bluuebunny/crossref_metadata_2025 huggingface dataset:
Tides for a convective Earth
Authors: Dehant V. , Defraigne P. , Wahr J. M.
Published: September 2002

In this paper we give values of the tidal gravimetric factor as well as of the Love numbers for the tidal surface displacement and for the tidal mass redistribution potential that are consistent with the presently adopted definitions. We present analytical expressions for these quantities and compute numerical values for two rotating, nonspherical Earth models. In the first model the Earth is everywhere in hydrostatic equilibrium, and the inner core and mantle are both elastic. In the second model the Earth is ellipsoidal with an inelastic mantle and with a nonhydrostatic initial state for which the effects of mantle convection and its associated boundary deformations are considered. This latter model is constrained to reproduce the observed free core nutation period and global Earth dynamical flattening.
Investigations of Fennoscandian glacial isostatic adjustment using modern sea level records
Authors: Davis J. L. , Mitrovica J. X. , Scherneck H.‐G. , Fan H.
Published: September 2002

We present a new method for determining the rates from a network of regional tide gauge data. The method involves using a model for the sea level observations (annual averages of relative sea level) that explicitly includes a term for the interannual sealevel variations, which are assumed to be constant for all sites in the network. The resulting simultaneous analysis of all the sea level observations yields sea level rates which are biased by the unknown temporal slope of the interannual variations. By differencing sea level rate estimates with respect to one site, “site‐referenced” sea level rates are calculated which are free from this bias. The differencing procedure also removes any signal due to a common sea level rate, such as might be associated with eustatic sea level rise. The resulting site‐referenced sea level rates therefore in principle reflect only the relative vertical crustal motions of the tide gauge sites with respect to the geoid. These site‐referenced sea level rates represent a potentially more accurate data set than have previously been used for sea level studies. We have used these rates to investigate glacial isostatic adjustment (GIA) in Fennoscandia. Specifically, we have used the site‐referenced sea level rates within the Baltic Sea to estimate adjustments to the ice history and Earth model which are used to calculate vertical rates of adjustment due to GIA, a method never before used for Fennoscandia. The parameters selected were lithospheric thickness, the viscosities of (assumed isoviscous) upper and lower mantles, and a Fennoscandian ice thickness scaling parameter. We present the results of this estimation procedure for a specific test ice model. We also find evidence that the sea level record for the site nearest the center of uplift, Furuögrund, may be contaminated by systematic errors of unknown origin. Data from this site have previously been used to determine a rate for the maximum uplift that we find is perhaps 1 mm yr−1, or ∼10%, too large.
Atmospheric torque on the Earth and comparison with atmospheric angular momentum variations
Authors: de Viron O. , Bizouard C. , Salstein D. , Dehant V.
Published: September 2002

The purpose of this paper is to compute atmospheric torques on the Earth, including the oceans, with an emphasis on the equatorial components. This dynamic approach is an alternative method to the classical budget‐based angular momentum method for viewing atmospheric effects on Earth's orientation in space. The expression of the total torque interaction between the atmosphere and the Earth is derived from the angular momentum balance equation. Such a torque is composed of three parts due to pressure, gravitation, and friction. Each of these torque components is evaluated numerically by a semi‐analytical approach involving spherical harmonic approximations, and their orders of magnitude are intercompared. For the equatorial components the pressure and gravitational torques have far larger amplitudes than that of the friction torque; these two major torques have the same order of magnitude but opposite signs, and the value of the sum of the torques is shown to be close to the equatorial components of the atmospheric angular momentum time derivative s, as would be expected in a consistent model‐based analysis system. The correlation between the two time series is shown to be very good at low frequency and decrease slowly with increasing frequency. The correlation is still significant (≥ 0.7) up to 0.5 cycle per day, but the correlation coefficient reduces to 0.5 at the diurnal frequency band, indicating the difficulty of calculating rapidly changing model‐based torques within an atmospheric analysis system.
Stress perturbations registered by jointing near strike‐slip, normal, and reverse faults: Examples from the Ebro Basin, Spain
Authors: Simón J. L. , Arlegui L. E. , Liesa C. L. , Maestro A.
Published: September 2002

Stress perturbation patterns created by coeval and preexisting fault planes are characterized by means of detailed mapping and analysis of joint sets in Miocene carbonates of the Tudela area. Strike and spacing of the pervasive N‐S joint set have been specially surveyed for this purpose; their spatial variations have been analyzed and interpreted in the light of different theoretical models proposed by other authors. Perturbations inferred near strike‐slip faults correspond to the classical examples related to compressional and extensional fault tips. A normal fault striking oblique to the horizontal stress axes deflects the latter toward making σ2 and σ3 trajectories parallel and perpendicular, respectively, to the fault strike, while causing an abrupt stress release within the footwall. Near a reverse fault with a small dextral component the deflection of σ1 trajectories is gentle, but a significant increase of differential stress occurs in the hanging wall.
A three‐dimensional fluid‐controlled earthquake model: Behavior and implications
Authors: Miller Stephen A. , Ben‐Zion Yehuda , Burg Jean‐Pierre
Published: September 2002

We describe the behavior of a three‐dimensional, fluid‐controlled fault model that couples the dominant mechanical effects of fluid within a cellular fault zone with shear stress accumulation from constant plate motion applied at the downward continuation of the fault. Improvements from a previous model include long‐term plate motion loading and porosity creation through dilatant slip, which allow the model to evolve to its steady state dynamic equilibrium. The examined results include slip and slip‐deficit accumulation, pore pressure buildup and release, stress states, the emergence of seismic scaling relationships, and frequency‐size statistics of model earthquakes. We find that asperities develop naturally within the model, reflecting the disorganization of the evolving stress state in Mohr space. The dynamical interaction of shear stress and effective normal stress perturbs the initial uniform stress state to a complex state that produces transient asperity development along the fault. These “Mohr‐space” asperities spontaneously evolve, disintegrate, reemerge, and migrate along the fault plane. The general model behavior is independent of the state of the fluid pressure. In four examined cases, which span the range of possible fault zone overpressures, the equilibrium condition is that which occupies all of the available Mohr space. Maximum slip deficits along the fault depend on the degree of fault weakness, ranging from about 3 m for a weak fault to over 30 m for a strong fault after 4000 years of model evolution. For events that breach the surface the seismic moment scales with the cube of the source dimension Mo ∼L3, which reflects the slipped area times the depth extent of the rupture. This scaling crosses isolines of stress drop. For confined events, Mo ∼L2 along isolines of stress drop, but no general scaling emerges. Clusters emerge between stress drop versus seismic moment and stress drop versus source dimension, with large events converging to average stress drops of about 8 MPa for a weak fault and about 20 MPa for a strong fault.
Uppermost mantle velocities and anisotropy beneath Europe
Authors: Hearn Thomas M.
Published: September 2002

Pn data collected within southern Europe and the Mediterranean are used to tomographically image variations in both seismic velocity and seismic anisotropy. Seismic anisotropy is an essential part of the inversion, and without it, several low velocity features within the uppermost mantle could not be properly imaged. The technically active mantle of southern Europe has much lower seismic velocities (7.6–8.1 km/s) than the more stable mantle of the sub‐African plate of the Adriatic sea (8.3 km/s). However, the most dramatic features within Europe's uppermost mantle relate to the Apennine, Dinaride, and Hellenide arcs. These arcs all have extremely low (<7.8 km/s) mantle velocities beneath them and considerable (>5%) amounts of arc‐parallel anisotropy. The Tyrrhenian and Aegean back arc regions also show low velocities (7.7–7.9 km/s) but less anisotropy. The same may be true for the Pannonian Basin, but the tomography has poorer resolution there. A model explaining these observations focuses on subducted water metasomatizing the mantle wedge. The addition of water causes melting, creates arc volcanism, lowers the seismic velocity, and enhances the formation of anisotropy due to preferential olivine orientation. For collisional arcs of the northern Mediterranean, arc‐parallel anisotropy has formed in response to compression across them and extension along them. Within back arc regions, water is no longer a major factor. Instead, convection associated with subduction and back arc extension controls the anisotropy.
Quantifying tectonic strain and magmatic accretion at a slow spreading ridge segment, Mid‐Atlantic Ridge, 29°N
Authors: Escartín J. , Cowie P. A. , Searle R. C. , Allerton S. , Mitchell N. C. , MacLeod C. J. , Slootweg A. P.
Published: September 2002

High‐resolution, deep‐towed side‐scan sonar data are used to characterize faulting and variations in tectonic strain along a segment of the slow spreading Mid‐Atlantic Ridge near 29°N. Sonar data allow us to identify individual fault scarps, to measure fault widths and spacing, and to calculate horizontal fault displacements (heave) and tectonic strain. We find that over long periods of time (>1 Myr on average), tectonic strain is ∼10% on average and does not vary significantly along axis. There is a marked asymmetry in tectonic strain that appears to be linked to asymmetric accretion along the whole segment, indicated by ∼50% lower tectonic strain on the east flank than on the west flank. These variations in tectonic strain do not correlate directly with changes in fault spacing and heave. Fault spacing and heave increase from the center of the segment toward the end (inside corner) on the west flank and from the outside to the inside corner across the axis. These parameters remain relatively constant along the segment on the east flank and across the axis at the segment center. Tectonic strain appears to be decoupled from magmatic accretion at timescales >1 Myr, as the decrease in magma supply from the segment center toward the end (inferred from variations in crustal thickness along the axis) is not correlated with a complementary increase in tectonic strain. Instead, tectonic strain remains relatively constant along the axis at ∼7% on the east flank and at ∼15% on the west flank. These results indicate that variations in fault development and geometry may reflect spatial differences in the rheology of the lithosphere and not changes in tectonic strain or magma supply along axis.
Identification of sources of potential fields with the continuous wavelet transform: Basic theory
Authors: Moreau Frédérique , Gibert Dominique , Holschneider Matthias , Saracco Ginette
Published: September 2002

The continuous wavelet transform is used to analyze potential fields and to locate their causative sources. A particular class of wavelets is introduced which remains invariant under the action of the upward continuation operator in potential field theory. These wavelets make the corresponding wavelet transforms easy to analyze and the sources' parameters (horizontal location, depth, multipolar nature, and strength) simple to estimate. Practical issues (effects of noise, choice of the analyzing wavelet, etc.) are addressed. A field data example corresponding to a near‐surface magnetic survey is discussed. Applications to the high‐resolution aeromagnetic survey of French Guyana will be discussed in the next paper of the series.
Grain boundary diffusion creep of synthetic anorthite aggregates: The effect of water
Authors: Dimanov A. , Dresen G. , Xiao X. , Wirth R.
Published: September 2002

To investigate the effect of trace amounts of water on plastic deformation of feldspar, we fabricated synthetic polycrystalline aggregates of pure anorthite from a glass. The glass powder was either densified and crystallized at 1473 K and 0.1 MPa or hot isostatically pressed at 1443 K and 300 MPa confining pressure. Fourier transform infrared spectrometry indicates a water content of 0.002–0.0035 wt % (300–550 H per 106 Si) for specimens prepared at atmospheric pressure. Hot‐isostatically pressed samples contain 0.05 wt % to 0.1 wt % (8000–15000 H per 106 Si), depending on whether they were crystallized from glass powder predried at 1073 K for 2–3 days or from glass powder as received. In the wet samples, <1 vol % glass was found. Transmission electron microscopy showed that two‐grain boundaries are glass‐free to a resolution of 5 nm. We performed creep experiments at 0.1 MPa pressure with temperatures ranging from 1373 to 1573 K and stresses ranging from 1 to 100 MPa. The mechanical data indicate grain boundary diffusion controlled creep with a stress exponent n = 1.0±0.1 and a grain size exponent m = −2.9±0.2. The activation energy for creep is 585±45 kJ mol−1 or 377±38 kJ mol−1 for dry or wet specimens, respectively. If extrapolated to natural temperatures (≅973–923 K), our mechanical data indicate 3–4 orders of magnitude difference in strength between dry samples and specimens containing trace amounts of water.
Aeromagnetic study of the continental crust of northeast Greenland
Authors: Schlindwein Vera , Meyer Uwe
Published: September 2002

A reconnaissance‐type aeromagnetic survey of northeast Greenland was undertaken by the Alfred Wegener Institute for Polar and Marine Research between 1993 and 1996. These data were compiled in a regional magnetic anomaly map at 3700 m altitude, covering the Caledonian orogen of east Greenland and parts of the Precambrian shield of north Greenland. The survey area was divided into regions with distinct magnetic anomalies. The interpretation of the magnetic anomalies is based on comparison with exposed geological structures and is locally supported by in situ measurements of magnetic susceptibilities. Middle Proterozoic magmatic rocks determine the magnetic character of the Precambrian shield of north Greenland. Rifting and downfaulting of the Precambrian basement at its northern and eastern margin are reflected by gradients in the magnetic field. The Caledonian fold belt of east Greenland could be subdivided into a northern only weakly magnetic part, where pervasive Caledonian reworking probably demagnetized the crust, and a southern part, where isolated positive anomalies occur. Magnetite survived Caledonian metamorphism in these areas. The sensitivity of magnetite to Caledonian metamorphism was recognized in several places in the Caledonides of east Greenland. This observation might be useful in determining the much debated degree of Caledonian reworking of the crystalline complexes of east Greenland.
Theoretical and experimental investigation of compaction bands in porous rock
Authors: Olsson William A.
Published: September 2002

Field investigators have recently discovered thin, tabular zones of pure compressional deformation that they called compaction bands. These bands were found in association with shear bands and were postulated to be genetically related to them. At the laboratory scale, compaction bands have been noticed in association with boreholes and preexisting, artificial shear cracks subjected to compressive stress fields. Natural compaction bands are noticeable in outcrops because of their resistance to weathering; however, they may be more difficult to discern on freshly cut rock surfaces such as drill core or borehole walls. Because of the much reduced porosity in the compaction bands, these structures are potentially important as permeability barriers in reservoirs and aquifers in porous rocks. For bands associated with boreholes, the crushed material can be washed into the borehole contributing to sand production and possibly altering the stability of the borehole. This paper examines a theoretical framework that explains these features as a constitutive instability leading to localized compaction in a way completely analogous to shear strain localization. Conventional triaxial experiments on Castlegate sandstone resulted in compaction bands. In addition, thick deformation bands having normals at low angles to the maximum compression are present in some specimens.
Crustal magnetization and long‐wavelength aeromagnetic anomalies of the Minto block, Quebec
Authors: Pilkington Mark , Percival John A.
Published: September 2002

Measured susceptibilities of >3500 samples from a 500×500 km area of the Minto block, Superior province, Quebec, have an average value of ∼0.03 SI (equivalent to induced magnetizations of ∼1.5 A/m). Charnockitic lithologies (igneous orthopyroxene‐bearing granodiorite, granite, diorite), that constitute large parts of the Minto region, yield the highest susceptibilities. The variations in susceptibility show a good correlation with the variations in the observed magnetic field, including those at long wavelengths (>100 km), suggesting that charnockitic and related granitoid rocks could be responsible for the long‐wavelength anomalies. Modeling of a magnetic profile perpendicular to the strike of mapped lithotectonic subprovinces in the Minto block supports this contention. Charnockitic lithologies are not usually considered a possible sources of long‐wavelength anomalies: mafic rock types are generally favored. The dominant Minto lithologies are more felsic but are more efficient in the production of magnetite from the available iron.
The connectivity of eddy variability in the Caribbean Sea, the Gulf of Mexico, and the Atlantic Ocean
Authors: Murphy Sylvia J. , Hurlburt Harley E. , O'Brien James J.
Published: September 2002

A set of numerical simulations is used to investigate the connectivity of mesoscale variability in the Atlantic Ocean, the Caribbean, and the Gulf of Mexico. The primitive equation models used for these simulations have a free surface and realistic coastline geometry including a detailed representation of the Lesser Antilles island arc. Two simulations have 1/4° resolution and include a 5.5‐layer reduced gravity and a 6‐layer model with realistic bottom topography. Both are wind forced and include the global thermohaline circulation. The third simulation is from a 1/2° linear wind‐driven model. In the two nonlinear numerical simulations, potential vorticity from decaying rings shed by the North Brazil Current retroflection can be advected through the Lesser Antilles. This potential vorticity acts as a finite amplitude perturbation for mixed barotropic and internal mode baroclinic instabilities, which amplify mesoscale features in the Caribbean. The eddies associated with the Caribbean Current are primarily anticyclonic and transit a narrow corridor across the Caribbean basin along an axis at 14° to 15°N with an average speed of 0.15 m/s. It takes them an average of 10 months to transit from the Lesser Antilles to the Yucatan Channel. Along the way, many of the eddies intensify greatly. The amount of intensification depends substantially on the strength of the Caribbean Current and is greatest during a multiyear period when the current is anomalously strong owing to interannual variation in the wind forcing. Some Caribbean eddies squeeze through the Yucatan Channel into the Gulf of Mexico, where they can influence the timing of Loop Current eddy‐shedding events. There is a significant correlation of 0.45 between the Loop Current eddy shedding and the eddies near the Lesser Antilles with a time lag of 11 months. However, Caribbean eddies show no statistically significant net influence on the mean eddy‐shedding period nor on the size and strength of shed eddies in the Gulf of Mexico. Additionally, no significant correlation is found between eddy shedding in the Gulf of Mexico and transport variations in the Florida Straits, although transport fluctuations in the Florida Straits at 27°N and the Yucatan Channel and showed a correlation of about 0.7 with a lag of 15 days. The linear solution exhibited a multiyear anomaly in the strength of the Caribbean circulation that was concentrated in the central and eastern Caribbean due to a multiyear anomaly in the wind field over the basin. In the nonlinear simulation this anomaly extended into the western Caribbean and across the entire Gulf of Mexico. This westward extension resulted from the nonlinearity and instability of the Caribbean Current, the westward propagation of the eddies, and the passage of Caribbean eddies through the Yucatan Channel into the Gulf of Mexico.
Characterization of aerosols at a coastal site near Vindeby (Denmark)
Authors: Vignati Elisabetta , de Leeuw Gerrit , Schulz Michael , Plate Elke
Published: September 2002

Aerosol particle size distributions and aerosol chemical composition were measured with optical particle counters and filter packs, respectively, on an offshore mast in the Danish coastal waters near Vindeby on the island of Lolland. The filter pack samples were analyzed for ions representative of marine and continental aerosol: Na+, Cl−, SO4−−, NO3−, and NH4+. The physical and chemical measurements are complementary, and the information on chemical composition, together with the local meteorological parameters, the synoptic situation, and air mass trajectories, is crucial for interpreting the observed variations in the particle concentrations. Considering these sets of data, four periods can be discerned during the experiment. Further, the aerosol concentrations vary with wind speed, as is expected for marine aerosols, depending strongly on the fetch. In the case of short fetch, the coarse aerosol fraction (diameter >1 μm) and the fine aerosol fraction (0.2–1 μm) decrease with wind speed. Processes like advection, diffusion, and dry deposition cannot explain the strong dependence of the concentrations on wind speed. Apparently, uncertainties in the source strength and origin of aerosol advected across the shore line, as well as processes affecting the aerosol concentrations over land, such as variability in the sources, washout by rain, and other removal and transport mechanisms, have much more influence than the transport and removal over water.
Tidal motions and tidally induced fluxes through La Línea submarine canyon, western Alboran Sea
Authors: Lafuente Jesús García , Sarhan Tarek , Vargas Manuel , Vargas Juan M. , Plaza Francisco
Published: September 2002

Detailed observations from two mooring lines deployed in La Línea submarine canyon, western Alboran Sea, are presented. This is a narrow canyon in the sense that its width is always less than the internal radius of deformation. Tidal currents within the canyon are polarized in the along‐canyon direction according to its narrow nature. They have considerable amplitude (values of around 0.5 m/s are often observed) and are forced by the internal pressure gradients associated with the baroclinic tide that is generated in the surroundings. Subsequent amplification of onshore baroclinic currents within the canyon accounts for the large amplitude observed. Cross‐shelf exchange through the canyon due to tidal motions is different from zero despite the close to zero mean of tidal currents. The explanation is based on the asymmetry of water properties flowing up‐canyon and down‐canyon (some sort of tidal rectification). Regarding the energy flux, the canyon seems to be an adequate conduit to carry energy to the shore. Estimations made from our observations indicate that energy input onto the shelf per unit length parallel to the shore at the canyon head is enough to maintain mixing on the shelf at intermediate depths.
Physical processes controlling the formation, evolution, and perturbation of the low‐salinity front in the inner shelf off the southeastern United States: A modeling study
Authors: Chen Changsheng , Zheng Lianyuan , Blanton Jackson O.
Published: September 2002

Physical processes that control the formation, evolution, and perturbation of the low‐salinity front over the inner shelf of the South Atlantic Bight have been examined using a fully three‐dimensional primitive equation and turbulent closure model. The model was forced by semidiurnal tides (M2, S2, and N2), climatological means of multiple river discharges, and upwelling‐favorable wind. This model has provided a reasonable simulation of the fortnightly and monthly variations of semidiurnal tides. Computed amplitudes and phases of tides show good agreement with observational data available at tidal gauge stations along the coast. Spatial structures of buoyancy currents are significantly modified by stratified tidal rectification. As a result, the southward residual current intensifies significantly at the front and reduces or reverses close to the coast. A weak velocity area forms in the downstream region of each river, which blocks the low‐salinity water to form multiple “tongue‐like” domes in the inner shelf. For a given springtime climatological upwelling‐favorable wind, isolated low‐salinity lenses can form episodically in two steps. At first, a geometrically controlling wave‐like frontal shape develops at the outer edge of the frontal zone as a result of the interaction between tides, multiple river discharges, and upwelling‐favorable wind. Then, isolated low‐salinity lenses form at the crest when water on the shoreward side of the crest is displaced by relatively high salinity water advected from the upstream trough south of the crest and diffused upward from the deeper region. Wind‐induced upwelling is noticeable to compensate for the water loss due to the near‐surface offshore Ekman transport, but it does not play a critical role in the formation of isolated lenses under the climatological conditions of river discharges and upwelling‐favorable winds over the inner shelf of the South Atlantic Bight.
Isopycnic modeling of the North Atlantic heat budget
Authors: Jones Ian , Leach Harry
Published: September 2002

A 2° resolution version of the Miami Isopycnic Coordinate Ocean Model is used within an idealized basin to examine the ocean heat budget. Simplified seasonal wind stress and thermohaline forcing, based on observational data of the North Atlantic, are used to drive the model. Following a 30‐year spin‐up, the model reaches a state of little annual heat content change, at which point a detailed study of the heat budget is performed. The heat budget is split into the components of surface forcing, diffusion, Ekman pumping, Ekman transport, and non‐Ekman advection. Both annual and seasonal results are obtained. Analysis of the heat budget reveals different annual balances to exist in different ocean regions. In the subpolar gyre the principal balance is between cooling caused by surface fluxes and warming due to geostrophic advection. However, in the subtropical gyre, where net surface fluxes are small, Ekman pumping balances geostrophic advection. As such, the Ekman pumping is seen to be important for supplying the necessary heat for subduction to take place. An investigation of the Ekman compensatory flow is undertaken. It is shown that the exact temperature and, consequently, the depth of this flow are important for determining the Ekman heat content change. The results here tentatively suggest this depth to be in the upper thermocline. Subduction rates are calculated and shown to be reasonable. The seasonal heat budget is dominated everywhere by surface fluxes. All other terms have negligible seasonal cycles, except for the Ekman terms, which exhibit a limited seasonal variation. In doing so the Ekman terms are seen to control the seasonal cycle of the transport of heat. This is due to their dependency on not only the time variation in wind stress but also the degree of stratification of the upper ocean.
Open‐ocean deep convection due to thermobaricity: 2. Numerical experiments
Authors: Akitomo Kazunori
Published: September 2002

Properties of open‐ocean deep convection due to thermobaricity at high latitudes have been investigated by executing three‐dimensional, nonhydrostatic numerical experiments in a rotating frame. As obtained with the scaling argument by Akitomo [this issue], the first type of deep convection, which is not so affected by thermobaricity, can occur in a homogeneous ocean with an intermediate water depth (∼2000 m) while the second type in a two‐layer ocean can occur through pure thermobaric instability. It is also confirmed that the former could occur in the Greenland Basin and the latter in the Weddell Sea, which suggests the different processes of deep and bottom water mass formation between the two regions. Dependence of convective properties on external parameters is basically the same with the scaling argument. However, some important differences from the theory are found in a homogeneous ocean. The convective size is determined by the product of characteristic velocity and timescale, whether the ocean is shallow or deep, rotating or nonrotating. This is possibly a feature common to nonlinear convective phenomena. The characteristic timescale is given by the shortest one of the diffusion time, the inertial period, and the period of any oscillatory feature accompanying convective motion. Reduced gravity and velocity scales depend on the ocean depth in a similar manner to the nonrotating scalings, even when the rotation is effective. As a result, the same relation is realized in the three‐dimensional regime as in the two‐dimensional regime, namely, that the convective size is proportional to the “Rossby radius of deformation.”
Settling‐driven convection: A mechanism of sedimentation from stratified fluids
Authors: Hoyal David C. J. D. , Bursik Marcus I. , Atkinson Joseph F.
Published: September 2002

Convection driven by sediment particles may play an important role in sedimentation from the base of buoyant (hypopycnal) plumes, for example, fluvial plumes in stratified estuaries and lakes, black smokers on the ocean floor, volcanic clouds, and coastal currents. In addition to the well‐known double‐diffusive convection mechanism, another mode of convective instability development is by settling across the density interface. We performed laboratory experiments to investigate this fingering/convective instability mechanism and its effect on particle distribution in the water column and deposition at the bed. A simple theoretical model of finger formation at a fluid density interface is developed based on an analogy with thermal/plume formation at a flat heated plate. This model, which involves a thickening interface layer that becomes gravitationally unstable relative to the ambient fluid, is in good agreement with measurements of finger size and instability wavelength from visualization experiments. Since fingering at the density interface drives larger‐scale convection in the fluid below, a mass balance model of the lower layer, assuming strong mixing (i.e., uniform sediment concentration) is successfully applied to predict sediment concentration in the water column and deposition at the bed. Strong mixing can be assumed since convective velocities are usually much greater than the particle fall velocities. As convection proceeds, the sediment concentrations in the two layers approach each other and convection will die out. Using the model equations, we develop analytical expressions for the time when convection ceases and the portion of sediment remaining in the water column.
Non‐Redfield carbon and nitrogen cycling in the Arctic: Effects of ecosystem structure and dynamics
Authors: Daly Kendra L. , Wallace Douglas W. R. , Smith Walker O. , Skoog Annelie , Lara Rubén , Gosselin Michel , Falck Eva , Yager Patricia L.
Published: September 2002

The C:N ratio is a critical parameter used in both global ocean carbon models and field studies to understand carbon and nutrient cycling as well as to estimate exported carbon from the euphotic zone. The so‐called Redfield ratio (C:N = 6.6 by atoms) [Redfield et al., 1963] is widely used for such calculations. Here we present data from the NE Greenland continental shelf that show that most of the C:N ratios for particulate (autotrophic and heterotrophic) and dissolved pools and rates of transformation among them exceed Redfield proportions from June to August, owing to species composition, size, and biological interactions. The ecosystem components that likely comprised sinking particles and had relatively high C:N ratios (geometric means) included (1) the particulate organic matter (C:N = 8.9) dominated by nutrient‐deficient diatoms, resulting from low initial nitrate concentrations (approximately 4 μM) in Arctic surface waters; (2) the dominant zooplankton, herbivorous copepods (C:N = 9.6), having lipid storage typical of Arctic copepods; and (3) copepod fecal pellets (C:N = 33.2). Relatively high dissolved organic carbon concentrations (median 105 μM) were approximately 25 to 45 μM higher than reported for other systems and may be broadly characteristic of Arctic waters. A carbon‐rich dissolved organic carbon pool also was generated during summer. Since the magnitude of carbon and nitrogen uncoupling in the surface mixed layer appeared to be greater than in other regions and occurred throughout the productive season, the C:N ratio of particulate organic matter may be a better conversion factor than the Redfield ratio to estimate carbon export for broad application in northern high‐latitude systems.
Caribbean Sea eddies inferred from TOPEX/POSEIDON altimetry and a 1/6° Atlantic Ocean model simulation
Authors: Carton James A. , Chao Yi
Published: September 2002

Large cyclonic and anticyclonic eddies are found in the Caribbean Sea. Analysis of sea level data from the TOPEX/POSEIDON altimeter shows that the eddies are quite regular, appearing at near 3‐month intervals west of the southern Lesser Antilles. These eddies progress westward at average speeds of 12 cm s−1, growing in amplitude up to 20 cm. Many eddies dissipate in the coastal waters of Nicaragua a half year after they appear. A 1/6° × 1/6° general circulation model of the Atlantic is shown to reproduce major features of the eddy life cycle, including their amplitudes, temporal scales, and propagation speed. Analysis of the model output further suggests that the eddies are mainly limited to the thermocline and above, with little phase lag in the vertical. The simulated eddies have sufficiently strong currents that the horizontal gradient of total vorticity changes sign, suggesting that conversions from mean to eddy kinetic energy may be contributing to their growth. Analysis of the simulation links eddies in the Caribbean with eddies formed outside the Caribbean at the confluence of the North Brazil Current and North Equatorial Countercurrent systems.
Simulation of the semidiurnal tides in the Strait of Gibraltar
Authors: Tejedor Luis , Izquierdo Alfredo , Kagan Boris A. , Sein Dmitry V.
Published: September 2002

The M2 and S2 surface tides in the Strait of Gibraltar are simulated using a two‐dimensional, nonlinear, boundary‐fitted coordinate model with a nominal resolution of ∼0.5 km. Good agreement is achieved with tide gauge and bottom pressure observations, as well as with current measurements made during the Gibraltar Experiment. The cotidal charts and the maps of tidal current ellipse parameters, which have been constructed on the basis of the model results, reproduce all of the known features of the spatial structure of the M2 and S2 tidal waves. These results also show that a ∼90° phase difference between tidal velocity and elevation is detected in much of the Strait of Gibraltar, thus suggesting a small mean tidal energy flux through the strait. The model results give evidence of the general direction for the M2 and S2 net tidal energy fluxes to the west. This finding is consistent with an observed southwestern tidal phase propagation and remains qualitatively unchanged when varying the strait's geometry as well as boundary and astronomical forcings.
Symmetric baroclinic instability during frontogenesis with horizontal density gradients and ageostrophic vertical shears
Authors: Mied Richard P. , Shen Colin Y. , Lindemann Gloria J. , Evans Thomas E.
Published: September 2002

This work examines the stability of surface frontogenesis in the presence of a horizontal density gradient and an ageostrophic current. We pose an initial value problem, in which two homogeneous bodies of water with different densities are separated by a horizontal transition region for the density. A surface current jet flows along the density front, and the geostrophic adjustment process is simulated using a fully nonlinear pseudo spectral numerical calculation in a 10 km × 30 m range and depth domain. We allow the evolution of the surface current jet but do not permit its variation in the y direction (perpendicular to the computational domain). A number of simulations are performed for a wide range of density differences and jet strengths. Surface frontogenesis and a tendency toward geostrophic adjustment of the initially ageostrophic fields do not always exhibit a smooth subsurface circulation accompanying the bunching of the surface isopycnals. Instead, a vortex is sometimes shed from the vicinity of the evolving front, and the isopycnals are distorted by this smaller‐scale vortical flow. To determine the source of the secondary symmetric baroclinic instability, the acceleration potential of the individual terms in the vorticity equation is calculated. The instability is caused by the vertical shear in the along‐front jet, which is intensified by the advection and vortex‐tilting processes during the frontogenesis. Although this vortex is left behind by the propagating hydraulic jump, it subsequently matures into a secondary hydraulic jump on its own. We found that in marginally unstable cases an increase in the kinematic viscosity can suppress its occurrence. Finally, we show that the unstable vortex is separate and distinct from the captured turbulent rotor which is thought to be locally trapped at a location just behind the propagating front.
Balloon‐borne in situ measurements of stratospheric H2O, CH4 and H2 at midlatitudes
Authors: Zöger M. , Engel A. , McKenna D. S. , Schiller C. , Schmidt U. , Woyke T.
Published: September 2002

Using a new Lyman α hygrometer, balloon‐borne measurements of H2O were performed on September 20, 1993 at midlatitudes (43° N). A cryogenic whole air sampler to measure CH4 and H2 amongst other long‐lived trace gases was operated on the same payload. The profile of the water vapor mixing ratio showed a well‐pronounced hygropause of 4 ppmv at θ = 420 K increasing to 6.0 ppmv at θ = 920 K. For θ > 400 K, total hydrogen ΣH2 = H2O + 2·CH4 + H2 appeared to be constant at a mean value of 7.72 ± 0.13 ppmv; the overall accuracy of this value is 0.65 ppmv. From the correlation functions of CH4 with H2 and H2O the water vapor yield from methane oxidation is determined with high precision to be 1.975 ± 0.030. The globally averaged water vapor mixing ratio entering the stratosphere from the troposphere was found to be 3.9 ± 1.0 ppmv. Intercomparison with H2O and CH4 profiles obtained nearby from the Halogen Occultation Experiment (HALOE) on board the UARS satellite showed good agreement within the experimental errors.
Multiplicity and stability of solutions for a simple Chapman and a more complex mesospheric photochemical model
Authors: Montecinos S. , Felmer P.
Published: September 2002

The goal of this article is to examine the validity of some properties of photochemical models, which were found numerically in recent studies: (1) the existence of periodic solutions and (2) the stability of these solutions. Even though we are interested in a more complete mesospheric model, we examine first the simple Chapman model and then we generalize the results to the complete one. We show that the 48‐hours periodic solution found in earlier work is orbitally stable, thus enhancing its physical significance. On the other hand, we find two new 24‐hours periodic solutions, which present ozone concentrations at much lower levels relative to the experimental data.
Processes determining the relationship between aerosol number and non‐sea‐salt sulfate mass concentrations in the clean and perturbed marine boundary layer
Authors: Van Dingenen Rita , Raes Frank , Putaud Jean‐Philippe , Virkkula Aki , Mangoni Monica
Published: September 2002

An evaluation of the indirect radiative forcing by aerosols requires knowledge about aerosol number densities, and more particularly the number of particles that can be activated in clouds. In this study we present a data set relating the total number (NTOT) and the number of particles with dry diameter > 80 nm (N>80) to the aerosol volume and non‐sea‐salt (nss) SO4= mass (MSO4). The data refer to submicron aerosol and have been obtained in both clean and polluted conditions in the North Atlantic marine boundary layer (MBL). Over this whole range, the relationships of both NTOT and N>80 versus MSO4 are close to linear. Detailed aerosol dynamics modeling shows that dilution of the initial pollution aerosol by entrainment of free tropospheric (FT) aerosol is the major process determining these relationships. Entrainment further explains our observation that the contribution of nss‐sulfate (i.e., (NH4)x(SO4)y) to the dry MBL aerosol mass decreases from over 85% near the continent to 45–70% in more remote and clean conditions, as smaller contributions of sulfate to the FT aerosol mass have been observed. Finally, the linear relationships between aerosol number and MSO4 suggest that the observed nonlinear relation between the number of cloud droplets and MSO4 must be mainly ascribed to nonlinearities in the cloud activation process.
Hydrocarbon measurements during tropospheric ozone depletion events: Evidence for halogen atom chemistry
Authors: Ramacher B. , Rudolph J. , Koppmann R.
Published: September 2002

During the Arctic Tropospheric Ozone Chemistry 1996 (ARCTOC 96) field campaign (March 29 to May 15, 1996), in situ measurements of C2‐C8 hydrocarbons, selected C1‐C2 halocarbons, and carbon monoxide were carried out at Ny Ålesund, Svalbard (78°55′N, 11°56′E). Two major tropospheric ozone depletions were observed during this period. In each case, concurrent depletion of alkanes and ethyne but no significant changes in benzene, chloromethane, or CO mixing ratios were detected. The change in the propane/benzene ratio can be used as evidence for the presence of chlorine radicals. Time integrated chlorine and bromine atom concentrations were calculated from the concentration changes of light alkanes and ethyne, respectively. At background ozone mixing ratios (O3 > 30 ppbv) our calculations yielded no significant integrated halogen atom concentrations (Cl: 5 ± 14 × 108 s cm−3, Br: 9 ± 42 × 1010 s cm−3). During major ozone depletion events, these values increase by more than a factor of 10 to values of about 1010 s cm−3 (Cl) and 5 × 1012 s cm−3 (Br). For such events the observed ozone losses can be explained quantitatively with these data. Our results show that free bromine atoms appear to be the major cause for ozone depletion (more than 92%). The contribution of chlorine atoms to the ozone loss is of the order of 1% or less. Highest integrated chlorine and bromine atom concentrations were found at lowest ozone mixing ratios and reached up to 1.4 × 1010 and 1.4 × 1013 s cm−3, respectively. A closer analysis reveals that during each ozone depletion event the integrated chlorine atom concentration increases earlier than the integrated bromine atom concentration and remains at high levels for a longer period of time. The bromine atom concentration starts to increase when ozone mixing ratios are below 15–20 ppbv and reaches very high levels for ozone <5 ppbv. The integrated chlorine concentration appears to be anticorrelated to the ozone mixing ratio (r2 = 0.811), whereas the integrated bromine concentration was found to be anticorrelated to the logarithm of the ozone mixing ratio (r2 = 0.895).
Chemical processing of biogenic hydrocarbons within and above a temperate deciduous forest
Authors: Makar P. A. , Fuentes J. D. , Wang D. , Staebler R. M. , Wiebe H. A.
Published: September 2002

A one‐dimensional canopy model was developed to study photochemical processes inside and above a mixed deciduous forest in southern Ontario. The Eulerian model made use of Lagrangian dispersion principles with a correction factor to incorporate the average ensemble time since emission to calculate atmospheric mixing; traditional diffusion methods were found to provide insufficient mixing to match the measurements. Neglecting chemical losses while making isoprene emission estimates was found to underestimate emission rates up to 40%. The ozone oxidation of biogenic and anthropogenic alkenes was found to be a potential source of hydroxyl (OH) and hydroperoxy (HO2) radicals during the morning and night. Reactions of HO2 with organic peroxy radicals formed by the OH oxidation of isoprene during the day were shown to be a significant source of organic peroxide formation above the canopy. The primary pathways of methacrolein and methylvinylketone formation were shown to be ozone oxidation of isoprene and hydroxyl radical oxidation of isoprene, respectively. Local ozone formation was shown to be limited by low mixing ratios of nitrogen oxides, despite high levels of isoprene present at the site.