bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of coupled diffusive and decompressive bubble growth in magmatic systems
Authors: Proussevitch Alexander A. , Sahagian Dork L.
Published: February 2004

Bubble growth in an ascending parcel of magma is controlled both by diffusion of oversaturated volatiles and decompression as the magma rises. We have developed a numerical model which explores the processes involved in water exsolution from basaltic and rhyolitic melts rising at a constant rate from magma chamber depths of 4 and 1 km. While the model does not attempt to simulate natural eruptions, it sheds light on the processes which control eruptive behavior under various conditions. Ascent rates are defined such that a constant rate of decompression dP/dt is maintained. A variety of initial ascent rates are considered in the model, from 1 m/s to 100 m/s for basalts, and from a few centimeters per second to 10 m/s for rhyolite, at the base of the conduit. The model results indicate that for any reasonable ascent rate, basaltic melt degasses at a rate sufficient to keep the dissolved volatile concentration at equilibrium with the decreasing ambient pressure. Rhyolitic melt reaches the surface at equilibrium if its ascent rate is less than 1 m/s, but it can erupt with high oversaturation at greater ascent rates. The latter may lead to explosive eruptions. If the ascent rate of rhyolite is 10 m/s or more, then melt barely degasses at all in the conduit and erupts with the highest oversaturation possible. For the case of slow magma rise, bubble growth is limited by decompression. For the case of rapid magma rise, bubble growth is limited by diffusion. The results of our simple model do not accurately simulate natural volcanic eruptions, but suggest that subsequent, more complex models may be able to simulate eruptions using the insights regarding diffusive and decompressive bubble growth processes explored in this study. Numerical modeling of volcanic degassing may eventually lead to better prediction of eruption timing, energetics and hazards of active volcanoes.
Erosion as a driving mechanism of intracontinental mountain growth
Authors: Avouac J. P. , Burov E. B.
Published: February 2004

In nature, mountains can grow and remain as localized tectonic features over long periods of time (>10 m.y.). By contrast, according to current knowledge of lithospheric rheology and neglecting surface processes, any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years. For example, assuming a quartz‐dominated crustal rheology, the relief of a range initially 3 km high and 300–400 km wide is reduced by half in about 15 m.y. as a result of lateral spreading of its crustal root. We suggest that surface processes might actually prevent such a “subsurface collapse.” Removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny. We performed a set of numerical experiments in order to validate this hypothesis. A section of a lithosphere, with a brittle‐elasto‐ductile rheology, initially loaded by a mountain range is submitted to horizontal shortening and to surface processes. If erosion is intense, material is removed more rapidly than it can be supplied by crustal thickening below the range, and the topography is rapidly smoothed. For example, a feature 3 km high and 300–400 km wide is halved in height in about 15 m.y. for an erosion coefficient k = 103 m2/yr (the erosion rate is of the order of a few 0.1 mm/yr). This regime might be called “erosional collapse.” If erosion is not active enough, the crustal root spreads out laterally and “subsurface collapse” occurs. In the third intermediate regime, removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow. In this “mountain growth” regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law. The erosion rate may be high (e.g., 0.5–0.9 mm/yr), close to the rate of tectonic uplift (e.g., 0.7–1.1 mm/yr), and few times higher than the rate of topographic uplift (0.15–0.2 mm/yr). These experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain. Surface processes must therefore be taken into account in the interpretation and modeling of long‐term deformation of continental lithosphere. Conversely, the mechanical response of the lithosphere must be accounted for when large‐scale topographic features are interpreted and modeled in terms of geomorphologic processes.
Geometry of the 1954 Fairview Peak‐Dixie Valley earthquake sequence from a joint inversion of leveling and triangulation data
Authors: Hodgkinson Kathleen M. , Stein Ross S. , Marshall Grant
Published: February 2004

In 1954, four earthquakes greater than Ms = 6.0 occurred within a 30‐km radius and in a period of 6 months. Elevation and angle changes calculated from repeated leveling and triangulation surveys which span the coseismic period provide constraints on the fault geometries and coseismic slip of the faults which were activated. The quality of the coseismic geodetic data is assessed. Corrections are applied to the leveling data for subsidence due to groundwater withdrawal in the Fallon area, and a rod miscalibration error of 150 ± 30 ppm is isolated in leveling surveys made in 1967. The leveling and triangulation observations are then simultaneously inverted using the single value decomposition (SVD) inversion method to determine fault geometries and coseismic slip. Using SVD, it is possible to determine on which faults slip is resolvable given the data distribution. The faults are found to dip between 50° and 80° and extend to depths of 5 to 14 km. The geodetically derived slip values are generally equal to, or greater than, the maximum observed displacement along the surface scarps. Where slip is resolvable the geodetic data indicates the 1954 sequence contained a significant component of right‐lateral slip. This is consistent with the N15°W trending shear zone which geodetic surveys have detected in western Nevada.
The 1994 Northridge, California, earthquake: Investigation of rupture velocity, risetime, and high‐frequency radiation
Authors: Hartzell Stephen , Liu Pengcheng , Mendoza Carlos
Published: February 2004

A hybrid global search algorithm is used to solve the nonlinear problem of calculating slip amplitude, rake, risetime, and rupture time on a finite fault. Thirty‐five strong motion velocity records are inverted by this method over the frequency band from 0.1 to 1.0 Hz for the Northridge earthquake. Four regions of larger‐amplitude slip are identified: one near the hypocenter at a depth of 17 km, a second west of the hypocenter at about the same depth, a third updip from the hypocenter at a depth of 10 km, and a fourth updip from the hypocenter and to the northwest. The results further show an initial fast rupture with a velocity of 2.8 to 3.0 km/s followed by a slow termination of the rupture with velocities of 2.0 to 2.5 km/s. The initial energetic rupture phase lasts for 3 s, extending out 10 km from the hypocenter. Slip near the hypocenter has a short risetime of 0.5 s, which increases to 1.5 s for the major slip areas removed from the hypocentral region. The energetic rupture phase is also shown to be the primary source of high‐frequency radiation (1–15 Hz) by an inversion of acceleration envelopes. The same global search algorithm is used in the envelope inversion to calculate high‐frequency radiation intensity on the fault and rupture time. The rupture timing from the low‐ and high‐frequency inversions is similar, indicating that the high frequencies are produced primarily at the mainshock rupture front. Two major sources of high‐frequency radiation are identified within the energetic rupture phase, one at the hypocenter and another deep source to the west of the hypocenter. The source at the hypocenter is associated with the initiation of rupture and the breaking of a high‐stress‐drop asperity and the second is associated with stopping of the rupture in a westerly direction.
A model of gas buildup and release in crater lakes
Authors: Chau H. F. , Kwok Peter K. , Mak Louis
Published: February 2004

The sudden release of carbon dioxide gas, which may accumulate gradually within the stratified water bodies of a crater lake, caused two fatal disasters in Cameroon during the past decade. We model the various processes that may have led to the release by considering (1) gas injection, (2) an internal triggering mechanism, (3) propagation of a disturbance after triggering, and (4) the forces that end the outburst. We suggest that the 1986 Lake Nyos outburst was triggered at the lake bottom. The final explosive stage of the release ended quickly when a sufficiently large volume of gas bubbles prevented circulation of water eddies within the lake. A cellular automaton model is used to estimate the amount of carbon dioxide gas released and the characteristic time interval between successive outbursts. If both the gas accumulation rate and the diffusion rate of carbon dioxide through water are constant, then rapid gas release will occur at fairly regular intervals. In which case, the amount of gas released from Lake Nyos is 0.17 ± 0.05 km3 at standard temperature and pressure each 37 ± 10 years. It is possible, however, that an external event could trigger a sudden release or that the diffusion rate of carbon dioxide may change as more gas accumulates, which could shorten the release time.
Combined laboratory and numerical studies of the interaction between buoyant and plate‐driven upwelling beneath segmented spreading centers
Authors: Magde Laura S. , Kincaid Chris , Sparks David W. , Detrick Robert S.
Published: February 2004

A combination of laboratory and numerical models are used to examine the mantle flow beneath a segmented ridge generated by the interaction of a linear, buoyant upwelling source with plate‐driven flow. In the absence of plate spreading, the linear buoyant source creates a very narrow (across‐axis), two‐dimensional upwelling pattern. The plate‐driven flow consists of a quasi‐linear sheet‐like upwelling that cuts beneath ridge‐transform inside corners and is not centered beneath the spreading segments. When buoyant and plate‐driven flows are combined, material rises beneath the inside corners and flows away from the axis asymmetrically. Near the ends of segments, this results in a geometrical misfit between the center of mantle upwelling and the ridge axis. If a similar pattern of mantle flow occurs beneath a segmented mid‐ocean ridge, the result will be a thinner crust toward segment ends and possibly a negative correlation between extent of mantle melting and average depth of melting. These results indicate that even with an essentially two‐dimensional source, in cases where it is oblique to the actual spreading segments, the upwelling beneath a segmented ridge will appear to be three‐dimensional along axis. Since slow spreading ridges are generally more segmented than fast spreading ridges, this effect is likely to be more important at slow spreading ridges.
Complexity of seismicity due to highly rate‐dependent friction
Authors: Cochard A. , Madariaga R.
Published: February 2004

We study a simple antiplane fault of finite length embedded in a homogeneous, isotropic, elastic solid in order to understand the origin of seismic source heterogeneity in the presence of nonlinear rate‐dependent friction. All the mechanical properties of the medium and friction are assumed to be homogeneous. Starting from a heterogeneous initial stress distribution, we apply a slowly increasing uniform stress load far from the fault and we simulate the seismicity for more than 20,000 events, in some cases. The style of seismicity produced by this model is determined by a control parameter which measures the degree of rate dependence of friction. For classical friction models with rate‐independent friction, no complexity appears and seismicity is perfectly periodic. For weakly rate‐dependent friction, seismicity becomes slightly nonperiodic but most events are still characteristic earthquakes. When friction is highly rate‐dependent, seismicity is completely irregular and ruptures of all sizes occur inside the fault. Highly rate‐dependent friction destabilizes the healing process, producing premature healing of slip and partial stress drop. Premature healing causes rupture to take the form of narrow, propagating slip episodes, the so‐called Heaton's [1990] pulses. Partial stress drop produces large variations in the state of stress which, in turn, produce earthquakes of different sizes. We make the conjecture that all models in which static stress drop is only a fraction of the dynamic stress drop produce stress heterogeneity.
A synthetic seismicity model for southern California: Cycles, probabilities, and hazard
Authors: Ward Steven N.
Published: February 2004

The absence of a long historical catalog of observed seismicity with which to constrain earthquake recurrence behaviors is a fundamental stumbling block to earthquake prediction in California. Conceding that this limitation is not likely to relax in the foreseeable future, alternative approaches must be sought to extend the catalog artificially. In this article, I evaluate the long‐term behaviors of earthquakes on a map‐like set of southern California faults through computer simulations that incorporate the physics of earthquake stress transfer and are constrained by excellent, but restricted, bodies of geological and seismological data. I find that model seismicity fluctuates on both short (decades) and long (centuries) timescales but that it possesses a well‐defined mean and standard deviation. Seismicity fluctuations correlate across different magnitudes, and the long‐term cycles of smaller events seem to lead cycles of larger events. Short‐period seismicity fluctuations do not exhibit this tendency, and short‐term changes in low‐magnitude (M5 +) seismicity are not likely to be an effective predictor of future large events, at least for the region as a whole. As do real faults, the model faults produce characteristic and power law quakes in variable ratios with diverse periodic and nonperiodic behaviors. Generally, larger events tend to occur quasi‐periodically, and smaller ones tend to cluster; however, only for a few earthquake classes and certain locations is recurrence notably non‐Poissonian. An important use of synthetic seismicity is in the construction of earthquake hazard maps because it firmly grounds previously ad hoc assumptions regarding frequency‐magnitude distributions, multiple‐segment failure statistics, and rupture extents, while satisfying a spectrum of geological constraints such as fault slip rate, segment recurrence interval, and slip per event. With its depth of temporal and spatial coverage, synthetic seismicity also provides a means to investigate the time dependence of seismic hazard. Because hazard likelihood is a concatenation of the recurrence statistics from many seismic sources, in only about 40–50% of the regions near the major faults do sequences of 0.1 g or 0.2 g exceedances differ from Poissonian.
Cross pressure and temperature derivatives of selected elastic moduli for olivine from gigahertz ultrasonic interferometry
Authors: Chen Ganglin , Yoneda Akira , Getting Ivan C. , Spetzler Hartmut A.
Published: February 2004

Techniques have been developed to measure the cross pressure and temperature dependence of elastic moduli of olivine using the gigahertz ultrasonic interferometric method in a gas pressure vessel (to 500 MPa) equipped with an internal heater (to 500 K). Special attention has been drawn to the effect of the bond which couples the sample to the buffer rod. We obtained ∂2C22/∂P∂T = (2.8 ± 0.9) × 10−3 K−1 for a San Carlos olivine sample with a lapped contact. Selective values of ∂2KT/∂P∂T (0, 0.5 × 10−3, 1.0 × 10−3 K−1) are used to calculate their effect on cell volume, bulk modulus, and bulk sound velocity for olivine at pressures to 14 GPa and temperatures to 1800 K (410‐km depth), using the high‐temperature form of the Birch‐Murnaghan equation of state. The largest difference between the cell volumes for ∂2KT/∂P∂T = 1 × 10−3 K−1 and for ∂2KT/∂P∂T = 0 is about 0.6% at the highest pressure and the highest temperature (14 GPa, 1800 K). Therefore PVT data with precision at least better than 0.6% are required to distinguish between the extreme values of ∂2KT/∂P∂T for olivine if the pressure and temperature data are free of errors. If the cross pressure and temperature derivative term is neglected (∂2KT/∂P∂T = 0) in extrapolating laboratory elasticity data to mantle pressure and temperature conditions, the bulk modulus of olivine can be underestimated by as much as 10% and the bulk sound velocity by 3% at the 410‐km seismic discontinuity. Thus the present study highlights the need to measure elastic properties of mantle candidate minerals at the P‐T conditions of the mantle transition zone in order to establish a more precise model of the mantle mineralogy.
Topographic perturbations of regional stresses and consequent bedrock fracturing
Authors: Miller Daniel J. , Dunne Thomas
Published: February 2004

Stresses at Earth's surface are profoundly influenced by topography. Simple models indicate that gravitational and regional stresses are concentrated, attenuated, even reversed within landforms. Such topographically induced stresses are potentially manifest as bedrock fractures. Fractures reduce rock mass strength, decrease erosional resistance, and create conduits for water. By creating fractures, topographically induced stresses can alter processes of mass wasting, bedrock incision, and groundwater flow. We use Savage et al.'s [1985] solution for stresses in symmetric ridges and valleys on an elastic half‐space in plane strain to assess topographic effects on regional and gravity‐induced stresses. We have extended their solution to include lateral loads that vary with depth, which better approximates states of stress found in Earth's crust and allows us to examine a greater range of stress regimes. We examine the calculated stress fields to assess, in a classic Mohr‐Coulomb context, the potential for consequent fracture. This model indicates that topographic relief can cause stresses of sufficient magnitude to break rock, creating fracture sets having a spatial distribution and orientation governed by landform shape and the regional state of stress. We find that in extensional tectonic regimes, topographically induced stresses favor surface‐parallel fractures through ridges and steeply dipping fractures through valleys. Conversely, in compressional regimes, topographically induced stresses favor steeply dipping fractures through ridges and surface‐parallel fractures through valleys. Such topographic interactions with regional stresses pose consequences for interpretations of fracture orientation, for assessments of slope stability, and for processes of landscape development.
Crustal deformation across and beyond the Los Angeles basin from geodetic measurements
Authors: Shen Zheng‐Kang , Jackson David D. , Ge Bob X.
Published: February 2004

We combine 6 years of Global Positioning System (GPS) data with 20 years of trilateration data and a century of triangulation, taped distance, and astronomic azimuth measurements to derive 66 interseismic station velocities in the greater Los Angeles region. We interpolate the velocities to construct a regional strain rate map beyond the Los Angeles basin. Our results generally agree with the model proposed by the Working Group on California Earthquake Probabilities in 1995. Important regional findings of this study are as follows: (1) There is a significant N‐S convergence and E‐W extension, about 0.22 and 0.17 ± 0.05/μstrain/yr, respectively, for the two components, along the southern frontal fault system of the San Gabriel Mountains. (2) The crustal deformation around the Big Bend of the San Andreas fault (SAF) cannot be explained solely by wrench‐style motion across the SAF. Remaining motion could be part of a NW‐SE extension which is the response to NE‐SW compression in the central Transverse Ranges region. Alternatively, it could be caused by left‐lateral faulting on an oblique blind thrust beneath the San Gabriel Mountains. (3) Low strain rates are found along the Elsinore fault and Newport‐Inglewood fault. (4) North‐south compression decreases from the Raymond Hill fault westward to the Santa Monica fault. There is little east‐west extension along this fault system.
Present‐day kinematics of Asia derived from geologic fault rates
Authors: Peltzer Gilles , Saucier François
Published: February 2004

Active fault geometry, available Quaternary rates on major faults, and the far‐field plate motion are used to determine crustal kinematics in the collision zone between India and Asia. Using a finite element formalism to construct a spherical shell model of the Asian continent with embedded faults, we seek a velocity solution approaching the motion of rigid blocks by minimizing the elastic strain energy in the fault‐bounded blocks. By doing so, we test the assumption that long‐term deformation within continents is mostly localized into major faults. In the solution, fault motion accounts for more than 80% of the deformation, allowing us to describe our velocity model in terms of quasi‐rigid block rotations on the sphere. South China is rotating clockwise about a pole located southwest of Borneo, implying an E‐ESE velocity vector of ∼11 mm/yr for a point at Shanghai, in agreement with the velocity vector determined by very long baseline interferometry [Heki et al., 1995]. The eastward movement of south China is accommodated by oblique extension along the Red River fault at a rate of 10 ± 5 mm/yr in the south and by the combination of left‐lateral strike‐slip motion on the Qinling‐Dabie Shan fault and the counterclockwise rotation of the Ordos and adjacent blocks in the north. The Tarim rotates clockwise with respect to Dzungaria about a pole located at 44°N, 92°E, consistent with increasing crustal shortening toward the west throughout the Tien Shan. Assuming incompressibility, a crustal volume budget over a domain encompassing the Tertiary mountain ranges in Asia shows that, over the last 10,000 years, 73 ± 4% of the north‐south shortening between India and Asia has been absorbed by thickening of the lithosphere, and 27% has been accommodated by lateral extrusion of continental blocks. The present‐day predominance of thickening in Asia results from the relatively slow eastward motion of south China, controlled by strike‐slip faulting in the Qinling Shan and oblique extension in northeastern China.
Spatial patterns in phytoplankton biomass and pigment distributions in the Ross Sea
Authors: DiTullio Giacomo R. , Smith Walker O.
Published: February 2004

The distribution of phytoplankton biomass and pigments was determined in the Ross Sea during late austral summer 1992. Large gradients in biomass were noted both in the east‐west and north‐south direction, with maximum particulate matter concentrations occurring in the southwest portion of the Ross Sea. Two xanthophyll pigments dominated the profiles, fucoxanthin (indicative of diatoms) and 19′ hexanoyloxyfucoxanthin (hex) (indicative of the prymnesiophyte Phaeocystis antarctica). Fucoxanthin concentrations were greatest near the coast of Victoria Land and within the northern transect, whereas hex concentrations were highest offshore in the SE Ross Sea. Particulate carbon:chlorophyll a (POC:chl) ratios in diatom‐dominated waters of the western Ross Sea were relatively high (210). The POC:chl ratio for Phaeocystis antarctica populations from the SE Ross Sea was substantially less (92) than the ratios observed in diatom‐dominated waters. Pigment 14C labeling indicated that phytoplankton carbon in the diatom‐dominated northern Ross Sea accounted for approximately 30% of the POC, with a phytoplankton C:chl ratio of 130. Short‐term (24 hour) vertical fluxes of pigments ranged from 3 to 40 μg chl a m−2 d−1. Diatom‐dominated regions had greater fluxes of phaeophorbides, suggesting that metazoans were the most important grazers at these locations. In contrast, the phaeophytin/total phaeopigment ratio was highest in waters dominated by Phaeocystis antarctica. The distribution of phytoplankton biomass and pigments revealed a spatially variable distribution of taxa, one which clearly has important consequences to food‐web dynamics, biogeochemical cycles, and vertical flux patterns in the Ross Sea.
Evidence for the influence of remote forcing on interdecadal variability in the southern Indian Ocean
Authors: Reason C. J. C. , Allan R. J. , Lindesay J. A.
Published: February 2004

Previous analysis of the Comprehensive Ocean‐Atmosphere Data Set and United Kingdom Meteorological Office Globally Integrated Sea‐Ice and Sea Surface Temperature data, when studied in terms of the four epochs 1900–1920, 1921–1941, 1942–1962, and 1963–1983, has indicated significant interdecadal variability in austral summer sea surface temperature (SST) and atmospheric circulation patterns over the wider Indian Ocean region. A global ocean general circulation model is used to investigate the dynamical response of the ocean to the observed interdecadal variability in the local winds and to various remote wind forcings. The potential thermodynamic effect of the winds on the surface heat flux is specifically excluded. Attention is focused on SST anomalies in the southern Indian Ocean, particularly the Agulhas Current, retroflection, and outflow zones, because this is where the observations exhibit the most prominent SST variability. When the observed Indian Ocean epoch winds are imposed, the dynamical response of the model leads to SST anomalies of the right sign as the observations but smaller in both magnitude and areal extent. When the magnitude of the mean Pacific winds is increased (decreased), the model responds dynamically by strengthening (weakening) the Indonesian throughflow, which then modulates the southern Indian gyre accordingly. The resulting SST anomalies in the southern Indian Ocean are larger and more widely distributed than those obtained with the local winds and bear greater resemblance to the observed patterns in this region. The results of this study suggest that modulations to the Indonesian throughflow can impact significantly on interdecadal variability in the southern Indian Ocean. Changes in the magnitude of the mean winds over the Pacific is one remote forcing mechanism demonstrated to modulate the Indonesian throughflow, and, hence, the Indian Ocean.
Oxygen isotopic identity of the Delaware Coastal Current
Authors: Khim Boo‐Keun , Krantz David E.
Published: February 2004

Results are presented and discussed from a preliminary study on the oxygen isotopic composition of coastal and inner shelf waters from the Middle Atlantic Bight between the mouths of Delaware Bay and Chesapeake Bay. The water samples were collected during the spring, April/May 1993, in conjunction with a detailed hydrographic survey of the Delaware Coastal Current. The relationship between salinity and δ18O, both of which are generally accepted conservative properties of seawater, allows identification of the fresh and saline mixing end‐members for the Delaware Coastal Current. The main source of freshwater to the Delaware Coastal Current is a mixture of runoff into Delaware Bay with an estimated δ18OS = 0 composition of −7.08‰. The freshwater input is dominated by Delaware River discharge with a mean δ18O of −8.0‰ based on previous studies; there appears to be a secondary contribution of freshwater that is isotopically heavier from coastal‐plain drainage. The extrapolated δ18OS = 32.35 value (−0.93‰) of the saline component implies exchange with Middle Atlantic Bight Shelf Water through along‐shelf and across‐shelf mixing processes. Limited data suggest that the oxygen isotopic composition of low‐salinity water exiting the Chesapeake Bay mouth is discernible compared with that of the Delaware Coastal Current.
Global validation of the along‐track scanning radiometer against drifting buoys
Authors: Harris A. R. , Saunders M. A.
Published: February 2004

The along‐track scanning radiometer (ATSR) was launched on the European Space Agency's first remote sensing satellite, ERS 1, on July 17, 1991. ATSR is designed to retrieve sea surface temperature (SST) to an accuracy of 0.25 K rms, which represents more than a factor of 2 improvement over any previously flown satellite radiometer. Early validation studies from limited regions suggest that ATSR is capable of measuring SST to near this design accuracy. We report a global validation study against quality‐controlled drifting buoys by examining 280 matchups worldwide with ATSR measurements at their full (1 km) resolution. We investigate optimizing the precision of ATSR using four different SST algorithms derived using a theoretical atmospheric transmission model, combined with various techniques to reduce remnant noise and other errors. We find that a “low‐noise” retrieval algorithm incorporating only the 3.7 and 11 μm nadir view channels gives the optimum precision, a global pixel precision of 0.26 K (or 0.25 K if 1/2° spatial averages are used). A standard deviation of 0.25 K against global drifting buoy data approaches the geophysical limit set by the inherent variability of the skin effect and by the buoy bulk temperature accuracy. Further progress will require comparison against quality in situ radiometer‐derived skin temperatures, although the problem of obtaining sufficiently large and diverse data sets will need to be addressed.
Using nonlinear forecasting to learn the magnitude and phasing of time‐varying sediment suspension in the surf zone
Authors: Jaffe Bruce E. , Rubin David M.
Published: February 2004

The time‐dependent response of sediment suspension to flow velocity was explored by modeling field measurements collected in the surf zone during a large storm. Linear and nonlinear models were created and tested using flow velocity as input and suspended‐sediment concentration as output. A sequence of past velocities (velocity history), as well as velocity from the same instant as the suspended‐sediment concentration, was used as input; this velocity history length was allowed to vary. The models also allowed for a lag between input (instantaneous velocity or end of velocity sequence) and output (suspended‐sediment concentration). Predictions of concentration from instantaneous velocity or instantaneous velocity raised to a power (up to 8) using linear models were poor (correlation coefficients between predicted and observed concentrations were less than 0.10). Allowing a lag between velocity and concentration improved linear models (correlation coefficient of 0.30), with optimum lag time increasing with elevation above the seabed (from 1.5 s at 13 cm to 8.5 s at 60 cm). These lags are largely due to the time for an observed flow event to effect the bed and mix sediment upward. Using a velocity history further improved linear models (correlation coefficient of 0.43). The best linear model used 12.5 s of velocity history (approximately one wave period) to predict concentration. Nonlinear models gave better predictions than linear models, and, as with linear models, nonlinear models using a velocity history performed better than models using only instantaneous velocity as input. Including a lag time between the velocity and concentration also improved the predictions. The best model (correlation coefficient of 0.58) used 3 s (approximately a quarter wave period) of the cross‐shore velocity squared, starting at 4.5 s before the observed concentration, to predict concentration. Using a velocity history increases the performance of the models by specifying a more complete description of the dynamical forcing of the flow (including accelerations and wave phase and shape) responsible for sediment suspension. Incorporating such a velocity history and a lag time into the formulation of the forcing for time‐dependent models for sediment suspension in the surf zone will greatly increase our ability to predict suspended‐sediment transport.
Near‐surface circulation of the Nordic seas as measured by Lagrangian drifters
Authors: Poulain P.‐M. , Warn‐Varnas A. , Niiler P. P.
Published: February 2004

In the period June 1991 to August 1993, 107 Argos tracked, drifters drogued to 15 m depth, were released in the Nordic seas (or Greenland, Iceland, and Norwegian Seas). The drifter movements revealed the strong and spatially confined current systems along the surface salinity fronts of the Iceland‐Faroe Frontal zone and of the Norwegian coast and along the continental margins and their extensions to the Barents Sea and Spitsbergen. The Norwegian Atlantic Current is composed of three distinct streams (two continental margin and one coastal branches) which join into one single swift mean current west of the Lofoten and Vesterålen Islands, where the strongest measured currents are in excess of 110 cm s−1. In addition to the general cyclonic gyre circulation in the Nordic seas, the drifters indicate smaller cyclonic circulation patterns in all the major subbasins, i.e., the Iceland plateau, the Norwegian, the Lofoten, and the Greenland basins. No surface signature of the East Icelandic Current is disclosed by the drifters. Interpolated and low‐pass‐filtered position data were used to construct maps of 15‐m‐depth ensemble mean velocity, velocity variability, and residence time. Vigorous eddy fields are dominant in the strong currents and in the Lofoten basin. Eulerian correlations indicate that they tend to propagate to the west. In contrast, the Iceland plateau appears quiescent, both in the mean and eddy velocities. Single‐particle diffusivities are computed and are found to be in the range 1–7 × 107 cm2 s−1. The corresponding Lagrangian timescale and space scale are 1–3 days and 10–40 km, respectively. These Lagrangian drifter measurements compose the first basin‐scale, accurate near‐surface velocity data set of the Nordic seas.
A model of the effects of horizontal density gradients on mixing processes in the oceanic surface layer
Authors: Liu Gang , Janowitz Gerald S. , Kamykowski Daniel
Published: February 2004

A diurnal Eulerian model of the upper mixed layer incorporating the effects of horizontal density gradients is developed based on that of Janowitz and Kamykowski [1991] and is utilized to study the effects of wind‐driven cross‐isopycnal transport on mixing processes in the layer. Four runs of the model are conducted for a constant horizontal density gradient of 2.5 × 10−5 kg m−4 with wind directions at 0°, 90°, 180°, and 270° to this gradient. Two runs are conducted with half this density gradient and wind directions 90° to the left and right of the gradient and one run with no density gradient. In all cases the model is first run with a constant heat loss rate of 150 W m−2 through the sea surface for 11 hours to simulate nighttime cooling and to provide initial conditions for daytime runs. The model is then run for 24 hours with a constant wind speed of 7 m s−1 and a maximum solar radiation of 900 W m−2 during the first 13 hours of the 24‐hour simulation. The results show that the shallowest mixed layer occurs when the wind direction is 90° to the left of the density gradient and the deepest layer occurs with the wind direction 90° to the right of the gradient. After 24 hours, the ratio of the deepest to the shallowest mixed layer depth is 2.35 for the stronger gradient and 1.83 for the weaker gradient. These results are explained by the effect of vertical shear of the wind‐driven advection of horizontal density variations, which changes the vertical density gradient and can destabilize or stabilize the mixed layer. The results indicate that wind‐driven, cross‐isopycnal transport can have significant effects on mixing processes in the layer. Dimensionless parameters are developed to estimate the relative importance of cross‐isopycnal, wind‐driven transport on mixed layer properties.
Simulation of annual plankton productivity cycle in the Black Sea by a one‐dimensional physical‐biological model
Authors: Oguz Temel , Ducklow Hugh , Malanotte‐Rizzoli Paola , Tugrul Suleyman , Nezlin Nikolai P. , Unluata Umit
Published: February 2004

The annual cycle of the plankton dynamics in the central Black Sea is studied by a one‐dimensional vertically resolved physical‐biological upper ocean model, coupled with the Mellor‐Yamada level 2.5 turbulence closure scheme. The biological model involves interactions between the inorganic nitrogen (nitrate, ammonium), phytoplankton and herbivorous zooplankton biomasses, and detritus. Given a knowledge of physical forcing, the model simulates main observed seasonal and vertical characteristic features, in particular, formation of the cold intermediate water mass and yearly evolution of the upper layer stratification, the annual cycle of production with the fall and the spring blooms, and the subsurface phytoplankton maximum layer in summer, as well as realistic patterns of particulate organic carbon and nitrogen. The computed seasonal cycles of the chlorophyll and primary production distributions over the euphotic layer compare reasonably well with the data. Initiation of the spring bloom is shown to be critically dependent on the water column stability. It commences as soon as the convective mixing process weakens and before the seasonal stratification of surface waters begins to develop. It is followed by a weaker phytoplankton production at the time of establishment of the seasonal thermocline in April. While summer nutrient concentrations in the mixed layer are low enough to limit production, the layer between the thermocline and the base of the euphotic zone provides sufficient light and nutrient to support subsurface phytoplankton development. The autumn bloom takes place sometime between October and December depending on environmental conditions. In the case of weaker grazing pressure to control the growth rate, the autumn bloom shifts to December–January and emerges as the winter bloom, or, in some cases, is connected with the spring bloom to form one unified continuous bloom structure during the January–March period. These bloom structures are similar to the year‐to‐year variabilities present in the data.
Directional spreading function of the sea wave spectrum at short scale, inferred from multifrequency radar observations
Authors: Caudal G. , Hauser D.
Published: February 2004

Several models of the directional spreading function of the sea wave spectrum have been proposed in the literature for low wavenumbers. In this paper we propose a high‐wavenumber extrapolation of those models, obtained by fitting a correcting term which vanishes at large scales (low wavenumbers). At short scales (high wavenumbers) the correcting term is constrained by multifrequency microwave observations of the normalized radar cross section σ° from P band (frequency = 0.43 GHz) up to Ka band (frequency = 34.43 GHz), together with optical observations of the sea surface slope variance. Two formulations are given, one providing a high‐wavenumber extrapolation to Apel's [1994] formulation and the other providing a high‐wavenumber extrapolation to Donelan et al.'s [1985] and Banner's [1990] formulation. The correcting term δ, expressed as a function of wavenumber k and wind speed U by means of six least squares fitted parameters, is found to vary strongly with k and slightly with U. Another simpler expression for δ, involving only the dependence with k by means of three fitted parameters, is therefore also proposed. According to our fitted model of the spreading function, there is a spectral region in the short gravity range where the sea spectrum shows only a weak dependence on the direction, in accordance with the previous models. However, unlike them, our model gives an increase of the anisotropy of the spreading function at higher wavenumbers, in such a way that the ratio between the cross‐wind and along‐wind spectral densities of the folded spectrum is reduced to no more than 35% at high wavenumbers (k ≈ 1000 rad/m). This increase of anisotropy at high frequency is in accordance with conclusions drawn by previous authors from radar backscatter data, which were, however, limited to narrow spectral bands since their analyses involved only single‐frequency radar data.
Detection of oceanic fronts at low grazing angles using an X band real aperture radar
Authors: Askari F. , Donato T. F. , Morrison J. M.
Published: February 2004

We examine the radar signatures and changes in the surface roughness associated with oceanic features in the low grazing angle (LGA) scattering regime. The X band (HH) radar signatures consist of high‐amplitude sea spikes, step changes in the normalized radar cross‐section (NRCS) modulations, and bright narrowbanded frontal structures. Using in situ observations coupled with airborne precision radiation thermometer (PRT‐5) data, we show that the step changes in radar cross‐section modulations are associated with either thermal stability‐induced stress variations or current velocity variations. Superimposed on the step changes are additional modulations that result from wave breaking and hydrodynamic straining. The amplitudes of the NRCS LGA measurements are compared with the predictions of four backscattering models: the Bragg, the tilted‐Bragg, the wedge, and the plume model. It is shown that while the simple Bragg model can describe the measurements to a limited degree, it generally tends to underpredict the results. Agreement is improved by including the tilt contribution from the longwave surface waves in the context of the composite scattering model. We use the wedge and plume models as the basis for explaining the cross sections associated with the high‐amplitude sea spikes. The wedge model is used to describe scattering from sharply crested waves, and the plume model is used to describe the extreme cross sections that are associated with breaking waves near the fronts. In describing the probability density function characteristics we show that the backscattering statistics exhibit “K distribution” behavior for the Gulf Stream current region and near‐frontal regions, while the cooler shelf waters have characteristics of an exponential distribution.
Long‐term acoustic measurement of temperature variations in the Pacific North Equatorial Current
Authors: Kaneko Arata , Zheng Hong , Nakano Iwao , Yuan Gang , Fujimori Hidetoshi , Yoneyama Kunio
Published: February 2004

A 247‐km path length sound transmission experiment using a single sound source sending to a remote receiver was performed during February–June 1992 along the latitudinal line of 10°N in the North Equatorial Current off Mindanao Island, Phillipines. The range‐averaged temperature was reconstructed at three layers (65–500 m, 500–1500 m, and 1500–3300 m) through the inverse analysis of travel time data obtained between the source and receiver. The temperature remained nearly constant during February–March and increased by 0.8°C for the upper and middle layers and 0.2°C for the lower layer in the subsequent months. A 0.8°C temperature increase was in good agreement with expendable bathythermograph results obtained at the start and finish of the experiment. National Oceanic and Atmospheric Administration advanced very high resolution radiometer images also show evidence of a warming of the surface water by 2.0°C during the same period. It is concluded that this acoustic experiment observed a warm event which took place in the western equatorial Pacific with the decay of the 1991–1992 El Niño.
Modeling the near‐freezing dichothermal layer in the Sea of Okhotsk and its interannual variations
Authors: Yang Jiayan , Honjo Susumu
Published: February 2004

The hydrographic structure in the Sea of Okhotsk is unique, especially regarding the existence of the Okhotsk Dichothermal Layer (ODTL), a near‐freezing layer beneath a highly seasonal surface layer. The formation of this cold layer is linked to extensive sea‐ice coverage in winter. Observations revealed strong signals of interannual variations of winter sea‐ice extent in this marginal sea and indicated a likely linkage with observed changes in temperature and salinity fields. In this paper, a 1.5‐dimensional coupled ocean‐ice model is used to study the formation of the ODTL and to test how external forcing fields affect this process. The model, despite its simplicity, simulates well the temperature and the salinity profiles that resemble observations. It is found that the ODTL is replenished during winter convection. During warm seasons, meltwater creates a very stable surface layer that effectively prevents mixing between warm surface water and cold water in the ODTL. Thus the seasonal variation of sea ice plays a leading role in maintaining the ODTL structure. Although the minimum temperature feature of the ODTL is robust in the model, the hydrographic structure in the Sea of Okhotsk is sensitive to interbasin exchanges with the North Pacific Ocean and the Sea of Japan and to some other external forcing fields. More interestingly, an interannual oscillation arises from this model even though the model forcing excludes any interannual variation. We concluded that such oscillation is due to the model's subharmonic response to the annual cycle of the forcing fields. It suggests that the seemingly zero‐net‐gain annual cycle of winter freezing and spring melting is actually an effective process transporting salt from surface to deeper layers and thus the coupled ocean‐ice's subharmonic response to this annual cycle is an important source for low‐frequency variations.
On the eddy field in the Agulhas Retroflection, with some global considerations
Authors: Schmitz William J.
Published: February 2004

Moored instrument data are used to describe and summarize selected characteristics of the observed time‐dependent flow at low frequencies, that is, the eddy field, in the Agulhas Retroflection. In addition, the basic features of the eddy kinetic energy (KE) distributions in the Agulhas Current System (ACS) are compared with those from analogous strong current regimes in other oceans, concentrating on those from the Gulf Stream System (GSS). Similarities between the eddy field in the ACS and the GSS are found to include the vertical structure of KE as well as its distribution with frequency. This correspondence between the eddy fields in the ACS and GSS is probably associated with a consonant physical mechanism (mixed barotropic/baroclinic instability) for eddy generation in the presence of comparable upper and intermediate layer mean flow characteristics. It is also pointed out (for the first time to my knowledge) that the upper 1000 m or so (σθ ≤ 27.4) of the ACS and GSS can have a very similar distribution of transport in potential density classes. Relationships between the eddy field and the general circulation for the ACS and GSS with the Kuroshio Current System and the North Brazil Current System are described in less detail and scope. It seems probable that the characteristics of the global eddy field are strongly affected by the interbasin‐scale upper and intermediate layer replacement flows associated with the global thermohaline circulation.