bluuebunny/crossref_metadata_2025 huggingface dataset:
Lead isotope systematics in Pacific hydrothermal sulfide deposits
Authors: Fouquet Y. , Marcoux E.
Published: February 2004

Sulfide samples from 12 hydrothermal volcanogenic massive sulfide (VMS) sites around the Pacific Ocean were analyzed for their lead isotope composition. They correspond to various geological environments, such as sediment‐starved ridges (East Pacific Rise (EPR), Galapagos, Juan de Fuca), sediment‐covered ridges, and back arc basins. Sulfides from all areas display remarkably homogeneous lead isotope compositions caused by hydrothermal circulation. Our results show that lead isotope compositions of sulfides have two major implications. The first aspect, at the scale of a volcanic segment, concerns metallogenic processes. Lead isotope compositions demonstrate that lead is leached from basalt in sediment‐starved ridges and that part is derived from sediment at sediment‐covered ridges. This conclusion confirms previous studies. At 13°N on the EPR, results are compatible with two separate hydrothermal systems for axial and off‐axial sulfide deposits. At this site, lead isotopes contribute to a better definition of the morphology of convective cells. Comparisons with the lead isotope composition of fossil VMS deposits open the discussion on the processes that produced highly homogeneous deposits in the same district. The second aspect, at a more regional scale, concerns the geodynamic and geochemical processes occurring at plate boundaries. Owing to homogenization through hydrothermal circulation, lead isotope compositions in VMS deposits can be considered as having the mean lead isotope composition of basalts from the same volcanic segment. In the Lau Basin, a slab with a Pacific lead isotope composition is subducted beneath the Indian Ocean lithosphere. The lead isotope composition of VMS samples illustrates the interaction of three contrasting end‐member sources: basalts from Pacific Ocean and Indian Ocean plates and pelagic sediments. Our results show the progressive northward decrease of Pacific arc influence with increasing age of the ridge and distance to the island arc.
Numerical simulation of elastic wave propagation using a finite volume method
Authors: Dormy Emmanuel , Tarantola Albert
Published: February 2004

Like the finite difference method, the finite volume method gives an approximate value for the derivative of a field at a given point using the values of the field at a few locations neighboring the point. The method uses the divergence theorem, considers a “finite volume” around the point and discretizes the surface bounding the volume. When the finite volumes considered are regular polyhedra, one obtains the expressions corresponding to standard centered finite differences, but the finite volume method is more general than the finite difference method because it may deal directly with irregular grids. It is possible to give a finite volume formulation of the elastodynamic problem, using dual volumes, that correspond, in the regular case, to the staggered grids used in the finite difference method. The scheme thus obtained is more general than the one obtained using finite differences, as the “grids” may be totally unstructured, but at the cost of having, in the general case, only a first‐order accuracy. Although the scheme is not consistent, numerical tests suggest that it is stable and convergent. This implementation of a finite volume method does not provide a way for a more general treatment of the boundaries than the conventional finite difference method.
Densification of quartz powder and reduction of conductivity at 700°C
Authors: Lockner David , Evans Brian
Published: February 2004

We have measured axial strain, volumetric strain, and electrical conductivity during the densification at 700°C of ultra‐fine quartz powder (5–10 μm diameter) saturated with distilled water. Individual experiments were run at confining pressures ranging from 200 to 370 MPa and pore pressures of 30, 100, and 200 MPa. During the experiments, which lasted from 10 hours to 8 days, the porosity decreased from an initial value of about 40% to final porosities ranging from 19% to as little as 8±1%. In all experiments, initial volumetric compaction rates were rapid (10−5 to 10−6 s−1) but decreased to between 10−7 and 10−8 s−1 after approximately 1 day. Electrical conductivity also decreased monotonically from 10−2 to 10−4 S/m during the experiments. We present a model in which changes in conductivity are controlled by constrictions in interconnecting channels, while porosity is controlled primarily by deposition of quartz in the pores. Both experimental and model results suggest a densification process in which conductivity reduces to matrix conductivity while leaving a residual porosity of 3–5%. In the Earth, the porosity at a given instant will be the net resultant of porosity reducing processes and porosity producing processes including fracturing. The rapid loss of conductivity and, by inference, permeability suggests that in the absence of processes which increase permeability, both of these properties should have very small values in the lower crust. Furthermore, these experiments lend support to arguments that time‐dependent compaction of fault gouge can play an important role in modifying fluid pressure and fault strength during the interseismic portion of the earthquake cycle for large earthquakes.
Travel time inversion with a priori information on illuminated parts of the reflectors
Authors: Jannaud L. R. , Delprat‐Jannaud F.
Published: February 2004

Since the solution of reflection tomography is underdetermined, the use of a priori information is essential to compute a model that is satisfactory from a geological point of view. We propose to compute a model that, classically, matches in a least squares sense the observed travel times and that also satisfies some additional useful constraints on the portions of reflectors illuminated by the rays. The implementation of this method requires the computation of the Jacobian of the impact point of a ray on a reflector with respect to the model. This computation is performed by the adjoint state technique, which efficiently produces exact derivatives. The utility of this approach is illustrated on a field data set acquired on fault structures. The method produces a model that matches the kinematic information under the constraint that certain reflectors do not cross the fault.
Dike intrusion as a trigger for large earthquakes and the failure of volcano flanks
Authors: Elsworth Derek , Voight Barry
Published: February 2004

Pore fluid pressures induced through dike intrusion have the capability to trigger large earthquakes and to initiate and sustain massive and catastrophic failure of volcano flanks. Suprahydrostatic pore pressures are generated as a result of both mechanical and thermal straining of the rock‐fluid medium. Mechanical strains and resulting pore pressures are described using an analogy to a moving volumetric dilation within a porous elastic medium. Thermal pore fluid pressures are simply represented by a static diffusive model subjected to uniform temperature rise at the dike interface. Resulting excess pore pressure distributions acting along the base of a wedge‐shaped slide block are used to define the stability of the flanks when subjected to the magma pressures that accompany intrusion. The destabilizing influence of mechanically induced pore pressures is maximized as the intruded width, or corresponding overpressure, of the dike is increased. For realistic parameter magnitudes in volcaniclastic materials, induced pore pressures are capable of initiating failure; however, the mechanical influence is restricted to the proximity of the intrusion, and by itself may not be capable of sustaining sliding motion once it is initiated. The destabilizing influence of thermally induced pore pressures is conditioned by the severity of thermal forcing, ratios of thermal and hydraulic diffusivities, and the time available for the combined thermal and pressure disturbance to propagate outward from the dike. Thermal pressurization of pore fluid extends more slowly than mechanical straining but is shown to be capable of developing massive uplift forces that could initiate and possibly sustain failure along downslope‐dipping failure planes. Deviatoric stress‐induced generation of pore pressures and frictional heating of pore fluids may act following slide initiation to maintain the impetus of flank failure and enable long runout over compressible marine sediments. In addition to the development of shallow failure, limited deep‐seated sliding may also result from the mechanical or thermal pore pressure loading mechanisms. In addition, pore pressures generated through intrusion or frictional heating may trigger, or amplify, large earthquakes, and these in turn may aid shallower flank slip through seismic ground accelerations and vibration‐induced pore pressure generation. These phenomenological models of pore pressure rise and stability control may aid comprehension of the cyclic growth, lateral expansion, and subsequent destruction of shield and stratovolcano flanks. Examples in this work refer particularly to oceanic volcanoes typified by the Hawaiian Islands and Réunion Island.
Stability of coda Q in the region of Parkfield, California: View from the U.S. Geological Survey Parkfield Dense Seismograph Array
Authors: Hellweg Margaret , Spudich Paul , Fletcher J. B. , Baker L. M.
Published: February 2004

Many investigators have proposed that changes in the rate at which the coda decays may be an intermediate term precursor to moderate‐to‐large earthquakes. Parkfield, California, on the San Andreas Fault, is a promising location for studying premonitory changes in coda Q, Qc, because a large earthquake is likely to occur there. We have investigated Qc using recordings from the U.S. Geological Survey Parkfield Dense Seismograph Array, which is a digital array with 14 triaxial sensors and an aperture of about 1 km. For each earthquake we can measure Qc from up to 42 recordings. Their average is more stable than the measurement from a single station. Using clustered seismicity, we have developed criteria for selecting events and reducing scatter in the measurement. The Qc value determined from a seismogram depends on the position and length of the analysis window. Thus Qc should always be measured from the same length window starting at the same lapse time regardless of the source location. In addition, the band‐limited signal‐to‐noise ratio at the end of the analysis window is important. Qc determined in two frequency bands, 4–8 Hz and 8–16 Hz, from a tight cluster of 26 events which occurred between December 1989 and January 1994 has not changed, despite M 4.7 and M 4.6 events in October 1992 and November 1993. Qc measured from local events (Δ < 60 km) in three frequency bands shows larger scatter but has also not changed during this period. For monitoring Qc, observations should include array averaged measurements from a single lapse time. Because Qc measurements made using an analysis window that starts at a constant multiple of the S wave lapse time depend on epicentral distance, a procedure combining the evaluation of the time and distance dependences of Qc also gives stable observations.
New seismic gap hypothesis: Five years after
Authors: Kagan Y. Y. , Jackson D. D.
Published: February 2004

We use earthquake data from 1989–1994 to test a forecast by Nishenko based on the seismic gap theory. We refer to this forecast as the “New Seismic Gap” hypothesis, because it is the first global forecast based on the seismic gap hypothesis that considers the recurrence time and characteristic earthquake magnitude specific to each plate boundary segment. Nishenko's forecasts gave probabilities that each of about 100 zones would be filled by characteristic earthquakes during periods of 5, 10, and 20 years beginning on the first day of 1989. Only the first of these can be tested now. We used three tests based on (1) the total number of zones filled by characteristic earthquakes, (2) the likelihood that the observed list of filled zones would result from a process with the probabilities specified in Nishenko's hypothesis, and (3) the likelihood ratio to that of a Poissonian null hypothesis. The null hypothesis uses a smoothed version of seismicity since 1977 and assumes a Gutenberg‐Richter magnitude distribution. We used both the Harvard Centroid moment tensor and the National Oceanic and Atmospheric Administration preliminary determination of epicenters catalogs in our test. We also used several different magnitude cutoffs in our tests, because Nishenko's forecast did not specify a clear relationship between the characteristic earthquake magnitude and the threshold magnitude for a successful prediction. Using a strict interpretation, that only earthquakes equal to or larger than the characteristic magnitude should be counted, both catalogs show only two qualifying earthquakes in the entire area covered by the forecast. The predicted number is 9.2, and the discrepancy is too large to result from chance at the 99% confidence level. The new seismic gap hypothesis predicts too many characteristic earthquakes for three reasons. First, forecasts were made for some zones specifically because they had two or more earthquakes in the previous centuries, biasing the estimated earthquake rate. Second, open intervals before the first event and after the last event are excluded in calculation of recurrence rate. Third, the forecast assumes that all slip in each zone is released in characteristic earthquakes of the same size, while in fact considerable slip is released by both smaller and larger earthquakes. The observed size distribution of earthquakes is inconsistent with the characteristic hypothesis: instead of a deficit of earthquakes above the characteristic limit, earthquake numbers are distributed according to the standard Gutenberg‐Richter relation. By lowering the magnitude threshold for qualifying earthquakes, it is possible to reduce the discrepancy between the observed and predicted number of earthquakes to an acceptable level. However, for every magnitude threshold we tried, the new seismic gap model failed the test on the number of filled zones, or the likelihood ratio test, or both, at least at the 95% confidence level.
Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic
Authors: Cande S. C. , Kent D. V.
Published: February 2004

Recently reported radioisotopic dates and magnetic anomaly spacings have made it evident that modification is required for the age calibrations for the geomagnetic polarity timescale of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted geomagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is consistent with astrochronology in the Pleistocene and Pliocene and with a new timescale for the Mesozoic.
The A‐type Mount Scott Granite sheet: Importance of crystal magma traps
Authors: Hogan John P. , Gilbert M. Charles
Published: February 2004

The presence of rapakivi feldspar and of distinctive porphyritic texture of Mount Scott Granite indicates a period of crystallization prior to final emplacement beneath an extensive penecontemporaneous rhyolite volcanic pile. Final crystallization conditions are interpreted to have been <50 MPa at depths < <1.4 km based on stratigraphic constraints. However, geobarometry based on the Al content of amphibole phenocrysts and comparison of granite compositions with phase relations in the SiO2‐NaAlSi3O8‐KAlSi3O8 ternary system both yield pressure estimates of ≈200 MPa. These pressure estimates are interpreted as plumbing the depth of a temporary storage chamber at ≈7–8 km. This depth coincides, in this case, both with the probable Proterozoic basement‐cover contact and with the calculated brittle‐ductile transition at time of ascent of Mount Scott magma. Although rising magma that fed the preceeding voluminous Carlton Rhyolite apparently rose unimpeded past these horizontal anisotropies, rising magma that formed Mount Scott Granite temporarily paused at this depth. Based on magmastatic calculations, we suggest that horizontal anisotropies (e.g., brittle‐ductile transition) become crustal magma traps where the magma driving pressure exceeds the lithostatic load when the anisotropy is encountered. During rifting, initial large influxes of magma may proceed passed crustal anisotropies but have the effect of increasing the relative magma driving pressure through reducing horizontal stress. Thus, magma driving pressure may eventually exceed the lithostatic load at the depth of the brittle‐ductile transition thereby activating this crustal magma trap. Ponding of magma at the brittle‐ductile transition chokes the eruption. Such a pause in magma supply rate may permit a return to initial stress conditions and deactivate the crustal magma trap. Once again magma will rise to the surface initiating a new magmatic cycle.
Ductile creep, compaction, and rate and state dependent friction within major fault zones
Authors: Sleep Norman H.
Published: February 2004

The shear traction on major strike‐slip faults during earthquakes is much lower than that expected on a frictionally sliding surface in equilibrium with hydrostatic pressure. The low shear traction is explained if the fluid pressure at the time of the earthquake is much greater than hydrostatic pressure. Ductile creep within mostly sealed fault zones compacts the matrix and thus increases fluid pressure between earthquakes. Frictional dilatancy during earthquakes decreases fluid pressure below hydrostatic, and over the earthquake cycle, the fault zone is in long‐term equilibrium with the country rock. This ductile mechanism is formally unified with rate and state theory for time‐dependent friction when the difference between a critical porosity where the rock loses all strength and the actual porosity of cracks is used as a state variable. This choice is justified by percolation theory of mostly broken lattices. Time‐dependent behavior associated with changes in normal traction in the laboratory is explained by the formalism. Instability (earthquakes) sometimes occurs in the numerical experiments. However, fairly small amounts of frictional dilatancy during initial frictional creep decrease fluid pressure and preclude unstable sliding. Two coupled mechanisms for producing dilatancy on faults once an instability is well underway are evident. (1) Expansion of pore fluids associated with frictional heating increases fluid pressure offsetting the effects of increased pore volume during earthquakes. There is some tendency for pore volume increase to balance fluid expansion so that fluid pressure stays relatively constant. (2) Production of isolated voids that do not immediately decrease fluid pressure throughout the fault zone during earthquakes can occur to the extent that the fault zone is not significantly strengthened. Although the extent of both processes is constrained by energy considerations, the variation of fluid pressure during earthquakes is not yet well enough understood to predict stress drop from observable material properties.
The flow system in the Japan Sea caused by a sea level difference through shallow straits
Authors: Ohshima Kay I.
Published: February 2004

We investigate how the current system through the Japan Sea is driven and what determines the volume transport. We suppose that a part of the difference in geopotential anomaly between the subtropical and subpolar gyre is converted into a barotropic sea level difference across the three shallow straits which connect the Japan Sea with the Pacific and that this difference is the primary driving force of the current system. Then, we examine the flow under the condition that there is a constant sea level difference between two oceans connected through a shallow strait. We found that the strait acts as a source of arrested shelf waves or steady coastal flows for the timescale beyond the inertial period; in the northern hemisphere, steady flows are established along the shelves with the coast to the right (left) in the ocean of low (high) sea level. We apply this notion to the current system through the Japan Sea. The Tsushima nearshore branch, the Tsugaru coastal mode, and the Soya Current can all be interpreted as a coastally trapped flow whose source is the upstream strait. Further, a series of northeastward flows along the South and East China Seas shelves should be interpreted as coastally trapped flows whose source is the downstream Tsushima Strait. Numerical model experiments incorporating the realistic topography also simulate the observed flow fields. The volume flux through each strait being limited geostrophically, relations between the sea level difference and volume transport can be represented by simple formulas.
Modeling the seasonal cycle of salinity in the Mackenzie shelf/estuary
Authors: Omstedt Anders , Carmack Eddy C. , Macdonald Robie W.
Published: February 2004

The freshwater content at the Mackenzie shelf/estuary is analyzed using observed and numerically simulated data. Field measurements cover the period from September 1986 to September 1987; calculations are based upon a time‐dependent, one‐dimensional model that treats the shelf as a single reservoir coupled to the surroundings through parameterizations of the inflows and outflows. The system is assumed to be controlled by transient Ekman flow dynamics, and forced by winds when the sea is ice free, and by tides when ice covered. Buoyancy fluxes due to river inflow, freezing and melting of ice, and outflow are added to the salinity conservation equation from which the freshwater content is calculated. Sensitivity studies from simulation of the 1986–1987 period show that the freshwater content of the Mackenzie shelf/estuary is highly influenced by freezing, ice advection off the shelf, and the wind‐driven transport, all of which work effectively to remove fresh water from the shelf. Ice keels in the inner shelf may also reduce the export of fresh water to the outer shelf. The sensitivity of the system to altered river inflow from either hydroelectric development or climate change is also examined. For example, upstream storage would increase winter inflows and thus decrease the shelf's capacity to ventilate the halocline, while inflow reduction would enhance shelf ventilation.
Incorporation of acoustic normal mode data into tomographic inversions in the Greenland Sea
Authors: Sutton P. , Morawitz W. M. L. , Cornuelle B. D. , Masters G. , Worcester P. F.
Published: February 2004

Acoustic normal mode group velocity data are extracted from tomographic receptions in the Greenland Sea using a combination of spatial filtering with data from a six‐element hydrophone array and variable time windowing. The mode group velocity data, together with ray travel time data, are used in inversions to obtain the range average sound speed profile. The modal data significantly improve near‐surface resolution, which is where the largest oceanographic signals occur. Inverse results using only acoustic data are consistent with point measurements, a Seasoar section, and sparse conductivity‐temperature‐depth data.
Mixing at Camarinal Sill in the Strait of Gibraltar
Authors: Wesson J. C. , Gregg M. C.
Published: February 2004

During the Gibraltar Experiment in 1985–1986, observations of shear, stratification, microstructure, and acoustic backscatter revealed a complex mixing regime at Camarinal Sill in the Strait of Gibraltar. The mixing is forced by strong semidiurnal tidal currents and weaker, but more steady, baroclinic flows. Flows are characterized as outflow or inflow, nearly equivalent to westward or eastward. Rising water at Gibraltar coincides with outflow at all depths. We observed two modes of outflow at the sill. In May 1986 the transition layer separating Atlantic and Mediterranean water was between 0.75 MPa and 1 MPa when we sampled east of the sill near neap tide. The transition turned down just before reaching the sill crest, where the flow became critical. Increasing shear soon destabilized the transition, producing billows which grew to 30–75 m on the west flank of the sill. The overturning generated intense turbulence, with peak dissipation rates, ε, exceeding 10−2 W kg−1. This turbulence rapidly thickened the transition from Δz = 30–50 m east of the sill to Δz = 130–150 m within 1–2 km west of the sill. From the rate of thickening, we estimate the rate of formation of new transitional water during neap tide as (0.8–1.2) × 105 m3 s−1. This outflow mode did not release eastward‐propagating internal bores when outflow turned to inflow, but the inflow did appear to arrest the deep outflow. In May, the largest average dissipation rate over and just west of the sill had the same magnitude as estimates of the net energy lost by adjustment of velocity and mass across the sill during outflow. Other averages are smaller than the energy loss, presumably because the limited sampling severely underestimates the average dissipation rate. During outflow, strong shears were observed to rise to 100 times those in the background internal wave field. However, probably owing to spatial offsets between the ship's acoustic Doppler current profiler and our tethered free‐fall profiler, the only statistically significant correlations were found between overturning scales measured directly and those estimated from ε in the same profile. We observed the other outflow mode in October 1985, when we profiled over the sill near spring tide and the transition was east of the sill, centered near 1.5 MPa. After encountering the sill, the transition rose sharply and remained within 10–20 m of the surface across the sill, plunging steeply downward just past the west edge of the sill. This was accompanied by intense turbulence and at least one rebound of the transition. We did not observe release of eastward propagating bores when inflow turned to outflow, but observations by Armi and Farmer (1988) in April 1986 found bores released at spring tide in regimes similar to the one we observed in October 1985.
On the electrokinetics of shear stress behavior in fluid‐mud suspensions
Authors: Aijaz Saima , Jenkins Scott A.
Published: February 2004

The variability of rheological properties in fluid‐mud suspensions is studied as a function of salinity and sediment concentration. It was found that the steady state shear stress increases exponentially with increasing sediment concentration and increases logarithmically by increasing the salinity of the suspension. An analytic model predicting shear stress as a function of electroviscous properties is developed. The model shows that the effect of salts in the suspension is to decrease the zeta potential by compressing the electric double layer and, thereby, elevating the shear stresses acting on the shearing planes at each particle‐fluid interface in the suspension. These stresses increase with the increase in salt content. The model incorporates the classical double‐layer theories of Gouy‐Chapman and the Helmholtz‐Smoluchowski theory for electrokinetics of charged particles. The model shows good correlation with experimental data at low sediment concentrations where the basic assumptions of the Gouy‐Chapman formulation are satisfied. At progressively higher sediment concentrations, there are increasingly larger departures between the model predictions and the experimental data due to violations of these basic assumptions. The advantages and limitations of the model are discussed.
The distribution of 14C and 39Ar in the Weddell Sea
Authors: Schlosser P. , Kromer B. , Weppernig R. , Loosli H. H. , Bayer R. , Bonani G. , Suter M.
Published: February 2004

Carbon 14 and 39Ar data from the Weddell Sea are presented and discussed. Values of Δ 14C and 39Ar are low in the winter mixed layer (Δ 14C ≈ −90 to −125‰; 39Ar ≈ 85 % modern). These low values are consistent with the surface layer dynamics which is dominated by entrainment of relatively old water of circumpolar origin and reduced gas exchange during sea ice cover. The Δ 14C and 39Ar values of the deep and bottom waters range from −160 to −150‰ and 38 to 57% modern, respectively. The Δ 14C values of Weddell Sea Bottom Water (WSBW) found in the central Weddell Sea along a 0° longitude section are only slightly higher than those of the overlying Weddell Sea Deep Water (WSDW) showing that the influence of bomb 14C on these waters is small. Part of the WSBW with higher Δ 14C values observed in the northwestern Weddell Sea seems to escape through the South Sandwich Trench, and part seems to mix from a boundary current into the central Weddell Sea. The observed 14C distribution is consistent with the hypothesis that Ice Shelf Water (ISW) is a source of WSBW. A simple conceptual model of the surface layer dynamics is used to estimate the prebomb Δ 14C values of Surface Water and Winter Water to be about −140 and −130‰, respectively. Using mixing ratios between WSDW and shelf water derived from temperature/salinity and 3He data, the prebomb Δ 14C values of WSBW are estimated to be −157‰ (potential temperature of WSBW: −0.7°C). The 39Ar concentration of WSBW with a potential temperature of −0.7°C is determined to be 57% modern. Bomb radiocarbon water column inventories are estimated and discussed.
Springtime microwave emissivity changes in the southern Kara Sea
Authors: Crane Robert G. , Anderson Mark R.
Published: February 2004

Springtime microwave brightness temperatures over first‐year ice are examined for the southern Kara Sea. Snow emissivity changes are revealed by episodic drops in the 37‐ to 18‐GHz brightness temperature gradient ratio measured by the Nimbus 7 scanning multichannel microwave radiometer. We suggest that the negative gradient ratios in spring 1982 result from increased scatter at 37 GHz due to the formation of a near‐surface hoar layer. This interpretation is supported by the results of a surface radiation balance model that shows the melt signature occurring at below freezing temperatures but under clear‐sky conditions with increased solar input to the surface. Published observations from the Greenland ice cap show a surface hoar layer forming under similar atmospheric conditions owing to the increased penetration and absorption of solar radiation just below the surface layer. In spring/early summer 1984 similar gradient ratio signatures occur. They appear to be due to several days of freeze‐thaw cycling following the movement of a low‐pressure system through the region. These changes in surface emissivity represent the transition from winter to summer conditions (as defined by the microwave response) and are shown to be regional in extent and to vary with the synoptic circulation.
A three‐dimensional model of the M4 tide in the Irish Sea: The importance of open boundary conditions and influence of wind
Authors: Davies A. M. , Lawrence J.
Published: February 2004

A fully nonlinear three‐dimensional hydrodynamic model of the Irish Sea is used to examine the processes determining the spatial variability of the M4 tide in the Irish Sea. The model is based upon a mixed spectral/finite difference approach, with an expansion of functions in the vertical (yielding a continuous current profile) and a finite difference grid in the horizontal. The model has a sufficiently fine grid to enable the nonlinear advective terms to be resolved in their finite difference form, and the spatial variability of the M4 tide to be represented. Computed M4 elevations and currents determined using a radiation open boundary condition with both M2 mid M4 elevation and current input and a flow dependent viscosity are found to be in satisfactory agreement with observations considering the limited grid resolution of the model, and the level of noise in the M4 tide as a percentage of the signal is much higher than for M2. This agreement is good enough to establish the credibility of using the model to examine the processes influencing the M4 tide, which are discussed in detail. The importance of an M4 current input to the radiation condition, and the difficulty of using an elevation specified open boundary condition to reproduce the M4 tide in this region are considered in some detail. The nonlinear advective terms are shown to have a limited local influence (namely, around headlands) in generating the M4 tide. The influence of wind effects upon M4 tidal currents, particularly through the nonlinear term associated with a time dependent eddy viscosity, is examined, and shown to produce a significant increase in the M4 near surface tidal current, and a modification of the current profile. In essence, the change of the M4 tidal current by meteorological effects will give rise to noise in any observed M4 current due to the random nature of the wind fields.
Detecting aliased tidal errors in altimeter height measurements
Authors: Schlax Michael G. , Chelton Dudley B.
Published: February 2004

A simple statistic is derived for quantifying the potential for the aliasing of tidal errors in a given linear estimate of sea surface height constructed from altimeter data. The existence of M2tidal constituent errors in Geosat data processed in the traditional way (i.e., with orbit errors removed using least squares fits to 1 cycle per revolution sinusoids) which are of sufficient magnitude to alias into apparently westward propagating ocean features is demonstrated by artificially inducing aliasing. The aliasing statistic presented here responds clearly to the induced aliasing and to actual aliasing caused by real data dropouts in the Geosat data. The potential for aliasing M2tidal errors is shown to vary with latitude depending on the time interval between ascending and descending ground tracks near the location of interest. The methods developed here are applied to Geosat data from the northeast Atlantic to demonstrate the presence of M2tidal error aliasing in those data.
Effect of topographic stress on circulation in the western Mediterranean
Authors: Alvarez Alberto , Tintoré Joaquín , Holloway Greg , Eby Michael , Beckers Jean Marie
Published: February 2004

The interaction of eddies with seafloor topography can exert large systematic forces on ocean circulation. Using a statistical mechanics approach it is possible to obtain a parametrization of this effect (the Neptune effect) for application in large‐scale ocean circulation models. Circulation of the western Mediterranean has been observed to follow a definite cyclonic path. Numerical models usually show good qualitative agreement for the large‐scale circulation but show systematic deficiencies at a subbasin scale. We have tested the importance of the Neptune effect on the circulation of the western Mediterranean Sea. To perform this test, different numerical experiments on western Mediterranean circulation were done with and without eddy‐topography interaction. As a first step we analyze the influence of the Neptune effect in an ideal western Mediterranean with closed straits. After these experiments the more realistic case of open straits is studied. All the experiments show that the Neptune effect may be a significant factor in the basin and subbasin scale circulation in the western Mediterranean Sea.
Preliminary assessment of the accuracy and precision of TOPEX/POSEIDON altimeter data with respect to the large‐scale ocean circulation
Authors: Stammer Detlef , Wunsch Carl
Published: February 2004

TOPEX/POSEIDON sea surface height measurements are examined for quantitative consistency with known elements of the oceanic general circulation and its variability. Project‐provided corrections were accepted but are tested as part of the overall results. The ocean was treated as static over each 10‐day repeat cycle and maps constructed of the absolute sea surface topography from simple averages in 2°×2° bins. A hybrid geoid model formed from a combination of the recent Joint Gravity Model‐2 and the project‐provided Ohio State University geoid was used to estimate the absolute topography in each 10‐day period. Results are examined in terms of the annual average, seasonal variations, and variations near the repeat period. Conclusions are as follows: the orbit error is now difficult to observe, having been reduced to a level at or below the level of other error sources; the geoid dominates the error budget of the estimates of the absolute topography; the estimated seasonal cycle is consistent with prior estimates; shorter‐period variability is dominated on the largest scales by an oscillation near 50 days in spherical harmonicsY1m(θ, λ) with an amplitude near 10 cm, close to the simplest alias of the M2tide. This spectral peak and others visible in the periodograms support the hypothesis that the largest remaining time‐dependent errors lie in the tidal models. Though discrepancies attributable to the geoid are within the formal uncertainties of the geoid estimates, removal of them is urgent for circulation studies. Current gross accuracy of the TOPEX/POSEIDON mission is in the range of 5–10 cm, distributed over a broad band of frequencies and wavenumbers. In finite bands, accuracies approach the 1‐cm level, and expected improvements arising from extended mission duration should reduce these numbers by nearly an order of magnitude.
Calibration of the TOPEX altimeter using a GPS buoy
Authors: Born George H. , Parke Michael E. , Axelrad Penina , Gold Kenneth L. , Johnson James , Key Kevin W. , Kubitschek Daniel G. , Christensen Edward J.
Published: February 2004

The use of a spar buoy equipped with a Global Positioning System (GPS) antenna to calibrate the height measurement of the TOPEX radar altimeter is described. In order to determine the height of the GPS antenna phase center above the ocean surface, the buoy was also equipped with instrumentation to measure the instantaneous location of the waterline, and tilt of the buoy from vertical. The experiment was conducted off the California coast near the Texaco offshore oil platform, Harvest, during cycle 34 of the TOPEX/POSEIDON observational period. GPS solutions were computed for the buoy position using two different software packages, K&RS and GIPSY‐OASIS II. These solutions were combined with estimates of the waterline location on the buoy to yield the height of the ocean surface. The ocean surface height in an absolute coordinate system combined with knowledge of the spacecraft height from tracking data provides a computed altimeter range measurement. By comparing this computed value to the actual altimeter measurement, the altimeter bias can be calibrated. The altimeter height bias obtained with the buoy using K&RS was −14.6±4 cm, while with GIPSY‐OASIS II it was −13.1±4 cm. These are 0.1 cm and 1.6 cm different from the −14.7±4 cm result obtained for this overflight with the tide gauge instruments located on Platform Harvest.
Observation and modeling of North Atlantic Deep Water Formation and its variability: Introduction
Authors: Stocker Thomas F. , Broecker Wallace S.
Published: February 2004

This Special Section is the result of a 2‐day miniconference held at Lamont‐Doherty Earth Observatory in November 1991. The purpose of that meeting was to review the current knowledge of North Atlantic Deep Water (NADW) formation and to discuss the role of the global circulation it sets up in the climate system [Broecker, 1991]. The present collection of papers, however, goes beyond the original scope of the meeting and is more up‐to‐date than a mere proceedings volume.

To obtain data on the formation and circulation of the deep water by classical Oceanographic observation methods is a difficult task. It is therefore necessary to try to fill the various gaps in knowledge by combining information from observations with what we can learn from an entire hierarchy of numerical models. This philosophy was behind the 1991 miniconference [Stocker and Broecker, 1992] and is also reflected in this special section. Although the degree of interaction must continue to increase, such meetings are valuable starting points for a closer communication between modelers and those collecting and producing the data which is so crucial for model validation. It is the purpose of this special section to make a contribution towards this aim.
Numerical simulation of a response of an oceanic front to an atmospheric frontal passage
Authors: Pereira Claudio Solano , Mascarenhas Affonso da S.
Published: February 2004

The influence of middle‐latitude cyclones and their associated cold fronts on open‐ocean fronts is studied with an embedded mixed layer ocean circulation model similar to that of Adamec et al. (1981). Finite differences with horizontal resolution of 5 km in a staggered grid are used to solve the model equations within a domain of total meridional extension of 300 km and 500 m depth. Three numerical experiments were performed to investigate the response of an oceanic front, similar to the South Atlantic subtropical convergence zone, to an atmospheric frontal passage. Under the effect of the cold air outbreak the thermal and dynamic structures of the upper layers are substantially modified with a higher sinking/cooling rate of the mixed layer. The sinking/cooling rate depends strongly on the magnitude of the thermal gradient at the top of the thermocline and on the mixed layer depth. Due to the differences in sinking/cooling rates, a single storm can move the oceanic front up to 20 km from its initial position. For low‐intensity winds, a change in the wind direction does not affect the thermal structure of the upper layer ocean, even in the frontal region. The mixed layer oscillations are entirely a function of the daily scale of the solar heating. For a sudden change in the wind direction, the response of the current velocity field in the mixed layer is almost instantaneous.
Global statistical analysis of TOPEX and POSEIDON data
Authors: Le Traon P. Y. , Stum J. , Dorandeu J. , Gaspar P. , Vincent P.
Published: February 2004

A global statistical analysis of the first 10 months of TOPEX/POSEIDON merged geophysical data records is presented. The global crossover analysis using the Cartwright and Ray (1990) (CR) tide model and Gaspar et al. (this issue) electromagnetic bias parameterization yields a sea level RMS crossover difference of 10.05 cm, 10.15 cm, and 10.15 cm for TOPEX‐TOPEX, POSEIDON‐POSEIDON, and TOPEX‐POSEIDON crossovers, respectively. All geophysical corrections give reductions in the crossover differences, the most significant being with respect to ocean tides, solid earth tide, and inverse barometer effect. Based on TOPEX‐POSEIDON crossovers and repeat‐track differences, we estimate the relative bias between TOPEX and POSEIDON at about −15.5±1 cm. This value is dependent, however, to the electromagnetic bias corrections used. An orbit error reduction method based on global minimization of crossover differences over one cycle yields an orbit error of about 3 cm RMS. This is probably an upper estimate of the orbit error since the estimation absorbs other altimetric signals. The RMS crossover difference is reduced to 8.8 cm after adjustment. A repeat‐track analysis is then performed using the CR tide model. In regions of high mesoscale variability, the RMS sea level variability agrees well with the Geosat results. Tidal errors are also clearly evidenced. A recent tide model (Ma et al., this issue) determined from TOPEX/POSEIDON data considerably improves the RMS sea level variability. The reduction of sea level variance is (4 cm)2 on average but can reach (8 cm)2 in the southeast Pacific, southeast Atlantic, and Indian Oceans. The RMS sea level variability thus decreases from 6 cm to only 4 cm in quiet ocean regions. The large‐scale sea level variations over these first 10 months most likely show for the first time the global annual cycle of sea level. Finally, we analyze the TOPEX and POSEIDON sea level anomaly wavenumber spectral characteristics. TOPEX and POSEIDON have identical spectral characteristics at low wavenumbers. For wavelengths shorter than 100 km, however, POSEIDON spectra are more energetic. This is probably related to the TOPEX tracker characteristics and to the way the acceleration correction is made in the geophysical data records. POSEIDON repeat‐track noise level is estimated at about 3 cm for a 1‐s average. The TOPEX repeat‐track noise level is about 1.8 cm RMS but this probably corresponds to averages over several seconds.
Life history of a cyclonic ring detached from the Kuroshio Extension as seen by the Geosat altimeter
Authors: Ichikawa Kaoru , Imawaki Shiro
Published: February 2004

The sea surface dynamic topography (SSDT; sea surface height relative to the geoid) is approximately estimated by combining the temporal fluctuation of SSDT determined from altimetry data with the climatological mean SSDT calculated from hydrographic data. This approximated SSDT, or composite SSDT, is obtained in the area southeast of Japan for the first year (1986–1987) of the Geosat Exact Repeat Mission. Variations in the Kuroshio Current System are well described by the composite SSDT. In particular, a complete life history of a cyclonic ring is observed for the first time in this region, namely, westward propagation of a southward meander of the Kuroshio Extension, its pinching‐off to form a cyclonic ring, the ring's westward movement, its coalescence with the Kuroshio, and its eastward advection. All available hydrographic data and cloud‐free satellite infrared images support the existence of this series of phenomena.
Spatial patterns in the length of the sea ice season in the Southern Ocean, 1979–1986
Authors: Parkinson Claire L.
Published: February 2004

The length of the sea ice season summarizes in one number the ice coverage conditions for an individual location for an entire year. It becomes a particularly valuable variable when mapped spatially over a large area and examined for regional and interannual differences, as is done here for the Southern Ocean over the years 1979–1986, using the satellite passive microwave data of the Nimbus 7 scanning multichannel microwave radiometer. Three prominent geographic anomalies in ice season lengths occur consistently in each year of the data set, countering the general tendency toward shorter ice seasons from south to north: (1) In the Weddell Sea the tendency is toward shorter ice seasons from southwest to northeast, reflective of the cyclonic ice/atmosphere/ocean circulations in the Weddell Sea region. (2) Directly north of the Ross Ice Shelf anomalously short ice seasons occur, lasting only 245–270 days, in contrast to the perennial ice coverage at comparable latitudes in the southern Bellingshausen and Amundsen Seas and in the western Weddell Sea. The short ice season off the Ross Ice Shelf reflects the consistently early opening of the ice cover each spring, under the influence of upwelling along the continental slope and shelf and atmospheric forcing from winds blowing off the Antarctic continent. (3) In the southern Amundsen Sea, anomalously short ice seasons occur adjacent to the coast, owing to the frequent existence of coastal polynyas off the many small ice shelves bordering the sea. Least squares trends in the ice season lengths over the 1979–1986 period are highly coherent spatially, with overall trends toward shorter ice seasons in the northern Weddell and Bellingshausen seas and toward longer ice seasons in the Ross Sea, around much of East Antarctica, and in a portion of the south central Weddell Sea.