bluuebunny/crossref_metadata_2025 huggingface dataset:
Pulse‐labeling studies of carbon cycling in arctic tundra ecosystems: Contribution of photosynthates to soil organic matter
Authors: Loya Wendy M. , Johnson Loretta C. , Kling George W. , King Jennifer Y. , Reeburgh William S. , Nadelhoffer Knute J.
Published: December 2002

To increase our understanding of carbon (C) cycling and storage in soils, we used 14C to trace C from roots into four soil organic matter (SOM) fractions and the movement of soil microbes in arctic wet sedge and tussock tundra. For both tundra types, the proportion of 14C activity in the soil was 6% of the total 14C‐CO2 taken up by plants at each of the four harvests conducted 1, 7, 21, and 68 days after labeling. In tussock tundra, we observed rapid microbial transformation of labile C from root exudates into more stable SOM. In wet sedge tundra, there appears to be delayed or indirect microbial use of root exudates. The net amount of 14C label transferred to SOM by the end of the season in both tundra types was approximately equal to the amount transferred to soils 1 day after labeling, suggesting that transfer of 14C tracer from roots to soils continued through the growing season. Overall, C inputs from living roots contributes 24 g C m−2 yr−1 in tussock tundra and 8.8 g C m−2 yr−1 in wet sedge tundra. These results suggest rapid belowground allocation of C by plants and subsequent incorporation of much of this C into storage in the SOM.
Association of sinking organic matter with various types of mineral ballast in the deep sea: Implications for the rain ratio
Authors: Klaas Christine , Archer David E.
Published: December 2002

We compiled and standardized sediment trap data below 1000 m depth from 52 locations around the globe to infer the implications of the Armstrong et al. [2002] “ballast” model to the ratio of organic carbon to calcium carbonate in the deep sea (the rain ratio). We distinguished three forms of mineral ballast: calcium carbonate, opal, and lithogenic material. We concur with Armstrong et al. [2002] that organic carbon sinking fluxes correlate tightly with mineral fluxes. Based on the correlations seen in the trap data, we conclude that most of the organic carbon rain in the deep sea is carried by calcium carbonate, because it is denser than opal and more abundant than terrigenous material. This analysis explains the constancy of the organic carbon to calcium carbonate rain ratio in the deep sea today, and argues against large changes in the mean value of this ratio in the past. However, sediment trap data show variability in the ratio in areas of high relative calcium carbonate export (mass CaCO3/mass ratio > 0.4), unexplainable by the model, leaving open the possibility of regional variations in the rain ratio in the past.
The wind‐driven, subtropical gyres and the solubility pump of CO2
Authors: Follows Michael J. , Ito Takamitsu , Marotzke Jochem
Published: December 2002

Using a suite of abiotic, ocean‐atmosphere, carbon cycle models we demonstrate that the representation of the ventilated thermocline leads to a significant enhancement of the sensitivity of atmospheric pCO2 to subtropical surface ocean properties. In particular, we study an idealized sector, ocean circulation and abiotic carbon cycle model with a coupled atmospheric reservoir and examine the solubility pump of CO2 in the subtropical oceans. We compare solutions for atmospheric pCO2 when driven only by buoyancy forces to those with both buoyancy and wind stress forcing. Introducing the wind stress leads to the formation of a subtropical gyre and the warm lens of the ventilated thermocline. This lens is depleted in carbon relative to the surrounding waters since its properties are inherited from the warm, subtropical surface ocean. It is undersaturated in carbon since subduction quickly follows the strong cooling in the western boundary current before equilibration with the overlying atmosphere can occur. Plausible wind stress patterns increase atmospheric pCO2, relative to the case without wind forcing, and double the sensitivity of atmospheric pCO2 to perturbations of the low latitude surface carbon system properties. We suggest that it is the resolution of the ventilated thermocline in global, three‐dimensional, ocean circulation models that enhances their sensitivity of atmospheric pCO2 to warm surface water properties relative to highly idealized box models.
Can eddies make ocean deserts bloom?
Authors: Oschlies Andreas
Published: December 2002

An eddy‐resolving coupled ecosystem‐circulation model of the North Atlantic is used to investigate the impact of mesoscale variability on the nitrate supply to the light‐lit euphotic zone. The focus is on the oligotrophic subtropical gyre where eddies have been suggested to reconcile apparently contradictory observational estimates of nutrient supply and export production. Comparison with observations indicates that the numerical model provides a realistic description of the subtropical eddy field and its interaction with biogeochemical tracers. The model results illustrate that the eddy‐induced nitrate flux into the euphotic zone is largest near the margins of the oligotrophic gyre where both vertical and lateral nutrient supply by eddies are effective. Typical values of simulated eddy‐induced nitrate supply are 0.05 mol m−2 yr−1, which is much lower than has been suggested previously. This new estimate of eddy‐induced nitrate supply is not sufficient to reconcile seemingly contradictory observational estimates of biological production in the subtropics. Alternative sources of fixed nitrogen, deviations from standard elemental stoichiometry, and possible effects of interannual variability will have to be considered in order to resolve apparent observational discrepancies in the oligotrophic subtropical gyres.
Modern and historic atmospheric mercury fluxes in both hemispheres: Global and regional mercury cycling implications
Authors: Lamborg C. H. , Fitzgerald W. F. , Damman A. W. H. , Benoit J. M. , Balcom P. H. , Engstrom D. R.
Published: December 2002

Using two different natural archiving media from remote locations, we have reconstructed the atmospheric deposition of mercury (Hg) over the last 800–1000 years in both hemispheres. This effort was designed (1) to quantify the historical variation and distributional patterns of atmospheric Hg fluxes in the midlatitudes of North America at Nova Scotia (N.S.) and at a comparable midlatitude region in the Southern Hemisphere at New Zealand (N.S.), (2) to identify and quantify the influence of anthropogenic and natural Hg contributions to atmospheric Hg fluxes, (3) to further investigate the suitability and comparability of our two selected media (lake sediments and ombrotrophic peat) for Hg depositional reconstructions, and (4) to assess the relative importance of wet and dry deposition to the study areas. Significant findings from the study include the following: (1) The lake sediments examined appear to faithfully record the contemporary flux of Hg from the atmosphere (e.g., 1997: N.S. Lakes: approximately 8 ± 3 μg m−2 yr−1; N.S. Rain: 8 μg m−2 yr−1). The upper 10 cm (approximately 10 yr) of ombrotrophic peat cores from Nova Scotia were dated using a biological chronometer (Polytrichum) and were also consistent with the flux data provided by current direct sampling of precipitation. These observations place limits on the contribution of dry deposition (40 ± 50% of wet flux). Unfortunately, the peat samples could not be dated below 10 cm. This was due to the apparent diagenetic mobility of the geochronological tracer (210Pb). (2) There is no evidence of a significant enhancement in the atmospheric Hg flux as a result of preindustrial (<1900 c.e. (Common Era)) activities such as the extensive Au and Ag mining in the Americas. (3) A factor of 3 and 5x increase in the deposition of Hg to the lake sediment archives was observed since the advent of the industrial revolution in New Zealand and Nova Scotia respectively, suggesting a worldwide increase in the atmospheric deposition of Hg. Furthermore, this increase is synchronous with increases in the release of CO2 from combustion of fossil fuels on a global scale. The magnitude of increase since industrialization appears larger in Nova Scotia than in New Zealand. This may be due to enhanced deposition of Hg as a result of either regional emission of Hg or enhanced regional oxidation of Hg°.
Ridge‐crossing mantle plumes and gaps in tracks
Authors: Sleep Norman H.
Published: December 2002

Hot spot tracks approach, cross, and leave ridge axes. The complications of this process make it difficult to determine the track followed by a plume and the evolution of its vigor. When a plume is sufficiently near the ridge axis, buoyant plume material flows along the base of the lithosphere toward the axis, forming an on‐axis hot spot. The track of the on‐axis hot spot is a symmetric V on both plates and an unreliable indication of the path followed by the plume. Aseismic ridges form more or less along flowlines from a plume to a ridge axis when channels form at the base of the lithosphere. A dynamic effect is that off‐axis hot spots appear to shut off at the time that an on‐axis hot spot becomes active along an axis‐approaching track. This produces a gap in the obvious track and a jump of the hot spot to the ridge axis. The gap results from the effects of ponded plume material on intraplate (membrane) stress. Membrane tension lets dikes ascend efficiently to produce obvious tracks of edifices. An off‐axis hot spot shuts down when the plume is sufficiently near the ridge axis that plume material flows there, putting the nearby lithosphere above the plume into compression, preventing dikes. In addition, the off‐axis thickness of plume material, which produces membrane tension, decreases as the slope of the base of the lithosphere increases beneath young lithosphere. Slow spreading rates favor gaps produced in this way. Gaps are observed near both fast and slow ridges.
Surface flow evolution throughout a glacier surge measured by satellite radar interferometry
Authors: Luckman Adrian , Murray Tavi , Strozzi Tazio
Published: December 2002

The surface flow evolution over 6 years of a glacier surge in Svalbard is revealed by satellite radar interferometry from ERS‐1 and ERS‐2. Eleven surface velocity maps, generated by combining multi‐temporal, differential interferograms utilising 36 ascending and descending‐pass images, give an unprecedented overview of the spatio‐temporal development of the surge. The surge is characterised by gradual initiation and termination phases, and flow restrictions appear to remain spatially consistent despite the relatively large changes in surface velocity. Results from this type of study are expected to help constrain physically‐based models of surge behaviour.
The 8‐hour tide in the Arctic mesosphere and lower thermosphere
Authors: Younger P. T. , Pancheva D. , Middleton H. R. , Mitchell N. J.
Published: December 2002

An all‐sky VHF meteor radar at Esrange (68°N, 21°E) near Kiruna in Northern Sweden has been used to investigate the 8‐hour tide in the Arctic mesosphere and lower thermosphere. We present a climatology of the 8‐hour tide over the period October 1999 to April 2001. The tide appears to be a persistent feature of the Arctic atmosphere, although a large day‐to‐day variability of the tidal amplitude is observed. At times the 8‐hour tide reaches amplitudes over 30 m s−1. The amplitude of the tide increases with height across the observed height range of ∼80–100 km. Monthly mean tidal amplitudes range from <2 m s−1 to values as large as 10 m s−1. A clear seasonal behavior is apparent with maximum amplitudes observed in the autumn. Vertical wavelengths are shortest in winter and spring (25–35 km) and longest in summer and autumn (50–90 km). At least on some occasions the vertical wave number relationships between the 8‐, 12‐, and 24‐hour tides suggest that the 8‐hour tide is being generated by nonlinear interaction between the 12‐ and 24‐hour tides.
Simultaneous acceleration and pitch angle scattering of field‐aligned electrons observed by the LEPA on CRRES
Authors: Abel G. A. , Fazakerley A. N. , Johnstone A. D.
Published: December 2002

Using data from the Low Energy Plasma Analyzer we present detailed pitch angle versus energy electron distributions at the time of field‐aligned electron events. We present three case studies, as well as the findings of a larger survey of 158 events. These distributions show that electrons are apparently scattered in pitch angle out of field‐aligned electron beams observed around 0.1–1 keV. Generally, this pitch angle scattering is accompanied by simultaneous acceleration. As electrons are scattered from the field‐aligned beam to pitch angles of ∼60°, they typically increase to energies of around 10 keV. The electrons are then seen to scatter to 90° pitch angle with no further change in energy. The observations suggest that this scattering and acceleration takes place near the geomagnetic equator and over a limited range of latitudes.
Wind survey of high‐speed bulk flows and field‐aligned beams in the near‐Earth plasma sheet
Authors: Raj Arjun , Phan Tai , Lin Robert P. , Angelopoulos V.
Published: December 2002

We have surveyed all high‐speed (>250 km/s) flows detected by Wind during its 17 perigee passes across the near‐Earth (xGSE = −25 to 0 RE) plasma sheet in the period between 1995 and 1997. By classifying high‐speed flow events based on their ion distribution characteristics rather than their plasma moments or the regions in which these flows were detected, we found that most (if not all) high‐speed flow ion distributions fall into two distinct categories: bulk flows and field‐aligned beams. We show that bulk flows are not simply the low‐latitude counterparts of field‐aligned beams. Field‐aligned beams have sharp cutoffs at low energies, occur within relatively steady magnetic field and plasma conditions, and are detected more often away from the neutral sheet. On the other hand, high‐speed bulk flows are well represented by a single drifting population and have their peak occurrence rate at the neutral sheet. Bulk flows are generally accompanied by large magnetic field fluctuations and sudden increase of energetic (up to 0.5 MeV) particle fluxes. They are often (but not always) associated with magnetic field dipolarization and plasma temperature enhancements. Little or no temperature enhancements are observed in cases where the spacecraft resides near the neutral sheet before the arrival of bursty bulk flows, suggesting that temperature enhancements seen in other bulk flow events may in part be a spatial effect instead of true heating of the plasma. Bulk flows are perpendicular to the magnetic field when detected at the neutral sheet but have a large field‐aligned component at higher magnetic latitudes. Field‐aligned bulk flows and field‐aligned beams are similar in terms of their velocity moments and may occur at the same magnetic latitudes but are easily distinguishable based on their ion distributions. We have used our categorization of bulk flow and field‐aligned beam events to search for the optimal moment‐based selection criteria to distinguish bulk flows from beams. We found that no single moment‐based parameter or threshold can serve to cleanly separate beam from bulk flow distributions because a range of values of these parameters exists where both types of fast flows are observed. We found perpendicular flow speed v⟂ > 250 km/s and plasma βxy (based on the x and y components of the magnetic field) > 2 to be the optimal moment‐based bulk flow selection criteria because they eliminate ∼95% of beam events while retaining ∼60% of bulk flow events. Finally, a previously reported dawn–dusk asymmetry in the occurrence of bursty bulk flows, with most events occurring in the premidnight sector, is confirmed by our survey. Field‐aligned beams, on the other hand, have no dawn–dusk bias.
Observation of a large density dropout across the magnetic field at 600 km altitude during the 6–7 April 2000 magnetic storm
Authors: Su S.‐Y. , Yeh H. C. , Chao C. K. , Heelis R. A.
Published: December 2002

The first Republic of China Satellite, ROCSAT‐1 at 600 km altitude observed two groups of equatorial spread‐F (ESF) plasma depletion structures located in either side of the midnight meridian during the 6–7 April 2000 magnetic storm recovery phase. In the group of postmidnight ESF irregularities, a large density dropout was observed in two consecutive ROCSAT orbits. This large density dropout structure is similar to the density depletion event observed by Defense Meteorological Satellite Program (DMSP) F9 spacecraft during the 13–14 March 1989 magnetic storm [Greenspan et al., 1991] except that the ROCSAT orbit traverses the magnetic field in the east‐west direction. The postmidnight group of depletions together with the large density dropout was observed to drift westward with the background plasma at ∼170 m s−1. We examine the properties of the large density dropout through the plasma data taken from the drift meter (DM) and retarding potential analyzer (RPA) sensors in three consecutive orbits. The apparent widening of the density depletion is explained by the different locations of the satellite within the depletion region. The extreme low‐density level inside the upwelling depletion indicates that of the nighttime F‐peak passed over the ROCSAT orbit at 600 km. The existence of NO+ ions inside the dropout is consistent with such an elevation of the F‐peak.
Relationship between the near‐Earth interplanetary field and the coronal source flux: Dependence on timescale
Authors: Lockwood M.
Published: December 2002

The Ulysses spacecraft has shown that the radial component of the heliospheric magnetic field is approximately independent of latitude. This has allowed quantification of the total open solar flux from near‐Earth observations of the interplanetary magnetic field. The open flux can also be estimated from photospheric magnetograms by mapping the fields up to the “coronal source surface” where the field is assumed to be radial and which is usually assumed to be at a heliocentric distance r = 2.5RS (a mean solar radius, 1RS = 6.96 × 108 m). These two classes of open flux estimate will differ by the open flux that threads the heliospheric current sheet(s) inside Earth's orbit at 2.5RS < r < 1R1 (where the mean Earth‐Sun distance, 1R1 = 1 AU = 1.5 × 1011 m). We here use near‐Earth measurements to estimate this flux and show that at sunspot minimum it causes only a very small (≈0.5%) systematic difference between the two types of open flux estimate, with an uncertainty that is of order ±24% in hourly values, ±16% in monthly averages, and between −6% and +2% in annual values. These fractions may be somewhat larger for sunspot maximum because of flux emerging at higher heliographic latitudes.
On the propagation of magnetohydrodynamic perturbations in a Harris‐type current sheet 1. Propagation on discrete modes and signal reconstruction
Authors: Fruit G. , Louarn P. , Tur A. , Le Quéau D.
Published: December 2002

We propose a study of the propagation of magnetohydrodynamic linear perturbations in a 1‐D model of the magnetotail current sheet. Starting from a mathematical analysis of the impulsive linear response of a Harris sheet to external excitations, we study the propagation of the different types of perturbations in the sheet. The structure of the discrete modes of propagation is analyzed, and their coupling to the initial perturbation is discussed. Using various types of initial perturbations (with different characteristic temporal and spatial scales, centered on the neutral sheet or shifted toward the lobe), we reconstruct the full spatial–temporal evolution of the magnetic, velocity, and pressure perturbations. We thus completely determined the perturbations that would be detected by a spacecraft in the tail if the propagation of the perturbations obeys to the ideal linear MHD equations. For example, it is shown that pulse‐like initial perturbations result in the propagation of long quasi‐monochromatic wave packets. The typical period is of the order of 1 min with a maximum amplitude for both the magnetic and thermal pressure perturbations that reach at maximum a few percent of the static value (for an initial pulse with maximum amplitude of the order of the total equilibrium pressure). The highest amplitudes are obtained with initial pulse with spatial scales of the order of the current sheet width.
Heliospheric magnetic field strength and polarity from 1 to 81 AU during the ascending phase of solar cycle 23
Authors: Burlaga L. F. , Ness N. F. , Wang Y.‐M. , Sheeley N. R.
Published: December 2002

The Voyager 1 (V1) observations of the heliospheric magnetic field strength B agree with Parker's model of the global heliospheric magnetic field from 1 to 81.0 AU and from 1978 to 2001.34 when one considers the solar cycle variations in the source magnetic field strength and the latitude/time variation in the solar wind speed. In particular, Parker's model, without adjustable parameters, describes the general tendency for B to decrease with increasing distance R from the Sun, the three broad increases of B around 1980, 1990, and 2000, and the minima of B in 1987 and 1997. During 1987 and 1997, B appears to be lower than Parker's model predicts, but that can be attributed to the presence of a heliospheric vortex street at these times and/or uncertainty in the observations. There is no evidence for a significant flux deficit increasing monotonically from 1 to 81.0 AU. By extrapolating these results and considering the limitations of the observations, V1 should continue to make useful measurements during the next few years at least. The magnetic field polarity in the distant heliosphere at V1 and Voyager 2 (V2) changed during the ascending phase of solar cycle 23. In the Northern Hemisphere, V1 observed a decrease in the percentage of positive polarities from ≈100% during 1997 to ≈50% during 2000. In the Southern Hemisphere, V2 observed the opposite behavior, an increase in the percentage of positive polarities from ≈0% during 1997 to ≈50% during 2000. The variation of magnetic polarity observed by V1 and V2 was caused by the increasing latitudinal width of the sector zone with increasing solar activity, which in turn was related to the increasing maximum latitudinal extent and the decreasing minimum latitudinal extent of the footprints of the heliospheric current sheet (HCS). There was a tendency for the speed and proton temperature to decrease and the density to increase at V2 from 1997 (when it observed flows from polar coronal holes) to 2001 (when it observed more complex and dynamic flows).
On the geometry of the external geomagnetic irregular variations
Authors: Bellanger E. , Blanter E. M. , Le Mouël J.‐L. , Mandea M. , Shnirman M. G.
Published: December 2002

Using a simple nonlinear signal analysis tool, we are able to picture some characteristics of the external geomagnetic irregular variations. No distinction between disturbed and quiet days is made. The irregular variations field thus obtained is found to have quite simple time and space properties.
Wind‐driven internal waves and Langmuir circulations in a numerical ocean model of the southern Baltic Sea
Authors: Axell Lars B.
Published: December 2002

A one‐dimensional numerical ocean model of the southern Baltic Sea is used to investigate suitable parameterizations of unresolved turbulence and compare with available observations. The turbulence model is a k‐ε model that includes extra source terms PIW and PLC of turbulent kinetic energy (TKE) due to unresolved, breaking internal waves and Langmuir circulations, respectively. As tides are negligible in the Baltic Sea, topographic generation of internal wave energy (IWE) is neglected. Instead, the energy for deepwater mixing in the Baltic Sea is provided by the wind. At each level the source term PIW is assumed to be related to a vertically integrated pool of IWE, E0, and the buoyancy frequency N at the same level, according to PIW(z) ∝ E0Nδ(z). This results in vertical profiles of ε (the dissipation rate of TKE) and Kh (the eddy diffusivity) according to ε ∝ Nδ and Kh ∝ Nδ−2 below the main pycnocline. Earlier observations are inconclusive as to the proper value of δ, and here a range of values of δ is tested in hundreds of 10‐year simulations of the southern Baltic Sea. It is concluded that δ = 1.0 ± 0.3 and that a mean energy flux density to the internal wave field of about (0.9 ± 0.3) × 10−3 W m−2 is needed to explain the observed salinity field. In addition, a simple wind‐dependent formulation of the energy flux to the internal wave field is tested, which has some success in describing the short‐ and long‐term variability of the deepwater turbulence. The model suggests that ∼16% of the energy supplied to the surface layer by the wind is used for deepwater mixing. Finally, it is also shown that Langmuir circulations are important to include when modeling the oceanic boundary layer. A simple parameterization of Langmuir circulations is tuned against large‐eddy simulation data and verified for the Baltic Sea.
Surface salinity advection in the tropical oceans compared with atmospheric freshwater forcing: A trial balance
Authors: Johnson Eric S. , Lagerloef Gary S. E. , Gunn John T. , Bonjean Fabrice
Published: December 2002

Climatological surface salinities and satellite‐derived surface currents are combined to estimate the horizontal and vertical time mean and seasonal salinity divergence in the tropical ocean's surface layer. The mean salinity divergence has magnitude and spatial patterns matching major features of an estimate of evaporation minus precipitation averaged from several atmospheric general circulation model reanalyses and satellite‐derived products (spatial correlation of 0.63). Thus a substantial portion of the atmospheric freshwater flux appears to be balanced by ocean transports in a relatively thin (32.5 m) surface layer. Large regional discrepancies remain, however, and their magnitude is greater than the scatter among the various atmospheric forcing products used in this study. The present salinity divergence calculation is imprecise because the mean salinity gradients have been weakened by large‐scale smoothing of sparse and unevenly sampled data. Further, other investigators show that higher‐frequency ocean processes (e.g., vertical and horizontal mixing of salinity) are significant at least locally. Nevertheless, several areas of consistent disagreement between mean flow salinity divergence and atmospheric forcing, such as excess evaporation in subtropical gyres, are suggestive of errors in the forcing. In particular, the shortfall of salinity convergence balancing high precipitation in the western Pacific is roughly consistent with regional surface flux corrections required to bring ocean general circulation model integrations into long‐term balance in this region. Thus this shortfall may reflect either overlarge precipitation estimates or a fundamental misunderstanding of how the ocean disperses this flux. Ultimately, further ocean physics must be addressed before firm conclusions can be drawn about the fraction of atmospheric freshwater input unaccounted for by the near‐surface mean flow and seasonal salinity divergence. High‐resolution space‐based salinity measurements will be necessary to provide the temporal and spatial coverage necessary to develop ocean salinity transport calculations into useful constraints on ocean processes, surface hydrological forcing, and model simulations.
Wintertime shoreward near‐surface currents south of Cape Hatteras
Authors: Savidge Dana K.
Published: December 2002

Cross‐isobath flow on continental shelves is of interest for a variety of reasons. Near Cape Hatteras, North Carolina, the transport of larval organisms, pollutants, and oceanic carbon budget constituents to and from the adjacent Albemarle and Pamlico Sounds may depend critically on cross‐isobath currents. Shoreward currents in the near‐surface waters south of Cape Hatteras are documented herein, on the basis of continuous 2‐year time series, encompassing all or part of three consecutive winters. Energetic shoreward currents exist ∼30% of the time from midfall through late spring. These currents are evident over the 20 and 35 m isobaths along a mooring line situated ∼40 km southwestward from Cape Hatteras. Shoreward velocities average ∼12 cm/s, and events persist from 0.5 to 4 days, occurring every 2.5–5 days, except in summer. These events often coincide with southwestward winds but occur under both upwelling and downwelling favorable conditions, such that Ekman veering in the surface layer does not account for the shoreward velocities. In winter the mooring line south of Cape Hatteras is frequently traversed by a strong temperature and salinity front, with light, relatively fresh, cold, stratified water on one side, and denser, more saline, warmer, unstratified water on the other. Hydrography and satellite sea surface temperature imagery help identify this front as the boundary between South Atlantic Bight and Mid‐Atlantic Bight coastal shelf waters, the “Hatteras Front.” Flow along the Hatteras Front where it crosses the shelf appears to account for the observed shoreward currents. The along‐shelf advection of the Hatteras Front may depend on both winds and Gulf Stream distance offshore.
Nonlinear energy transfer within the oceanic internal wave spectrum at mid and high latitudes
Authors: Hibiya Toshiyuki , Nagasawa Maki , Niwa Yoshihiro
Published: December 2002

In order to examine how the energy supplied by M2 internal tides cascades through the local internal wave spectrum down to dissipation scales, two sets of numerical experiments are carried out where the Garrett‐Munk‐like quasi‐stationary internal wave spectra at 49°N (experiment I) and 28°N (experiment II), respectively, are first reproduced and then perturbed instantaneously in the form of an energy spike at the lowest vertical wave number and M2 tidal frequency. These experiments attempt to simulate the nonlinear energy transfer within the quasi‐stationary internal wave fields near the Aleutian Ridge and the Hawaiian Ridge, respectively, both of which are generation regions of large‐amplitude M2 internal tides. In experiment I, the energy spike stays at the lowest wave number, where it is embedded and the spectrum remains quasi‐stationary after the energy spike is injected. In experiment II, in contrast, the energy level at high horizontal and vertical wave numbers rapidly increases after the injection of the energy spike, exhibiting strong correlation with the enhancement of high vertical wave number, near‐inertial current shear. This implies that as the high vertical wave number, near‐inertial current shear is intensified, high horizontal wave number internal waves are efficiently Doppler shifted so that the vertical wave number rapidly increases and enhanced turbulent dissipation takes place. The elevated spectral density in the high vertical wave number, near‐inertial frequency band, which plays the key role in cascading energy to dissipation scales, is thought to be caused by parametric subharmonic instability. In experiment I, in contrast, the M2 tidal frequency is 1.2 times the inertial frequency at 49°N so that M2 internal tide is free from parametric subharmonic instability. Accordingly, even though significant M2 internal tidal energy may be generated, it is not available to support local deep water mixing.
Fire effects on surface‐atmosphere energy exchange in Alaskan black spruce ecosystems: Implications for feedbacks to regional climate
Authors: Chambers S. D. , Chapin F. S.
Published: December 2002

Although fire is crucial to the functioning and diversity of boreal forests, the second largest biome on Earth, there are few detailed studies of the effects of disturbance on surface‐atmosphere interactions in these regions. We conducted tower‐based micrometeorological measurements in summer over six recently burned black spruce stands that varied in age between 0 and 14 years. Results are presented for nonprecipitating conditions. There was an initial reduction of minimum albedo following fire from ∼0.09 to 0.06 followed by a rapid increase to 0.135 as the cover of successional vegetation increased. For clear‐sky conditions near local solar noon, the combined effects of increased surface temperature due to a reduced surface‐atmospheric coupling and albedo reduced midday net radiation by 9.3% (∼70 W m−2) for the first decade of succession. The average daily net radiation declined by 5.5%. Near noon, the relative partitioning of net radiation into ground heat flux doubled compared to nearby unburned stands, although on a daily basis the increase was only significant for the first few years following fire. Reduced net radiation, enhanced ground heat flux, and reduced Bowen ratio values following the first decade of succession could lead to a midday reduction of approximately 80 W m−2 in sensible heat flux compared to an unburned stand. Across a fire‐scar boundary, the contrast in sensible heating and roughness is sufficiently large to induce mesoscale circulations and possibly trigger convective development. Because fire scars frequently exceed 104 ha, locally cooler conditions would prevail with suppressed planetary boundary layer development. Given the abundance of fire disturbance throughout interior Alaska, these effects may contribute to the regional climate patterns.
Analysis of lake ice dynamics and morphology on Lake El'gygytgyn, NE Siberia, using synthetic aperture radar (SAR) and Landsat
Authors: Nolan Matt , Liston Glen , Prokein Peter , Brigham‐Grette Julie , Sharpton Virgil L. , Huntzinger Rachel
Published: December 2002

A time series of more than 450 combined ERS‐2, Radarsat‐1, and Landsat‐7 scenes acquired between 1998 and 2001 was analyzed to develop a fairly complete picture of lake ice dynamics on Lake El'gygytgyn, NE Siberia (67.5°N, 172°E). This 14‐km3 lake partially fills a meteorite impact crater formed 3.6 million years ago and is home to a paleoenvironmental coring project. The duration of lake ice cover and the onset of lake ice breakup are important both to interpretations of the archived sediment core record and to future drilling projects that will use the ice as a stable platform. Ice formation, snowmelt, and ice breakup likely occur in late October, mid‐May, and early July, respectively. These data were used to validate a one‐dimensional energy‐balance lake ice model, which can now be used to hindcast paleoclimate based on core proxy information. Synthetic aperture radar (SAR) backscatter from the lake ice also revealed unusual spatial variations in bubble content, which were found to indicate the level of biological productivity in the sediments directly beneath the ice, with the highest productivity located in the shallowest (0–10 m) as well as the deepest (170–175 m) regions of the lake. Seismic data indicates that the backscatter anomaly above the deepest water is collocated with the central peak of the impact crater, 500 m below the surface. Several hypotheses are presented to explain this anomaly. Regardless of cause, the fact that large spatial variations in biological productivity exist in the lake has important implications for selecting the locations of future sediment cores.
Analysis of a multiyear global vegetation leaf area index data set
Authors: Buermann Wolfgang , Wang Yujie , Dong Jiarui , Zhou Liming , Zeng Xubin , Dickinson Robert E. , Potter Christopher S. , Myneni Ranga B.
Published: December 2002

The analysis of a global data set of monthly leaf area index (LAI), derived from satellite observations of normalized difference vegetation index (NDVI) for the period July 1981 to September 1994, is discussed in this paper. Validation of this retroactive, coarse resolution (8 km) global multiyear data set is a challenging task because repetitive ground measurements from all representative vegetation types are not available. Therefore the magnitudes and interannual variations in the derived LAI fields were assessed as follows. First, the use of a NDVI‐based algorithm, as opposed to a more physically based approach, is estimated to result in relative errors in LAI of about 10–20%, which is comparable to the mean uncertainty of AVHRR NDVI data. Second, the satellite LAI values compared reasonably well to ground measurements from three field campaigns. Third, comparison with an existing multiyear LAI data set showed qualitative agreement with regards to interannual variability, although the LAI values of the earlier data were consistently larger than those derived here. Fourth, interannual variations in LAI were evaluated through correlations with climate data sets, e.g., sea surface temperatures and precipitation in tropical semiarid regions known for ENSO impacts, temperature dependence of vegetation growth, and therefore LAI, in the northern latitudes. The general consistency between these independent data sets imbues confidence in the LAI data set, at least for use in large‐scale modeling studies. Finally, improvements in near‐surface climate simulation are documented in a companion article when satellite LAI values were used in a global climate model. The data set is available to the community via our Web server (http://cybele.bu.edu).
Impacts of NOx emissions from subsonic aircraft in a global three‐dimensional chemistry transport model including plume processes
Authors: Kraabøl Anne Gunn , Berntsen Terje K. , Sundet Jostein K. , Stordal Frode
Published: December 2002

A three‐dimensional (3‐D) global chemistry transport model (CTM) has been used to study the impact of NOx emissions from subsonic aircraft on the NOx and ozone concentrations in the troposphere. Plume scale processes have been included by a dispersion model which modifies the NOx emissions to include formation of nitrogen reservoir species and nonlinear chemical effects on ozone production and loss within the plume regime. These modifications were varied as a function of time of day of emissions, latitude, season, temperature, atmospheric turbulence and background concentrations of NOx and ozone. With a global emission rate of 0.7 Tg(N) yr−1 from aircraft and including plume scale effects in the 3‐D CTM, we estimate a maximum ozone perturbation at 250 hPa from 5.5 to 10.5 ppbv north of 40°N in May. In January the maximum is lower (3–4 ppbv) and more confined to a zonal band between 35 and 50°N. During July the influence of vertical mixing is found to reduce the maximum at midlatitudes, and the largest perturbations (6.5–8 ppbv) are found in the polar region. The use of modified aircraft emissions had a significant influence on the estimated NOx and ozone increases, and the largest effects were found in May. Compared with an unmodified NOx emission from aircraft, the NOx perturbations at cruise altitude decreased by 25–35% over the eastern U.S., the North Atlantic Ocean, and Europe. The corresponding decrease in the ozone perturbation was 15–18% at northern middle and high latitudes.
Two‐dimensional model simulations of the QBO in ozone and tracers in the tropical stratosphere
Authors: Fleming Eric L. , Jackman Charles H. , Rosenfield Joan E. , Considine David B.
Published: December 2002

Meteorological data from the United Kingdom Meteorological Office (UKMO) and constituent data from the Upper Atmospheric Research Satellite (UARS) are used to construct yearly zonal mean dynamical fields for the 1990s for use in the NASA/Goddard Space Flight Center (GSFC) two‐dimensional (2‐D) chemistry and transport model. This allows for interannual dynamical variability to be included in the model constituent simulations. In this study, we focus on the tropical stratosphere. We find that the phase of quasi‐biennial oscillation (QBO) signals in equatorial CH4 and profile and total column O3 data are resolved quite well using this empirically based 2‐D model transport framework. However, the QBO amplitudes in the model constituents are systematically underestimated relative to the observations at most levels. This deficiency is probably due in part to the limited vertical resolutions of the 2‐D model and the UKMO and UARS input data sets. We find that using different heating rate calculations in the model affects the interannual and QBO amplitudes in the constituent fields, but has little impact on the phase. Sensitivity tests reveal that the QBO in transport dominates the ozone interannual variability in the lower stratosphere, with the effect of the temperature QBO being dominant in the upper stratosphere via the strong temperature dependence of the ozone loss reaction rates. We also find that the QBO in odd nitrogen radicals, which is caused by the QBO modulated transport of NOy, plays a significant but not dominant role in determining the ozone QBO variability in the middle stratosphere. The model mean age of air is in good overall agreement with that determined from tropical lower‐middle stratospheric OMS balloon observations of SF6 and CO2. The interannual variability of the equatorial mean age in the model increases with altitude and maximizes near 40 km, with a range of 4–5 years over the 1993–2000 time period.
Particle characteristics from the spectra of polar mesospheric clouds
Authors: Carbary J. F. , Morrison D. , Romick G. J.
Published: December 2002

Spectrographic imagers on the Midcourse Space Experiment (MSX) satellite have obtained the first spectra of polar mesospheric clouds (PMCs) from the far ultraviolet to the visible (164–585 nm). Several hundred PMC spectra were extracted over a 3 week period during the summer of 1999. To improve statistics, the spectra were normalized and averaged within 32 groups of clouds having common latitudes and scattering angles. The subsequent spectra, from three spectrographic imagers, produced a PMC signal that fell into the noise at wavelengths short of 200 nm and showed Earth albedo effects longward of ∼315 nm. The spectra between these limiting wavelengths were subjected to a least squares analysis to determine characteristics of the scattering particles assuming a lognormal distribution of Mie scatterers made of ice. Averaged over all PMCs, the distribution mode (size) was 65.2 ± 2.2 nm and the distribution dispersion (width) was 1.15 ± 0.04. Over the period of observation, neither the mode nor the dispersion exhibited systematic variations in time or latitude, although less intense clouds at lower latitudes may have been undersampled.
A test of three‐dimensional radiative transfer simulation using the radiance signatures and contrasts at a high latitude coastal site
Authors: Ricchiazzi Paul , Payton Allison , Gautier Catherine
Published: December 2002

A three‐dimensional Monte Carlo radiation code is used to analyze surface observations of sky radiance at a high latitude coastal site. Observations were made at Palmer Station, Antarctica, under uniform marine stratus clouds during both ice‐free and iced‐in ocean conditions. The model results using actual snow/ocean area coverage show good qualitative agreement with all‐sky images and high spectral resolution radiometer scans. When viewing over the island, toward the northeast, observations and modeling indicate that sky radiance in the visible bands is up to a factor of two greater than for directions toward the southwest, viewing out over open ocean. Owing to a decrease in snow reflectance at longer wavelengths, modeled and observed radiance distributions in the near infrared tend to be much more symmetric. Results of the modeling study indicate that details of the snow BRDF, or island topography, have a very small effect on model predictions. In addition, the model indicates that the angular distribution of sky radiance is fairly insensitive to cloud optical depth (assumed homogeneous) and is independent of cloud height as long as cloud layer thickness is not much larger than the cloud base height.
Large‐scale mass balance effects of blowing snow and surface sublimation
Authors: Déry Stephen J. , Yau M. K.
Published: December 2002

This study examines the effects of surface sublimation and blowing snow on the surface mass balance on a global and basin scale using the European Centre for Medium‐Range Weather Forecasts (ECMWF) Re‐Analysis (ERA15) data at a resolution of 2.5° that span the years 1979–1993. The combined processes of surface and blowing snow sublimation are estimated to remove 29 mm yr−1 snow‐water equivalent (swe) over Antarctica, disposing about 17 to 20% of its annual precipitation. In the Northern Hemisphere, these processes are generally less important in continental areas than over the frozen Arctic Ocean, where surface and blowing snow sublimation deplete upward of 100 mm yr−1 swe. Areas with frequent blowing snow episodes, such as the coastal regions of Antarctica and the Arctic Ocean, are prone to a mass transport >100 Mg m−1 yr−1. Although important locally, values of the divergence of mass through wind redistribution are generally 2 orders of magnitude less than surface and blowing snow sublimation when evaluated over large areas. For the entire Mackenzie River Basin of Canada, surface sublimation remains the dominant sink of mass as it removes 29 mm yr−1 swe, or about 7% of the watershed's annual precipitation. Although the first of its kind, this study provides only a first‐order estimate of the contribution of surface sublimation and blowing snow to the surface mass balance because of limitations with the data set and some uncertainties in the blowing snow process.
Comparison of airborne measured and calculated spectral actinic flux and derived photolysis frequencies during the PEM Tropics B mission
Authors: Shetter R. E. , Cinquini L. , Lefer B. L. , Hall S. R. , Madronich S.
Published: December 2002

Scanning actinic flux spectroradiometer (SAFS) instruments were deployed on the National Aeronautics and Space Administration (NASA) DC‐8 and P‐3B research aircraft for the Pacific Exploratory Mission in the Tropical Pacific (PEM Tropics B mission). The SAFS instruments measured the wavelength‐dependent downwelling and upwelling actinic flux from 282 to 420 nm. From the actinic flux spectra, photolysis frequencies of a number of molecules were calculated. Flight tracks during the PEM Tropics B mission varied in altitude from the surface up to 12 km, with much time spent above, below, and among broken and scattered clouds. Comparisons of the measured photolysis frequencies to the photolysis frequencies calculated with the National Center for Atmospheric Research (NCAR) tropospheric ultraviolet visible (TUV) radiative transfer model, assuming clear skies, demonstrate some expected differences in this complex radiation environment, while in general the agreement between the measurements and the model is within 15%. Differences between the measurements and the model are expected in and around clouds, because the radiation model is not equipped to input broken and scattered cloud effects, except on an individual case‐by‐case basis. The differences in the composite mean ratio of SAFS measurement to TUV model for total photolysis frequencies vary from 6% to 18% for jNO2 and from 6% to 11% for jO(1D) for both aircraft. The differences between the measurements and the models represent the accuracy that can be expected when using photolysis frequencies from clear‐sky radiative transfer modeling in regional and global atmospheric chemistry models for regions with broken and scattered clouds in the troposphere.