bluuebunny/crossref_metadata_2025 huggingface dataset:
Volcanic eruption plume top topography and heights as determined from photoclinometric analysis of satellite data
Authors: Glaze Lori S. , Wilson Lionel , Mouginis‐Mark Peter J.
Published: September 2002

Photoclinometry, a shape‐from‐shading technique, is applied to satellite data to determine the three‐dimensional height and morphology of the upper optical surface of a plume from the eruption of Redoubt Volcano, Alaska. The technique can be applied to visible images of volcanic plumes that have uniform scattering properties and is most effective on images with moderate incident Sun angles and plume transport perpendicular to the azimuth of the Sun. The two most significant sources of error are the finite resolution of the image data and the choice of image data number (DN) value for a flat plume element. Provided the plume spans at least 10 image pixels, useful results can be obtained for the central portion of the plume. The closest approximation to a flat plume element is found to be the visible DN value corresponding to the coldest pixel along a transect. Errors in the element altitude due to uncertainty in the DN value for a flat pixel amount to ∼10–20 m per pixel and are cumulative along the plume transect. For Redoubt this results in an uncertainty of approximately 300 m at the highest points. The method indicates that the Redoubt plume rose to an altitude of approximately 3 km while traveling a distance of 150 km from the vent and that the surface topography of the plume exhibits influences of the ambient wind through simple wave structures. This technique will have a tremendous impact on studies of plume spreading dynamics and the time‐integrated evolution of plumes.
Numerical simulations of granular shear zones using the distinct element method: 1. Shear zone kinematics and the micromechanics of localization
Authors: Morgan Julia K. , Boettcher Margaret S.
Published: September 2002

Two‐dimensional numerical simulations were conducted using the distinct element method (DEM) to examine the influences of particle size distribution (PSD) and interparticle friction μp on the nature of deformation in granular fault gouge. Particle fracture was not allowed in this implementation but points in PSD space were examined by constructing assemblages of particles with self‐similar size distributions defined by the two‐dimensional power law exponent D. For these numerical “experiments,” D ranged from 0.81 to 2.60, where D=1.60 represents the two‐dimensional equivalent of a characteristic PSD to which cataclastically deforming gouge is thought to evolve. Experiments presented here used μp values of 0.10 and 0.50 and were conducted using normal stress σn on the shear zone walls of 70 MPa. Shear strain within these simulated assemblages was accommodated by intermittent displacement along discrete slip surfaces, alternating between broadly distributed deformation along multiple slip planes and highly localized deformation along a single, sharply defined, subhorizontal zone of slip. Slip planes corresponded in orientation and sense of shear to shear structures observed in natural gouge zones, specifically Riedel and Y shears; the oblique Riedel shears showed more extreme orientations than typical, but their geometries were consistent with those predicted for low‐strength Coulomb materials. The character of deformation in the shear zone varied with PSD due to changes in the relative importance of interparticle slip and rolling as deformation mechanisms. A high degree of frictional coupling between large rolling particles in low D (coarse‐grained) assemblages resulted in wide zones of slip and broadly distributed deformation. In higher D assemblages (D >= 1.60), small rolling particles self‐organized into columns that separated large rolling particles, causing a reduction in frictional resistance within the deforming assemblage. This unusual particle configuration appears to depend on a critical abundance of small particles achieved at D ≈ 1.60 and may enable strain localization in both real and simulated granular assemblages.
Upper mantle velocity structure beneath southern Africa from modeling regional seismic data
Authors: Zhao Ming , Langston Charles A. , Nyblade Andrew A. , Owens Thomas J.
Published: September 2002

The upper mantle seismic velocity structure beneath southern Africa is investigated using travel time and waveform data which come from a large mine tremor in South Africa (mb 5.6) recorded by the Tanzania broadband seismic experiment and by several stations in southern Africa. The waveform data show upper mantle triplications for both the 410‐ and 670‐km discontinuities between distances of 2100 and 3000 km. Auxiliary travel time data along similar profiles obtained from other moderate events are also used. P wave travel times are inverted for velocity structure down to ∼800‐km depth using the Wiechert‐Herglotz technique, and the resulting model is evaluated by perturbing it at three depth intervals and then testing the perturbed model against the travel time and waveform data. The results indicate a typical upper mantle P wave velocity structure for a shield. P wave velocities from the top of the mantle down to 300‐km depth are as much as 3% higher than the global average and are slightly slower than the global average between 300‐ and 420‐km depth. Little evidence is found for a pronounced low‐velocity zone in the upper mantle. A high‐velocity gradient zone is required above the 410‐km discontinuity, but both sharp and smooth 410‐km discontinuities are permitted by the data. The 670‐km discontinuity is characterized by high‐velocity gradients over a depth range of ∼80 km around 660‐km depth. Limited S wave travel time data suggest fast S wave velocities above ∼150‐km depth. These results suggest that the bouyant support for the African superswell does not reside at shallow depths in the upper mantle.
Initiation of subduction at Atlantic‐type margins: Insights from laboratory experiments
Authors: Faccenna Claudio , Giardini Domenico , Davy Philippe , Argentieri Alessio
Published: September 2002

We have performed scaled lithospheric experiments to simulate the behavior of a ocean‐continent plate system subjected to compressional strain over a geological timescale. Experiments have been constructed using sand and silicone putty, representing the brittle upper crust and the ductile lower crust/upper mantle, respectively; the layers floated on glucose syrup simulating the asthenosphere. Compressional stress is achieved by displacing a piston at constant velocity perpendicular to the plate margin. We investigate the influence of four parameters: (1) the negative buoyancy of oceanic lithosphere, (2) the horizontal body forces between continent and ocean, and (3) the brittle and (4) the ductile strength of the passive margin. Two numbers express the importance of these parameters: the Argand number (Ar), representing the ratio between the body force of continent and its integrated strength, and the buoyancy number (F), representing the ratio between the buoyancy force of ocean and its ductile resistance. We obtain three scenarios. In experiments with Ar 3 and F < 1 the ocean deforms by distributed folds, resembling the undulations observed in the Indian Ocean. In experiments with Ar 7 and F >1 the continent collapses toward the ocean, producing back‐arc extension and subduction, simulating the post‐Alpine Neogene evolution of the Mediterranean area. In experiments with Ar 3 and F > 1 the passive margin slowly evolves toward trench nucleation with the formation of a viscous mantle instability. We conclude that the latter model can be applied to the evolution of Atlantic‐type margins, where there is evidence of this ongoing process.
On relating apparent stress to the stress causing earthquake fault slip
Authors: McGarr A.
Published: September 2002

Apparent stress τa is defined as
, where
is the average shear stress loading the fault plane to cause slip and η is the seismic efficiency, defined as Ea/ W, where Ea is the energy radiated seismically and W is the total energy released by the earthquake. The results of a recent study in which apparent stresses of mining‐induced earthquakes were compared to those measured for laboratory stick‐slip friction events led to the hypothesis that
. This hypothesis is tested here against a substantially augmented data set of earthquakes for which
can be estimated, mostly from in situ stress measurements, for comparison with τa. The expanded data set, which includes earthquakes artificially triggered at a depth of 9 km in the German Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland (KTB) borehole and natural tectonic earthquakes, covers a broad range of hypocentral depths, rock types, pore pressures, and tectonic settings. Nonetheless, over ∼14 orders of magnitude in seismic moment, apparent stresses exhibit distinct upper bounds defined by a maximum seismic efficiency of ∼0.06, consistent with the hypothesis proposed before. This behavior of τa and η can be expressed in terms of two parameters measured for stick‐slip friction events in the laboratory: the ratio of the static to the dynamic coefficient of friction and the fault slip overshoot. Typical values for these two parameters yield seismic efficiencies of ∼0.06. In contrast to efficiencies for laboratory events for which η is always near 0.06, those for earthquakes tend to be less than this bounding value because Ea for earthquakes is usually underestimated due to factors such as band‐limited recording. Thus upper bounds on
appear to be controlled by just a few fundamental aspects of frictional stick‐slip behavior that are common to shallow earthquakes everywhere. Estimates of
from measurements of τa for suites of earthquakes, using
, are found to be comparable in magnitude to estimates of shear stress on the basis of extrapolating in situ stress data to seismogenic depths.
Source of the climate signal recorded by magnetic susceptibility variations in Indian Ocean sediments
Authors: Hounslow Mark W. , Maher Barbara A.
Published: September 2002

The origins of magnetic susceptibility variations in deep‐sea sediments from the Indian Ocean (Ocean Drilling Program) (ODP Leg 117) have been examined to identify the specific connections between climate shifts and sediment magnetic properties. In a previous study, the magnetic susceptibility variations in ODP Hole 722B were identified as an outstanding proxy paleoceanographic record, despite evidence of postdepositional loss of magnetic minerals through reductive diagenesis. This paradox is addressed by assessing the contributions of ferrimagnetic detrital iron oxides, bacterial magnetosomes, and paramagnetic detrital Fe silicate minerals to the magnetic susceptibility signal. In addition to detailed magnetic analyses, mineralogical, morphological, and grain size data have been obtained from representative magnetic extracts. For Hole 722B, we find that magnetic responses to climate change result from (1) fluctuations in the volume magnetic susceptibility, which is primarily controlled by carbonate dilution; (2) a ferrimagnetic signal, which is restricted to the upper 7 meters below sea floor (mbsf) and which largely reflects source area aridity; and (3) a paramagnetic susceptibility record below 7 mbsf, which is coincident in frequency with variations in lithogenic grain size.
Analysis of stress‐induced oval fractures in a borehole at Deep Sea Drilling Project Site 504, eastern equatorial Pacific
Authors: Morin Roger H. , Flamand Rock
Published: September 2002

Deep Sea Drilling Project (DSDP) Hole 504B is located in the eastern equatorial Pacific Ocean and extends to a total depth of 2111 m beneath the seafloor (mbsf). Several acoustic televiewer logs have been obtained in this well during successive stages of drilling, and the resulting digital images have revealed numerous oval‐shaped fractures seemingly etched into the borehole wall. A theoretical examination of these stress‐induced features identifies a unique and ephemeral set of stress distributions and magnitudes that are necessary for their production. Consequently, the ovals provide a basis for quantifying the magnitudes and orientations of the maximum and minimum horizontal principal stresses, SH and Sh at this site. Vertical, truncated breakouts and horizontal tensile fractures define the spatial boundaries of the ovals. Explicit criteria for their occurrence are combined with estimates for various physical properties of the rock to yield a range of possible values for the horizontal principal stresses. The conspicuous oval geometry is completed by a curved fracture that joins the vertical and horizontal components. Its degree of curvature is delineated by the modified Griffith failure criterion and is directly related to the principal stress difference (SH − Sh). Matching a series of type curves corresponding to specific values for (SH − Sh) with the actual undistorted well bore images allows the magnitude of the stress difference to be further constrained. With a value for (SH − Sh) of 45 ± 5 MPa the individual magnitudes of SH and Sh are determined more precisely. Final estimates for the horizontal principal stresses in DSDP Hole 504B at a depth of 1200 mbsf are 141 MPa ≤ SH ≤ 149 MPa and 91 MPa ≤ Sh ≤ 109 MPa. Stress magnitudes derived from this approach rely heavily upon the values of a variety of physical properties, and complementary laboratory measurements performed on relevant rock samples provide critical information. Uncertainties in estimating these properties translate into less precise predictions of principal stresses.
Foreshock occurrence before large earthquakes
Authors: Reasenberg Paul A.
Published: September 2002

Rates of foreshock occurrence involving shallow M ≥ 6 and M ≥ 7 mainshocks and M ≥ 5 foreshocks were measured in two worldwide catalogs over ∼20‐year intervals. The overall rates observed are similar to ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span. The observed worldwide rates were compared to a generic model of earthquake clustering based on patterns of small and moderate aftershocks in California. The aftershock model was extended to the case of moderate foreshocks preceding large mainshocks. Overall, the observed worldwide foreshock rates exceed the extended California generic model by a factor of ∼2. Significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region, with the rate before thrust events higher and the rate before strike‐slip events lower than the worldwide average. Among the thrust events, a large majority, composed of events located in shallow subduction zones, had a high foreshock rate, while a minority, located in continental thrust belts, had a low rate. These differences may explain why previous surveys have found low foreshock rates among thrust events in California (especially southern California), while the worldwide observations suggests the opposite: California, lacking an active subduction zone in most of its territory, and including a region of mountain‐building thrusts in the south, reflects the low rate apparently typical for continental thrusts, while the worldwide observations, dominated by shallow subduction zone events, are foreshock‐rich. If this is so, then the California generic model may significantly underestimate the conditional probability for a very large (M ≥ 8) earthquake following a potential (M ≥ 7) foreshock in Cascadia. The magnitude differences among the identified foreshock‐mainshock pairs in the Harvard catalog are consistent with a uniform distribution over the range of observation.
Geodetic estimates of fault slip rates in the San Francisco Bay area
Authors: Savage J. C. , Svarc J. L. , Prescott W. H.
Published: September 2002

Bourne et al. [1998] have suggested that the interseismic velocity profile at the surface across a transform plate boundary is a replica of the secular velocity profile at depth in the plastosphere. On the other hand, in the viscoelastic coupling model the shape of the interseismic surface velocity profile is a consequence of plastosphere relaxation following the previous rupture of the faults that make up the plate boundary and is not directly related to the secular flow in the plastosphere. The two models appear to be incompatible. If the plate boundary is composed of several subparallel faults and the interseismic surface velocity profile across the boundary known, each model predicts the secular slip rates on the faults which make up the boundary. As suggested by Bourne et al., the models can then be tested by comparing the predicted secular slip rates to those estimated from long‐term offsets inferred from geology. Here we apply that test to the secular slip rates predicted for the principal faults (San Andreas, San Gregorio, Hayward, Calaveras, Rodgers Creek, Green Valley and Greenville faults) in the San Andreas fault system in the San Francisco Bay area. The estimates from the two models generally agree with one another and to a lesser extent with the geologic estimate. Because the viscoelastic coupling model has been equally successful in estimating secular slip rates on the various fault strands at a diffuse plate boundary, the success of the model of Bourne et at. [1998] in doing the same thing should not be taken as proof that the interseismic velocity profile across the plate boundary at the surface is a replica of the velocity profile at depth in the plastosphere.
Mixing and compositional stratification produced by natural convection: 1. Experiments and their application to Earth's core and mantle
Authors: Jellinek A. Mark , Kerr Ross C. , Griffiths Ross W.
Published: September 2002

An extensive series of laboratory experiments is used to quantify the circumstances under which fluids can be mixed by natural convection at high flux Rayleigh number. A compositionally buoyant fluid was injected at a fixed rate into an overlying layer of ambient fluid from a planar, horizontally uniform source. The nature of the resulting compositional convection was found to depend on two key dimensionless parameters: a Reynolds number Re and the ratio U of the ambient fluid viscosity to the input fluid viscosity. Increasing the Reynolds number corresponded to increasing the vigor of the convection, while the viscosity ratio was found to determine the spacing between plumes and whether buoyant fluid rose as sheets (U < 1) or axisymmetric plumes (U > 1). From measurements of the final density profile in the fluid after the experiments we quantified the extent to which buoyant liquid was mixed in terms of a thermodynamic mixing efficiency E. The mixing efficiency was found to be high (E > 0.9) when either the Reynolds number was large (Re > 100) or the viscosity ratio was small (U < 0.2) and was found to be low (E < 0.1) when both Re < 1 and U > 200. The amount of mixing was related to whether ascending plumes generated a large‐scale circulation in the ambient fluid. When our results are applied to the differentiation of the Earth's core, we suggest that the convection resulting from the release of buoyant residual liquid into the liquid outer core due to crystallization at the boundary between the inner and the outer core will probably lead to nearly complete mixing. In the dynamically very different context of the mantle, mantle plumes are predicted to ascend through the mantle and pond beneath the lithosphere, whereas convection driven by the subduction of oceanic lithosphere is expected to produce moderate to extensive mixing of the mantle. When the competing plate and plume modes of mantle convection are considered together, we find that owing to a larger driving buoyancy flux, the plate‐scale flow will destroy any stratification at the top of the mantle produced by mantle plumes. Applying our results to the “stagnant lid” style of thermal convection predicted to occur in the mantles of the Moon, Mercury, Mars, Venus, and pre‐Archean Earth, we expect the respective flows to produce minor thermal stratification at the respective core‐mantle boundaries. In part 2 of this study [Jellinek and Kerr, this issue] we apply our results to the differentiation of magma chambers and komatiite lava flows.
Experiments in migration processing of SS precursor data to image upper mantle discontinuity structure
Authors: Shearer Peter M. , Flanagan Megan P. , Hedlin Michael A. H.
Published: September 2002

Long‐period SS precursors result from underside reflections off upper mantle discontinuities. By grouping and stacking global seismic data by SS bounce point location it is possible to map lateral variations in depths to the 410‐ and 660‐km discontinuities, a process analogous to common midpoint (CMP) stacking in reflection seismology. Because this method assumes horizontal reflectors, energy arriving from dipping or intermittent reflectors may not be correctly imaged. To address this possibility, we experiment with techniques based on migration processing of shallow seismic reflection data. The problem is complicated by the uneven distribution of sources and receivers for the SS precursor observations, but the data are sufficiently dense beneath the northwest Pacific Ocean that reasonably good coverage can be obtained for this region. We parameterize the model as a grid of point scatterers in latitude, longitude, and depth (from the surface to 1000 km depth) and compute travel times from each grid point to the source and receiver locations. These times are used to construct a matrix equation that yields predicted SS precursor waveforms from the assumed scatterers. To recover the model, we experiment with both simple back projection and full inversions using a conjugate gradient method. Tests on noise‐free synthetic data (generated using the same source‐receiver distribution as the actual data) suggest that detailed resolution of discontinuity structure is possible, at horizontal scales much smaller than the Fresnel zone. However, the real data do not produce coherent results unless some degree of horizontal smoothing is imposed, at least partially defeating the purpose of this approach. Results for the northwest Pacific find structure on the 410‐ and 660‐km discontinuities and hints of intermittent reflectors at other depths. Random resampling tests, however, suggest that most of these features are not reliably resolved, with the exception of a depression on the 660‐km discontinuity seen in the northwest Pacific. Our experiments show that it is unlikely that small‐scale structure on the 660‐km discontinuity near subducting slabs causes significant bias in maps of the large‐scale 660‐km topography derived from long‐period SS precursor observations.
Phytoplankton taxonomic variability in nutrient utilization and primary production in the Ross Sea
Authors: Arrigo Kevin R. , DiTullio Giacomo R. , Dunbar Robert B. , Robinson Dale H. , VanWoert Michael , Worthen Denise L. , Lizotte Michael P.
Published: September 2002

Patterns of nutrient utilization and primary productivity (PP) in late austral spring and early summer in the southwestern Ross Sea were characterized with respect to phytoplankton taxonomic composition, polynya dynamics, and upper ocean hydrography during the 1996–1997 oceanographic program Research on Ocean‐Atmosphere Variability and Ecosystem Response in the Ross Sea. Phytoplankton biomass in the upper 150 m of the water column ranged from 40 to 540 mg chlorophyll a (Chl a) m−2, exceeding 200 mg Chl a m−2 everywhere except the extreme northern and eastern boundaries of the Ross Sea polynya. Diatom biomass was greatest in the shallow mixed layers of Terra Nova Bay, while the more deeply mixed waters of the Ross Sea polynya were dominated by Phaeocystis antarctica. Daily production computed from the disappearance of NO3 (1.14 g C m−2 d−1) and total dissolved inorganic carbon (TDIC, 1.29 g C m−2 d−1) is consistent with estimates made from an algorithm forced with satellite measurements of Chl a (1.25 g C m−2 d−1) and from measurements of 14C uptake (1.33 g C m−2 d−1). Phytoplankton PP in the Ross Sea averaged 100 g C m−2 yr−1 during 1996–1997. Despite the early formation of the Terra Nova Bay polynya the diatom bloom there did not reach its peak PP until middle to late January 1997 (most likely because of more intense wind mixing in November), ∼6 weeks after the P. antarctica bloom in the Ross Sea polynya had reached the same stage of development. From 70 to 100% of the C and N deficits in the upper 150 m could be accounted for by particulate organic matter, indicating that there had been little dissolved organic matter production or export of particulate material prior to our cruise. This suggests that early in the season, PP and zooplankton grazing are decoupled in the southwestern Ross Sea. The NO3∶PO4 disappearance ratio in waters dominated by P. antarctica (19.0±0.61) was significantly greater than in waters where diatoms were most common (9.52±0.33), and both were significantly different from the Redfield N∶P ratio of 16. Vertical profiles of TDIC suggest that P. antarctica took up 110% more CO2 per mole of PO4 removed than did diatoms, an important consideration for climate models that estimate C uptake from the removal of PO4.
Prochlorococcus growth rates in the central equatorial Pacific: An application of the ƒmax approach
Authors: Liu Hongbin , Landry Michael R. , Vaulot Daniel , Campbell Lisa
Published: September 2002

Minimum daily growth rates of Prochlorococcus were estimated for the central equatorial Pacific (12°S–12°N, 140°W) during El Niño (February‐March 1992) and normal upwelling (August‐September 1992) conditions. Growth rate estimates were based on the percentages of cells in the S and G2 division phases at dawn (∼0700 LT) and dusk (∼1800 LT) as approximate values for ƒmin and ƒmax, respectively. During both environmental conditions, depth‐integrated growth rates of Prochlorococcus were higher in oligotrophic waters at the northern end of the transect (0.36–0.52 d−1) and decreased to a minimum at the equator. The lowest growth rates were found at the equator during El Niño and at 2°N during normal upwelling, where a large biomass of buoyant diatoms had accumulated in the vicinity of a convergent front. Prochlorococcus growth rate reached a high of 0.64 d−1 at 1°S and maintained a moderate rate (0.36–0.49 d−1) throughout the southern end of the transect. An inverse relationship was found between the contribution of Prochlorococcus to the total primary production and nitrate concentration as well as total primary production measured by the 14C method. While Prochlorococcus is a dominant primary producer in tropical and subtropical open‐ocean ecosystems generally, it is relatively more important in oligotrophic waters than in the nutrient rich equatorial upwelling zone.
Geometry, migration, and evolution of wave orbital ripples at LEO‐15
Authors: Traykovski Peter , Hay Alex E. , Irish James D. , Lynch James F.
Published: September 2002

Observations of the temporal evolution of the geometric properties and migration of wave‐formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements. Six weeks of bedform observations were taken at the sandy (medium to coarse sized sand) LEO‐15 site located on Beach Haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar. During this period, six tropical storms, several of hurricane strength, passed to the east of the study site. Ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed. The predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3/4 of the wave orbital diameter. Although orbital diameters become larger than 130 cm during the maximum wave event, it is unclear if a transition to non‐orbital scaling is occurring. Ripple migration is found to be directed primarily onshore at rates of up to 80 cm/day. Suspended transport due to wave motions, calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements, are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration. Thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near‐bottom suspended transport. Bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates. Sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction, as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave‐to‐current velocities.
A preliminary model study of the large‐scale seasonal cycle in bottom pressure over the global ocean
Authors: Ponte Rui M.
Published: September 2002

Output from the primitive equation model of Semtner and Chervin [1992] is used to examine the seasonal cycle in bottom pressure pb over the global ocean. Effects of the volume‐conserving formulation of the model on the calculation of pb are considered. The estimated seasonal, large‐scale pb signals have amplitudes ranging from less than 1 cm over most of the deep ocean to several centimeters over shallow, boundary regions. Variability generally increases toward the western sides of the basins and is also larger in some Southern Ocean regions. An oscillation between subtropical and higher latitudes in the North Pacific is clear. Comparison with barotropic simulations indicates that, on basin scales, seasonal pb variability is related to barotropic dynamics and the seasonal cycle in Ekman pumping and results from a small, net residual in mass divergence from the balance between Ekman and Sverdrup flows.
Rainfall measurements with navigational radar
Authors: Lebedev Ivan , Tomczak Matthias
Published: September 2002

A novel approach to measuring rainfall in the ocean is presented. The method is based on the remote sensing of rain by navigational radar and subsequent calibration of the radar signal against the upper ocean salinity changes. The estimate of the area‐average precipitation is obtained as the product of the calibration factor and the fraction of area covered by rainfall. We describe an application of this technique in the field during a recent voyage of the Australian R/V Franklin to the eastern tropical Indian Ocean. It is suggested that with further refinement of radar signal processing, navigational radar can be used to routinely measure open ocean precipitation from research vessels and ships of opportunity.
Anomalous southward intrusions of the Oyashio east of Japan: 2. Two‐layer numerical model
Authors: Sekine Yoshihiko
Published: September 2002

Anomalous southward intrusions of the Oyashio along the Japanese Coast in the northwestern Pacific occur from winter to late spring, with cold sea surface temperatures exerting various influences on the temperature in Japan and on aquaculture. A two‐layer numerical model is employed to examine the interannual response of the ocean to the observed wind stress change over 1961–1987, with special reference to the occurrence of the anomalous southward Oyashio intrusion. Since 1971, anomalous southward Oyashio intrusions were observed in winter of 1974, 1981, 1984 and 1986, and the significant southward shift of the Oyashio is well simulated in these years. Because the observed southward latitude of no wind stress curl in the North Pacific and the Sverdrup transport show clear simultaneous correlation with the calculated southernmost latitude of the subarctic circulation, the anomalous southward intrusion of the Oyashio is found to be essentially due to the barotropic response of the ocean to the change in the wind stress characterized by the southward development of the enhanced Aleutian low. The model also shows that a baroclinic response from the eastern boundary is mainly confined to a region east of 140°W, and an uplift of the interface in a central area of the subarctic circulation is also formed by the divergence of the surface Ekman layer in winter of the anomalous southward Oyashio intrusion. The calculated upper layer thickness south of the subarctic circulation has a clear positive simultaneous correlation with the observed southernmost latitude of the Oyashio defined by the 5°C isotherm at a depth of 100 m, which has been used as an index. However, the observed southernmost latitude of the Oyashio follows the observed latitude of no wind stress curl and the Sverdrup transport by ∼2–4 months, while the calculated southernmost latitude of the subarctic circulation shows simultaneous correlation with no wind stress curl and the Sverdrup transport. From this, it is pointed out that the observed southernmost latitude of the Oyashio defined by the 5°C isotherm at a depth of 100 m lags by ∼2–4 months the barotropic response of the ocean which appears in the surface pressure fields and upper layer velocity.
Ocean general circulation model sensitivity to forcing from scatterometer winds
Authors: Milliff Ralph F. , Large William G. , Morzel Jan , Danabasoglu Gokhan , Chin Toshio M.
Published: September 2002

Enhanced global surface wind fields are constructed from a blend of NASA scatterometer (NSCAT) and ERS 2 scatterometer data and National Centers for Environmental Prediction (NCEP) analyses, at 6‐hour intervals, for a repeatable annual cycle from August 1996 through July 1997. Wind field properties (wind speed, zonal and meridional wind stresses, wind stress curl, and kinetic energy input) for the enhanced winds are compared with the NCEP analyses for the same time period. Large‐scale, zonal patterns dominate annual average difference maps for wind speed and both components of wind stress. Wind stress curl differences are largest in the subpolar North Atlantic and in the midlatitudes of the southern hemisphere. The importance of wind field differences are measured by their impacts on the response of an ocean general circulation model (OGCM). Twin experiments are performed using the National Center for Atmospheric Research Climate System Model ocean component in stand‐alone mode. The annual mean OGCM responses to the enhanced winds and to surface winds from the NCEP analyses are compared for barotropic stream function, surface velocities, upper ocean upwelling, sea surface temperature (SST), surface heat flux, meridional overturning stream function, and total northward heat transport. Differences in the annual mean responses are attributable to differences in the mean forcing and not to the mesoscale signal that is present in the enhanced winds but not resolved by the OGCM. Differences in SST and surface heat flux are partitioned between local thermodynamic balances and balances involving ocean dynamics as well. Northward heat transport differences of order −0.2 PW in the southern hemisphere midlatitudes are consistent with weaker eastward and southward stresses in the westerlies and a 27% reduction in kinetic energy input from enhanced winds. This difference in northward heat transport is compensated in the southern hemisphere tropical Pacific where surface stresses and the OGCM surface response to enhanced winds are more westward and more divergent, resulting in 27% greater kinetic energy input. An extensive appendix details the realistic high‐wavenumber character of the enhanced winds.
Uncertainty of South China Sea prediction using NSCAT and National Centers for Environmental Prediction winds during tropical storm Ernie, 1996
Authors: Chu Peter C. , Lu Shihua , Liu W. Timothy
Published: September 2002

Error propagation from winds to ocean models was numerically investigated using the Princeton Ocean Model (POM) for the South China Sea with 20‐km horizontal resolution and 23 σ levels conforming to a realistic bottom topography during the lifetime of tropical cyclone Ernie (November 4–18, 1996). Numerical integration was divided into preexperimental and experimental stages. The preexperiment phase generates the initial conditions on November 1 for the sensitivity experiment. During the experimental stage the POM was integrated from November 1 to 30, 1996 under National Centers for Environmental Prediction (NCEP) reanalyzed surface fluxes along with two surface wind data sets, namely, the daily averaged interpolated NASA scatterometer winds and the NCEP winds. The relative root‐mean‐square differences fluctuate from 0.5 to 1.0 for winds, 0.25 to 0.7 for surface elevations, 0.47 to 1.02 for surface currents, and 0 to 0.23 for surface temperatures. This indicates that the model has less uncertainty overall than the wind fields used to drive it, which in turn suggests that the ocean modeling community may progress without waiting for the atmospheric modelers to build the perfect forecast model.
Optical impacts of oceanic coccolithophore blooms
Authors: Tyrrell T. , Holligan P. M. , Mobley C. D.
Published: September 2002

Blooms of coccolithophores, particularly those of the species Emiliania huxleyi, cause light in the surface ocean to behave in an unusual fashion, producing distinctive bright “white waters,” apparent from ships and readily detected by remote sensing. The brightness is caused by scattering of light from calcium carbonate platelets (coccoliths). Here we present the results of a modeling study, giving precise calculations of how the coccolith light scattering changes the behavior of light in the water. The results from a Monte Carlo optical model are closely compared to data from the CD60 cruise for a coccolithophore bloom south of Iceland in 1991 [Holligan et al., 1993], and the model is then used to extrapolate from the observational data to predict diverse optical properties that were not measured. Model performance was also tested by comparison of results with those from other, more established optical models. The model results demonstrate clearly that coccoliths cause (1) an increase in the emergent flux (the water‐leaving radiance), (2) brighter, more intensely heated water in the top few meters, and (3) darker, less intensely heated water deeper down. Implications of these effects for phytoplankton productivity and for climatology are discussed. Coccolith light scattering is estimated to contribute to global annually averaged planetary albedo by a maximum of ∼0.13%, equivalent to only a small globally averaged radiative forcing of ∼0.22 W m−2.
Diagnosing a meander of the shelf break current in the Middle Atlantic Bight
Authors: Pickart Robert S. , Torres Daniel J. , McKee Theresa K. , Caruso Michael J. , Przystup Jennifer E.
Published: September 2002

Two wintertime hydrographic crossings of the shelf break current in the Middle Atlantic Bight are analyzed. Separated by 3 days, the two sections sampled opposite phases of a meander of the current. The shipboard acoustic Doppler current profiler (ADCP) data reveal a strong, convergent jet during the first crossing, transporting 0.32 Sv of water equatorward. During the second crossing the transport is the same but the jet is weak and divergent. The measured Rossby numbers imply that the jet is significantly nonlinear in the convergent state. The associated potential voracity Q distributions differ substantially; the core of the weak jet is characterized by a region of uniform Q, whereas the strong jet contains no such feature. Surface thermal imagery indicates that the leading edge of a steep meander trough was sampled during the first crossing, followed by a broad crest during the second crossing. This is consistent with the convergent versus divergent nature of the flow in the two sections. A nearby Gulf Stream ring likely caused the steepening of the trough. After integrating the spatially low‐passed thermal wind shear, the resulting geostrophic sections are compared to the similarly low‐passed ADCP fields. This, together with the scales of the meander deduced from the surface imagery, indicates that the trough was in gradient wind balance, whereas the crest was predominantly geostrophic. These observations are consistent with the structure and dynamics of modeled baroclinic jets.
Particle fluxes during austral spring and summer in the southern Ross Sea, Antarctica
Authors: Asper Vernon L. , Smith Walker O.
Published: September 2002

The flux of particles from the euphotic zone through 200 m was investigated on the Ross Sea continental shelf during two cruises, the first in November‐December 1994 and the second in December 1995 and January 1996. An assessment of surface layer phytoplankton biomass and productivity was made simultaneously. Particle flux was measured using floating sediment traps whose collection efficiency was assessed rigorously. Phytoplankton biomass and productivity increased rapidly in November‐December, and biomass was maximal in mid‐December. Thereafter productivity appeared to decline substantially. Biomass declined as well, but not as rapidly as productivity. Vertical flux rates were low early in the bloom period, averaging 457 mg m−2 d−1, but increased markedly in late December and January (mean = 1160 mg m−2 d−1). Daily losses due to vertical flux represented only 2.3% of the surface layer particulate organic carbon standing stock. Measured particle fluxes were greater than those observed previously, and this is attributed to the period and depths sampled as well as to the care taken to ensure accuracy of sample collection. As in other regions, vertical flux of biogenic material is coupled with surface layer production and biomass. In our study area, however, a distinct temporal lag is introduced between surface production and flux at depth as a result of the temporal characteristics of the dominant mechanism generating large particles (aggregation) as well as the characteristic species of the region (Phaeocystis antarctica).
Eolian‐transported freshwater diatoms and phytoliths across the equatorial Atlantic record: Temporal changes in Saharan dust transport patterns
Authors: Romero Oscar E. , Lange Carina B. , Swap Robert , Wefer Gerold
Published: September 2002

The deposition of freshwater diatoms (FD) and phytotliths (P) was determined from sediment traps for a wide region (20°N–7°S) in the tropical and equatorial Atlantic, along a N‐S transect in the eastern equatorial Atlantic around 10°W and in the western equatorial Atlantic around 25°W. These siliceous organisms are derived from the Sahara and Sahel regions of Africa, and eolian transport with direct settling over the open ocean is assumed to be the transport agent. Depositional rates of FD and P revealed strong coupling with seasonal changes in Saharan dust transport that are associated with seasonal precipitation patterns, major wind systems, and the geographical extension of the dust plume across the Atlantic. Mean daily fluxes were highest south of Cape Verde (FD = 9 × 104 valves m−2 d−1; P = 2 × 104 bodies m−2 d−1), moderately high off Cape Blanc and in the Guinea Basin north of the equator (of the order of 3 × 104 valves m−2 d−1 for FD, and 0.7 × 104 bodies m−2 d−1 for P), and consistently low south of the equator and in the western equatorial Atlantic. In traps north of the equator, seasonal changes were marked. Aulacoseira granulata and A. islandica were the most abundant FD in the traps, regardless of trap location and season. However, the number of FD species was higher in the Cape Blanc and Cape Verde areas. The morphological diversity (shape and size) of the P assemblage decreased with increasing distance from the African continent. Patterns of FD and P accumulation rates in surface sediments coincided with those in the traps. Robust freshwater diatom and phytolith records associated with seasonal eolian transport from Saharan and Sahelian regions into the Atlantic furnish clues that can help in our present understanding of the processes linking transport between land, atmosphere, and ocean.
Carbon disulfide in the North Atlantic and Pacific oceans
Authors: Xie Huixiang , Moore Robert M.
Published: September 2002

Carbon disulfide (CS2) was determined in surface waters of the North Atlantic and Pacific Oceans. The mean concentrations (and ranges) of CS2 in open ocean waters were 13.4 (7.8–26.1) pM S (picomol sulfur per liter) for the North Atlantic and 14.6 (7.2–27.5) pM S for the Pacific. The concentrations in the coastal waters of the North Atlantic averaged 26.4 pM S and ranged from 17.9 to 40.4 pM S. Warm waters generally contained higher levels of CS2 than did cold waters. All the study areas were found to be supersaturated in CS2 relative to the atmosphere based on calculations from published CS2 mixing ratios in the marine boundary layer and their Henry's law constants. Sea‐to‐air fluxes of CS2 were estimated using exchange velocities for spot and climatological wind speeds. The global oceanic flux extrapolated from this study is 0.18 Tg CS2 yr−1 and in the range 0.13–0.24 Tg CS2 yr−1. It is suggested that microbial processes, photochemical reactions, and phytoplankton activity are potential sources for oceanic CS2.
Quasi‐stationary wave variability in NSCAT winds
Authors: Milliff Ralph F. , Hoar Timothy J. , van Loon Harry , Raphael Marilyn
Published: September 2002

Planetary and interseasonal properties of the NASA scatterometer (NSCAT) winds are explored in the context of the quasi‐stationary waves (QSWs) of the southern hemisphere. The QSWs are examined by means of zonal asymmetries of 3‐month running averages of the meridional velocity derived from NSCAT. The study period spans the entire NSCAT record from September 15, 1996, through June 29, 1997. Meridional winds from the European Space Agency ERS 2 scatterometer are used to augment 3‐month averages centered on August 1996 through May 1997. The time period corresponds to the transition from year −1 to 0 of the 1997 warm event in the Southern Oscillation. Comparisons are made with QSW signals in geopotential height anomalies from the National Centers for Environmental Prediction/National Center for Atmospheric Research Climate Data Assimilation System. The zonal anomalies of meridional wind from NSCAT are shown to be in approximate geostrophic balance with zonal gradients in the zonal anomalies of geopotential height at 500 and 1000 hPa.
Western Pacific interannual variability associated with the El Niño‐Southern Oscillation
Authors: Wang Chunzai , Weisberg Robert H. , Virmani Jyotika I.
Published: September 2002

Observations of sea surface temperature (SST), sea level pressure (SLP), surface wind, and outgoing longwave radiation (OLR) show that the El Niño‐Southern Oscillation (ENSO) displays western Pacific anomaly patterns in addition to eastern Pacific anomaly patterns. During the warm phase of ENSO, warm SST and low SLP anomalies in the equatorial eastern Pacific and low OLR anomalies in the equatorial central Pacific are accompanied by cold SST and high SLP anomalies in the off‐equatorial western Pacific and high OLR anomalies in the off‐equatorial far western Pacific. Also, while the zonal wind anomalies over the equatorial central Pacific are westerly, those over the equatorial far western Pacific are easterly. The nearly out‐of‐phase behavior between the eastern and western tropical Pacific is also observed during the cold phase of ENSO, but with anomalies of opposite sign. These western Pacific interannual anomaly patterns are robust features of ENSO, independent of data sets. It is argued that equatorial easterly (westerly) wind anomalies over the far western Pacific during the warm (cold) phase of ENSO are initiated by off‐equatorial western Pacific cold (warm) SST anomalies, and that these winds are important for the evolution of ENSO. An atmosphere model is employed to demonstrate that small off‐equatorial western Pacific cold (warm) SST anomalies (compared to those in the east) are sufficient to produce equatorial easterly (westerly) wind anomalies as observed over the far western Pacific. The coupled ocean‐atmosphere model of Zebiak and Cane is then modified to investigate the evolution of the western Pacific interannual anomaly patterns in a coupled ocean‐atmosphere system, by including a meridional structure to the subsurface temperature parameterization in the western Pacific. The modified model produces both western and eastern Pacific interannual anomaly patterns.