bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics of the exterior cusp for steady southward interplanetary magnetic field: Interball observations
Authors: Fedorov A. , Dubinin E. , Song P. , Budnick E. , Larson P. , Sauvaud J.‐A.
Published: February 2004

High‐latitude observations of plasma flow across the magnetopause are made by the Interball‐Tail satellite near the polar cusp for steady southward and dawnward interplanetary magnetic field. Three‐dimensional proton distribution functions measured in this region contain the features predicted by the existing reconnection models. D‐shaped distributions in magnetospheric parts of the field lines with cutoff near zero velocity parallel to the magnetic field and tablet‐shaped distributions tailward from the cusp are observed and can be interpreted in terms of a gradually diminishing ion injection. Besides these predicted signatures, a new type of proton distribution with large pitch angles is also observed. We present observations of suprathermal ions and electrons with peaks at ∼ 90° pitch angles on the high‐latitude magnetospheric field lines adjacent to the magnetopause. These particles show a sharp change in fluxes on the magnetospheric side, indicating that they are probably quasi‐trapped in the cusp field. We also present the results of a simulation that uses the Toffoletto‐Hill model of the open magnetosphere to trace the particle transport from the magnetosheath to magnetospheric locations of Interball. The simulation successfully reproduces the main predicted features of distribution functions as well as distributions peaked at large pitch angles. The results show that for steady southward interplanetary magnetic field with a large dawnward component, the entry of magnetosheath plasma into the magnetosphere can be described in terms of open topology of field lines near the high‐altitude cusp.
Super Dual Auroral Radar Network observations of velocity‐divergent structures in the F region ionosphere
Authors: André R. , Villain J.‐P , Krassnosel'skikh V. , Hanuise C.
Published: February 2004

This paper describes Super Dual Auroral Radar Network observations of an unusual mesoscale (scale size L ≈ 300 km) structure in the auroral convection pattern. This structure is characterized by an expansion motion of the plasma in a plane perpendicular to the magnetic field and then by an anomalously high velocity divergence. This study describes those observations and eliminates possible artifacts in the data analysis. Because the radar data strongly suggest that the structure is located in the F region, the observations are thus in opposition with the well‐known divergence‐free motion hypothesis. They are interpreted in terms of an ion demagnetization process, in which the collisionless ions become locally collisional in the F region.
Polar spacecraft based comparisons of intense electric fields and Poynting flux near and within the plasma sheet‐tail lobe boundary to UVI images: An energy source for the aurora
Authors: Wygant J. R. , Keiling A. , Cattell C. A. , Johnson M. , Lysak R. L. , Temerin M. , Mozer F. S. , Kletzing C. A. , Scudder J. D. , Peterson W. , Russell C. T. , Parks G. , Brittnacher M. , Germany G. , Spann J.
Published: February 2004

In this paper, we present measurements from two passes of the Polar spacecraft of intense electric and magnetic field structures associated with Alfven waves at and within the outer boundary of the plasma sheet at geocentric distances of 4–6 RE near local midnight. The electric field variations have maximum values exceeding 100 mV/m and are typically polarized approximately normal to the plasma sheet boundary. The electric field structures investigated vary over timescales (in the spacecraft frame) ranging from 1 to 30 s. They are associated with strong magnetic field fluctuations with amplitudes of 10–40 nT which lie predominantly in the plane of the plasma sheet and are perpendicular to the local magnetic field. The Poynting flux associated with the perturbation fields measured at these altitudes is about 1–2 ergs cm−2 s−1 and is directed along the average magnetic field direction toward the ionosphere. If the measured Poynting flux is mapped to ionospheric altitudes along converging magnetic field lines, the resulting energy flux ranges up to 100 ergs cm−2s−1. These strongly enhanced Poynting fluxes appear to occur in layers which are observed when the spacecraft is magnetically conjugate (to within a 1° mapping accuracy) to intense auroral structures as detected by the Polar UV Imager (UVI). The electron energy flux (averaged over a spatial resolution of 0.5° ) deposited in the ionosphere due to auroral electron beams as estimated from the intensity in the UVI Lyman‐Birge‐Hopfield‐long filters is 15–30 ergs cm−2s−1. Thus there is evidence that these electric field structures provide sufficient Poynting flux to power the acceleration of auroral electrons (as well as the energization of upflowing ions and Joule heating of the ionosphere). During some events the phasing and ratio of the transverse electric and magnetic field variations are consistent with earthward propagation of Alfven surface waves with phase velocities of 4000–10000 km/s. During other events the phase shifts between electric and magnetic fields suggest interference between upward and downward propagating Alfven waves. The E/B ratios are about an order of magnitude larger than typical values of c/Σp, where Σp is the height integrated Pedersen conductivity. The contribution to the total energy flux at these altitudes from Poynting flux associated with Alfven waves is comparable to or larger than the contribution from the particle energy flux and 1–2 orders of magnitude larger than that estimated from the large‐scale steady state convection electric field and field‐aligned current system.
Fermi acceleration of suprathermal solar wind oxygen ions
Authors: Freeman T. J. , Parks G. K.
Published: February 2004

The Wind spacecraft has observed numerous sunward bursts of ≈2 MeV ions upstream of the Earth's bow shock. These energetic particles are not protons or alpha particles and are probably oxygen ions. The bursts typically last several minutes at the highest energies, but they can last for tens of minutes at intermediate energies (tens to hundreds of keV). In this paper, Wind observations of the November 30, 1994, bursts and numerical particle simulations are presented that demonstrate that suprathermal solar wind O6+ ions, which have undergone Fermi acceleration between the bow shock and interplanetary magnetic field (IMF) rotations, are the most likely source of these bursts. Each burst either coincides with or is closely followed by a large IMF rotation. By using measured magnetic field data the timing of the bursts detected by Wind is reproduced in the simulation. Simulated spectra of H+, He2+, and O6+ fluxes fit the observed spectra, assuming an increase of ∼2 orders of magnitude in the high‐energy tail of the solar wind oxygen distribution, relative to the average solar wind abundance of oxygen. Such enhancements in CNO group ions above 40 keV/nucleon were measured by the Suprathermal and Energetic Particle (STEP) instrument during this interval. This study concludes by predicting that ion composition and charge state measurements will show these bursts to be solar wind oxygen ions in high charge states.
Two‐dimensional inverse modeling of sedimentary basin subsidence
Authors: Poplavskii Konstantin N. , Podladchikov Yuri Y. , Stephenson Randell A.
Published: February 2004

Recently developed mathematical models of subsidence in extensional basins provide a good basis for forward modeling. However, the large number of parameters to be specified by the user makes it difficult not only to do the modeling itself, but also to judge the meaning of the results. We present a new method for automatic searching of the best‐fitting parameter set of a 2D basin formation model. Weighted goal functions are used in the minimization process by the inverse problem solver algorithm. The nominal parameter set in the present case includes profiles of the crustal and subcrustal thinning factors, and the level of lithosphere necking. The method was tested on synthetic data with parameters that were perturbed beforehand. With some restrictions, the algorithms are capable of resolving such perturbations. The inversion technique has been applied to two profiles crossing the Dnieper‐Donets Basin (Ukraine). The crustal thinning factors obtained argue for a scissors‐like style of basin opening. High subcrustal thinning values are necessary in order to explain the abnormally thick Carboniferous section in the basin. These values should be treated as cumulative ones due to the currently unresolvable influence of other rifting‐related processes, particularly phase transitions.
Late Mesoproterozoic magnetostratigraphic results from Siberia: Paleogeographic implications and magnetic field behavior
Authors: Gallet Yves , Pavlov Vladimir E. , Semikhatov Mikhail A. , Petrov Peter Yu.
Published: February 2004

We present a magnetostratigraphic study of the late Mesoproterozoic Malgina and Linok Formations, located along the southeastern (Uchur‐Maya region) and northwestern (Turukhansk region) margins of the Siberian craton, respectively. Biostratigraphic, radiometric, and chemostratigraphic data indicate that these formations are likely coeval between 1050 and 1100 Ma. Paleomagnetic analyses reveal a high‐temperature component carried by magnetite and/or hematite. This component yields positive fold and reversal tests, together with a positive conglomerate test for the Malgina Formation, which indicates that the magnetization was acquired during or soon after sediment deposition. The mean paleomagnetic direction obtained from the Uchur‐Maya region, which is unambiguously representative of the Siberian craton, indicates that it could not have been part of Rodinia at that time if Siberia was located in the Southern Hemisphere and if we assume that Laurentia and Siberia were connected along their present northern shorelines. We emphasize that Siberia could have been part of Rodinia during the late Mesoproterozoic if southern Siberia was joined to the northern part of Laurentia as recently proposed by Rainbird et al. [1998]. If true, placing the Siberian craton in the Southern Hemisphere implies that the magnetic polarity of the ∼1000 Ma Laurentian paleomagnetic poles must be switched. Our data also show the occurrence of at least 15 symmetric geomagnetic field reversals, indicating that the paleomagnetic results from the late Mesoproterozoic Keweenawan lavas do not reflect a worldwide and persistent asymmetric field during the Proterozoic.
Strain partitioning during oblique plate convergence in northern Sumatra: Geodetic and seismologic constraints and numerical modeling
Authors: McCaffrey Robert , Zwick Peter C. , Bock Yehuda , Prawirodirdjo Linette , Genrich Joachim F. , Stevens Colleen W. , Puntodewo S. S. O. , Subarya Cecep
Published: February 2004

Global Positioning System (GPS) measurements along the subduction zone of northern Sumatra (2°S to 3°N) reveal that the strain associated with the oblique convergence of the Australian plate with Eurasia is almost fully partitioned between trench‐normal contraction within the forearc and trench‐parallel shear strain within a few tens of kilometers of the Sumatran fault Kinematic analyses of interplate earthquake slip vectors provide slip rates on the Sumatran fault within a few millimeters per year of GPS and geologic rates, giving us more confidence in the use of slip vectors for inferring slip partitioning elsewhere. The inferred slip rate on the Sumatran fault is ∼1/3 less than the full margin parallel component of plate motion. An across‐forearc rotation in the slip vectors suggests that the missing arc‐parallel shear occurs seaward of the geodetic network, between the forearc islands and the trench. Simple finite element models are used to explore the conditions under which the change in the principal strain rate directions between the forearc and the arc region can occur. Modeling suggests that neither a preexisting strike‐slip fault nor a zone of thermally induced lithospheric weakness in the overriding plate is needed for strain partitioning to occur. In general, forearc slivers form over the region of interplate coupling and are driven along strike by the basal shear. A volcanic arc can help the partitioning process by localizing the margin‐parallel shear strain in the upper plate if its crust and mantle are weaker than its surroundings. Interplate slip vectors and geodetic results from Sumatra together suggest that the highest coupling on the plate boundary occurs beneath and seaward of the forearc islands, consistent with inferences about the rupture zones of great nineteenth century earthquakes there. The Sumatra example suggests that geodetic measurements of interseismic, margin‐parallel shear strain at oblique convergent margins can be used to map the landward extent of the relatively high basal stress beneath the overriding plate if one can correct for strain localization caused by weak upper plate strike‐slip faults.
Structural controls on the continent‐ocean transition in the northern Gulf of California
Authors: Nagy Elizabeth A. , Stock Joann M.
Published: February 2004

In the Gulf of California the Pacific‐North America plate boundary changes character from an oceanic‐type spreading center and transform fault system (to the south) to a region of diffuse continental deformation (to the north). The presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric, heat flow, gravity, or seismic data which indicate significant differences north and south of latitude ∼30°N. We suggest instead that north of ∼30°N a continent‐ocean transition begins which we name the Wagner Transition Zone (WTZ). Diffuse deformation characterizes the WTZ where slip occurs along reactivated north to NNW striking normal faults developed during late Miocene or Pliocene ENE directed extension. Transtensional deformation varies from ENE directed extension along dip‐slip faults in the west to dextral shear along the coast to dextral‐oblique slip along inferred north to NNW striking faults submerged in the northern gulf. By accounting for rotational and extensional plate motion deformation in northeastern Baja California, vector constraints require that submerged structures accommodate ∼30 mm/yr of slip in a direction slightly clockwise of the relative plate motion direction. The juxtaposition of the discrete spreading center system in the central gulf with the diffuse WTZ appears to have been a stable configuration since 4–6 Ma, perhaps controlling the evolution of spreading center jumps between Upper and Lower Tiburón and Delfin basins due to the juxtaposition of kinematically partitioned structural domains. Different histories of prerift extension and subduction‐related arc magmatism along the length of the gulf, partly related to the migration of the Rivera triple junction, may explain the location of the continent‐ocean transition.
A forward model of three‐dimensional fracture orientation and characteristics
Authors: Tuncay K. , Park A. , Ortoleva P.
Published: February 2004

A new forward modeling approach to simulate the extension/closure and orientation statistics of evolving fracture networks is presented. The model is fully dynamical and couples fracturing to other processes. Thus, fracturing affects hydrology and, in turn, its development is affected by fluid pressure. In this way, highly pressured fluids can enhance their own migration while low‐pressured ones may become trapped. Fracturing affects the stress tensor through rock volumetric changes and stress affects fracture dynamics. Thus, to predict fracture dynamics, we coevolve a set of fracture variables simultaneously with mass, momentum, and energy conservation equations for the solid and multiple fluid phases. In our computational approach, a representative set of putative fractures of a range of orientations is introduced. The time‐dependent properties of each realized fracture is calculated by using the stress component normal to its fracture plane, pressure, and rock properties. The volumetric strain caused by fracturing allows for a self‐limiting feedback that we account for using a nonlinear incremental stress rheology. The anisotropic fracture permeability is obtained using the predicted fracture network statistics. Thus, the coupling between rock deformation, notably fracturing, and hydrology is accounted for as is that between fracturing and stress. The statistical aspect of fracture network dynamics is described by assuming a probability distribution characterizing variations in rock strength within a nominally uniform lithology. The dependence of fracture density and length on the rate of fluid pressure or stress variation is thereby captured. Embedding the model in a three‐dimensional basin finite element simulator, we illustrate the dynamical nature of the location and character of fracture zones in a sedimentary basin.
Physical and stable isotopic properties and growth processes of sea ice collected in the southern Sea of Okhotsk
Authors: Ukita Jinro , Kawamura Toshiyuki , Tanaka Noriyuki , Toyota Takenobu , Wakatsuchi Masaaki
Published: February 2004

This study presents physical and stable isotopic properties of sea ice in the southernmost section of the Sea of Okhotsk on the basis of observations and measurements made in 1995 and 1996 midwinter cruises. Structural analysis revealed that ice was predominantly of the granular type. Saucer‐shaped cores and wedges were seen in the cross sections of the pancakes, which indicate a rafting event in the early stage of the pancake formation. The fraction of snow with respect to the total ice mass was estimated at ∼8% from independent δ18O and density measurements. From these findings we conclude that the initial growth mechanisms of the sea ice over this region are by the frazil pancake cycle with rafting and by the snow ice formation. Analysis also shows that the formation of the snow ice induces ∼10% more salt flux through the additional freezing of the seawater. In a short (synoptic) timescale this implies that snow becomes a negative (salting) rather than a positive (freshening) source for the buoyancy flux. These results suggest critical influence of the rapid formation of the frazil pancakes and snow ice under stormy conditions on the salt and buoyancy fluxes over the region.
River runoff, sea ice meltwater, and Pacific water distribution and mean residence times in the Arctic Ocean
Authors: Ekwurzel Brenda , Schlosser Peter , Mortlock Richard A. , Fairbanks Richard G. , Swift James H.
Published: February 2004

Hydrographic and tracer data collected during ARK IV/3 (FS Polarstern in 1987), ARCTIC91 (IB Oden), and AOS94 (CCGS Louis S. St‐Laurent) expeditions reveal the evolution of the near‐surface waters in the Arctic Ocean during the late 1980s and early 1990s. Salinity, nutrients, dissolved oxygen, and δ18O data are used to quantify the components of Arctic freshwater: river runoff, sea ice meltwater, and Pacific water. The calculated river runoff fractions suggest that in 1994 a large portion of water from the Pechora, Ob, Yenisey, Kotuy, and Lena Rivers did not flow off the shelf closest to their river deltas, but remained on the shelf and traveled via cyclonic circulation into the Laptev and East Siberian Seas. River runoff flowed off the shelf at the Lomonosov Ridge and most left the shelf at the Mendeleyev Ridge. ARCTIC91 and AOS94 Pacific water fraction estimates of Upper Halocline Water, the traditionally defined core of the Pacific water mass, document a decrease in extent compared to historical data. The front between Atlantic water and Pacific water shifted from the Lomonosov Ridge location in 1991 to the Mendeleyev Ridge in 1994. The relative age structure of the upper waters is described by using the 3H‐3He age. The mean 3H‐3He age measured in the halocline within the salinity surface of 33.1±0.3 is 4.3±1.7 years and that for the 34.2±0.2 salinity surface is 9.6±4.6 years. Lateral variations in the relative age structure within the halocline and Atlantic water support the well‐known cyclonic boundary current circulation.
Flow and water property structures between the Bering Sea and Fiji in the summer of 1993
Authors: Roden Gunnar I.
Published: February 2004

An overview is given of the top‐to‐bottom flow, thermohaline, oxygen, and nutrient structures between the Bering Sea and the Fiji islands, based on high‐resolution field measurements along 179°E. The zonal flow is dominated by a banded mesoscale structure, reflecting the highly sheared structure and instabilities of the major ocean currents crossing the date line. The major midlatitude currents are 100–200 km wide, have maximum speeds of ∼40 cm s−1, and extend to ∼2000 m, when measured by the 5 cm s−1 isotach. El Niño conditions in 1993 caused a merger of the North Equatorial Countercurrent with the Equatorial Undercurrent, leading to a 600 km wide band of equatorial eastward flow (40–60 cm s−1) in the upper 300 m. Beneath, a 400 km wide and 1.2 km deep westward jet (10–30 cm s−1) is found. Between 1500 and 3500 m the equatorial flow is predominately to the southeast, and between 3500 and 5000 m it is toward the northwest. Consonant with banded shear flow and flow‐topography interactions, the physical and chemical property distributions along 179°E are complicated, reflecting the effects of differential property advection, mixing, and water mass transformation. Well‐defined property fronts occur in shear zones between currents of different origin. The thermohaline, oxygen, and nutrient tongues associated with North and South Pacific intermediate and deep waters have considerable internal structure, indicative of mesoscale processes and contorted flow paths. The abyssal flow near the date line indicates northward movement of lower circumpolar water to the Hess rise, whence it turns westward and north, west of the Emperor Seamounts; properties east of this seamount chain are very uniform. The deep and abyssal waters of the isolated Aleutian Basin differ substantially from those in the adjacent Pacific, by being warmer (Δ = 0.1°C), less saline (Δ = 0.01), poorer in oxygen (Δ = 50 μmol kg−1), and much richer in silicate (Δ = 80 μmol kg−1). An isolated basin within the Wake‐Necker Ridge also shows some unusual bottom water characteristics below sill depth.
Virtual wave stress and mean drift in spatially damped surface waves
Authors: Weber Jan Erik
Published: February 2004

The concept of virtual wave stress (VWS) is applied to spatially attenuated, deepwater surface gravity waves. With particular emphasis on laboratory wave tank measurements it is pointed out that VWS induces a mean Eulerian drift current that increases with time. This can be important for the determination of the wave‐induced drift current, especially when the surface contains thin slicks of contaminating material. A novel formulation is derived that relates VWS to the lateral divergence of the mean wave momentum flux. It is suggested that this formulation can be helpful in determining the mean drift current in the presence of surface slicks as well as the mean volume flux associated with deepwater waves that break in a limited spatial region.
Quantifying mesoscale variability in ocean transient tracer fields
Authors: Haine Thomas W. N. , Gray Suzanne L.
Published: February 2004

A powerful way to test the realism of ocean general circulation models is to systematically compare observations of passive tracer concentration with model predictions. The general circulation models used in this way cannot resolve a full range of vigorous mesoscale activity (on length scales between 10–100 km). In the real ocean, however, this activity causes important variability in tracer fields. Thus, in order to rationally compare tracer observations with model predictions these unresolved fluctuations (the model variability error) must be estimated. We have analyzed this variability using an eddy‐resolving reduced‐gravity model in a simple midlatitude double‐gyre configuration. We find that the wave number spectrum of tracer variance is only weakly sensitive to the distribution of (large scale slowly varying) tracer sources and sinks. This suggests that a universal passive tracer spectrum may exist in the ocean. We estimate the spectral shape using high‐resolution measurements of potential temperature on an isopycnal in the upper northeast Atlantic Ocean, finding a slope near k−1.7 between 10 and 500 km. The typical magnitude of the variance is estimated by comparing tracer simulations using different resolutions. For CFC‐ and tritium‐type transient tracers the peak magnitude of the model variability saturation error may reach 0.20 for scales shorter than 100 km. This is of the same order as the time mean saturation itself and well over an order of magnitude greater than the instrumental uncertainty.
Separating in situ and terrigenous sources of absorption by dissolved materials in coastal waters
Authors: Twardowski Michael S. , Donaghay Percy L.
Published: February 2004

It is presently unclear to what extent coastal and oceanic chromophoric dissolved organic matter (CDOM) is remnant of diluted inputs from the terrestrial biosphere, is the product of in situ biological processes, or is a derivative of both sources. The development of a persistent phytoplankton thin layer (2–4 m thick) in East Sound, Washington, provided an opportunity to study the link between CDOM formation and in situ phytoplankton production. Spectral CDOM absorption ag(λ), spectral particulate absorption ap(λ), and hydrographic parameters were simultaneously recorded in high‐resolution vertical profiles. Significant fine‐scale variability was observed in ag(412). The majority of this fine structure covaried with salinity, consistent with mixing between an oceanic end member water type (low CDOM, high salinity) and a riverine one (high CDOM, low salinity). During the development of the thin layer, deviations from this ag(412)‐salinity relationship were observed. To isolate only the changes in ag(412) resulting from in situ processes, the initial state, represented by the ag(412)‐salinity relationship, was subtracted from the measured ag(412). The remaining “in situ change” or residual ag(412) fraction was linearly correlated with oxygen supersaturation and ap(440). Above the phytoplankton layer centered at 3–6 m the ag residual was negative, suggesting removal by photodegradation. A positive ag residual within the thin layer is the first direct evidence for rapid in situ CDOM production associated with phytoplankton primary production. Although the in situ fraction was 10% or less of the total CDOM absorption in East Sound during this period, in the open ocean this fraction may comprise nearly all of the absorption by dissolved materials.
Characterization and validation of the heat storage variability from TOPEX/Poseidon at four oceanographic sites
Authors: Polito Paulo S. , Sato Olga T. , Liu W. Timothy
Published: February 2004

Oceanic heat storage and heat storage rate are correlated to the sea surface height anomaly signals from the TOPEX/Poseidon altimeter data from 1992 to 1998. The height anomaly data are decomposed through two‐dimensional finite impulse response filtering. The signal components are the basin‐scale (seasonal), westward propagating (Rossby waves), eastward propagating (Kelvin waves), and mesoscale eddies and a small‐scale residual. The sum of these filtered components does not depart significantly from the original signal. Correlations and rms differences are calculated to compare heat storage estimates on the basis of altimeter and in situ data at four sites: Tropical Atmosphere‐Ocean Array, Hawaii Ocean Time‐series Program, California Cooperative Oceanic Fisheries Investigations, and hydrostation “S.” The heat storage anomaly differences range from 52 to 64×107 J m−2, and the correlations range from 0.67 to 0.88. The heat storage rate rms differences range from 97 to 228 W m−2, and the correlations range from 0.66 to 0.82. The fraction of the variance associated with Rossby waves varies from 13% in the equatorial Pacific to 62% in the tropical North Pacific. Conversely, in the same regions the nonpropagating signal goes from 74 to 13%.
A wind comparison study using an ocean general circulation model for the 1997–1998 El Niño
Authors: Hackert Eric C. , Busalacchi Antonio J. , Murtugudde Ragu
Published: February 2004

Predictions of the 1997–1998 El Niño exhibited a wide range of forecast skill that were dependent, in part, on the wind‐driven initial conditions for the ocean. In this study the results of a reduced gravity, primitive equation, sigma coordinate ocean general circulation model are compared and contrasted when forced by several different wind products for the 1997–1998 El Niño/La Niña. The different wind products include atmospheric model winds, satellite wind products, and a subjective analysis of ship and in situ winds. The model results are verified against fields of observed sea level anomalies from TOPEX/Poseidon data, sea surface temperature analyses, and subsurface temperature from the Tropical Atmosphere‐Ocean buoy array. Depending on which validation data type one chooses, different wind products provide the best forcing fields for simulating the observed signal. In general, the model results forced by satellite winds provide the best simulations of the spatial and temporal signal of the observed sea level. This is due to the accuracy of the meridional gradient of the zonal wind stress component that these products provide. Differences in wind forcing also affect subsurface dynamics and thermodynamics. For example, the wind products with the weakest magnitude best reproduce the sea surface temperature (SST) signal in the eastern Pacific. For these products the mixed layer is shallower, and the thermocline is closer to the surface. For such simulations the subsurface thermocline variability influences the variation in SST more than in reality. The products with the greatest wind magnitude have a strong cold bias of >1.5°C in the eastern Pacific because of increased mixing. The satellite winds along with the analysis winds correctly reproduce the depth of the thermocline and the general subsurface temperature structure.
Geographical distribution of new production in the western/central equatorial Pacific during El Niño and non‐El Niño conditions
Authors: Turk Daniela , Lewis Marlon R. , Harrison Glen W. , Kawano Takeshi , Asanuma Ichio
Published: February 2004

Rates of new production in the warm waters of the tropical Pacific and their seasonal to interannual variability are estimated by measurement of 15N‐nitrate uptake along the equator in the western and central (145°E–165°W) equatorial Pacific. Measurements were carried out during non‐El Niño conditions and two examples of anomalous conditions: the moderate El Niño in 1994–1995 and the strong El Niño in 1997–1998. Variations in new production are explained in relation to changes in physical, chemical, and biological environments during the El Niño‐Southern Oscillation cycle. During non‐El Niño conditions in the western region the rates of integrated new production over the euphotic zone were low (0.15 mmol m−2 d−1) and likely limited by availability of nitrate. In the central region, new production was higher (1.37 mmol m−2 d−1) owing to upwelling of nitrate‐rich subsurface waters. The east‐west asymmetry of observed variables in the western and central equatorial Pacific is altered during El Niño conditions owing to the eastward expansion of the western Pacific Warm Pool. In the western region, during the moderate El Niño in 1994–1995, the integrated new production increased slightly compared to non‐El Niño conditions, while during the strong El Niño in 1997–1998, it was higher by a factor of 10 than in non‐El Niño conditions. In the central region during the 1994–1995 El Niño, the rates of new production markedly decreased compared to non‐El Niño conditions. During the El Niño in 1997–1998, the integrated rates of new production were comparable to non‐El Niño conditions, but the vertical distribution showed a downward displacement of maximum rates. The results of our study suggest that interannual variations in new production in the western and central equatorial Pacific correlate well with the change of the nitracline depth during the eastward expansion of the Warm Pool and depend strongly upon the severity of the El Niño event.
Oxygen isotope/salinity relationship in the northern Indian Ocean
Authors: Delaygue Gilles , Bard Edouard , Rollion Claire , Jouzel Jean , Stiévenard Michel , Duplessy Jean‐Claude , Ganssen Gerald
Published: February 2004

We analyze the surface δ18O‐salinity relationships of the Bay of Bengal and the Arabian Sea, in the northern Indian Ocean, known for their contrasting hydrological conditions. New measurements of these tracers show a very low δ18O‐salinity slope associated with the strong dilution in the Bay of Bengal, but a slope more typical of this latitude in the Arabian Sea. Although this region is marked by a complex monsoonal regime, numerical modeling using a box model and a general circulation model is able to capture the δ18O‐salinity slope and its geographical variation. Both models clearly show that the low δ18O‐salinity slope is due to the evaporation‐minus‐precipitation balance, with an important contribution of the continental runoff in the Bay of Bengal. Although the low value of these slopes (∼0.25) makes past salinity reconstructions uncertain, insight into the Last Glacial Maximum conditions shows a probable stability of these slopes and limited error on paleosalinity.
Indonesian Throughflow in a coupled climate model and the sensitivity of the heat budget and deep overturning
Authors: Banks Helene T.
Published: February 2004

The circulation of the Indian Ocean in the Hadley Centre Climate Model is characterized by a very strong Indonesian Throughflow of O(24 Sv) compared with observational estimates of O(10 Sv) and very weak deep inflow across 32°S compared with observational estimates. The model throughflow has an upper core and a lower core of approximately equal magnitudes. We examine the driving terms for the throughflow and find that the upper core is driven by the wind stress curl and the lower core is driven by bottom pressure torques. The heat gained by the Indian Ocean from the atmosphere is approximately zero in the model, which is at odds with the observational estimates, which indicate a net heating of the Indian Ocean of between 0.4 and 1.2 PW. We hypothesize that the small heat flux is a direct consequence of the large throughflow and weak overturning. Two sensitivity experiments with different throughflows are described. We find that the magnitude of the deep overturning in the coupled model is strongly related to the magnitude of the lower core of the throughflow. The deep overturning increases from 3 to 9 Sv, as the magnitude of the lower core is reduced from 12 to 0 Sv. We find that the sensitivity of the heat gain of the Indian Ocean to the magnitude of the throughflow is O(0.005 PW) per sverdrup. This weak sensitivity may be due to the fact that the changes in the throughflow in the sensitivity experiments take place in the lower (colder) core.
Improved electromagnetic bias theory: Inclusion of hydrodynamic modulations
Authors: Elfouhaily T. , Thompson D. R. , Chapron B. , Vandemark D.
Published: February 2004

The modulation of short ocean waves by longer ones is a likely contributor to the radar altimeter's electromagnetic ranging bias (EM bias). An analytic model to account for this component of the EM bias is developed here under a two‐scale two‐dimensional hydrodynamic assumption. Following the principle of wave action balance, a standard hydrodynamic modulation transfer function is used to establish that the longer modulating waves enter the EM bias formulation not only through their elevation and slope variables but also through their quadratures. These latter contributions help to explain the role of long‐wave slope and orbital velocity fields within the EM bias problem. Simplified analytical expressions are derived using linear Gaussian statistics for both modulating and modulated waves. For the sake of completeness an outline of the possible extension to nonlinear interacting waves is provided.
Interannual variability of summer sea ice thickness in the Siberian and central Arctic under different atmospheric circulation regimes
Authors: Haas Christian , Eicken Hajo
Published: February 2004

Extensive drill hole and electromagnetic induction measurements of sea ice thickness in the Siberian and central Arctic Seas in the summers of 1993, 1995, and 1996 reveal significant interannual variability. In the Laptev Sea, minimum and maximum modal first‐year ice thicknesses amounted to 1.25 and 1.85 m in 1995 and 1996, respectively. Ice thickness correlates with ice extent, which reached a record minimum in August 1995 and was well above average in 1996. These differences are explained by the strength and location of a summer cyclonic atmospheric circulation pattern affecting both ice advection and surface melt. From drifting buoys deployed in 1995 and satellite radar backscatter data, first‐ and second‐year ice regimes are delineated. Differences in first‐year ice backscatter coefficients between 1993, 1995, and 1996 are explained by differences in level ice surface roughness. The Lagrangian evolution of ice thickness between 1995 and 1996 is studied. While the shape of the thickness distribution does not change significantly, the mean (modal) ice thickness of the ice field increases from 1.80 m (1.25 m) in 1995 to 2.86 m (2.25 m) in 1996. The thickness distribution of second‐year ice in 1996 closely agrees with that of level multiyear ice downstream in the Transpolar Drift obtained in 1991. In 1996, mean level ice thickness increases at 0.23 and 0.16 m deg−1 with latitude in the Kara and Laptev Sea sectors of the Arctic Ocean, respectively.
Pathways of cross‐frontal exchange in the North Atlantic Current
Authors: Dutkiewicz Stephanie , Rothstein Lewis , Rossby Tom
Published: February 2004

The North Atlantic Current (NAC) forms part of the boundary between the subtropical and subpolar gyres in the North Atlantic Ocean. The current has topographically controlled stationary meanders that appear to grow and decay. A region east of the current in the Newfoundland Basin contains water of mixed subpolar/subtropical properties, suggesting that there is exchange across the NAC. This study considers data from isopycnal RAFOS floats launched in the NAC region from 1993 to 1995. We use the RAFOS data to define the “frontal zone” as a pressure range where the jet is most likely to be found. This definition requires a latitudinal dependence as the NAC shoals to the north. Floats shallower and deeper than this range are defined to be on the subpolar and subtropical side, respectively. These definitions are used to estimate mixing that occurs between the current and its surroundings and to estimate the relative quantity of exchange of water parcels between the two gyres. Only small quantities of mass exchange from one gyre to the other are found, but there is a distinct asymmetry leading to a mean flux from the subpolar to subtropical sides. We also find that floats spend significant time in the frontal region and are frequently exchanged between fast and slow moving waters, particularly at the meander extrema. Diffusion, while in the jet, leads to eddy cross‐frontal exchange which is important for the exchange of properties across the NAC.
Isopycnal Lagrangian statistics from the North Atlantic Current RAFOS float observations
Authors: Zhang Huai‐Min , Prater Mark D. , Rossby Tom
Published: February 2004

One hundred isopycnal floats were tracked on the 27.2 and 27.5 σθ surfaces in the Newfoundland Basin (NFB) from July 1993 to July 1995 to study the current structure and exchanges of waters between the subtropical and subpolar gyres. The float‐mapped mean flow consists of weak flows in the NFB and a strong boundary current (the North Atlantic Current (NAC)), which separates from the boundary at the Northwest Corner, becoming a diffusive zonal drift. The NAC meanders are linked to topography and have similar patterns on the two isopycnals despite the fact that the upper layer velocities are twice as fast as the lower layer ones. Perturbation velocity from the mean is used to compute isopycnal turbulent dispersion and diffusivity. This large data set allows us to resolve a narrow mean NAC and results in a Gaussian turbulence. The turbulence approximately follows the classic Taylor dispersion theory. Integral timescales and length scales and turbulent isopycnal diffusivity are of 1.5–2.5 days, 20–30 km, and (1 − 7) × 103 m2 s−1, respectively. The timescale increases with depth and decreases with latitude, the length scale decreases with depth and longitude, and the diffusivity decreases with depth and from NAC to NFB. Compared to previous results from surface drifters and isobaric floats, our isopycnal statistics are more isotropic and agree better with the Taylor dispersion theory because (1) the mean velocity has a better resolution and (2) the isopycnal floats are better Lagrangian followers. The diffusivity scales better with the rms velocity and length scale than with the velocity variance and timescale.
On the shape and widening of salt marsh creeks
Authors: Fagherazzi Sergio , Furbish David Jon
Published: February 2004

We have developed a model that simulates aspects of initial channel formation in a youthful salt marsh environment. The model mimics the evolution of the cross section of a channel by coupling calculations of bottom shear stresses caused by tidal motions with erosion, taking into account the deposition of cohesive sediments. The simulations characterize flow in a reference cross section that includes an incipient channel zone and a marsh surface zone, with assigned water surface level and initial bottom elevation. This model mimics key characteristics of salt marshes where discharges due to tidal motion repeat in time with approximately the same magnitude and water surface level. Significant reductions in the tidal prism due to increasing bottom elevation above mean sea level, however, are not treated. Rather, the model is suitable for youthful salt marshes where relatively large water depths are maintained. Prolonged deposition reduces the area available for flow and thereby changes the shear stress distribution at the bottom, leading locally to erosion and alteration of the channel cross section. The simulations suggest that two mechanisms contribute to the longitudinal widening exhibited by salt marsh channels, which typically is disproportionately greater than that exhibited by river channels. The short duration of the maximum discharge (spring tide) and corresponding erosion rates, when compared with deposition rates, prevent the channel from reaching a deep, narrow equilibrium configuration. Furthermore, autoconsolidation of cohesive sediments, often occurring in salt marsh environments, leads to a downward increase in the resistance of the sediment to erosion. As scour occurs locally, the flow encounters more resistant sediment layers; so rather than deepening the channel over a narrow zone, flow and bottom stresses become more uniformly distributed leading to a wider channel than would otherwise occur in the absence of autoconsolidation. Based on flow and sediment properties estimated for the Venice Lagoon, Italy, simulations are consistent with observations of salt marsh creeks at this location.
East Australian Current volume transports at 30°S: Estimates from the World Ocean Circulation Experiment hydrographic sections PR11/P6 and the PCM3 current meter array
Authors: Mata Mauricio M. , Tomczak Matthias , Wijffels Susan , Church John A.
Published: February 2004

Between September 1991 and March 1994 the oceanic region off the east coast of Australia at 30°S was the subject of an intense observational program Part of the World Ocean Circulation Experiment, the objective was to improve our understanding of the dynamics of the East Australian Current (EAC) and to measure its volume and energy flux. The two main components were the Pacific Current Meter Array 3 (PCM3) with six moorings spanning a total distance of 120 km, and repeat, high‐resolution hydrographic surveys. The long‐term average fromPCM3 shows the EAC as a narrow, swift, and highly variable current centered at 40 km from the coast with strong shear in the upper 1000 m and mean southward velocities of 0.6 m s−1 in the core. The measurements also revealed a thick countercurrent underneath the poleward flow. The mean volume transport for the entire section covered by the array (between the coast and 154.4°E) was 22.1±4.6 Sv toward the south with an rms variability of 30 Sv. Combining the data from the PCM3 array with the hydrographic sections generated nine detailed snapshots of the absolute geostrophic current field. The snapshots were used to evaluate the reliability of the PCM3 data for determining volume transports. The rms difference between transports derived from direct current observations and from hydrographie data was 3.8 Sv, indicating that the PCM3 array can be used to describe the current variability despite the sparse spatial distribution of velocity measurements. Transport variability was dominated by periods between 90 and 140 days, which are attributed to eddy shedding in the region. Occasional intense northward flow events were observed where total transports reached up to 50 Sv northward. On those occasions the southward boundary current changed from being surface intensified and mainly baroclinic to northward and more barotropic, related to the northward migration past the array of the EAC coastal separation point.
Saharan dust and Florida red tides: The cyanophyte connection
Authors: Walsh John J. , Steidinger Karen A.
Published: February 2004

Prediction of the consequences of harmful algal blooms for humans and other vertebrates is constrained by an inadequate understanding of the factors that promote their initiation. A simple exponential growth model of net production is used for analysis of four time series at different sampling intervals over ∼40 years of red tide strandings, associated fish kills, and concomitant dust loadings on the West Florida shelf. At least large summer blooms of a toxic dinoflagellate Gymnodinium breve appear to be primed regularly by an aeolian supply of nutrients. Wet deposition of Saharan mineral aerosols may alleviate iron limitation of diazotrophic cyanophytes, which in turn fuel the nitrogen economy of red tides in the eastern Gulf of Mexico. Vagaries of the wind‐induced circulation and of selective grazing pressure on phytoplankton competitors within phosphorus replete coastal waters then determine each year the residence times for exposure of G. breve‐mediated neurotoxins to fish, manatees, and humans along the southeastern United States.