction at high background Richardson number. These data support the findings of Polzin et al. [1995], who argue that the dissipation rate is most sensitive to variations in wave frequency.
Dynamics of the Brazil‐Malvinas Confluence based on inverted echo sounders and altimetry
Authors: Goni Gustavo , Kamholz Scott , Garzoli Silvia , Olson Donald
Published: February 2004

We use data from Geosat altimeter and from 10 inverted echo sounder (IES) moorings deployed in the SW Atlantic Ocean off the Argentine continental shelf to investigate several aspects of the dynamics of the upper layer in the Brazil‐Malvinas Confluence region. We use the altimeter data to estimate the sea height anomalies at each IES location and use the IES data to compute the upper layer thickness, taken in this work to go to the depth of the 8°C isotherm. We first discuss the sea height and upper layer thickness variations caused by the passage of the Brazil Current, Malvinas Current, and warm anticyclonic and cold cyclonic eddies. We introduce a two‐layer model in which we decompose the sea height into its baroclinic and barotropic contributions. We then propose a method to monitor the thickness of the upper layer and the barotropic and baroclinic transports as a function of the sea height anomalies and the statistics of the upper layer thickness and reduced gravity for the region. We compute the reduced gravity values from the slope of a linear fit between the sea height anomalies and the upper layer thicknesses. We estimate the reduced gravity values for this region to range from 0.005 to 0.011 m s−2. We also estimate the mean barotropic sea height difference using two methods: conservation of mass and conservation of potential vorticity. Finally, we compute the time series for the baroclinic and barotropic transports during the Geosat Exact Repeal Mission time period. Our results suggest that the mean baroclinic transport in the upper layer decreases from 12 Sv at around 35°S to 7 Sv at 37°S. Our results also indicate that there is a significant barotropic contribution to the upper layer transport in the confluence region.
The determination of the large‐scale circulation of the Pacific Ocean from satellite altimetry using model Green's functions
Authors: Stammer Detlef , Wunsch Carl
Published: February 2004

A Green's function method for obtaining an estimate of the ocean circulation using both a general circulation model and altimetric data is demonstrated. The fundamental assumption is that the model is so accurate that the differences between the observations and the model‐estimated fields obey a linear dynamics. In the present case, the calculations are demonstrated for model/data differences occurring on a very large scale, where the linearization hypothesis appears to be a good one. A semi‐automatic linearization of the Bryan/Cox general circulation model is effected by calculating the model response to a series of isolated (in both space and time) geostrophically balanced vortices. These resulting impulse responses or “Green's functions” then provide the kernels for a linear inverse problem. The method is first demonstrated with a set of “twin experiments” and then with real data spanning the entire model domain and a year of TOPEX/POSEIDON observations. Our present focus is on the estimate of the time‐mean and annual cycle of the model. Residuals of the inversion/assimilation are largest in the western tropical Pacific, and are believed to reflect primarily geoid error. Vertical resolution diminishes with depth with 1 year of data. The model mean is modified such that the subtropical gyre is weakened by about 1 cm/s and the center of the gyre shifted southward by about 10°. Corrections to the flow field at the annual cycle suggest that the dynamical response is weak except in the tropics, where the estimated seasonal cycle of the low‐latitude current system is of the order of 2 cm/s. The underestimation of observed fluctuations can be related to the inversion on the coarse spatial grid, which does not permit full resolution of the tropical physics. The methodology is easily extended to higher resolution, to use of spatially correlated errors, and to other data types.
Pacific low‐latitude western boundary currents and the Indonesian throughflow
Authors: Lukas Roger , Yamagata Toshio , McCreary Julian P.
Published: February 2004

In this introductory paper, we review observations and theories of the low‐latitude western boundary currents in the Pacific Ocean and the Pacific‐to‐Indian Ocean throughflow, including the new results presented in this special section. In addition, we summarize issues and questions that remain to be addressed. In particular, we point to the need for completing the zero‐order description of the mean and seasonal variation of the circulations of the region.
The evolution of the bottom boundary layer on the sloping continental shelf: A numerical study
Authors: Middleton John F. , Ramsden David
Published: February 2004

Using the Mellor‐Yamada level II closure scheme, a numerical study is made of the bottom boundary layer on a uniform, stratified shelf with slope α. For constant interior along‐slope currents ug and molecular background diffusion, a new timescale for shutdown of bottom stress is derived for the downwelled layer as well as a new estimate for the maximum height of the upwelled layer, hU = (CD/fN)1/2ug(1 + S1/2)−1 where CD = 2.5 × 10−3 is a drag coefficient, f and N are the Coriolis and buoyancy frequencies, and S = (Nα/f)2 is the Burger number. The Richardson number is also shown to fix the thermal‐wind shear at the top of the upwelled layer to be approximately
, while within the layer, a depth‐averaged form of the geostrophic balance is shown to hold. In the case of large interior vertical diffusivities (10−4 m2s−1), the fluxes of momentum and buoyancy into the interior are quantified and shown to result in the arrest (enhancement) of shutdown in the upwelling (downwelling) boundary layers. Advective restratification of mixed water is not found to occur, and for an 8‐day periodic interior current ug(t), the thermal‐wind shear that remains during the downwelling and upwelling phases can result in a 1.2‐day lag of interior current with bottom stress. Bottom stress and the cross‐slope Ekman flux are found to be surprisingly symmetric during the upwelling and downwelling phases of the interior current. The lag with bottom stress and effects of shutdown are shown to persist in the presence of tidal currents and large background diffusion and may well be significant on the continental shelf.