bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of upper mantle discontinuities near Northwestern Pacific Subduction Zones using precursors to sSH
Authors: Zhang Zhi , Lay Thorne
Published: February 2008

Teleseismic long‐period World‐Wide Standard Seismograph Network (WWSSN) tangential component recordings of deep and intermediate depth earthquakes are analyzed for the presence of sS precursors, denoted sxS, signifying underside reflections from discontinuities at depth x below the sS reflection point above the source. These sS precursors can be used to place constraints on upper mantle discontinuities in the vicinity of subduction zones. The clearest precursor is usually the smS phase, the underside reflection from the Moho, which is observed for a number of events in the northwestern Pacific for paths reflecting under the Sea of Okhotsk and North Korea. The amplitude and timing of smS relative to sS are modeled using synthetic seismograms to determine the shear wave impedance contrast at the Moho and the crustal thickness in these regions. The results are compared with previous work on pP precursors reflected from the Moho for similar paths, with consistent crustal thickness being found. smS is strong for continental reflection points under North Korea, which has laterally varying 31 to 36 km crustal thickness and 31.2% to 18% shear wave impedance contrast at the Moho. In the northern Sea of Okhotsk region the crustal thickness is around 21 to 29 km with 26% to 28% shear wave impedance contrast at the Moho. Stacked smS waveforms for the Sea of Okhotsk vary from strong, isolated arrivals in the north to weak, poorly defined precursors toward the south, reflecting a transition from continental to oceanic crust. smS is not clearly separated from sS for oceanic paths under the Izu and Japan regions, and our procedure cannot resolve oceanic Moho properties unless broadband data are used. Other precursors, sxS, are much weaker than smS and are difficult to identify in individual waveforms. We use slant stacking to enhance the signal to noise and search for precursors with various slownesses, using recordings from 13 deep events. Four SH wave reflectors may exist below the Moho in our study area. The shallowest is the “80‐km” discontinuity, which varies in depth laterally, being near 80 to 85 km below the Sea of Okhotsk, near 66 km beneath Izu Japan, and near 90 km under North Korea. A reflector near 200 to 210 km depth is indicated by data from Izu Japan, but there is no evidence for any shear wave impedance contrast around this depth under North Korea or the Sea of Okhotsk. Weak evidence is found for a “330‐km” discontinuity in these regions, with the depth of a few percent impedance contrast varying from 325 to 335 km. Relatively strong and consistent arrivals indicate the presence of a “400‐km” discontinuity, with the depth varying from 380 to 400 km, perhaps indicating slight elevation of the olivine→ β phase transformation near the slabs.
Shallow structure of oceanic crust in the western North Atlantic from seismic waveform inversion and modeling
Authors: Minshull Timothy A. , Singh Satish C.
Published: February 2008

Seismic reflection data from Mesozoic oceanic crust in the western North Atlantic, in the vicinity of the Blake Spur Fracture Zone, have imaged a number of short, subhorizontal events in the upper 1 km of the crystalline crust. Wide‐angle expanding spread profile (ESP) data from the same region indicate the presence of first‐order velocity discontinuities in this depth interval. Using ESP data reprocessed to enhance precritical reflection events, we investigate the relationship between the precritical and postcritical reflections with three different approaches: amplitude analysis, forward modeling, and waveform inversion. The precritical reflections have high apparent reflection coefficients (∼ 0.1) and may be divided into two types: those which can be traced out to critical ranges (5–6 km), representing velocity steps, and those which fade out at ranges of 3–4 km, representing shorter‐wavelength velocity variations. Synthetic seismogram modeling indicates that the velocity steps occur within a 50 m depth interval, while the shorter‐wavelength features may correspond to high‐ or low‐velocity zones with a thickness of 20 m (10% of the dominant seismic wavelength) or less. We investigate the velocity structure in detail by full waveform inversion of a selected portion of ESP data, transformed into the intercept time‐slowness domain. The inversion method minimizes the sample‐by‐sample misfit between the data and reflectivity synthetics in a least squares sense. A conjugate gradient algorithm was used, starting from a velocity structure derived from forward modeling of wide‐angle ESP data. The inversion tightly constrains short‐wavelength components of the velocity structure, and indicates that intracrustal reflections result both from major velocity steps and from zones of alternating high and low velocity. Synthetic seismograms generated from sonic log data from Deep Sea Drilling Project hole 418A show reflections of similar amplitude and character to those seen in the ESP data. These reflections probably originate from rather subtle features of the velocity‐depth profile, related primarily to changes in porosity in the pillow lava sequence. A deeper reflector, 800 ms below oceanic basement in the Blake Spur Fracture Zone, may correspond to the top of a zone of serpentinized upper mantle.
Some observations on the effects of shear stress on a polymorphic transformation in perovskite‐structured lead‐zirconate‐titanate ceramic
Authors: Zeuch David H. , Montgomery Stephen T. , Keck Jeffrey D.
Published: February 2008

We performed a series of hydrostatic and constant‐stress‐difference (CSD) experiments at room temperature on modified lead‐zirconate‐titanate (PZT 95/5‐2Nb) ceramic in order to quantify the influence of shear stress on the displacive, and possibly martensitic, first‐order, ferroelectric/rhombohedral → antiferroelectric/orthorhombic phase transformation. In hydrostatic compression, the transformation began at approximately 260 MPa and was incompletely reversed upon return to ambient conditions. Strains associated with the transformation were isotropic, both on the first and subsequent hydrostatic cycles. Results for the CSD tests were quite different. First, the confining pressure and mean stress at which the transition begins decreased approximately linearly with increasing stress difference. Second, we observed that the rate of transformation apparently decreased with increasing shear stress and the accompanying purely elastic shear strain. This result contrasts with the almost universal assertion that shear stresses accelerate reaction and transformation kinetics. Finally, strain was not isotropic during the transformation: axial strains were greater and lateral strains smaller than for the hydrostatic case, though volumetric strain behavior was comparable for the two types of tests. However, this last effect does not appear to be an example of transformational plasticity but, rather, a “one‐time” occurrence: no additional unexpected strains accumulated during subsequent cycles through the transition under nonhydrostatic loading. If subsequent hydrostatic cycles were performed on samples previously run under CSD conditions, strain anisotropy was again observed, indicating that the earlier superimposed shear stress produced a permanent mechanical anisotropy in the material. The mechanical anisotropy probably results from a crystallographic preferred orientation that developed during the transformation under shear stress.
Upper mantle Q structure beneath the East Pacific Rise
Authors: Ding Xiao‐Yang , Grand Stephen P.
Published: February 2008

A model of Qβ as a function of depth has been determined for the upper mantle beneath the East Pacific Rise. The model is appropriate for shear waves with periods from roughly 15 to 30 s. The amplitudes of multibounce SH phases which propagate within the upper mantle were used to constrain the Q structure. The multibounce phase amplitudes were measured with respect to the S wave amplitude for each seismogram. The data consist of S, SS, SSS, and SSSS waves for distances from 30° to 80°. The sources are earthquakes which occurred on transform faults associated with the East Pacific Rise and the seismograms were recorded by World‐Wide Standard Seismograph Network and Canadian Seismic Network stations in western North America. Mantle beneath oceanic crust less than 15 Ma and tectonically active continental crust is sampled. Since the S wave data primarily sample the deep mantle, the resulting upper mantle Q model depends on the Q structure assumed for the lower mantle. We assumed a constant Q of 312 for the mantle beneath 650 km depth. With this assumption, the upper mantle phase amplitudes require a Q of about 70 in the upper 150 km of the mantle. From 150 to 400 km depth, Q increases to 180. The Q in the transition zone, from 400 to 650 km depth, was found to be 180, significantly lower than the lower mantle Q. The resulting Q model is consistent with several surface wave studies of mantle attenuation beneath the Pacific. However, our results predict a QScS (210) which is significantly higher than many direct measurements of ScS attenuation indicate. Furthermore, the large range in measured QScS is inconsistent with lateral Q variations being confined to the upper mantle if the region we have studied represents the low Q extreme of upper mantle. We conclude that large lateral variations in Q exist in the lower mantle.
Deformation from 1973 to 1987 in the epicentral area of the 1959 Hebgen Lake, Montana, earthquake (Ms = 7.5)
Authors: Savage J. C. , Lisowski M. , Prescott W. H. , Pitt A. M.
Published: February 2008

A 40‐km aperture trilateration network centered on the 1959 Hebgen Lake earthquake epicenter has been surveyed in 1973, 1974, 1976, 1978, 1981, 1984, and 1987. The deformation inferred from those surveys is described roughly by a uniaxial, 0.266 ± 0.014 μstrain/yr, N15°E ± 1°extension that is uniform in both time and space. That extension is orthogonal to the strike (N78°W ± 5°) of the 1959 rupture plane inferred from focal mechanism solutions. The absence of strain accumulation in the N75°W direction suggests that the source of deformation must be greatly elongated in that direction. Looked at in greater detail, the deformation is found to be concentrated in the northern part of the network. The zone of concentrated deformation coincides with a part of a well‐defined trend in seismicity that extends at least 100 km N75°W from the Sour Creek resurgent dome in the Yellowstone caldera. Extension perpendicular to that trend is indicated by focal mechanism solutions. Thus, the zone of rifting identified within the Hebgen Lake network apparently extends east southeast to the Yellowstone caldera. About 8 mm/yr extension is observed across a 20‐km width within the zone.
Seismic events with implosional focal mechanisms in the Coeur d'Alene Mining District, northern Idaho
Authors: Stickney Michael C. , Sprenke Kenneth F.
Published: February 2008

Rockbursts are a major hazard in the Coeur d'Alene mining district of northern Idaho. To study the source mechanisms of mining‐related seismicity, we operated 15 portable seismographs in the Coeur d'Alene mining district for one week in September 1990. We found the velocity model of the district to consist of a halfspace with a P wave velocity of 5.74 km/s and an S wave velocity of 3.47 km/s. In‐mine seismic monitoring systems provided excellent control on the velocity model and most hypocenter locations. Twenty‐one events, with sufficient focal sphere coverage for fault plane solutions, were recorded. Eleven of the events produced all‐dilatational first motions; for all events, 90% of first motions observed were dilatational. Ten events produced unsolvable first‐motion data in terms of the usual double‐couple mechanism, but seven of these unsolvable events were found consistent with an implosional source and two others were consistent with a shear‐implosional source. Our results provide firm observational evidence that dilatational first motions are predominant in this deep metal‐mining district, probably the result of implosional and shear‐implosional source mechanisms.
A high‐resolution seismic reflection and gravity survey of Quaternary deformation across the Wasatch Fault, Utah
Authors: Stephenson W. J. , Smith R. B. , Pelton J. R.
Published: February 2008

High‐resolution reflection seismic and detailed gravity data were acquired across an exposure of the Wasatch fault, Utah, near a trench excavated for dating of Quaternary fault displacement. The seismic data across the Quaternary Wasatch fault are interpreted to show three subparallel surfaces dipping from 70°W at the surface to an estimated 45°W at 40 m, displaced unconsolidated sediments, and colluvial material abutting the main fault. Detailed gravity data were of assistance in mapping the sediment‐bedrock interface from the rangefront across the Wasatch Fault Zone. A combined high‐resolution seismic and gravity interpretation reveals that bedrock is 80 m deeper in the hanging wall than in the footwall, suggesting that displacement has occurred on several en echelon blocks in the near‐surface of the Wasatch fault. Movement on the most recent Wasatch fault trace began prior to deposition of Pleistocene lacustrine sediments. The twelvefold, high‐resolution reflection seismic data were recorded and processed to achieve vertical resolution of approximately 1 m and horizontal resolution (Fresnel zone radii) of as good as 5 m on the deepest reflecting interfaces at 40 m. Reasonably high frequencies and good bandwidth (about 80–300 Hz) in the stacked seismic data permitted a direct comparison to the trenched cross section. Good correlation between the trench stratigraphy and structure and the seismic data suggests high‐resolution seismic data can be a valuable tool for analyzing near‐surface faulting in unconsolidated sediment and for locating potential trench sites.
A model function of the global bomb tritium distribution in precipitation, 1960–1986
Authors: Doney Scott C. , Glover David M. , Jenkins William J.
Published: February 2008

We have developed, using the World Meteorological Organization/International Atomic Energy Agency (WMO/IAEA) tritium data set, a global model function for predicting the annual mean concentration of decay‐corrected bomb tritium (TU81N) in precipitation over the time period 1960–1986. The model consists of two reference time histories or factors, calculated from a factor analysis of the zonally averaged global data set, and global maps of the two spatial coefficents associated with the factors. By combining the reference curves with the appropriate coefficent values taken from these maps, an estimate of the tritium time history for particular locations can be produced. The predicted error for the integrated tritium concentration from the annual model is approximately 3–10% and, in a comparison with previous work (Weiss and Roether, 1980), provides a statistically better fit of the tritium data at most stations. We also discuss the seasonal cycle of tritium in precipitation and present estimates for the amplitude and phase for tritium using a simple seasonal model. At most of the WMO/IAEA monitoring stations, our four‐parameter seasonal model accounts for greater than 80% of the variance in the monthly observations.
Measurements of the internal wave wake of a ship in a highly stratified sea loch
Authors: Watson G. , Chapman R. D. , Apel J. R.
Published: February 2008

An internal wave experiment was conducted in Loch Linnhe, Scotland, in which three ships were used to generate internal wave wakes. Synthetic aperture radar images of the wakes confirmed the applicability of the kinematic theory of Keller and Munk for the geometry of the wave pattern. The waves were measured in situ using three‐axis current meters and other instrumentation. The waves were quite well represented by linear propagation theory, although the dependence of amplitude on depth did not agree precisely. The time‐varying frequency of the waves was determined from the data in two different ways, including a Hubert transform method, and agreed well with that predicted by the theory. The subsurface data were used to estimate the surface current and its gradient (the strain rate). Any dependence of wave amplitude on ship speed was weak enough to be obscured by variations due to changes in the stratification. Amplitude estimates of waves from the 1400‐ and 5300‐tonne ships were similar. Those from the 29,000‐tonne ship were 2–3 times greater.
A description of water types on the Mackenzie Shelf of the Beaufort Sea during winter
Authors: Moore R. M. , Melling H. , Thompson K. R.
Published: February 2008

For a number of years during the 1980s, observations of the physical and chemical properties of seawater in the southeastern Beaufort Sea have been acquired in late winter. The most complete data set, from 1987, has been used in a comparison of winter and summer [Macdonald et al., 1989] water properties in the area. Most obvious is an increase in the salinity of surface waters in winter. The magnitude of this increase varies dramatically from year to year. Part of the increase is a consequence of brine rejection during the growth of sea ice, and part is associated with an intrusion over the shelf of a water mass of high nutrient and low oxygen concentrations which is a feature of the entire western Arctic Ocean. Principal component analysis was used to allow all five chemical tracers to be combined and viewed simultaneously. The properties of the upper 120 m are found to lie, to a close approximation, on a plane. This leads us to a simple interpretation based on a three‐component mixing model involving river runoff, water from the nutrient maximum, and an offshore near‐surface component. It is shown that the best fit plane occupied by arctic surface waters in the Beaufort Sea closely matches that defined by the influences of river inflow, of the freeze‐melt cycle, and of photosynthesis and respiration. However, the effects of freezing/melting and of river inflow cannot be clearly distinguished using the chosen suite of tracers. It has been determined that if the waters of the upper 250 m are to be represented in the same manner, a fourth end member is required.
Acoustical measurements of air bubbles bursting at a water surface: Bursting bubbles as Helmholtz resonators
Authors: Spiel Donald E.
Published: February 2008

Evidence is presented that air bubbles bursting at the surface of water are Helmholtz resonators. The detection by an acoustic transducer of the aerosols generated by the collapsing bubble is also demonstrated. The use of this detector to measure such parameters as bubble collapse time, the number and spacing of the jet drops, their ejection velocities, and their size is discussed.
Assimilation of altimeter data in a global third‐generation wave model
Authors: Lionello P. , Günther H. , Janssen P. A. E. M.
Published: February 2008

We investigated the effect of the assimilation of altimeter satellite data in the third‐generation ocean wave model WAM. We used a sequential method, where analyzed significant wave height fields are created by optimum interpolation, and the analyzed values are then used to construct the analyzed wave spectrum. The method provides also an estimate of the surface stress showing the possibility of using the analysis of the wave spectrum to derive an analyzed surface stress field. In a first set of numerical experiments, the data, provided by the Seasat altimeter, have been assimilated in the WAM model for 1½ days. The comparison between model results and satellite data during the continuation of the run shows a positive and persistent impact of the assimilation. In a second set of numerical experiments, Geosat altimeter data were assimilated for 10 days and the resulting analysis was compared with buoy data. Although the assimilation improves the model results, it is not capable of compensating the differences between model and buoys. Some failures are clearly derived from the absence in the satellite data of the high‐wave events that were reported by the buoys. Other failures may be the consequence of an excessive swell attenuation in the WAM model, which compromises the effect of a previous correction. In fact, the comparison of WAM model results with altimeter data suggests that there is a tendency of the model to overevaluate initially the wind sea, and successively to overestimate the decay of the wave energy, when the waves leave the area of the storm.
The global structure of the annual and semiannual sea surface height variability from Geosat altimeter data
Authors: Jacobs Gregg A. , Born George H. , Parke Mike E. , Allen Patrick C.
Published: February 2008

The global structure of the annual and semiannual sea surface height variability is constructed from the first 2 years of the Geosat Exact Repeat Mission (ERM) altimeter data. The GEM‐T2 orbits available for the first 2 years of the ERM have an accuracy of 40 cm RMS. Residual orbit error is modeled as one cycle per orbital revolution and is removed from collinear differences of arcs made of data from one full orbital revolution of the satellite. The error in the Schwiderski M2 tidal model must be estimated due to the fact that the tidal variability aliases to 1.15 cycles per year (cpy), and this frequency is not separable from 1 cpy with the 2 years of data. An estimate of the M2 tidal error is made based on the particular temporal and spatial aliasing of the tide. With this error removed, a least squares fit of sine and cosine waves with annual and semiannual frequencies is made to the time series at every point along the ground track of the satellite. This produces sine and cosine coefficients at the ground track points which are interpolated to a regularly spaced ½° grid over the globe. From the sine and cosine maps, amplitude and phase may be obtained. Interpolation of the sine and cosine coefficients using different spatial scales is done to better observe the large‐scale phase changes and to remove small‐scale noise. Results show the phase relationships between major current systems, large‐scale variations near the equator in the Intertropical Convergence Zone (ITCZ), a 180° phase difference between the northern and southern hemispheres for the annual variability, large‐scale westward propagating waves, and other large‐scale gyre features.
Modeling the effects of buoyancy on the evolution of geophysical boundary layers
Authors: Baum Eric , Caponi Enrique A.
Published: February 2008

A simple modeling based on the Imperial College k‐ε turbulence description is shown to be capable of describing the highly anisotropic phenomena associated with the strong buoyancy effects encountered in upper ocean (and atmospheric boundary layer) environments. Both diurnal stratification and transition to buoyant mixing (from that driven by wind shear) are described in considerable detail, provided that the modeling of the turbulent Prandtl number and of the buoyant generation term in the ε model equation are evaluated in a suitably realistic and consistent way. The resulting model equations are suitable for simulating the evolution of the ensemble‐averaged properties of the oceanic mixed layer in response to a specified history of interfacial shear stress and heat flux. The model is tested by comparing the predicted mixed layer evolution with the dissipation rate measurements of Shay and Gregg.
The origin of green icebergs in Antarctica
Authors: Kipfstuhl J. , Dieckmann G. , Oerter H. , Hellmer H. , Graf W.
Published: February 2008

A comparison of samples from a translucent green iceberg with a core from the Ronne Ice Shelf revealed an excellent agreement in isotopic composition, crystal structure, and incorporated sediment particles. Marine shelf ice which constitutes the basal portion of some ice shelves is considered to be the source of green icebergs. It most likely results from “ice pump” processes which produce large amounts of ice platelets in the water column beneath ice shelves. These subsequently accumulate and become compacted into bubble‐free, desalinated ice. Iceberg and drift‐buoy trajectories indicate that green icebergs observed in the Weddell Sea originate from the Amery Ice Shelf rather than from the Ronne Ice Shelf, although this ice shelf is also a potential source.
The distribution of aluminium in the Greenland Sea and its relationship to ventilation processes
Authors: Measures C. I. , Edmond J. M.
Published: February 2008

During two cruises to the Greenland Sea in 1988 and 1989 a total of 690 on‐board aluminium (Al) determinations were performed on samples collected from profiles located along four transects in the Greenland Sea, as part of the Greenland Sea Project. Al concentrations range from extremely low values (0.4 nM) in the Polar waters of the East Greenland Current draining the Arctic Ocean, to 17.9 nM in the newly formed Norwegian Sea Deep Water (NSDW) at the periphery of the Greenland gyre. The Al content in the component water masses serves to constrain the make‐up of these water masses from the various mixing products of the region. The enrichment of Al in the NSDW both at its site of formation within the Greenland Sea (up to 17.9 nM) and in the Lofoten Basin of the Norwegian Sea (up to 40.3 nM) leads to the conclusion that there must be an additional, external, source of Al to these water masses. The apparent inability of water column or sedimentary diagenetic processes to provide this enrichment and the observation of enriched Al values associated with a high‐salinity shelf brine leads us to the conclusion that these brines may be responsible for the observed enrichments. The high concentration of Al in the Greenland Sea Deep Water compared with the water masses that produce it, suggests that Al may be acting as an inverse tracer of the processes that ventilate this basin by convective overturn, i.e. the Al concentration which increases with time, as a result of continuous brine or other external inputs, is reduced during convective events. It is also argued that this flushing of elevated Al water during convective events may have a similar counterpart in the Iceland Sea and that this mechanism could go some way toward solving the Al deficit that exists between the enriched North Atlantic Deep Water and its depleted precursor water masses observed in the intermediate waters of the Greenland and Iceland seas.
Aggregation of fine particles at the sediment‐water interface
Authors: Stolzenbach Keith D. , Newman Kathleen A. , Wong Charles S.
Published: February 2008

The presence of a bottom sediment layer agitated by mechanical stirring or by resident organisms (tubificid oligochaetes) significantly increases the rate at which fine (1 μm) cohesive particles are removed from suspension in laboratory columns. Measured rates of particle removal are equivalent to deposition velocities ranging from 0.23 m day−1 to 0.41 m day−1. These rates are an order of magnitude faster than deposition by gravitational settling or coagulation with larger particles in the water column as observed in experimental controls. It is hypothesized that the increased removal rate is the result of aggregation in a sediment layer at the bed‐water interface characterized by loosely bound (fluffy), porous material hydrodynamically coupled to the water column. According to this hypothesis particle removal occurs when motion of the overlying water or organism activity causes suspended fine particles to collide with and stick to the interfacial sediment. This new hypothesis is supported by the mass and size distribution of tracer particles recovered in cores and sediment traps at a coastal site and by theoretical estimates of interfacial aggregation rates.
Excess 210Po and nutrient recycling within the California Coastal Transition Zone
Authors: Kadko David
Published: February 2008

Profiles of 210Pb, 210Po, and nutrients were measured within a cold, chlorophyll‐rich filament observed in satellite images off Point Arena, California. The most striking observation was the large excess of 210Po in the surface water of near‐shore filament stations. This excess 210Po, typically associated with the breakdown of particles in the upper thermocline, is indicative of the transport of recently upwelled water seaward within the filament. Its large inventory and association with NO2 within the filament surface is consistent with the filament being part of a continuous, meandering, southward flowing jet within the California Current. Nutrients and biomass are “pumped” along the meander path by the interaction of biological processes (production, grazing) and vertical motion (upwelling, downwelling). Products of particle breakdown, such as NO2 and 210Po, are continuously recycled and accumulate along the meander path. The implication is that within the coastal transition zone, some portion of the nutrients incorporated into new production has also been regenerated along the meandering flow. The excess 210Po is removed from the chlorophyll‐rich filament at a rate which is approximately an order of magnitude greater than 210Po removal from open ocean surface water. This suggests that the scavenging rate of 210Po, like that of 234Th, varies with the rate of primary production.
Trajectories in Gulf Stream meanders
Authors: Cushman‐Roisin Benoit
Published: February 2008

A simple model based on conservation of stream function and potential vorticity is used to derive theoretical estimates for the horizontal and vertical displacements of fluid particles in Gulf Stream meanders. These displacements are given as functions of the meander's curvature and meridional displacement. The theory also provides an estimate of the translation speed of the meander. Finally, comparisons with RAFOS float trajectories and observational estimates of the components of the potential‐vorticity budget validate the theory.
Short‐period fluctuations in meander of the Kuroshio's path off Cape Shiono Misaki
Authors: Kimura Shingo , Sugimoto Takashige
Published: February 2008

Direct current measurements in the strong stream layer of the Kuroshio were carried out to investigate short‐period fluctuations of the small‐scale meander of the Kuroshio at two mooring stations off Cape Shiono Misaki in 1985–1986. The Kuroshio had a stable nonlarge meander path and flowed eastward parallel to the mooring stations during a large part of the observations, which were appropriate conditions for temporal analysis. Temperature and velocity variations reveal the existence of dominant periodic fluctuations in the small‐scale meander of the Kuroshio's path and its frontal disturbances. A fluctuation with a period of 17–19 days and 400‐km wavelength is found in association with small‐scale meander of the Kuroshio's path. Periodic fluctuations of 5–8 days and 10–12 days are also recognized and have 100‐km and 200‐km wavelength, respectively. Satellite imagery shows that the two periodic fluctuations are dominant in frontal regions as plumes from the north wall of the Kuroshio. The fluctuations with the three periodic bands propagating downstream at about 20 cm s−1 and transitions of wavelike disturbances of the Kuroshio front observed by satellite suggest that all of the propagations are subject to the barotropic component of the eastward velocity. The temporal and spatial characteristics of the Kuroshio meander show good correspondence with other observations in upstream regions of the Kuroshio. The phase speed of wavelike perturbations well recognized in the Gulf Stream region is approximately twice as large as that of Kuroshio perturbations. The difference seems to be caused by differences of water depth beneath the two streams.
Optical absorption spectra of waters from the Orinoco River outflow: Terrestrial input of colored organic matter to the Caribbean
Authors: Blough N. V. , Zafiriou O. C. , Bonilla J.
Published: February 2008

An extensive series of optical absorption spectra were recorded for waters in the eastern Caribbean Sea, the Gulf of Paria, and the Orinoco River Estuary during the high‐flow period of the Orinoco River in the Fall of 1988. Evidence for high levels of dissolved, colored organic matter (COM) was found throughout the eastern Caribbean, with these levels increasing substantially at stations nearer the Gulf of Paria and the Orinoco River. The dependence of the absorption at 300 nm (a300) on salinity exhibited conservative mixing behavior for transects through the Gulf of Paria. In contrast, transects through the Orinoco Estuary revealed that a300 remained approximately constant or slightly increased with increasing salinity up to 27‰, after which it decreased linearly with increasing salinity. This nonconservative behavior was not produced by COM release from suspended particulate matter at higher salinity, nor did it appear to originate from the sediment. Below 30‰, the slopes of the log‐linearized absorption spectra were independent of salinity (0.0140 nm−1). At higher salinities, the slopes apparently increased, suggesting that the COM is modified. Based on specific absorption coefficients measured for COM isolated from this area, COM input by the Orinoco River is estimated to be ∼ 2.5 × 1012 g C/yr, which represents about 1% of the total global transport of dissolved organic carbon to the ocean. This input is likely to influence substantially the carbon cycle, photochemical properties, and optical characteristics of the waters in this region.
Carbonyl compounds in the lower marine troposphere over the Caribbean Sea and Bahamas
Authors: Zhou Xianliang , Mopper Kenneth
Published: February 2008

Formaldehyde, acetaldehyde, acetone, and butanone were measured in 59 samples of marine air. Average clean air background concentrations were about 0.55, 0.50, 0.38, and 0.03 ppbv, respectively, in agreement with past measurements. The formaldehyde concentration is also in agreement with that predicted from photooxidation of methane and other locally derived organic matter. Formaldehyde and acetaldehyde showed strong diurnal variations throughout the 12‐day sampling period. Photo‐chemical oxidation of locally derived organic matter, such as nonmethane hydrocarbons and long‐chained lipids, appears to be the major source for both formaldehyde and acetaldehyde in the lower marine boundary layer. Acetone, on the other hand, showed weaker diurnal fluctuations, consistent with its significantly longer lifetime in the troposphere. Sinks for carbonyl compounds in the lower marine boundary layer are less clearly known. Our results suggest that photolysis, reaction with free radicals, and deposition at the sea surface are minor, short‐term sinks during our study. The main sink appears to be vertical mixing, probably followed by photolysis in the upper marine boundary layer and free troposphere.
Gulf Stream meanders over steep topography
Authors: Pickart Robert S. , Watts D. Randolph
Published: February 2008

Observations of the Gulf Stream thermocline as it meanders over the continental slope off Cape Hatteras are presented. Daily vertical sections of temperature and geostrophic velocity to 1500 m are calculated from an array of inverted echo sounders aligned across the Gulf Stream. These accompany time series of deep currents from a simultaneous array of bottom current meters. The vertical sections are computed using a previously derived technique which assumes the Gulf Stream variability is primarily first baroclinic mode. The sections reveal that the Gulf Stream front systematically shoals when it meanders offshore, an effect not observed farther downstream. This is accompanied by a compression of the main thermocline that in turn increases the core velocity of the jet but has little effect on overall transport. To explain this shoaling, the Gulf Stream is treated as a two‐layer flow; the lower layer vorticity balance suggests that the shoaling is caused by deep water columns preserving their layer thickness in the presence of the steep topography.
Comparison of special sensor microwave imager vector wind stress with model‐derived and subjective products for the tropical Pacific
Authors: Busalacchi Antonio J. , Atlas Robert M. , Hackert Eric C.
Published: February 2008

A new source of vector wind stress data is assessed relative to existing analyses of the surface wind field. The large‐scale variability of vector wind stress generated by Atlas et al. (1991) and based on the special sensor microwave imager (SSM/I) remotely sensed observations of surface wind speed is compared with five operational and subjectively analyzed wind products across the tropical Pacific basin for the first year of SSM/I, July 1987 through June 1988. The conventional wind stress data considered are the operational wind products from European Centre for Medium Range Weather Forecasts (ECMWF), National Meteorological Center (NMC), and Fleet Numerical Oceanography Center (FNOC), and the subjective analyses from Florida State University and the University of Hawaii. The spatial and temporal variability of the zonal component, meridional component, and curl of the wind stress are examined relative to their future use in wind‐driven ocean model studies of tropical Pacific Ocean circulation. The basin‐scale structure of the SSM/I data fall within a range bracketed by ECMWF, the University of Hawaii, NMC, and the Florida State products. The SSM/I data are shown to be most similar to the ECMWF analysis and the subjective analysis of satellite‐derived cloud motion wind vectors (SAWIN) performed at the University of Hawaii. The basin‐wide mean rms difference between the SSM/I data and these two products is 0.18 dyn cm−2 for τx and 0.10–0.12 dyn cm−2 for τy. In order to place these differences within context, the basin‐wide mean standard deviation for the temporal variability is found to be 0.23–0.26 dyn cm−2 for τx and 0.15–0.16 dyn cm−2 for τy. On regional scales, some of the differences among these products are greater than 0.3 dyn cm−2. The ECMWF and SAWIN analyses are the most similar (basin‐wide mean rms difference τx = 0.16 dyn cm−2, τy = 0.11 dyn cm−2) of the 15 possible product versus product comparisons. This may indicate a high weight given to cloud motion wind vectors in the ECMWF analysis for data sparse regions of the tropical Pacific. The wind data from the FNOC analysis forecast system in use in 1987–1988 (and since upgraded) was the most dissimilar wind product. The relatively dense space‐time coverage of the SSM/I satellite data (order of 27 observations per 2° × 2.5° grid square every 1.5 days), together with the large‐scale similarity with conventional wind products, suggests that the SSM/I‐based analysis represents a new source of surface wind information suitable for ocean modeling studies. As a result of the potential demonstrated here, strong consideration should be given to the use of these wind data in forcing ocean circulation modeling studies. Furthermore, the prospects of processing the surface wind speed retrievals from spaceborne passive and active microwave sensors dating back for more than 10 years should be examined.
Statistical and dynamical analysis of internal waves on the continental shelf of the Middle Atlantic Bight from space shuttle photographs
Authors: Zheng Quanan , Yan Xiao‐Hai , Klemas Vic
Published: February 2008

By interpreting two space shuttle photographs taken with a Linhof camera on June 8, 1991, a total of 34 internal soliton packets on the continental shelf of the Middle Atlantic Bight are recognized. The internal soliton field has a three‐level structure: packet groups with average wavelength of 17.5 km, packets with average wavelength of 7.9 km, and solitons with average wavelength of 0.6 km. Using the finite‐depth theory we derive that the maximum amplitude of solitons is 5.6 m, the phase speed is 0.42 m/s, and the period is 23.8 min. The frequency distribution of solitons is triple‐peaked at 1.9 × 10−4 Hz, 3.0 × 10−4, and 6.9 × 10−4 Hz. Substituting statistical results of number of solitons in a packet into the fission law, we find that the upper and the lower edges of the shelf break are the primary and the secondary generation sources of internal solitons, respectively. This reveals that the sharp change in the bottom topography is a key condition for the soliton fission or disintegration. Calculations show that the group period of solitons is 12.5 hours coinciding with that of local semidiurnal tides. This fact confirms that the tides are a dominant generation force for internal solitons on the continental shelf.
Estimates of currents in the nearshore ocean region using interferometric Synthetic Aperture Radar
Authors: Shemer L. , Marom M. , Markman D.
Published: February 2008

Microwave remote sensing of the ocean surface using Synthetic Aperture Radar (SAR) is of great interest due to its high resolution and the potential of nearly instantaneous coverage of large areas. The experience accumulated in ocean imaging by SAR indicates that there are substantial difficulties in interpretation of the obtained images of ocean wave systems. Ocean surface currents are almost impossible to obtain from SAR imagery. The present paper reports on measurements of ocean surface currents based on imaging of the nearshore regions of the Monterey Bay using an interferometric SAR. The method is a modification of the conventional SAR which employs two spatially separated antennas. This modification allows one to construct an interferogram from the two complex images obtained by each antenna, which provides direct mapping of the observed surface velocities. The present results demonstrate the advantages of this novel technique in studying nearshore ocean surface dynamics. In particular, spatial distribution of the longshore current velocity is presented.
The effect of roughness elements on wind erosion threshold
Authors: Raupach M. R. , Gillette D. A. , Leys J. F.
Published: February 2008

A theory is developed to describe the dependence upon roughness density of the threshold friction velocity ratio Rt, the ratio of the threshold friction velocity of an erodible surface without roughness to that of the surface with nonerodible roughness present. The roughness density is quantified by the frontal area index λ. The prediction is Rt = (1 − mσλ)−½(1 + mβλ)−½, where β is the ratio of the drag coefficient of an isolated roughness element on the surface to the drag coefficient of the substrate surface itself; σ is the basal‐to‐frontal area ratio of the roughness elements; and m (< 1) is a parameter accounting for differences between the average substrate surface stress and the maximum stress on the surface at any one point. The prediction is well verified by four independent data sets.
Polar stratospheric clouds at the South Pole in 1990: Lidar observations and analysis
Authors: Collins Richard L. , Bowman Kenneth P. , Gardner Chester S.
Published: February 2008

In December 1989 a Rayleigh/sodium lidar (589 nm) was installed at the Amundsen‐Scott South Pole station, and was used to measure stratospheric aerosol, temperature, and mesospheric sodium profiles through October 1990. Observations of stratospheric aerosol and temperature are presented in this paper. Polar stratospheric clouds (PSCs) were first observed in late May at about 20 km. As the lower stratosphere cooled further, PSCs were observed throughout the 12–27 km altitude region, and remained there from mid‐June until late August. Observations in early September detected no PSCs above 21 km. An isolated cloud was observed in mid‐October. Throughout the winter the clouds had small backscatter ratios (< 10). Observations made at two wavelengths in July show that the clouds are predominately composed of nitric acid trihydrate with associated Angstrom coefficients between 0.2 and 3.7. Comparison of the lidar data and balloon borne frost point measurements in late August indicate that the nitric acid mixing ratio was less than 1.5 ppbv. Observations over periods of several hours show downward motions in the cloud layers similar to the phase progressions of upwardly‐propagating gravity waves. The vertical phase velocities of these features (≈ 4 cm/s) are significantly faster than the expected settling velocities of the cloud particles. Both the backscatter ratio profiles and the radiosonde horizontal wind profiles show 1–4 km vertical structures. This suggests that the kilometer‐scale vertical structure of the PSCs is maintained by low frequency gravity waves propagating through the cloud layers.
Error analysis of ClO, O3, and H2O abundance profiles retrieved from millimeter wave limb sounding measurements
Authors: Aellig Christoph P. , Kämpfer Niklaus , Bevilacqua Richard M.
Published: February 2008

A rigorous error analysis is applied to the retrieval of chlorine monoxide (ClO), ozone (O3), and water vapor (H2O) profiles from millimeter wave spectra measured with a limb sounding instrument at 204, 184, and 183 GHz. The effect of different height resolutions on the accuracy and precision is studied. The nature of the different error sources and their contributions to the total error are discussed. A quantitative analysis for a specific limb sounding experiment, the MAS on the ATLAS spacelab shuttle program, is presented. The accuracy of the profiles to be retrieved is 30% for ClO around 35 km (for an altitude resolution of 5 km and an integration time of 200 s per tangent height). O3can be retrieved at an accuracy of 5 to 9% between 18 and 60 km (3 km; 1–100 s, depending on height). H2O can be retrieved at an accuracy of 4 to 9% between 20 and 75 km (3 km; 1–100 s). For O3, measurements at a resolution of 1 km are possible between 18 and 50 km, for H2O between 20 and 60 km.
The seasonal variation of nonmethane hydrocarbons in the free troposphere over the North Atlantic Ocean: Possible evidence for extensive reaction of hydrocarbons with the nitrate radical
Authors: Penkett S. A. , Blake N. J. , Lightman P. , Marsh A. R. W. , Anwyl P. , Butcher G.
Published: February 2008

Detailed observations have been made of the seasonal variation in concentration of a wide range of nonmethane hydrocarbons over the North Atlantic Ocean in air with a predominantly polar maritime origin. The results bear out many of the findings of our previous studies [Lightman et al., 1990] with the observation of a concentration maximum in the winter and a minimum in the summer. The more recent results indicate that the amplitude of the seasonal cycle is remarkably constant. Also the total concentration of reactive carbon in the various forms of nonmethane hydrocarbons probably exceeds 20 ppbC. This constitutes a substantial reservoir of material which could take part in ozone production in the relatively remote troposphere, if sufficient nitrogen oxides are copresent. The hydrocarbon composition of air in winter is strongly dependent on its origin, with tropical air having a composition similar to polar air in the spring months. The relative magnitude of the seasonal cycles for some hydrocarbons is proportional to their rates of reaction with hydroxyl radicals. This is true for straight chain paraffins up to C5, acetylene and benzene, and it suggests that the removal of these molecules from the atmosphere occurs predominantly by reaction with hydroxyl radicals. For other molecules, particularly branched chain paraffins and substituted aromatic molecules, there is evidence that other removal processes are operating in competition with the hydroxyl radical, especially in winter. Arguments are advanced that in the case of the branched chain paraffins this may be caused by reaction with nitrate radicals.