bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of lithospheric and mantle stratification on global post‐seismic deformation
Authors: Sabadini R. , Vermeersen L. L. A.
Published: February 2004

A new class of multilayered, viscoelastic Earth models based on PREM, with an incompressible, linear, viscoelastic Maxwell rheology, is applied to the modeling of co‐seismic and post‐seismic deformation following large earthquakes. The novelty of our approach stands on the usage, for the first time in post‐seismic studies, of a fully analytical scheme based on normal mode theory that allows to deal with some of the complexities of the real Earth, such as lithospheric and mantle layering, sphericity and self‐gravitation. For a dip‐slip source, used as a test case, elastic layering of the lithosphere is necessary for the modeling of realistic patterns of post‐seismic deformation, although only a few layers, namely three, are sufficient to reproduce correct deformation patterns, in consequence of the fact that below the crust, the lithosphere inferred from PREM is rather homogeneous. The viscosity structure of the mantle has a major influence on post‐seismic deformation in the far field, at distances from the epicenter of the order of 100 km. At least for lithospheric sources, precise modeling of post‐seismic deformation does not require for the mantle the same fineness of layering that is necessary for the lithosphere. Our findings are key for future analyses of the co‐seismic and post‐seismic effects of large earthquakes, in view of a correct interpretation of VLBI, GPS and Satellite Differential Radar Altimetry data.
The magnetic field and magnetosphere of Ganymede
Authors: Kivelson M. G. , Khurana K. K. , Coroniti F. V. , Joy S. , Russell C. T. , Walker R. J. , Warnecke J. , Bennett L. , Polanskey C.
Published: February 2004

Within Jupiter's magnetosphere, Ganymede's magnetic field creates a mini‐magnetosphere. We show that the magnetic field measured during Galileo‧s second pass by Ganymede, with closest approach at low altitude almost directly over the moon's polar cap, can be understood to a large measure in terms of the structure of a vacuum superposition model of a uniform field and a Ganymede‐centered dipole field. Departures from the simple model can be attributed principally to magnetopause currents. We show that the orientation of the observed magnetopause normal is qualitatively consistent with expectations from the vacuum superposition model. The magnetopause currents inferred from the inbound boundary crossing are closely related to expected values, and the magnetic structure of the boundary is similar to that observed at the magnetopause of Earth. We use the vacuum magnetic field model to infer the magnetic field near Ganymede's surface, and thereby predict the particle loss cones that should be present along the spacecraft trajectory. By mapping a fraction of the corotation electric field into the polar cap, we determine expected flow velocities near closest approach to Ganymede as a function of reconnection efficiency. We conclude by discussing prospects for measurements on Galileo's remaining passes by Ganymede.
Northern boundary currents and adjacent recirculations off southwestern Australia
Authors: Hufford Gwyneth E. , McCartney Michael S. , Donohue Kathleen A.
Published: February 2004

Full‐depth hydrographic and velocity measurements along 115°E south of Australia (WHP section I9S) reveal an active northern boundary current regime within 200–300 km of the Australian continental shelf and an offshore recirculation regime within the South Australian Basin. In the upper ocean an oft‐described westward‐flowing northern boundary current, the Flinders Current, extends through the thermocline and the salinity minimum strata of Antarctic Intermediate Water, with an opposing eastward recirculation to its south. The new data shows that beneath the Flinders Current, an eastward‐flowing deep northern boundary current transports water of subtropical Indian Ocean origin supplied by southward deep flow along the western continental slope of Australia in the Perth Basin. Between these northern boundary currents and the Antarctic Circumpolar Current, thermocline and deep geostrophic shears indicate weak recirculations, eastward at thermocline levels and westward in the deep water. Lowered Acoustic Doppler current profiler data reveal stronger organized recirculations to both the Flinders Current and the deep northern boundary current, with the westward deep recirculation exceeding the eastward transport of the deep northern boundary current and thus representing a net supply of deep water to the Perth Basin.
Ion heating in oblique low‐Mach number shocks
Authors: Gedalin M.
Published: February 2004

The contribution of the directly transmitted ions to ion heating in the low‐Mach number oblique shock is determined analytically. We derive approximate expressions for the pressure tensor inside the ramp of a low‐Mach number low‐β shock. In the thin shock limit the ion state equation p ∝ n³ is recovered within the ramp, which corresponds to the complete demagnetization of ions and effective one‐dimensional behavior. We derive the estimate for the maximum downstream temperature of the heated ion distribution, which depends only on the magnetic compression ratio and cross‐shock potential, and may be verified observationally.
The distribution of sulfur dioxide and other infrared absorbers on the surface of Io
Authors: Carlson R. W. , Smythe W. D. , Lopes‐Gautier R. M. C. , Davies A. G. , Kamp L. W. , Mosher J. A. , Soderblom L. A. , Leader F. E. , Mehlman R. , Clark R. N. , Fanale F. P.
Published: February 2004

The Galileo Near Infrared Mapping Spectrometer was used to investigate the distribution and properties of sulfur dioxide over the surface of Io, and qualitative results for the anti‐Jove hemisphere are presented here. SO2, existing as a frost, is found almost everywhere, but with spatially variable concentration. The exceptions are volcanic hot spots, where high surface temperatures promote rapid vaporization and can produce SO2‐free areas. The pervasive frost, if fully covering the cold surface, has characteristic grain sizes of 30 to 100 µm, or greater. Regions of greater sulfur dioxide concentrations are found. The equatorial Colchis Regio area exhibits extensive snowfields with large particles (250 to 500 µm diameter, or greater) beneath smaller particles. A weak feature at 3.15 µm is observed and is perhaps due to hydroxides, hydrates, or water. A broad absorption in the 1 µm region, which could be caused by iron‐containing minerals, shows a concentration in Io's southern polar region, with an absence in the Pele plume deposition ring.
Impact of the extratropical Pacific on equatorial variability
Authors: Lysne JoAnn , Chang Ping , Giese Benjamin
Published: February 2004

An ocean general circulation model (OGCM) is used with a data assimilation routine to explore how the higher latitudes in the Pacific affect equatorial variability. The assimilation of observed temperature data in the extratropical regions generates decadal variability on the equator, indicating that the subsurface thermal structure of the tropics may be modulated by low‐frequency variability in the higher latitudes. While preliminary analyses show that midlatitude temperature anomalies can be advected to the subtropics along isopycnal surfaces, it is also demonstrated that a wave mechanism may be one of the dominant physical processes by which information is carried from the higher latitudes to the equatorial region.
Growing the Earth's D″ layer: Effect of density variations at the core‐mantle boundary
Authors: Kellogg Louise H.
Published: February 2004

The D″ region of the mantle has been interpreted as a variable‐thickness layer of hot, chemically heterogeneous material at the base of the mantle. This paper reports the results of numerical simulations testing whether growth of a chemically distinct layer at the base of the mantle is possible by influx of molten iron alloy from the core to the mantle. A finite‐element model of thermo‐chemical mantle convection was used to simulate exchange of material between the mantle and core. Growth of a significant layer at the base of the mantle depends on several factors; we focus on the effect of density of the material in the chemical boundary layer. Relatively low density material was swept away from the core‐mantle boundary by mantle upwellings. Very high density material, in contrast, was not swept away, but flowed into the mantle too slowly to form a layer as thick as D″. For moderate density contrasts between material in a chemical boundary layer and the overlying mantle, a laterally heterogeneous layer formed. The stable layer results in an elevated temperature contrast between the core and the mantle. High temperatures in this layer are associated with broad regions of elevated temperature in the overlying mantle.
An assessment of along‐strike fluid and heat transport within the Barbados Ridge Accretionary Complex: Results of preliminary modeling
Authors: Screaton Elizabeth , Ge Shemin
Published: February 2004

In this investigation, we examined the effects of the three‐dimensional geometry of the Barbados Ridge accretionary complex on decollement zone fluid flow and thermal transport. Modeling results indicate that fluid preferentially escapes the decollement zone through the thin sediment layer north of the Tiburon Rise. This “drain” produces a northerly (along‐strike) component of fluid flow in the vicinity of the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) transect. Depending on pore‐pressure ratios on the arcward boundary of the complex, the along‐strike component can reach half the magnitude of the component perpendicular to the deformation front. Thermal advection along oblique flow paths would result in significantly higher decollement temperatures at the deformation front than indicated by cross‐sectional models constructed perpendicular to the deformation front.
Bromine, iodine, and chlorine in single aerosol particles at Cape Grim
Authors: Murphy D. M. , Thomson D. S. , Middlebrook A. M.
Published: February 2004

Using a laser ionization mass spectrometer, the chemical compositions of single particles larger than 0.16 µm diameter were measured in the marine boundary layer (MBL) at Cape Grim, Tasmania during the ACE‐1 experiment. Chlorine was observed in nearly all of the particles. Bromine was internally mixed with amounts near the sea water mole fraction in most particles. The aerosol mole fraction of iodine was similar to bromine, so iodine was highly enriched relative to sea water. Iodine was not uniformly mixed among all particles. Iodine‐rich particles included sea salt and metals. Iodine was also correlated with the presence of organics in the spectra. Chlorine, bromine and iodine were anti‐correlated with sodium sulfate. Since particles accumulate sulfate in the MBL, this anti‐correlation suggests that the aerosols were losing all three halogens as they aged.
Elasticity of natural majorite and ringwoodite from the catherwood meteorite
Authors: Sinogeikin Stanislav V. , Bass Jay D. , Kavner Abby , Jeanloz Raymond
Published: February 2004

Sound velocities and elastic moduli of natural polycrystalline majorite (Mg0.78Fe0.21Ca0.01)SiO3, and ringwoodite (Mg0.75Fe0.25)2SiO4 from the Catherwood meteorite were measured by Brillouin spectroscopy. These are the first acoustic measurements for such natural high‐pressure phases with Fe contents comparable to that of the Earth's mantle. The adiabatic bulk modulus of majorite is Ks=164(4) GPa, and the shear modulus is μ=87(2) GPa, which are typical values for garnet‐structured silicates. Both the elastic moduli and the sound velocities of natural cubic majorite are slightly lower than values for aluminosilicate garnets with comparable Fe contents. Moreover, the bulk and shear moduli of natural majorite are indistinguishable from those of pure Mg majorite, and the dependence of these moduli on Fe content is small. In contrast, ringwoodite (Ks=193(3), μ=113(2)) exhibits a pronounced change in the aggregate elastic moduli and sound velocities with increasing Fe content. The addition of 10% Fe to γ‐phase changes the velocity by an amount comparable to the velocity change across the β→γ transition in Mg2SiO4. It is therefore necessary to account for the effect of Fe in constructing mineralogical models of the transition zone.
A new look at the nature of comet Halley's LF electromagnetic waves: Giotto observations
Authors: Tsurutani Bruce T. , Lakhina Gurbax S. , Neubauer Fritz M. , Glassmeier Karl‐Heinz
Published: February 2004

All of the comet Halley high‐time resolution magnetic field data have been examined to determine the nature of the “turbulence” and its difference from that of comets Giacobini‐Zinner and Grigg‐Skjellerup. Although much of the wave appears unpolarized, occasionally there are intervals of clear order. We find several interesting new wave polarizations: arc‐, left‐hand/arc‐ (sunglass) and left‐hand circular polarized waves (in the spacecraft frame). The former two types have separations of ∼120 s between individual pulses, indicating that the waves are generated from pickup of the H2O group cometary ions. The third type of waves occurs in a wave‐train and may be a detached whistler packet. The unusual polarizations could be caused by wave refraction in the highly turbulent (and high β) Halley environment or by nonlinear evolution due to strong growth rates. It is noted that some of the large amplitude waves are non‐planar. These results are further details of the “linear polarization” of Halley waves determined by previous coherency analyses, and may explain some of the evolution leading to its plasma turbulence.
Wavelet analysis of dynamical processes in cirrus
Authors: Demoz Belay B. , Starr David O'C. , Chan K. Roland , Bowen Stuart W.
Published: February 2004

Vertical air velocity observations obtained from the NASA DC‐8 on 21 April 1996 are analyzed using wavelet and spectral analysis methods. Wavelet analysis yielded a very useful characterization of the complex dynamical structures encountered above, below and within the cirrus clouds observed on this flight. Intermittent small groups of apparent gravity waves were found at length scales ranging from 2 km to greater than 40 km. Within the cloud, patches of convective cells were also found. The hierarchy of dynamical structures was well discriminated by the analysis.
Micron‐sized dust particles detected in the outer solar system by the Voyager 1 and 2 plasma wave instruments
Authors: Gurnett D. A. , Ansher J. A. , Kurth W. S. , Granroth L. J.
Published: February 2004

During the Voyager 1 and 2 flybys of the outer planets it has been demonstrated that the plasma wave instrument can detect small dust particles striking the spacecraft. In this paper, we examine the Voyager plasma wave data for dust impacts in the interplanetary medium at heliocentric radial distances ranging from 6 to 60 astronomical units (AU). The results show that a small but persistent level of dust impacts exists out to at least 30 to 50 AU. The average number density of these particles is about 2 × 10−8 m−3, and the average mass of the impacting particles is believed to be a few times 10−11 g, which corresponds to particle diameters in the micron range. Possible sources of these particles are planets, moons, asteroids, comets, and the interstellar medium. Of these, comets appear to be the most likely source. The number densities are only weakly dependent on ecliptic latitude, which indicates that the particles probably do not originate from planets, moons, or asteroids. Comparisons with interstellar dust fluxes measured in the inner regions of the solar system by the Ulysses spacecraft indicate that the particles are not of interstellar origin.
Comparison of finite difference‐ and pseudospectral methods for convective flow over a sphere
Authors: Fornberg Bengt , Merrill David
Published: February 2004

For modeling convective flows over a sphere or within a spherical shell, pseudospectral (PS) methods are in general far more cost‐effective than finite difference‐ (or finite element) methods. This study confirms this, and proceeds by comparing a Fourier‐PS implementation (based on a longitude‐latitude grid) with one using spherical harmonics. For similar resolutions, these are found to be of similar accuracy. However, the former is computationally about ten times faster.
On the role of ocean‐atmosphere interaction in midlatitude interdecadal variability
Authors: Weng Wenjie , Neelin J. David
Published: February 2004

A simple midlatitude ocean‐atmosphere model is used to investigate possible roles of coupled feedback in interdecadal climate variability over the North Atlantic and North Pacific oceans. Stochastic forcing by atmospheric internal variability maintains variance in both uncoupled and coupled cases. Coupling contributes to creating an interdecadal mode with distinct spatial pattern and preferred time scale, seen as a broad spectral peak. A near‐analytic solution for the coupled interdecadal mode suggests that the most important parameter in determining the period is the zonal length scale of the atmospheric wind stress feedback over the region. Subject to this scale, the period is then determined by oceanic Rossby wave dynamics, which tends to give westward propagation in subsurface fields. The dipolar sea surface temperature (SST) anomalies are generated primarily by the advection of climatological SST by geostrophic current. Although the magnitude of the feedback of SST to atmospheric response is much smaller than atmospheric internal variability, its effects are significant.
Recent results with an MF radar at McMurdo, Antarctica: Characteristics and variability of motions near 12‐hour period in the mesosphere
Authors: Fritts David C. , Riggin Dennis M. , Balsley Ben B. , Stockwell Robert G.
Published: February 2004

We present an analysis of the first wind measurements obtained with an MF (medium frequency) radar installed at McMurdo Station, Antarctica in January 1996. The largest amplitude motions are the quasi‐12‐hr motions, with additional periodicities at 24 hr, ∼10 hr and 2 to 10 days. The quasi‐12‐hr motions occur at discrete periods distinct from 12 hr and exhibit pronounced amplitude and vertical wavenumber modulations at planetary wave periodicities. During the months analyzed in this report (mid‐January through April 1996) the 12‐hr wave is by far the dominant motion the mesosphere. The source of this wave is unknown, but our results provide additional support for the suggestion that such motions arise through nonlinear interactions involving the migrating semidiurnal tide and low‐frequency planetary wave motions.
Observations of electron velocity distribution functions in the solar wind by the WIND Spacecraft: High angular resolution Strahl measurements
Authors: Fitzenreiter R. J. , Ogilvie K. W. , Chornay D. J. , Keller J.
Published: February 2004

The WIND Solar Wind instrument (SWE) includes a sensor especially configured to measure the solar wind strahl. The strahl is the excess electron (≥100 eV) halo component of the solar wind most closely aligned with the magnetic field. Strahl electrons originate in the inner corona and move freely out to 1 A.U., providing information on the state of the corona. Electron data acquired during seven solar rotations show that the electron velocity distributions are most anisotropic and the strahl flux most intense during high speed streams. The strahl angular width is smallest when the solar wind velocity is largest, approximately 5° in width at 600 eV, and becomes much wider when the velocity is low. Coincident with the strahl is a small reverse electron flux which does not form a beam but fills the field of view. This variability of the flux and angular distribution of the strahl and anti‐strahl suggests that a pitch angle scattering process may be acting to broaden the strahl and that the anti‐strahl may be due to backscattering of the strahl.
High resolution climate record from the Beijing area during the Last Glacial‐Interglacial Cycle
Authors: Liu X. M. , Hesse P. , Liu T. S. , Bloemendal J.
Published: February 2004

The Zhaitang loess section, located in the western suburbs of Beijing has yielded a very high resolution magnetic susceptibility record for the last‐glacial and interglacial period. All substages can be clearly defined in marine isotope stage (MIS) 5 (S1) and even also in substage 5a, demonstrating comparable detail with Vostok and parts of the GRIP ice record. Rapid climate shifts during the Eemian period may have extended to continental Asia, but exerted only a weak influence on the Zhaitang record. Abrupt climate transitions (5b/5c/5d/5e) recorded in this section from the NE edge of the Plateau show more detrial than most other Chinese loess records. These three well defined peaks in substage 5a, imply sensitivity of the east Asian monsoon to rapid hemisphere‐scale climate change and the prospect of finding more information on orbital and non‐orbital events in the loess sequence.
Melting and crystal structure of iron at high pressures and temperatures
Authors: Shen Guoyin , Mao Ho‐kwang , Hemley Russell J. , Duffy Thomas S. , Rivers Mark L.
Published: February 2004

High‐pressure melting, phase transitions and structures of iron have been studied to 84 GPa and 3500 K with an improved laser heated diamond anvil cell technique and in situ high P‐T x‐ray diffraction. At pressures below 60 GPa, the lower bound on the melting curve is close to those measured by Boehler [1993] and Saxena et al. [1993]; however, at pressures above 60 GPa our data indicate melting at higher temperatures than these studies, but still lower than the melting curve of Williams et al [1990]. The ε‐γ‐1 triple point is 60(±5) GPa and 2800(±200) K, based on our data of the ε‐γ phase transition and the observation of melting by in situ x‐ray diffraction. No solid phases other than ε‐Fe and γ‐Fe were observed in situ at high temperatures (>1000 K) and pressures to 84 GPa. However, the diffraction patterns of temperature quenched products at high pressure can be fit to other structures such as dhcp.
The nutrient, salinity, and stable oxygen isotope composition of Bering and Chukchi Seas waters in and near the Bering Strait
Authors: Cooper Lee W. , Whitledge Terry E. , Grebmeier Jacqueline M. , Weingartner Tom
Published: February 2004

Seawater nutrient, salinity, and oxygen 18 data collected from 1990 to 1993 in the Bering and Chukchi Seas were used to identify potential sources of nutrients and water masses that result in formation of the Arctic Ocean upper halocline and its associated nutrient maximum. Water matching the δ18O values of the Arctic Ocean upper halocline and containing sufficient, or a nearly sufficient, nutrient and salinity concentration was collected in subsurface waters in the summer in portions of the Bering Sea, particularly the Gulf of Anadyr. However, nutrient concentrations significantly declined in this north flowing water over the shallow continental shelf before it reached the Bering Strait, as a consequence of biological utilization, and dilution with nutrient‐poor and oxygen 18‐depleted fresh water. Therefore it does not appear likely that the flow of unaltered water through the Bering Strait in the summer plays a critical role in the formation of the Arctic Ocean upper halocline. The role of other mechanisms for contributing Pacific‐derived waters to the Arctic Ocean nutrient maximum is considered.
Large‐scale circulation and water mass distribution in the Arctic Ocean from model results and observations
Authors: Gerdes Rüdiger , Schauer Ursula
Published: February 2004

We combine results from hydrographic measurements and numerical models to investigate the circulation in the Arctic Ocean. Observations exhibit a marked difference in the temperature and salinity distribution west and east of the St. Anna Trough. West of the St. Anna Trough the warm and saline core of the Fram Strait branch is found close to the shelf break. East of the St. Anna Trough the highest temperatures and salinities are found further off shore. A colder, relatively fresh layer of Barents Sea outflow displaces the Fram Strait branch. The numerical model reproduces these hydrographic structures. It depicts the Barents Sea branch as the most important inflow into the Arctic with respect to heat, fresh water, and volume. The divergence of the heat transport in the Barents Sea is as great as the net heat transport into the Arctic Ocean. A large part of the freshwater input takes place in the area of the Barents/Kara Sea. The model transports associated with the Fram Strait branch are smaller because most of the Atlantic water recirculates in Fram Strait. Bottom topography plays a crucial role for the model circulation. Our results support previous conceptual models of the Atlantic layer circulation in the Arctic Ocean. The model circulation is mainly thermohaline driven. Ventilation times for the Eurasian and Canadian Basins below 1‐km depth are estimated as 220 and 420 years, respectively.
A numerical study of wind‐induced, near‐inertial oscillations over the Texas‐Louisiana shelf
Authors: Chen Changsheng , Xie Liusen
Published: February 2004

The wind‐induced, near‐inertial oscillations over the Texas‐Louisiana shelf were studied using a two‐dimensional primitive equation model forced by an impulse of the surface wind. Numerical experiments reproduced the cross‐shelf structure of observed near‐inertial oscillations. For a given spatially uniform impulse of the wind, the model predicted the large near‐inertial oscillations that were surface‐intensified and characterized by a first baroclinic mode in the vertical. The oscillations reached a maximum kinetic energy near the shelf break and decayed gradually toward the coast but rapidly offshore. The magnitude and cross‐shelf distribution of the wind‐induced, near‐inertial energy changed with wind direction. Effects of nonlinear advection on near‐inertial oscillations were largest near the shelf break and decreased both onshore and offshore where the bottom topography was gentle or flat. They were stronger during the wind event and weaker after the wind ceased. Nonlinear interaction of tides and wind was weak on the shelf. However, it tended to provide a positive net contribution to near‐inertial currents for both cases where tides were in and out of phase with winds. A simple theory suggests that the cross‐shelf variation of near‐inertial oscillations was controlled by the cross‐shelf gradient of surface elevation and the vertical gradient of Reynolds stress.
Calculation of solar‐induced fluorescence in surface and subsurface waters
Authors: Culver Mary E. , Perry Mary Jane
Published: February 2004

Solar‐induced chlorophyll a fluorescence can be observed in up welling irradiance spectra, but two major corrections are necessary before the signal may be used to detect phytoplankton pigment or primary production. A two‐component correction model was developed for use in both surface and subsurface waters. The first component separates up welling irradiance resulting from fluorescence from total up welling irradiance using a simple linear approximation between two wavelengths on either side of the chlorophyll a emission spectrum. Fluorescence accounted for 10 to 40% of total upwelled irradiance at the surface and up to 80% in subsurface waters. The second component corrects for attenuation of irradiance between the phytoplankton source and the detector using attenuation terms that incorporate the angular distributions of solar excitation and isotropic fluorescence emission irradiances. Results from our model compare favorably with models rigorously derived for surface application, and ‐ because it is not based on the single‐scattering assumption, ‐ our model also can be used in subsurface waters. The diverse water masses sampled (fjord, coastal, and open ocean) belonged to a single optical type, according to a classification using Kd(490) and aphy(490) (rather than chlorophyll a); these results indicate that our model can be used in diverse environments to extract a corrected fluorescence signal from upwelling irradiance spectra.
El Niño in the 1990s
Authors: Goddard Lisa , Graham Nicholas E.
Published: February 2004

The evolution of the unusual warming events of the first half of the 1990s is discussed based on the National Center for Environmental Prediction ocean re‐analysis data, covering 1980–1995. The warmings occurring in 1991–1992, 1993, and 1994 are viewed as individual episodes, rather than as one long El Niño. It is shown that the warm episodes of the 1990s differ from previous El Niños in two important ways. First, subsurface anomalies are more strongly coupled to the mixed layer across the equatorial Pacific in the 1990s than in earlier years. Second, a persistent warm SST anomaly in the central Pacific has been instrumental in initiating warm events. Both of these factors interfere with the dominant mode of variability in the tropical Pacific air‐sea system, namely the delayed oscillator mechanism. We postulate that the low skill of dynamical atmosphere‐ocean models in predicting the recent warmings is related to a weakened delayed oscillator mechanism.
Spectral analysis of tsunami waves: Separation of source and topography effects
Authors: Rabinovich Alexander B.
Published: February 2004

A new approach is proposed to separate the influence of source and topography in observed tsunami spectra and to reconstruct the initial source spectrum. The method assumes a linear tide gauge response to external forcing and is based on comparative analysis of tsunami and background spectra. Evaluation of the ratio of tsunami to background spectra gives functional characteristics that are invariant of station location and dependent only on the source parameters. Three events, the Urup tsunami of December 22, 1991 (northwest Pacific), the Hokkaido Southwest (Okushiri) tsunami of July 12, 1993 (Sea of Japan), and the Shikotan tsunami of October 4, 1994 (northwest Pacific), are used as examples of the proposed approach. The source spectra reconstructed from the analysis of different tide gauges were found to be in good agreement with each other and with the seismological, geodetic, and field survey data.
Temporal and spatial variabilities of the South China Sea surface temperature anomaly
Authors: Chu Peter C. , Lu Shihua , Chen Yuchun
Published: February 2004

In this study we use the National Centers for Environmental Prediction (NCEP) monthly sea surface temperature (SST) fields (1982–1994) to investigate the temporal and spatial variabilities of the South China Sea (SCS) warm/cool anomalies. Three steps of analysis were performed on the data set: ensemble mean

, composite analysis to obtain the monthly mean anomaly relative to the ensemble mean
, and empirical orthogonal function (EOF) analysis on the residue data relative to
. The ensemble mean SST field
has a rather weak horizontal gradient: 29°C near the Borneo coast to 25°–26°C near the southeast China coast. Two areas of evident SST anomalies were found in the monthly
variation: west of Borneo‐Palawan Islands (WBP) and southeast of the southern Vietnam coast (SVC). Four patterns, monsoon and transition each with two out‐of‐phase structures, were found. During the spring‐to‐summer transition (March to May) the warm anomaly is formed in the northern SCS with
located at 112°–119°30′E, 15°–19°30′N. During the fall‐to‐winter transition (October to November) the northern SCS (north of 12°N) cool anomaly is formed in November with
located at 108°–115°, 13°–20°N. We performed an EOF analysis on the residue data relative to
in order to obtain transient and interannual variations of the SST fields. EOF1 accounts for 47% of the variance and represents the northern SCS warm/cool anomaly pattern. EOF2 accounts for 14% of the variance and represents the southern SCS dipole pattern. Strong northern SCS warm anomaly (1°C warmer) appears during October–November 1987 and January–February 1988, and strong northern SCS cool anomaly (1°C cooler) occurs during March 1986 and November 1992. Furthermore, a strong cross correlation between wind stress curl and SST anomalies, computed from the European Centre for Medium‐Range Weather Forecast analyzed wind stress data and the NCEP SST data for different lags, shows the existence of an air‐sea feedback mechanism in the SCS deep basin.
Regional identification in the tropical Atlantic Ocean of residual tide errors from an empirical orthogonal function analysis of TOPEX/POSEIDON altimetric data
Authors: Arnault S. , Le Provost C.
Published: February 2004

Two years (October 1992 to September 1994) of TOPEX/POSEIDON data have been processed over the tropical Atlantic Ocean to produce monthly sea level anomalies. Four different oceanic tide corrections have been applied in order to quantify the effect of this correction on the large‐scale altimetrically determined sea level variability and to identify potential residual tide errors. Two tide corrections are drawn from the U.S. University of Texas tide model, and two others are drawn from the French Grenoble tide model. Comparison of the altimetric sea level anomalies with tide gauge data gives RMS differences of between 2 and 5 cm, equivalent to those already observed by several authors during recent TOPEX/POSEIDON studies. However, this paper demonstrates that the 2‐monthly oscillations, which were previously identified in investigations using old versions of tide corrections, still remain, to some extent independently of the new tide models being used. These 2‐monthly oscillations are not only located along the western boundary and therefore cannot be attributed to 40‐ to 60‐day oscillations observed in several mooring records in that area. An empirical orthogonal function analysis of the altimetric data clearly shows that these oscillations are due to tide aliasing. This approach also emphasizes the improvement finalized by the more recent versions of the University of Texas and the Grenoble tide models. However, the most convincing result of this analysis is that it allows regional quantification of the impact of tidal residual error aliasing which persists even when the more sophisticated tide models are used.
Comparisons of mesoscale variability in the Semtner‐Chervin 1/4° model, the Los Alamos Parallel Ocean Program 1/6° model, and TOPEX/POSEIDON data
Authors: McClean Julie L. , Semtner Albert J. , Zlotnicki Victor
Published: February 2004

Measures of mesoscale variability in the Semtner‐Chervin 1/4° and the Los Alamos Parallel Ocean Program (POP) 1/6° models were compared with those obtained from TOPEX/POSEIDON (T/P) data. The objectives of these comparisons were two‐fold: the first was to validate the models using altimetry as a measure of the variability of the real ocean, and the second was to evaluate the effect of increased model resolution/decreased horizontal friction. Mesoscale root‐mean‐square (rms) sea surface height (SSH), eddy kinetic energies, and length scales were used to quantify the mesoscale variability. Results showed that the models reproduced the distribution and much of the magnitude of this variability associated with the major current systems; however, in the oceans' interiors the magnitude was underrepresented. The 1/6° and 1/4° models were found to explain about 60% and 50% of the global T/P variability, respectively. Estimates of eddy kinetic energy (and rms velocities) from T/P and the models were compared, demonstrating that the models were less energetic than the T/P fields. Independent comparisons were made with lagrangian drifters in the Pacific basin. Excellent agreement was found between the total POP velocity fields and the drifter data in the tropics, where the T/P geostrophic values were too high due to error amplification by the 1/ƒ factor. In the midlatitudes, the drifter values exceeded those derived from the total model velocities; the T/P results lay between the two. Differences are attributed to the drifter analysis choices and possible residual noise in the altimetry data. The effect of increased resolution/decreased friction was best seen in the length scales where the POP scales agreed more closely with the T/P values than with the 1/4° model The distribution and magnitude of the POP length scales were generally in agreement with the T/P values between 10° and 40°. Near the equator, discrepancies were due to the long equatorial and instability waves, whose long wavelengths were too great to be resolved by the the combination of noise in the altimeter slopes, and the particular definition of length scale chosen.
Sulfur‐containing species (methanesulfonate and SO4) over the last climatic cycle in the Greenland Ice Core Project (central Greenland) ice core
Authors: Legrand Michel , Hammer C. , De Angelis M. , Savarino J. , Delmas R. , Clausen H. , Johnsen S. J.
Published: February 2004

A high‐resolution profile covering the last two centuries and a discontinuous study spanning the complete last glacial‐interglacial cycle of methanesulfonate (MSA) (CH3SO3−) and sulfate were obtained along Summit (central Greenland) ice cores. MSA concentrations were close to 4±1.4 ng g−1 from 1770 to 1870 A.D. and 3 ng g−1 in 1900, and exhibited a well‐marked decreasing trend from 1945 to the present. These changes of Summit snow MSA concentrations between 1770 and 1945 are discussed in terms of possible modulation of dimethylsulfide (DMS) marine emissions influencing the Greenland Ice Sheet by past climatic fluctuations in these regions. The decrease of MSA levels in Summit snow layers deposited since 1945 suggests either a decline in marine biota at high northern latitudes or a changing yield of MSA from DMS oxidation driven by modification of the oxidative capacity of the atmosphere in response to increasing anthropogenic NOx, and hydrocarbon emissions. While interglacial ice concentrations of MSA and sulfate are close to 2.9±1.9 ng g−1 and 27±10 ng g−1, respectively, reduced MSA (1.2±0.7 ng g−1) and enhanced sulfate (55±19 ng g−1) levels characterized the early Holocene stage (9000 to 11,000 years B.P.). MSA concentrations in glacial ice remain similar to the ones observed during interglacial stages. In contrast, sulfate levels are strongly enhanced (243±84 ng g−1) during the last glacial maximum (14,400 to 15,700 B.P.) compared with the interglacial ones. These variations of sulfur‐containing species in response to past climatic conditions are similar to those found in other Greenland cores. In contrast, they are different from those revealed in the Antarctic Vostok ice core, where colder climates were associated with an increase by a factor of 5 and 2 in MSA and sulfate concentrations, respectively. These glacial‐interglacial changes are discussed in terms of present and past contributions of marine DMS emissions versus other sulfate sources such as volcanic emissions and continental dust to the Greenland precipitation.
Variability of surface layer hydrography in the tropical Pacific Ocean
Authors: Ando Kentaro , McPhaden Michael J.
Published: February 2004

The purpose of this study is to examine the variation of ocean surface layer hydrography on interannual timescales in the tropical Pacific Ocean using conductivity‐temperature‐depth measurements from 1976 to 1994. We demonstrate that associated with interannual variations in atmospheric forcing, there were distinct changes in mixed layer temperature, salinity, depth, and barrier layer thickness between normal, El Niño, and La Niña time periods. During El Niño a warm, fresh mixed layer accompanied by an underlying barrier layer develops in the central and eastern Pacific in association with increased precipitation and reduced trade wind forcing. Conversely, during La Niña, when unusually cold conditions prevail in the central and eastern equatorial Pacific and the warm pool is confined to the far western Pacific, a thick barrier layer is found west of 160°E. Statistically, we demonstrate that barrier layers occur with increasing frequency as mixed layer temperatures increase from 20°C to 30°C, suggesting that barrier layer thickness may be one of the factors affecting surface temperatures. The physical mechanism underlying this relationship is likely to be related to the reduced efficiency of vertical turbulent mixing in cooling the surface via entrainment of thermocline water when the barrier layer is thick. Changes in mixed layer temperature, on the other hand, can affect precipitation and therefore mixed‐layer salinity, leading to the possibility of feedbacks on interannual timescales involving the mixed‐layer temperature balance and the hydrologic cycle over the ocean. The extent to which such feedbacks, if operative, may influence the detailed evolution of large‐scale, lower‐frequency variability in the tropical Pacific needs to be critically assessed in the context of coupled ocean‐atmosphere models.
The size distribution of microparticles from selected segments of the Greenland Ice Core Project ice core representing different climatic periods
Authors: Steffensen Jørgen Peder
Published: February 2004

The size distribution of insoluble microparticles (dust) in the particle size interval 0.4–6.0 μm radius has been measured in more than 1400 samples from the Greenland Ice Core Project (GRIP) deep ice core from Summit, Greenland. The samples were taken from ice core segments representing 32 climatic periods including ice from the Eem interglacial and ice from below the Eemian ice. The mean dust volume distributions from the climatic periods are compared, and the relationship of total dust mass to the concentration of Ca2+ and to the stable isotopic composition (δ18O) is investigated. The dust volume distributions are found to be lognormal in the size interval 0.4–2.0 μm with nearly identical shapes. Changes of the lognormal part of the volume distributions are found to be connected to the total dust mass in a systematic way. The total dust mass is correlated to the Ca2+ concentration, and data do not show any enrichment in Ca2+ from exposed continental shelves due to lower sea levels during the last glacial maximum. The total dust mass is strongly connected to δ18O. The volume distribution of the particles in the size interval 2.0–6.0 μm is found to be almost the same in most periods. The exceptions are the “cold” periods in the last part of the last glacial period, where the volume of these particles is higher than in most periods, and the periods from the Eemian and just above the Eemian where the volume of these particles is lower. The volume distributions of both the Eemian “warm” periods and the “cold” Eemian events are different from the distributions in ice from both below and above the Eemian ice. Climate changes appear to have modified the processes of production, transport, modification, and deposition of the dust aerosol in the same way over the last 120,000 years or more.
Sea level variability and surface eddy statistics in the Mediterranean Sea from TOPEX/POSEIDON data
Authors: Iudicone Daniele , Santoleri Rosalia , Marullo Salvatore , Gerosa Paolo
Published: February 2004

Two years of TOPEX/POSEIDON altimeter data have successfully been used to study the mesoscale field in the Mediterranean and to investigate the seasonal and year‐to‐year variability of the sea level and eddy statistics in this basin. The mesoscale field described by TOPEX/POSEIDON revealed a strong, but subbasin dependent, seasonal signal. Year‐to‐year variations are also evident in terms of both intensity and position of the main mesoscale features. Crossover analysis indicated the existence of a meridional transport of eddy momentum away from the Algerian current due to the northward migration of mesoscale eddies. A comparison between mesoscale features detected by the altimeter and contemporaneous features observed using advanced very high resolution radiometer (AVHRR) sea surface temperature has been made for the following Mediterranean subbasins: the Algerian basin, the Levantine basin, and the Tyrrhenian Sea. The results definitively prove the direct relation between sea level anomalies and the Mediterranean eddy field.
Nonlinear shoaling of directionally spread waves on a beach
Authors: Herbers T. H. C. , Burton M. C.
Published: February 2004

The shoaling of directionally spread surface gravity waves on a gently sloping beach with straight and parallel depth contours is examined with weakly dispersive Boussinesq theory. In this second‐order theory, energy is transferred from the incident waves to components with both higher and lower frequencies in near‐resonant nonlinear triad interactions. Directional spreading of the incident waves causes a weak detuning from resonance that is of the same order as the detuning owing to dispersion. Boussinesq theory predictions of the evolution of a single triad (i.e., two primary wave components shoaling from deep water forcing a secondary wave component) are compared to predictions of dispersive finite depth theory for a typical range of beach slopes, incident wave amplitudes, frequencies, and propagation directions. The dependencies of the predicted secondary wave growth on primary wave incidence angles are in good agreement. Whereas the sum frequency response is insensitive to the (deep water) spreading angle of the primary waves, the difference frequency (infragravity) response is significantly reduced for large spreading angles. A stochastic formulation of Boussinesq wave shoaling evolution equations is derived on the basis of the closure hypothesis that phase coupling between quartets of wave components is weak. In this approximation the second‐ and third‐order statistics of random, directionally spread shoaling waves are described by a coupled set of evolution equations for the frequency alongshore wavenumber spectrum and bispectrum. It is shown that a smooth overlap with solutions of dispersive finite depth theory exists in the limit of small beach slope and weak nonlinearity.
Heat fluxes of the Indian Ocean from a global eddy‐resolving model
Authors: Garternicht U. , Schott F.
Published: February 2004

The output of the global eddy‐resolving ¼° ocean model of Semtner/Chervin (run by the Naval Postgraduate School, Monterey, California) has been used to study the oceanic temperature and heat flux in the Indian Ocean. The meridional heat flux in the northern Indian Ocean is at the low end of the observed values. A vertical overturning cell in the upper 500 m is the main contributor to the annual mean meridional heat flux across 5°S, whereas the horizontal gyre circulation, confined to the upper 500 m, dominates north of the equator. The change of monsoon winds is manifested in a reversal of the meridional circulation throughout the whole water column. The most notable result is a strong linear relationship of the meridional temperature flux and the zonal wind stress component north of 20°S. The model's Pacific‐Indian Ocean throughflow across the section at 120°E accounts for −8.8±5.1 Sv (1 Sv≡106 m3 s−1). A strong interannual variability during the model run of 3 years shows a maximum range of 12 Sv in January/February and a minimum during March through June. The inflow from the Pacific into the Indian Ocean results in a total annual mean temperature flux of −0.9 PW (1 PW≡1015 W). In the model the temperature flux from the Pacific through the Indian Ocean to the south dominates in comparison with the input of solar heat from the northern Indian Ocean.