bluuebunny/crossref_metadata_2025 huggingface dataset:
Precipitation characteristics over land from the NOAA‐15 AMSU sensor
Authors: Ferraro Ralph R. , Weng Fuzhong , Grody Norman C. , Zhao Limin
Published: February 2004

Window channel measurements from the NOAA‐15 Advanced Microwave Sounding Unit (AMSU) are used to retrieve rain rates. This study focuses on the rain rate retrievals over land and uses measurements at 89 and 150 GHz to detect a scattering signal by millimeter‐sized ice particles under precipitating atmospheres. An operational algorithm using the AMSU‐A module (89 GHz) implemented in August 1999 is also described. The algorithms are calibrated with hourly surface rain observations. It is shown that identification of stratiform rain can be dramatically improved using measurements at 150 GHz from the AMSU‐B sensor. Improved spatial resolution of AMSU‐B is a major factor contributing to this algorithm.
Improving ocean angular momentum estimates using a model constrained by data
Authors: Ponte Rui M. , Stammer Detlef , Wunsch Carl
Published: February 2004

Ocean angular momentum (OAM) calculations using forward model runs without any data constraints have recently revealed the effects of OAM variability on the Earth's rotation. Here we use an ocean model and its adjoint to estimate OAM values by constraining the model to available oceanic data. The optimization procedure yields substantial changes in OAM, related to adjustments in both motion and mass fields, as well as in the wind stress torques acting on the ocean. Constrained and unconstrained OAM values are discussed in the context of closing the planet's angular momentum budget. The estimation procedure yields noticeable improvements in the agreement with the observed Earth rotation parameters, particularly at the seasonal timescale. The comparison with Earth rotation measurements provides an independent consistency check on the estimated ocean state and underlines the importance of ocean state estimation for quantitative studies of the variable large‐scale oceanic mass and circulation fields, including studies of OAM.
Stratospheric influence on upper tropospheric tropical cirrus
Authors: Boehm Matthew T. , Verlinde Johannes
Published: February 2004

Radiosonde data from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program Nauru99 experiment revealed waves with downward phase propagation from the lower stratosphere into the upper troposphere. The occurrence of cirrus clouds and the tropical tropopause structure were closely related to these waves. This study reveals a close relationship between upper tropospheric cirrus and large‐scale dynamics, suggesting that tropical cirrus cannot be studied in isolation from planetary scale forcing.
Bacterial growth in supercooled cloud droplets
Authors: Sattler Birgit , Puxbaum Hans , Psenner Roland
Published: February 2004

It is well known that the atmosphere is a conveyor of microorganisms, and that bacteria can act as ice or cloud condensation nuclei, but clouds have not been considered as a site where organisms can live and reproduce. Here we show that bacteria in cloud droplets collected at high altitudes are actively growing and reproducing at temperatures at or below 0°C. Since ∼60% of the earth surface is covered by clouds, cloud water should be considered as a microbial habitat.
Satellite observations of upper stratospheric and mesospheric OH: The HOxdilemma
Authors: Conway Robert R. , Summers Michael E. , Stevens Michael H. , Cardon Joel G. , Preusse Peter , Offermann Dirk
Published: February 2004

We report the first observations of the vertical distribution of hydroxyl (OH) from the upper stratosphere to the mesopause. The Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) made these measurements in August 1997. The data confirm the results from the earlier November 1994 MAHRSI mission that were confined to altitudes above 50 km, namely that mesospheric OH densities are 25 to 35% lower than predicted by standard photochemical theory. However, the new observations show that below 50 km the OH density increases rapidly and at 43 km altitude it is larger than that expected from standard theory. This represents a serious dilemma for our understanding of odd‐hydrogen chemistry because the same key reactions are thought to dominate OH/HO2 partitioning in both regions. We show that neither standard photochemical theory nor any previously proposed changes are adequate to explain the OH observations in both the upper stratosphere and mesosphere.
Development of the polar vortex in the 1999–2000 Arctic winter stratosphere
Authors: Manney G. L. , Sabutis J. L.
Published: February 2004

The 1999–2000 Arctic stratospheric vortex was unusually cold, especially in the early winter lower stratosphere, with a larger area near polar stratospheric cloud formation temperatures in Dec and Jan, and much lower temperatures averaged over Nov–Jan, than any previously observed Arctic winter. In Nov and early Dec, there was a double jet in the upper stratosphere, with the anticyclone cutoff in a region of cyclonic material. By late Dec, there was a discontinuous vortex, large in the upper stratosphere, small in the lower stratosphere; evolving to a strong, continuous, relatively upright vortex by mid‐Jan. This vortex evolution in 1999–2000 is typical of that in other cold early winters. Despite unusually low temperatures, the lower stratospheric vortex developed more slowly than in previous unusually cold early winters, and was weaker than average until late Dec.
Sea surface slope as an estimator of the Kuroshio volume transport east of Taiwan
Authors: Yang Yih , Liu Cho‐Teng , Lee Thomas N. , Johns William , Li Hsien‐Wen , Koga Momoki
Published: February 2004

The Kuroshio volume transport (KVT) at 24.5°N near Taiwan was directly measured for the first time during the period from September 1994 to May 1996 through the channel between Taiwan and Iriomote, Japan. Adjusted sea level difference anomaly (SLDA) across the Kuroshio is examined as a proxy for long‐term monitoring of KVT. Regression analyses were performed on the directly measured KVT and SLDA between Ishigaki and sea level stations on the Taiwan side (Keelung, Kenfang and Suao). The best estimators of KVT are: (1) KVT1=0.32×SLDA1 (Ishigaki‐Suao) + 20.85 Sυ±2.3 Sυ for daily KVT and (2) KVT10=0.37 × SLDA10 (Ishigaki‐Suao) + 20.87 Sυ±1.4 Sυ for 10‐day low‐passed KVT.
Tsunami scattering provinces in the Pacific Ocean
Authors: Mofjeld H. O. , Titov V. V. , González F. I. , Newman J. C.
Published: February 2004

We use a scattering index to identify the regions in the Pacific Ocean where topographic features scatter significant tsunami energy. Based on linear wave theory, the index is computed from the Smith/Sandwell topography. Consistent with numerical simulations, it shows that there is a narrow band of strong scatterers running across the ocean from the northwest (Emperor Seamount Chain) to the southeast (Easter Island Fracture Zone). The eastern Pacific is nearly devoid of scatterers, except for this band and isolated features along the eastern margin. To the west of the band lies a region with moderate scattering; the strongest scattering occurs in the southwestern Pacific. The Pacific is rimmed by island arcs and shallow continental shelves that also trap and scatter tsunamis. These results show that numerical models of trans‐Pacific tsunamis must resolve the effects of the small‐scale topography in order to accurately simulate their wave patterns and amplitudes.
1997–98: Unprecedented thermal stress to coral reefs?
Authors: Lough J. M.
Published: February 2004

Mass bleaching is a stress response of corals subjected to warmer‐than‐normal seawater temperatures during the warm season. During 1997–98 there were unprecedented numbers of reports of bleaching on many of the world's coral reefs. Observational evidence suggests an increase in frequency of mass coral bleaching events since the late 1970s. Two indices of warm season sea surface temperatures (SSTs; SST maximum anomaly and degree‐months) are presented for 47 reef sites where bleaching occurred during 1997–98. The level of thermal stress at the vast majority of these coral reef sites during 1997–98 was unmatched in the period 1903–99. Warm season SSTs at these coral reef sites have significantly warmed over this period and the frequency of warm season SST extremes has increased since the late 1970s. Continued warming of tropical SSTs, as is likely due to the enhanced greenhouse effect, will increase the level of thermal stress to coral reefs. Increased frequency of bleaching events will reduce corals' capacity to recover and may significantly alter the make‐up of present day coral reef ecosystems.
A census of North Brazil Current Rings observed from TOPEX/POSEIDON altimetry: 1992–1998
Authors: Goni Gustavo J. , Johns William E.
Published: February 2004

Six years of TOPEX/POSEIDON altimeter data are used to investigate the formation of rings and eddies shed by the North Brazil Current. Upper layer thickness maps were used to identify 34 of these features formed in the North Brazil Current retroflection region, an average of more than 5 rings and eddies per year. The ensemble of ring trajectories closely parallels the 500 m isobath, and one out of six rings penetrate into the Caribbean Sea through the southern Lesser Antilles. The rest of the rings and eddies follow a northern trajectory past Barbados once they reach 58W. Their estimated mean translation speed is 14 km/day and their mean length scale is approximately 100 km. Our results suggest that the formation rate of NBC rings and eddies is nearly twice that previously thought, and that they may account for more than 1/3 of the interhemispheric transport within the Atlantic meridional overturning cell.
Temperature profile retrieval from surface to mesopause by combining GNSS Radio Occultation and Passive Microwave Limb Sounder Data
Authors: von Engeln Axel , Bühler Stefan , Kirchengast Gottfried , Künzi Klaus
Published: February 2004

A theoretical study on the combination of a passive microwave and a GNSS radio occultation instrument, mounted on the International Space Station (ISS), was performed. Both instruments allow to deduce the atmospheric temperature profile. We combined realistically simulated data of the two highly synergistic sensors with the optimal estimation method (OEM) and retrieved single joint temperature profiles from 0 to 90 km altitude for which we found accuracies <1 K below 35 km and <4 K in the mesosphere, respectively. In addition, we performed simultaneous tropospheric water vapor and temperature retrievals leading to a water vapor accuracy <10 to 20 % at altitudes below 5 to 8 km. The OEM allows to optimally exploit the synergy in the data and maximizes retrieval accuracy over the full range from surface to mesopause. It seems worthwhile to install this type of instrument combination on the ISS or other low‐Earth‐orbit (LEO) platforms.
Inversion of Pn travel times for lateral variations of moho geometry beneath the central Andes and comparison with the receiver functions
Authors: Baumont David , Paul Anne , Zandt George , Beck Susan L.
Published: February 2004

We inverted the Pn travel times to characterize the geometry of the Moho along a profile across the Central Andes (20°S) where previous workers have estimated the crustal thickness using receiver functions. Contrary to receiver functions, this technique is not sensitive to the crustal Vs. Therefore, the comparison of the two approaches provides valuable complementary information. Overall, our results are in good agreement with those based on receiver functions. However, some important discrepancies are observed beneath the Western Cordillera and the Subandes, where we find crusts 10‐km thinner than in previous models. We confirm that the central part of the orogen appears to be isostatically compensated by the presence of a thick crust. However, at both edges, the topography probably requires additional support, low‐density mantle beneath the Western Cordillera and a strong flexural support of the Brazilian shield beneath the Subandes.
A new global Earth's gravity field model from satellite orbit perturbations: GRIM5‐S1
Authors: Biancale Richard , Balmino Georges , Lemoine Jean‐Michel , Marty Jean‐Charles , Moynot Bernard , Barlier Francois , Exertier Pierre , Laurain Olivier , Gegout Pascal , Schwintzer Peter , Reigber Christoph , Bode Albert , König Rolf , Massmann Franz‐Heinrich , Raimondo Jean‐Claude , Schmidt Roland , Yuan Zhu Sheng
Published: February 2004

A new model of the Earth's gravity field, called GRIM5‐S1, was prepared in a joint German‐French effort. The solution is based on satellite orbit perturbation analysis and exploits tracking data from 21 satellites to solve simultaneously for the gravitational and ocean tide potential and tracking station positions. The satellite‐only solution results in a homogeneous representation of the geoid with an approximation error of about 45 cm in terms of 5×5 degree block mean values, and performs globally better in satellite orbit restitution than any previous gravity field model. The GRIM5 normals, which were generated taking into account the latest computational standards, shall be the reference for use during the coming geopotential satellite mission CHAMP and should provide new standards in computing orbits of next altimetric missions like Jason and ENVISAT. The GRIM5‐S1 normals also give the basis for the tracking/surface data combined solution GRIM5‐C1.
Frequency dependent Lg attenuation in south‐central Alaska
Authors: McNamara D. E.
Published: February 2004

The characteristics of seismic energy attenuation are determined using high frequency Lg waves from 27 crustal earthquakes, in south‐central Alaska. Lg time‐domain amplitudes are measured in five pass‐bands and inverted to determine a frequency‐dependent quality factor, Q(f), model for south‐central Alaska. The inversion in this study yields the frequency‐dependent quality factor, in the form of a power law: Q(f)=Q0 fη = 220(±30) f0.66(±0.09) (0.75≤f≤12Hz). The results from this study are remarkably consistent with frequency dependent quality factor estimates, using local S‐wave coda, in south‐central Alaska. The consistency between S‐coda Q(f) and Lg Q(f) enables constraints to be placed on the mechanism of crustal attenuation in south‐central Alaska. For the range of frequencies considered in this study both scattering and intrinsic attenuation mechanisms likely play an equal role.
On the temporal character and regionality of the Arctic Oscillation
Authors: Overland James E. , Adams Jennifer Miletta
Published: February 2004

Decadal differences between the 1990s and 1980s in winter (JFM) sea‐level pressure and 300 mb zonal winds have an Arctic‐centered character with nearly equal contributions from the Atlantic and Pacific sectors. In contrast, the differences between positive and negative Arctic Oscillation (AO) composites defined from monthly values of Principal Components from the same period have similar magnitudes in the Pacific and Arctic, but have an additional large North Atlantic Oscillation (NAO) signature in the Atlantic sector. Thus Arctic changes on decadal scales are more symmetric with the pole than suggested by the standard AO index definition. Change point analysis of the AO shows that a shift in value near 1989 is an alternate hypothesis to a linear trend. Analysis of zonal and meridional winds by longitudinal sectors shows the importance of the standing wave pattern in interpreting the AO, which supplements the view of the AO as a simple zonal average (annular) mode.
Minimum relative entropy: Theory and application to surface temperature reconstruction from borehole temperature measurements
Authors: Kennedy Paula L. , Woodbury Allan D. , Wang Kelin
Published: February 2004

In this paper we extend the minimum relative entropy (MRE) method to reconstruct ground surface temperature changes (GST) from borehole temperature measurements (BHT). The application of MRE to recovering GST is promising and provides an alternative to other inverse methods in geophysics. The relative entropy formulation provides the advantage of allowing for a prior bias in the estimated pdf and ‘hard’ bounds if desired. Test cases showed good recoveries of the GST. The method was utilized in recovering GST from two data sets in Canada. The Lac Dufault data gave very consistent results for different choices of a priori information and bounds. The Mariner results were not as good quality. This method has only recovered past ground surface temperatures, which does not directly provide information regarding climate change. However, results contained herein show fairly uniform temperatures until the past 100 to 500 years and in more recent times about a 4°C rise in temperature, consistent with previous published results.
Is upper stratospheric chlorine decreasing as expected?
Authors: Waugh Darryn W. , Considine David B. , Fleming Eric L.
Published: February 2004

The monthly‐mean total chlorine abundance (ClT) at 55 km inferred from HALOE HCl observations increases from 1992 to 1997 and then subsequently decreases. The pre‐1997 increase is consistent with surface measurements of ClT time‐lagged by around 6 years. However, a decrease after 1997 is inconsistent with such a time lag, which would predict a peak in late 1999. Accounting for stratospheric mixing processes produces an expected stratosphere ClT which is in agreement with the HALOE ClT time series considering the uncertainty in the HALOE data. However, the peak in ClT is still predicted to occur in later 1999 rather than 1997. We find that reasonable low frequency changes in transport, chlorine partitioning, anomalous buildup of organic chlorine at 55 km, and tropospheric rainout of inorganic chlorine do not reconcile the expected and HALOE ClT time series. At present, we are unable to explain how upper stratospheric ClT could decrease as early as 1997.
Warm and cold water routes of an O.G.C.M. thermohaline conveyor belt
Authors: Speich Sabrina , Blanke Bruno , Madec Gurvan
Published: February 2004

A global general circulation model analyzed with a Lagrangian methodology is used to describe and quantify the paths, transports, and characteristics of the “warm” waters forming the upper branch of the conveyor belt in the North Atlantic Ocean. The total transport for this branch turns out to be 17.8 Sv in the North Atlantic at 20°N: 11.8 Sv are composed of waters coming from the two classical origins, the Drake Passage and the Indonesian Throughflow, which contribute with 6.5 and 5.3 Sv respectively. The remaining 6 Sv find their origins partly in the passage between Antarctica and the Australian Continent (with 3.1 Sv) and partly in the Indo‐Atlantic sector itself (i.e., with 2.9 Sv). The geographical structure of the different routes emphasizes the role of the Southern Ocean and large‐scale current systems in water mass transformation and distribution.
The generation of internal tides at the Hawaiian Ridge
Authors: Merrifield Mark A. , Holloway Peter E. , Johnston T. M. Shaun
Published: February 2004

Previous analyses of altimeter and acoustic data indicate that the Hawaiian Ridge is a significant generation site for semidiurnal (M2 period) internal waves owing to tidal flow over topography. Such waves likely play an important role connecting the dissipation of tidal energy to ocean mixing. Here a primitive equation numerical model with realistic stratification, bathymetry, and tidal forcing is used to simulate the generation of these waves in a manner consistent with altimeter sea surface observations. The highest amplitude waves originate at three generation sites located at both the island and seamount portions of the chain. These sites have in common both enhanced flows across ridge‐shaped topography and maxima in model calculated body force which couples barotropic and baroclinic motions. The model vertical structure suggests the presence of multiple dynamical modes within 100 km of the ridge and not just the lowest modes that have been detected at the sea surface.
Melt intrusion as a trigger for lithospheric foundering and the eruption of the Siberian flood basalts
Authors: Tanton Linda T. Elkins , Hager Bradford H.
Published: February 2004

Any viable model of the Siberian flood basalt (SFB) eruption must provide for a massive pulse of magma, initially erupted below sea level. We propose as a triggering mechanism a limited precursory melt that intrudes and heats the mantle lithosphere, lowering its viscosity, and increasing its density as the melt freezes into eclogite. This warm, dense mantle lithosphere is then removed via a Rayleigh‐Taylor instability that creates surface subsidence. Removal of the mantle lithosphere lengthens the melting column, and the melt volume of the SFB can be produced in less than a million years. The model is permissive of the existing geologic data for the SFB, which rule out a traditional hot, deep mantle plume. Numerical models demonstrate that the ability of a mantle upwelling to remove lithosphere is dependent primarily upon the lithospheric rheology, not on the temperature or size of the upwelling.
Normal stress distribution of rough surfaces in contact
Authors: Hansen Alex , Schmittbuhl Jean , Batrouni G. George , de Oliveira Fernando A.
Published: February 2004

We study numerically the stress distribution on the interface between two thick elastic media bounded by interfaces that include spatially correlated asperities. The interface roughness is described using the self‐affine topography that is observed over a very wide range of scales from fractures to faults. We analyse the correlation properties of the normal stress distribution when the rough surfaces have been brought into full contact. The self affinity of the rough surfaces is described by a Hurst exponent H. We find that the normal stress field is also self affine, but with a Hurst exponent H‐1. Fluctations of the normal stress are shown to be important, especially at local scales with anti‐persistent correlations.
Generation of cyclonic eddies by the Agulhas Current in the Lee of the Agulhas Bank
Authors: Penven P. , Lutjeharms J. R. E. , Marchesiello P. , Roy C. , Weeks S. J.
Published: February 2004

Anti‐cyclonic rings are shed from the Agulhas Current at its retroflection. They subsequently drift off into the South Atlantic. Smaller, cyclonic eddies have also been observed in this region. The origin of these latter eddies has remained unknown. We present model results that indicate that the configuration of the southern Agulhas Current and the poleward termination of the continental shelf of Africa allows shedding of cyclonic lee eddies by a flow detachment process. Hydrographic data, thermal infrared satellite images and altimetric observations are furnished that show that this model simulation is consistent with the characteristics and the behaviour of cyclonic eddies in the region.
The Tropical Western Hemisphere Warm Pool
Authors: Wang Chunzai , Enfield David B.
Published: February 2004

The Western Hemisphere warm pool (WHWP) of water warmer than 28.5°C extends from the eastern North Pacific to the Gulf of Mexico and the Caribbean, and at its peak, overlaps with the tropical North Atlantic. It has a large seasonal cycle and its interannual fluctuations of area and intensity are significant. Surface heat fluxes warm the WHWP through the boreal spring to an annual maximum of SST and areal extent in the late summer/early fall, associated with eastern North Pacific and Atlantic hurricane activities and rainfall from northern South America to the southern tier of the United States. SST and area anomalies occur at high temperatures where small changes can have a large impact on tropical convection. Observations suggest that a positive ocean‐atmosphere feedback operating through longwave radiation and associated cloudiness is responsible for the WHWP SST anomalies. Associated with an increase in SST anomalies is a decrease in atmospheric sea level pressure anomalies and an anomalous increase in atmospheric convection and cloudiness. The increase in convective activity and cloudiness results in less longwave radiation loss from the surface, which then reinforces SST anomalies.
Rotation and plate locking at the Southern Cascadia Subduction Zone
Authors: McCaffrey Robert , Long Maureen D. , Goldfinger Chris , Zwick Peter C. , Nabelek John L. , Johnson Cheryl K. , Smith Curt
Published: February 2004

Global Positioning System vectors and surface tilt rates are inverted simultaneously for the rotation of western Oregon and plate locking on the southern Cascadia subduction thrust fault. Plate locking appears to be largely offshore, consistent with earlier studies, and is sufficient to allow occasional great earthquakes inferred from geology. Clockwise rotation of most of Oregon about a nearby pole is likely driven by collapse of the Basin and Range and results in shortening in NW Washington State. The rotation pole lies along the Olympic‐Wallowa lineament and explains the predominance of extension south of the pole and contraction north of it.
IB and NIB Hypotheses and their possible discrimination by GRACE
Authors: Foldvary Lorant , Fukuda Yoichi
Published: February 2004

The NASA, the GFZ and the DLR plan the GRACE satellite mission to obtain an accurate gravity field after every 2‐4 weeks. Because of its extreme high precision, GRACE is expected to determine the temporal variations of the gravity fields due to time varying geophysical phenomena. Among them, the effects of the atmospheric surface pressure have the largest signals and we investigated its effects mainly from the viewpoint of degree amplitudes. Behaviour of atmospheric variations over oceanic areas is unknown. The response of the ocean is essentially important not only for the corrections of the atmospheric effects on gravity fields, but also for many other studies such as satellite altimetry, crustal deformation and the Earth rotations. We proposed and applied several ocean response models, i.e., IB, NIB, and intermediate ones, and evaluated the degree power differences between each one of them. The results show that almost all the differences are distinguishable by GRACE.
Triggered slip: Observations of the 17 August 1999 Izmit (Turkey) Earthquake using radar interferometry
Authors: Wright Tim , Fielding Eric , Parsons Barry
Published: February 2004

We use Synthetic Aperture Radar interferometry (InSAR) to map the displacement field of the 17 August 1999 Izmit earthquake, which largely conforms to that predicted for an elastic upper crust. We determine the earthquake source parameters and show that slip continues farther west than the mapped fault ruptures. We also show that additional sub‐surface displacements occurred on parallel strands of the North Anatolian Fault Zone. We argue that this was caused by changes in static stress accompanying the mainshock, or by the dynamic release of regional stresses.
Retrieval of aerosol properties over the ocean using multispectral and multiangle Photopolarimetric measurements from the Research Scanning Polarimeter
Authors: Chowdhary Jacek , Cairns Brian , Mishchenko Michael , Travis Larry
Published: February 2004

The evaluation of the direct and indirect aerosol forcings of climate requires precise knowledge of the aerosol optical thickness, size distribution, chemical composition, and number density. Global monitoring of these parameters from satellites requires instruments that make full use of the information content of the scattered solar radiation. In this paper we analyze multispectral and multiangle photopolarimetric observations performed with an airborne version of the Earth Observing Scanning Polarimeter over the ocean and demonstrate their exceptional retrieval potential. Using the 0.865‐ and 2.250‐µm channels, we are able to determine the parameters of a bimodal size distribution, identify the presence of water‐soluble and sea salt particles, and retrieve the optical thickness and column number density of aerosols. We also demonstrate that less comprehensive measurements by existing instruments would fail to provide retrievals of comparable completeness and accuracy.
Topography and penetration of the Greenland Ice Sheet measured with Airborne SAR Interferometry
Authors: Dall Jørgen , Madsen Søren Nørvang , Keller Kristian , Forsberg Rene
Published: February 2004

A digital elevation model (DEM) of the Geikie ice cap in East Greenland has been generated from interferometric C‐band synthetic aperture radar (SAR) data acquired with the airborne EMISAR system. GPS surveyed radar reflectors and an airborne laser altimeter supplemented the experiment. The accuracy of the SAR DEM is about 1.5m. The mean difference between the laser heights and the SAR heights changes from 0 m in the soaked zone to a maximum of 13 m in the percolation zone. This is explained by the fact that the snow in the soaked zone contains liquid water which attenuates the radar signals, while the transparency of the firn in the percolation zone makes volume scattering dominate at the higher elevations. For the first time, the effective penetration has been measured directly as the difference between the interferometric heights and reference heights obtained with GPS and laser altimetry.
Subglacial sediments: A regional geological template for ice flow in West Antarctica
Authors: Studinger Michael , Bell Robin E. , Blankenship Donald D. , Finn Carol A. , Arko Robert A. , Morse David L. , Joughin Ian
Published: February 2004

We use aerogeophysical data to estimate the distribution of marine subglacial sediments and fault‐bounded sedimentary basins beneath the West Antarctic Ice Sheet (WAIS). We find that significant ice flow occurs exclusively in regions covered by subglacial sediments. The onsets and lateral margins of ice streams coincide with the limit of marine sediments. Lateral margins are also consistently linked with fault‐bounded basins. We predict that the inland migration of ice streams B and C1 towards the ice divide outside the region covered by marine or rift sediments is unlikely. The subglacial geology has the potential to modulate the dynamic evolution of the ice streams and the WAIS.
Satellite altimeter monitoring the Kuroshio Transport south of Japan
Authors: Imawaki Shiro , Uchida Hiroshi , Ichikawa Hiroshi , Fukasawa Masao , Umatani Shin‐ichiro ,   the ASUKA Group
Published: February 2004

In 1993–1995, we carried out observations of the Kuroshio south of Japan, including direct current measurements and repeated hydrographic surveys along a satellite track of the TOPEX/POSEIDON altimeter. The velocity field of the Kuroshio is determined by geostrophic calculation using the repeated hydrographic survey data, referenced to velocities observed at mid and abyssal depths. The volume transport of the Kuroshio is estimated from this velocity field. The estimated transports of the Kuroshio have a high correlation with sea‐surface height differences across the Kuroshio. Having this relationship and using the altimeter data, we obtained a time series of the Kuroshio transport over seven years at ten‐day intervals. The Kuroshio transport, excluding contributions by local recirculations, is estimated to be 42 × 106 m³/sec on average. The correlation between sea‐surface height difference and transport provides a practical method of long‐term monitoring of the Kuroshio transport using satellite altimetry.
Resolution of tomographic models of the mantle beneath Iceland
Authors: Keller William R. , Anderson Don L. , Clayton Robert W.
Published: February 2004

The locations of volcanic islands may be controlled by thin or extending parts of the lithosphere over a partially molten asthenosphere [Anderson and Bass, 1984; Favela and Anderson, 2000], by edge effects near the boundaries of thick cratonic lithosphere [Anderson, 1998], or by narrow jets of hot mantle rising from deep within the mantle [Campbell and Griffiths, 1992; Morgan, 1971; Wilson, 1986]. Many hotspots are found on or near ridges, at lithospheric discontinuities, or in extensional environments, so high resolution seismic images are required to determine whether it is lithospheric structure, stresses in the lithosphere, or the deep mantle that is the controlling factor for the location of these volcanoes. In this study, we perform a simple experiment in which we use basic geometrical arguments to better understand the resolution of tomographic images of the upper 400 km of the mantle under Iceland. Our results indicate that a narrow, deep seated mantle plume is not required in order to explain the observed travel time delays in this region. Results of tomographic inversions are often viewed as unique; however, recent seismic studies of the Icelandic Hotspot have illustrated the non‐unique nature of these models.
Ion recombination in aircraft exhaust plumes
Authors: Sorokin A. , Mirabel P.
Published: February 2004

In this article, a model which examines the evolution of ion concentrations in a hot aircraft exhaust plume on the ground is proposed. The model includes plume dilution and ion‐ion recombination with coefficients which vary with temperature. A comparison of the model is made with the available ground‐based experimental data obtained on the ATTAS research aircraft engines. From this comparison, an ion emission index of the order of 8 1016 ions/kg(fuel) inferred.
Further evidence of ice thinning in the Arctic Ocean
Authors: Wadhams Peter , Davis Norman R.
Published: February 2004

A sea ice thickness profile, obtained in September 1996 from the Eurasian Basin of the Arctic Ocean between Fram Strait and the North Pole, was compared with a profile obtained in the same region in September‐October 1976. A decline in mean ice draft of 43% was observed over the 20‐year interval, in agreement with changes observed in other parts of the Arctic Ocean by Rothrock et al. [1999].
Seismically ”Fast” Geodynamic Mantle Models
Authors: Davies J. Huw , Bunge Hans‐Peter
Published: February 2004

We show that biased sampling of Earth structure by body‐waves provides an additional explanation for the fact that short period body‐wave seismic velocity models are faster than long‐period free‐oscillation models (apparent dispersion). We do this by tracing a set of body‐waves used in global tomography studies through synthetic seismic models derived from mantle circulation models. The histograms of the arrival time residuals have a negative mean for all the models investigated. We interpret that this results from a predominance of rays sampling the fast structures of subduction zones due to the concentration of sources there. The interpretation successfully passes two tests; the first showed that the signal is tectonically controlled, while the second involved breaking the correlation between ray paths and structures when no bias is found. The negative mean implies that Earth as sampled by body‐waves is fast compared to an average reference Earth (e.g. as measured from free‐oscillations). This effect (and others) will need to be quantified before attenuation can be extracted from the apparent dispersion.
Isotopic measurements of stratospheric ozone
Authors: Krankowsky Dieter , Lämmerzahl Peter , Mauersberger Konrad
Published: February 2004

Isotope ratios of stratospheric ozone samples collected during four balloon flights are reported. In an altitude range between 22 and 33 km all ratios show enrichments between 7 and 11%, somewhat lower for 49O3 than for 50O3. Those enrichments are in very good agreement with results from laboratory isotope studies when stratospheric pressure and temperatures are included. The new data presented do raise questions about past stratospheric ozone isotope measurements which sometimes showed high values never observed in the laboratory or tropospheric environment.
Paleoflexure
Authors: Albert Richard A. , Phillips Roger J.
Published: February 2004

We tested the assumption that flexural parameters measured in the present represent the lithosphere's mechanical response at its time of loading. We employed finite element models of cooling and non‐cooling versions of one‐ and two‐dimensional temperature distributions in a topographically loaded elastoviscoplastic oceanic lithosphere. Effective elastic thickness (Te) values of the cooling and non‐cooling versions of a 1‐D halfspace temperature field showed a 3% and 7% decrease, respectively, over about 40 Myr and supported the notion that the lithosphere's mechanical state at the time when loading ceased was effectively “frozen in” as it cooled. The second set of models used a high‐temperature perturbation beneath the load to simulate a magma conduit, with cooling and non‐cooling conduit models exhibiting decreases in Te of 10% and 12%, respectively. As post‐loading creep was responsible for the decreases in Te, weaker rheology or higher temperature gradients would exacerbate the effects found here.
A 14.6 kyr record of nitrogen flux from desert soil profiles as inferred from vadose zone pore waters
Authors: Hartsough Peter , Tyler S. W. , Sterling J. , Walvoord M.
Published: February 2004

A 14.6 kyr chronology of infiltration developed from deep vadose (unsaturated) zone cores in southern Nevada is presented to assess the chronology of nitrogen loss from the active rooting zone. Soil water chemistry in the deep vadose zone is first used to develop a chronology of paleohydrology and subsequently nitrogen fluxes. While elevated nitrogen concentrations (as NO3) are commonly found in desert vadose zones, NO3/Cl ratios indicate that nitrate behaved conservatively beneath the active rooting zone. Mean nitrogen fluxes from the active root zone range from 103 to 108 mg/m²/yr and appear relatively constant over time, in spite of dramatic climate and vegetation changes. The long‐term loss of nitrogen from the active rooting zone implies that nitrogen cycling processes have been relatively constant since the Pleistocene/Holocene transition, that the biological community may not be primarily limited by the availability of nitrogen, and that the loss of nitrogen into the vadose zone should be considered in desert ecosystem productivity studies.
A study of the 1999 monsoon rainfall in a mountainous region in central Nepal using TRMM products and rain gauge observations
Authors: Barros A. P. , Joshi M. , Putkonen J. , Burbank D. W.
Published: February 2004

Raingauge data from the 1999 monsoon were compared with precipitation derived from the precipitation radar (PR) and the microwave imager instruments on board the Tropical Rainfall Measuring Mission (TRMM) satellite. The raingauges are part of a new hydrometeorological network installed in the Marsyandi river basin, which extends from the edge of the Tibetan Plateau to the Gangetic basin. TRMM‐derived precipitation showed better detection of rain at low altitude stations as compared with high elevation stations, with good scores for the PR product for rain rates >0.5 mm/hr. The 3D PR rain rates suggest strong interaction between mesoscale convective systems and steep terrain at elevations of 1–2 km, which is consistent with the very high rainfall measured at those locations. Analysis of the raingauge data shows that even at altitudes as high as 4,000 m the cumulative monsoon rainfall is comparable to the highest amount recorded in the Indian subcontinent.
Elevated mixing ratios of surface ozone over the Arabian Sea
Authors: Lal Shyam , Lawrence Mark G.
Published: February 2004

Surface O3 levels exceeding 70 nmol/mol, the highest observed to date over the Indian Ocean, were recorded in the Arabian Sea marine boundary layer during the Indian Ocean Experiment (INDOEX) in 1999. Contrasted with this, available O3 observations from nearby coastal cities show generally lower values, rarely exceeding 50 nmol/mol, and averaging 25–35 nmol/mol during daytime, and 5–15 nmol/mol during nighttime. A 3D photochemical model simulates the high marine boundary layer O3 values relatively well, but considerably overestimates the urban O3 values. Surface wind fields show that the air masses with elevated O3 frequently come from India, implying that rapid photochemical production is likely to be occurring in the continental outflow. Furthermore, downwards transport of O3‐rich air masses from the free troposphere may also be important in producing some of the observed high‐O3 events. Implications of these observations are discussed.
mountain torques and atmospheric oscillations
Authors: Lott François , Robertson Andrew W. , Ghil Michael
Published: February 2004

Theoretical work and general circulation model (GCM) experiments suggest that the midlatitude jet stream's interaction with large‐scale topography can drive intraseasonal oscillations in large‐scale atmospheric circulation patterns. In support of this theory, we present new observational evidence that mountain‐induced torques play a key role in 15–30‐day oscillations of the Northern Hemisphere circulation's dominant patterns. The affected patterns include the Arctic Oscillation (AO) and the Pacific‐North‐American (PNA) pattern. Positive torques both accelerate and anticipate the midlatitude westerly winds at these periodicities. Moreover, torque anomalies anticipate the onsets of weather regimes over the Pacific, as well as the break‐ups of hemispheric‐scale regimes.
Seasonal transition in gravity wave activity during the springtime stratospheric vortex breakdown
Authors: Duck Thomas J. , Whiteway James A.
Published: February 2004

Profiles of temperature and wind measured by meteorological balloons are used to examine the changes in gravity wave activity at high latitudes during a final springtime breakdown of the Arctic stratospheric vortex. The measurements show that gravity wave potential energy densities in the lower stratosphere decreased dramatically as the cyclonic wintertime vortex gave way to the weak anticyclonic summertime circulation. The reduction in gravity wave activity after the vortex breakdown is attributed primarily to increased critical level filtering of orographic waves and lowered stratospheric wind speeds.
Intense seismic scattering near the Earth's core‐mantle boundary beneath the Comoros hotspot
Authors: Wen Lianxing
Published: February 2004

I report strong seismic PKP precursors recorded in the seismic stations in Tanzania for earthquakes occurred in the Fiji subduction zone. The observed PKP precursors show clear onsets and rapid variations of amplitude for different events and stations. I develop a technique to use the precursor onset times to determine the location of seismic scatterers and apply forward modeling of precursor amplitudes to place constraints on the magnitude of seismic anomalies. The travel time analysis indicates that these recorded PKP precursors are caused by the seismic scattering in the lowermost 360 km of the mantle beneath the Comoros hotspot. The amplitudes of the observed PKP precursors suggest a rather complex seismic structure with a P velocity variation of at least 8%, likely associated with partial melt.
Natural signals in the MSU lower tropospheric temperature record
Authors: Michaels Patrick J. , Knappenberger Paul C.
Published: February 2004

Volcanic and ENSO forcings explain nearly two‐thirds (63.5%) of the variance in monthly global temperature anomalies in the MSU lower tropospheric temperature observations from 1979 through 1999. While the raw trend in the drift‐adjusted 1979–99 global data is a statistically significant warming of 0.055°C/decade, the combination of volcanism and ENSO account for 25.4% of this trend, or 0.014°C/decade. The remaining trend, 0.041°C/decade, is left unexplained, likely comprising a combination of factors including the anthropogenic alterations to the collection of the earth's greenhouse gases, solar variability, and other forcings internal to the earth/atmosphere system. A recent widely cited attempt to explain the discrepancy between the rate of the observed increase in lower tropospheric MSU temperatures and the much larger value predicted by general circulation models weakens considerably when the known natural forcing mechanisms are more completely specified.
Intraseasonal eddies in the Sulawesi Sea simulated in an ocean general circulation model
Authors: Masumoto Y. , Kagimoto T. , Yoshida M. , Fukuda M. , Hirose N. , Yamagata T.
Published: February 2004

The intraseasonal variability associated with mesoscale eddies in the Sulawesi Sea simulated in a high resolution ocean general circulation model is described in detail. The cyclonic eddies, with a diameter of about 400 km, are generated at the entrance of the Sulawesi Sea between the Mindanao and the Halmahera Islands with 40 days interval. They are associated with a high speed (> 20 cm/s) down to 1000 m level. The anticlockwise circulation in the Sulawesi Sea, reported so far in both models and observations, may be a long time‐averaged image of the above energetic eddies. The intraseasonal eddies significantly affect the volume transport through passages in the northern part of the Indonesian archipelago. The intraseasonal transport variation, however, is highly damped within the Indonesian seas in the present model.
CO2 levels required for deglaciation of a “near‐snowball” Earth
Authors: Crowley Thomas J. , Hyde William T. , Peltier W. Richard
Published: February 2004

Geologic evidence suggests that in the Late Neoproterozoic (∼600 Ma) almost all land masses were glaciated, with sea‐level glaciation existing even at the equator. A recent modeling study has shown that it is possible to simulate an ice‐covered Earth glaciation with a coupled climate/ice‐sheet model. However, separate general circulation model experiments suggest that a second solution may exist with a substantial area of ice free ocean in the tropics. Although 0.1 to 0.3 of an atmosphere of CO2 (∼300 to 1000 X) is required for deglaciation of a “Snowball Earth,” the “exit” CO2 levels for an open water solution could be significantly less. In this paper we utilize a coupled climate/ice sheet model to demonstrate four points: (1) the open water solution can be simulated in the coupled model if the sea ice parameter is adjusted slightly; (2) a major reduction in ice volume from the open water/equatorial ice solution occurs at a CO2 level of about 4X present values—about two orders of magnitude less than required for exit from the “hard” snowball initial state; (3) additional CO2 increases are required to get fuller meltback of the ice; and (4) the open water solution exhibits hysteresis properties, such that climates with the same level of CO2 may evolve into either the snowball, open water, or a warmer world solution, with the trajectory depending on initial conditions. These results set useful targets for geochemical calculations of CO2 changes associated with the open‐water solution.
Tropical Pacific/Atlantic Ocean interactions at multi‐decadal time scales
Authors: Latif Mojib
Published: February 2004

Analysis of sea surface temperature (SST) observations suggests a pan‐oceanic interaction between the tropical Pacific and the Atlantic Ocean at multi‐decadal time scales, such that periods of anomalously high SSTs in the tropical Pacific are followed by a basin‐wide SST dipole in the Atlantic Ocean with a time delay of a few decades. The SST anomaly structure in the Atlantic Ocean is reminscent of variations in the North Atlantic thermohaline circulation. The two ocean basins are linked through an “atmospheric bridge” involving anomalous fresh water fluxes. Based on the observational findings, the Atlantic thermohaline circulation may strengthen during the next decades in response to the strong decades‐long increase in tropical Pacific SST, which will have strong impacts on the climates of North America and Europe through changes in the North Atlantic Oscillation (NAO).