bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and dynamics of the Uranian magnetotail: Results from hot plasma and magnetic field observations
Authors: Kane M. , Mauk B. H. , Keath E. P. , Krimigis S. M.
Published: February 2008

Analysis of the Voyager 2 low‐energy charged particle (LECP) and magnetic field (MAG) data in the Uranian magnetotail has shown this system to have many features similar to those seen at Earth. Isotropization of core magnetosphere 28–43 keV ions has been observed at the trapping boundary upon entry into the magnetotail near L=17, an effect seen in ion populations at Earth and attributed to neutral sheet scattering. An extensive and persistent plasma sheet boundary layer was found to contain field‐aligned ion streams with a variety of angular and energetically dispersive effects. Streaming distributions in the magnetotail horn region map to the auroral emission region observed by the Voyager ultraviolet spectrometer (UVS). A quiet‐time near‐tail region is distinguished from a distant disturbed region, where evidence of a substorm is observed. There is evidence suggesting that the distant plasma sheet is not corotating with the planet. Proton density gradients in this disturbed region indicate the presence of detached plasma structures. During what appears to be an occuring substorm, energy dispersionless field‐aligned streaming, seen within the neutral sheet region, indicates close proximity to a substormlike acceleration region. Extensive observations of the Earth's magnetotail provide a model for ordering a variety of particle and magnetic phenomena observed in the Uranian magnetotail.
Atoll stratigraphy as a record of sea level change: Problems and prospects
Authors: Lincoln Jonathan M. , Schlanger Seymour O.
Published: February 2008

Stratigraphic hiatuses and solution unconformities in the subsurface of Enewetak Atoll, northern Marshall Islands, record periods of atoll emergence during low stands of sea level. Changes in sea level are also recorded in the atoll subsurface by variations in the rate of sediment accumulation relative to the subsidence rate of the underlying volcanic edifice. Past sea levels can be derived from atoll stratigraphy by correcting the present depth of dated subsurface horizons for thermal subsidence and lithospheric flexure since the time of deposition. A correction for depositional paleodepth may also be necessary. As a result of erosion and nondeposition during periods of emergence, the history of sea level derived in this manner is discontinuous. Past sea levels derived from atoll stratigraphy can only be estimated to within ±50 m relative to present sea level owing to uncertainties in the corrections for subsidence and flexure; however, the minimum magnitude of sea level falls estimated from stratigraphic hiatuses can be estimated to within ±10 m. Owing to limited fossil‐based age resolution, only long‐term sea level trends can be deduced from sediments dated by means of biostratigraphy. Based on the biostratigraphic ages of subsurface horizons at Enewetak, we can discern very little long‐term change in sea level from late Eocene through late Oligocene, a rise to ∼110 m above present sea level in the early Miocene, a long‐term fall of ∼170 m through middle and late Miocene time, and a long‐term rise of ∼60 m from the end of the Miocene to present. Resolution of the sea level history recorded beneath mid‐ocean atolls may be improved by determining the age of shallow‐marine carbonates by means of strontium isotope stratigraphy. Our interpretation of past sea levels based on 87Sr/86Sr chronostratigraphy from Enewetak confirms the long‐term sea level trends inferred from biostratigraphic subsurface ages. In addition, we interpret three Oligocene sea level falls with minimum magnitudes of 30–50 m at ∼35, 33–30, and ∼25 Ma and four Miocene sea level falls greater than 30–95 m at 16–14, ∼12, ∼10, and ∼5 Ma. These estimates of the timing and magnitude of Oligocene and Miocene sea level changes, derived from Enewetak Atoll stratigraphy, are compared with sea level histories derived from continental margin stratigraphy and from ice volume changes inferred from deep‐sea foraminifera δ18O records.
Large‐scale stratigraphic architecture, eustatic variation, and unsteady tectonism: A theoretical evaluation
Authors: Jordan T. E. , Flemings P. B.
Published: February 2008

We incorporate a process‐based depositional model with basin subsidence models to predict stratigraphic records. This allows us to investigate the importance of subsidence geometry on coastal stratigraphy and thus to characterize and compare the stratigraphic architecture of two categories of tectonic basins. The models demonstrate that the correlation of stratigraphic sequences to eustatic cycles is not the same in passive margin basins as in foreland basins and that in a foreland basin, the record of episodic tectonism is distinct from that of eustatic sea level change. In the model, sediment transport is approximated by slope‐controlled diffusion; nonmarine transport is treated as more efficient diffusion than marine transport. Three different subsidence and sediment supply models are examined: a simple passive margin basin, a simple foreland basin, and then a foreland basin for which vertical motions are driven by thrust shortening that is compensated flexurally and for which sediment supply is related to relief. Predicted passive margin stratigraphies, for cases of varying eustatic sea level, are similar to those of natural basins and include progradational packages and subaerial unconformities, which are used to define sequence boundaries that form during sea level fall. We explore the timing relationships between stratigraphic features and a sinusoidal sea level history, showing that the phase relationship depends on subsidence, sediment flux, efficiency of sediment transport, and period and amplitude of sea level. When the basic geometry of the basin is inverted, placing the sediment supply on the side with maximum subsidence as is the case in foreland basins, the sequence character changes markedly: subaerial erosion does not generate unconformities. In the models of a dynamic foreland basin, sediment supply and subsidence are linked to the structure of the flanking thrust belt and are not necessarily constant. For steady thrusting and variable sea level, unconformities that define sequence boundaries form only on the distal or forebulge side of the basin, and the ages of the sequence boundaries correlate to limes of rising sea level. In cases of constant sea level but variable thrusting, subaerial unconformities are cut locally on both the proximal margin of the basin and the distal margin of the basin, yet the ages of the proximal margin and distal margin unconformities are out of phase in the tectonic cycle: erosion is most pronounced during quiescence on the proximal side and during thrusting on the distal side.
The role of the sediment load in sequence stratigraphy: The influence of flexural isostasy and compaction
Authors: Reynolds David J. , Steckler Michael S. , Coakley Bernard J.
Published: February 2008

Simplified, physically based models are utilized in order to examine the effects of sea level, sedimentation, tectonic subsidence, isostatic compensation, and compaction on development of unconformities. Unconformities divide volumes of relatively conformable sediments defining the time and spatial extent of sedimentary sequences. The model parameters are varied both individually and in concert in order to isolate their contributions to sequence architecture. These models reveal the importance of sedimentation and subsidence rates, as well as sea level amplitude and rate, in determining the type and extent of sequence boundary formed. Sedimentation and subsidence rate can vary significantly within and between margins, producing different sequence boundaries given an identical eustatic sea level fall. It is shown that sea level amplitude plays an important role in determining which systems tracts are present and sequence boundary timing for type 1 sequences and that sea level rate controls the same attributes in type 2 sequences. Isostatic compensation and compaction, which have been described as secondary effects, are shown to have considerable influence on sequence architecture. These two processes, due to isostatic response to sediment loading and sediment self‐loading, initiate feedback by enhancing and partitioning accommodation space. These additional effects invalidate application of the equilibrium point as a guide to where and how all accommodation space is created. The manner in which flexure distributes accommodation space is a function of lithospheric rigidities: higher rigidities partition space laterally, producing wide shelves which favor type 1 sequence boundaries, whereas lower rigidities partition space vertically forming narrow shelves which favor type 2 sequence boundaries. Compaction generates a net landward shift of the shelf edge and favors type 2 sequence development Both isostatic compensation and compaction introduce time delays in sequence development. One consequence of compaction is to rotate sedimentary packages, generating growth and leakage of anticlinal structures as fluids are driven from the underlying section. This process has important implications for hydrocarbon migration in compaction‐dominated systems, such as the Gulf of Mexico margin.
Forced nutations of the Earth: Influence of inner core dynamics: 1. Theory
Authors: Mathews P. M. , Buffett B. A. , Herring T. A. , Shapiro I. I.
Published: February 2008

Gravitational and pressure couplings between the solid inner core and the rest of the Earth give rise to torques through which the inner core influences the nutational motions of the Earth. In view of the very small magnitude of the moment of inertia of the inner core relative to that of the the Earth as a whole, one expects from physical considerations that inclusion of the inner core in the dynamics should lead to a new nutational normal mode with a natural frequency not too far from that of the free core nutation, and to an associated weak resonance in the amplitude of forced nutations. We present here a treatment of the nutation problem for an oceanless, elastic, spheroidally stratified Earth, with the dynamical role of the inner core explicitly included in the formulation. As a preliminary to the setting‐up of dynamical equations, we devote some attention to a careful definition of a suitable coordinate system and of certain basic dynamical variables. We use the approach of Sasao et al. (1980), with their system of dynamical equations enlarged by the inclusion of two additional equations which are needed to describe the rotational motion of the inner core. An extension and sharpening of a line of reasoning employed by them enables us to derive expressions for the torques which couple the mantle and the fluid outer core to the solid inner core. Solving the enlarged system of equations, we show that a new nearly diurnal eigenfrequency does emerge; a rough estimate places it not very far from the prograde annual tidal excitation frequency, so that possible resonance effects on nutation amplitudes need careful consideration. Another eigenfrequency, attributable to a wobble of the inner core, is also found; its value is estimated to be a few times smaller than the wobble frequency that the inner core would have in the absence of couplings to the rest of the Earth. Considering an expansion, in terms of resonance contributions, of the amplitude of forced nutations normalized relative to that for a corresponding rigid Earth model, we indicate how the coefficients in the expansion are related to those in expansions of the type used by Wahr (1981b). Finally, we discuss the problem of comparing observed nutation amplitudes with those computed on the basis of Earth models generated from seismological data, with special reference to the fact that the dynamical ellipticity of the Earth, as computed from published Earth models which assume the condition of hydrostatic equilibrium, differs significantly from that determined from the precession constant. Numerical results, corrections for unmodeled effects, and comparison with observational results will be dealt with in accompanying papers.
The effects of the solid inner core and nonhydrostatic structure on the Earth's forced nutations and Earth tides
Authors: de Vries Dan , Wahr John M.
Published: February 2008

Recent highly accurate nutation results from very long baseline interferometry (VLBI) disagree with the 1980 International Astronomical Union (IAU) nutation model by about 2 milliarcseconds (mas). Most of the discrepancy is at the retrograde annual frequency, and it has been interpreted as being due to a nonhydrostatic core‐mantle boundary (CMB) ellipticity [Gwinn et al., 1986]. The ellipticity affects the eigenfrequency of the free core nutation (FCN) normal mode, predominantly a relative rotation about an equatorial axis between the fluid outer core (OC) and the solid mantle. This interpretation in terms of a nonhydrostatic CMB shape is consistent with inferences from diurnal tidal gravity measurements [Levine et al., 1986; Neuberg et al., 1987]. The theory that this interpretation is based on assumes that the inner core (IC)is a fluid. Furthermore, the Earth is presumed to be everywhere hydrostatically prestressed, and so it is inconsistent to use results from this theory to infer information about nonhydrostatic structure. In this paper, we extend the theoretical results to include the effects of a solid IC, and of nonhydrostatic structure. The presence of the IC is responsible for a new, nearly diurnal, prograde normal mode (for a hydrostatically prestressed Earth, the frequency is (1 – 1/471) cycles per day) that involves a relative rotation between the IC and OC about an equatorial axis. We call this newly modeled normal mode the free inner core nutation (FICN). The FICN causes a perturbation in the prograde semiannual nutation amplitude of close to 0.2 mas, but the contribution to the retrograde annual nutation is only 0.003 mas, not enough to explain the VLBI discrepancy. Its effects on the diurnal Earth tide are negligible. Nonhydrostatic structure affects the eigenfrequency of this mode and causes the nutation and Earth tide amplitudes to depend on IC‐OC boundary, CMB, and outer surface topography at all spatial scales. The dependence, though, falls off as the (l ‐ 1)th power of the ratio of the IC radius divided by the OC radius, for the simplified case of a homogeneous, rigid IC and mantle, and homogeneous, incompressible OC (l, here, is the angular order in the spherical harmonic expansion of the boundary shape). For small l, nonhydrostatic effects on the FICN are of the same order as hydrostatic effects for this specialized case. Neither the IC nor the nonhydrostatic terms significantly affect the conclusions about the values of the CMB ellipticity.
Permafrost response to surface temperature change and its implications for the 40,000‐year surface temperature history at Prudhoe Bay, Alaska
Authors: Harrison W. D.
Published: February 2008

The geothermal data of Lachenbruch et al. [1982] in ice‐bearing permafrost at Prudhoe Bay are interpreted in terms of the temperature history on a time scale of 40,000 years or longer. To this end, two theories are developed: a “forward” theory to calculate the response of ice‐bearing permafrost to a surface temperature disturbance, and an “inverse” theory to calculate parameters characterizing the surface temperature history from suitable measurements in this permafrost. The inverse theory shows that two pieces of information characterizing the surface temperature history on a long time scale, and the equilibrium geothermal heat flux, can usually be determined but the results depend upon the quality of the data, the structure of the surface temperature disturbance, and the timing of the measurements; the memory of the permafrost may range from near zero to hundreds of thousands of years. The geothermal data indicate that the Prudhoe Bay ground surf ace temperature has a 40,000‐year average of −10.9±3.0°C, and a value at glacial maximum of −10.9±4.6°C. The limits correspond to the estimated uncertainties in the data, and they depend somewhat on the structure of the surface temperature disturbance over the last 40,000 years or so. The −10.9°C temperature is the same as that which prevailed before the 1.8 K warming of the past century. The limits placed on the variation of the ground surface temperature by this interpretation are not in accord with limits placed by some existing interpretations of other paleoclimatic indicators. It is concluded that a consistent understanding of climate history and geothermal regime in northern Alaska does not yet exist.
Three‐dimensional analysis of gravity anomalies in the Mark Area, Mid‐Atlantlc Ridge 23°N
Authors: Morris E. , Detrick R. S.
Published: February 2008

We have carried out a three‐dimensional analysis of the gravity field over the Mid‐Atlantic Ridge at Kane (MARK) area, a well‐studied portion of the Mid‐Atlantic Ridge south of the Kane fracture zone between 23°40′N and 22°30′N. The Mid‐Atlantic Ridge in this area is composed of two distinct spreading segments separated by a small nontransform discontinuity at 23°15′N. The mantle Bouguer anomalies in the MARK area are dominated by V‐shaped bands of anomalous mass and small‐amplitude gravity lows and highs located both on and off‐axis, which indicate that the anomaly distribution is more complex than the simple bulls eye pattern described along some other parts of the Mid‐Atlantic Ridge. Within the Kane fracture zone, and in the ridge‐transform intersection area, mantle Bouguer anomalies do not accurately reflect the extent of crustal thinning that has been documented seismically. This is attributed to lower average crustal and mantle densities, due to fracturing and alteration, within the fracture zone and the inability of gravity data to resolve the abrupt change to thinner fracture zone crust which is confined to a narrow zone along the fracture zone trough and beneath the nodal basin at the ridge‐transform intersection. Outside of the Kane fracture zone the magnitude of the crustal thickness variations predicted solely from the residual gravity anomalies compare reasonably well with the crustal structure derived from seismic refraction studies. The largest residual gravity anomalies, implying significant reductions in crustal thickness, are observed over the small nontransform offset at 23° 15′N. The anomalous mass associated with this feature forms a v‐shape that can be traced well off‐axis suggesting that these discontinuities are fundamental ridge segmentation boundaries that can persist for millions of years. The distinctive V‐shaped pattern of gravity anomalies in the MARK area reflects systematic crustal thickness variations related to the northward migration of this nontransform offset over the past 3–4 m.y.
Paleointensity of the Earth's magnetic field recorded by two Late Quaternary volcanic sequences at the Island of La Réunion (Indian Ocean)
Authors: Chauvin Annick , Gillot Pierre‐Yves , Bonhommet Norbert
Published: February 2008

A paleomagnetic study has been undertaken on two late Quaternary volcanic sequences from the Piton de la Fournaise volcano (island of La Réunion) in order to provide absolute paleointensities of the Earth's magnetic field from a site in the southern hemisphere. The pattern of secular variation recorded by both sequences clearly indicates that the extrusion rate of the lava flows was not constant with time. A detailed investigation of the magnetic properties of the samples was carried out in parallel with paleointensity experiments which were performed with the Thellier method. Samples with minimum overprint, composed by single‐domain or pseudo‐single‐domain magnetite showing good thermal stability and a narrow blocking temperature spectrum were the most successful during Thellier's experiments. Values of the paleofield strength from 19 to 55 mT have been obtained on 23 flows among the 30 which were sampled. Few variations (7.5–9.9×1022 A m2) are observed in the virtual dipole moments (VDMs) determined on the youngest sequence (5–12 ka). Those values are within the range of fluctuations of the worldwide variations of VDMs for this period. On the other hand, data from the oldest sequence (82–98 ka) are the first paleointensity values obtained in that time period, and the observed VDMs extend from 4.1 to 8.8×1022 A m2. These results suggest that for this time interval, the field did not depart from its average behavior.
Paleomagnetism of the Silurian volcanism at Almaden, southern Spain
Authors: Perroud Hervé , Calza Françoise , Khattach Driss
Published: February 2008

The Silurian volcanism of the Almaden mining district, southern Spain, has been the object of a very detailed paleomagnetic study. The 175 cores (25 sites) sampled all along the eastern termination of the Almaden syncline were subjected to magnetic susceptibility measurements, thermomagnetic cycles, progressive alternating field and thermal demagnetizations as well as microscopic observations. The magnetic mineralogy of these volcanics flows is dominated by magnetite in at least two very distinct forms, including small cubic distributed grains and large elongated crystals. The observed anisotropy of magnetic susceptibility varies according to the relative abundance of the two magnetite phases, showing a complete inversion of the susceptibility tensors. This anisotropy has been related to the emplacement of the flows on a gentle slope dipping ∼ 10° toward the southwest, implying that their volcanic source was located to the northeast. The paleomagnetic analysis has revealed two characteristic components of magnetization: the first (direction D=109°, I=+11°, α95=9°, pole position at 11°S, 69°E, dp=5°, dm=9°) yields a negative fold test and is interpreted as a Late Paleozoic partial overprint; its declination is 46° away of the expected Late Paleozoic field direction, which confirms that rotations are still occurring after the main folding phases of the Variscan orogeny; the second (direction D=62°, I=−36°, α95=14°, pole position at 8°N, 118°E, dp=9°, dm=13°) is prefolding as evident from the significant clustering when applying appropriate tectonic corrections, and we interprete it as the original magnetization of Silurian age. The compilation of all Paleozoic paleomagnetic results from Spain reveals that the Iberian Meseta has been moving along with Gondwana at high drift rates throughout Ordovician and Silurian times.
Lithospheric extension near Lake Mead, Nevada: A model for ductile flow in the lower crust
Authors: Kruse Sarah , McNutt Marcia , Phipps‐Morgan Jason , Royden Leigh , Wernicke Brian
Published: February 2008

Small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions. In this study we examine the boundary between the unextended Colorado Plateau and a strongly extended domain in the Basin and Range Province in the Lake Mead (Nevada) region. Bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small. Analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly dependent on the channel thickness and on the length scale of flow required. Finite element modeling of Newtonian flow and power law creep shows that flow over the length scale of the eastern Basin and Range (500 km or more) corresponding to upper crustal extension by a factor of 1.4–3 over 10 m.y. requires effective viscosities less than 1018–1020 Pa s for ductile channels 10–25 km thick. Flow over shorter length scales (150 km) may be accommodated with effective viscosities as high as 1021 Pa s. Modeling suggests that these effective viscosities may be sustained by lower crustal material deforming at laboratory‐derived power law creep rates. The longer‐scale flow may require elevated crustal temperatures (more than 700°C), depending on the composition and material properties assumed. Under the boundary conditions assumed in this study the linear viscous flow models yield a satisfactory approximation to deformation by power law creep. This work suggests that flow in the lower crust may be a viable mechanism for producing small variations in total crustal thickness between strongly extended and less extended regions, and thereby explaining the relative uniformity in gravity and topography between such regions.
Orbital forcing of low‐frequency glacioeustasy
Authors: Matthews R. K. , Frohlich Cliff
Published: February 2008

The role of orbital forcing in glacioeustasy has long presented a major problem regarding the reconciliation of data‐oriented seismic sequence stratigraphy with process‐oriented dynamic stratigraphy. Dynamic stratigraphy commonly cites orbital forcing periods of approximately 20,000 years (precession), 40,000 years (tilt), and 100,000 years (eccentricity) which are too fine scale to be resolved consistently by seismic sequence stratigraphy. Seismic sequence stratigraphy consistently identifies seemingly eustatic events on a scale of millions of years which have no heretofore easily identifiable causal mechanisms. In this paper, we propose a geologic mechanism and realistic quantitative construct to explain how Antarctic ice volume varies as a function of orbital forcing in the Tertiary. We demonstrate that this mechanism, which has a nonlinear response to long‐period modulation of the orbital forcing time series, can produce major glacioeustatic events with quasi‐periodicities of the order of 2 m.y. We use a FORTRAN program, STRATA‐various, to construct a two‐dimensional forward model demonstrating that this proposed mechanism can produce a synthetic sequence stratigraphy which bears strong resemblance to the generalizations of seismic sequence stratigraphy. We acknowledge important achievements of seismic sequence stratigraphy. Nevertheless, we propose that the seismic sequence stratigraphy concepts of long‐term and short‐term eustatic curves be replaced by independent estimates of tectonoeustasy and glacioeustasy based upon data sets which are, wherever possible, independent of seismic sequence stratigraphy. Likewise, we propose that qualitative generalizations be replaced with explicit forward models as the targets for model/data convergence.
Tidal responses and earthquake‐related changes in the water level of deep wells
Authors: Igarashi George , Wakita Hiroshi
Published: February 2008

Monitoring of the water level has been continued in 100‐m‐deep and 500‐m‐deep wells in the southern Kanto District, Japan, to examine the relationship between changes in the well water level and earthquake occurrence. By using a Bayesian approach, the observed data were decomposed into four components: tidal response, barometric response, trend (drift), and irregular (noise) components. Loading effects of the ocean tide on aquifer dilatation were evaluated by using Schwiderski's ocean tide models and the harmonic constants compiled by the International Hydrographic Bureau. Well‐aquifer properties were examined based on the tidal and barometric responses of the water level. Coseismic water level steps were detected in both wells for two earthquakes during the observation period from April 1985 to December 1989. By calibrating the strain sensitivity of the well water levels based on the tidal responses, strain changes equivalent to the coseismic steps in the water levels were estimated. The coseismic strain changes obtained for the deeper well can be explained reasonably by earthquake fault models in a viscoelastic half‐space. However, those observed in the shallower well were too large and in the wrong direction to be explained by the earthquake fault models.
Paleomagnetism of the Patagonian Plateau Basalts, southern Chile and Argentina
Authors: Butler Robert F. , Hervé Francisco , Munizaga Francisco , Beck Myrl E. , Burmester Russell F. , Oviedo Eduardo S.
Published: February 2008

A total of 505 paleomagnetic samples were collected from 65 sites (volcanic flows) of the Patagonian plateau basalts at four locations in southern Chile and Argentina. K/Ar analyses indicate that 38 flows from two locations form a Late Cretaceous group (64–79 Ma), while the remaining 27 flows are Eocene in age (42–56 Ma). Progressive demagnetization indicates that a characteristic remanent magnetization (ChRM) has been successfully isolated from 59 flows. Rock‐magnetic properties and analysis of ChRM directions within and between sites allow the secure inference that the ChRM is a thermoremanent magnetization acquired during original cooling. The Eocene flows yield 15 independent samplings of the paleomagnetic field. However, these data fail the reversals test and probably do not adequately average geomagnetic secular variation. The Late Cretaceous flows yield 18 independent samplings of the paleomagnetic field, six of normal polarity and 12 of reversed polarity. These data pass the reversals test and have angular dispersion of virtual geomagnetic poles (VGPs) consistent with adequate sampling of geomagnetic secular variation. The mean of these 18 VGPs yields a Late Cretaceous paleomagnetic pole for South America: latitude = 78.7°S; longitude = 358.4°E; A95 = 6.3° (K = 31.6; S = 14.5°). This pole is consistent with similar age poles from North America and Africa when the Atlantic is reconstructed to 70 Ma. Contrary to previous indications, the South American apparent polar wander path does not reach the present rotation axis until after Late Cretaceous time.
A study of the use of scatterometer data in the European Centre for Medium‐Range Weather Forecasts operational analysis‐forecast model: 2. Data impact
Authors: Anderson D. , Hollingsworth A. , Uppala S. , Woiceshyn P.
Published: February 2008

Data from the Seasat scatterometer are assimilated into the ECMWF analysis‐forecast model. To assess the impact of using Seasat A satellite scatterometer (SASS) data, comparison is made between analyses which use this data type and those which do not, as well as between forecasts from these analyses. The differences in the northern hemisphere are found to be small, but sizeable differences occur in the southern hemisphere. Some synoptic comparisons are made between SASS data and the model analyses, as well as data impact on forecasts for the QE‐II and Ark Royal storms.
Modulated flows beneath wind‐ruffled, mechanically generated water waves
Authors: Jiang Jih‐Yih , Street Robert L.
Published: February 2008

Data taken beneath wind‐ruffled, mechanically generated water waves in both the fixed‐probe and the wave‐following coordinate systems were analyzed. A phase‐averaging technique was used to define the component associated with the monochromatic, mechanically generated waves and to separate that component from the fluctuating one which is associated with ripples and turbulence. When observed in the wave‐following frame, maxima of the fluctuating normal stresses occur near the wave crest; the fluctuating shear stress is positive and possesses a peak on the windward face of the mechanically‐generated wave. The data in fixed‐probe and wave‐following measurements are consistent when an appropriate transformation is used between the two. The orbital velocities of waves were computed by our implementation of the nonlinear theory of Dean. Then, the modulated mean current was obtained by subtracting the orbital velocity of the wave and the time‐averaged velocity from the phase‐averaged velocity. The results show strong modulation in the current boundary layer. The horizontal component of the modulated mean current is positive in the trough of the carrier‐wave profile and negative near the crest. The range of fluctuation in a wave profile decays with depth.
Turbulent mixing near the Yermak Plateau during the Coordinated Eastern Arctic Experiment
Authors: Padman Laurie , Dillon Thomas M.
Published: February 2008

During the ice camp component of the Coordinated Eastern Arctic Experiment (CEAREX) in March–April 1989, 1500 profiles of temperature, conductivity, and velocity shear microstructure were obtained near the Yermak plateau. The dominant signal in isopycnal displacements was the diurnal variability near the plateau, consistent with the enhanced diurnal currents found in this region previously from the Fram III and Fram IV ice camps. The dissipation rate of turbulent kinetic energy ε reached maximum values of about 10−6 W kg−1 in two distinct regions, the surface mixing layer and the pycnocline between 120 and 220 m. There was a significant component of diurnal variability in ε in each region. In the surface layer, ε was proportional to the cube of the ice‐relative current speed at 30 m below the ice, and it decayed approximately exponentially with increasing depth. Major mixing events in the pycnocline were correlated with large‐amplitude, short‐duration, isopycnal displacements. The lowest dissipation rates were recorded over the deep Nansen basin, north of the plateau, and in the subsurface core of a submesoscale baroclinic eddy in northern Fram Strait. The time‐averaged vertical eddy diffusivity near the slope of the plateau was about 2.5×10−4 m2 s−1 in the pycnocline above the Atlantic layer, implying an upward heat flux of 25 W m−2, although only a small fraction of this heat reached the base of the mixed layer. The results confirm that current interactions with steep topography are critical to the modification of Atlantic Water in the eastern Arctic Ocean.
The aerodynamic roughness of different types of sea ice
Authors: Guest Peter S. , Davidson Kenneth L.
Published: February 2008

The aerodynamic roughness for all major types of sea ice is specified on the basis of surface layer measurements from ship and ice floe platforms. Median neutral surface drag coefficients, Cdn×1000, for grease and nilas ice are 0.7 and 1.6. Small, medium, large, and fused pancake ice have Cdn equal to 0.9, 1.6, 2.4, and 1.9. Young ice Cdn ranges from 2.3 to 3.1. Very smooth first‐year or multiyear ice with no pressure ridges has Cdn equal to 1.5. Pack ice has median values of 2.0 and 2.2 for first‐year and multiyear ice. Marginal ice zone ice averages 3.1 and 3.4 (first‐year, multiyear) if unaffected by wave action. Wave‐affected ice has Cdn values of 4.2 and 4.6 (first‐year, multiyear). Extremely rough multiyear ice in intense shear or compaction regions has Cdn equal to 8.0. Open ocean areas within 100 km of an ice edge averaged 1.8, while ice‐free water downwind of ice, with small or no waves is 1.4. These values can be adjusted if heat fluxes are present. The accuracy of the Cdn averages is estimated to be 20% or better.
Microwave and physical properties of sea ice in the winter Marginal Ice Zone
Authors: Tucker W. B. , Grenfell T. C. , Onstott R. G. , Perovich D. K. , Gow A. J. , Snuchman R. A. , Sutherland L. L.
Published: February 2008

Surface‐based active and passive microwave measurements were made in conjunction with ice property measurements for several distinct ice types in the Fram Strait during March and April 1987. Synthetic aperture radar imagery downlinked from an aircraft was used to select study sites. The surface‐based radar scattering cross section and emissivity spectra generally support previously inferred qualitative relationships between ice types, exhibiting expected separation between young, first‐year and multiyear ice. Gradient ratios, calculated for both active and passive data, appear to allow clear separation of ice types when used jointly. Surface flooding of multiyear floes, resulting from excessive loading and perhaps wave action, causes both active and passive signatures to resemble those of first‐year ice. This effect could possibly cause estimates of ice type percentages in the marginal ice zone to be in error when derived from aircraft‐ or satellite‐borne sensors.
Evidence for stability enhancement of sea ice in the Greenland and Labrador Seas
Authors: Marsden R. F. , Mysak L. A. , Myers R. A.
Published: February 2008

Malmberg proposed in 1969 that anomalously low surface (upper 200 m) salinities in the Iceland seas during the late 1960s could increase the stability of the water column sufficiently to prevent deep convection. The increased stability would then enhance the formation of sea ice during winter in this region because even at the freezing point (−1.8°C), the surface water density is sufficiently low to prevent mixing with the water below. Hence only a small portion of the water column need be cooled before freezing can occur and ice formation is enhanced. Here we explore this concept further by examining hydrographic data from three locations bordering Greenland and Iceland and then compare these with colocated sea ice concentrations. For the period 1953–1980, it is shown that the lag correlations between surface salinity and sea ice anomalies show a common structure for the case where salinity anomalies lead sea ice anomalies. It is shown that this correlation structure can be reproduced by a simple statistical feedback model using a nonstationary negative feedback coefficient to account for salinity forcing of ice anomalies. The data presented are all consistent with the concept of stability enhancement of sea ice formation.
Relationship between gas exchange, wind speed, and radar backscatter in a large wind‐wave tank
Authors: Wanninkhof Richard H. , Bliven L. F.
Published: February 2008

Gas transfer velocities across the air‐water interface are correlated with wind speed, friction velocity, and radar backscatter from the surface in a large wind‐wave tank in Delft, the Netherlands. The rates of sulfur hexafluoride and nitrous oxide exchange were measured at wind speeds ranging from 3.5 m/s to 20 m/s, and with mechanically generated waves in the 100 m long, 8 m wide, and 0.7 m deep tank. Gas transfer velocities were related to wind speed with a power law dependence. Gas transfer showed a linear dependence with friction velocity. An exponential relationship between gas transfer and average radar backscatter cross section fits the wind tunnel data well for microwave units operating at 13.5 and 35 GHz in VV and HH polarization. The relative rates of gas exchange of the two gases confirm an inverse square root dependence on the Schmidt number at intermediate wind speeds. A slight enhancement of sulfur hexafluoride exchange compared to nitrous oxide exchange was evident when breaking waves were present.
Iron availability, nitrate uptake, and exportable new production in the subarctic Pacific
Authors: Banse Karl
Published: February 2008

Rates of nitrate uptake in bioassays from the open Gulf of Alaska by Martin et al. (1989) are reinterpreted. During the exponential phase of phytoplankton growth, iron addition affected the rate of nitrate uptake significantly at only one of the three stations (position of the former ocean weather station “Papa”); results from one station (northern) are not interpretable. Appreciable nitrate uptake and algal growth in all controls during the first few days appears to have consumed a few to several times as much dissolved iron as was found initially; this suggests high solubilization (regeneration) rates or uptake of iron moities other than iron measured as dissolved. Exponential increases of biomass at demonstrated reduced grazing made the iron demand in the controls outrun the supply after the first few days, in contrast to the iron treatments. It is suggested that in the field, grazing normally seems to prevent the phytoplankton from reaching concentrations that reduce the iron (and nitrate) to levels that depress division rates drastically. It is unclear whether the flourishing of medium‐sized diatoms in the assays reflects removal of iron limitation on division rates of these rare species or exclusion of grazers from the containers. Although nitrate uptake may be equated with new production, the export to depth of new organic material (and hence of CO2) is not predictable from assays in small (liter) containers, especially so because the role of the large grazers on the size composition of the phytoplankton and the production of large sinking particles is not evaluated.
The response of the Delaware estuary to the combined forcing from Chesapeake Bay and the ocean
Authors: Wong Kuo‐Chuin
Published: February 2008

Observational evidence suggests that a substantial fraction of the subtidal volume exchange in the Delaware estuary is associated with the exchange through the Chesapeake and Delaware Canal, which connects the upper reach of Delaware estuary to the head of Chesapeake Bay. An analytical model is developed to examine the effect of the combined forcing from Chesapeake Bay and the ocean on the barotropic volume exchange within the Delaware estuary. The relative importance of these two forcing mechanisms depends strongly on the time scales involved. At the semidiurnal tidal frequency, remote forcing from Chesapeake Bay is not important to the exchange process in the Delaware estuary, and the response of the estuary is predominantly driven by forcing from the ocean. At subtidal frequencies, however, forcing from Chesapeake Bay via the canal is just as effective as forcing from the ocean in producing volume exchange within part of the Delaware estuary. Furthermore, the presence of the canal significantly modifies the way in which the Delaware estuary responds to forcing from the ocean.
Gravity current propagation along an incline in the presence of boundary mixing
Authors: Noh Y. , Fernando H. J. S.
Published: February 2008

The effects of boundary mixing on a continuous gravity current (a plume) propagating along an incline were investigated using a laboratory experiment. Boundary mixing was generated by oscillating the incline along its plane. It was observed that when the background turbulence is strong, such typical characteristics of the gravity current as the “raised‐head” structure and the sharp front disappear. Quantitative measurements on the frontal velocity, density structure, and the thickness of the gravity current were made, and their dependencies on the intensity of turbulence at the boundary are presented.
An intense acoustic scattering layer at the top of a mid‐ocean ridge hydrothermal plume
Authors: Thomson Richard E. , Gordon R. Lee , Dolling Adrian G.
Published: February 2008

We have used a tethered oceanic profiling package consisting of a conductivity‐temperature‐depth (CTD) sensor, transmissometer and acoustic Doppler current profiler (ADCP) to investigate the plume emanating from a known hydrothermal vent site at a depth of 2200 m near the crest of Endeavour Ridge in the northeast Pacific Ocean. Acoustic profiles obtained in August 1988 about 3 km to the southwest of the vent site revealed the presence of an intense scattering layer centered at approximately 1870 m depth near the top of the neutrally buoyant portion of the plume. Peak intensity anomalies in the 100‐m‐thick scattering layer ranged from 10 to 15 dB and were of comparable magnitude to upper ocean values observed in the euphotic zone. Below the top of the plume, backscatter intensities were significantly below background levels. We speculate that the strong backscatter layer consists of highly mobile macrozooplankton which congregate near the top of the plume to take advantage of the increased concentrations of nutrients originating with vertical buoyant transport of chemosynthetic bacteria and fine‐grain particles from the vent site. At the same time, the animals are able to avoid toxic levels of hydrothermally derived metals and sulfur inherent to the main body of the plume.
Intercomparing shortwave radiation codes for climate studies
Authors: Fouquart Y. , Bonnel B. , Ramaswamy V.
Published: February 2008

As a second step of the international program of Intercomparison of Radiation Codes Used in Climate Models (ICRCCM), an intercomparison of shortwave radiation models was initiated. Among the 26 codes that participated in the comparison were very detailed (line‐by‐line), narrow‐band (high‐spectral resolution), as well as highly parameterized (low‐spectral resolution) models. A considerable spread was detected in the response of these models to a set of well‐defined atmospheric profiles. Substantial discrepancies exist among models even for the simplest case of pure water vapor absorption with standard deviation ranging from 1% to 3% for the downward fluxes at the surface and from 6% to 11% for the total atmospheric absorption. The divergences in downward surface flux increase to nearly 4% when all absorbers and the molecular scattering are considered. In cloudy conditions the divergences range from 4% to 10%, depending on the cloud optical thickness. Another major uncertainty that has been identified is the spectral averaging of the scattering properties which can result in very significant errors for low spectral resolution codes. Since these errors appear to be systematic, they may induce unrealistic feedback mechanisms in numerical climate models. The amplitude of the differences between models is in many cases larger than the accuracy required for the achievements of several objectives of the World Climate Research Program. While reference solutions for the absorption and scattering in atmospheres can be obtained based on the state‐of‐the‐art spectroscopic knowledge and rigorous computational techniques, the absolute tests of the validity of the radiation algorithms would be comprehensive field experiments in which the radiative and all relevant atmospheric parameters are measured to a high degree of accuracy.
The effects of geomagnetic disturbances and atmospheric tides upon middle atmosphere winds and MF radar scatter at Saskatoon (52°N, 107°W, 61° geomagnetic)
Authors: Manson A. H. , Meek C. E.
Published: February 2008

The radar‐derived winds (60–110 km) at Saskatoon (52°N, 107°W, 61° geomagnetic) for the summer and winter months of 1982/1983 are separated into categories according to the daily planetary magnetic index Ap: quiet <7(Q), moderate 7–19(M), and disturbed >19(D). The data yield, which is directly related to the scatter of radar pulses from the middle atmosphere, is much enhanced for both the moderate and disturbed categories during nighttime and early morning hours. The seasonal and height variations of radar scatter are related to particle precipitation characteristics. Above 90 km there are also small changes in the mean winds and in the amplitudes of both 24‐ and 12‐hour tidal oscillations (10%) due to variations in magnetic activity. The small phase changes identified for both tidal oscillations show interannual variation and are most likely due to data selection effects. The wind data are further separated according to the amplitudes of the daily tides (greater or less than 20 m/s). The yield of radar data is significantly greater when the tides are large for each season and disturbance category. This effect is related to the dynamic stability of the atmosphere and the increase of turbulence and hence radar scatter due to the large tidal amplitude.