bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of annual signals on geodetic velocity
Authors: Blewitt Geoffrey , Lavallée David
Published: October 2002

Our analysis of Global Positioning System (GPS) site coordinates in a global reference frame shows annual variation with typical amplitudes of 2 mm for horizontal and 4 mm for vertical, with some sites at twice these amplitudes. Power spectrum analysis confirms that GPS time series also contain significant power at annual harmonic frequencies (with spectral indices 1 < α < 2), which indicates the presence of repeating signals. Van Dam et al. [2001] showed that a major annual component is induced by hydrological and atmospheric loading. Unless accounted for, we show that annual signals can significantly bias estimation of site velocities intended for high accuracy purposes such as plate tectonics and reference frames. For such applications, annual and semiannual sinusoidal signals should be estimated simultaneously with site velocity and initial position. We have developed a model to calculate the level of bias in published velocities that do not account for annual signals. Simultaneous estimation might not be necessary beyond 4.5 years, as the velocity bias rapidly becomes negligible. Minimum velocity bias is theoretically predicted at integer‐plus‐half years, as confirmed by tests with real data. Below 2.5 years, the velocity bias can become unacceptably large, and simultaneous estimation does not necessarily improve velocity estimates, which rapidly become unstable due to correlated parameters. We recommend that 2.5 years be adopted as a standard minimum data span for velocity solutions intended for tectonic interpretation or reference frame production and that we be skeptical of geophysical interpretations of velocities derived using shorter data spans.
Methods for inferring regional surface‐mass anomalies from Gravity Recovery and Climate Experiment (GRACE) measurements of time‐variable gravity
Authors: Swenson Sean , Wahr John
Published: October 2002

The Gravity Recovery and Climate Experiment, GRACE, will deliver monthly averages of the spherical harmonic coefficients describing the Earth's gravity field, from which we expect to infer time‐variable changes in mass, averaged over arbitrary regions having length scales of a few hundred kilometers and larger, to accuracies of better than 1 cm of equivalent water thickness. These data will be useful for examining changes in the distribution of water in the ocean, in snow and ice on polar ice sheets, and in continental water and snow storage. We describe methods of extracting regional mass anomalies from GRACE gravity coefficients. Spatial averaging kernels were created to isolate the gravity signal of individual regions while simultaneously minimizing the effects of GRACE observational errors and contamination from surrounding glacial, hydrological, and oceanic gravity signals. We then estimated the probable accuracy of averaging kernels for regions of arbitrary shape and size.
Late Quaternary slip rates across the central Tien Shan, Kyrgyzstan, central Asia
Authors: Thompson Stephen C. , Weldon Ray J. , Rubin Charles M. , Abdrakhmatov Kanatbek , Molnar Peter , Berger Glenn W.
Published: October 2002

Slip rates across active faults and folds show that late Quaternary faulting is distributed across the central Tien Shan, not concentrated at its margins. Nearly every intermontane basin contains Neogene and Quaternary syntectonic strata deformed by Holocene north‐south shortening on thrust or reverse faults. In a region that spans two thirds of the north‐south width of the central Tien Shan, slip rates on eight faults in five basins range from ∼0.1 to ∼3 mm/yr. Fault slip rates are derived from faulted and folded river terraces and from trenches. Radiocarbon, optically stimulated luminescence, and thermoluminescence ages limit ages of terraces and aid in their regional correlation. Monte Carlo simulations that sample from normally distributed and discrete probability distributions for each variable in the slip rate calculations generate most likely slip rate values and 95% confidence limits. Faults in basins appear to merge at relatively shallow depths with crustal‐scale ramps that underlie mountain ranges composed of pre‐Cenozoic rocks. The sum and overall pattern of late Quaternary rates of shortening are similar to current rates of north‐south shortening measured using Global Positioning System geodesy. This similarity suggests that deformation is concentrated along major fault zones near range‐basin margins. Such faults, separated by rigid blocks, accommodate most of the shortening in the upper crust.
Preferred crystal orientations due to melt convection during directional solidification
Authors: Bergman Michael I. , Cole David M. , Jones Jackson R.
Published: October 2002

Directional solidification of salt water results in a preferred crystallographic orientation, with ice platelets growing with their short dimension (the c axis of the hexagonal close packed (hcp) ice crystal) transverse to the growth direction. In the absence of a mean saltwater flow parallel to the solidification interface, the ice platelets, and hence the c axes, are arbitrarily oriented in the plane transverse to growth. Here we report on laboratory experiments on salt water solidifying horizontally from a vertical chill plate. This geometry yields a convectively driven, organized fluid flow parallel to the solidification interface, which we found causes the platelets to trend parallel to the flow direction. This contrasts with previous field and laboratory observations of ice solidifying vertically from salt water in the presence of an externally forced flow, such as an ocean current, where the c axes lie parallel to the flow direction. The results support the hypothesis that transverse textures result from a hydrodynamic interaction with hcp dendritic platelets but highlight the difference between flow driven internally versus externally to the dendritic mushy layer. They also suggest that convective fluid flow may be the cause for the fabric observed in resolidified cracks in sea ice sheets and perhaps that fluid motion in the Earth's outer core may play a role in the seismic anisotropy of the hcp iron inner core.
Global Omori law decay of triggered earthquakes: Large aftershocks outside the classical aftershock zone
Authors: Parsons Tom
Published: October 2002

Triggered earthquakes can be large, damaging, and lethal as evidenced by the1999 shocks in Turkey and the 2001 earthquakes in El Salvador. In this study, earthquakes withMs≥ 7.0 from the Harvard centroid moment tensor (CMT) catalog are modeled as dislocations to calculate shear stress changes on subsequent earthquake rupture planes near enough to be affected. About 61% of earthquakes that occurred near (defined as having shear stress change ∣Δτ∣ ≥ 0.01 MPa) theMs≥ 7.0 shocks are associated with calculated shear stress increases, while ∼39% are associated with shear stress decreases. If earthquakes associated with calculated shear stress increases are interpreted as triggered, then such events make up at least 8% of the CMT catalog. Globally, these triggered earthquakes obey an Omori law rate decay that lasts between ∼7–11 years after the main shock. Earthquakes associated with calculated shear stress increases occur at higher rates than background up to 240 km away from the main shock centroid. Omori's law is one of the few time‐predictable patterns evident in the global occurrence of earthquakes. If large triggered earthquakes habitually obey Omori's law, then their hazard can be more readily assessed. The characteristic rate change with time and spatial distribution can be used to rapidly assess the likelihood of triggered earthquakes following events ofMs≥ 7.0. I show an example application to theM= 7.7 13 January 2001 El Salvador earthquake where use of global statistics appears to provide a better rapid hazard estimate than Coulomb stress change calculations.
Petrology of Alarcon Rise lavas, Gulf of California: Nascent intracontinental ocean crust
Authors: Castillo P. R. , Hawkins J. W. , Lonsdale P. F. , Hilton D. R. , Shaw A. M. , Glascock M. D.
Published: October 2002

The Alarcon Basin in the southern Gulf of California originated by intracontinental rifting of the southwestern margin of North America and has evolved into an intercontinental ocean basin by 3.7 m.y. of seafloor spreading. Lava samples collected on the axial ridge along Alarcon Rise, two of the many near‐axis seamounts on the northwest flank of the rise, and an abyssal hill on 0.6 Ma crust on the southeast flank were examined petrographically and analyzed for major and trace element and He‐Sr‐Nd‐Pb isotope composition. Most samples are typical mid‐ocean ridge basalt (MORB), but rocks at the axial ridge site closest to continent are more differentiated. Rocks from the axial ridge and abyssal hill are genetically related through separate episodes of fractional crystallization. Seamount samples are relatively more primitive than seafloor samples and may have been formed by smaller degree of melting of the same mantle source as the seafloor lavas. Tamayo transform fault has no apparent effect on the geochemistry of axial ridge lavas erupted close to it; this is contrary to the postulated transform fault effect. Alarcon Basin differs from the Salton Trough and Guaymas Basin in that its youngest seafloor shows no evidence for contamination with continental crust or a subduction component. We propose that Pacific MORB source mantle underlies the Gulf of California because the western North American margin has overridden the Pacific suboceanic mantle. Miocene rifting and Pliocene spreading in Alarcon Basin have created a window allowing the most recent MORB melts to rise uncontaminated to the surface.
Origin of the West Taiwan basin by orogenic loading and flexure of a rifted continental margin
Authors: Lin A. T. , Watts A. B.
Published: October 2002

Seismic and well data suggest that the West Taiwan basin developed by orogenic loading and flexure of a rift‐type continental margin. The most likely source of the loading is Taiwan, where oblique convergence between the Eurasian and Philippine Sea plates has produced an orogenic belt up to 300 km in length, 100 km in width, and 4 km in height. Flexure modeling shows that surface loading is unable to explain the depth of the West Taiwan basin. Other, subsurface or buried loads are required. Combined surface and buried loading explains the depth and width of the basin. It also accounts for a Bouguer gravity anomaly “high” and flanking “low” over the orogenic belt, a lateral offset of 20–30 km between the peak topography and the maximum depth to the seismic Moho, and evidence for tectonic uplift in the Penghu Islands. The depth of the base of the foreland sequence in the northern part of the West Taiwan basin can be explained well by an elastic plate model with an effective elastic thickness, Te, of 13 km. While this value is low when compared to most other foreland basins, it is within the range of values derived from rifted continental margins. The northern part of the West Taiwan basin unconformably overlies a passive margin sequence and therefore appears to have inherited the long‐term (>1 Myr) flexural properties of the margin. In the southern part of the basin, however, the depth to the base of the foreland sequence dips too steeply to be explained by elastic plate models. This part of the basin therefore appears to be yielding rather than flexing. Differences in the flexural behavior along strike of the West Taiwan foreland basin lithosphere are reflected in seismicity patterns west of the thrust front. The northern part of the basin is associated with a low level of seismic activity, while the south correlates with an abundance of earthquakes, especially at shallow (<25 km) depths. There is a cluster of earthquakes along two extensional faults that were active during rifting of the underlying margin. Therefore lithospheric flexure and fault reactivation may be important contributors to the seismicity of the Taiwan region.
A short, reverse polarity interval within the Jaramillo subchron: Evidence from the Jingbian section, northern Chinese Loess Plateau
Authors: Guo Bin , Zhu Rixiang , Florindo Fabio , Ding Zhongli , Sun Jimin
Published: October 2002

A high‐resolution paleomagnetic and rock magnetic investigation supported by scanning electron microscopy observations, X‐ray diffraction determinations, and anisotropy of magnetic susceptibility analyses has been carried out on a loess‐paleosol sequence from Jingbian (37.4°N, 108.8°E), at the northern extremity of the Chinese Loess Plateau. The results indicate that the magnetic assemblage is dominated by pseudo‐single‐domain magnetite with associated maghemite and traces of a higher‐coercivity phase, probably hematite. The Brunhes‐Matuyama and upper Jaramillo transitions have been carefully documented and a record of a short reverse polarity interval (SRI), entirely within the Jaramillo normal polarity subchron, has been obtained. The duration of the SRI, recorded over a stratigraphic interval of 48 cm, is 10.1 kyr (29 kyr if the polarity transitions are included). These results seem to exclude that rock magnetic variations are adversely affecting the recording process and compel us to reconsider the Jaramillo subchron as characterized by a pair of normal polarity intervals interrupted by a short reverse polarity interval.
Pervasive mantle plume head heterogeneity: Evidence from the late Cretaceous Caribbean‐Colombian oceanic plateau
Authors: Kerr Andrew C. , Tarney John , Kempton Pamela D. , Spadea Piera , Nivia Alvaro , Marriner Giselle F. , Duncan Robert A.
Published: October 2002

In SW Colombia picritic pillow lavas and tuffs, as well as breccias composed of picritic clasts, occur interspersed with basalts of the Central Cordillera and represent accreted portions of the ∼90 Ma Colombian/Caribbean oceanic plateau (CCOP). We present new geochemical data for these picrites and high‐MgO basalts from SW Colombia, along with new data from Deep Sea Drilling Project Leg 15 drill sites. The 40Ar/39Ar ages for the CCOP in the Central Colombian Cordillera range from 87 to 93 Ma. Both SW Colombia picrites and Leg 15 basalts are compositionally diverse and range from reasonably enriched ((La/Nd)n > 1 and (εNd)i < +4.1) to relatively depleted ((La/Nd)n < 1 and (εNd)i > +8.0). Nb/Y and Zr/Y systematics suggest that the depleted component is not depleted MORB mantle, but is an intrinsic part of the plume. The bulk of the CCOP compositions can be explained by mixing between this depleted mantle and a HIMU component. However, radiogenic isotope systematics indicate the presence of an EM2 (or possibly EM1) component within the plume. Mantle melt modeling suggests that the enriched magma types are the product of deeper, small degree melting of a pervasively heterogeneous plume comprising a refractory matrix with enriched streaks/blobs, whereas shallower, more extensive melting, results in the formation of relatively depleted magmas.
Triggering of the 1999 MW 7.1 Hector Mine earthquake by aftershocks of the 1992 MW 7.3 Landers earthquake
Authors: Felzer Karen R. , Becker Thorsten W. , Abercrombie Rachel E. , Ekström Göran , Rice James R.
Published: October 2002

There is strong observational evidence that the 1999 MW 7.1 Hector Mine earthquake in the Mojave Desert, California, was triggered by the nearby 1992 MW 7.3 Landers earthquake. Many authors have proposed that the Landers earthquake directly stressed the Hector Mine fault. Our model of the Landers aftershock sequence, however, suggests there is an 85% chance that the Hector Mine hypocenter was actually triggered by a chain of smaller earthquakes that was initiated by the Landers main shock. We perform our model simulations using the Monte Carlo method based on the Gutenberg‐Richter relationship, Omori's law, Båth's law, and assumptions that all earthquakes, including aftershocks, are capable of producing aftershocks and that aftershocks produce their own aftershocks at the same rate that other earthquakes do. In general, our simulations show that if it has been more than several days since an M ≥ 7 main shock, most new aftershocks will be the result of secondary triggering. These secondary aftershocks are not physically constrained to occur where the original main shock increased stress. This may explain the significant fraction of aftershocks that have been found to occur in main shock stress shadows in static Coulomb stress triggering studies.
Experimental investigation of volcanic particle aggregation in the absence of a liquid phase
Authors: James M. R. , Gilbert J. S. , Lane S. J.
Published: October 2002

Understanding the dispersal and deposition of fine‐grained silicate particles from volcanic plumes is key to interpreting ash fall deposits and predicting hazards for future eruptions. It is known that many of these particles fall incorporated into delicate, dry aggregates whose sedimentation characteristics have not been previously investigated. Here we present the results of laboratory experiments on aggregates of small, dry silicate particles produced by the fragmentation of pumice collected from the 18 May 1980 Mount St. Helens fall deposit. The aggregation process is driven by electrostatic charges naturally imparted to the particles during the fracture process. For particle fall distances of ∼1 m, images of the in‐flight aggregates show that they commonly have irregular shapes and are up to ∼800 μm in size. Strobe photography was used to determine aggregate fall velocities and, by representing aggregates as falling spheres, suggested that they had densities of ∼100–200 kg m−3. Comparable densities were obtained from experiments where equivalent fall velocities were assumed for aggregates and single particles which had been transported similar distances within a horizontal airflow. These dispersal experiments produced bimodal particle size distributions, similar to those observed in the 18 May 1980 Mount St. Helens deposits, and suggest that the aggregates were composed mainly of particles <70 μm in diameter. Our experimental results are in agreement with aggregate size and density estimates previously used within several theoretical plume sedimentation models in order to explain some features of natural ash deposits.
Methods for resolving the origin of large igneous provinces from crustal seismology
Authors: Korenaga Jun , Kelemen Peter B. , Holbrook W. Steven
Published: October 2002

We present a new quantitative framework to understand the process of mantle melting based on the velocity structure of igneous crust. Our approach focuses on the lower crustal section, which is expected to be least affected by porosity and seawater alteration, especially for thick igneous crust. Our methodology is thus best for constraining the origin of large igneous provinces. First, a quantitative relation between bulk crustal velocity and mantle melting parameters is established on the basis of data from mantle melting experiments. Second, we show how lower crustal velocity can be used to place bounds on the expected range of bulk crustal velocity, despite ambiguity in crustal emplacement processes. By modeling fractional crystallization processes at a range of crustal pressures, these bounds are derived as a function of the proportion of lower versus upper crust. Finally, a simple mantle melting model is constructed to illustrate the effects of potential temperature, active upwelling, and a preexisting lithospheric lid on predicted crustal thickness and velocity. As an example, this new interpretation is applied to a seismic transect across the southeast Greenland margin to constrain mantle dynamics during the opening of the North Atlantic. Some complicating factors in seismological inference on mantle melting process, such as the possibilities of subcrustal igneous fractionation and mantle source heterogeneity, are also discussed with this example.
Geoid profile determination by direct integration of GPS inertial navigation system vector gravimetry
Authors: Jekeli Christopher , Kwon Jay H.
Published: October 2002

Analogous to astrogeodetic methods, the geoid undulation may be determined along profiles by integrating the horizontal components of the gravity vector estimated by airborne GPS inertial navigation system. This has the advantage of obviating large regional surveys of the vertical gravity component that are required in the usual boundary value problem approach. Two data sets were tested, one over the Canadian Rocky Mountains and one over the North Atlantic Ocean. Comparisons with the Canadian geoid model and an Arctic geoid model, respectively, showed better than 10 cm (SD) agreement in the relative geoid, with resolution judged to be between 10 and 20 km. The geoid estimation was found to be sensitive to the curvature of the geopotential field in the mountainous region, and, in the ocean area, the estimates clearly revealed the difference between the geoid and the mean sea surface at this resolution and accuracy.
Oceanic age and transient tracers: Analytical and numerical solutions
Authors: Wunsch Carl
Published: October 2002

Transient tracers and the closely related “age” tracers exhibit a rich physical and mathematical structure even for problems of one space dimension. This richness tends to make interpretation of observations, which are inevitably thin in both space and time, difficult, in contrast to the situation in modeling studies. At least six different timescales and corresponding space scales can appear in one‐dimensional problems. In higher dimensions the number of scales increases. Several examples of analytical and numerical solutions are explored for the light they cast on understanding a fluid flow. Boundary Green functions emerge as the fundamental physical/mathematical link between interior tracer distributions and surface and other boundary variations. With transient tracers in inverse calculations one should normally use the underlying tracer distributions to attempt to solve for fundamental fluid properties, such as the mixing coefficients, rather than ambiguous “ventilation” times, which among other problems, may be determined only by the detection threshold and are often mainly functions of the tracer decay constant rather than of fluid properties. Tracers that are transient only through stochastic boundary conditions show that large‐scale space/time patterns can emerge in the tracer field, having little or no clear connection to the underlying fluid flow.
Boundary layer intrusions from a sloping bottom: A mechanism for generating intermediate nepheloid layers
Authors: McPhee‐Shaw Erika E. , Kunze Eric
Published: October 2002

Laboratory experiments were used to investigate the growth of intrusions due to internal‐wave reflection from a sloping boundary. When normalized by the incident energy density flux, the average intrusion spreading velocity was found to be a linear function of the frequency ratio ω/ωc, where ω is the frequency of the incident wave and ωc is the critical frequency, at which the wave characteristic has the same angle as the bottom slope. Evenly spaced layers, indicating thin perturbations in the background density gradient, developed within the mixing region and spread into the tank interior. The vertical spacing of these layers also bore a linear relationship to ω/ωc. A linear model of internal‐wave reflection suggests that these layers may be related to an isopycnal displacement, or overturn, scale. Intrusion growth occurred at a range around the critical frequency and was strongest at slightly supercritical conditions. A balance relating the spreading rate of intrusions to the divergence of energy density flux across the boundary layer is derived. Fitting the laboratory results to this theoretical prediction suggested a weak net buoyancy flux. This balance might be of use in predicting spreading rates of intermediate nepheloid layers generated by internal‐wave mixing at oceanic margins.
Arctic Ocean mixed‐layer eddy generation under leads in sea ice
Authors: Smith David C. , Lavelle J. W. , Fernando H. J. S.
Published: October 2002

Convection in the Arctic Ocean mixed layer occurs under openings in sea ice where new sea ice is formed. The process of brine rejection during ice formation creates negatively buoyant fluid at the surface of ice leads. When the fluid convects downward the ocean mixed layer below the lead is stirred vertically and, through planetary rotational effects, along‐lead flows at the lead edges are induced. These vertical and lateral motions have been previously studied using two‐dimensional numerical ocean models and observed in field programs such as LeadEx. In this paper the generation of Arctic Ocean eddies through dynamic instabilities is investigated using a three‐dimensional, nonhydrostatic ocean model. Results suggest that salt‐enriched vortices can develop at the base of the mixed layer as a consequence of brine rejection, but the vortex formation requires a time period of several days, a time long enough that the progenitor lead would likely have closed. The scale of the eddies is found to depend on the magnitude of the buoyancy forcing, the width of the lead, and the duration of the buoyancy forcing. The number of vortices produced depends on lead width and duration of buoyancy forcing but is less sensitive to the magnitude of the buoyancy forcing. Vortex sizes, scaled appropriately, and vortex numbers are compared with those found in recent laboratory studies of convectively driven flows from line segment sources.
Two‐layer tidal modeling of the Yellow and East China Seas with application to seasonal variability of the M2 tide
Authors: Kang Sok Kuh , Foreman Michael G. G. , Lie Heung‐Jae , Lee Jae‐Hak , Cherniawsky Josef , Yum Ki‐Dai
Published: October 2002

The baroclinic response of tide and tidal currents in the Yellow and East China Seas is investigated using a two‐layer numerical model. Seasonal variability in the M2 tide, especially the smaller summer amplitudes prevailing along the Korea/Tsushima Strait [Kang et al., 1995], is investigated by a series of numerical experiments with varying degrees of stratification specific to winter and summer. Model results show that the summer amplitudes of the M2 tide around the southwestern tip of the Korean peninsula and Korea/Tsusima Strait decrease, with a peak decrease of about 14 cm off the southwestern tip of the Korean peninsula, while the summer amplitudes in other coastal regions tend to increase. This seasonal variability generally coincides with the observations. These models results indicate that seasonal stratification has several noticeable effects on the tides, including varying degrees of current shear, varying frictional dissipation, and varying barotropic energy flux. In particular, it drives complicated seasonal variability in the M2 tide, with a peak amplitude modulation of nearly 5% off the southwestern tip of the Korean peninsula. The seasonal variation of barotropic M2 energy flux through the eastern entrance of the Yellow Sea is thought to induce the corresponding variability in the M2 amplitude in the Korea/Tsusima Strait, with smaller amplitudes found in the summer.
Systematic microstructure variability in double‐diffusively stable coastal waters of nonuniform density gradient
Authors: Duda Timothy F. , Rehmann Chris R.
Published: October 2002

Conductivity microstructure, water velocity, and stratification were measured during a tow‐yo transect near the New England shelf/slope front in early August 1997. Velocity data were collected with an acoustic Doppler profiler on the ship. The other data were collected with a towed platform. Estimates of χ, the rate of dissipation of temperature variance, were computed from the conductivity data with vertical resolution of 0.3 m. Relationships between χ and shear, temperature gradient, buoyancy frequency (N), and gradient Richardson number (Ri) were explored, with special focus on measurements taken in waters stable to double‐diffusive processes (to avoid ambiguity of interpretation) and exhibiting variable density gradient (N ranging from 5 to 40 cph). For this subset of data, χ computed from data grouped into classes of local mean temperature gradient (/dz) was proportional to /dz to the 0.7 power, which is consistent with diapycnal thermal eddy diffusivity K being proportional to (/dz)−1.3 within the framework of the Osborn‐Cox model that relates χ and the mean temperature gradient to the heat flux. No correlation between K and Ri was observed, with Ri computed at 4 m vertical scale, so that systematic inhomogeneous large‐scale forcing is not responsible for a false correlation of K and /dz. Water mass salinity characteristics in the area caused N2 to be proportional to (/dz)4/5, rather than to /dz as in the isohaline case, giving rise to the steep inverse relation K = 10−10N−3.3, with N in radians/s and diffusivity in m2/s. The fit K = 2 × 10−9N−2.5 results if one questionable data ensemble is disregarded. These relations are comparable to results obtained previously from the near‐bottom tow data. They are not intended to be universal formulae but are meant to describe the conditions we encountered. They are not expected to hold at high and low values of N outside of our measurement range. An interpretation is that under these conditions the less strongly stratified (lower N) layers in this shelf area are more prone to instability of the larger‐scale shear than the intervening interfaces, with the subsequent greater energy dissipation in the layers leading to higher buoyancy flux KN2 in the layers than in the interfaces.
Mixing layers and coherent structures in vegetated aquatic flows
Authors: Ghisalberti Marco , Nepf Heidi M.
Published: October 2002

To date, flow through submerged aquatic vegetation has largely been viewed as perturbed boundary layer flow, with vegetative drag treated as an extension of bed drag. However, recent studies of terrestrial canopies demonstrate that the flow structure within and just above an unconfined canopy more strongly resembles a mixing layer than a boundary layer. This paper presents laboratory measurements, obtained from a scaled seagrass model, that demonstrate the applicability of the mixing layer analogy to aquatic systems. Specifically, all vertical profiles of mean velocity contained an inflection point, which makes the flow susceptible to Kelvin‐Helmholtz instability. This instability leads to the generation of large, coherent vortices within the mixing layer (observed in the model at frequencies between 0.01 and 0.11 Hz), which dominate the vertical transport of momentum through the layer. The downstream advection of these vortices is shown to cause the progressive, coherent waving of aquatic vegetation, known as the monami. When the monami is present, the turbulent vertical transport of momentum is enhanced, with turbulent stresses penetrating an additional 30% of the plant height into the canopy.
Formation of the Haida‐1998 oceanic eddy
Authors: Crawford W. R. , Cherniawsky J. Y. , Foreman M. G. G. , Gower J. F. R.
Published: October 2002

Two large, mesoscale, anticyclonic eddies formed along the west coast of the Queen Charlotte Islands of western Canada in early 1998. Altimetry measurements from TOPEX/Poseidon and ERS‐2 satellites suggest that these eddies first appeared near Cape St. James at the southern tip of the islands. The eddies merged in June to form “Haida‐1998,” the highest eddy ever observed in the region. Currents near Cape St. James in winter follow complicated patterns attributed to tidal rectification and pressure‐driven outflow from Hecate Strait. The adjustment of these flows to the bathymetric features likely contributes to the formation of Haida Eddies. Eddies that first appear farther north along the west coast of the Queen Charlotte Islands are set up by other processes, such as baroclinic instability.
Simulation of the Great Bay Estuarine System: Tides with tidal flats wetting and drying
Authors: Ertürk Ş. N. , Bilgili A. , Swift M. R. , Brown W. S. , Çelikkol B. , Ip J. T. C. , Lynch D. R.
Published: October 2002

Realistic lunar tides of the Great Bay Estuarine System have been simulated using a fixed boundary finite element numerical model as described by Ip et al. [1998]. It is a two‐dimensional, nonlinear, time‐stepping model with a groundwater component coupled to a kinematic force balance to facilitate the realistic drainage and filling of elements during ebb and flood, respectively. The numerical model reproduces the observed M2 tides as described by Swift and Brown [1983], and it successfully captures qualitatively correct residual currents and transports, realistic mass‐conserving flooding and dewatering of the tidal flats, and current asymmetry between flood and ebb. The simulation results were sensitive to local bathymetry and the implemented friction law. The accuracy of the model is demonstrated by comparison with the 1975 Great Bay study [Swift and Brown, 1983] in terms of tidal elevations at 14 tidal stations and 4 cross‐sectionally averaged current measurements in the estuary. Quantitatively, the model results show good agreement with observations, displaying correlation coefficients of ≥0.96 in surface elevation and ≥0.95 in averaged current, with average RMS errors of 0.12 m and 0.26 m s−1, respectively. In addition, tidal flat hydrodynamics, characteristic distributions of residual current, sediment bed load transport, and influence of topography on the overall circulation in the region are also discussed.
Global ocean circulation during 1992–1997, estimated from ocean observations and a general circulation model
Authors: Stammer D. , Wunsch C. , Giering R. , Eckert C. , Heimbach P. , Marotzke J. , Adcroft A. , Hill C. N. , Marshall J.
Published: October 2002

A three‐dimensional oceanic state is estimated for the period 1992–1997 as it results from combining large‐scale ocean data sets with a general circulation model. At the cost of increased computational load, the estimation (assimilation) method is chosen specifically so that the resulting state estimate is consistent with the model equations, having no artificial sources or sinks. To bring the model into close agreement with observations, its initial temperature and salinity conditions are permitted to change, as are the time‐dependent surface fluxes of momentum, heat and freshwater. Resulting changes of these “control vectors” are largely consistent with accepted uncertainties in the hydrographic climatology and meteorological analyses. The assimilation procedure is able to correct for many of the traditional shortcomings of the flow field by changing the surface boundary conditions. Changes in the resulting flow field are predominantly on the gyre scale and affect many features that are often poorly simulated in traditional numerical simulations, such as the strengths of the Gulf Stream and its extension, the Azores Current and the anticyclonic circulation associated with the Labrador Sea. Tests of the results and their consistency with prior error assumptions show that the constrained model has moved considerably closer to the observations imposed as constraints, but has also moved closer to independent data from the World Ocean Circulation Experiment not used in the assimilation procedure. In some regions where the comparisons remain indeterminate, not enough ocean observations are available, and it is difficult to ascribe the residuals to either the model or the observations. Although problems remain, a useful first solution to the global time‐dependent ocean state estimation problem has been found. The estimates will continue to improve through the evolution of numerical models, computer power increases, more data, and more efficient estimation methods.
Radiatively driven convection in ice‐covered lakes: Observations, scaling, and a mixed layer model
Authors: Mironov Dmitrii , Terzhevik Arkady , Kirillin Georgy , Jonas Tobias , Malm Joakim , Farmer David
Published: October 2002

Penetrative convection is discussed where the instability is driven by radiative heating of water below the temperature of maximum density. Convection of this type occurs in ice‐covered freshwater lakes in late spring, when the snow cover vanishes and solar radiation is absorbed beneath the ice cover. The vertical temperature structure, bulk mixed layer scaling, and mixed layer deepening are examined for a number of temperate and polar lakes. A bulk mixed layer scaling for this type of convection is based on energy arguments underlying the classical Deardorff convective scaling. The depth of the convective layer serves as an appropriate length scale. However, a modified scale that takes account of the energetics of a distributed radiation source term replaces the surface buoyancy flux velocity scale used by Deardorff. The scaling compares favorably with large‐eddy simulations of turbulence kinetic energy (TKE) and with both observations and large‐eddy simulations of the TKE dissipation rate. Mixed layer deepening is simulated with a model of convection beneath lake ice. The model describes the structure of the stably stratified layer just beneath the ice with a stationary solution to the heat transfer equation; the structure of the entrainment layer is parameterized with a zero‐order jump approach. The entrainment equation is derived from the mixed layer TKE budget and bulk mixed layer scaling. Entrainment regimes characteristic of convection beneath ice are analyzed. It is shown that if the Deardorff convective velocity scale is replaced with a scale incorporating the distributed buoyancy flux, the entrainment equation describing atmospheric and oceanic convective boundary layers also applies beneath the ice. Model predictions compare well with data from observations in a number of lakes. We propose and compare with observations an extension of the mixed layer model that allows for the inclusion of salinity. Although the salt concentration is low in most temperate and polar lakes, its dynamical effect can be significant close to the temperature of maximum density.
Medium‐term bar behavior and associated sediment transport at Hasaki, Japan
Authors: Kuriyama Y.
Published: October 2002

The medium‐term movement of a longshore bar and the associated cross‐shore sediment transport were investigated using beach profile data obtained nearly daily for 8 years from the seaward foot of a dune to a water depth of about 5 m at Hazaki Oceanographical Research Station (HORS). Bar crests were found to move seaward repeatedly with a period of a year. The duration time of the seaward bar migration is shorter than those on the other coasts in the United States, the Netherlands, and New Zealand. Although bars migrated in one direction, seaward, the cross‐shore sediment transport rate associated with the bar migrations fluctuated seaward and shoreward. Seaward sediment transport occurred on and around bar crests, whereas shoreward sediment transport occurred in trough regions. The cross‐shore sediment transport was active when the offshore wave energy was large, from winter to spring and in autumn. However, the direction of the sediment transport in a region where bars were most developed was different during the two periods; it was seaward from winter to spring and shoreward in autumn. A numerical model for beach profile change confirmed that the development and decay of a bar was caused by the spatial and temporal variations of the cross‐shore sediment transport rate.
Wind stress forcing of the Oregon coastal ocean during the 1999 upwelling season
Authors: Samelson R. , Barbour P. , Barth J. , Bielli S. , Boyd T. , Chelton D. , Kosro P. , Levine M. , Skyllingstad E. , Wilczak J.
Published: October 2002

The wind stress forcing of the Oregon coastal ocean during June–August 1999 is estimated from a regional mesoscale atmospheric model and from satellite scatterometer observations, supplemented by moored and coastal surface data and by a land‐based wind profiler. Both the mean and variable components of model alongshore wind stress increase by factors of 3–4 from north to south along the Oregon coast. There is evidence of orographic intensification near Cape Blanco, which is supported by previous aircraft and ship observations during August 1995. The systematic southward increase of southward stress will drive enhanced ocean upwelling along the southern Oregon coast and suggests the new hypothesis that systematic variations in local wind stress may contribute to the observed offshore displacement of the coastal upwelling jet in this region. It is inferred from an analysis of the model and surface data that ocean upwelling modifies coastal surface air temperatures by 1–5°C over timescales of 12–24 hours.
Toward an optimal inversion method for synthetic aperture radar wind retrieval
Authors: Portabella M. , Stoffelen A. , Johannessen J. A.
Published: October 2002

In recent years, particular efforts have been made to derive wind fields over the oceans from synthetic aperture radar (SAR) images. In contrast with the scatterometer, the SAR has a higher spatial resolution and therefore has the potential to provide higher resolution wind information. Since there are at least two geophysical parameters (wind speed and wind direction) modulating the single SAR backscatter measurements, the inversion of wind fields from SAR observations has an inherent problem of underdetermination. Moreover, this modulation is highly nonlinear, further complicating the inversion. Lorenc [1986] presented a general statistical approach to solve inversion problems (including underdetermined problems) in meteorological analysis. We propose a SAR wind retrieval method based on this general approach. This simplified method combines the SAR information with some background information coming from high‐resolution limited area model to retrieve the most probable wind vector, assuming that all sources of information contain errors and that these are well characterized. We then evaluate two different SAR wind retrieval methods. The first one is commonly used by the SAR community and is based on a combination of a wind streak detection algorithm (wind direction retrieval) and a C band model inversion (wind speed retrieval). The second one is the new method we propose, based on the general statistical approach. We show the potential problems and limitations of using any of these methods and show how the second method can potentially contribute to a significant improvement in SAR wind retrieval. The new method prepares the ground for the assimilation of SAR data in high‐resolution numerical weather prediction models.
A freshwater jet on the east Greenland shelf
Authors: Bacon Sheldon , Reverdin Gilles , Rigor Ignatius G. , Snaith Helen M.
Published: October 2002

In August 1997, RRS Discovery cruise 230 (World Ocean Circulation Experiment (WOCE) section A25) ran a hydrographic section into Cape Farewell on the southern tip of Greenland. The closest approach to the shore was 2 nm in a water depth of 160 m over the east Greenland shelf. Analysis of the hydrographic data (conductivity‐temperature‐depth (CTD), vessel‐mounted acoustic Doppler current profiler, and thermosalinograph) has revealed a current flowing southwestward, ∼15 km wide, 100 m deep, and centered ∼10 km offshore. We believe it to be driven by meltwater runoff from Greenland. This feature, which we call the East Greenland Coastal Current (EGCC), carries a little less than 1 Sv (106 m3 s−1) with peak current speeds of ∼1 m s−1 at the surface. The center of the EGCC lies on a salinity front with maximum salinity contrast ∼4 practical salinity units (psu) between coast and shelf break and between surface and bottom. A spot value of freshwater transport is 0.06 Sv (1800 km3 yr−1), which is equivalent to ∼30% of the Arctic freshwater gain. The presence of the EGCC and its continuity up the east Greenland coast as far as Denmark Strait is confirmed in satellite sea surface temperature images and surface drifter tracks. We estimate the sensitivity of its freshwater flux to changes in melt season mean surface air temperature to be >25% per 1°C.
Experimental study of nearshore dynamics on a barred beach with rip channels
Authors: Haller Merrick C. , Dalrymple Robert A. , Svendsen Ib A.
Published: October 2002

Wave and current measurements are presented from a set of laboratory experiments performed on a fixed barred beach with periodically spaced rip channels using a range of incident wave conditions. The data demonstrate that the presence of gaps in otherwise longshore uniform bars dominates the nearshore circulation system for the incident wave conditions considered. For example, nonzero cross‐shore flow and the presence of longshore pressure gradients, both resulting from the presence of rip channels, are not restricted to the immediate vicinity of the channels but instead are found to span almost the entire length of the longshore bars. In addition, the combination of breaker type and location is the dominant driving mechanism of the nearshore flow, and both are found to be strongly influenced by the variable bathymetry and the presence of a strong rip current. The depth‐averaged currents are calculated from the measured velocities assuming conservation of mass across the measurement grid. The terms in both the cross‐shore and longshore momentum balances are calculated, and their relative magnitudes are quantified. The cross‐shore balance is shown to be dominated by the cross‐shore pressure and radiation stress gradients in general agreement with previous results, however, the rip current is shown to influence the wave breaking and the wave‐induced setup in the rip channel. Analysis of the longshore balance shows that the overall strength of the longshore feeder currents is governed by a balance between longshore pressure gradient forcing and an opposing radiation stress gradient. In addition, the longshore feeder currents show maxima in the bar trough region, providing experimental evidence that longshore pressure gradients can shift longshore current maxima shoreward from the bar crest. Finally, since the longshore radiation stress gradient in the surf zone is governed by the amount of wave dissipation on the rip current, there exists a positive feedback mechanism whereby a strong rip current can weaken the radiation stress gradient opposing the feeder currents and lead to even stronger feeder currents and rips.