bluuebunny/crossref_metadata_2025 huggingface dataset:
The time‐averaged paleomagnetic field 0–5 Ma
Authors: McElhinny M. W. , McFadden P. L. , Merrill R. T.
Published: February 2004

Persistent departures from the geocentric axial dipole field model of the time‐averaged paleomagnetic field over the past 5 Myr have been analyzed using oceanic data from deep‐sea cores and continental data from igneous rocks and sediments. The data set comprises the equivalent of 9490 spot readings of the field (5831 normal and 3659 reverse) from 930 groups of data. Variations in declination anomalies between groups of data strongly suggest the existence of second‐order tectonic effects (small rotations) that make it difficult, if not impossible, to identify any nonzonal terms that might exist. Inclination anomalies have therefore been modeled in terms of low‐degree zonal harmonics. The calculated inclination anomalies within 10° latitude bands are widely scattered (for both oceanic and continental data) and do not appear to have been drawn from distributions with common means. We attribute this large scatter to previously undetected or uncorrected second‐order tectonic effects in the continental data or, in the oceanic data, to small departures from the vertical in deep‐sea cores. If it is assumed that these effects are random between groups of data and without a systematic bias, they can be averaged out to a large extent if there are sufficient numbers of groups within each latitude band. This method also has the major advantage that it maximizes time averaging of the field. When this is done, an acceptable fit to the inclination anomaly data is found using a zonal harmonic model. Although the point estimates for the reverse polarity zonal quadrupole term are consistently larger than those for the normal polarity zonal quadrupole term, the difference is not significant and is likely due to contamination effects. This finding is true for a combination of all the data or for separate combinations of continental igneous rocks and ocean sediments. This conclusion differs from all previous analyses. The major differences occur in model estimates of the zonal octupole term, the estimates varying widely depending on the particular data combination. We show that false octupole terms can be introduced by several factors, including inclination errors in sediments and lavas, the use of unit vectors in the analyses, and incomplete magnetic cleaning, particularly of reversely magnetized rocks. Thus we cannot impute a geomagnetic significance to the estimates of a zonal octupole term. We suggest that the best estimates for the zonal quadrupole G2 = g20 / g10 are obtained from the combined Brunhes age igneous rocks and oceanic normal data (0.033 ± 0.019) and for the combined Matuyama age igneous rocks and oceanic reverse data (0.042 ± 0.022). When these two sets are combined, the overall best estimate of G2 is 0.038 ± 0.012.
Mantle source heterogeneity and melting processes beneath seafloor spreading centers: The East Pacific Rise, 18°–19°S
Authors: Niu Yaoling , Waggoner D. Guy , Sinton John M. , Mahoney John J.
Published: February 2004

We present new major and trace element and Nd‐Pb‐Sr isotopic data on samples from the East Pacific Rise (EPR) axis and nearby seamounts in the Hump area, 18°–19°S. Most samples studied are normal mid‐ocean ridge basalt (N‐MORB); four samples from the southern seamounts are enriched MORB (E‐MORB). Dredge 52 samples from a southern seamount are depleted in incompatible elements yet possess “enriched” isotopic signatures. Except for the dredge 52 samples, all the samples show significant correlations between isotopic ratios and ratios of incompatible elements; that is, incompatible elements and isotopes are coupled. Sr and Nd isotopic ratios correlate significantly better with ratios of moderately incompatible elements than with ratios involving highly incompatible ones (e.g., Rb, Nb, and K) which appear to be “overenriched”. Both isotopic and incompatible element ratios also correlate with the extent of melting calculated from major elements. We interpret these correlations as mixing trends resulting from melting of a heterogeneous source containing enriched (“plume‐like”) domains of variable sizes. Overenrichment of highly incompatible elements in E‐MORB appears to be recent and is best explained by low‐degree‐melt infiltration in the source region prior to major melting events. The low‐degree melts are primarily derived from isotopically N‐MORB mantle. This low‐degree melt process also explains the incompatible element‐isotope decoupling throughout the EPR between 13° and 23°S. The dredge 52 samples too are consistent with such a process, but their immediate source is a site of low‐degree melt generation.
Strombolian explosions: 2. Eruption dynamics determined from acoustic measurements
Authors: Vergniolle S. , Brandeis G. , Mareschal J.‐C.
Published: February 2004

Strombolian activity consists of a series of explosions due to the breaking of a large overpressurized bubble at the surface of the magma column. Acoustic pressure due to sound waves has been measured and analyzed at Stromboli for more than 50 explosions. Three parts can be distinguished in the acoustic pressure waveform, which are related to the behavior of the bubble before, during, and after its bursting. Before the sharp rise in acoustic pressure, the signal is dominated by waves with a frequency of 2 Hz, which develop on the nose of the bubble. They produce sound in air by imposing a rapid motion to the interface, and one could detect a bubble travelling in the uppermost 30 m of the magma column. When the bubble reaches the air‐magma interface, its strong vibration, driven by a large overpressure inside the gas, generates the main event with a frequency around 9 Hz. After the bubble has burst, kinematic waves of frequency around 4.5 Hz are the main source of sound. They develop at the surface of the magma left on the conduit side. The three types of motion, although determined independently, give consistent results. Furthermore, combining the results obtained for the two types of kinematic waves, the magma viscosity is estimated to be of 300 ± 65 Pa s, which is in good agreement with petrological constraints and corroborates the validity of our analysis. This suggests that acoustic measurements constitute a powerful tool in the understanding of eruption dynamics.
Effects of chemical environment on dislocation creep of quartzite
Authors: Post Alice D. , Tullis Jan , Yund Richard A.
Published: February 2004

The water‐related chemical parameter that affects dislocation creep in quartzite has been determined from variations in sample strength and microstructure with chemical environment in buffered deformation and hydrostatic annealing experiments. Samples were weld‐sealed in double capsules;, , and were buffered using solid oxygen buffers, AgCl or CO2. Black Hills quartzite was deformed at 900°C and 1.5 × 10−5s−1. Two samples were deformed at ∼1700 MPa confining pressure at constant and , with and varying over 8 and 15 orders of magnitude, respectively. Both samples deformed by climb‐accommodated dislocation creep with flow stresses of 300 MPa. Two additional samples were deformed at ∼700 MPa at constant lower than for the 1700‐MPa samples, with varying over 2 orders of magnitude. Both samples faulted with a peak strength of ∼800 MPa. These four experiments suggest no dependence of dislocation creep strength on , or ; instead, a strong dependence of strength on is inferred. Previously deformed samples of Heavitree quartzite were hydrostatically annealed for 4 days at 800°C and 1200 or 500 MPa confining pressure, varying and over 2.5 and 1 order of magnitude, respectively. The microstructures of these samples show increased rates of dislocation climb and grain boundary migration with increasing but no dependence on . These buffered experiments indicate that dislocation creep is affected by alone and suggest that the exponent for the term in the power law creep flow law is >2.
Mid‐ocean ridge eruptive vent morphology and substructure: Evidence for dike widths, eruption rates, and evolution of eruptions and axial volcanic ridges
Authors: Head James W. , Wilson Lionel , Smith Deborah K.
Published: February 2004

High‐resolution side scan sonar data of Mid‐Atlantic Ridge (MAR) inner valley floor axial volcanic ridges reveal details of their architectural elements. We develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features. Inhibition of gas exsolution, lack of magma disruption, and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling, leading to more rapid centralization of eruptions along the widest places in the dike. Dikes are predicted to initially feed eruptions from fissure vents, producing lines of hummocky ridges; centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts. The widest dikes are predicted to produce smooth flows up to several kilometers in length, which should pond in adjacent lows. Edifice sizes predicted on the basis of volume fluxes, and flow lengths implied by the widths of dikes observed on the seafloor, in Iceland and in ophiolite complexes, are in quantitative agreement with the dimensions of observed features. Each ridge is made up of the products of a variety of these individual dike‐emplacement and extrusive events involving various dike widths, cooling times, and eruption durations. On the basis of typical MAR spreading rates, about one dike emplacement event would be expected every 40 years; since not all dikes reach the surface, the actual mean interval between eruptions will exceed this. MAR dikes should solidify between average dike emplacement events. Dike emplacement events are more frequent on the East Pacific Rise (EPR) and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes. Differences between the morphology of the MAR axial volcanic ridge and the sheet flow dominated EPR are attributed to on average wider dikes erupting with greater frequency along the EPR.
Geyser periodicity and the response of geysers to deformation
Authors: Ingebritsen S. E. , Rojstaczer S. A.
Published: February 2004

Numerical simulations of multiphase fluid and heat transport through a porous medium define combinations of rock properties and boundary conditions which lead to geyser‐like periodic discharge. Within the rather narrow range of conditions that allow geyser‐like behavior, eruption frequency and discharge are highly sensitive to the intrinsic permeabilities of the geyser conduit and the surrounding rock matrix, to the relative permeability functions assumed, and to pressure gradients in the matrix. In theory, heat pipes (concomitant upward flow of steam and downward flow of liquid) can exist under similar conditions, but our simulations suggest that the periodic solution is more stable. Simulated time series of geyser discharge are chaotic, but integrated quantities such as eruption frequency and mass discharge per eruption are free of chaos. These results may explain the observed sensitivity of natural geysers to small strains such as those caused by remote earthquakes, if ground motion is sufficient to induce permeability changes. Changes in geyser behavior caused by minor preseismic deformation, periodic surface loading, and Earth tides are more difficult to explain in the context of our current model.
Very long baseline interferometry determination of neglected tidal terms in high‐frequency Earth orientation variation
Authors: Gipson John M.
Published: February 2004

I present a model of high‐frequency earth orientation parameter (HF‐EOP) variation derived from 15 years of very long baseline interferometry (VLBI) data. This model uses twice as much data as previously published VLBI models. I estimate the coefficients of HF‐EOP at all tides with a magnitude greater than 5 mm in the tidal potential. This includes sidebands of the larger tides K1, O1, M2, and N2. I compare this model with other empirical models derived from VLBI and satellite laser ranging (SLR) data and also with predictions for HF‐EOP variation derived from models of the ocean. This model has the best agreement of any VLBI model with the independent SLR results: The RMS level of agreement of the coefficients is 1.2 μs in UT1 and 9 microseconds of arc (μas) in polar motion (PM). This model also has the best agreement of any VLBI or SLR model with the predictions of the best ocean model: The RMS level of agreement of the coefficients are 1.1 μs in UT1 and 6.5 μas in PM. I also compare the predictions of this model with the hourly measurements of EOP from the CONT94 VLBI campaign. The residuals are 8.9 μs in UT1 and 222 μas in PM, which are consistent with the formal errors of the measurements.
Crystallization and gravitationally controlled ponding during the formation of mound springs, terraces, and “black smoker” flanges
Authors: Kerr Ross C. , Turner J. Stewart
Published: February 2004

We present a series of laboratory experiments in which hot, dense aqueous solutions were cooled and crystallized by contact with cold ambient solutions as they flowed on horizontal, sloping, or nearly vertical boundaries. In each case, the crystals grew to form “dams” on the boundary, behind which the input fluid ponded before overflowing the rim. On a sloping boundary, simultaneous crystallization of a number of dams led to the growth of terraces that are reminiscent of natural travertine and sinter terraces. This phenomenon is not critically dependent on the composition, concentration, or flow rate of the fluid. If the fluid overflowing the rim becomes undersaturated, uniform crystallization is replaced by dissolution that soon focuses at one point of the rim, leading to a “breakout” of the ponded fluid. In an appendix, we also examine quantitatively the flow of water over the rim of mound springs and terraces and show that surface tension can determine both the two‐dimensional volume flow rate and the length of rim overflowed.
Slopes, lava flow volumes, and vent distributions on Volcán Fernandina, Galápagos Islands
Authors: Rowland Scott K.
Published: February 2004

Digital elevation data from TOPSAR, an airborne synthetic aperture radar system that uses interferometry to derive topography, are used to determine slope distributions and lava flow thicknesses on Fernandina Volcano, Galápagos Islands. Four extracaldera slope regions are defined (from the coast inland): A coastal plain (average slope ∼2°), an apron (6°–12°), steep slopes 250–600 m high (20°–43°), and a 0.5‐ to 1‐km‐wide summit platform (∼10°). Lava flows and vents are mapped using Shuttle Imaging Radar‐C (SIR‐C), SPOT, Landsat Thematic Mapper (TM), and air photos. Over 500 flows are identified and categorized as young, intermediate, or old based on albedo, vegetation cover, and margin preservation. By area, young flows constitute 55% of the island, 34% are intermediate, and 7% are old. The aa:pahoehoe ratio of the young flows is 85:15, whereas for the intermediate flows it is 58:42. Of the 423 vents that were classified, 236 are radial and 80% of these are in the apron, 143 are arcuate and 94% of these are in the summit platform, and 46 have transitional orientations and are about equally divided between the steep slopes and summit platform; 95% of all vents are within 13 km of the caldera center. TOPSAR data allow flow volumes to be estimated, and young flows range from <0.01 to 0.12 km3, with a total volume of 2.3 km3. By volume, 91% of the young lava erupted from radial vents below the steep slopes, many of which are concentrated within the SE apron about 5–6 km from the caldera. Similar concentrations to the NE, NW, and SW consist of young and intermediate flows. Different proportions of lava flows and vents form the different slope regions; the coastal plain averages 0.1 vents/km2 and the slightly steeper apron averages 0.6 to 0.9 vents/km2, increasing inland. The summit platform averages 4.7 vents/km2, and this concentration supports previously proposed mechanisms for producing higher elevations and steeper slopes in the central part of the volcano. Temporal changes in the plumbing system and/or magma supply, rate are suggested by the change in aa:pahoehoe ratio; it appears that in the past, low effusion rate eruptions were more common (perhaps from a filled caldera), whereas more recently, high effusion rate eruptions have dominated.
Expulsion of abnormally pressured fluids along faults
Authors: Roberts Sheila J. , Nunn Jeffrey A. , Cathles Larry , Cipriani Francois‐Dominique
Published: February 2004

Numerical simulations of fluid flow and heat transport in the South Eugene Island minibasin, offshore Louisiana, show that expulsion of geopressured fluids along faults can produce temperature and pressure anomalies similar to those observed in the area. In the simulations, abnormally pressured fluid moves along the fault through a fracture network. A thermal anomaly forms adjacent to the fault, while a larger fluid pressure anomaly extends into sediments on either side. Results from constant fault permeability simulations indicate that (1) geopressured sediments must be relatively permeable (5 × 10−17 m2) for expulsion to occur, (2) the size of thermal anomalies depend on the depth to which the fault is hydraulically open, and (3) fluid is vertically transported into shallow sediments when fault permeability is high, while lateral transport along deeper sands dominates when fault permeability is low. Excess fluid pressure in abnormally pressured sediments drops to half its original value throughout much of the minibasin after 10,000 years of expulsion; the associated thermal anomaly is also larger than observations, suggesting expulsion is not continuous. Variable fault permeability simulations, in which compaction of fault zone sediments closes the fracture network, indicate that fault permeability decreases by 1–2 orders of magnitude 1–200 years after expulsion begins. Thermal and baric anomalies from variable permeability simulations are smaller than from constant permeability simulations and are more consistent with available data. Faults must remain permeable for 20–30 years to produce thermal and baric anomalies similar to those observed in the area.
Surface currents in the area of the Indo‐Pacific throughflow and in the tropical Indian Ocean observed with surface drifters
Authors: Michida Yutaka , Yoritaka Hiroyuki
Published: February 2004

The location data set of 31 surface drifters deployed in the Indian Ocean between 1990 and 1993 are analyzed to illustrate the surface current field and its seasonal changes in the tropical Indian Ocean and the area of Indo‐Pacific throughflow. Trajectories of drifters, daily drifting velocities, and grid‐averaged velocities are described and compared with published climatologies of the surface currents in the Indian Ocean and numerically simulated flows in the Indonesian Seas. The surface current field and its seasonal changes in the Indian Ocean derived from drifter data are generally consistent with the published climatologies. Zonal velocities computed from the drifter data set in the equatorial jet, which appears twice a year in the transition periods of the monsoon, are greater than previous estimations. Surface currents flowing from the Indian Ocean toward the Indonesian Seas in boreal winter, in the opposite direction from that observed during other seasons, are traced with drifters deployed in the region south of the Indonesian archipelago. In particular, a drifter passing from the Indian Ocean to the Flores Sea through Lombok Strait in December 1993 is consistent with numerical modeling which shows northward flow in the Lombok Strait in boreal winter.
Field study of wave attenuation on an offshore coral reef
Authors: Hardy Thomas A. , Young Ian R.
Published: February 2004

A major field experiment was conducted which obtained measurements of the attenuation and transformation of short gravity waves as they cross the windward edge of an offshore coral reef. Water level data were collected for over 3000 individual time series during a wide range of environmental conditions. At the innermost measurement site, which is located on the horizontal reef flat after the completion of wave breaking, the upper bound of significant and maximum wave heights is limited to less than 40% and 60% of the reef flat water depth, respectively. As with significant wave height, both the shape and energy level of the reef flat spectra are strongly affected by changes in reef flat water depth. For higher tide levels the spectra on the reef flat closely mimic the corresponding incident spectra. However, the attenuation is greater for both lower frequencies and higher‐energy portions of the spectra. This causes the reef flat spectra to be broader than those measured windward of the reef. At lower water levels, considerable energy losses due to wave breaking and bottom friction occur. Most of the energy loss comes from the vicinity of the spectral peak, and energy shifts to harmonics of the peak of the spectrum can be seen.
Factors influencing the distribution of diatoms and other algae in the Ross Sea
Authors: Leventer Amy , Dunbar Robert B.
Published: February 2004

Quantitative microscopic analyses of sediment trap samples collected in the Ross Sea between January 1990 and February 1992 reveal striking temporal and spatial differences in algal bloom composition and size. Trap samples from the southwestern Ross Sea (site A, 76°30′S, 167°30′E) were dominated by the diatom Fragilariopsis curia (maximum of 92%), a species associated with both sea ice and the retreating ice edge in the Ross Sea. This species was probably seeded by melting congelation ice. Highest flux of diatom valves to both upper and lower traps in the 1991–1992 season occurred as a distinct event in mid‐February 1991, after which flux decreased by 1 to 3 orders of magnitude. As overall flux decreased, an increase in relative abundance of Chaetoceros resting spores and the open water species Thalassiosira antarctica was observed in the lower trap, suggesting some localized lateral advection. In the south central Ross Sea (site B, 76°30′, S 175°W), algal diversity was much higher, with a greater contribution of nondiatom material. The prymnesiophyte Phaeocystis antarctica, dinoflagellates, and cysts of unknown affinity were much more common than at site A, as were the diatoms Fragilariopsis cylindrus, Fragilariopsis kerguelensis, and Thalassiosira gracilis. Highest diatom flux occurred later (February 20 to March 6,1991) at this site and was an order of magnitude lower as compared to maximum flux at site A. Distinct differences in bloom size and composition between sites A and B may be a function of upper water column structure, differences in the amount of sea ice melting, and the type of sea ice present at the time of ice breakout (congelation versus pack ice). Despite rich surface productivities, lower silica flux to the seafloor at site B results from the higher proportion of nonsiliceous algae. At site C (72°30′S, 172°30′E), in the northwestern Ross Sea, diatoms again dominated algal flux; however, assemblage composition differed from that observed at both sites A and B. Relative abundance of F. curta was lower, averaging 60–70%, while the remainder of the assemblage was made up of F. cylindrus, other more robust species of Fragilariopsis, and Chaetoceros resting spores. Algal flux at site C is intermediate in style between sites A and B. Significantly, spatial differences in diatom assemblages noted above appear to be reflected in seafloor surface sediments, suggesting that downcore diatom data provide an interpretable record of paleoproductivity in the Ross Sea.
Bar/trough generation on a natural beach
Authors: Thornton E. B. , Humiston R. T. , Birkemeier W.
Published: February 2004

Mechanisms for bar/trough generation are examined using velocities measured in the field applied to theBowen[1980]/Bailard[1981] energetics‐based sediment transport model. Measurements consist of a cross‐shore array of nine electromagnetic current meters spanning the surf zone and daily bathymetric surveys during a 10‐day period during which two storms occurred, when the bathymetry evolved from a three‐dimensional terrace to a well‐developed linear bar. The model predicts bed and suspended load transport separately based on various velocity moments. The velocities are partitioned into mean currents, low‐frequency infragravity and shear instabilities (<0.05 Hz), and high‐frequency short waves and turbulence (>0.05 Hz) to determine the relative importance of various mechanisms to the total transport. Velocity moments are computed over 90‐min intervals to resolve tidal fluctuations. Tidal signatures were apparent in all modes of transport. Predicted transport rates are integrated and compared with daily cross‐shore bathymetric profiles (averaged over a 400‐m length of beach). The suspended load terms were an order of magnitude greater than bed load terms owing to the low fall velocity of the fine‐grain sand within the surf zone. Model results for this experiment indicate the dominant mechanism for bar development was sediments mobilized by the strong longshore current and incident short waves within the surf zone and transported offshore by the mean undertow and shoreward transport onshore due to short wave velocity skewness. Using standard coefficients, the model correctly predicted the first‐order movement of the bar during storms, but underpredicted trough development, and did not always perform well during mild wave conditions.
Interaction of fluctuating river flow with a barotropic tide: A demonstration of wavelet tidal analysis methods
Authors: Jay David A. , Flinchem Edward P.
Published: February 2004

Wavelet transforms provide a valuable new tool for analysis of tidal processes that deviate markedly from an assumption of exact periodicity inherent in traditional harmonic analysis. A wavelet basis adapted to nonstationary tidal problems is constructed and employed to analyze the modulation of the external tide in a river by variations in streamflow. Interaction of a surface tide with river flow is the best available demonstration of the continuous wavelet transform (CWT) methods developed. It is the simplest and perhaps the only nonstationary tidal process for which both sufficient data and dynamical understanding exist to allow detailed comparisons between CWT analyses and analytical predictions of the response of tides to nontidal forcing. Variations at upriver locations of low‐frequency elevation (river stage ZR) and three tidal species are deduced from cross‐sectionally integrated equations. For landward propagation in a channel of constant cross section with quadratic friction, the log of the amplitude of the diurnal (D1), semidiurnal (D2) and quarterdiurnal (D4) elevations should vary at far upriver locations with the square root of the river flow (QR;), and river stage (ZR) should depend on the square of river flow. Convergent geometry and species‐species frictional interactions modify these predictions somewhat. CWT analyses show that the predicted amplitude behavior for the tidal species is approximately correct. Best results are obtained for the dominant, dynamically simplest processes (ZR and D2). In the past, further progress in understanding river tides has been limited by a lack of data analysis tools. Data analysis tools are now clearly better than the available analytical solutions.
Effects of wave breaking on the surface drift
Authors: Melsom Arne
Published: February 2004

The present paper is an investigation of the motion induced in the ocean by breaking waves. The theory that is presented here addresses two related problems concerning such motion. First, several parameterizations for transfer of momentum during breaking are examined. Second, attempts are made to describe the surface drift and the vertical structure of motion that results from multiple breaking events. It is shown that the various parameterizations of breaking events are robust in the sense that they qualitatively and quantitatively give similar results. However, for description of the vertical structure of the current, the Reynolds stresses must be parameterized. For this purpose, we will here adopt the concept of eddy viscosity. It is demonstrated that wave breaking enhances motion close to the surface and retards motion in the deeper parts of the ocean. Also, the drift induced by multiple events of breaking is significant, and it is only slightly deflected from the direction of wave propagation.
Predictability experiments in the North Atlantic Ocean: Outcome of a quasi‐geostrophic model with assimilation of TOPEX/POSEIDON altimeter data
Authors: Brasseur P. , Blayo E. , Verron J.
Published: February 2004

In a recent paper, Blayo et al. [1994] described the simulation of the North Atlantic general circulation during the period October 1992 to September 1993, as obtained from the assimilation of TOPEX/POSEIDON (T/P) altimeter data in a high‐resolution, quasi‐geostrophic model of the Atlantic basin between 20°N and 60°. In line with this paper, the purpose of the present study is to investigate the usefulness of the quasi‐geostrophic model for predicting the time evolution of the mesoscale flow field from the perspective of operational oceanography. Forecast experiments have been conducted to address the following three questions: (1) how does the model with assimilation differ from a so‐called “pure forecast” trajectory, as produced in the absence of altimeter data; (2) can the model evolution controlled with T/P altimetric residuals be considered as a decent description of the actual ocean evolution; (3) is the assimilation useful to predict the future of the system at short and medium ranges? The simulations consist of twin experiments obtained from the assimilation of subsets of the original T/P data set (covering, for instance, only part of the total duration of the reference experiment) or from the assimilation of synthetic altimeter data extracted from the reference simulation. In addition, predictability experiments have been carried out to determine the doubling timescale of small errors in the specification of the model initial conditions. The statistics derived from the simulations indicate that the assimilation of altimetry is efficient in reconstructing the past and the present state of the ocean on an operational basis. The procedure is shown to be able to recover both the surface and the deep flow during the assimilation period. The transport of the Gulf Stream is kept consistent with observational evidences as long as altimeter data are taken into account. However, forecasting mesoscale eddies is realistic only for a couple of weeks in advance. It is found that the averaged doubling time of initial errors is about 20 days, and credit of forecasts over more than 2 months in advance is questionable. As expected, predictions of the future are bounded by the predictability timescales inherent to the physical nature of the system.
Factors affecting suspended‐solids concentrations in South San Francisco Bay, California
Authors: Schoellhamer David H.
Published: February 2004

Measurements of suspended‐solids concentration (SSC) were made at two depths at three sites in South San Francisco Bay (South Bay) to determine the factors that affect SSC. Twenty‐eight segments of reliable and continuous SSC time series data longer than 14 days were collected from late 1991 or 1992 through September 1993. Spectral analysis and singular spectrum analysis were used to relate these data segments to time series of several potential forcing factors, including diurnal and semidiurnal tides, the spring‐neap tidal cycle, wind shear, freshwater runoff, and longitudinal density differences. SSC is greatest during summer, when a landward wind shear is applied to South Bay by the afternoon sea breeze. About one half the variance of SSC is caused by the spring‐neap cycle, and SSC lags the spring‐neap cycle by about 2 days. Relatively short duration of slack water limits the duration of deposition of suspended solids and consolidation of newly deposited bed sediment during the tidal cycle, so suspended solids accumulate in the water column as a spring tide is approached and slowly deposit as a neap tide is approached. Perturbations in SSC caused by wind and local runoff from winter storms during the study period were usually much smaller than SSC variations caused by the spring‐neap cycle. Variations of SSC at the study sites at tidal timescales are tidally forced, and nonlinear physical processes are significant. Advective transport dominates during spring tides, when water with higher SSC due to wind wave resuspension is advected to the main channel from shallow water, but, during neap tides, advective transport is less significant. The findings of this and other studies indicate that the tidally averaged transport of suspended solids responds to seasonal variations of wind shear in South Bay.
A new method to determine near‐sea surface air temperature by using satellite data
Authors: Konda Masanori , Imasato Norihisa , Shibata Akira
Published: February 2004

We present a new algorithm with which to determine near‐sea surface air temperature from satellite observations. A relationship between air temperature, sea surface temperature (SST), wind speed, and humidity is obtained from the aerodynamical equation and the bulk formula. We solve air temperature from this relationship by giving other variables with the observations and without explicit assumption of boundary layer parameters. Our new method is validated using observed monthly mean data at the Japan Meteorological Agency (JMA) and Tropical Ocean‐Global Atmosphere (TOGA)‐Tropical Atmosphere Ocean (TAO) buoys. Air temperature and sensible heat flux can be determined with accuracies of 0.0° ± 1.2°C and 0.1 ± 8.7 W/m2, respectively, when SST, wind speed, and humidity are given by in situ observations. In order to retrieve near‐sea surface air temperature and sensible heat flux from satellite data, SST is obtained from multi channel sea surface temperature (MCSST) of advanced very high resolution radiometer (AVHRR), wind speed is obtained from special sensor microwave/imager (SSM/I), and humidity is obtained from SSM/I together with the empirical equation proposed by Liu [1986]. An error of monthly mean satellite‐derived air temperature is −0.3° ± 3.1°C, and that of sensible heat flux is 10.0 ± 37.6 W/m2. Errors of both satellite‐derived values are enlarged, possibly because the satellite observations have seasonably systematic error. Time evolution of the interannual variation of air temperature estimated by satellite agrees well with that of in situ measurements. The estimation error of the interannual variation of air temperature is 0.0° ± 1.5°C, and that of sensible heat flux is 0.3 ± 19.0 W/m2. At an interannual timescale the satellite‐derived sensible heat flux catches the time evolution of the in situ observation. Both the air temperature and the sensible heat flux obtained by using our method have fewer errors than estimations made by assuming relative humidity as the climatological mean value.
Inferring multiscale structure in atmospheric turbulence using satellite‐based synthetic aperture radar imagery
Authors: Mourad Pierre D.
Published: February 2004

This paper describes the multiscale structure in synthetic aperture radar (SAR) backscatter seen in an image of a cold‐air outbreak as being due to the influence on the surface of the ocean of three scales of interacting atmospheric turbulence. A low‐resolution (pixels have side lengths of 100 m) SAR image shows the first class of backscatter pattern to be broken lines of enhanced radar backscatter (σ), of order 1 dB above the background, with an average cross‐wind scale of 5 km and downwind lengths of many tens of kilometers. These are associated with quasi‐two‐dimensional roll vortices. The second class of radar backscatter pattern is two‐dimensional backscatter regions whose linear aggregrations constitute the lines of backscatter. They are, on average, 1 km wide in the cross‐wind direction and are 2.5 km long in the downwind direction. A higher‐resolution image (pixels have side lengths of 12.5 m) of the same area shows a third class of structure: smaller‐scale regions of enhanced backscatter up to several hundred meters in length downwind and greater than 100 m across wind that are associated with 7‐dB variations in σ in the unaveraged image. The modulated, fine‐scale backscatter structure compares favorably in both scale and effect to observations of the modulation of “microfronts” in the atmospheric surface layer by kilometer‐scale “inactive eddies,” the latter being of uncertain origin. Therefore the hypothesis offered here is that the fine‐scale SAR structure is caused by the influence of atmospheric microfronts on the ocean surface, while the two‐dimensional, kilometer‐scale backscatter features are the signature of inactive eddies linearly aggregated by or possibly intrinsic to the roll vortices.
Waves and currents over a fixed rippled bed: 1. Bottom roughness experienced by waves in the presence and absence of currents
Authors: Mathisen Paul Peter , Madsen Ole Secher
Published: February 2004

Several theoretical models have been advanced for turbulent boundary layers associated with wave and current flows over a rough bed. Although these models differ, particularly in the sophistication with which turbulence is modeled, they all share one common feature: the bottom roughness is characterized by a single roughness length scalekn. However, no experimental data have been presented which accurately characterize the bottom roughnesses for wave and current boundary layers such that this basic assumption can be considered verified. This paper and a companion paper [Mathisen and Madsen, this issue] present results of an experimental study to verify this basic assumption. In this paper, wave roughnesses (i.e., bottom roughnesses experienced by pure waves and waves in the presence of a current) are compared with the roughness experienced by a pure current. The roughnesses for pure currents and pure waves are shown to be the same if determination of the pure wave roughness from measurements of wave energy dissipation includes the phase difference between the near‐bottom horizontal orbital velocity and bottom shear stress. When these analysis procedures are extended to combined wave‐current flows, the roughness experienced by waves in the presence of a current is shown to be that obtained for pure waves.
A current velocity index of ocean variability
Authors: Thomson Richard E. , Ware Daniel M.
Published: February 2004

We use daily time series of the longshore component of residual (detided) current velocity υ(z, t) measured at several depths at a single location on the continental slope to construct a current velocity index, Iυ ≡ α|υ′s∂υ/∂z|∂υ/∂z, of coastal ocean variability. Here z is depth, t is time, and α = −sgn(f)[(f2 + δ2)συ]−1 is a normalization factor constructed from the local Coriolis parameter f, a friction parameter δ, and the standard deviation συ of the near‐surface current velocity,
, where υ′s is the fluctuation about the time series mean
; sgn(f) = +1 for the northern hemisphere and −1 for the southern hemisphere. The time series of vertical shear, ∂υ/∂z ≈ Δυ/Δz, is derived from the velocity difference Δυ between simultaneous current meter records separated by a vertical distance Δz within the upper half of the water column. The index characterizes low‐frequency baroclinic variability over the continental margin and is linked, through use of the thermal wind relation, to instability processes that effect the transfer of potential energy stored in the mean cross‐slope density field to the kinetic energy of mesoscale meanders and eddies. For eastern boundary regions the index ranges from large positive values during summer upwelling conditions to large negative values during winter downwelling conditions. The “transition” seasons, spring and fall, are characterized by Iυ ≈ 0. We demonstrate use of the index by applying it to the continental margin of southwest Vancouver Island using current velocity data collected from October 1989 to March 1995 at depths of 35, 100, 175, and 400 m at a long‐term mooring site. Results enable us to specify the start and end times of the oceanic seasons and to quantify the summer upwelling and winter downwelling intensities for the region. Gaps in the current meter time series prompted us to examine records of coastal sea level, sea surface temperature, sea surface salinity, and “Bakun” upwelling index (the wind‐driven cross‐shore component of surface Ekman transport) as possible surrogates for the velocity index. None of these time series were acceptable. Although the Bakun index is a logical substitute for the velocity index, differences between the two indices are significant and suggest that current velocity data are needed for accurate delineation of seasonal oceanic variability along continental margins.
Cleavage of a Gulf of Mexico loop current eddy by a deep water cyclone
Authors: Biggs D. C. , Fargion G. S. , Hamilton P. , Leben R. R.
Published: February 2004

Eddy Triton, an anticyclonic eddy shed by the Loop Current in late June 1991, drifted SW across the central Gulf of Mexico in the first 6 months of 1992, along the “southern” of the three characteristic drift paths described by Vukovich and Crissman [1986] from their analyses of 13 years of advanced very high resolution radiometer sea surface temperature data. An expendable bathythermograph (XBT) and conductivity‐temperature‐depth (CTD) transect of opportunity through Triton at eddy age 7 months in January 1992 found that eddy interior stood 23 dyn. cm higher than periphery; this gradient drove an anticyclonic swirl transport of 9–10 Sv relative to 800 dbar. At eddy age 9–10 months and while this eddy was in deep water near 94°W, it interacted with a mesoscale cyclonic circulation and was cleaved into two parts. The major (greater dynamic centimeters) piece drifted NW to end up in the “eddy graveyard” in the NW corner of the gulf, while the minor piece drifted SW and reached the continental margin of the western gulf off Tuxpan. This southern piece of Eddy Triton then turned north to follow the 2000‐m isobath to about 24°N and later coalesced with what remained of the major fragment. Because Eddy Triton's cleavage took place just before the start of marine mammals (GulfCet) and Louisiana‐Texas physical oceanography (LATEX) field programs, the closely spaced CTD, XBT, and air dropped XBT (AXBT) data that were gathered on the continental margin north of 26°N in support of these programs allow a detailed look at the northern margin of the larger fragment of this eddy. Supporting data from the space‐borne altimeters on ERS 1 and TOPEX/POSEIDON allow us to track both pieces of Eddy Triton in the western Gulf and follow their spin down in dynamic height, coalescence, and ultimate entrainment in January 1993 into another anticyclonic eddy (Eddy U).
Observations of recurrent mesoscale eddies in the eastern Mediterranean
Authors: Matteoda Anna M. , Glenn Scott M.
Published: February 2004

A Mediterranean Sea observation program, conducted in the early 1990s by the Naval Oceanographic Office, included temperature aerial surveys, the deployment of satellite‐tracked drifting buoys, and the real‐time analysis of satellite infrared imagery. Over 4 years of weekly front and eddy analyses generated from the satellite imagery and over 100 drifting buoy trajectories were examined here for recurrent eddies in the eastern Mediterranean. Five recurrent mesoscale eddies found in the satellite imagery remained relatively stationary with the standard deviation of the eddy center locations comparable to the mean eddy diameter. Three of the recurrent eddies were seeded with satellite‐tracked drifting buoys. One buoy circulated within each of two anticyclones for several weeks, allowing application of a kinematic feature model to the buoy trajectories for the extraction of propagation and circulation characteristics. The Ierapetra anticyclone remained southwest of the mean satellite‐derived center, rotating slowly clockwise about 1.2° d−1 with a swirl velocity that increased linearly with distance from the center nearly to its outer edge. The Pelops anticyclone exhibited a slight westward propagation tendency, its major axis rotated clockwise about 4° d−1, and the swirl velocity had a broad maximum well inside the eddy that was nearly independent of radius. Inertial waves were observed in two of the buoy trajectories, with the differences in the observed inertial periods consistent with the effects of eddy circulation on near‐inertial waves.
Mixing of chlorophyll from the Middle Atlantic Bight cold pool into the Gulf Stream at Cape Hatteras in July 1993
Authors: Wood A. Michelle , Sherry Nelson D. , Huyer Adriana
Published: February 2004

In July 1993 we collected hydrographic data and information on chlorophyll distribution on the continental shelf north of Cape Hatteras and across the shelf break at Cape Hatteras. The data show that a warm, transparent mixed layer lies over much colder, euphotic, chlorophyll‐rich bottom water on the shelf. This layer has temperature and salinity properties characteristic of the Middle Atlantic Bight (MAB) cold pool, a distinctive mass of cold bottom water formed when cold water from the Gulf of Maine and Scotian Shelf is isolated from surface water by vernal warming and seasonal stratification [Houghton et al., 1982]. The constant density of this chlorophyll‐rich water (σθ = 25.0–25.6) combined with a strong chlorophyll gradient along the 25 σθ isopycnal at the shelf break indicates that chlorophyll advected off the shelf at Cape Hatteras in July 1993. TS diagrams further indicate that cold pool water, and the chlorophyll it contained, mixed into upper levels of the Gulf Stream. Thus the MAB may contribute to the nutrient budget of Atlantic surface waters through a long loop of circulation that transports deep water from the Labrador Sea to Cape Hatteras.
Horizontal mixing in lakes
Authors: Peeters F. , Wüest A. , Piepke G. , Imboden D. M.
Published: February 2004

Horizontal mixing in the upper hypolimnion of lakes far from the boundaries was studied in lake basins with surface areas between 5 and 220 km2 by observing the growth of the concentration distribution of the fluorescent dye sodium fluorescein (uranin). In each of the eight experiments, between 0.2 and 2 kg of uranin was instantaneously released into the lake in the appropriate depth (between 15 and 25 m) in such a way to keep the initial cloud size as small as possible. The horizontal extension of the cloud was repeatedly determined by integration of numerous vertical profiles. These surveys served to test theoretical models for horizontal mixing. The temporal development of the size and of the variances along the principal axes of the tracer concentration distribution was the main property considered here. The experiments cover a range of cloud sizes between 3 × 102 and 3 × 105 m2. None of them support the hypothesis that cloud size grows with elapsed time raised to the power of 3 as predicted by the inertial subrange turbulence model first applied to dispersion by Batchelor [1950]. The shear‐diffusion model of Carter and Okubo [1965] was found to provide a good description of the development of cloud size with time. This model also accounts for the fact that the tracer distributions were not radially symmetric. Effective horizontal diffusivities lie between 0.02 and 0.3 m2 s−1. Reevaluation of published data from experiments in Lake Ontario [e.g., Murthy, 1976] and in the ocean [e.g., Okubo, 1971] supports both the applicability of the shear‐diffusion model and the doubts raised against the appropriateness of the inertial subrange model for scales up to 1000 m.