bluuebunny/crossref_metadata_2025 huggingface dataset:
surface and deep water changes in the subpolar North Atlantic during Termination II and the Last Interglaciation
Authors: Bauch Henning A. , Erlenkeuser Helmut , Jung Simon J. A. , Thiede Jörn
Published: September 2002

To reconstruct the history of water mass exchange between the NE Atlantic and the Nordic seas, sediment cores from ∼2 km water depth were studied across Termination II (TII) and through the last interglaciation (MIS5e). During early TII the sudden appearance of the low‐latitude planktonic foraminifera Beella megastoma is noted in both regions along with a steep decrease in benthic foraminiferal δ18O. Since other proxies indicate that surface waters were cold and stratified because of meltwater, conditions which prevented near‐surface thermohaline circulation and vertical convection in the Nordic seas, water mass exchange between the two areas occurred at the subsurface. During later TII, surface conditions changed, and this subsurface circulation style was eventually replaced by vertical convection. In the Nordic seas, B. megastoma vanished from the record together with ice‐rafted debris (IRD) at the end of TII, while subpolar foraminiferal abundance rose. Peak interglacial conditions with intensive vertical convection now fully developed, generating a bottom water temperature gradient of ∼4°C between the two areas. However, surface water temperatures deteriorated in the Nordic seas already notably before IRD recurred, and δ18O increased at the end of MIS5e.
The role of oceanic forcing in mid‐Holocene northern hemisphere climatic change
Authors: Kerwin Michael W. , Overpeck Jonathan T. , Webb Robert S. , DeVernal Anne , Rind David H. , Healy Richard J.
Published: September 2002

Previous model investigations suggested that changes in orbital forcing and feedbacks associated with northward expansion of the boreal forest were both required to explain the full magnitude of enhanced high‐latitude Northern Hemisphere summer warming at 6 ka, thus implying that biospheric feedbacks (decreased planetary albedo associated with forest expansion) may be large in the future [Foley et al., 1994; TEMPO (Testing Earth System Models with Paleo‐Observations), 1996]. Before the magnitude of past biospheric and other feedbacks can be estimated with confidence, however, the role of realistic high‐latitude oceanic forcing (i.e., sea surface temperature (SST) increases and sea ice reductions) should also be considered. Here we review existing paleoceanographic observations that suggest portions of the North Atlantic were up to 4°C warmer than today at 6 ka. We then combine our estimates of mid‐Holocene North Atlantic SST and sea ice conditions with new climate model simulations to suggest that a significant portion of high‐latitude summer warming at 6 ka in the Northern Hemisphere can be accounted for by altered orbital forcing, SSTs, and sea ice relative to today. Our results underscore the importance of incorporating realistic boundary conditions when using paleoenvironmental data to evaluate the climate system's sensitivity to altered forcing and suggest that high‐latitude oceanic feedbacks were major contributors to enhanced high‐latitude summer warming in the Northern Hemisphere at 6 ka.
Permeability of the continental crust: Implications of geothermal data and metamorphic systems
Authors: Manning C. E. , Ingebritsen S. E.
Published: September 2002

In the upper crust, where hydraulic gradients are typically <1 MPa km−1, advective heat transport is often effective for permeabilities k ≥ 10−16 m² and advective mass (solute) transport for k ≥ 10−20 m². Regional‐scale analyses of coupled groundwater flow and heat transport in the upper crust typically infer permeabilities in the range of 10−17 to 10−14 m², so that heat advection is sometimes significant and solute advection should nearly always be significant. Analyses of metamorphic systems suggest that a geochemically significant level of permeability can exist to the base of the crust. In active metamorphic systems in the mid to lower crust, where vertical hydraulic gradients are likely >10 MPa km−1, the mean permeabilities required to accommodate the estimated metamorphic fluid fluxes decrease from ∼10−16 m² to ∼10−18 m² between 5‐ and 12‐km depth. Below ∼12 km, which broadly corresponds to the brittle‐plastic transition, mean k is effectively independent of depth at ∼10−18.5±1 m² Consideration of the permeability values inferred from thermal modeling and metamorphic fluxes suggests a quasi‐exponential decay of permeability with depth of log k ≈ −3.2 log z ‐ 14, where k is in meters squared and z is in kilometers. At mid to lower crustal depths this curve lies just below the threshold value for significant advection of heat. Such conditions may represent an optimum for metamorphism, allowing the maximum transport of fluid and solute mass that is possible without advective cooling.
Inverse‐scattering method for dielectric objects based on the reconstruction of the nonmeasurable equivalent current density
Authors: Caorsi Salvatore , Gragnani Gian Luigi
Published: September 2002

Usual inverse‐scattering methods for the reconstruction of dielectric objects are limited by the information that can be collected at the measurement locations. In this paper, an inverse‐scattering method is proposed that allows one to reconstruct the unmeasured components of the equivalent current density and then to improve the dielectric reconstruction. The mathematical formulation is provided, and the theory is supported by the results of numerical simulations.
Present‐day motion of the Sierra Nevada block and some tectonic implications for the Basin and Range province, North American Cordillera
Authors: Dixon Timothy H. , Miller Meghan , Farina Frederic , Wang Hongzhi , Johnson Daniel
Published: September 2002

Global Positioning System (GPS) data from five sites on the stable interior of the Sierra Nevada block are inverted to describe its angular velocity relative to stable North America. The velocity data for the five sites fit the rigid block model with rms misfits of 0.3 mm/yr (north) and 0.8 mm/yr (east), smaller than independently estimated data uncertainty, indicating that the rigid block model is appropriate. The new Euler vector, 17.0°N, 137.3°W, rotation rate 0.28 degrees per million years, predicts that the block is translating to the northwest, nearly parallel to the plate motion direction, at 13–14 mm/yr, faster than previous estimates. Using the predicted Sierra Nevada block velocity as a kinematic boundary condition and GPS, VLBI and other data from the interior and margins of the Basin and Range, we estimate the velocities of some major boundary zone faults. For a transect approximately perpendicular to plate motion through northern Owens Valley, the eastern California shear zone (western boundary of the Basin and Range province) accommodates 11±1 mm/yr of right‐lateral shear primarily on two faults, the Owens Valley‐White Mountain (3±2 mm/yr) and Fish Lake Valley (8±2 mm/yr) fault zones, based on a viscoelastic coupling model that accounts for the effects of the 1872 Owens Valley earthquake and the rheology of the lower crust. Together these two faults, separated by less than 50 km on this transect, define a region of high surface velocity gradient on the eastern boundary of the Sierra Nevada block. The Wasatch Fault zone accommodates less than 3±1 mm/yr of east‐west extension on the eastern boundary of the Basin and Range province. Remaining deformation within the Basin and Range interior is also probably less than 3 mm/yr.
Approximate confidence intervals for design floods for a single site using a neural network
Authors: Whitley Robert , Hromadka T. V.
Published: September 2002

A basic problem in hydrology is the computation of confidence levels for the value of the T‐year flood when it is obtained from a log Pearson III distribution using the estimated mean, estimated standard deviation, and estimated skew. Here we give a practical method for finding approximate one‐sided or two‐sided confidence intervals for the 100‐year flood based on data from a single site. These confidence intervals are generally accurate to within a percent or two, as tested by simulations, and are obtained by use of a neural network.
Evaluating the use of “goodness‐of‐fit” Measures in hydrologic and hydroclimatic model validation
Authors: Legates David R. , McCabe Gregory J.
Published: September 2002

Correlation and correlation‐based measures (e.g., the coefficient of determination) have been widely used to evaluate the “goodness‐of‐fit” of hydrologic and hydroclimatic models. These measures are oversensitive to extreme values (outliers) and are insensitive to additive and proportional differences between model predictions and observations. Because of these limitations, correlation‐based measures can indicate that a model is a good predictor, even when it is not. In this paper, useful alternative goodness‐of‐fit or relative error measures (including the coefficient of efficiency and the index of agreement) that overcome many of the limitations of correlation‐based measures are discussed. Modifications to these statistics to aid in interpretation are presented. It is concluded that correlation and correlation‐based measures should not be used to assess the goodness‐of‐fit of a hydrologic or hydroclimatic model and that additional evaluation measures (such as summary statistics and absolute error measures) should supplement model evaluation tools.
A new modeling approach for water resources policy analysis
Authors: Simonovic Slobodan P. , Fahmy Hussam
Published: September 2002

Water resources policy analysis deals with the protection of people from the harmful effects of water and assurance of a consistent, adequate supply of usable water. Population and regulatory pressures, political and economic instabilities, and climatic variations can all be expected to further stress water supply resources. Developing policy for managing water systems for human needs in such an environment is difficult, slow, and very costly. The approach to water resources policy analysis developed in this paper is that of the rational decision maker who lays out goals and uses logical processes to explore the best way to reach those goals. The decision maker may be an individual or a group. The emphasis in this paper is on how water resources decisions ought to be analyzed and made. In establishing this framework we are proposing integration of object‐oriented modeling approach with systems analysis. Our concern here is with how the water policy analysis process should be structured to best address a policy choice, and with the object‐ oriented model that will aid understanding and prediction. The proposed approach is illustrated in the paper by the case study of water resources policy analysis for Egypt.
Coastline regression of the Romagna Region, Italy, due to natural and anthropogenic land subsidence and sea level rise
Authors: Gambolati Giuseppe , Teatini Pietro , Tomasi Lucio , Gonella Marco
Published: September 2002

The Romagna coastal area in the Northern Adriatic Sea has experienced in recent times continuous changes because of its precarious environment and low ground elevation above mean sea level (msl). Major processes that may influence the stability of the coast profile include land subsidence of both natural and anthropogenic origin and the msl rise caused by global climate change. According to the most accredited modeling predictions msl is expected to rise by almost 0.5 m over the next century because of the greenhouse effect. Natural land subsidence is the result of deep downward tectonic movement and consolidation of geologically recent deposits. It may be estimated in the range of 2–2.5 mm/yr in the Ravenna area and twice as much in the Po River delta. Anthropogenic land subsidence is primarily related to groundwater pumping from the upper fresh water aquifer system and gas production from Plio‐Pleistocene reservoirs. Geodetic surveys from 1953 to 1990 provide documentary evidence of cumulative land settlement exceeding 0.8 m and 1.2 m at Marina di Ravenna and Cesenatico, respectively. In this study we estimate both natural and anthropogenic land subsidence for the years 2015, 2050, and 2100 with the aid of ad hoc finite element simulation models. The use of these predictions together with the expected msl rise shows that many present lowlands may be permanently submerged at the end of the next century. The extent of the flooded area of the Romagna coastal region can be as much as 690 and 910 km2, using optimistic and pessimistic land subsidence scenarios, respectively. A local detailed analysis indicates that the areas around the cities of Ravenna and Cesenatico may be seriously affected by sea water ingression while the city of Rimini is well protected because of its relatively high elevation above msl.
Isotope hydrology of southern Nevada groundwater: Stable isotopes and radiocarbon
Authors: Davisson M. L. , Smith D. K. , Kenneally J. , Rose T. P.
Published: September 2002

A new δ18O map of southern Nevada groundwater shows a systematic decrease in δ18O of ∼5‰ from 36° to 39°N latitude. The variation is consistent with higher‐latitude recharge following continuous flow paths along north‐south trending graben valleys and systematically increasing in δ18O due to mixing with lower‐latitude, higher‐δ18O recharge. The data do not suggest that large masses of groundwater with unusually low δ18O values were recharged during the last pluvial period as suggested by previous workers. Many δ18O‐δD pairs in groundwater indicate variable amounts of evaporation relative to the global meteoric water line. The precipitation rate and type (rain versus snow) for a given geographic area controls the extent of evaporation. A “model” δ18O value was calculated from evaporated groundwaters by subtracting postcloud evaporation. The distribution of these model δ18O values suggest new regional groundwater flow paths previously undocumented. Dissolved inorganic carbon in groundwaters collected from deep regional flow systems typically has low 14C concentrations (≤12% modern carbon). In contrast, groundwaters collected from the carbonate rock of the Spring Mountains and alluvium of Forty Mile Canyon have higher 14C contents, indicating more recent recharge. Because of nonlinear mixing of 14C, groundwater in the regional flow system likely acquired most of its observable 14C from mixtures of young, locally recharged groundwater.
A Bayesian Approach to parameter estimation and pooling in nonlinear flood event models
Authors: Campbell Edward P. , Fox David R. , Bates Bryson C.
Published: September 2002

A Bayesian procedure is presented for parameter estimation in nonlinear flood event models. We derive a pooling diagnostic using Bayes factors to identify when it is reasonable to pool model parameters across storm events. A case study involving a quasi‐distributed, nonlinear flood event model and five watersheds in the southwest of Western Australia is presented to illustrate the capabilities and utility of the procedure. The results indicate that Markov chain Monte Carlo methods based on the Metropolis‐Hastings algorithm are useful tools for parameter estimation. We find that pooling is not justified for the model and data at hand. This suggests that current practices in nonlinear flood event modeling may be in need of urgent review.
TARSO modeling of water table depths
Authors: Knotters Martin , De Gooijer Jan G.
Published: September 2002

Threshold autoregressive self‐exciting open‐loop (TARSO) models are fitted to six time series of water table depths with precipitation excess as input variable. Basically, these models are nonlinear in structure because they incorporate several regimes which are separated by so‐called thresholds. For each well a subset TARSO ((SS)TARSO) model is selected using a Bayes information criterion (BIC). (SS)TARSO models are used to simulate realizations of water table depths with lengths of 30 years, from which characteristics such as durations of exceedance are computed. The simulation performance of the fitted (SS)TARSO models is compared with results obtained from transfer function noise (TFN) models, dynamic regression (DR) models, and with a physical descriptive model, called SWATRE, extended with additional autoregressive moving average(ARMA) processes for the noise (SWATRE+ARMA). As compared to the linear TFN and DR models the (SS)TARSO models perform better because they incorporate several regimes. These regimes are the result of different soil layers or drainage levels. Furthermore, it is interesting that (SS)TARSO models show a good relative performance as compared to the SWATRE+ARMA models. A possible reason may be that inputs of SWATRE are uncertain.
Estimation of the unsaturated hydraulic conductivity from the pressure distribution in a centrifugal field
Authors: Basha H. A. , Mina N. I.
Published: September 2002

Most measurements of the steady state unsaturated hydraulic conductivity require the measurement of the pressure and the flow rate in a laboratory column under normal drainage conditions. However, centrifugal techniques allow the creation of experimental conditions that are more suitable for fast and accurate measurements of the hydraulic conductivity function. Explicit algebraic equations for the parameters were developed for various forms of the hydraulic conductivity function, including the log linear form, the power form, and the more general inverse‐power form. Parameter estimation of the unsaturated hydraulic conductivity function is accomplished through the use of exact and approximate solutions derived from the governing one‐dimensional unsaturated flow equation for a soil medium experiencing a centrifugal force. The exact solution pertains to the log linear hydraulic conductivity function. The approximate solutions were obtained through perturbation expansion for the other forms of the hydraulic conductivity. The approximate solutions showed good agreement with the numerical solution for two soil types and various experimental conditions. The parameter estimation procedure was tested using a hypothetical pressure distribution and was found to give fairly accurate reproduction of the assumed unsaturated hydraulic conductivity.
River flood forecasting with a neural network model
Authors: Campolo Marina , Andreussi Paolo , Soldati Alfredo
Published: September 2002

A neural network model was developed to analyze and forecast the behavior of the river Tagliamento, in Italy, during heavy rain periods. The model makes use of distributed rainfall information coming from several rain gauges in the mountain district and predicts the water level of the river at the section closing the mountain district. The water level at the closing section in the hours preceding the event was used to characterize the behavior of the river system subject to the rainfall perturbation. Model predictions are very accurate (i.e., mean square error is less than 4%) when the model is used with a 1‐hour time horizon. Increasing the time horizon, thus making the model suitable for flood forecasting, decreases the accuracy of the model. A limiting time horizon is found corresponding to the minimum time lag between the water level at the closing section and the rainfall, which is characteristic of each flooding event and depends on the rainfall and on the state of saturation of the basin. Performance of the model remains satisfactory up to 5 hours. A model of this type using just rainfall and water level information does not appear to be capable of predicting beyond this time limit.
Evidence for nonlinear, diffusive sediment transport on hillslopes and implications for landscape morphology
Authors: Roering Joshua J. , Kirchner James W. , Dietrich William E.
Published: September 2002

Steep, soil‐mantled hillslopes evolve through the downslope movement of soil, driven largely by slope‐dependent transport processes. Most landscape evolution models represent hillslope transport by linear diffusion, in which rates of sediment transport are proportional to slope, such that equilibrium hillslopes should have constant curvature between divides and channels. On many soil‐mantled hillslopes, however, curvature appears to vary systematically, such that slopes are typically convex near the divide and become increasingly planar downslope. This suggests that linear diffusion is not an adequate model to describe the entire morphology of soil‐mantled hillslopes. Here we show that the interaction between local disturbances (such as rainsplash and biogenic activity) and frictional and gravitational forces results in a diffusive transport law that depends nonlinearly on hillslope gradient. Our proposed transport law (1) approximates linear diffusion at low gradients and (2) indicates that sediment flux increases rapidly as gradient approaches a critical value. We calibrated and tested this transport law using high‐resolution topographic data from the Oregon Coast Range. These data, obtained by airborne laser altimetry, allow us to characterize hillslope morphology at ≈2 m scale. At five small basins in our study area, hillslope curvature approaches zero with increasing gradient, consistent with our proposed nonlinear diffusive transport law. Hillslope gradients tend to cluster near values for which sediment flux increases rapidly with slope, such that large changes in erosion rate will correspond to small changes in gradien. Therefore average hillslope gradient is unlikely to be a reliable indicator of rates of tectonic forcing or baselevel lowering. Where hillslope erosion is dominated by nonlinear diffusion, rates of tectonic forcing will be more reliably reflected in hillslope curvature near the divide rather than average hillslope gradient.
An anaerobic field injection experiment in a landfill leachate plume, Grindsted, Denmark: 2. Deduction of anaerobic (methanogenic, sulfate‐, and Fe (III)‐reducing) redox conditions
Authors: Albrechtsen Hans‐Jørgen , Bjerg Poul L. , Ludvigsen Liselotte , Rügge Kirsten , Christensen Thomas H.
Published: September 2002

Redox conditions may be environmental factors which affect the fate of the xenobiotic organic compounds. Therefore the redox conditions were characterized in an anaerobic, leachate‐contaminated aquifer 15–60 m downgradient from the Grindsted Landfill, Denmark, where an field injection experiment was carried out. Furthermore, the stability of the redox conditions spatially and over time were investigated, and different approaches to deduce the redox conditions were evaluated. The redox conditions were evaluated in a set of 20 sediment and groundwater samples taken from locations adjacent to the sediment samples. Samples were investigated with respect to groundwater chemistry, including hydrogen and volatile fatty acids (VFAs) and sediment geochemistry, and bioassays were performed. The groundwater chemistry, including redox sensitive species for a large number of samples, varied over time during the experimental period of 924 days owing to variations in the leachate from the landfill. However, no indication of change in the redox environment resulting from the field injection experiment or natural variation was observed in the individual sampling points. The methane, Fe(II), hydrogen, and VFA groundwater chemistry parameters strongly indicated a Fe(III)‐reducing environment. This was further supported by the bioassays, although methane production and sulfate‐reduction were also observed in a few samples close to the landfill. On the basis of the calculated carbon conversion, Fe(III) was the dominant electron acceptor in the region of the aquifer, which was investigated. Because of the complexity of a landfill leachate plume, several redox processes may occur simultaneously, and an array of methods must be applied for redox characterization in such multicomponent systems.
Combined spatial and kalman filter estimation of optimal soil hydraulic properties
Authors: Cahill Anthony T. , Ungaro Fabrizio , Parlange Marc B. , Mata Michael , Nielsen Donald R.
Published: September 2002

A method for determining optimal parameters for a field‐scale hydraulic conductivity function is presented and tested on soil moisture and matric potential data measured at several locations in a field drainage experiment. The change in moisture content over time at the individual locations is modeled using Richards' equation, and an optimization for the hydraulic conductivity parameters is performed using a merit function derived from the Kalman filter, which allows consideration of measurement and process noise. The spatial correlation among the different measurement points is explicitly taken into account using the covariance between points in the calculation of the process noise covariance matrix. It is shown that the standard deviation of the effective hydraulic conductivity function estimated by the Kalman filter method applied to all measurements is significantly less than the standard deviations estimated by simple averaging of the parameters derived using other methods applied to the individual point moisture time series.
The relationship between soil heterotrophic activity, soil dissolved organic carbon (DOC) leachate, and catchment‐scale DOC export in headwater catchments
Authors: Brooks P. D. , McKnight D. M. , Bencala K. E.
Published: September 2002

Dissolved organic carbon (DOC) from terrestrial sources forms the major component of the annual carbon budget in many headwater streams. In high‐elevation catchments in the Rocky Mountains, DOC originates in the upper soil horizons and is flushed to the stream primarily during spring snowmelt. To identify controls on the size of the mobile soil DOC pool available to be transported during the annual melt event, we measured soil DOC production across a range of vegetation communities and soil types together with catchment DOC export in paired watersheds in Summit County, Colorado. Both surface water DOC concentrations and watershed DOC export were lower in areas where pyrite weathering resulted in lower soil pH. Similarly, the amount of DOC leached from organic soils was significantly smaller (p < 0.01) at sites having low soil pH. Scaling point source measurements of DOC production and leaching to the two basins and assuming only vegetated areas contribute to DOC production, we calculated that the amount of mobile DOC available to be leached to surface water during melt was 20.3 g C m−2 in the circumneutral basin and 17.8 g C m−2 in the catchment characterized by pyrite weathering. The significant (r2=0.91 and p < 0.05), linear relationship between over‐winter CO2 flux and the amount of DOC leached from upper soil horizons during snowmelt suggests that the mechanism for the difference in production of mobile DOC was heterotrophic processing of soil carbon in snow‐covered soil. Furthermore, this strong relationship between over‐winter heterotrophic activity and the size of the mobile DOC pool present in a range of soil and vegetation types provides a likely mechanism for explaining the interannual variability of DOC export observed in high‐elevation catchments.
Methane production, consumption, and air‐sea exchange in the open ocean: An Evaluation based on carbon isotopic ratios
Authors: Holmes M. Elizabeth , Sansone Francis J. , Rust Terri M. , Popp Brian N.
Published: September 2002

The concentration of methane and its isotopic composition did not appear to vary significantly in the subtropical North Pacific during 1996–1997. Methane enters the atmosphere via the upper mixed layer at a rate of 1.6 μmol m−2d−1 (±0.1) with a δ13C value of approximately −42‰ (±1.5), with no apparent seasonal variation. In comparison, the sea‐air flux of methane in the Sargasso Sea was between 1.6 and 4.4 μmol m−2d−1 with an isotopic composition between −43 and −45‰. Excess methane in surface waters appears to be generated throughout the upper 300 m of the water column by bacterial methanogenesis. The methane concentration maxima occur at the pycnocline, suggesting that the maxima are supported by methanogenesis in suspended particles that accumulate at these depths. Particle incubation experiments show that methane production may occur in these microenvironments. The lack of a diurnal signal implies that methane production and consumption in the ocean is independent of day‐night cycles such as photosynthesis, grazing and vertical migration of zooplankton. The flux to the atmosphere appears to be the main sink for methane in the upper ocean; microbial oxidation and downward eddy diffusion account for only 3 and 6%, respectively, of the total methane loss from the upper 300 m. Below that depth, concentrations decrease and isotopic ratios vary with depth due to bacterial oxidation and mixing of water masses of different ages and different histories of methane input.
A global, open‐ocean model of carbonyl sulfide and its air‐sea flux
Authors: Preiswerk Dominic , Najjar Raymond G.
Published: September 2002

A global, open‐ocean, climatological model of marine carbonyl sulfide (OCS) with diurnal, seasonal, vertical and latitudinal resolution is presented and analyzed. The model includes the processes of photchemical production, air‐sea gas exchange, vertical turbulent mixing, and hydrolysis. In accordance with observations, the model predicts the highest surface OCS concentrations during the summer in high latitudes. The model with the best guess set of parameters predicts the open ocean to be a sink of OCS (2.3 Gmol yr−1) but severely underpredicts surface OCS concentrations in most locations. When a simple temperature‐dependent dark production parameterization is included, the ocean becomes a source of OCS (2.1 Gmol yr−1). In addition to dark production, the model is most sensitive to the surface concentration of colored dissolved organic matter (CDOM) and the spectral slope of CDOM absorption.
Net community production and oxygen fluxes in the Nordic Seas based on O2 budget calculations
Authors: Falck Eva , Gade Herman G.
Published: September 2002

Historical hydrographic data from 1955 to 1988 are applied to an oxygen model for the euphotic zone in an attempt to estimate the net community production in the Nordic Seas (Greenland, Iceland, and Norwegian Seas). The model requires prior knowledge of the net annual oxygen flux through the sea surface, which is achieved by an oxygen budget based on volume transports from the literature and observed oxygen concentrations. A net annual uptake of approximately 2.3 mol O2 m−2 is needed to balance the budget, indicating that the region acts as a sink for atmospheric oxygen. An oxygen budget for the euphotic zone is then compiled on the basis of observed monthly mean changes in oxygen content, which are assumed to be balanced by net air‐sea oxygen exchange and net community production. Vertical exchange through the bottom of the euphotic zone and horizontal transports are ignored. The mean net community production in the Nordic Seas is found to be 36 g C m−2 y−1, but this result is sensitive to variations in the calculated oxygen fluxes which arise from the uncertainty of the parameterization of the air‐sea gas exchange.
How strong is the Harvardton‐Bear Constraint?
Authors: Broecker Wallace , Lynch‐Stieglitz Jean , Archer David , Hofmann Matthias , Maier‐Reimer Ernst , Marchal Olivier , Stocker Thomas , Gruber Nicolas
Published: September 2002

We compare the sensitivity of the partial pressure of CO2 in the warm surface ocean and atmosphere to the influence of the ocean’s cold water outcrops in a wide spectrum of models. While in simple box models the cold ocean dominates, in three‐dimensional ocean general circulation models, this influence is considerably smaller, suggesting that exchange processes between the warm and cold regime in the real ocean are extremely important in determining the distribution of chemical properties.
Impacts of seasonal covariance on CO2 inversions
Authors: Law Rachel M. , Rayner Peter J.
Published: September 2002

This paper investigates the impact of seasonal covariances between transport and concentrations on inverse estimates of CO2 sources with an atmospheric transport model. We use two atmospheric transport models differing only in their parameterizations of subgrid vertical diffusion. The models have similar transport characteristics for nonseasonal sources but different covariances between seasonal sources and transport. Four mass balance inversions have been performed with different model combinations to estimate 1981–1996 CO2 sources. The cases indicate “typical” and “upper bound” impacts due to seasonal covariance and observing network size. Typical differences are small on semihemispheric scales for a network of approximately 20–25 oceanic and coastal sites. Upper bound differences are large, particularly in the partitioning of sources between land and ocean. This behavior is model dependent. The interannual variability of the four inversions is similar. This suggests that estimates of interannual variability of sources are more robust to uncertainties in transport than long‐term mean estimates.
Transport of airborne lithogenic material down through the water column in two contrasting regions of the eastern subtropical North Atlantic Ocean
Authors: Bory Aloys J.‐M. , Newton Philip P.
Published: September 2002

Downward particle fluxes were measured using deep‐moored sediment traps deployed in two regions of contrasting primary productivity levels (mesotrophic and oligotrophic) of the eastern subtropical North Atlantic Ocean. The high percentage of lithogenic material (∼20–30% on average) in the paniculate matter collected shows the regional significance of the atmospheric dust inputs originating from West Africa. The magnitudes of lithogenic and biogenic fluxes decrease ∼5–6 and ∼8–9 fold, respectively, from near the African margin (mesotrophic region) to the remote open ocean (oligotrophic region). These trophic differences seem to give rise to differences in the characteristics of the downward transport of lithogenic material. At the oligotrophic site, the relatively low and slow export of biogenic matter apparently limits and delays the removal of lithogenic particles delivered to surface waters from the atmosphere. In contrast, the higher biological activity in the mesotrophic region seems to provide persistent conditions for an efficient and faster downward transport of the deposited lithogenic particles, and the temporal variability of lithogenic fluxes largely reflects that of the atmospheric dust inputs. Thus whether the temporal variability of the exported lithogenic flux in the water column follows that of the atmospheric deposition appears to depend on the trophic status. In the mesotrophic region the oft‐observed linear relationship between lithogenic and particulate organic matter (hereinafter POM) fluxes breaks down at high POM fluxes. This observation adds weight to the idea that linear relationships between POM fluxes and some candidate proxies for POM transfer cannot be assumed when POM export is large. A high mesoscale variability of biogenic, but not lithogenic, fluxes in the water column of the mesotrophic region underscores the relevance of mesoscale studies for regional estimates of export of biogenic material.
Climatic changes along the northwestern African Continental Margin over the last 30 kyrs
Authors: Ternois Yann , Sicre Marie‐Alexandrine , Paterne Martine
Published: September 2002

Two sediment cores were investigated to study the regional climatic variability of the NW African upwelling over the late Quaternary. Biomarker data and Sea Surface Temperatures (SSTs) predicted from alkenones at 25°N and 21°N exhibited distinct features. The amplitude of the last deglaciation was estimated to 4.5°C at 25°N and 2–2.5°C at 21°N. At 25°N, terrestrial and marine glacial inputs were higher than Holocene ones as a result of a strengthening of the trade winds and intensification of oceanic production. In contrast, at 21°N, warmer SSTs and lower organic carbon and alkenone productions during the last glacial suggest a regression of the upwelling and therefore a change of wind regime. Low glacial n‐alkanols are consistent with the migration of the vegetation belt during the maximum of African aridity, while their decrease towards the core‐top may be indicative of anthropogenic disturbances.
The January 1998 Earthquake swarm at Axial Volcano, Juan de Fuca Ridge: Hydroacoustic evidence of seafloor volcanic activity
Authors: Dziak Robert P. , Fox Christopher G.
Published: September 2002

On January 25, 1998 at 11:33Z, an intense swarm of earthquakes began within the summit caldera of Axial Volcano, central Juan de Fuca Ridge (JdFR). The earthquake swarm was detected using the U.S. Navy hydrophones in the northeast Pacific Ocean. The earthquake swarm lasted 11 days and produced 8247 detected (1037 located) earthquakes. During the first two days of the swarm, earthquake activity migrated from the summit caldera ∼50 km along the south rift zone of Axial Volcano at rates of 0.92±0.13 and 0.23±0.09 m s−1. Earthquake epicenter migration is characteristic of a lateral magma dike injection. The largest three swarm earthquakes occurred within the caldera. Their timing and mechanisms are consistent with adjustment of the caldera floor as magma is removed from beneath the summit. Earthquakes with short T‐wave rise times (shallow focal depths) cluster at two spots along the summit and south rift zone, and are considered possible seafloor eruption sites. In situ ground deformation and hydrothermal plume monitors, and later shipboard observations, confirmed the occurrence of a seafloor volcanic eruption at the volcano's summit.
Throughflow within Makassar Strait
Authors: Gordon Arnold L. , Susanto R. Dwi , Ffield Amy
Published: September 2002

From late November 1996 to early July 1998 velocity measurements were made at two moorings within a constriction in Makassar Strait near 3°S. The 1997 average throughflow is 9.3 Sv, with an uncertainty of about ±2.5 Sv depending on how the surface flow is taken into account. The results show that throughflow within Makassar Strait can account for all of the Pacific to Indian interocean transport. The correlation of transport to ENSO may be as high as 0.73, though the time series is too short to say this with assurance. Most of the remaining variance is explained by the annual cycle, with a June maximum and December minimum. A strong intra‐seasonal event occurs from late May to July, 1997.
Microbial biomass in the hydrothermal plumes associated with the 1998 Axial Volcano Eruption
Authors: Cowen James P. , Shackelford Rachel , McGee Donald , Lam Phyllis , Baker Edward T. , Olson Eric
Published: September 2002

The Axial Response Team (ART‐1) documented greatly intensified chronic‐style hydrothermal plumes at Axial Volcano following a 1998 eruptive event. Significantly higher numbers of bacteria were found in the plume versus background stations and depths, due largely to 5 samples with high numbers (to 1.8×105 /ml) in far field stations. Highest ratios (0.20) of metal depositing capsuled bacteria to total bacteria were found in the near‐field (over caldera) plume. An unusual capsule form (FeCap), that dominated the near field plume capsule populations, was present in all plume samples, but were absent in background samples. Multi‐cell filaments, metal‐encrusted sheathed clusters and matrix‐enmeshed colonies, all uncommon in the water column, were also present in plume samples.
Influence of lateral boundaries on mesoscale variability
Authors: Wang Liping , Chao Yi , Koblinsky Chester , Howden Stephen
Published: September 2002

Using the Florida Strait as a prototype, it is shown that lateral boundaries can have a significant dynamic influence on mesoscale variability, provided the distance between the lateral boundaries is very close to the critical width for baroclinic instability (
is the internal Rossby radius of deformation). As a result, the close relation between strong background currents and mesoscale variability found in open‐ocean settings breaks down. In addition, straits that are narrow enough can very effectively block through‐strait movement of strong mesoscale variability.
Oceanic control of decadal North Atlantic sea level pressure variability in winter
Authors: Latif Mojib , Arpe Klaus , Roeckner Erich
Published: September 2002

The predictability of winter‐time North Atlantic sea level pressure (SLP) variability has been investigated by means of an ensemble of integrations with an atmospheric general circulation model (AGCM) forced by observed sea surface temperatures (SSTs) for the period 1951–1994. The results imply that the SLP variations on timescales of several years to decades may be predictable, provided the SST anomalies themselves used to drive the AGCM can be predicted. The model, however, suffers from systematic errors, and the simulated centers of action are shifted relative to those observed.
Oceanic influence on the North Atlantic Oscillation and associated northern hemisphere climate variations: 1959–1993
Authors: Mehta Vikram M. , Suarez Max J. , Manganello Julia V. , Delworth Thomas L.
Published: September 2002

The North Atlantic Oscillation (NAO) exhibits variations at interannual to multidecadal time scales and is associated with climate variations over eastern North America, the North Atlantic, Europe, and North Africa. Therefore, it is very important to understand causes of these NAO variations and assess their predictability. It has been hypothesized, based on observations, that sea surface temperature (SST) and sea‐ice variations in the North Atlantic Ocean influence the NAO. We describe results of an ensemble of sixteen experiments with an atmospheric general circulation model in which we used observed SST and sea‐ice boundary conditions globally during 1949–1993. We show that multiyear NAO and associated climate variations can be simulated reasonably accurately if results from a large number of experiments are averaged. We also show that the ambiguous results of previous NAO modeling studies were strongly influenced by the ensemble size, which was much smaller than that in the present study. The implications of these results for understanding and predictability of the NAO are discussed.
Modeling the relaxation of red sprite plasma
Authors: Nunn David , Rodger Craig J.
Published: September 2002

Red sprites consist of multiple ionised columns extending above a thunderstorm from ∼30 km to ∼90 km. Electron densities in these columns are very much larger than the ambient background, perhaps fives orders of magnitude at 70 km. These highly ionized structures cause observable perturbations in subionospheric VLF transmissions known as “VLF Sprites”. Three models of initial sprite electron density are considered, and using a realistic ionization relaxation model the time dependence of electron density is derived. A 3‐D Born propagation code of is used to compute the time profile of a VLF sprite. Two profiles show good agreement with the time signature experimentally observed, in that scattered amplitude and phase decrease logarithmically with time. These simulations provide insight into the nature and structure of sprite columns, and indicate an additional constraint which should be applied to red sprite creation models.
May 4, 1998 storm: Observations of energetic ion composition by POLAR
Authors: Chen Jiasheng , Fritz Theodore A.
Published: September 2002

A major geomagnetic storm occurred at 5:00–12:05 UT on May 4, 1998. During this period, the magnetosphere was compressed and eroded; POLAR traveled in its outbound orbit from the equatorial radiation belt to the cusp and crossed the magnetopause into the magnetosheath. Two CEP‐like (Cusp Energetic Particle) events with two to three orders of magnitudes enhancements of MeV ion fluxes were measured. The first event had a peak flux higher than that of the intense outer radiation belt in the equatorial plane. The shape of the measured energy spectra (in the unit of keV/e) were ion species dependent. For He+/He+ + ratio at 18–269 keV/e, the difference between the radiation belt and the magnetosheath can be by a factor of as large as 62. In the magnetosheath, the 18–269 keV/e ion composition are variable, indicating a mass dependent acceleration process. These MeV ions may be energized by a mechanism in the cusp responsible for the CEP events previously reported.
Quasiperiodic oscillations observed at the edge of an auroral arc by auroral turbulence 2
Authors: Ivchenko Nickolay , Marklund Göran , Lynch Kristina , Pietrowski David , Torbert Roy , Primdahl Fritz , Ranta Aarne
Published: September 2002

The Auroral Turbulence II (AT2) sounding rocket carried three payloads into the auroral ionosphere where they crossed several arc structures. At the border of an auroral arc a quasiperiodic structure was observed by the magnetic and electric field instruments as well as by the particle detectors. The variations were temporal oscillations, but existed only in a narrow (≈ 7 km) region transverse to the arc, with a correlation length along the arc of at least several km. The relation between the electric and magnetic field amplitude indicates the Alfvénic nature of the variations. Field aligned electron precipitation is correlated to the field variations. The narrow band nature of the oscillations and frequency around 0.6 Hz is consistent with waves confined in the ionospheric Alfvén resonator.
Excitation of short‐scale density structures by drift waves during ionospheric heating
Authors: Antani S. N. , Guzdar P. N.
Published: September 2002

In this paper we investigate the role of dissipative drift instability in exciting “needle‐like” short scale density structures recently observed during ionospheric heating [Kelley et al.,, 1995; Franz et al., 1999]. A two‐fluid model for a warm, collisional plasma column, representing a typical density striation is used to derive a dispersion relation for drift modes. It is found that unstable drift modes can be excited. More accurate nonlocal eigenmode analysis reveals that for Arecibo parameters, the m = 3 mode is most unstable. Mixing‐length estimate of the density spectrum indicates a k−2 scaling in reasonable agreement with the observed spectrum.
Plasma mass density in the plasmatrough: Comparison using ULF waves and CRRES
Authors: Loto'aniu T. M. , Waters C. L. , Fraser B. J. , Samson J. C.
Published: September 2002

Observations of ULF field line resonances detected by seven magnetometers in the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) array were used to estimate equatorial cold plasma mass densities in and around the plasmatrough region as a function of both radial distance and time of day. The magnetometers ranged in latitude from Pinawa (L=4.3) to Rankin Inlet (L=12.4). The equatorial cold plasma mass densities were estimated using the T89 magnetic field model, an R−4 plasma density distribution function and the wave equation from cold plasma MHD theory. These ground based plasma mass density estimates were compared to those obtained from the CRRES spacecraft. Typical values obtained from the ground based measurements were ∼2 H+cm−3 at L ∼10 to ∼10 H+cm−3 at L ∼ 7. The data recorded on 3 October, 1990 showed excellent agreement with the spacecraft observations for 8.5 < L < 10. It was found that the differences between ground based estimates of plasma mass density and the values derived from spacecraft observations increased in regions of large plasma mass density spatial gradients. This result may be explained in terms of the spacing of magnetometers in the ground based array.
Change in photospheric magnetic flux during coronal mass ejections
Authors: Lara Alejandro , Gopalswamy Nat , DeForest Craig
Published: September 2002

We report on the variations of photospheric magnetic flux during coronal mass ejections (CMEs). Using magneograms from the SOHO/MDI instrument, we have computed the magnetic flux of 7 active regions and one disappearing filament region associated with CMEs. When we plotted the flux versus time over a period of few days before, during and after the CME event, we found changes in the mean magnetic flux per pixel (∼4 arcsec²), ranging from ∼0.4 to ∼3.1 × 1017 Mx, in structures of size smaller than the active region. Flare onsets and the filament disappearance clearly occurred during periods of significant variations in the measured magnetic flux.