bluuebunny/crossref_metadata_2025 huggingface dataset:
Response of the magnetospheric convection to sudden interplanetary magnetic field changes as deduced from the evolution of partial ring currents
Authors: Hashimoto Kumiko K. , Kikuchi Takashi , Ebihara Yusuke
Published: December 2002

We examined the response of the partial ring current (PRC) to sudden changes in the interplanetary magnetic field (IMF) Bz for two PRC events, in order to investigate the response of the convection in the near‐Earth magnetotail to IMF changes. We found that ground magnetic changes due to the PRC started to develop simultaneously at all local times with a time lag of 5–11 min from the growth of the ionospheric convection as detected by the polar cap magnetometers. The ground magnetometers furthermore showed that the asymmetric ring current changed into a symmetric ring current with no significant time lag (<1 min) when the convection electric field started to decay because of the northward turning of the IMF. The temporal and spatial evolution of the plasma pressure and the ground magnetic perturbations due to the PRC are simulated for the events involved using a particle simulation model. The simulation results agree well with the observations in the temporal variations and their local time dependence. The quick development and reconfiguration of the observed PRC suggest that the ionosphere played a crucial role in driving the convection in the near‐Earth magnetotail and in the inner magnetosphere. In particular, the immediate reconfiguration of the PRC caused by the northward turning of the IMF implies nearly instantaneous propagation of the ionospheric convection electric field to the inner magnetosphere and subsequent quick change in the ring current configuration. We discuss the propagation of the convection electric field to the near‐Earth magnetotail through the ionosphere after its propagation along the magnetic field lines from the generator in the dayside magnetosphere.
Coupling between pressure solution creep and diffusive mass transport in porous rocks
Authors: Gundersen Elisabeth , Renard François , Dysthe Dag Kristian , Bjørlykke Knut , Jamtveit Bjørn
Published: December 2002

Pressure solution is widely regarded as a mechanism of ductile deformation in the upper crust. It is driven by stress differences and its rate is affected by temperature, grain size, and fluid chemistry. Pressure solution involves dissolution at grain contacts under high stress and precipitation at grain contacts on pore surfaces under low stress, leading to porosity reduction by precipitation in the pore space or by grain indentation. For a system closed at a grain scale, pressure solution is traditionally described by a mechanism involving three steps: (1) dissolution at intergranular interfaces, (2) diffusion of solutes inside the contact between two grains, and (3) precipitation on the surface of the grains in contact with the pore fluid. In this paper we propose a model where we have added a fourth step to this process, diffusive transport to other open pores, to account for the macroscopic diffusion of solutes in pore fluids, such that the deformation is not closed at the grain scale. In this model, differences in mineral solubility due to variations in stress and grain size produce concentration gradients which drive diffusive mass transport. The interaction between pressure solution at a grain scale and transport over distances of several grains can lead to the amplification of initial porosity heterogeneities and subsequent localization of deformation. Regions of intense dissolution compact and form “bands” in close proximity to regions where the porosity reduction is mainly due to cementation. Pressure solution augmented by large‐scale diffusional transport will cause mass transport from fine‐grained to coarse‐grained rock volumes. We show that such processes are important during both diagenesis of sediments and compaction of fault gouge.
Microstructures and lattice fabrics in the Hilti mantle section (Oman Ophiolite): Evidence for shear localization and melt weakening in the crust–mantle transition zone?
Authors: Dijkstra Arjan H. , Drury Martyn R. , Frijhoff René M.
Published: December 2002

Experimental studies have shown that olivine aggregates with ≥4% melt are significantly weaker than melt‐free aggregates. However, questions remain as to the importance of melt weakening in nature. In several studies, melt weakening has been invoked to explain patterns of mantle flow in the Oman Ophiolite. In this paper, we reinvestigate evidence for melt weakening in the Hilti mantle section using structural and microstructural methods. The average olivine grain size increases with depth below the crust–mantle boundary. This is related to a change from equigranular‐to‐porphyroclastic microstructures at shallow levels to coarse porphyroclastic microstructures at depth. A strong foliation and high degree of recrystallization in the upper part of the mantle section are interpreted as the strong imprint of localized deformation at stresses of ∼4–10 MPa. Lattice orientation data show that the high‐strain peridotites have recorded top‐to‐the‐west shear; reversed shear senses were found deeper in the section. Since there is petrographic evidence for melt in the high‐strain zone, strain localization was probably caused by a melt‐related weakening mechanism. High melt contents required for melt weakening suggest that melt accumulated just below the crust–mantle boundary. We conclude that melt weakening is probably related to enhancement of grain boundary rather than intragranular deformation processes. Effective viscosities <2 × 1016 Pa·s may locally exist in the uppermost mantle beneath ridges if melt weakening occurs. Although our results agree with those of previous studies in the Hilti Massif, we conclude that both the previously published active flow model and a ridge compression model can account for them.
Dynamic shear rupture interactions with fault bends and off‐axis secondary faulting
Authors: Poliakov Alexei N. B. , Dmowska Renata , Rice James R.
Published: December 2002

On the basis of elastodynamic stress fields for singular crack and nonsingular slip‐weakening models of propagating rupture, we develop preliminary answers to such questions as follows: If a rupturing fault is intersected by another, providing a possible bend in the failure path, when will stressing be consistent with rupture along the bend? What secondary fault locations and orientations, in a damaged region bordering a major fault, will be stressed to failure by the main rupture? Stresses that could initiate rupture on a bend are shown to increase dramatically with crack speed, especially near the limiting speed (Rayleigh for mode II, shear for mode III). Whether a bend path, once begun, can be continued to larger scales depends on principal stress directions and ratios in the prestress field. Conditions should often be met in mode II for which bend paths encouraged by stressing very near the rupture tip are discouraged by the larger‐scale stressing, a basis for intermittent rupture propagation and spontaneous arrest. Secondary failure in the damage zone likewise increases markedly as the limiting speed is approached. Such may make the fracture energy much greater than for slip on a single surface. The extent of secondary faulting is strongly affected by prestress directions and the ratio of residual to peak strength. For mode II, prestress controls whether activation occurs primarily on the extensional side, which we show to be the typical case, or on the compressional side too. Natural examples are consistent with the concepts developed.
A spatial random field model to characterize complexity in earthquake slip
Authors: Mai P. Martin , Beroza Gregory C.
Published: December 2002

Finite‐fault source inversions reveal the spatial complexity of earthquake slip over the fault plane. We develop a stochastic characterization of earthquake slip complexity, based on published finite‐source rupture models, in which we model the distribution of slip as a spatial random field. The model most consistent with the data follows a von Karman autocorrelation function (ACF) for which the correlation lengths a increase with source dimension. For earthquakes with large fault aspect ratios, we observe substantial differences of the correlation length in the along‐strike (ax) and downdip (az) directions. Increasing correlation length with increasing magnitude can be understood using concepts of dynamic rupture propagation. The power spectrum of the slip distribution can also be well described with a power law decay (i.e., a fractal distribution) in which the fractal dimension D remains scale invariant, with a median value D = 2.29 ± 0.23, while the corner wave number kc, which is inversely proportional to source size, decreases with earthquake magnitude, accounting for larger “slip patches” for large‐magnitude events. Our stochastic slip model can be used to generate realizations of scenario earthquakes for near‐source ground motion simulations.
Effect of humidity on granular friction at room temperature
Authors: Frye Kevin M. , Marone Chris
Published: December 2002

We report on laboratory experiments designed to investigate the microphysical processes that result in rate‐ and state‐dependent friction behavior. We study the effect of relative humidity (RH) (<5 to 100%) in velocity stepping tests (10–20 μm/s) and slide‐hold‐slide (SHS) tests (3–1000 s) on 3 mm thick layers of quartz and alumina powders sheared at 25 MPa normal stresses. Granular powders are conditioned in situ under controlled RH to create new surface area before shearing. We find a transition from velocity strengthening to velocity‐weakening frictional behavior as RH increases. The transition occurs at 30–35% RH for quartz and 55–60% RH for alumina. Frictional healing is negligible at low humidity and increases with increasing RH for both materials. The coefficient of sliding friction is independent of humidity. We use normal stress vibrations in SHS tests to isolate chemically assisted healing mechanisms operative within contact junctions from compaction induced granular strengthening. We find that reorganization of granular particles influences friction but that chemically assisted mechanisms dominate. Our data show that rate‐ and state‐dependent friction behavior for granular materials, including time‐dependent healing and steady state velocity dependence, is the result of chemically assisted mechanisms that can be reduced or turned off at low humidity at room temperature in quartz and alumina.
The Northeast Nevada Volcanic Field: Magnetic properties and source implications
Authors: Palmer H. C. , MacDonald W. D.
Published: December 2002

Magnetic susceptibility and remanence studies on late Eocene ash flow deposits of the Northeast Nevada Volcanic Field (NENVF) were undertaken at 39 sites in four sampling localities: Dolly Varden Mountains (20 sites), Nanny Creek area of the Pequop Mountains (11 sites), eastern Windermere Hills (7 sites), and southern Snake Mountains (1 site). These deposits span a narrow age range from 41.08 to 39.08 Ma based on40Ar/39Ar data [Brooks et al., 1995a,1995b] and on new K‐Ar ages presented here. Characteristic remanences reveal that all ignimbrites sampled are reversely magnetized. The characteristic directions of remanent magnetization do not significantly deviate from the expected direction computed from the 42 Ma mean pole for North America, indicating that no major vertical axis rotations have affected the sampling regions since Eocene time. Anisotropy of magnetic susceptibility studies, undertaken to decipher flow trends and possible source areas for these ash flow tuff units of the NENVF, are consistent with a source region near the Toano Range. This places the ash flow source area east of Nanny Creek, the type area for the NENVF. The suspected source region lies close to the Wells fault system. However, Tertiary detachment faulting, Basin and Range normal faulting, erosion, and sedimentation have obscured the suspected source region. A more clearly defined zone of Eocene igneous activity, in the central Pilot Range to the east, cannot be ruled out as a potential source area.
First‐principles lattice dynamics and thermoelasticity of MgSiO3 ilmenite at high pressure
Authors: Karki Bijaya B. , Wentzcovitch Renata M.
Published: December 2002

Phonon dispersions of MgSiO3 ilmenite, a major mantle mineral, are calculated at high pressures by using first‐principles density functional perturbation theory. Our results for the zone center modes and their initial pressure dependencies agree very well with the experimental Raman data. However, comparison with infrared data shows some discrepancy. The calculated eigenvectors indicate that low‐frequency modes are primarily associated with the stretching of weak Mg‐O bonds, whereas high‐frequency modes are dominated by the stretching of stiff Si‐O bonds. All the mode frequencies are shown to increase monotonically with increasing pressure and no soft modes exist over the pressure regime studied (−11 to 33 GPa). Several thermodynamic properties including the thermal equation of state, thermal expansivity, entropy, and heat capacity that can be useful to better understand the contribution of the silicate ilmenite phase to the composition and dynamics of the Earth's mantle are successfully presented at geophysically relevant pressure‐temperature conditions.
Diffusion creep of anorthite‐quartz aggregates
Authors: Xiao X. , Wirth R. , Dresen G.
Published: December 2002

We investigated plastic flow of synthetic two‐phase aggregates composed of anorthite and 0–75 wt % quartz. The aggregates deform predominantly by grain boundary diffusion creep at temperatures of 1173–1473 K, strain rates of 10−7–10−4 s−1, and at confining pressure of 0.1 and 300 MPa. Strength of the aggregates increases with increasing content of quartz particles that behave as almost rigid inclusions. Strengthening is more pronounced for samples deformed at 0.1 MPa than at 300 MPa confining pressure. Predictions of composite strength based on self‐consistent continuum models are in good agreement with the experimental data from uniaxial experiments at quartz fractions <35 wt %. However, for larger quartz fractions the models underestimate the composite strength as they do not capture nonlocal mechanical interactions between quartz particles. The creep data from samples deformed in triaxial tests at 300 MPa pressure closely follow the isostress bound up to 50 wt % quartz. We suggest that load transfer to quartz inclusions is reduced in samples with >5000 H/106Si at 300 MPa confining pressure in agreement with observed microstructures. Transmission electron microscope observations indicate local gradients in deformation around quartz particles dominantly in samples deformed at atmospheric pressure.
Glacial isostatic adjustment and the radial viscosity profile from inverse modeling
Authors: Kaufmann Georg , Lambeck Kurt
Published: December 2002

A formal inverse procedure is used to infer radial mantle viscosity profiles from several observations related to the glacial isostatic adjustment process. The data sets consist of Late Pleistocene and Holocene sea level data from Scandinavia, the Barents Sea, Central Europe, Canada, and the far field, as well as observations of changes in the Earth's rotation and gravitational field, and present‐day uplift and gravity changes in Scandinavia. Inferences of mantle viscosity are robust against assumptions such as the a priori viscosity model and model discretization. However, the quality of ice sheet reconstruction remains crucial for the inverse inference. The importance to discuss regional mantle viscosity models in view of the lateral variability in mantle properties has been evident. Our inference suggests a two order of magnitude increase of mantle viscosity with depth, and volume‐averaged upper and lower mantle viscosities around 7 × 1020 and 2 × 1022 Pa s, respectively. Mantle viscosity does not need to increase sharply across the 660‐km seismic discontinuity. The viscosity profiles suggested are also able to reconcile the large‐scale geoid anomaly related to mantle convection.
A reversal of the Earth's magnetic field recorded in mid‐Miocene lava flows of Gran Canaria: Paleointensities
Authors: Leonhardt R. , Soffel H. C.
Published: December 2002

An extensive paleointensity study was carried out on an approximately 14.1 Myr old reverse‐to‐normal transition of the geomagnetic field. A total of 188 samples from a volcanic sequence on Gran Canaria (Canary Islands) were subjected to Thellier‐type paleointensity determinations. The paleodirectional results of this sequence are reported by Leonhardt et al. [2002]. A modified Thellier technique, which facilitates the recognition of MD tails and the formation of new magnetic remanences with higher blocking temperatures than the actual heating step, was used on the majority of the samples. We obtained reliable paleointensity results for 35% of the 87 sampled lava flows. In general, the obtained field intensities are lower than expected for the mid‐Miocene. This observation is very likely related to a long‐term reduction of the field close to transitions. Very low paleointensities with values <5 μT were obtained during an excursion, preceding the actual transition, and also close to significant changes of the local field directions. These are interpreted as nondipolar components becoming dominant for short periods and provoking a rapid change of field directions. During the transition, 15 successive lava flows recorded similar local field directions corresponding to a cluster of virtual geomagnetic poles close to South America. Chronologically, within this cluster, the paleointensity increases from about 9 to 28 μT followed by a decrease back to approximately 9 μT. This variation of paleointensity between lava flows with similar direction indicates independent records of the geomagnetic field and gives strong evidence for a stable transitional state that lasted a significant period.
A multicomponent coupled model of glacier hydrology 1. Theory and synthetic examples
Authors: Flowers Gwenn E. , Clarke Garry K. C.
Published: December 2002

Basal hydrology is acknowledged as a fundamental control on glacier dynamics, especially in cases where surface meltwater reaches the bed. For many glaciers at midlatitudes, basal drainage is influenced by subaerial, englacial, and subsurface water flow. One of the major shortcomings of existing basal hydrology models is the treatment of the glacier bed as an isolated system. We present theoretical and computational models that couple glacier surface runoff, englacial water storage and transport, subglacial drainage, and subsurface groundwater flow. Each of the four model components is represented as a two‐dimensional, vertically integrated layer that communicates with its neighbors through water exchange. Governing equations are derived from the law of mass conservation and are expressed as a balance between the internal distribution of water and external sources. The numerical exposition of this theory is a time‐dependent finite difference model that can be used to simulate glacier drainage. In this paper we outline the theory and conduct simple tests using an idealized glacier geometry. In the companion paper, the model is tailored to Trapridge Glacier, Yukon Territory, Canada, where results are compared with measurements of subglacial water pressure.
Ar/Ar ages from transitionally magnetized lavas on La Palma, Canary Islands, and the geomagnetic instability timescale
Authors: Singer B. S. , Relle M. K. , Hoffman K. A. , Battle A. , Laj C. , Guillou H. , Carracedo J. C.
Published: December 2002

A detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of Barranco de los Tilos on the island of La Palma, Canary Islands, reveals a complex, temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events. The Matuyama‐Brunhes (M‐B) reversal is recorded in five transitionally magnetized lava flows from the north (TN) section. The isochrons obtained from three of the lower four M‐B lavas are defined by 14 incremental heating experiments that, together with a previous age determination, yielded a weighted mean of 798.4 ± 6.2 ka (all uncertainties ±2σ). In addition, a 780.3 ± 10.3 ka isochron was determined for the overlying transitionally magnetized flow, indicating that it was erupted during a distinctly younger portion of the transition. Near the base of the south (TS) section one finds a sequence of weakly magnetized flows associated with virtual geomagnetic pole (VGP) positions in the southwest Indian Ocean between latitudes 56°S and 65°S, suggesting instability of the geomagnetic field beyond that of typical secular variation. 40Ar/39Ar isochrons from three of these flows, defined by 11 separate incremental heating experiments, gave a weighted mean of 822.2 ± 8.7 ka. This anomalous field behavior recorded 24 ± 11 kyr prior to the M‐B reversal may coincide with an event featured in several marine sediment records. Directly above two normal polarity flows (40Ar/39Ar isochrons of 751.9 ± 18.1 ka and 675.0 ± 15.7 ka) are nine transitionally magnetized lavas having magnetization directions associated with low to midlatitude VGPs spanning 23°–60°N. These flows are then capped by a single flow possessing normal polarity. Based on 12 incremental heating experiments, 40Ar/39Ar isochrons of five of these nine lavas, along with the uppermost flow, gave a weighted mean age of 580.2 ± 7.8 ka for this period of transitional to normal field behavior. From these same transitional lavas, Quidelleur et al. [1999] reported three unspiked K‐Ar ages with a weighted mean of 602 ± 24 ka and proposed a new event called the “La Palma” excursion. However, the 40Ar/39Ar age presented here is three times more precise than the K‐Ar age and is indistinguishable at the 95% confidence level from the 40Ar/39Ar age of a lava from the Snake River Plain, Idaho, that originally defined the Big Lost event. Transitional field behavior of similar age observed in astronomically dated marine cores further establishes that the Big Lost event recorded at La Palma was indeed global in extent. Rigorous temporal and geomagnetic constraints for several additional periods of geomagnetic field instability during the last several million years will comprise a geomagnetic instability timescale that can be factored confidently into models of the dynamo process.
Variation in the position of the upwelling front on the Oregon shelf
Authors: Austin Jay A. , Barth John A.
Published: December 2002

As part of an experiment to study wind‐driven coastal circulation, 17 hydrographic surveys of the middle to inner shelf region off the coast of Newport, OR (44.65°N, from roughly the 90 m isobath to the 10 m isobath) were performed during Summer 1999 with a small, towed, undulating vehicle. The cross‐shelf survey data were combined with data from several other surveys at the same latitude to study the relationship between upwelling intensity and wind stress field. A measure of upwelling intensity based on the position of the permanent pycnocline is developed. This measure is designed so as to be insensitive to density‐modifying surface processes such as heating, cooling, buoyancy plumes, and wind mixing. It is highly correlated with an upwelling index formed by taking an exponentially weighted running mean of the alongshore wind stress. This analysis suggests that the front relaxes to a dynamic (geostrophic) equilibrium on a timescale of roughly 8 days, consistent with a similar analysis of moored hydrographic observations. This relationship allows the amount of time the pycnocline is outcropped to be estimated and could be used with historical wind records to better quantify interannual cycles in upwelling.
Sea surface salinity changes in the East China Sea during 1997–2001: Influence of the Yangtze River
Authors: Delcroix Thierry , Murtugudde Raghuram
Published: December 2002

The role of the Yangtze River discharge in changing sea‐surface salinity (SSS) in the East China Sea during the 1997–2001 periods is investigated by using (1) in situ SSS measurements along a merchant ship track running from Taiwan strait to Korea strait, (2) ocean general circulation model (OGCM) simulations and specific sensitivity studies, and (3) in land precipitation data to infer estimates of the river discharge in absence of gauged station data. The river effects on 1997–2001 sea surface temperature (SST) and surface currents are also examined. Both in situ SSS‐SST measurements and OGCM simulations clearly indicate that the integration of the 1997 drought signal and 1998–1999 flooding signal over the catchment area of the Yangtze River are manifest in the ocean where local precipitation played a rather modest role. Sensitivity studies with the OGCM with and without the river discharge further reveal that these signals can extend over several hundred kilometers off the river mouth, consistent with the along track in situ measurements. In particular, SSS along the ship track can drop from about 34 psu in August 1997 to less than 26 psu in July 1998 (almost the same for 1999) at the latitude of the river mouth although located 500 km to the west of the ship track. Model results indicate that the dynamic and thermodynamic effects of the freshwater input from the river may be phase lagged by up to a season with maximum surface warming occurring in the summer months when the river discharge is maximum whereas the largest differences in mixed layers with and without the river input occur during later winter and late autumn. The possible positive feedback between the increase of river runoff yielding warm SST anomaly off the river mouth and so enhanced atmospheric convection and precipitation is discussed.
Steric sea level variations during 1957–1994: Importance of salinity
Authors: Antonov John I. , Levitus Sydney , Boyer Timothy P.
Published: December 2002

Spatially averaged (50°S–65°N) temperature and salinity changes in the 0–3000 m layer during the 1957–1994 period resulted in a sea level rise at a mean rate of about 0.55 mm per year. About 10% of this rate is due to a decrease of the volume mean salinity. The magnitude of total steric sea level (TSSL) changes and the ratio of thermosteric and halosteric anomalies to TSSL anomaly are nonuniform geographically. Salinity effects are critically important to the TSSL changes in some regions of the ocean. For example, the thermosteric anomaly is nearly compensated by the halosteric anomaly in the subpolar North Atlantic. This fact will cause erroneous heat content estimates based on altimetric observations from space if a climatological salinity is assumed. Based on the present historical archive of salinity data, a decrease in global mean salinity has occurred. This increase of fresh water would cause sea level rise at a rate of 1.3 ± 0.5 mm/yr if the added water comes from sources other than floating sea ice.
Sea level and wind stress curl in the Tuvalu Region of the South Pacific during the 1997/1998 El Niño
Authors: Luick John L.
Published: December 2002

Pronounced low sea levels during the 1997/1998 El Niño are analyzed for the effects of local wind stress curl and baroclinic Rossby waves. The sea levels analyzed were produced by a data‐assimilating numerical model (not part of this study), and the wind stress curl is based on the wind stress field used to force the model. Frequency and phase as a function of time are inferred by convolving Morlet wavelets with the sea level and wind stress curl data. At certain times and locations the phase relationships indicate reasonable agreement with a solution to the vorticity equation and identify areas of Rossby wave generation and propagation. In the initial stage of the El Niño the sea level declined as a result of regional variability, but in the mature stage the decline was due to interannual, basin‐scale processes.
Sea ice of the northern Canadian Arctic Archipelago
Authors: Melling Humfrey
Published: December 2002

Existing information concerning the pack ice and relevant climate variables of the Canadian Arctic Archipelago north of Parry Channel is summarized. This knowledge is enhanced by newly available data on ice thickness derived from 123,703 drill holes completed during the 1970s. Pack ice in this area is a mix of multiyear, second‐year, and first‐year ice types, with the latter subordinate except in the southeast. Ice remains land fast for more than half the year, and summertime ice concentration is high (7–9 tenths). In a typical year, less than 20% of the old ice and 50% of the first‐year ice melt. There are large interannual fluctuations in ice coverage and some suggestion of a decadal cycle. The average ice thickness in late winter is 3.4 m but subregional means reach 5.5 m. The pack is a mix of two populations, one consisting largely of multiyear ice imported from the zone of heavy ridging along the periphery of the Beaufort gyre and the other consisting of a mix of relatively undeformed first‐year, second‐year, and multiyear ice types that grow and age within the basin. The ice of the Sverdrup Basin is strongly influenced by a flux of heat (approximately 10 W m−2) that originates in the Atlantic‐derived waters of the Arctic Ocean. The drift of ice through the basin is controlled in the present climate by the formation of stable ice bridges across connecting channels. The drift is episodic. Relaxation of these controls in a warmer climate may cause deterioration in ice conditions in Canadian Arctic waters.
Temporal and spatial variability of the sea surface salinity in the Nordic Seas
Authors: Furevik Tore , Bentsen Mats , Drange Helge , Johannessen Johnny A. , Korablev Alexander
Published: December 2002

In this paper, the temporal and spatial variability of the sea surface salinity (SSS) in the Nordic Seas is investigated. The data include a Russian hydrographical database for the Nordic Seas and daily to weekly observations of salinity at Ocean Weather Station Mike (OWSM) (located at 66°N, 2°E in the Norwegian Sea). In addition, output from a medium‐resolution version of the Miami Isopycnic Coordinate Ocean Model (MICOM), forced with daily National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data, is used to complement the analysis of the temporal and spatial fields constructed from the observational data sets. The Nordic Seas show a strong seasonal variability in the vertical density stratification and the mixed layer (ML) depth, with a weak stratification and a several hundred meters deep ML during winter and a well‐defined shallow ML confined to the upper few tens of meters during summer. The seasonal variability strongly influences the strength of the high‐frequency variability and to what extent subsurface anomalies are isolated from the surface. High‐frequency variability has been investigated in terms of standard deviation of daily SSS, calculated for the different months of the year. From observations at OWSM, typical winter values range from 0.03 to 0.04 psu and summer values range from 0.06 to 0.07 psu. Results from the model simulation show that highest variability is found in frontal areas and in areas with strong stratification and lowest variability in the less stratified areas in the central Norwegian Sea and south of Iceland. Investigation of the interannual variability over the last 50 years shows a marked freshening of the Atlantic Water in the Norwegian and Greenland Seas. Moreover, the strength of the southern sector of the Polar front, as defined by the 34.8–35.0 psu isohalines along the western boundary of the inflowing Atlantic Water, undergoes significant interannual variability with gradient stretching reaching up to 300 km. In comparison, the variability in the strength of the eastern front and northern sector of the Polar front, seemingly controlled by the shelf break off Norway and the ridge between the Norwegian and the Greenland Seas, typically undergoes stretching only between 60 and 80 km. The investigation also demonstrates that the low‐frequency variability in the upper ocean density field in the Greenland Sea, a key factor for the deep water convection, is governed by the variability in the sea surface field. Since the early 1960s, there has been a negative trend in the salinity, probably contributing to the observed decrease in the deep water production in that period.
A modeling study of shelf circulation off northern California in the region of the Coastal Ocean Dynamics Experiment 2. Simulations and comparisons with observations
Authors: Gan Jianping , Allen J. S.
Published: December 2002

This is the second part of a modeling study of wind‐forced flow on the continental shelf off northern California in the region (37°–40°N) of the Coastal Ocean Dynamics Experiment (CODE). Gan and Allen [2002] analyzed the shelf flow response to idealized wind stress forcing in a process‐oriented study. The study here applies forcing from observed winds and heat flux for April–May 1982 and compares the model results with moored current and temperature measurements. The Princeton Ocean Model (POM) is used in a three‐dimensional limited area domain with a high‐resolution curvilinear grid (approximately 1 km horizontal spacing, 60 vertical levels) and realistic coastline and bottom topography. The objectives of the study are to simulate the response of the shelf circulation field to time‐varying observed wind stress and heat flux, to compare model results with oceanographic observations to establish confidence in the model, and to subsequently analyze the model fields and the model dynamical balances to help understand the behavior of the observed flow. The model variables show overall good agreement with corresponding observations. Similar to the conclusions by Gan and Allen [2002], it is found that the alongshore variability of upwelling is mainly controlled by the interaction of the wind‐forced shelf flow with the coastline and bottom topography. Different dynamical regimes in the regions north and south of the coastal capes formed by Pt. Reyes and Pt. Arena and in the more uniform region between these capes are identified and investigated. The results demonstrate that the coastal capes play a dominant role in causing alongshore variability of the upwelling flow, including the setup of an alongshore pressure gradient that forces northward currents during relaxation of southward upwelling favorable winds. An analysis of the balance of terms in the equation for potential temperature indicates that across‐shore temperature advection is the major contributor to the cooling of coastal water during upwelling, with a larger magnitude to the south of the capes. To the north of the capes, however, alongshore temperature advection is the dominant contributor to the colder water near the coast.
Observing the pole tide with satellite altimetry
Authors: Desai Shailen D.
Published: December 2002

Almost 9 years of sea surface height observations from the TOPEX/Poseidon (T/P) satellite altimetry mission are used to observe the geocentric pole tide deformations of the sea surface. If the oceans are assumed to have an equilibrium response, then satellite altimeters effectively observe the equipotential surface that is associated with the solid Earth and ocean pole tide deformations. The long‐wavelength component of the geocentric pole tide deformations at the Chandler wobble period is observed from T/P altimetry to be consistent with the theoretical self‐consistent equilibrium response of the ocean pole tide. The geocentric pole tide explains 70% of the variance in the degree 2 order 1 spherical harmonic component of the residual sea surface heights that are observed by T/P, after removing the seasonal, inverse barometer, and lunisolar tidal effects. If the long‐wavelength component of the ocean pole tide is assumed to have an equilibrium response at the Chandler wobble period, then satellite altimetry proves to be another geodetic technique that can be used to estimate the Love number k2 at that period.
Seasonal variation and instability nature of the North Pacific Subtropical Countercurrent and the Hawaiian Lee Countercurrent
Authors: Kobashi Fumiaki , Kawamura Hiroshi
Published: December 2002

The relation between baroclinic instability of the North Pacific Subtropical Countercurrent (STCC) and the Hawaiian Lee Countercurrent (HLCC) and seasonal evolution of mesoscale sea surface height (SSH) variability was investigated by using historical hydrographic data and altimeter‐derived SSH anomaly data during 1992–2000. The annual and monthly mean dynamic height climatologies show two robust eastward‐flowing countercurrents. The STCC extends typically along 24°N from 130°E to 160°W and slightly shifts to the north as it flows toward the east, and the HLCC is located typically along 20°N and extends from about 150°E to just west of the Hawaiian Islands. Seasonal variations of the STCC in the western North Pacific and the HLCC west of the Hawaiian Island, where the eddy variability is relatively larger than the background, were quite different: the STCC (HLCC) is strong in winter (summer) to summer (winter) and weak in fall (spring). In contrast, seasonal evolution of mesoscale SSH variability has almost the same cycle in the two countercurrent regions. The eddy kinetic energy (EKE) is a maximum in spring and a minimum in fall. The dominant zonal scales are shortest in late winter and become longer with time. A linear quasigeostrophic stability analysis using a three‐layer model indicates that instability properties are very similar between the two countercurrent regions. The STCC and the HLCC become baroclinically more unstable during winter and late fall to winter, respectively, due to strong vertical velocity shear and weak stratification between the STCC/HLCC and underlying westward flow, and the most unstable wave during this season has an e‐folding timescale of about 2–3 months and a zonal wavelength of about 300–400 km. The same behavior of the seasonal evolution of the mesoscale variability in the two countercurrent regions is a manifestation of these similarities. In terms of increasing nonlinearity of the unstable waves and an inverse cascade of the eddy energy, we discussed generation processes of the mesoscale SSH disturbances.
Deep convection as a source of new particles in the midlatitude upper troposphere
Authors: Twohy Cynthia H. , Clement Charles F. , Gandrud Bruce W. , Weinheimer Andrew J. , Campos Teresa L. , Baumgardner Darrel , Brune William H. , Faloona Ian , Sachse Glen W. , Vay Stephanie A. , Tan David
Published: December 2002

A case study of new particle formation in the region downwind of a mesoscale convective system stretching across much of the central United States is presented. Airborne measurements were made of condensation nuclei (CN), cloud particle surface area, water vapor, and other gases. CN concentrations were greatly enhanced above and downwind of the cirrus anvil, with maximum concentrations of 45,000 per standard cm3. Volatility and electron microscope measurements indicated that most of the particles were likely to be small sulfate particles. The enhancement extended over at least a 600‐km region. Multivariate statistical analysis revealed that high CN concentrations were associated with surface tracers, as well as convective elements. Convection apparently brings gas‐phase particle precursors from the surface to the storm outflow region, where particle nucleation is favored by the extremely low temperatures. Simple calculations showed that deep convective systems may contribute to a substantial portion of the background aerosol in the upper troposphere at midlatitudes.
Chemical compositions in convective and nonconvective regions during PEM‐Tropics B
Authors: Cubukcu Nihat , Krishnamurti T. N. , Dougherty Ralph C.
Published: December 2002

Four DC‐8 flights during Pacific Exploratory Mission (PEM)‐Tropics B are analyzed in terms of geographical location, meteorological conditions, and chemical compositions. The meteorological conditions are determined using the meteorological analysis of the Florida State University Global Spectral Model, trajectory analysis, and in situ measurements. Typical undisturbed meteorological conditions are found over the northeastern Pacific between 30° and 15°N and southeast of Hawaiian Islands during flights 5 and 6. Disturbed meteorological conditions are found over western and central equatorial Pacific especially over the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ) during flights 7 and 10. Comparison of the chemical concentrations between disturbed and undisturbed regions revealed remarkable differences. The distribution of the chemical constituents is found to be closely linked to the atmospheric state. Large amounts of pollutants are found to be capped at lower levels due to climatologically persistent trade wind inversion in the undisturbed regions. Transient convective systems are found to be the primary entity that destroys the inversion, causing the vertical transport of the chemicals. The disturbed regions displayed more homogeneous vertical distributions of chemicals resulting from low‐level convergence and associated convection. Trajectory analysis showed that in undisturbed regions centered north of Hawaiian Islands, and in disturbed regions, the interaction between lower‐level easterlies and upper‐level westerlies plays a significant role on atmospheric chemistry over these regions.
Airborne sampling of aerosol particles: Comparison between surface sampling at Christmas Island and P‐3 sampling during PEM‐Tropics B
Authors: Dibb Jack E. , Talbot Robert W. , Seid Garry , Jordan Carolyn , Scheuer Eric , Atlas Elliot , Blake Nicola J. , Blake Donald R.
Published: December 2002

Bulk aerosol sampling of soluble ionic compounds from the NASA Wallops Island P‐3 aircraft and a tower on Christmas Island during PEM‐Tropics B provides an opportunity to assess the magnitude of particle losses in the University of New Hampshire airborne bulk aerosol sampling system. We find that most aerosol‐associated ions decrease strongly with height above the sea surface, making direct comparisons between mixing ratios at 30 m on the tower and the lowest flight level of the P‐3 (150 m) open to interpretation. Theoretical considerations suggest that vertical gradients of sea‐salt aerosol particles should show exponential decreases with height. Observed gradients of Na+ and Mg2+, combining the tower observations with P‐3 samples collected below 1 km, are well described by exponential decreases (r values of 0.88 and 0.87, respectively), though the curve fit underestimates average mixing ratios at the surface by 25%. Cascade impactor samples collected on the tower show that >99% of the Na+ and Mg2+ mass is on supermicron particles, 65% is in the 1–6 micron range, and just 20% resides on particles with diameters larger than 9 microns. These results indicate that our airborne aerosol sampling probes must be passing particles up to at least 6 microns with high efficiency. We also observed that nss SO42− and NH4+, which are dominantly on accumulation mode particles, tended to decrease between 150 and 1000 m, but they were often considerably higher at the lowest P‐3 sampling altitudes than at the tower. This finding is presently not well understood.
Forest leaf area density profiles from the quantitative fusion of radar and hyperspectral data
Authors: Treuhaft Robert N. , Asner Gregory P. , Law Beverly E. , Van Tuyl Steven
Published: December 2002

The leaf area density (LAD) of a forest is an important indicator of forest biomass and is therefore pertinent to monitoring carbon sequestration and change. Quantitative physical models were used to estimate forest LAD from radar and hyperspectral airborne remote sensing observations. A parameter‐estimation technique based on physical models minimizes the need for in situ observations and thereby facilitates global remote sensing of forest structure. Using data from the NASA Airborne Synthetic Aperture Radar (AIRSAR) and the NASA Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) over three forest plots in Central Oregon, parameters were estimated separately from the radar and hyperspectral data and then combined to form LAD. Gaussian relative LAD profiles were estimated from multialtitude interferometric and polarimetric AIRSAR data. Leaf area indices (LAI) were estimated from AVIRIS data and used to normalize the relative density profiles to produce LAD as a function of height. LAD was also determined from field measurements of geometric tree properties and LAI. LADs in the three forest plots were in the 0.02–0.18 m2 m−3 range, with LAIs in the range 0.8–2.4 m2 m−2. The agreement between the remotely sensed and field‐measured LAD was typically 0.02 m2 m−3 but occasionally as high as 0.06 m2 m−3, which was within a 1–2 standard error range. More complex models for the remotely sensed relative density, along with more diverse radar observation strategies, will improve LAD accuracy in the future. LAD estimation will also improve when radar, hyperspectral, and other relevant remote sensing data sets are combined in a single parameter‐estimation process, as opposed to the separate estimations performed in this first LAD demonstration.
Low frequency variability of tropical cyclone potential intensity 2. Climatology for 1982–1995
Authors: Bister Marja , Emanuel Kerry A.
Published: December 2002

In this note, we describe the method of calculating a climatology of tropical cyclone potential intensities for the years 1982–1995, as displayed athttp://www.mit.edu/∼emanuel/pcmin/climo.html. The climatology has been constructed from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis and the Reynolds' Global Sea Surface Optimum Interpolation (OI) temperature analysis. We estimate errors in potential intensities resulting from known problems in the data and discuss the results.
Black carbon in aerosol during BIBLE B
Authors: Liley J. Ben , Baumgardner D. , Kondo Y. , Kita K. , Blake D. R. , Koike M. , Machida T. , Takegawa N. , Kawakami S. , Shirai T. , Ogawa T.
Published: December 2002

The Biomass Burning and Lightning Experiment (BIBLE) A and B campaigns over the tropical western Pacific during springtime deployed a Gulfstream‐II aircraft with systems to measure ozone and numerous precursor species. Aerosol measuring systems included a MASP optical particle counter, a condensation nucleus (CN) counter, and an absorption spectrometer for black carbon. Aerosol volume was very low in the middle and upper troposphere during both campaigns, and during BIBLE A, there was little aerosol enhancement in the boundary layer away from urban areas. In BIBLE B, there was marked aerosol enhancement in the lowest 3 km of the atmosphere. Mixing ratios of CN in cloud‐free conditions in the upper troposphere were in general higher than in the boundary layer, indicating new particle formation from gaseous precursors. High concentrations of black carbon were observed during BIBLE B, with mass loadings up to 40 μg m−3 representing as much as one quarter of total aerosol mass. Strong correlations with hydrocarbon enhancement allow the determination of a black carbon emission ratio for the fires at that time. Expressed as elemental carbon, it is about 0.5% of carbon dioxide and 6% of carbon monoxide emissions from the same fires, comparable to methane production, and greater than that of other hydrocarbons.