bluuebunny/crossref_metadata_2025 huggingface dataset:
Impact of the large‐scale Arctic circulation and the North Water Polynya on nutrient inventories in Baffin Bay
Authors: Tremblay Jean‐Éric , Gratton Yves , Carmack Eddy C. , Payne Christopher D. , Price Neil M.
Published: October 2002

The distributions of nitrate, phosphate, and silicate in northern Baffin Bay were determined from 90 bottle casts taken between April 11 and July 21, 1998. During late spring, low‐salinity Arctic water entered northern Smith Sound and mixed with Baffin Bay water (BBW) within the North Water Polynya. The Arctic water originated from the Bering Sea and contained high concentrations of phosphate and silicate (referred to as silicate‐rich Arctic water (SRAW)). The distribution of the two water masses was established using a new tracer, Siex, which showed moderate penetration of SRAW into Smith Sound during April and a very strong incursion in May and June, consistent with the intensification of southward current velocities. Biological depletion of macronutrients in BBW began in April and continued until nitrate was exhausted from the upper mixed layer in early June. Beneath the Polynya the deep waters (>450 m) showed a marked increase in nutrient concentration toward the bottom, which was most pronounced in the south and much stronger for silicate than nitrate and phosphate. The silicate enrichment was consistent with dissolution of diatom‐derived biogenic silica in deep waters. The results indicate that the North Water acts as a silicate trap in which the biota differentially transports surface silicate to depth, thereby influencing local and downstream nutrient signatures.
Laboratory experiments on mesoscale vortices colliding with a seamount
Authors: Cenedese Claudia
Published: October 2002

Interaction between a vortex and a right vertical cylinder was investigated in the laboratory for both a self‐propagating vortex and one advected by a background uniform flow. In the former case, experiments were carried out with a sloping bottom in order to simulate the β plane. In the latter case the bottom was flat and a cylinder was towed, with a uniform speed, through a fluid otherwise at rest and into a stationary vortex. In both cases, after a cyclonic vortex came in contact with the cylinder, fluid peeled off the outer edge of the vortex and went around the cylinder with a counterclockwise velocity vs as predicted by the circulation equation. This fluid formed a new cyclonic vortex in the wake of the cylinder, and bifurcation of the original vortex into two vortices occurred provided 400 ≤ Re ≤ 1100, where the Reynolds number Re = vsLmax/ν and Lmax is the larger of the vortex or the cylinder diameter. This result is in agreement with previous studies of uniform flow past a cylinder in a rotating environment, and therefore we suggest that the new vortex in the wake of the cylinder was formed like those in the well‐known Karman vortex street. Experiments have been carried out systematically by varying D/d, the ratio of the cylinder diameter to the vortex diameter and the geometry of the encounter. The results suggest that the presence of a background flow enhances the bifurcation mechanism. A good agreement between the laboratory experiments and the observation of a meddy bifurcating after collision with the Irving Seamount in the Canary Basin, suggests that the oceanic vortex‐bifurcation process is similar to that observed in the laboratory experiments.
Evidence of abyssal eddies in the Brazil Basin
Authors: Weatherly Georges , Arhan Michel , Mercier Herle , Smethie William
Published: October 2002

We report evidence of two deep cyclonic and two deep anticyclonic submesoscale eddies from World Oceanographic Circulation Experiment hydrographic casts made in the Brazil Basin. We infer that three of these were likely formed in or near the Deep Western Boundary Current (DWBC) of North Atlantic Deep Water (NADW), and thus had traveled eastward after formation. These eddies appear to be a new way for transporting NADW away from the DWBC to the ocean interior. One of the apparent cyclonic eddies appeared to be laterally in contact with one of the anticyclonic eddies. About 10 days later an attempt was made to resample the apparent eddies that had been in contact. These observations, although limited, are interpreted to indicate that they survived the encounter, that the cyclonic eddy had now moved to be beneath the anticyclonic one with each being somewhat thinner, and that they produced a new anticyclonic eddy by partially merging. Deep float observations [Hogg and Owens, 1999] partially support the second inference.
Seasonality of the ventilation of the tropical Atlantic thermocline in an ocean general circulation model
Authors: Lazar Alban , Inui Tomoko , Malanotte‐Rizzoli Paola , Busalacchi Antonio J. , Wang Liping , Murtugudde Ragu
Published: October 2002

This paper presents an analysis of subduction and advection within the shallow subtropical‐tropical connections (STC) of the Atlantic Ocean. We carried out a kinematic study of the seasonal cycle of subduction, entrainment, and subsurface circulation in an upper Atlantic Ocean general circulation model (GCM), with a particular emphasis on the southern STC that is less studied but more important for the water supply of the equatorial upwelling. It is found that the southern STC, like the northern, has a typical water mass formation rate of 25–50 m/year, depending on the wind product used. This formation lasts 1 to 2 months in the south (longer in the northern tropics) and starts from early winter to late spring, in contrast to the common view of the end of March start. Such a large span of starting time is explained by a key control mechanism of the wind through the spatial distribution and seasonal variations of its speed. Each of these results stresses the importance of the variability as well as the uncertainties of the wind forcing for understanding the sustenance and variability of the characteristics of the tropical and equatorial thermocline. The slow currents within the thermocline branches of the STC may play a role in setting an interannual to decadal timescale for tropical or equatorial SST variability; we estimate an Atlantic STC timescale associated with the main advective pathways that at maximum reaches 5 years.
Decadal variations in Labrador Sea ice cover and North Atlantic sea surface temperatures
Authors: Deser Clara , Holland Marika , Reverdin Gilles , Timlin Michael
Published: October 2002

The spatial and temporal evolution of winter sea ice anomalies in the Labrador Sea and associated sea surface temperature (SST) variations in the North Atlantic are documented for three periods of above‐normal ice cover: 1972–1974, 1983–1985, and 1990–1992. These events are notable for their winter‐to‐winter persistence, despite the fact that the ice margin retreats to northern Baffin Bay each summer, and for their spatial evolution, progressing from the northern Labrador Sea to the southern tip of Newfoundland over a 3 year period. Above‐normal sea ice is consistently accompanied by below‐normal SSTs in the subpolar Atlantic: the latter persist 1–3 years after the decay of the ice anomalies and in some cases exhibit a tendency for eastward movement across the gyre. Spring–summer freshwater anomalies at 100 m depth in the West Greenland Current are found to precede by ∼8 months the initial occurrence of above‐normal ice cover in the northern Labrador Sea. The role of atmospheric forcing in the joint evolution of anomalous sea ice and SST is assessed by means of an ice‐ocean mixed layer model forced with observed air temperature and wind fields. The model results indicate that thermodynamic atmospheric forcing accounts for much of the winter‐to‐winter persistence and spatial evolution of the ice and concurrent SST anomaly patterns. However, the subsequent persistence of SST anomalies in the subpolar region is not well simulated, suggesting that oceanic processes omitted from the simple slab mixed layer formulation play a contributing role.
Coriolis effects and the thermal bar
Authors: Farrow Duncan E. , McDonald N. Robb
Published: October 2002

A model for the thermal bar system in the rotating frame that includes unsteady inertia is formulated. Asymptotic solutions are found to the initial value problem in the frictionless, small bottom slope limit. These solutions include inertial oscillations that are significant enough to reverse the circulation ahead of the thermal bar. These asymptotic solutions are compared with numerical solutions of the full model that includes friction. The consequences of both sets of results on the thermal bar in lakes is discussed.
Variability of Antarctic sea ice 1979–1998
Authors: Zwally H. Jay , Comiso Josefino C. , Parkinson Claire L. , Cavalieri Donald J. , Gloersen Per
Published: October 2002

The principal characteristics of the variability of Antarctic sea ice cover as previously described from satellite passive microwave observations are also evident in a systematically calibrated and analyzed data set for 20.2 years (1979–1998). The total Antarctic sea ice extent (concentration >15%) increased by 11,180 ± 4190 km2 yr−1 (0.98 ± 0.37% (decade)−1). The increase in the area of sea ice within the extent boundary is similar (10,860 ± 3720 km2 yr−1 and 1.26 ± 0.43% (decade)−1). Regionally, the trends in extent are positive in the Weddel Sea (1.4 ± 0.9% (decade)−1), Pacific Ocean (2.0 ± 1.4% (decade)−1), and Ross (6.7 ± 1.1% (decade)−1) sectors, slightly negative in the Indian Ocean (−1.0 ± 1.0% (decade)−1), and strongly negative in the Bellingshausen‐Amundsen Seas sector (−9.7 ± 1.5% (decade)−1)). For the entire ice pack, ice increases occur in all seasons, with the largest increase during fall. On a regional basis the trends differ season to season. During summer and fall the trends are positive or near zero in all sectors except the Bellingshausen‐Amundsen Seas sector. During winter and spring the trends are negative or near zero in all sectors except the Ross Sea, which has positive trends in all seasons. Components of interannual variability with periods of about 3–5 years are regionally large but tend to counterbalance each other in the total ice pack. The interannual variability of the annual mean sea ice extent is only 1.6% overall, compared to 6–9% in each of five regional sectors. Analysis of the relation between regional sea ice extents and spatially averaged surface temperatures over the ice pack gives an overall sensitivity between winter ice cover and temperature of −0.7% change in sea ice extent per degree Kelvin. For summer some regional ice extents vary positively with temperature, and others vary negatively. The observed increase in Antarctic sea ice cover is counter to the observed decreases in the Arctic. It is also qualitatively consistent with the counterintuitive prediction of a global atmospheric‐ocean model of increasing sea ice around Antarctica with climate warming due to the stabilizing effects of increased snowfall on the Southern Ocean.
Simulations of ice‐ocean dynamics in the Weddell Sea 2. Interannual variability 1985–1993
Authors: Timmermann R. , Hellmer H. H. , Beckmann A.
Published: October 2002

Investigations of sea ice‐ocean interaction on the continental shelf in the southwestern Weddell Sea reveal a strong correlation between fluctuations of atmospheric forcing and sea ice formation. Anomalies of meridional wind stress in the inner Weddell Sea are consistent with the phase of the Antarctic Circumpolar Wave. Positive anomalies of northward wind stress cause an increase of sea ice export in the same year and of sea ice formation in the following year, leading to an increased production of High‐Salinity Shelf Water. Driven by a varying zonal density distribution over the continental shelf, the circulation in the Filchner‐Ronne Ice Shelf cavity fluctuates between two modes, each of which features a characteristic distribution of basal freezing and melting regions. Thus signals of interannual atmospheric variability propagate into the deep ocean and the sub‐ice shelf cavities.
Observations of the internal tide and associated mixing across the Malin Shelf
Authors: Rippeth Tom P. , Inall Mark E.
Published: October 2002

An energetic internal tide was observed at two contrasting shelf continental sites to the west of Scotland. The first site was 5 km shoreward of the continental shelf break, and the second was 45 km directly to the east of the first site. Each site was instrumented with a seabed mounted acoustic Doppler current profiler and thermistor chain for a 2 week period in July 1996, when thermal stratification was well developed. In addition to the measurements of the evolution of the water column structure and flow field, two series of measurements of the rate of dissipation of turbulent kinetic energy were made at each of the sites. The barotropic tide was found to contain an unusually strong diurnal component, thought to be due to a coastally trapped wave; as a result, there is a significant diurnal component to the baroclinic energy spectra. Significant peaks are also present in the baroclinic energy spectra at interaction frequencies between the main semidiurnal (M2) and diurnal (K1) tidal constituents at both sites. Observations of profiles of the rate of dissipation of turbulent kinetic energy show a significant proportion of the measured rate of dissipation took place within the thermocline. Close to the shelf edge, detailed observations of a hydraulic jump and nonlinear internal waves are analyzed to show significantly stronger mixing at this site (characterized by diffusion coefficients of 1.6 and 6.1 × 10−4 m2 s−1) than at the site farther onshore (where the diffusion coefficient is estimated to be 0.7 × 10−4 m2 s−1).
Spatial and temporal variability of tropospheric ozone (O3) in the boundary layer above the Aegean Sea (eastern Mediterranean)
Authors: Kouvarakis G. , Vrekoussis M. , Mihalopoulos N. , Kourtidis K. , Rappenglueck B. , Gerasopoulos E. , Zerefos C.
Published: October 2002

To study the spatial and temporal variability of tropospheric ozone in the marine boundary layer over the Aegean Sea (eastern Mediterranean), an O3 analyzer has been installed onboard of a passenger vessel traveling on a regular basis in the Aegean Sea (from Heraklion/Crete 35°30′N, 25°13′E to Thessaloniki 40°64′N, 22°97′E) during a period of 14 months (August 1999 to November 2000). In addition, O3 data have been obtained on a daily basis at the regional background station of Finokalia (Crete; 35°30′N, 25°70′E) since September 1997, short‐term measurements of O3 were performed over Crete during the PAUR II campaign (May 1999), and the first O3 data from a rural area (40°32′N, 23°50′E) close to Thessaloniki at the north of Greece have been collected from March 2000 to January 2001. This survey extensive points out the existence of a well‐defined seasonal cycle in boundary layer O3 with maximum in summer both above the Aegean Sea and at Finokalia. However, the seasonal signal (defined as the summer/winter ratio) is not constant and varies as a function of air mass origin from 1.33 to 1.15 for the N‐NE and SW‐S sectors, respectively, in line with the geographical location of the O3 precursor sources. Our data show the absence of any significant longitudinal gradient over Crete at least during spring and autumn and the absence of significant latitudinal gradient between the north and south Aegean Sea during all seasons for air masses having similar origin. The above results indicate that long‐range transport is the main factor accounting for the elevated O3 levels above the eastern Mediterranean Sea. Thus (1) O3 data from Finokalia, where the longest time series are available for the area, have regional significance and (2) over the entire Aegean basin, ozone values are above the 32 ppbv European Union (EU) phytotoxicity limit throughout the year and above the 53 ppbv EU health protection limit most of the time during the dry season of the year. The very significant correlation between black carbon (BC) and O3 observed during an intensive campaign in May 2000 provides an indication that the high O3 concentrations at Finokalia resulted from ageing of air masses strongly affected by combustion processes.
Integrated atmospheric water vapor estimates from a regional GPS network
Authors: Baltink Henk Klein , van der Marel Hans , van der Hoeven André G. A.
Published: October 2002

Integrated atmospheric water vapor (IWV) estimates from a 15‐station‐wide network of Global Positioning System (GPS) receivers have been collected continuously since November 1997. The core of this network consists of five stations of the active GPS reference system in the Netherlands. A network with sufficient long baselines was chosen to secure the absolute accuracy of the GPS IWV data. Rapid satellite orbits available 12 to 24 hours after data acquisition are used in the processing of the GPS data, and IWV estimates are available with a typical delay of 1 day. Comparison of the GPS IWV data with data retrieved from a water vapor radiometer and radiosondes shows a good agreement. Different network configurations and processing strategies have been investigated to optimize the network and processing for future near‐real‐time use. In near‐real‐time applications, only predicted orbits are available; however, the accuracy of the predicted orbits is, in general, not sufficient for accurate IWV retrieval. We tested whether orbit relaxation, i.e., the simultaneous adjustment of orbit parameters during the processing of the GPS data, could increase the accuracy of the IWV estimates. During an experiment with orbit relaxation applied to predicted orbits a significant improvement of the accuracy of the GPS IWV data was found. The accuracy was comparable to GPS IWV data retrieved with final orbits, the most accurate orbit data available. Results of the experiments and the analysis of operational acquired data are presented.
A new Chemical Lagrangian Model of the Stratosphere (CLaMS) 1. Formulation of advection and mixing
Authors: McKenna Daniel S. , Konopka Paul , Grooß Jens‐Uwe , Günther Gebhard , Müller Rolf , Spang Reinhold , Offermann Dirk , Orsolini Y.
Published: October 2002

Recent satellite observations and dynamical studies have demonstrated the existence of filamentary structures in chemical tracer fields in the stratosphere. It is also evident that such features are often below the spatial resolution of the highest‐resolution Eulerian models that have been used up to the present time. These observations have motivated the development of a novel Chemical Lagranigan Model of the Stratosphere (CLaMS) that is based on a Lagrangian transport of tracers. The description of CLaMS is divided into two parts: Part 1 (this paper) concentrates on the Lagrangian dynamics, i.e., on the calculation of trajectories and on a completely new mixing algorithm based on a dynamically adaptive grid, while part 2 describes the chemical integration and initialization procedure. The mixing of different air masses in CLaMS is driven by the large‐scale horizontal flow deformation and takes into account the mass exchange between the nearest neighbors determined by Delaunay triangulation. Here we formulate an isentropic, i.e., two‐dimensional version of the model and verify the mixing algorithm using tracer distributions measured during the space shuttle CRISTA‐1 experiment where highly resolved stratospheric structures were observed in early November 1994. A comparison of the measured Southern Hemispheric N2O distribution with CLaMS results allows the intensity of simulated mixing to be optimized. The long‐term robustness of the transport scheme is investigated in a case study of the 1996–1997 Northern Hemisphere polar vortex. This study further provides a dynamical framework for investigations of chemical arctic ozone destruction discussed in part 2.
Stable carbon isotopic compositions of light hydrocarbons over the western North Pacific and implication for their photochemical ages
Authors: Saito Takuya , Tsunogai Urumu , Kawamura Kimitaka , Nakatsuka Takeshi , Yoshida Naohiro
Published: October 2002

Light hydrocarbons collected over the western North Pacific in canisters during an oceanographic cruise in May 1999 were analyzed for the mixing ratios and the stable carbon isotopic compositions (δ13C). Their mixing ratios decreased during the passage of a warm front. By contrast, they increased after the passage of a cold front. The δ13C values of ethane were found to be in a range of −26 to −19‰ and to be systematically heavier when the mixing ratios (range of 1.7–0.6 ppbv) became lower. The δ13C values of ethane over the open ocean generally showed heavier values than those previously reported in the source regions, suggesting an isotopic fractionation during a long‐range atmospheric transport. In contrast, δ13C values of i‐ and n‐butanes varied in a wide range (−37 to −16‰ and −37 to −21‰, respectively); however, their averaged values are comparative to the isotopic source compositions. Likewise, no systematic relationships were found between the mixing ratios and the δ13C values of the butane isomers. For n‐butane the observed relationship was substantially different from that predicted by kinetic isotope effect by the reaction with OH radicals probably due to the contribution of effective mixing process. From the changes in the δ13C values for ethane and n‐butane the averaged photochemical ages were calculated to be 49 ± 26 days and 2 ± 6 days, respectively. The difference in the estimated ages is attributed to the different reactivity of hydrocarbons. The average ages for n‐butane were found to be consistent with numbers of days, which the air mass took for the transport from the continent to the sampling site over the ocean, on the basis of backward trajectory analysis. This study suggests that the photochemical ages are useful for assessing the importance of recent emission in the marine atmosphere.
Turbulence parameters in the tropical troposphere and lower stratosphere
Authors: Satheesan K. , Krishna Murthy B. V.
Published: October 2002

Using the mesosphere‐stratosphere‐troposphere (MST) radar observations in the troposphere and lower stratosphere at a tropical station, Gadanki, the rate of turbulent kinetic energy dissipation ϵ, as estimated by the spectral width method, the power method, and the variance method, has been compared. It is found that ϵ from the spectral width method and variance method agree quite well under low background wind conditions, whereas under high background wind conditions, ϵ from the spectral width method appears to be an underestimate. The ϵ from the power method is generally lower than that from the other two methods. Adopting the variance method, the turbulence parameters ϵ and the eddy diffusivity for momentum Km have been estimated for tropospheric and lower stratospheric regions over Gadanki for the period 18 January 1999 to 5 March 1999, making use of the vertical wind observations from MST radar on each night for ∼2 hours. Average altitude profiles of ϵ and Km for the winter season are presented. The turbulence parameter profiles reveal a prominent peak in the upper troposphere. From Richardson number considerations the observed turbulence is attributed mainly to wind shear.
Estimation of aerosol opacity and land surface bidirectional reflectance from ATSR‐2 dual‐angle imagery: Operational method and validation
Authors: North Peter R. J.
Published: October 2002

Imagery from the second Along Track Scanning Radiometer (ATSR‐2) is analyzed to reveal that a separation of the atmospheric from the land surface signal is possible given two near‐simultaneous images at differing slant paths. The method is based on the inversion of a simple physical model of light scattering by the land surface. The result is an image of the land surface reflectance that is relatively uncontaminated by atmospheric scattering and a separate image of the atmospheric aerosol loading. Unlike single‐image satellite methods for aerosol estimation, no assumptions are required about the land surface spectral properties, allowing for quantitative mapping of aerosol over an arbitrary land surface. Validation is performed over boreal forest by comparison with data from a network of Sun photometers. Satellite estimates of aerosol optical depth agree with field measurements to within 0.02 at 550 nm. An important consequence is that measurements can be compared over time, and small changes in land surface reflectance can be separated from atmospheric changes.
Measurement and interpretation of isoprene fluxes and isoprene, methacrolein, and methyl vinyl ketone mixing ratios at the PROPHET site during the 1998 Intensive
Authors: Apel E. C. , Riemer D. D. , Hills A. , Baugh W. , Orlando J. , Faloona I. , Tan D. , Brune W. , Lamb B. , Westberg H. , Carroll M. A. , Thornberry T. , Geron C. D.
Published: October 2002

Mixing ratios of isoprene, methyl vinyl ketone (MVK), and methacrolein (MACR) were determined continuously during an 8‐day period in the summer of 1998 at a rural forested site located within the University of Michigan Biological Station (UMBS). The measurements were obtained as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) study. Fluxes of isoprene were concurrently measured at a nearby tower (AmeriFlux, located 132 m north‐northeast of the PROPHET tower). Following the study, 1‐km‐resolution emission estimates were derived for isoprene within a 60‐km radius of the tower using forest density estimates (Biogenic Emissions Inventory System (BEIS3) model). Measured isoprene fluxes at the site compared well with modeled isoprene fluxes when using BEIS3 and a detailed leaf litter‐fall data set by tree species from the UMBS site. Mean midday (1000–1400 LT) mixing ratios for isoprene, MACR, and MVK were 1.90 ± 0.43, 0.07 ± 0.01, and 0.14 ± 0.04 ppbv, respectively. Median midday mixing ratios of these compounds were 1.96 ± 0.26, 0.06 ± 0.02, and 0.10 ± 0.02 ppbv, respectively. Ratios of the isoprene oxidation products to isoprene are understood in the context of previous laboratory and field measurement studies of these compounds and a simple consecutive reaction scheme model. Results of the model indicate that the air masses studied represented relatively fresh emissions with a photochemical age of measured isoprene between 3.6 and 18 min, which is significantly less than the photochemical lifetime of isoprene (τ = 45 min at [OH] = 3.35 × 106 molecules cm−3). Thus a large portion of the isoprene that reaches the manifold has not had time to react completely with OH, yielding lower than expected ratios based on model calculations that do not explicitly take this into account. A rapid decrease in isoprene mixing ratios was observed soon after sunset, followed by a slower decay throughout the rest of the night. Emission maps were generated indicating that isoprene fluxes are highest in the immediate vicinity of the tower compared to the surrounding area of the site. Thus vertical diffusion and advection from the surrounding region are postulated to cause the observed initial rapid decrease in isoprene at the site. The second isoprene decay may be due to chemistry and/or dynamics, but the effects cannot be separated with the available data.
Temporal patterns, sources, and sinks of C8‐C16 hydrocarbons in the atmosphere of Mace Head, Ireland
Authors: Sartin, J. H. , Halsall, C. J. , Robertson, L. A. , Gonard, R. G. , MacKenzie, A. R. , Berresheim, H. , Hewitt C. N.
Published: October 2002

During the 1999 New Particle Formation and Fate in the Coastal Environment (PARFORCE) field campaign, 16 C8‐C16 volatile organic compounds (VOCs) were identified in the coastal atmosphere of Mace Head, Ireland. Sampling took place over 24 days, with 12 VOCs routinely quantified. Concentrations were observed in the low <10–150 parts per trillion by volume range, with levels typically in the order of aldehydes > ketones ≥ n‐alkanes. Concentrations of these compounds were also measured in shoreline surface seawater. No relationship was observed between atmospheric concentrations and high/low tide events. Many VOCs revealed a temporal pattern in the atmosphere, with highest concentrations measured during the early morning and lowest concentrations in the afternoon. The strongest pattern was observed for the n‐alkanes. However, this was dependent on the prevailing air mass direction and the local meteorology. A Lagrangian box model was applied to assess this diurnal cycle, using seawater emissions as a source (based on the seawater concentrations and observed wind speeds), and depletion via OH radicals and dilution by entrainment as sinks (using measured [OH] and boundary layer height data). The model gave good agreement to the observed concentrations for selected air mass types, predicting the daytime decrease in VOC concentrations due to OH radical chemistry and boundary layer growth, and the subsequent increase in VOC concentrations toward evening as both oxidation chemistry diminished and the mixing layer height fell.
Mineralogy of Saharan dust transported over northwestern tropical Atlantic Ocean in relation to source regions
Authors: Caquineau Sandrine , Gaudichet Annie , Gomes Laurent , Legrand Michel
Published: October 2002

On the basis of daily Saharan dust samples collected at Sal Island (Cape Verde Archipelagos) and Barbados (Caribbean Sea) over 3 years, this study focuses on the mineralogical signature of the African sources providing dust over the tropical North Atlantic Ocean. First, the sources of the collected dust were localized by using relative clays abundance (illite‐to‐kaolinite ratio) combined with Meteosat infrared imagery, horizontal visibility, and backward trajectories of dusty air masses. Then, each identified source was linked to a single value of the illite‐to‐kaolinite ratio. Those results highlight that the clay content of the emitted dust depends directly on both the latitude and the longitude of the source. Dust originating from northwestern sources exhibits illite‐to‐kaolinite ratios higher than those from Sahelian regions. Likewise, illite‐to‐kolinite ratio decreases from west to east.
Determination of subpixel fractions of nonforested area in the Amazon using multiresolution satellite sensor data
Authors: Hagen, S. C. , Braswell, B. H. , Frolking, S. , Salas, W. A. , Xiao X.
Published: October 2002

We present the results of an analysis that combines coarse and fine spatial resolution remote sensing data to reconstruct time series of nonforested area in a region of the Brazilian Amazon. We used a 10‐year sequence (1989–1998) of Landsat Thematic Mapper data for one scene (∼30,000 km2) in Rondônia, Brazil, to parameterize a regression model that uses subsets of AVHRR GAC reflectance data. This model detected information on interannual changes in nonforested area with a combination of detail and coverage greater than would normally be available using either data set alone. Within this domain we retrieved nonforest cover fraction in a cross‐validation test with coefficients of determination (R2) of 0.32 at 8–km resolution and 0.64 at 48‐km resolution. At the 48‐km resolution the model captures interannual variability and trends of fractional cover within individual pixels that changed considerably over the 10 years of this analysis. The results for this region suggest that retrospective analyses will benefit from further development of techniques combining AVHRR, TM, and other prior information.
Measurements of gas‐phase hydrogen peroxide and methyl hydroperoxide in the coastal environment during the PARFORCE project
Authors: Morgan R. B. , Jackson A. V.
Published: October 2002

Ambient gas‐phase hydrogen peroxide (H2O2) and organic hydroperoxides (ROOHs) were measured at Mace Head Atmospheric Monitoring Station during September 1998 and June 1999. These measurements formed part of the New Particle Formation and Fate in the Coastal Environment (PARFORCE) campaign with the intention of assisting with the elucidation of potential oxidation pathways that lead to the production of new particles. A reflux concentrator sampled ambient air prior to on‐site analysis by a peroxidase fluorescence technique with specific peroxide detection. Hydrogen peroxide, methyl hydroperoxide (CH3OOH), and several hydroperoxide species were detected. The highest H2O2 concentrations were observed during periods of clean maritime air, and a significant correlation exists between H2O2 concentration and tidal cycle during days when air masses had advected over inter tidal zones upwind of Mace Head. Peak H2O2 concentrations were observed around low tide on these days, coincident with particle events.
Impact of land use/land cover change on regional hydrometeorology in Amazonia
Authors: Baidya Roy Somnath , Avissar Roni
Published: October 2002

A high‐resolution mesoscale model was used to investigate the impact of deforestation in Amazonia. Coherent mesoscale circulations were triggered by the surface heterogeneity; synoptic flow did not eliminate the circulations but advected them away from the location where they were generated. This was substantiated by satellite‐derived cloud images. These circulations affected the transport of moisture and heat at the synoptic scale and can affect climate. Adequate parameterizations for these processes should be included in GCMs for more accurate climate simulations.
Particle size distribution of organic primary and secondary aerosol constituents in urban, background marine, and forest atmosphere
Authors: Kavouras Ilias G. , Stephanou Euripides G.
Published: October 2002

Polynuclear aromatic hydrocarbons (PAHs), n‐alkanes, n‐alkanals, n‐alkanols, saturated and unsaturated carboxylic acids, α, ω‐dicarboxylic acids, and carbonyl and carboxylic photooxidation products of monoteprenes were determined in particle‐sized aerosols of urban (Heraclion, Island of Crete, Greece), background marine (Island of Crete, Greece), and forest (Northern Greece and Portugal) atmospheres. The n‐alkanes were mostly associated with fine particles in the urban and forest aerosol, and their mass mean aerodynamic diameter (MMAD) calculated over the whole size range (total MMAD) was 0.45 μm and 0.63 μm, respectively. In the background marine aerosol, n‐alkanes were more evenly distributed, and their MMAD was 2.00 μm, because of physical changes occurring during their long‐range transport. Similar observations have been done for PAHs and n‐alkanals. Conversely, the most biogenic compound class, namely n‐alkanols, were evenly associated in the urban, background marine, and forest aerosol, between fine and coarse particles, and their corresponding total MMAD was 2.45, 2.69, and 1.67 μm, respectively. The total MMAD of n‐alkanoic acids was 0.71, 0.62, and 0.91 μm in the urban, background marine, and forest aerosol, respectively. Several compounds associated with photochemical reactions in the atmosphere were detected in urban marine and forests aerosol in the fine and ultrafine fraction, showing the low total MMAD (0.28–0.77 μm) in all aerosol types.
Modeling cloud effects on hydrogen peroxide and methylhydroperoxide in the marine atmosphere
Authors: Kim Cheol‐Hee , Kreidenweis Sonia M. , Feingold Graham , Frost Gregory J. , Trainer Michael K.
Published: October 2002

Hydrogen peroxide (H2O2) and methylhydroperoxide (CH3OOH) are studied with a coupled gas phase and aqueous phase chemical model representing a remote nonprecipitating cloudy boundary layer. Cloud interactions may deplete or enhance H2O2 but have a minor effect on CH3OOH. Therefore two primary questions are addressed: (1) do nonprecipitating clouds perturb the ratio of H2O2/CH3OOH, and if so, (2) what is the rate of reestablishment of this ratio to clear‐sky levels following cloud contact. The results show that the rate of recovery of the ratio of H2O2 to Ch3OOH after perturbation by cloud interactions depends on NOx (=NO + NO2) mixing ratios and on the time of day that cloud is encountered. When cloud contact is followed by a significant period of daylight, recovery to precloud values is rapid; however, when cloud contact occurs during the late afternoon or night, recovery can take up to 24 hours under high NOx conditions. Sensitivity tests show that in‐cloud heterogeneous conversion of HNO3 to aerosol has a small but detectable effect (∼10%) on the recovery of the ratio. Neglecting dry deposition of H2O2 and HNO3 increases the predicted ratio H2O2/CH3OOH in clear air prior to cloud contact, and has a small effect on the relative recovery rate of the ratio. In‐cloud consumption of H2O2 by SO2 suppresses the postcloud ratio by ∼40% relative to that in the base case for low levels of SO2 (∼200 ppt), with a more pronounced effect on the ratio and its rate of recovery for [SO2] ∼1 ppb. Because of the uncertainties associated with measurement of peroxides, and the dependence of the recovery of the ratio on the time of cloud contact, it is suggested that measurements of the ratio be considered judiciously and that they may not be of broad utility in predicting recent cloud contact.
Planetary cratering: A probabilistic approach
Authors: Ward Steven N.
Published: October 2002

I develop several statistical indices of cratering on planetary surfaces based on Poissonian probability. These cratering formulas, being both analytic and probabilistic, have advantages over numerical or nonprobabilistic approaches in ease of calculation, clarity of interpretation, and evident flexibility. Specific indices developed include the fraction of a planet's surface expected to be cratered N occasions over a given time interval, expected uncertainty in crater coverage, the depth and distribution of developed megaregolith, and the evolution in crater population. For instance, under current conditions, 15% of the Earth's surface should have been cratered one or more times in the past 3 billion years. In the median case, ejecta from these impacts would have blanketed the planet to a depth of 313 m. These indices, of course, depend upon asteroid flux and the minimum asteroid size imposed by atmospheric filtering. Analytical formulas make it simple to account for such variations by feeding in their history from current conditions to those present on the early Earth. If, as is thought, Earth's bolide flux rate has decreased by a factor of 10,000 from its formation until now, then 99.99% of the Earth's surface should have been cratered one or more times in the past 3 billion years. In the first 100 Ma of the early Earth, 90% of its the surface would have suffered more than 50 impacts, even considering the protection of the atmosphere. Ramifications of these bombardment statistics pertain to the survivability of crustal fragments and early life forms.
Emplacement of giant radial dikes in the northern Tharsis region of Mars
Authors: Scott Evelyn D. , Wilson Lionel , Head James W.
Published: October 2002

Three distinct sets of graben are associated with the volcano Alba Patera on Mars. One set, approximately circumferential to the edifice, has long been accepted to have formed as a tectonic response to an extensional stress regime associated with the evolution of the Alba Patera edifice. A second set includes mainly linear structures interpreted by many workers to have formed in response to very large‐scale regional stresses. We infer that the third set of graben, all of which are relatively linear, none of which are strictly parallel to members of the second set, and many of which contain numerous pit craters, formed above long (∼1000 km), laterally propagating regional dikes emanating from a volcanic center located to the south within the Tharsis region. The expected geometries of such dikes (several hundred meters depth to dike top, ∼20 km depth to dike base, mean dike width ∼30–90 m) are modeled on the assumption that they were fed from a shallow magma reservoir centered on a neutral buoyancy horizon, expected to be present at a depth of ∼10 km on Mars. The volumes of magma in the dikes are consistent with a reservoir similar in size to those inferred to be present under the Tharsis shield volcanoes provided that the dikes were emplaced during caldera collapse episodes. The sizes of the graben associated with these dikes are consistent with the relaxation, during or immediately after dike emplacement, of preexisting regional extensional stresses of a few tens of MPa.
A tenuous dust ring of Jupiter formed by escaping ejecta from the Galilean satellites
Authors: Krivov Alexander V. , Krüger Harald , Grün Eberhard , Thiessenhusen Kai‐Uwe , Hamilton Douglas P.
Published: October 2002

This paper focuses on the dust environment between the orbits of the Galilean moons of Jupiter. Recent discovery of dust clouds around the Galilean satellites formed by impact ejecta from hypervelocity impacts of interplanetary micrometeoroids [Krüger et al., 1999d] suggests that a fraction of the ejected particles may escape from the source satellites into circum‐Jovian orbits. We estimate production rates and study dynamical evolution of the escaping ejecta, controlled by gravitational, radiation pressure, and electromagnetic forces, to show that grains larger than several tenths of a micrometer in radius are likely to stay in bound orbits around Jupiter for tens or hundreds of years until they either are lost to collisions with the satellites or Jupiter or are ejected to interplanetary space. It is concluded that these small debris form a broad dust ring with number densities up to ∼103 km−3, extending at least from Europa's orbit outward beyond the orbit of Callisto. Our results are consistent with in situ measurements of the Galileo spacecraft. We analyze impact events recorded by the Galileo dust detector from 1996 through 2001 and find more than 200 events outside the orbit of Europa, compatible with impacts of particles orbiting Jupiter in prograde orbits. An empirical dust number density distribution derived from these data agrees quite well with the theoretical one.
Lunar Prospector neutron spectrometer constraints on TiO2
Authors: Elphic R. C. , Lawrence D. J. , Feldman W. C. , Barraclough B. L. , Gasnault O. M. , Maurice S. , Lucey P. G. , Blewett D. T. , Binder A. B.
Published: October 2002

Lunar Prospector neutron spectrometer measurements of the epithermal and thermal neutron leakage fluxes are used to provide constraints on TiO2 abundances in lunar surface materials. We use FeO abundance estimates based on both Clementine spectral reflectance techniques and preliminary Lunar Prospector gamma ray spectrometer determinations to first establish a model thermal neutron absorption due to all major elements except titanium. Then we remove the additional absorbing effects due to the rare earth elements gadolinium and samarium by using Lunar Prospector gamma ray spectrometer thorium abundances as a rare earth element proxy. The result can be compared to the ratio of epithermal to thermal neutron fluxes, which point to the presence of the additional thermal neutron absorber, titanium. We can derive abundance estimates of TiO2 and compare to other estimates derived spectroscopically. Our results show a significantly lower abundance of TiO2 than has been derived using Clementine data.
Strike‐slip motion and double ridge formation on Europa
Authors: Nimmo Francis , Gaidos Eric
Published: October 2002

There is abundant observational evidence for strike‐slip displacement on the surface of Europa. Strike‐slip motion between crustal blocks produces shear heating and an increase in temperature. We model the shear heating within the ice crust using a two‐dimensional, finite difference formulation, with a near‐surface brittle layer of constant specified thickness and a Newtonian ductile layer beneath. We obtain a maximum temperature anomaly of 66 K for a brittle layer thickness of 2 km and shear velocity of 6 × 10−7 m s−1. Such a velocity is appropriate for diurnal (85 hour) tidal motion. The local increase in temperature may cause ∼100 m uplift around the shear zone through thermal buoyancy. The stresses required to produce velocities of order 10−7 m s−1 are similar to estimates for present‐day tidal stresses on Europa (104–105 Pa). Brittle layer thicknesses >2 km are unlikely to persist at active shear zones because of the effect of shear heating. Shear velocities greater than or equal to ∼10−6 m s−1 will give rise to melting at shallow depths. The removal of material by downwards percolation of meltwater may cause surface collapse along the shear zone; inward motion, leading to compression, may also result. The combination of thermally or compression‐induced uplift and melt‐related collapse may be responsible for the pervasive double ridges seen on Europa's surface.
Ratio of sodium to potassium in the Mercury exosphere
Authors: Potter A. E. , Anderson C. M. , Killen R. M. , Morgan T. H.
Published: October 2002

The ratio of sodium to potassium in the exosphere of Mercury is highly variable and averages about 100, considerably larger than for the Moon. We considered the possibility that this high value results from potassium loss rates that are larger than sodium loss rates. The principal loss processes are the sweeping of hot atoms off the planet by solar radiation acceleration and the capture of metal photoions by the solar wind. We found that radiation acceleration losses are not sufficiently different for sodium and potassium to explain the high ratio. Capture of sodium and potassium photoions by the solar wind is thought to be the major loss process. However, there is a competing process for the ions, which is collision with the surface, followed by neutralization. These atoms can return to the exosphere. The net steady state density of sodium and potassium in the exosphere depends on the relative efficiencies of solar wind capture and surface recycling. We propose that the photoion loss and recycling rates are sufficiently different for sodium and potassium to account for the high ratio. The higher mass of the potassium atom relative to sodium results in a smaller scale height and a larger gyroradius for potassium relative to sodium, which may result in more rapid net loss of potassium.
Changes in planktonic assemblages during sapropel S5 deposition: Evidence from Urania Basin area, eastern Mediterranean
Authors: Corselli C. , Principato M. S. , Maffioli P. , Crudeli D.
Published: October 2002

A micropaleontological study of planktonic assemblages on the partially laminated sapropel S5 (late Pleistocene, marine isotope stage (MIS) 5e) was performed in two piston cores from Urania Basin area (eastern Mediterranean, west of Crete): UM94PC16 and UM94PC31 recovered during a PALEOFLUX Project Cruise. The abundance of Florisphaera profunda indicates the development of a deep chlorophyll maximum (DCM) before the anoxic condition at bottom were established, whereas patterns of upper photic zone coccoliths suggest extreme oligotrophy in surface water. The short appearance of Globorotalia scitula and the presence of Globigerinoides ruber in the lower part of sapropel testify to a thermal stratification, also recorded by changes in primary producers. During G. scitula occurrence, diatoms, mainly represented by Pseudosolenia calcar‐avis, appear and bloom because of their capability in using nutrients from DCM. Scanning electron microscope analyses performed on selected intervals from UM94PC16 show that the sapropel is organized in microlaminae mostly composed by siliceous microfossils. In particular, sapropel S5 could be related to an enhanced nutrient availability in the lower‐middle part of the photic zone, stratified conditions, and a higher continental input.