bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic channel waves in the accretionary prism of the Middle America Trench
Authors: Shapiro N. M. , Campillo M. , Singh S. K. , Pacheco J.
Published: February 2004

A visual examination of seismograms of earthquakes located near the Middle America Trench and recorded on VBB seismographs along the Pacific coast of Mexico, when bandpass filtered between 6 and 30 s, shows the presence of two types of waves. The first is a short pulse and the second is a dispersed wavetrain. The first type of wave is quickly attenuated, while the second type propagates without significant attenuation. We applied a frequency‐time analysis to identify the nature of these waves. A comparison of period‐group velocity diagrams for signals recorded on different stations shows that the first arrival is a Rayleigh wave propagating in the continental plate, while the second one propagates along the trench in the thickened sediment wedge of the accretionary prism. We have also measured group velocities of Rayleigh wave propagating in the Cocos plate far from the coast. At periods between 15 and 20 s, the group velocities of the wave propagating along the trench are lower than the velocity of Rayleigh waves propagating in adjacent continental and oceanic plates. We conclude that the dispersed wavetrain observed near the coast is a wave trapped in a channel formed by low‐velocity sediments of the accretionary prism.
Galileo plasma wave observations near Europa
Authors: Gurnett D. A. , Kurth W. S. , Roux A. , Bolton S. J. , Thomsen E. A. , Groene J. B.
Published: February 2004

In this paper we present results from the Galileo plasma wave instrument during the first two flybys of Europa, which occurred on December 19, 1996, and February 20, 1997. Strong whistler‐mode noise was observed in the vicinity of Europa during both flybys. Emission at the upper hybrid resonance frequency, fUH, and a propagation cutoff at the local electron plasma frequency, fpe, provided measurements of the local electron number density. The electron density measurements show a region of highly disturbed plasma in the vicinity of Europa with density enhancements ranging from about 30 to 100 cm−3 above the ambient Jovian magnetospheric background, which in both cases was about 80 cm−3.
Evidence for multiple ejecta: April 7–11, 1997, ISTP Sun‐Earth connection event
Authors: Berdichevsky D. , Bougeret J.‐L. , Delaboudinière J.‐P. , Fox N. , Kaiser M. , Lepping R. , Michels D. , Plunkett S. , Reames D. , Reiner M. , Richardson I. , Rostoker G. , Steinberg J. , Thompson B. , von Rosenvinge T.
Published: February 2004

Evidence is presented that the enhanced geomagnetic activity, on April 10–11, 1997, was caused by one of two ejecta that left the Sun at ≈ 14 UT on April 7. This ejecta was not directly detected at the Earth. The evidence for this interpretation is based on WIND spacecraft observations in the solar wind (SW). It is consistent with: (i) measured velocities of the coronal mass ejections from the SOHO coronagraph; (ii) the initial propagation speed of the shock generated in this event, estimation from type II radio burst observations from the WAVES instrument on WIND, and (iii) the time profile of energetic ions observed by EPACT on WIND. This locally unobserved ejecta (moving at 600 to 700 kms−1) generated a fast shock which accelerated ions to several tens of MeV/amu. The inferred passage of the first ejecta close to Earth (on April 10 to 11) is based on the observation of an interplanetary shock (IS) ahead of a field and plasma compressional region where the draping of the SW flow and possibly the changes in the direction of the IMF are consistent with a location northward of a faster ejecta. This ejecta was responsible for disturbed SW conditions including approximately ten hours of southward orientation of the interplanetary magnetic field (IMF) and a ram pressure many times above normal. The slower moving ejecta was directed toward Earth and was observed with WIND from about 0550 until 1500 UT on April 11. It had a strong northward IMF and produced density enhancements which elevated the ram pressure to more than four times above normal.
Applying AVO analysis to GPR data
Authors: Baker Gregory S.
Published: February 2004

For reflected‐wave phenomena in both seismology and ground‐penetrating radar (GPR), the amplitudes of the reflected waves depend on the incidence angles and the physical properties of the media above and below an interface. Amplitude variation with offset (AVO) analysis employs knowledge of reflected‐wave phenomena to constrain the physical properties of two media at an interface. Ostrander (1984) demonstrated that AVO analysis can be used on seismic reflection data in certain instances to obtain information about the subsurface not revealed by traveltime‐offset relationships (i. e., traditional seismic cross sections). Quantifiable information about the physical properties of the subsurface may then be extracted. AVO analysis, proven for a decade to work well in reflection seismology, may be useful in near‐surface hydrogeological situations when applied carefully to GPR data to better constrain the presence or absence of nonaqueous phase liquid contaminants (NAPLs) in the subsurface.
Preservation of glaciochemical time‐series in snow and ice from the Penny Ice Cap, Baffin Island
Authors: Grumet Nancy S. , Wake Cameron P. , Zielinski Greg A. , Fisher David , Koerner Roy , Jacobs John D.
Published: February 2004

A detailed investigation of major ion concentrations of snow and ice in the summit region of Penny Ice Cap (PIC) was performed to determine the effects of summer melt on the glaciochemical time‐series. While ion migration due to meltwater percolation makes it difficult to confidently count annual layers in the glaciochemical profiles, time‐series of these parameters do show good structure and a strong one year spectral component, suggesting that annual to biannual signals are preserved in PIC glaciochemical records.
Laboratory and in situ evidence for the presence of ice particles in a PMSE region
Authors: Zadorozhny A. M. , Vostrikov A. A. , Witt G. , Bragin O. A. , Dubov D. Yu. , Kazakov V. G. , Kikhtenko V. N. , Tyutin A. A.
Published: February 2004

An interpretation is made of rocket data obtained from an electric field mill (EFM) sensor during the international rocket‐radar campaign NLC‐91. The interpretation is based on a laboratory study of the interaction of a water particle beam with the EFM recovered after the rocket flight. Both rocket and laboratory data show that the field mill is sensitive to ice microparticle impacts and that perturbations in the EFM signal observed in the vicinity of noctilucent clouds and an enhanced radar echo (PMSE) layer are caused not only by atmospheric electric fields but also by the impact of solid particles. The altitude range of the impact signatures suggests the presence of ice particles in the PMSE region above the height of the optically detected noctilucent clouds. The analysis of the rocket data on the basis of the laboratory results allows us to estimate sizes of these PMSE particles to be 13–40 nm.
A quiescent state of 3 to 8 MeV radiation belt electrons
Authors: Selesnick R. S. , Blake J. B. , Kolasinski W. A. , Fritz T. A.
Published: February 2004

During a ∼3 month period in mid‐1996 outer radiation belt electrons in the energy range from ∼ 3 to 8 MeV were diffusing inward and decaying in intensity with no internal or external source. Measurements from the HIST instrument on POLAR are used to constrain a model for time dependent lossy radial diffusion of these electrons, and to obtain estimates of a parameterized radial diffusion coefficient and lifetime. For lower energy electrons, of ∼ 1 to 3 MeV, a source at L > 6 is apparent throughout most of the same period.
MHD simulation of a rapidly rotating magnetosphere interacting with the external plasma flow
Authors: Miyoshi T. , Kusano K.
Published: February 2004

The nonlinear interaction of a rapidly rotating magnetosphere with the external supersonic plasma flow is investigated by using the three‐dimensional global magneto‐hydrodynamic (MHD) simulation. The results of the simulations clearly indicate that various dawn‐dusk asymmetries are generated due to the effect of the rotation. Particularly, the location of the plasmoid, which is generated in the tail region of the rotating magnetosphere, is quite different from the case without rotation. It is also found that distinctive magnetospheric structures such as the plasma disc are largely modified in response to the dynamic pressure in the external plasma flow.
Long‐term ozone decline over the Canadian Arctic to early 1997 from ground‐based and balloon observations
Authors: Fioletov V. E. , Kerr J. B. , Wardle D. I. , Davies J. , Hare E. W. , McElroy C. T. , Tarasick D. W.
Published: February 2004

Column ozone measurements in the Canadian High Arctic (north of 70°N) started in 1957 and there have been regular ozone sonde flights there since 1966. Column ozone values over the High Arctic were as much as 45% below normal for some days in March 1997. During March 1996 arctic ozone values were also very low. In both years, temperatures in the arctic stratosphere were extremely cold and, especially in 1997, the vortex wind pattern was unusual for the arctic and quite similar to the antarctic spring vortex. Similar cold stratospheric temperatures were present in 1967, but ozone deviations were much smaller. Despite the very low values in the High Arctic during March 1997, the average column ozone from three Canadian sites in the 50–60°N latitude range was only 3.5% below normal.
balance velocities of the Greenland Ice Sheet
Authors: Joughin Ian , Fahnestock Mark , Ekholm Simon , Kwok Ron
Published: February 2004

We present a map of balance velocities for the Greenland ice sheet. The resolution of the underlying DEM, which was derived primarily from radar altimetery data, yields far greater detail than earlier balance velocity estimates for Greenland. The velocity contours reveal in striking detail the location of an ice stream in northeastern Greenland, which was only recently discovered using satellite imagery. Enhanced flow associated with all of the major outlets is clearly visible, although small errors in the source data result in less accurate estimates of the absolute flow speeds. Nevertheless, the balance map is useful for ice‐sheet modelling, mass balance studies, and field planning.
A model study of the potential role of the reaction BrO + OH in the production of stratospheric HBr
Authors: Chipperfield M. P. , Shallcross D. E. , Lary D. J.
Published: February 2004

We have used a constrained one‐dimensional photochemical model to investigate the effect of a potential minor channel of the fast reaction between BrO + OH to produce HBr. There is no direct evidence for this reaction but the analogous yield of HCl from ClO + OH is thought to be about 5%. With only a 1–2% yield of HBr the modelled HBr mixing ratio between 20–30 km increases from around 0.5 parts per 1012 by volume (pptv) to 1–2 pptv. This brings the model into agreement with recent balloon‐borne observations of stratospheric HBr. Should BrO + OH produce HBr with around 1–2% yield then this reaction will dominate HBr production between 20–35 km. As the main loss of HBr is reaction with OH this will lead to steady state HBr:BrO partitioning which is independent of other species, and temperature.
Ab initio elasticity of three high‐pressure polymorphs of silica
Authors: Karki Bijaya B. , Stixrude Lars , Crain Jason
Published: February 2004

Full elastic constant tensors of three high‐pressure polymorphs of silica: stishovite, CaCl2‐type and columbite‐type (α‐PbO2 structure); are determined at lower mantle pressures from first‐principles using the plane wave pseudopotential method within the local density approximation. The calculated zero pressure athermal elastic moduli are within a few percent of the experiments. We find that the elastic properties of silica are strongly pressure dependent. The shear wave velocity decreases rapidly (by 60%) and the anisotropy increases rapidly (by a factor of five) between 40 and 47 GPa prior to the transition from stishovite to the CaCl2 structure at 47 GPa. At this phase transition, the isotropically averaged shear wave velocity changes discontinuously by 60%, while the S‐wave polarization anisotropy decreases by a factor of two. The transformation of the CaCl2 phase to the columbite phase at 98 GPa is accompanied by a discontinuous change of 1–2% in elastic wave velocity and decrease by a factor of two in anisotropy. We suggest that even a small amount of silica in the lower mantle may contribute significantly to observed seismic anisotropy, and may provide an explanation of observed seismic reflectivity near 1000 km.
The δ18O record along the Greenland Ice Core Project deep ice core and the problem of possible Eemian climatic instability
Authors: Johnsen Sigfús J. , Clausen Henrik B. , Dansgaard Willi , Gundestrup Niels S. , Hammer Claus U. , Andersen Uffe , Andersen Katrine K. , Hvidberg Christine S. , Dahl‐Jensen Dorthe , Steffensen Jørgen P. , Shoji Hitoshi , Sveinbjörnsdóttir Árny E. , White Jim , Jouzel Jean , Fisher David
Published: February 2004

Over 70,000 samples from the 3029‐m‐long Greenland Ice Core Project (GRIP) ice core drilled on the top of the Greenland Ice Sheet (Summit) have been analyzed for δ8O. A highly detailed and continuous δ8O profile has thus been obtained and is discussed in terms of past temperatures in Greenland. We also discuss a three‐core stacked annual δ8O profile for the past 917 years. The short‐term (<50 years) variability of the annual δ8O signal is found to be 1‰ in the Holocene, and estimates for the coldest parts of the last glacial are 3‰ or higher. These data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the Eemian. Spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the δ8O signal, which for the Holocene ice is found to be considerably stronger than expected. We suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix. Deconvolution techniques were employed for restoring with great confidence the highly attenuated annual δ8O signal in the Holocene. We confirm earlier findings of dramatic temperature changes in Greenland during the last glacial cycle. Abrupt and strong climatic shifts are also found within the Eem/Sangamon Interglaciation, which is normally recorded as a period of warm and stable climate in lower latitudes. The stratigraphic continuity of the Eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not able to reconcile.
Hydrography of the Romanche and Chain Fracture Zones
Authors: Mercier Herlé , Morin Pascal
Published: February 2004

The Romanche and Chain Fracture Zones (RFZ and CFZ) are known as pathways for the Lower North Atlantic Deep Water (LNADW) and the Antarctic Bottom Water (AABW) from the western trough to the eastern trough of the equatorial Atlantic Ocean across the Mid‐Atlantic Ridge. We report hydrographic observations describing the evolution of the LNADW and AABW properties during their transit through the equatorial fracture zones. To the west of the RFZ and CFZ sills, the LNADW and AABW cores are separated by a deep thermocline (pycnocline) which marks the transition between the two water masses. The deep thermocline is eroded eastward and vanishes in the eastern basin. This zonal asymmetry is the signature of a zonal baroclinic pressure gradient associated in each fracture zone with eastward flows of LNADW and AABW below 3800 m depth. During the eastward transit through the RFZ, the near‐bottom AABW becomes less dense (saltier ∼0.07° and warmer ∼0.7°C), richer in oxygen ∼0.3 mL L−1, but less rich in silicate ∼30 μmol L−1. Simultaneously, the LNADW core becomes fresher and colder at a given density, and the oxygen maximum characterizing the LNADW core weakens by ∼ 0.2 mL L−1. We argue that the property changes within the AABW and LNADW are due to intense vertical mixing. In addition to vertical mixing with the AABW, it seems likely that isopycnal mixing of the LNADW with the Eastern Basin Deep Water contributes to the property evolution of the LNADW exiting the RFZ. In the CFZ, evidence is also given of mixing occuring within the AABW and the LNADW. A sudden change in the AABW properties observed across the CFZ main sill suggests the blocking of the eastward progression of the densest AABW by the CFZ main sill. However, this dense water can reach the eastern basin by flowing northeastward through valleys, the sills of which are deeper than thoses of the CFZ. Finally, a simple hydraulic control model applied to the RFZ and CFZ flows predicts a transport of 1.4 (0.1) 106 m3 s−1 of water colder than 1.9°C across the RFZ (CFZ) main sill.
The Greenland Ice Sheet Project 2 depth‐age scale: Methods and results
Authors: Meese D. A. , Gow A. J. , Alley R. B. , Zielinski G. A. , Grootes P. M. , Ram M. , Taylor K. C. , Mayewski P. A. , Bolzan J. F.
Published: February 2004

The Greenland Ice Sheet Project 2 (GISP2) depth‐age scale is presented based on a multiparameter continuous count approach, to a depth of 2800 m, using a systematic combination of parameters that have never been used to this extent before. The ice at 2800 m is dated at 110,000 years B.P. with an estimated error ranging from 1 to 10% in the top 2500 m of the core and averaging 20% between 2500 and 2800 m. Parameters used to date the core include visual stratigraphy, oxygen isotopic ratios of the ice, electrical conductivity measurements, laser‐light scattering from dust, volcanic signals, and major ion chemistry. GISP2 ages for major climatic events agree with independent ages based on varve chronologies, calibrated radiocarbon dates, and other techniques within the combined uncertainties. Good agreement also is obtained with Greenland Ice Core Project ice core dates and with the SPECMAP marine timescale after correlation through the δ18O of O2. Although the core is deformed below 2800 m and the continuity of the record is unclear, we attempted to date this section of the core on the basis of the laser‐light scattering of dust in the ice.
Accuracy assessment of recent ocean tide models
Authors: Shum C. K. , Woodworth P. L. , Andersen O. B. , Egbert G. D. , Francis O. , King C. , Klosko S. M. , Le Provost C. , Li X. , Molines J‐M , Parke M. E. , Ray R. D. , Schlax M. G. , Stammer D. , Tierney C. C. , Vincent P. , Wunsch C. I.
Published: February 2004

Over 20 global ocean tide models have been developed since 1994, primarily as a consequence of analysis of the precise altimetric measurements from TOPEX/POSEIDON and as a result of parallel developments in numerical tidal modeling and data assimilation. This paper provides an accuracy assessment of 10 such tide models and discusses their benefits in many fields including geodesy, oceanography, and geophysics. A variety of tests indicate that all these tide models agree within 2–3 cm in the deep ocean, and they represent a significant improvement over the classical Schwiderski 1980 model by approximately 5 cm rms. As a result, two tide models were selected for the reprocessing of TOPEX/POSEIDON Geophysical Data Records in late 1995. Current ocean tide models allow an improved observation of deep ocean surface dynamic topography using satellite altimetry. Other significant contributions include their applications in an improved orbit computation for TOPEX/POSEIDON and other geodetic satellites, to yield accurate predictions of Earth rotation excitations and improved estimates of ocean loading corrections for geodetic observatories, and to allow better separation of astronomical tides from phenomena with meteorological and geophysical origins. The largest differences between these tide models occur in shallow waters, indicating that the current models are still problematic in these areas. Future improvement of global tide models is anticipated with additional high‐quality altimeter data and with advances in numerical techniques to assimilate data into high‐resolution hydrodynamic models.
A unified directional spectrum for long and short wind‐driven waves
Authors: Elfouhaily T. , Chapron B. , Katsaros K. , Vandemark D.
Published: February 2004

Review of several recent ocean surface wave models finds that while comprehensive in many regards, these spectral models do not satisfy certain additional, but fundamental, criteria. We propose that these criteria include the ability to properly describe diverse fetch conditions and to provide agreement with in situ observations of Cox and Munk [1954] and Jähne and Riemer [1990] and Hara et al. [1994] data in the high‐wavenumber regime. Moreover, we find numerous analytically undesirable aspects such as discontinuities across wavenumber limits, nonphysical tuning or adjustment parameters, and noncentrosymmetric directional spreading functions. This paper describes a two‐dimensional wavenumber spectrum valid over all wavenumbers and analytically amenable to usage in electromagnetic models. The two regime model is formulated based on the Joint North Sea Wave Project (JONSWAP) in the long‐wave regime and on the work of Phillips [1985] and Kitaigorodskii [1973] at the high wavenumbers. The omnidirectional and wind‐dependent spectrum is constructed to agree with past and recent observations including the criteria mentioned above. The key feature of this model is the similarity of description for the high‐ and low‐wavenumber regimes; both forms are posed to stress that the air‐sea interaction process of friction between wind and waves (i.e., generalized wave age, u/c) is occurring at all wavelengths simultaneously. This wave age parameterization is the unifying feature of the spectrum. The spectrum's directional spreading function is symmetric about the wind direction and has both wavenumber and wind speed dependence. A ratio method is described that enables comparison of this spreading function with previous noncentrosymmetric forms. Radar data are purposefully excluded from this spectral development. Finally, a test of the spectrum is made by deriving roughness length using the boundary layer model of Kitaigorodskii. Our inference of drag coefficient versus wind speed and wave age shows encouraging agreement with Humidity Exchange Over the Sea (HEXOS) campaign results.
Observations of the oceanic thermal skin in the Atlantic Ocean
Authors: Donlon Craig J. , Robinson Ian S.
Published: February 2004

An extensive set of measurements made along a transect of the Atlantic Ocean covering a latitude range of between 52°N and 20°S have been used to investigate the characteristics of the bulk (BSST) minus skin (SSST) sea surface temperature difference (ΔT). ΔT is highly variable and has an overall mean value of 0.35 K±0.35 K, a daytime mean value of 0.49 K±0.39 K and a nighttime mean value of 0.27 K ±0.28 K. ΔT in all cases has no dependence on the amount of cloud cover. At low wind speeds, ΔT increases as the wind speed increases. At a wind speed >10 m s−1, wind induced turbulence reduces ΔT to a mean value of zero, and there is no evidence of any further wind speed dependency. The fact that a negligible ΔT exists in high wind speed conditions suggests that it can be appropriate to use BSST observations to validate satellite SSST measurements and that these are the preferred conditions in which to merge satellite‐derived SSST and pseudo‐BSST data. The variability of ΔT found in the data indicate that it is unwise to assume a mean global ΔT of 0.3 K. A comparison between several parameterizations of ΔT shows that such parameterizations are unable to describe the ΔT values found in this data set.
A study of the evolution of the under‐ice water layer during summer melt phase in an Arctic bay
Authors: Rajan Subramaniam D. , Laible Henry , Tucker Walter B.
Published: February 2004

An acoustic ice tomography experiment which extended over a 12‐month period was conducted in the Sabine Bay area of the Canadian Archipelago. Acoustic transmitter and receiver arrays were deployed in the ice for this experiment. The vertical arrays penetrated through the ice and extended into the water column. The acoustic data collected from the transmitter/receiver pairs in the water column are analyzed to investigate the changes in the sound speed structure of the water column. Analysis of the acoustic data shows significant changes in the sound speed structure in the upper 2 m of the water column during the summer melt phase. We associate these changes in sound speed with the influx of fresh water caused by the melting of ice cover, snow, and runoff from rivers. Other features of the acoustic data suggest (1) that the fresh water remained stagnant and the ice growth during the winter months was due to the freezing of the fresh water and (2) that ice formed around the transducers as the fresh water came into contact with it. As the study presented in this paper was not one of the goals of the tomography experiment, support measurements to validate the conclusions from the acoustic data, such as measurement of temperature and salinity in the water column, were not made. However, the analysis presented is a demonstration of the usefulness of this technique to monitor the changes that take place in the region beneath the ice cover.
Deterministic decomposition of deep water short‐crested irregular gravity waves
Authors: Prislin Igor , Zhang Jun , Seymour Richard J.
Published: February 2004

A method is developed for the deterministic decomposition of nonlinear short‐crested irregular ocean waves accurate up to second order in wave steepness. Initially, an alternative maximum likelihood method is used to determine the directional spreading of wave energy. Then a least squares fitting scheme is used to calculate the initial phases of directional free‐wave components. The effects of nonlinear wave‐wave interactions among these free‐wave components are calculated using a conventional second‐order wave‐wave interaction model and subtracted from the measurements. The final results are obtained through an iterative process of computing the free‐wave components and their nonlinear interactions. Given an ocean wave field defined by multiple feed‐point measurements, the method is capable of decomposing the wave field into a set of directional free‐wave components. On the basis of the derived free‐wave components, the wave characteristics of the wave field can be predicted in the vicinity of the measurements. The method has been applied to two sets of ocean wave pressure measurements which were collected from an array of sensors mounted on an offshore production platform near the California coast. Satisfactory agreement between the predictions based on the decomposed free‐wave components and related measurements indicates the proposed method is reliable. It is expected that this method will have a variety of applications to ocean science and engineering.
Combining multiple altimeter missions
Authors: Jacobs G. A. , Mitchell J. L.
Published: February 2004

Viewing altimeter data only at the points where separate altimeter missions' ground tracks cross provides a method to observe long time period sea surface height (SSH) variations and avoids many of the problems inherent in combining separate altimeter data sets through an independently determined geoid. TOPEX/POSEIDON (T/P) data over the time period from January 1, 1993, to December 31, 1995, form a mean SSH that is used as a reference by other altimeter data sets. A least squares analysis of the mean T/P SSH determines the portion of the Geographically Correlated Orbit Error (GCOE) that may be observed through crossover differences and removes this portion of the GCOE. The analysis removes errors of 0.86 cm RMS at 1 cycle per orbit revolution (cpr) and indicates negligible errors at higher frequencies. After the GCOE removal, the accuracy of the T/P reference mean is better than 1 cm RMS as measured by crossover differences. The GCOE contained in the Geosat‐Exact Repeat Mission (ERM) and ERS 1 data with orbit solutions using the Joint Gravity Model (JGM) 3 is evaluated through an adjustment to the T/P reference mean surface. The Geosat‐ERM data indicate a bias of about 28 cm averaged over the globe, and the ERS 1 bias is 44 cm. The T/P data used here is not corrected for the oscillator drift correction error so that the actual bias is less by about 13 cm. Both the Geosat‐ERM and ERS 1 GCOE are mainly 1 cpr. GCOE estimates at frequencies above 1 cpr indicate little actual orbit error but are more correlated to instrument correction errors (particularly water vapor). Simultaneous T/P and ERS 1 SSH anomalies to the T/P mean indicate good correlation.
HF radar comparisons with moored estimates of current speed and direction: Expected differences and implications
Authors: Graber Hans C. , Haus Brian K. , Chapman Rickey D. , Shay Lynn K.
Published: February 2004

The validation of estimates of ocean surface current speed and direction from high‐frequency (HF) Doppler radars can be obtained through comparisons with measurements from moored near‐surface current meters, acoustic Doppler current profilers, or drifters. Expected differences between current meter (CM) and HF radar estimates of ocean surface vector currents depend on numerous sources of errors and differences such as instrument and sensor limitations, sampling characteristics, mooring response, and geophysical variability. We classify these sources of errors and differences as being associated exclusively with the current meter, as being associated exclusively with the HF radar, or as a result of differing methodologies in which current meters and HF radars sample the spatially and temporally varying ocean surface current vector field. In this latter context we consider three geophysical processes, namely, the Stokes drift, Ekman drift, and baroclinicity, which contribute to the differences between surface and near‐surface vector current measurements. The performance of the HF radar is evaluated on the basis of these expected differences. Vector currents were collected during the High Resolution Remote Sensing Experiment II off the coast of Cape Hatteras, North Carolina, in June 1993. The results of this analysis suggest that 40%–60% of the observed differences between near‐surface CM and HF radar velocity measurements can be explained in terms of contributions from instrument noise, collocation and concurrence differences, and geophysical processes. The rms magnitude difference ranged from 11 to 20 cm s−1at the four mooring sites. The average angular difference ranged between 15° and 25° of which about 10° is attributed to the directional error of the radar current vector estimates due to the alignment of the radial beams.
New aerosol robust sea surface temperature algorithms for the along‐track scanning radiometer
Authors: Brown S. J. , Harris A. R. , Mason I. M. , Závody A. M.
Published: February 2004

The along‐track scanning radiometer (ATSR) was the first spaceborne radiometer to possess a dual view angle capability designed to improve the accuracy of the remote sensing of sea surface temperature (SST). We present the results of comparing SST retrieved by using “single‐look” and “dual‐look” atmospheric correction algorithms applied to ATSR data collected around the British Isles from April to July 1992. We find that the retrievals using the single‐look algorithms are consistently cooler (mean value 0.67 K) than the dual‐look retrieved temperatures, the rms spread of the differences being 0.2 K. Potential causes of these differences could be either a previously predicted wind‐induced (surface emissivity related) error for the dual‐look algorithm or stratospheric volcanic aerosols, produced by the eruption of Mt. Pinatubo in June 1991, mainly affecting the single‐look algorithm. To determine the dominant cause of the observed differences, we develop three new dual‐look algorithms: one robust against surface emissivity variations, one against aerosol variations, and one against both of these effects. Comparing the results yielded by these algorithms indicates that the retrieved SST differences are due to aerosol variations affecting the single‐look retrievals and that the surface emissivity effects on the dual‐look algorithm are less than 0.1 K. We use this result to develop global dual‐look ATSR SST algorithms that are very robust against aerosols to a precision of 0.08 K under a wide range of aerosol loadings. The results are particularly important for the early part of the ATSR mission, when the current dual‐look SST retrievals could be affected by Pinatubo aerosols. Furthermore, the high spatial and temporal variations in stratospheric aerosols seen in our study indicate that such algorithms are essential for long‐term bias‐free global SST monitoring.
Physics and observational signatures of free, forced, and frictional stratified seamount‐trapped waves
Authors: Codiga Daniel L.
Published: February 2004

To interpret measurements at Cobb Seamount, the observational signatures in stratified seamount‐trapped waves, with emphasis on forcing and frictional influences, are demonstrated for bathymetry and stratification representing a typical midlatitude seamount. Inviscid waves propagate azimuthally with standing wave radial and vertical structure. This includes current ellipses parallel or perpendicular to isobaths with uncorrelated radial and azimuthal velocities, nearly horizontal depth regions within which currents rotate in time clockwise or counterclockwise, and velocity turning with depth alternately clockwise and counterclockwise in successive quarter periods. Waves are explained conceptually in terms of “stratified slope‐Kelvin waves” (plane, propagating, ƒ plane topographic Rossby waves in stratification over a planar sloping bottom). An inviscid stratified seamount‐trapped wave is a cross‐isobath (radial and vertical) mode, of equal amplitude stratified slope‐Kelvin waves propagating upslope and downslope, that resonates in the azimuthally reentrant waveguide of the sloping seamount sides. Cross‐isobath structure is controlled by (1) ray refraction by slope angle variations, (2) Airy function behavior near turning points where bottom slope decreases to the ratio between wave and buoyancy frequencies, and (3) geometric compression by cylindrical geometry. In a stationary forced damped wave maintained by ambient tidal currents against dissipation concentrated near the summit where currents are strongest, cross‐isobath standing wave structure is broken with rays carrying energy upslope dominating; currents propagate phase downward and outward in addition to azimuthally, have ellipses oriented across isobaths with positively correlated radial and azimuthal velocities, and are dominated by clockwise rotation in time and counterclockwise turning with depth.
Steric and wind‐induced changes in TOPEX/POSEIDON large‐scale sea surface topography observations
Authors: Stammer Detlef
Published: February 2004

Three years of TOPEX/POSEIDON altimeter data are analyzed with respect to large‐scale fluctuations in sea surface height observations and underlying physical processes. In midlatitudes the most conspicuous feature in the large‐scale changes in sea surface height are related to surface buoyancy fluxes, predominantly surface heat fluxes. The next prominent variability on subannual timescales is the adjustment of the ocean to varying winds stress fields in terms of planetary waves. The degree to which basin‐scale sea surface height variations relative to the steric component can be described as a time‐dependent Sverdrup balance is addressed for the Pacific Ocean. If a flat bottom ocean is assumed, a significant correlation of meridional mass transport variations inferred from TOPEX/POSEIDON and a simple Sverdrup model can be found in the Pacific Ocean north of 40°N, but it fails elsewhere. Numerical ocean general circulation models show a good agreement with the TOPEX/POSEIDON observations in all those components of ocean variability and allow further insight into the dynamics that govern observed sea surface height changes.
Wind wave tank measurements of wave damping and radar cross sections in the presence of monomolecular surface films
Authors: Gade Martin , Alpers Werner , Hühnerfuss Heinrich , Lange Philipp A.
Published: February 2004

Measurements of the damping of small gravity and gravity‐capillary water surface waves covered with monomolecular organic films of different viscoelastic properties were performed in the wind wave tank facility of the University of Hamburg. The wind speed dependence of the radar cross sections for X and Ka band was measured with upwind looking microwave antennas. It is shown that Marangoni damping theory, which describes the damping of water surface waves by viscoelastic surface films, is not the only damping mechanism in wind wave tank experiments where the wind sea is not fully developed. The other source terms of the action balance equation, i.e., the energy input into the water waves from the wind, the nonlinear wave‐wave interaction, and the dissipation by wave breaking, are affected differently by the various substances. It is hypothesized that this difference is caused by the different viscoelastic properties of the substances, i.e., by the different intermolecular interactions of the film molecules. A slight dip in the wind dependence of the radar cross section at Ka band at wind speeds of 8–9 m/s was measured, which corresponds to comparable reductions of the mean squared wave height and wave slope. Polarization ratios (i.e., the ratios of the radar backscatter at vertical and horizontal polarization) higher than those predicted by simple Bragg scattering theory for X band at low wind speeds and different incidence angles are explained within a (three‐scale) composite‐surface model. At higher wind speeds, where the polarization ratio decreases rapidly, breaking by wedges and spilling breakers is hypothesized to become more dominant. The dependence of the polarization ratio on the coverage of the water surface with a slick is explained qualitatively by means of the composite‐surface model. Finally, it is stated that wind wave tank measurements in the presence of monomolecular surface films are useful for the verification of theories concerning radar backscattering, wave damping, and wind‐wave and wave‐wave interactions.
Large‐scale estimation of transport from the Pacific to the Indian Ocean
Authors: Potemra James T. , Lukas Roger , Mitchum Gary T.
Published: February 2004

The objective of this model‐data intercomparison is to determine the feasibility of deriving an useful index for fluctuations in the Pacific to Indian Ocean throughflow volume transport. Due to insufficient direct observations and the present limitations in numerical models, accurate estimation of variations in the throughflow transport on seasonal to interannual timescales is not yet possible; however, an index based on weighted, monthly mean sea level anomalies in different regions of the western Pacific and eastern Indian Oceans is presented. Numerical model results and sea level from the TOPEX/POSEIDON altimeter show that the large/scale pressure gradient forcing of the throughflow is controlled by the Pacific Ocean side on interannual timescales, and by a combination of Indian Ocean and Pacific Ocean processes on seasonal to annual timescales. The model throughflow is maximum in boreal summer (11 Sv) and minimum in boreal winter (4 Sv) with a 9‐year mean of 7.4 Sv. These values are within the range of various estimations of throughflow transport, and they agree in phase. Of the 7.4 Sv model transport, almost 1.8 Sv is due to direct, local wind forcing (based on Ekman calculations). Interannual fluctuations from El Niño‐Southern Oscillation activity are associated with increases in throughflow transport during cold events and decreases in transport during warm events. Using empirical orthogonal function analysis and results from previous studies, an index of the throughflow variability is developed using model sea level and model transport. Sea level in four regions is found to be sufficient to index the model throughflow variations: south of Java, northwest of Australia, in the Pacific warm pool and off the coast of the Philippines. A regression technique applied to the model sea level at these locations yields an index which correlates with the model throughflow at 0.83. The same weights applied to sea level in similar regions from the TOPEX/POSEIDON altimeter correlate with the model throughflow transport at a level of 0.78.
Zonal displacement of the western equatorial Pacific “fresh pool”
Authors: Delcroix Thierry , Picaut Joël
Published: February 2004

The distribution of salt in the tropical Pacific Ocean and its variability are potentially important in better understanding the El Niño Southern Oscillation (ENSO) ocean‐atmosphere coupled system. Complementary data sets (including sea surface salinity (SSS) derived from a ship‐of‐opportunity network, Geosat sea level and derived zonal surface currents, and outgoing longwave radiation‐derived precipitation, evaporation, and cruise measurements along the 165°E longitude) are used to describe and understand SSS changes in the western equatorial Pacific over several ENSO cycles. It is first shown that the 1974–1989 zonal displacements of the eastern edge of the western equatorial Pacific “fresh pool” (SSS < 35), marked by a salinity front and closely related to the eastern edge of the warm pool, were dominated by interannual variations that are highly correlated with the Southern Oscillation Index (SOI). Owing to the availability of basin‐wide Geosat surface current data, it is then shown that the eastward extension of the eastern edge of the fresh pool during the 1986–1987 El Niño, as well as the westward retreat during the 1988–1989 La Niña, were chiefly the result of zonal salt advection in the equatorial band. Although the equatorial upwelling extended toward the western Pacific during La Niña, its effectiveness in changing the near‐surface salinity (and temperature) is found questionable. Bearing in mind the uncertainty in determining open‐ocean precipitation, evaporation, and mixed‐layer depth, it appears that the net freshwater flux probably played a minor role in the zonal displacements of the eastern edge of the fresh pool, as compared to zonal salt advection. The roles of first baroclinic Kelvin and first meridional mode Rossby waves are discussed considering that they account in a large part for zonal advection.
Satellite images of upwellings and cold filament dynamics as transient effects of violent air‐sea interactions downstream from the island of Sardinia (western Mediterranean Sea)
Authors: Salusti E.
Published: February 2004

A systematic analysis of thermal satellite images of upwellings and cold filament dynamics downstream from the island of Sardinia (western Mediterranean Sea), covering the years 1984, 1985, 1989, 1990, 1993, and 1994, is discussed. For this sea the main transient atmospheric forcing comes from bursts of the mistral, a north‐western wind which, downstream from Sardinia, affects a rather calm basin, the Tyrrhenian Sea. In early summer, particularly clear effects of atmospheric forcings such as gyres, upwellings, and cold filaments can be observed. Our analysis provides not only a systematic insight into these phenomena, but also some experimental evidence of a novel mechanism of cold filament formation and evolution. Indeed, in July 1989, a nice set of satellite images shows a cold filament rooted to the shallowest part of the wide shelf off Olbia, which meanders offshore for a few days under the effect of the mistral and of the general circulation. It is of interest to underline how stably the filament is rooted to the shelf so that in only one image it appears as an elongated straight segment. Also, the filament's potential vorticity is discussed in order to gain some insight into shelf water fluxes, detachment depths, and velocities relative to such phenomena.
Evolution of nonlinear internal waves in the East and South China Seas
Authors: Liu Antony K. , Chang Y. Steve , Hsu Ming‐K. , Liang Nai K.
Published: February 2004

Synthetic Aperture Radar (SAR) images from ERS‐I have been used to study the characteristics of internal waves northeast and south of Taiwan in the East China Sea, and east of Hainan Island in the South China Sea. Rank‐ordered packets of internal solitons propagating shoreward from the edge of the continental shelf were observed in the SAR images. On the basis of the assumption of a semidiurnal tidal origin, the wave speed can be estimated and is consistent with the internal wave theory. By using the SAR images and hydrographic data, internal waves of elevation have been identified in shallow water by a thicker mixed layer as compared with the bottom layer on the continental shelf. The generation mechanism includes the influences of the tide and the Kuroshio intrusion across the continental shelf for the formations of elevation internal waves. The effects of water depth on the evolution of solitons and wave packets are modeled by the nonlinear Kortweg‐deVries (KdV) type equation and linked to satellite image observations. The numerical calculations of internal wave evolution on the continental shelf have been performed and compared with the SAR observations. For a case of depression waves in deep water, the solitons first disintegrate into dispersive wave trains and then evolve to a packet of elevation waves in the shallow water area after they pass through a “turning point” of approximately equal layer depths that has been observed in the SAR image and simulated by the numerical model. The importance of the dissipation effect in the coastal area is also discussed and demonstrated.
Recalculations of pack ice and lead surface energy budgets during the Arctic Leads Experiment (LEADEX) 1992
Authors: Persson P. Ola G. , Ruffieux Dominique , Fairall C. W.
Published: February 2004

Because of the discovery of two errors in the calculations of long‐wave radiative fluxes from the Eppley radiometers, a recalculation of the ice pack and surface energy budgets during the Arctic Leads Experiment (LEADEX) was necessary. The error principally contributing to the observed differences was in the relationships that have been used for obtaining long‐wave radiative fluxes from Eppley radiometers for the past 20 years. This paper focuses on the impacts on the LEADEX data. The main impacts of the corrections of these errors are decreases in the amplitudes of the diurnal variations of the long‐wave fluxes and the skin temperature; an increase in the estimate of the sensible heat transfer coefficient (CH) and the removal of its apparent diurnal cycle; and an interpretation of a more stable near‐surface background atmosphere during the deployment to Lead 4. Average 10‐m neutral values of the drag coefficient (CD) and CH are now found to be 1.46×10−3 and 1.40×10−3, respectively.