bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic velocity structure at a gas hydrate reflector, offshore western Colombia, from full waveform inversion
Authors: Minshull T. A. , Singh S. C. , Westbrook G. K.
Published: February 2004

Seismic reflection profiles across many continental margins have imaged bottom simulating reflectors (BSRs), which have been interpreted as being formed at the base of a methane hydrate stability field. Such reflectors might arise either from an impedance contrast between high‐velocity, partially hydrated sediments and water‐saturated sediments or from a contrast with partially gas‐saturated sediments. These alternatives may be hard to distinguish by conventional amplitude‐versus‐offset or waveform modeling approaches. Here we investigate the origin of a high amplitude BSR in the accretionary wedge offshore of western Colombia by seismic waveform inversion. The inversion procedure consists of three steps: firstly, determination of root‐mean‐square velocities and hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptical trajectories in the intercept time‐slowness domain; secondly, determination of accurate interval velocities between these reflectors by a Monte Carlo search for maximum energy; and thirdly, a waveform fit in the frequency‐slowness domain, using differential reflectivity seismograms and a conjugate‐gradient optimization algorithm to minimize the sample‐by‐sample waveform misfit between data and synthetic. At two locations, near a structural high, we find a ∼30‐m thick low‐velocity zone beneath the BSR, with the properties of a partially gas‐saturated zone, while at a third location, where the BSR amplitude is lower, we find no evidence for anomalously low velocities. The preferential development of the BSR in structures that would tend to intercept fluid flow or migrating gas and the presence of free gas beneath the BSR indicate a mechanism of BSR formation in which free methane gas migrates upward into the hydrate stability field or is carried there in advecting pore water.
Crustal structure beneath Long Valley Caldera from modeling of teleseismic P wave polarizations and Ps converted waves
Authors: Steck Lee Karl , Prothero William A.
Published: February 2004

In this study we present new constraints on the nature of the low‐velocity zone beneath Long Valley caldera, based on the measured propagation directions of teleseismic P waves and on modeling of P to S converted waves. The low‐velocity body is a large asymmetrical volume which deepens to the east, extending from depths of 7 to 30 km. It contains lower velocities than originally proposed by earlier teleseismic studies. In particular, there is a tabular feature between 7 and 11 km depth that has a reduction in velocity of about 30%. These low velocities imply a much greater percentage of melt in the crust beneath Long Valley caldera than previously estimated. Array analysis of large delayed arrivals identifies them to be Ps converted waves from the shoulders and roof of this tabular zone. These conversions bound the depth to the magma chamber roof to be within about 10 km of the surface. Our results are consistent with elements from several other studies, and we present an integrated and improved model of crustal structure at Long Valley. The concordance of the deeper low‐velocity zones with regional structural trends implies that the shallow low velocity feature is a cupola on top of an asymmetric diapiric ridge rising up from the migmatized lower crust of the Basin and Range. We present two contrasting interpretations of the geometry of low‐velocity zones in the crust: one implies a time invariant magma chamber and conduit system for Long Valley caldera, the other implies an evolution of that system from a simple vertical regime to its current asymmetrical geometry.
A comparison of mechanical thickness estimates from trough and seamount loading in the southeastern Gulf of Alaska
Authors: Harris Robert N. , Chapman David S.
Published: February 2004

The southern portion of the Kodiak‐Bowie seamount chain in the southeastern Gulf of Alaska presents a unique opportunity to investigate loading on young oceanic lithosphere. Oceanic lithosphere younger than 25 Ma is loaded both by sediments in a deep offshore trough and by seamounts 100–200 km offshore. Free‐air gravity anomaly values associated with the trough range from −90 mGal over the trough to +25 mGal over the offshore flexural high, whereas values associated with the volcanic loading range from +80 mGal over the seamounts to −10 mGal over their associated moats. These anomalies are modeled first using elastic beam theory. Gravity anomalies associated with the trough are modeled in terms of elastic bending of a sediment filled trench. Elastic thicknesses associated with the trough range from 12 to 22 km with an estimated uncertainty of ±5 km. Seamounts are approximated as a series of stacked finite cylinders. Gravity highs over the seamounts are used to model density, while gravity patterns associated with the moat and peripheral bulge are used to model the elastic thickness along the seamount chain. Elastic thicknesses along the seamount chain range from 2 to 5 km (estimated uncertainty range 0 to 7 and 0 to 10 km, respectively). To investigate differences in elastic thickness estimates between the trough and seamount models, we incorporate the effects of finite yield strength. Curvatures implied by the elastic beam models are used to calculate mechanical bending stresses. Bending moments calculated from elastic beam models and yield envelope models agree for flexure models at the Queen Charlotte Trough and most seamounts. The mechanical thickness corresponds to the depth to the 700°C isotherm assuming a dry olivine rheology and a simple cooling model for oceanic lithosphere. Our results imply that the strength of the lithosphere calculated for both seamount and trench loading corresponds to the same isotherm.
Stress perturbations associated with active faults penetrated by boreholes: Possible evidence for near‐complete stress drop and a new technique for stress magnitude measurement
Authors: Barton Colleen A. , Zoback Mark D.
Published: February 2004

Detailed studies of stress‐induced wellbore breakouts in wells drilled through active faults reveal stress field discontinuities that are apparently associated with recent fault movements. These discontinuities are expressed as localized rotations in wellbore breakout orientation in the vicinity of the fault penetrated by the borehole. This phenomenon is observed in a variety of tectonic environments and rock types. Utilizing cases where relatively complete knowledge of the horizontal principal stresses is available from in situ measurements, we use three‐dimensional dislocation modeling to demonstrate that these discontinuities can be explained as the superposition of a reference stress state and a perturbation caused by movement on preexisting faults. Case studies from normal, strike‐slip and reverse faulting stress states indicate that nearly complete stress drop is required to match the observed breakout orientation anomalies. Hydraulic fracturing data independently confirm the occurrence of near‐complete stress drop on some faults penetrated by drilling. Modeling of the observed interactions between breakouts and fractures can also be used to obtain information about the magnitude of in situ stress.
Geochemical effects of dynamic melting beneath ridges: Reconciling major and trace element variations in Kolbeinsey (and global) mid‐ocean ridge basalt
Authors: Devey C. W. , Garbe‐Schönberg C.‐D. , Stoffers P. , Chauvel C. , Mertz D. F.
Published: February 2004

Zero‐age basalts dredged from the Kolbeinsey Ridge directly north of Iceland are mafic quartz tholeiites (MgO 6–10 wt. %), strongly depleted in incompatible elements. Fractionation‐corrected Na2O contents (“Nag”) are amongst the lowest found on the global ridge system, implying that the degree of partial melting at Kolbeinsey is amongst the highest for all mid‐ocean ridge basalt (MORB). In contrast, the basalts show large ranges of incompatible‐element ratios (e.g., K2O/TiO2 of 0.01 to 0.12 and Nd/Sm of 2.1 to 2.9) not related to variations in radiogenic isotope ratios; this suggests recent enrichment/depletion events associated with small‐degree partial melting as their cause, rather than long‐lived source heterogeneity. Tholeiitic MORB from many regions globally show similar or more extreme variations in K2O/TiO2. Dynamic melting of an adiabatically upwelling source can reconcile these conflicting indications of the degree of melting. Through dynamic melting, the incompatible elements are partially separated into different melt fractions based on their bulk partition coefficients, more incompatible elements being concentrated in deeper, smaller‐degree partial melts. The final erupted magma is a mix of melts from all depths in the melting column. The concentration of highly incompatible elements in the mix will be very sensitive to the physical processes allowing the deep melts to separate and migrate to the site of mixing, and small fluctuations in the efficiency of the separation process can account for the large range of trace element ratios seen at Kolbeinsey. The major element chemistry of the erupted mix (and Na8) is much more robust, depending mainly on the integrated total amount of melting. The large variations of incompatible element ratios seen at Kolbeinsey, and in MORB in general, therefore give no information about the total degree of melting occurring beneath the ridge, nor do they require a heterogeneous source.
Degree 12 model of shear velocity heterogeneity in the mantle
Authors: Su Wei‐jia , Woodward Robert L. , Dziewonski Adam M.
Published: February 2004

We obtain a three‐dimensional (3D) model of shear wave velocity heterogeneity of the Earth's mantle by inverting a large set of seismic data consisting of 27,000 long‐period seismograms and 14,000 travel time observations. About 60% of the data has been collected through the efforts of several research groups and used in earlier studies. The new data, which come from stations of different seismic networks including the Chinese Digital Seismographic Network (CDSN) and Geoscope, are extracted to provide sampling of mantle heterogeneity as uniform as possible. Because of the improved data coverage, we expand our model to degree 12 in spherical harmonics to describe horizontal variations, and to order of 13 in Chebyshev polynomials to describe radial variations. The resulting model shows a clear pattern of slower‐than‐average shear velocities at shallow depths underlying the major segments of the world‐wide ridge system. These anomalies extend to depths greater than 300 km and in some cases appear to continue into the lower mantle. There is also a good correlation between the major continental shields and fast‐velocity perturbations at depths extending to 300–400 km. Some of the continental “roots” extend to depths greater than in other studies. The pattern of heterogeneity is more complex in the midmantle, where the power spectrum is almost flat and has a relatively low amplitude; therefore the results in this depth range should be interpreted with caution. The pattern of the heterogeneity indicates a rapid change at a depth of about 1700 km. At this depth, the power spectrum of the model shifts from one which is almost flat in the midmantle to that dominated by degrees 2 and 3; this pattern then continues to the core‐mantle boundary (CMB). The model is dominated by a few megastructures of velocity heterogeneity below the depth of 2000 km, in agreement with previous studies. Among these megastructures are the “Pangea Trough,” “Great African Plume,” and “Equatorial Pacific Plume Group.” The model predicts well the large‐scale pattern of observed S, SS absolute travel times, and SS‐S, ScS‐S differential travel times. It also predicts well the waveforms of mantle wave and body wave. We compare our model with several other recently published models. There is generally a good agreement in the long‐wavelength pattern of the models, especially at shallow depths and near the CMB. However, the amplitude as well as the pattern of shorter‐wavelength features are in some cases quite different.
The dependence of microwave backscatter from the sea on illuminated area: Correlation times and lengths
Authors: Plant William J. , Terray Eugene A. , Petitt Robert A. , Keller William C.
Published: February 2004

During the Synthetic Aperture Radar (SAR) and X band Ocean Nonlinearities‐Forschungsplattform Nordsee experiment, we mounted two continuous wave microwave systems on an elevator on the German Research Platform Nordsee for the purpose of investigating the dependence of microwave backscatter from the sea surface on illuminated area. The two systems operated at X and Ka bands (10 and 35 GHz) and collected HH and VV polarized backscattered signals simultaneously. The elevator system allowed us to vary the altitude of the two microwave systems above the sea surface from 7.5 to 27 m, always in the far field of the antennas. Most data were collected at a 45° incidence angle, which implied that the Ka band system illuminated areas from 0.4 to 6.0 m2 while the X band system viewed spots between 2.9 and 41.3 m2. We examined the dependence of the normalized radar cross section (σ0), its variance, and the bandwidth of the Doppler spectrum on illuminated areas. We were unable to detect any dependence of σ0 on area but found a definite decrease in its variance as area increased. At X band the variance divided by the square of σ0, the normalized variance, decreased from values near 12 for small areas to values near 2 for large areas. At Ka band, corresponding values were 40 and 2. The normalized variance was always slightly larger for HH polarization. By fitting the area dependence of the normalized variance to available theory, we deduce that correlation lengths are on the order of 10 times the microwave wavelength at both X and Ka band. Values for the normalized variance of an elementary scattering facet were also inferred and are presented in this paper. From the Doppler bandwidths we obtained radial velocity spreads over the illuminated areas and found that they agreed well at X and Ka band. These velocity spreads, which are inversely proportional to the correlation time of the backscatter, increased rapidly with illuminated area for small areas but tended to level off to values of about 0.5 m s−1 at large areas. This implies a decorrelation time for large illuminated areas of about 10 ms at X band and 3 ms at Ka band but somewhat larger values for small areas. The dependence of the velocity spread was found to be well explained by theory if an intrinsic velocity spread of 0.07 m s−1 was used to represent scatterer lifetime effects.
Surface circulation variability in the Balearic Basin
Authors: García M. J. López , Millot C. , Font J. , García‐Ladona E.
Published: February 2004

The Balearic Basin is defined as a wide region of the western Mediterranean located in between the Liguro‐Provençal and Algerian basins. The dynamics of the Liguro‐Provençal Basin, in the north, is dominated by the dense water formation process associated with a permanent circulation along the continental slope: the Northern Current. The Algerian Basin, in the south, is dominated by intense mesoscale eddies and their interactions with the unstable Algerian Current. The Balearic Basin is a transition region between these two different dynamic regimes. It includes the Catalan Sea, located between the Balearic Islands and the Iberian peninsula, and the wide Gulf of Valencia in the south of this sea. Some new ideas about the surface circulation variability have been inferred from a series of NOAA satellite advanced very high resolution radiometer images, taken mainly from the autumn‐winter period during 1981–1988, and also from various in situ data. The Northern Current, which flows southward along the Iberian peninsula slope, is covered in summer by a warm surface layer spreading over the whole Catalan Sea. This local warming creates one of the most intense thermal fronts in the western Mediterranean, the Pyrenees Front. Near the Balearic Islands, the flow through the sills of recent Atlantic water, partially deflected by anticyclonic eddies from the Algerian Current, creates the Balearic Front and contributes to its mesoscale variability. This variability appears to be much more intense than previously described. The Balearic Front is clearly recognized as the westward continuation of the North Balearic Front, already defined in the open sea as the northern limit of the recent Atlantic water reservoir. Finally, the Gulf of Valencia is frequently influenced by water entering from the Algerian Basin, which appears to be important in the disruption of the Northern Current and the formation of the Balearic Current, the geostrophic flow associated with the Balearic Front.
Coastal zone color scanner “system calibration”: A retrospective examination
Authors: Evans Robert H. , Gordon Howard R.
Published: February 2004

During its lifetime the coastal zone color scanner (CZCS) produced approximately 66,000 images. These have been placed in an archive of “raw” radiance (sensor counts) in a subsampled format that is easily accessible. They have also been processed to form global fields, at reduced resolution, of normalized water‐leaving radiance, phytoplankton pigments, and diffuse attenuation coefficient. Using this archive, we have tried to characterize some aspects of the “system calibration” for the 8‐year lifetime of CZCS. Specifically, we have assumed that the sensitivity of the red band decayed in a simple manner similar to the well‐known long‐term degradation of the shorter‐wavelength bands, and we examined the sensitivity of the green and yellow bands by computing the globally averaged water‐leaving radiance, over 10‐day periods, for all of the imagery. The results provide evidence that in addition to the long‐term degradation, short‐term (2 weeks to 1 month) variations in the radiometric sensitivity of these bands started in early fall 1981 and continued for the rest of the mission. In contrast, the data suggest the absence of such variations prior to August 1981. It is reasonable to believe that the sensitivity of the blue (and probably the red) band underwent such variations as well; however, our methodology cannot be used to study the other bands. Thus after these fluctuations began, the actual values of CZCS‐estimated pigment concentrations at a given location should be viewed with skepticism; however, the global patterns of derived pigment concentration should be valid. Had an extensive set of surface measurements of water‐leaving radiance, e.g., from moored buoys or drifters, been available during the CZCS mission, these fluctuations could have been removed from the data set, and this would have greatly increased its value. The lessons learned from CZCS, that is, the requirement of good radiometric calibration and stability and the necessity of “sea truth” stations to monitor the performance of the system (sensor plus algorithms), are being applied to the seaviewing wide‐field‐of‐view sensor (SeaWiFS) scheduled for launch in August 1993.
Gulf of California biogeographic regions based on coastal zone color scanner imagery
Authors: Santamaría‐del‐Angel Eduardo , Alvarez‐Borrego Saúl , Müller‐Karger Frank E.
Published: February 2004

Topographically, the Gulf of California is divided into a series of basins and trenches that deepen to the south. Maximum depth at the mouth is greater than 3000 m. Most of the northern gulf is less than 200 m deep. The gulf has hydrographic features conducive to high primary productivity. Upwelling events have been described on the basis of temperature distributions at the eastern coast during winter and spring and at the western coast during summer. Tidal amplitude may be as high as 9 m in the upper gulf. On the basis of discrete phytoplankton sampling, the gulf was previously divided into four geographic regions. This division took into consideration only the space distribution, taxonomic composition, and abundance of microphytoplankton. With the availability of the coastal zone color scanner (CZCS) imagery, we were able to include the time variability of pigments to make a more detailed biogeographic division of the gulf. With weekly composites of the imagery, we generated time series of pigment concentrations for 33 locations throughout the gulf and for the whole life span of the CZCS. The time series show a clear seasonal variation, with maxima in winter and spring and minima in summer. The effect of upwelling at the eastern coast is clearly evident, with high pigment concentrations. The effect of the summer upwelling off the Baja California coast is not evident in these time series. Time series from locations on the western side of the gulf also show maxima in winter and spring that are due to the eddy circulation that brings upwelled water from the eastern side. Principal‐component analysis was applied to define 14 regions. Ballenas Channel, between Angel de la Guarda and Baja California, and the upper gulf always appeared as very distinct regions. Some of these 14 regions relate to the geographic distributions of important faunal groups, including the benthos, or their life cycles. For example, the upper gulf is a place for reproduction and the nursery of many fish species, marine mammals and birds are specially abundant in Ballenas Channel, sardine spawning mostly occurs in the central gulf in spring, and shrimp are abundant off mainland Mexico.
Direct skin friction measurements and stress partitioning over movable sand ripples
Authors: Zhenlin Li Michael
Published: February 2004

The separation of skin friction from total boundary layer shear stress for flows over bed forms is critical in erosion and sediment transport studies. Close‐to‐bed velocity profiles and skin friction shear velocities have been measured over mobile sand ripples of different geometries in a series of flume experiments to investigate the effects of form drag and flow stress partitioning. As expected, higher and longer ripples produce stronger form drag and hence higher total shear velocities for a given mean flow velocity, but the skin friction shear velocity was found to decrease with the increase in ripple height and length. For a fixed ripple geometry, the ratio of the skin friction to the total shear stress is also found to increase with the flow strength. Results of the present study together with data from previous investigations are used to test and extend the stress partition model of Smith and McLean (1977) for rippled beds. The Smith and McLean model is found to give reasonable partition of the skin friction from the total shear stress over mobile sand ripples. Overall empirical relationships based on total shear velocity and ripple geometry have also been obtained to understand the mechanisms of skin friction variation and to simplify the stress partitioning in bottom boundary layer flows over sand ripples.
Observations of turbulence in the surf zone
Authors: George Ron , Flick R. E. , Guza R. T.
Published: February 2004

Turbulence generated by waves breaking on a natural beach is examined using hotfilm anemometer data. Turbulence intensity is estimated from dissipation rates determined from wavenumber spectra of short (1/8 s) hotfilm time series. The resulting Froude‐scaled turbulence intensities are relatively uniform between the seabed and the wave trough level and are similar in vertical structure but lower in magnitude than in existing laboratory studies. The magnitudes of the turbulence intensities observed in both the field and laboratory are consistent with an existing macroscopic model of bore dissipation in the surf zone. Scaling by this bore model relates turbulence intensities generated by monochromatic waves in small‐scale laboratory experiments to those generated by random waves in the natural surf zone.
Model simulations of the tides in the Barents Sea
Authors: Gjevik B. , Nøst E. , Straume T.
Published: February 2004

A large‐scale numerical model covering the Nordic Seas and the Arctic Ocean with 25–12.5‐km grid resolution has been applied to study the tides in the Barents Sea. Tidal charts for elevation and phase are presented for the four tidal constituents M2 S2, N2, and K1. Computed harmonic constants for elevation and current have been compared with observations from 46 coastal and pelagic tidal stations. The agreement between model and field data is good for most stations with an overall standard deviation of ±2–5 cm for amplitude, ±18°–30° for phase, and ±1–3 cm/s for current speed. Calculated tidal residual currents show anticyclonic eddy circulation over the Svalbard Bank and around Bear Island and Hopen. Particle trajectories and streaklines due to the tidal flow over Svalbardbanken have been calculated, and the computed streaklines are found to agree well with observed rifts in the ice cover due to stranded icebergs.
Ocean surface wind retrievals from special sensor microwave imager data with neural networks
Authors: Stogryn A. P. , Butler C. T. , Bartolac T. J.
Published: February 2004

Several fully connected, feed forward neural networks trained on a set of special sensor microwave imager examples matched with buoy winds have yielded retrieval accuracies considerably better than those achieved by the current operational method. Equations and coefficients for using two of these networks, each with four input brightness temperatures and a hidden layer containing two neurodes, are given for implementation in wind retrieval codes. The first demonstrated an rms retrieval error of 1.41 m/s at a reference height of 19.5 m using an independent data set representing clear sky conditions. The second network yielded rms retrieval accuracies of 2.39 m/s under adverse weather conditions. This represents a factor of more than 2 improvement over the alternate algorithms that were examined for nonclear conditions.
Factors affecting the estimate of primary production from space
Authors: Balch W. M. , Byrne C. F.
Published: February 2004

Remote sensing of primary production in the euphotic zone has been based mostly on visible‐band water‐leaving radiance measured with the coastal zone color scanner. There are some robust, simple relationships for calculating integral production based on surface measurements, but they also require knowledge of photoadaptive parameters such as maximum photosynthesis which currently cannot be obtained from space. A 17,000‐station data set is used to show that space‐based estimates of maximum photosynthesis could improve predictions of ψ, the water column light utilization index, which is an important term in many primary productivity models. Temperature is also examined as a factor for predicting hydrographic structure and primary production. A simple model is used to relate temperature and maximum photosynthesis; the model incorporates (1) the positive relationship between maximum photosynthesis and temperature and (2) the strongly negative relationship between temperature and nitrate in the ocean (which directly affects maximum growth rates via nitrogen limitation). Since these two factors relate to carbon and nitrogen, “balanced carbon/nitrogen assimilation” was calculated assuming the Redfield ratio. It is expected that the relationship between maximum balanced carbon assimilation versus temperature is concave‐down, with the peak dependent on nitrate uptake kinetics, temperature‐nitrate relationships, and the carbon/chlorophyll ratio. These predictions were compared with sea truth data. The minimum turnover time for nitrate was also calculated using this approach. Lastly, sea surface temperature gradients were used to predict the slope of isotherms (a proxy for the slope of isopycnals in many waters). Sea truth data show that at size scales of several hundred kilometers, surface temperature gradients can provide information on the slope of isotherms in the top 200 m of the water column. This is directly relevant to the supply of nutrients into the surface mixed layer, which is useful for predicting integral biomass and primary production.
A salinity front and current rip near Cape Hatteras, North Carolina
Authors: Marmorino G. O. , Trump C. L.
Published: February 2004

Shipboard hydrographic and acoustic Doppler current profiler data collected in September 1991 show a shallow front separating brackish coastal water from denser Gulf Stream water advancing onto the continental shelf near Cape Hatteras. The surface expression of the front has the hallmarks of a current rip: a corrugated line of flotsam (indicating surface convergence) and an adjoining band of short, steep waves (indicating wave‐current interaction). Sinking flow of 15 cm/s and a 10‐dB change in acoustic backscatter strength occur over a cross‐front distance of 10–20 m. While northward surface currents in the Gulf Stream water move at about 60 cm/s, the front advects northward at only 35 cm/s; this difference implies frontal propagation to the south relative to the deeper fluid at a speed consistent with theoretical expectations. The surface convergence across the front is estimated to be about 50 cm/s, and the horizontal surface strain rate is estimated to be 0.025–0.05 s−1. This supports the large strain rate needed by Jansen et al. (1993) to simulate the O(10 dB) radar modulations measured at the rip by Askari (1992). Similar current rips may account for curvilinear high‐backscatter features appearing in historical synthetic aperture radar images of the shelf area north of Cape Hatteras.
Correcting infrared satellite estimates of sea surface temperature for atmospheric water vapor attenuation
Authors: Emery William J. , Yu Yunyue , Wick Gary A. , Schluessel Peter , Reynolds Richard W.
Published: February 2004

A new satellite sea surface temperature (SST) algorithm is developed that uses nearly coincident measurements from the microwave special sensor microwave imager (SSM/I) to correct for atmospheric moisture attenuation of the infrared signal from the advanced very high resolution radiometer (AVHRR). This new SST algorithm is applied to AVHRR imagery from the South Pacific and Norwegian seas, which are then compared with simultaneous in situ (ship based) measurements of both skin and bulk SST. In addition, an SST algorithm using a quadratic product of the difference between the two AVHRR thermal infrared channels is compared with the in situ measurements. While the quadratic formulation provides a considerable improvement over the older cross product (CPSST) and multichannel (MCSST) algorithms, the SSM/I corrected SST (called the water vapor or WVSST) shows overall smaller errors when compared to both the skin and bulk in situ SST observations. Applied to individual AVHRR images, the WVSST reveals an SST difference pattern (CPSST‐WVSST) similar in shape to the water vapor structure while the CPSST‐quadratic SST difference appears unrelated in pattern to the nearly coincident water vapor pattern. An application of the WVSST to week‐long composites of global area coverage (GAC) AVHRR data demonstrates again the manner in which the WVSST corrects the AVHRR for atmospheric moisture attenuation. By comparison the quadratic SST method underestimates the SST corrections in the lower latitudes and overestimates the SST in the higher latitudes. Correlations between the AVHRR thermal channel differences and the SSM/I water vapor demonstrate the inability of the channel difference to represent water vapor in the midlatitude and high latitudes during summer. Compared against drifting buoy data the WVSST and the quadratic SST both exhibit the same general behavior with relatively small differences with the buoy temperatures.
Tidal propagation in strongly convergent channels
Authors: Friedrichs Carl T. , Aubrey David G.
Published: February 2004

Simple first‐ and second‐order analytic solutions, which diverge markedly from classical views of cooscillating tides, are derived for tidal propagation in strongly convergent channels. Theoretical predictions compare well with observations from typical examples of shallow, “funnel‐shaped” tidal estuaries. A scaling of the governing equations appropriate to these channels indicates that at first order, gradients in cross‐sectional area dominate velocity gradients in the continuity equation and the friction term dominates acceleration in the momentum equation. Finite amplitude effects, velocity gradients due to wave propagation, and local acceleration enter the equations at second order. Applying this scaling, the first‐order governing equation becomes a first‐order wave equation, which is inconsistent with the presence of a reflected wave. The solution is of constant amplitude and has a phase speed near the frictionless wave speed, like a classical progressive wave, yet velocity leads elevation by 90°, like a classical standing wave. The second‐order solution at the dominant frequency is also a unidirectional wave; however, its amplitude is exponentially modulated. If inertia is finite and convergence is strong, amplitude increases along channel, whereas if inertia is weak and convergence is limited, amplitude decays. Compact solutions for second‐order tidal harmonics quantify the partially canceling effects of (1) time variations in channel depth, which slow the propagation of low water, and (2) time variations in channel width, which slow the propagation of high water. Finally, it is suggested that phase speed, along‐channel amplitude growth, and tidal harmonics in strongly convergent channels are all linked by morphodynamic feedback.
Passive microwave observations of the Weddell Sea during austral winter and early spring
Authors: Grenfell T. C. , Comiso J. C. , Lange M. A. , Eicken H. , Wensnahan M. R.
Published: February 2004

The results of multispectral passive microwave observations (6.7 to 90‐GHz) are presented from the cruises of the FS Polarstern in the Weddell Sea from July to December 1986. This paper includes primarily the analysis of radiometric observations taken at ice station sites. Averaged emissivity spectra for first‐year (FY) ice were relatively constant throughout the experiment and were not statistically different from FY ice signatures in the Arctic. Detailed ice characterization was carried out at each site to compare the microwave signatures of the ice with the physical properties. Absorption optical depths of FY ice were found to be sufficiently high that only the structure in the upper portions of the ice contributed significantly to interstation emissivity variations. The emissivities at 90‐GHz, e(90), had the greatest variance. Both e(90) at vertical polarization and GRe(90,18.7) (defined as [ev(90)‐ev(18.7)]/ev[(90)+ev(18.7)]) depended on the scattering optical depth which is a function of the snow grain diameter and layer thickness. The variance showed a latitude dependence and is probably due to an increase in the strength of snow metamorphism nearer the northern edge of the ice pack. The contribution of variations of near‐surface brine volume to the emissivity was not significant over the range of values encountered at the station sites. Emissivity spectra are presented for a range of thin ice types. Unsupervised principal component analysis produced three significant eigenvectors and showed a separation among four different surface types: open water, thin ice, FY ice, and FY ice with a thick snow cover. A comparison with SMMR satellite data showed that averaged ice concentrations derived from the ship's ice watch log were consistent with the satellite concentrations. The surface based emissivities for FY ice were also compared with emissivities calculated from scanning multichannel microwave radiometer (SMMR) satellite radiances. Best agreement was found at 6.7 and 10‐GHz, while at 18 and 37‐GHz, SMMR emissivities were slightly lower than surface based results. For the three lower frequencies agreement was found within a confidence limit of 95% and for 37‐GHz within about 90%.
Remote sensing of ocean waves: The Surface Wave Process Program experiment
Authors: Goldstein R. M. , Li F. , Smith J. , Pinkel R. , Barnett T. P.
Published: February 2004

An airborne synthetic aperture radar (SAR) has been used as an interferometer to obtain direct, calibrated measurements of the ocean wind wave directional spectrum. Flights over the same area from three different directions produced reasonably consistent results. The SAR‐derived wave data agree well with simultaneous observations by a conventional wave measuring system and with a novel in situ acoustic Doppler system.
Light reflection from sea ice during the onset of melt
Authors: Perovich Donald K.
Published: February 2004

A knowledge of the reflection of light from a sea ice cover is important for the interpretation of remote sensing imagery at visible and near‐infrared wavelengths, for climatological studies involving the energy balance of the polar regions, and for understanding radiative transfer in sea ice. Spectral measurements of albedo, bidirectional reflectance distribution function, and polarized reflectance were made for Arctic sea ice conditions found during the onset of melt. The wavelength region studied was from the visible to the near infrared (400–1000 nm). Results are presented for the five following surface types: (1) dry snow, (2) dry snow with a glazed surface, (3) bare ice, (4) blue ice, and (5) melt pond. Results indicate that spectral albedos decrease at all wavelengths as the melt season progresses and the surface conditions evolve from (1) through (5) and that the decrease is most pronounced at longer wavelengths. Reflectance data suggest that at most angles, reflectance has the same spectral shape as albedo; at 30° zenith, reflectance is for the most part azimuthally isotropic; and at 60° zenith there is a significant specular component at 0° azimuth, especially for the bare ice cases. Light reflected at 60° zenith angle was strongly polarized. In general, light reflected from snow‐free ice was more strongly polarized, with the degree of polarization increasing with wavelength.
The classification of Arctic sea ice types and the determination of surface temperature using advanced very high resolution radiometer data
Authors: Massom Robert , Comiso Josefino C.
Published: February 2004

The accurate quantification of new ice and open water areas and surface temperatures within the sea ice packs is a key to the realistic parameterization of heat, moisture, and turbulence fluxes between ocean and atmosphere in the polar regions. Multispectral NOAA advanced very high resolution radiometer/2 (AVHRR/2) satellite images are analyzed to evaluate how effectively the data can be used to characterize sea ice in the Bering and Greenland seas, both in terms of surface type and physical temperature. The basis of the classification algorithm, which is developed using a late wintertime Bering Sea ice cover data, is that frequency distributions of 10.8‐μm radiances provide four distinct peaks, representing open water, new ice, young ice, and thick ice with a snow cover. The results are found to be spatially and temporally consistent. Possible sources of ambiguity, especially associated with wider temporal and spatial application of the technique, are discussed. An ice surface temperature algorithm is developed for the same study area by regressing thermal infrared data from 10.8‐ and 12.0‐μm channels against station air temperatures, which are assumed to approximate the skin temperatures of adjacent snow and ice. The standard deviations of the results when compared with in situ data are about 0.5 K over leads and polynyas to about 0.5–1.5 K over thick ice. This study is based upon a set of in situ data limited in scope and coverage. Cloud masks are applied using a thresholding technique that utilizes 3.74‐ and 10.8‐μm channel data. The temperature maps produced show coherence with surface features like new ice and leads, and consistency with corresponding surface type maps. Further studies are needed to better understand the effects of both the spatial and temporal variability in emissivity, aerosol and precipitable atmospheric ice particle distribution, and atmospheric temperature inversions.
Seismic‐acoustic sensing of sea ice wave mechanical properties
Authors: Xie Yunbo , Farmer David M.
Published: February 2004

Sea ice inhomogeneities can be probed by seismic and acoustic waves. Observations of the seismic‐acoustic response of first‐year sea ice to artificially generated impacts created by dropping a lead ball from a fixed height show that an ice ridge in the path of a propagating seismic wave dampens the wave. This damping effect is found to be sensitive to the angle of incidence, especially at high frequencies (∼100 Hz). Both the longitudinal (P wave) and horizontally polarized shear (SH) wave speeds are found to be much more variable in the vicinity of the ridge; outside of the ridged area the shear wave speed is relatively uniform, varying by only 13% in contrast to the 27% variability of the P wave speed. The Crary wave, which is a special case of the vertically polarized wave (SV), is most easily induced by seafloor‐reflected acoustic waves penetrating the ice at large grazing angles. When an ice sheet is struck with a vertical blow, it radiates sound with a dipole pattern; the radiated frequency also depends on the radiation angle with higher‐frequency energy projected preferentially downward. Most of the potential energy resulting from artificial impacts is dissipated through damaging the ice surface, with only a small fraction being converted into acoustic and seismic waves.
Eddy kinetic energy and momentum flux in the Southern Ocean: Comparison of a global eddy‐resolving model with altimeter, drifter, and current‐meter data
Authors: Wilkin John L. , Morrow Rosemary A.
Published: February 2004

The ability of a seasonally forced high‐resolution global ocean general circulation model to simulate eddy variability and associated energy and momentum transfer processes in the Southern Ocean is assessed by comparing model statistics with observations. The observations include Geosat altimeter data analyzed for surface velocity variance at satellite ground track crossover points, current‐meter data from the Agulhas and Campbell plateaus, and surface drifter data in the Tasman Sea. In western boundary currents and energetic regions of the Antarctic Circumpolar Current model eddy kinetic energy is lower than observed by typically a factor of 4, and in less energetic regions by a factor of 10. Differences in the location and extent of energetic regions are related to smoothness of the model bathymetry and other features of the model configuration. Eddy momentum flux divergence and eddy to mean kinetic energy conversion at the surface are diagnosed from the model. These show regions where eddy activity accelerates the mean flow through instability processes. Observational estimates of these terms are computed using mean flow gradients from hydrography climatology and altimeter eddy statistics. Several features of the spatial distribution of the observational estimates are consistent with the model and suggest that future calculations of mean currents from altimeter data will allow direct computation of eddy to mean current momentum and energy conversion terms.
Acoustic detection of ice crystals in Antarctic waters
Authors: Penrose John D. , Conde M. , Pauly T. J.
Published: February 2004

During the voyage of the RSV Aurora Australis to the region of Prydz Bay, Antarctica in January–March 1991, ice crystals were encountered at depths from the surface to 125‐m in the western area of the bay. On two occasions, crystals were retrieved by netting, and echo sounder records have been used to infer additional regions of occurrence. Acoustic target strength estimates made on the ice crystal assemblies encountered show significant spatial variation, which may relate to crystal size and/or aggregation. Data from a suite of conductivity‐temperature‐depth casts have been used to map regions of the study area where in situ water temperatures fell below the computed freezing point. Such regions correlate well with those selected on the basis of echogram type and imply that ice crystals occurred at depth over large areas of the bay during the cruise period. The ice crystal distribution described is consistent with that expected from a plume of supercooled water emerging from under the Amery Ice Shelf and forming part of the general circulation of the bay. The magnitude of the supercooled water plume is greater than those reported previously in the Prydz Bay region. If misinterpreted as biota on echo sounder records, ice crystals could significantly bias biomass estimates based on echo integration in this and potentially other areas.
Microwave backscatter from the sea surface: Bragg scattering by short gravity waves
Authors: Poulter E. M. , Smith M. J. , McGregor J. A.
Published: February 2004

Resonant Bragg scattering forms the basis for the composite or dual‐scale model for microwave backscatter from the sea surface. The scatterers are short surface waves that are spatially resonant with the incident electromagnetic waves. Bragg scattering is most easily identified when the intrinsic velocity of the scattering agents can be deduced and compared with theoretical surface wave phase velocities. In this paper we present S band (wavelength 10 cm) microwave backscatter data taken at low grazing angles for three situations where the scatterer velocity could be separated from the surface currents. These include a situation where pack ice acts as an additional tracer on the water surface, a wind direction reversal, and an azimuthal scan during low wind speed conditions where two Bragg peaks are visible in the Doppler spectra. These data show that under these conditions the scattering is dominated by propagating 5‐cm resonant waves. Doppler spectra recorded at low grazing angles are interpreted in terms of the angular distribution of the scattering waves, the contributions of wave orbital velocities, and the effects of geometric shadowing.
Numerical studies of eddy shedding in the Gulf of Mexico
Authors: Dietrich David E. , Lin Charles A.
Published: February 2004

We model the eddy shedding phenomenon in the Gulf of Mexico using the Sandia Ocean Modeling System. In the first part of the study a parameter sensitivity study is performed using a rectangular basin two‐level model. In particular, the sensitivity of the model eddy shedding to inflow/outflow conditions, horizontal resolution (80, 40, and 20 km), and reduced gravity is examined. The results are interpreted in terms of the quasi‐geostrophic vertical motion equation and vorticity conservation. In the second part of the paper we include realistic coastlines and bathymetry in the model. A horizontal resolution of 20 km is used together with 16 vertical levels. Various observed features of the eddy shedding are simulated by the model. A boundary current which follows topographic contours is generated around the Gulf because of the splitting of the flows associated with the loop current and the shed eddy. This current is likely to be important in the dissipative stages of the eddies in the western Gulf. In addition, our results suggest that higher‐order baroclinic modes are important in the dissipation of loop current eddies.