Convergent Front Research Articles

Devonian Andean‐type convergence in the southern Dunhuang block (NW China): Petro‐structural, metamorphic P–T and geochronological constraints

AbstractArchean to Palaeoproterozoic basement rocks exposed in the Dunhuang block in NW China were affected by Palaeozoic crustal reworking, as constrained by previous zircon U–Pb geochronological investigations. However, relationships between the Palaeozoic metamorphic ages, P–T evolution and deformational history of the region remain ambiguous. In order to address this issue, P–T–t–D paths of paragneisses from the basement of the Hongliuxia belt in the southern Dunhuang block were investigated. Inclusions in garnet and kyanite from the paragneisses are considered as vestiges of Palaeozoic M1 metamorphism corresponding to initiation of the prograde evolution. The earliest continuous metamorphic fabric is an originally steep N–S striking foliation S2. This fabric was reworked by vertical folds F3 associated with the development of a ubiquitous steep, mainly south‐dipping, E‐W striking axial planar foliation S3. The S2 foliation in paragneisses is mainly associated with Grt–St–Ky–Sil–Bt–Ms–Pl–Qz–Rt assemblages in samples from the western domain and with Grt–Ky–Sil–Bt–Kfs–Pl–Qz–Rt assemblages in samples from the northeastern domain of the Hongliuxia belt. The S3 foliation is associated with Grt–Sil–St–Bt–Ms–Pl–Qz–Ilm assemblages in the western domain and with Grt–Sil–Bt–Ms–Pl–Qz–Kfs–Ilm assemblages in the northeastern domain, followed by growth of chlorite in both domains. Early prograde stage (M1) from 4.0–6.5 kbar and 540–560°C to metamorphic peak (M2a) at 9–10 kbar and ~650–675°C is mainly recorded by paragneisses from the western domain. Subsequent decompression is initially accompanied by heating (M2b) constrained to 6.5–7 kbar and 675–710°C in the western domain, and to 6–6.5 kbar and ~730°C in the northeastern domain, followed by cooling (M3) through 4–6.5 kbar and 550–650°C till late chloritization (late M3). In situ U–Pb dating of monazite combined with monazite trace‐element compositions suggests that prograde evolution (M1) most likely started at c. 406 Ma, peak‐P conditions (M2a) were reached at 400–394 Ma, decompression associated with heating (M2b) took place at 393–391 Ma, and cooling (M3) during exhumation probably lasted from 380 to 354 Ma. The prograde metamorphism probably reflects burial during underthrusting of neighbouring continental basement (the Alxa block or an equivalent) below the Dunhuang block. This event culminated in pure shear thickening (D2a) of the whole supra‐subduction margin followed by minor heating and exhumation (D2b). The D3‐M3 event is interpreted as reflecting exhumation during orthogonal shortening of the system, possibly in response to an independent orogenic cycle. Combined with the available regional data, this study reveals the existence of a complex tectono‐metamorphic evolution for the Dunhuang block characterized by two distinct orogenic phases with (i) the thickening of a previously thinned arc‐back‐arc crust recorded in the northern and central belts at 420–410 Ma in the pro‐wedge side of the active margin (Sanweishan phase), followed by (ii) the 410–390 Ma thickening in the retro‐wedge side (Hongliuxia phase). Such a tectonic evolution of the whole Dunhuang block resembles Andean‐type migration of crustal thickening from the convergent front to hinterlands. The D3‐M3 event, potentially responsible for the juxtaposition of rocks from different geological occurrences and depths, is seemingly independent from this Andean‐type orogenic cycle.

Structure, geochronology, and petrogenesis of Permian peraluminous granite dykes in the southern Chinese Altai as indicators of Altai−East Junggar convergence

How and at which thermal conditions the convergence between the Chinese Altai and East Junggar operated remain poorly understood. This issue is addressed in the current study by focusing on the timing and petrogenesis of syntectonic granite dykes from the representative areas of Fuyun (convergent front) and Kalasu-Aletai (Chinese Altai interior). It is shown that Fuyun and Kalasu-Aletai dykes are fractionated I- and S-type granites, with zircon and monazite U-Pb ages of 300−291 Ma and 281−265 Ma, respectively. Geochemically, the Fuyun dykes have lower contents of aluminous (ASI: 0.97−1.13) and light rare earth element-enriched features, while the Kalasu-Aletai dykes have ASI = 1.01−2.17 and show overall flat rare earth element patterns with tetrad effects. The Fuyun dykes exhibit less evolved Sr-Nd isotopic characteristics (87Sr/86Srinitial: 0.7039−0.7048, εNd(t): + 5.7 to + 6.1) with respect to those of the Kalasu-Aletai dykes (87Sr/86Srinitial: 0.6978−0.7183, εNd(t): −7.6 to +3.0). The Fuyun and Kalasu-Aletai dykes are geochemically compatible with isotopically less evolved East Junggar arc components and heterogeneous Ordovician wedge sediment of the Chinese Altai, respectively, implying genetic links. We propose that the late Paleozoic Chinese Altai−Junggar convergence created a local perturbation of weak mantle beneath the southern Chinese Altai, causing partial melting of the underthrusting East Junggar and the overriding Altai components successively. The resulting magmas were emplaced along northward propagating syn-tectonic tensional fractures perpendicular to the Chinese Altai−East Junggar deformation front that serves as an excellent indicator of the convergent-shortening process.

Thermochemical release mechanisms and circumferential initiation affecting detonation in an aluminized explosive

ABSTRACT A modified thermochemical energy term and γ-law extension are incorporated into the JWL equation of state for predicting PBXN-111 detonation behavior under self- and externally supported conditions. The modifications resolve previous speculations regarding the causes affecting non-scalability of the reaction zone and detonation velocity with respect to explosive diameter, and a problem encountered to quantify increased detonation power gain by circumferential initiation. The modifications accurately consider “static effects” of size and the additional hydrodynamic compression resulting from a consecutive 2-step shock jump during convergent front collapse, leading to an elevated von Neumann spike and Chapman-Jouguet state between which physical and chemical transformations take place affecting shock regeneration and detonation power release. Necessary modifications to the CYLEX test are recommended to ensure that wall expansions are measured under steady-state conditions.

The Major Two‐stage Shortening Deformation of the Northern Tibet and Tian Shan Area Since the Latest Oligocene

AbstractIt seems to be progressively recognized that the stress of the India‐Asia convergent front can be transferred rapidly through the southern and central Tibetan lithosphere to the northern Tibet, hence leading to the crustal thickening deformation there during or immediately after the onset of the India‐Asia collision (ca. 55 Ma). This study focuses on the late Cenozoic deformation and tectonic uplift of the northern Tibet and Tian Shan area. Detailed compilations of a variety of proxy data from sediments and bedrocks suggest that the northern Tibet and Tian Shan area underwent one stage of approximately synchronous widespread contractile deformation since 25–20 Ma, which seemed to decrease at circa 18 Ma as revealed by low‐temperature thermochronological data. The latest Oligocene‐early Miocene was also significant basin‐forming episodes when many intermontane subbasins began to receive syntectonic sedimentation in the northeastern Tibet. Subsequently, the other phase of compressional deformation began to encroach more widely into the northern Tibet and Tian Shan area in episodic steps or continuously from 16–12 Ma to present.

Stratification in the presence of an axial convergent front: Causes and implications

Flood tide salinity stratification in the presence of an axial convergent front is revealed with new data. The data were collected during sampling campaigns in the Conwy estuary, North Wales, the location of a previous study for axial convergent fronts. The stratification, with a maximum observed vertical density difference of 5 kg m−3, begins as soon as the saline intrusion arrives and lasts throughout the flood tide. The flood stratification is shown to be caused by a tidal straining-type process. The along-channel shear is modified by cross-channel currents developed during the formation of the convergent front such that surface currents are smaller than those at mid-depth. The modified shear interacts with the horizontal density gradient to form the stratification on the flood tide. Implications for the turbulence and sediment transport regime in the estuary are discussed.

LAGRANGIAN MEASUREMENTS OF TURBULENT DISSIPATION OVER A SHALLOW TIDAL FLAT FROM PULSE COHERENT ACOUSTIC DOPPLER PROFILERS

We present high resolution (25 mm spatial, 8 Hz temporal) profiles of velocity measured over a shallow tidal flat using pulse-coherent Acoustic Doppler Profilers mounted on surface drifters. The use of Lagrangian measurements mitigated the problem of resolving velocity ambiguities, a problem which often limits the application of high-resolution pulse-coherent profilers. Turbulent dissipation rates were estimated from second-order structure functions of measured velocity. Drifters were advected towards, and subsequently trapped on, a convergent surface front which marked the edge of a freshwater plume. Measured dissipation rates increased as a drifter deployed within the plume approached the front. A drifter then propagated with and along the front as the fresh plume spread across the tidal flats. Near-surface turbulent dissipation measured at the front roughly matched a theoretical mean-shear-cubed relationship, whereas dissipation measured in the stratified plume behind the front was suppressed. After removal of estimates affected by surface waves, near-bed dissipation matched the velocity cubed relationship, although scatter was substantial. Dissipation rates appeared to be enhanced when the drifter propagated across small subtidal channels.

A numerical study of marine larval dispersal in the presence of an axial convergent front

Estuarine axial convergent fronts generate strong secondary (cross-estuary) flows which can be important for larval dispersal and settlement, and have been shown in this research to aid estuarine retention of natal populations which will promote production. This paper explores several larval migratory strategies, synchronised by sensory cues such as pressure, velocity, salinity and solar radiation, in relation to an estuarine axial convergent front – an important circulatory mechanism that forms in many coastal regions where larvae are concentrated; hence these results have implications for fisheries management. A three-dimensional hydrodynamic model is applied to an idealised channel, parameterised from observations of a well documented axial convergent front in the Conwy Estuary, UK. The model simulates the bilateral cross-sectional circulation of the front, attributed to the interaction of lateral shear of the longitudinal currents with the axial salinity gradient. Axial surface convergence develops during the flood phase of the tide and (weaker) surface divergence during the ebb phase. Lagrangian Particle Tracking Models subsequently use velocities from a range of simulated tidal and climatic scenarios to track larvae in the estuary. The results show that axial convergent fronts aid estuarine retention of larvae. Specifically retention is enhanced for larvae that experience tidal stream transport and diel migration. Tidally-synchronised larvae exhibit the strongest landward dispersal while the modelled copepod (a combination of tidal and salinity cues) exhibits ‘full retention’ in the mid-estuary release location. Finally, the vertical migration speed of the larvae must exceed the background vertical velocities in order to get the greatest enhancement of retention.

Pulse signal detection in a focal area of a convergent wave front

Diffraction effects, taking place during nonlinear transformations in inhomogeneous acoustic fields, are experimentally investigated. The case of a convergent spherical wave front propagating in a uniform nonlinear medium, detection of an acoustic field in a focus, and receiving of the detected signal in the region of the initial wave front aperture are considered. A spherical piezoceramic transducer is used in the experiments as a focusing device. Broad-angle “nonlinear scattering” signals have been recorded at the experimental facility where a pulsed mode of focused transducer operation in water is implemented. The dependence of the amplitude of the signal, detected in the focal area, and its shape on the scattering direction, as well as on the distance between the focus and the receiving point, are studied.

Experimentation and direct numerical simulation of self-similar convergent detonation wave

The propagation of self similar convergent detonation wave in TATB-based explosive composition was studied both experimentally and numerically. The device constists in a 50 mm cylinder of TATB surrounded by an HMX tube. The detonation in HMX overdrives the detonation in TATB which adapts to the propagation velocity with a convergent front at centerline. We measured a curvature of κ = −21.2 m−1 for propagation velocity of 8750 m/s, which extends the knowledge of the (Dn ,κ ) law. A wide ranged EOS/reaction rate model inspired from previous work of Wescott et al. was calibrated to reproduce both the run-to-detonation distance and the newly extended (Dn ,κ ) law for the 1D sligthly curved detonation theory. 2D Direct Numerical Simulations (DNS) were made on fine resolved mesh grid for the experimental configuration and for various driver velocities. The simulation reproduces the experimental data both qualitatively (overall detonation structure) and quantitatively (κ = −25.4 m−1 ).

Northern Monterey Bay upwelling shadow front: Observations of a coastally and surface‐trapped buoyant plume

During the upwelling season in central California, northwesterly winds along the coast produce a strong upwelling jet that originates at Point Año Nuevo and flows southward across the mouth of Monterey Bay. A convergent front with a mean temperature change of 3.77 ± 0.29°C develops between the warm interior waters and the cold offshore upwelling jet. To examine the forcing mechanisms driving the location and movement of the upwelling shadow front and its effects on biological communities in northern Monterey Bay, oceanographic conditions were monitored using cross‐shelf mooring arrays, drifters, and hydrographic surveys along a 20 km stretch of coast extending northwestward from Santa Cruz, California, during the upwelling season of 2007 (May–September). The alongshore location of the upwelling shadow front at the northern edge of the bay was driven by: regional wind forcing, through an alongshore pressure gradient; buoyancy forces due to the temperature change across the front; and local wind forcing (the diurnal sea breeze). The upwelling shadow front behaved as a surface‐trapped buoyant current, which is superimposed on a poleward barotropic current, moving up and down the coast up to several kilometers each day. We surmise that the front is advected poleward by a preexisting northward barotropic current of 0.10 m s−1 that arises due to an alongshore pressure gradient caused by focused upwelling at Point Año Nuevo. The frontal circulation (onshore surface currents) breaks the typical two‐dimensional wind‐driven, cross‐shelf circulation (offshore surface currents) and introduces another way for water, and the material it contains (e.g., pollutants, larvae), to go across the shelf toward shore.

A numerical study of lateral grain size sorting by an estuarine front

A two-dimensional non-hydrostatic model of baroclinic flow was applied to an estuarine cross-section. The model was driven by lateral variations in density and produced the classical bilateral cross-sectional recirculation of an axial convergent front. Simulations of the erosion, transport and deposition of sediment by the frontal secondary flows were applied to a range of grain sizes. The model predicted considerable lateral variation in grain size deposition across the frontal recirculation zone, analogous to the lateral grain size sorting which occurs in river meanders. The sorting primarily took place between the frontal surface convergence and the bankward limit of recirculation. A series of sensitivity tests revealed that the contribution of the front to lateral grain size sorting was strongly influenced by the lateral channel slopes and lateral density gradient. The results from this numerical study support previous suggestions (based on observations of near-surface discontinuities in sediment concentration across the frontal interface) that fronts may act as sieves within the estuarine sediment transport system.

Cenozoic tectonic evolution of Qaidam basin and its surrounding regions (Part 1): The southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin

Cenozoic Qaidam basin, the largest active intermountain basin inside Tibet, figures importantly in the debates on the history and mechanism of Tibetan plateau formation during the Cenozoic Indo-Asian collision. To determine when and how the basin was developed, we conducted detailed geologic mapping and analyses of a dense network of seismic reflection profiles from the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin. Our geologic observations provide new constraints on the timing and magnitude of Cenozoic crustal thickening in northern Tibet. Specifically, our work shows that the southernmost part of the Qilian Shan-Nan Shan thrust belt and contractional structures along the northern margin of Qaidam basin were initiated in the Paleocene-early Eocene (65–50 Ma), during or immediately after the onset of the Indo-Asian collision. This finding implies that stress was transferred rapidly through Tibetan lithosphere to northern Tibet from the Indo-Asian convergent front located >1000 km to the south. The development of the thrust system in northern Qaidam basin was driven by motion on the Altyn Tagh fault, as indicated by its eastward propagation away from the Altyn Tagh fault. The eastward lengthening of the thrust system was spatially and temporally associated with eastward expansion of Qaidam basin, suggesting thrust loading was the main control on the basin formation and evolution. The dominant structure in northern Qaidam basin is a southwest-tapering triangle zone, which started to develop since the Paleocene and early Eocene (65–50 Ma) and was associated with deposition of an overlying southwest-thickening, growth-strata sequence. Recognition of the triangle zone and its longevity in northern Qaidam basin explains a long puzzling observation that Cenozoic depocenters have been located consistently along the central axis of the basin. This basin configuration is opposite to the prediction of classic foreland-basin models that require the thickest part of foreland sediments deposited along basin edges against basin-bounding thrusts. Restoration of balanced cross sections across the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin suggests that Cenozoic shortening strain is highly inhomogeneous, varying from ~20% to >60%, both vertically in a single section and from section to section across the thrust belt. The spatially variable strain helps explain the conflicting paleomagnetic results indicating different amounts of Cenozoic rotations in different parts of Qaidam basin. The observed crustal shortening strain also implies that no lower-crustal injection or thermal events in the mantle are needed to explain the current elevation (~3000–3500 m) and crustal thickness (45–50 km) of northern Qaidam basin and the southern Qilian Shan-Nan Shan thrust belt. Instead, thrusting involving continental crystalline basement has been the main mechanism of plateau construction across northern Qaidam basin and the southern Qilian Shan-Nan Shan region.

Observations and numerical modelling of a non-buoyant front in the Tay Estuary, Scotland

Acoustic Doppler current profiler (ADCP) and density data have been collected for a section of front which consistently occurs on the flood tide along a break in bathymetry in the Tay Estuary. Lateral velocity shear in a vertical profile through the front was measured to be 0.52 s −1. An estuarine cross-sectional numerical model was developed with buoyancy-driven flow. Results from the numerical model showed that shears of such magnitude cannot be produced by buoyancy alone. Instead, a hypothesis was devised for the generation of the bathymetry-aligned front, and tested using the numerical model. The flooding current flows over sandbanks at the southern bank of the estuary and is then directed over (rather than along) the bathymetry break due to a sudden topographic restriction at the Tayport Narrows. Due to tidal phase effects, this overbank flow has a lower density than the ambient main channel water, hence behaving as a buoyant plume. The plume entrains higher density bottom water and a recirculation cell is set up in the lee of the bathymetry break. A surface convergent front occurs because a corresponding towards-bank flow (confirmed by field data) occurs in the centre of the channel. The numerical model was applied to this configuration using suitable initial and boundary conditions based on field observations. Lateral velocity profiles and the strength of shear show good agreement with the field data. It is suggested that the presence of a density gradient is required to generate the front but is not the main driving force.

Short‐wave directional properties in the vicinity of atmospheric and oceanic fronts

The author estimated and investigated mean wave directions and directional spread parameters of 0.505 Hz ocean waves using data from 24.5 MHz‐HF ocean radar. The author described temporal and spatial variabilities of the short‐wave field. The short‐wave direction is a good indication of wind direction. The spread parameter was related to the variability of wind direction. The author examined the case that an atmospheric front or an oceanic front exists in the HF ocean radar observation area. In the case of an atmospheric front the short‐wave direction changed with the passage of the atmospheric front. Furthermore, the complicated frontal structure, which cannot be verified from a weather chart, was revealed. Narrow directional spread was observed in the vicinity of the convergent front, which was reproduced using a simplified model based on the wave action conservation law. The result shows that wave refraction associated with a frontal convergent zone is important in determining the spatial variability of short‐wave directional properties.

Physical control of biological processes in the central equatorial Pacific Ocean

A five-component (phytoplankton, zooplankton, ammonium, nitrate, detritus) physical–biological model was developed to investigate the effects of physical processes on daily to interannual time scales, on the lower trophic levels of the central equatorial Pacific. Many of the biological processes included in the ecosystem model respond to environmental fluctuations with time scales between 1 and 10 d, which are not typically resolved by basin- to global-scale circulation models. Therefore, the equatorial Pacific ecosystem model is forced using daily information (solar radiation, velocity, temperature) from the Tropical Atmosphere Ocean (TAO) mooring array. The ecosystem model also requires vertical velocity information which necessitated the development of a method for computing daily vertical velocities from the TAO array. Much of the variability in primary production, plankton and nutrient concentrations observed in 1992 during the US Joint Global Ocean Flux Study Equatorial Pacific Process Study time-series cruises (TS1 and TS2), is well reproduced in the model simulations. Simulations demonstrate that lower primary productivities during TS1 as compared to TS2 result from the deeper thermocline that persisted during TS1 as a result of El Niño conditions; however, because of the simultaneous reduction in grazing pressure, simulated chlorophyll levels are similar for these two time periods. Simulations of this single-species ecosystem model successfully reproduce data collected both during and after the El Niño, suggesting that species composition changes are not of first-order importance when examining the effects of the 1991–92 El Niño on the equatorial Pacific ecosystem. A 60–70% increase in chlorophyll concentration and a 400% increase in the chlorophyll contribution by diatoms was associated with the passage of a tropical instability wave (20-d period) across the study site during TS2. This period of high chlorophyll concentration and diatom abundance coincided temporally with strong northward velocities and strong downwelling velocities in the upper euphotic zone. Observations and simulations suggest that this increase in chlorophyll concentration and change in species composition not only results from in situ diatom growth stimulated by increased iron concentrations, but also results from the advection of diatoms toward the convergent front located along the leading (western) edge of the instability wave. Equatorially trapped internal gravity waves can also stimulate in situ phytoplankton growth as high-frequency vertical motions introduce limiting micronutrients, such as iron, into the euphotic zone. Because iron can be taken up by the picoplankton on time scales much shorter than the wave period (6–8 days), these waves may provide a mechanism for effecting a large flux of iron into the euphotic zone. Exclusion of these high-frequency motions results in an iron flux to the euphotic zone that may be underestimated by more than 30%.

Diffraction tomography based on McCutchen’s theorem

An early paper by McCutchen [J. Opt. Soc. Am.54, 240 (1964)] relates the modulation of a convergent spherical wave front to the Fourier transform of the complex amplitude near the geometrical focal point. This implies additional useful Fourier relationships between the wave-front modulations and the cross sections of the diffracted field, which contain the geometrical focal point. We show how these relations can be applied to diffraction tomography. To make use of McCutchen’s relations, particular emphasis is given to the analysis of diffraction tomography with point-source illumination. We derive a sufficient condition under which linear tomographic reconstruction can be applied to arbitrary incident fields and synthetic apertures. This suggests a modified filtered backpropagation algorithm. In addition, we use the results of McCutchen’s paper to obtain information about the symmetry of the object from the scattered field.

Prochlorococcus growth rates in the central equatorial Pacific: An application of the ƒmax approach

Minimum daily growth rates of Prochlorococcus were estimated for the central equatorial Pacific (12°S–12°N, 140°W) during El Niño (February‐March 1992) and normal upwelling (August‐September 1992) conditions. Growth rate estimates were based on the percentages of cells in the S and G2 division phases at dawn (∼0700 LT) and dusk (∼1800 LT) as approximate values for ƒmin and ƒmax, respectively. During both environmental conditions, depth‐integrated growth rates of Prochlorococcus were higher in oligotrophic waters at the northern end of the transect (0.36–0.52 d−1) and decreased to a minimum at the equator. The lowest growth rates were found at the equator during El Niño and at 2°N during normal upwelling, where a large biomass of buoyant diatoms had accumulated in the vicinity of a convergent front. Prochlorococcus growth rate reached a high of 0.64 d−1 at 1°S and maintained a moderate rate (0.36–0.49 d−1) throughout the southern end of the transect. An inverse relationship was found between the contribution of Prochlorococcus to the total primary production and nitrate concentration as well as total primary production measured by the 14C method. While Prochlorococcus is a dominant primary producer in tropical and subtropical open‐ocean ecosystems generally, it is relatively more important in oligotrophic waters than in the nutrient rich equatorial upwelling zone.

Comparison of airborne synthetic aperture radar imagery with in situ surface‐slope measurements across Gulf Stream slicks and a convergent front

Synthetic aperture radar (SAR) imagery showing two types of features near the inshore edge of the Gulf Stream are compared with nearly simultaneous estimates of the sea surface slope field derived from optical shipboard measurements. One class of feature consists of a set of narrow, dark lines having radar signal modulations of about −10 dB at L band. These modulations are comparable to those observed in the in situ wave‐slope data over wavenumbers corresponding to the radar‐resonant Bragg waves. These modulations are also predicted by a simplified equilibrium wave spectrum model using a surface elasticity of about 25 mN/m (as determined from in situ measurements of surface tension) and a wind friction velocity of about 10 cm/s (from buoy and shipboard measurements). These results support Lyzenga and Marmorino's [1998] inference that the Gulf Stream dark lines represent relatively passive, biogenically derived surfactant slicks that are advected and strained by the large‐scale surface flow field. The second class of feature consists of a pair of bright‐dark signatures over the shallow continental shelf region near Cape Hatteras, North Carolina. These features are characterized by signal modulations of about +4 and −4 dB in both the L band SAR and in situ data. These modulations appear to be due to the interaction of surface waves with the current convergence and divergence regions associated with a rotary circulation in a shallow convergent front. Model calculations of this interaction by Lyzenga [1998] indicate that the positive perturbations associated with current convergence should be relatively independent of the wavenumber, which is in approximate agreement with the in situ measurements of the present paper. The model also indicates that the negative perturbations associated with the region of current divergence should decrease for wavenumbers higher than L band. That trend is observed in the SAR data but not in the in situ data, which show significant perturbations at higher wavenumbers. It is conjectured that this is due to the effects of surfactants and either along‐front or time variability in the frontal dynamics, which are not included in the model. Additional comparisons of this kind should be useful to further test and improve physical models of near‐surface phenomena.

Feeding by larval cod in different water‐masses on Western Bank, Scotian Shelf

Gut contents were obtained from 1406 cod larvae from 94 stations in seven water masses related to a gyre around Western Bank, Scotian Shelf, 22 November–16 December 1992. Initial samples were from: well‐mixed water over the bank crest (CW); a surrounding convergent FRONT; relatively cold, fresh water (CFW) largely east of CW; warmer, salty water (WSW) west of CW and FRONT. After a storm on 3–6 December, samples were from CW and CFW displaced south‐east on the bank and, after further winds 11–12 December, from CW displaced north‐west off the bank. Zooplankton biomass (300–333 μm mesh, mostly Calanus copepodids) did not differ among water masses, but larval concentrations were significantly higher in FRONT than elsewhere. The small‐copepod diets of larvae varied among water masses, partly attributable to larval growth during the sampling period. Numbers of prey in guts, and indices of fullness and digestion, varied among water masses. More reliably, after ANCOVAs significant independent variables were: overwhelmingly time of day (maximum prey numbers and fullness at ∼19:00) and larval size; water mass; weaker interactions of the above among themselves and with sample depth and date; a very weak negative turbulence‐index effect on gut prey numbers in depth‐stratified samples. After ANCOVAs, larvae from prestorm CW had significantly higher prey numbers and fullness than did those from FRONT, WSW, and CFW. Larvae in CFW were significantly fuller when sampled closer to sites of former CW after the storm. Although numbers of prey in larvae advected off the bank in CW decreased significantly, prey averaged larger, so that gut fullness did not decrease. We conclude that larvae were best fed in the `centre' of the Western Bank gyre, but not greatly affected by subsequent displacement off the bank.

Polarimetric analysis and modeling of multifrequency SAR signatures from Gulf Stream fronts

Using airborne synthetic aperture radar data from the 1990 Gulf Stream Experiment, this paper investigates the polarization and wavelength dependence of radar signatures for narrow fronts with converging flows occurring within the Gulf Stream. The signal-to-background ratios of the cross-polarization backscatter return from a convergent front were found much higher than those of copolarization returns, when the flight path is crossing the front. However, a second convergent front, imaged at 45/spl deg/, showed that the signal-to-background ratios are nearly equal for co- and cross-polarizations. A polarimetric procedure, which has been successfully used to measure terrain slopes and to generate elevation maps, is applied to the convergent front to explain the polarization and imaging geometry dependence of these radar responses. A theoretical modeling of radar modulation using an ocean wave model and a composite-Bragg scattering model, which incorporates the effect of breaking waves, was developed. Calculations with the model agree reasonably well with the radar measurements at various polarizations for three radar frequencies: P-band (68 cm in wavelength), L-band (24 cm), and C-band (5.7 cm).