bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations on the long‐period variability of the Gulf Stream downstream of Cape Hatteras
Authors: Vazquez Jorge
Published: February 2008

To examine the long‐period variability of the Gulf Stream, sea level residuals relative to a 2‐year mean sea level in the Gulf Stream downstream of Cape Hatteras (between 75°W and 60°W longitude) are used. Residuals, as derived from Geosat altimetry between November 1986 and December 1988, were gridded in space and time at a temporal resolution of 10 days and spatial resolution of 1/4°. Complex empirical orthogonal function (CEOF) analysis was applied to the data set to extract the spatially correlated signal with the original data sub sampled to 1/2°. In addition to determining the space‐time scales and propagation characteristics of the different modes, wavenumber‐frequency spectral techniques were used to separate the variability into propagating and stationary components. The CEOF technique applied to the data set indicated that the first four CEOF modes accounted for 60% of the variability and were found to be above the noise level 99% of the time. CEOF 1 was associated with westward propagation at 5 km/d at a wavelength of 2000 km and eastward propagation at 1–2 km/d centered at a 500‐km wavelength. This first CEOF is in good agreement with thin‐jet equivalent barotropic models which predict westward propagation for wavelengths greater than 1130 km. A deflection of the wavelike pattern at 65°W also indicates a possible topographic effect. A simple scaling of the effect of topography indicates that for length scales longer than the internal Rossby radius of deformation, the topographic term is at least of the same order of magnitude as the beta effect. The second CEOF was more broadbanded in wavenumber space, with eastward propagation occurring in a wavenumber‐frequency band between 300 and 1400 km and 0.5 and 2.0 cycles/yr. The third CEOF is similar in structure to the first, but with less energy. CEOF 4 was clearly identifiable with higher frequencies than the first three with westward propagation at 4 km/d. The spatial location of this mode along with the westward propagation indicates possible influences from eddy‐stream interactions. Thus topography, Rossby wave dynamics, and eddy‐stream interactions all appear to have a significant role in determining the space‐time scales and propagation properties of the long‐period response of sea level in the Gulf Stream.
Large variations of surface ocean radiocarbon: Evidence of circulation changes in the southwestern Pacific
Authors: Druffel Ellen R. M. , Griffin Sheila
Published: February 2008

Radiocarbon (Δ14C) and stable isotope (δ18O and δ13C) records are presented for biannual samples from a 323‐year banded coral series collected from the southern Great Barrier Reef, Australia. The high‐precision Δ14C record contains variations on an interannual timescale, that are particularly large between A.D. 1680 and 1730. By comparison with tree ring Δ14C records [Stuiver and Quay, 1980; M. Stuiver, personal communication, 1992), it is clear that these shifts were not caused by changes in the Δ14C of atmospheric CO2. Changes in vertical mixing and large scale advective changes involving source waters to the western Coral Sea region are likely processes that could account for these large Δ14C variations. Most low Δ14C values for the period A.D. 1635‐1875 coincide with El Niño/Southern Oscillation (ENSO) events as reported by Quinn et al. [1987] for the eastern tropical Pacific. However, ENSO does not explain all of the variations, especially during 1875–1920 when Δ14C values remained high. Cross‐spectral analysis of the early half of the Δ14C and δ18O records (A.D. 1635–1795) reveals that the 6‐year period is coherent; this coherency is not present in the latter half (A.D. 1797–1957) of the isotope records. These data support the concept of century timescale changes in the nature of ENSO, as it is manifest in the southwestern Pacific. Our coral record shows no evidence of a Suess effect, the lowering of Δ14C from late 1800s through 1955 due mainly to CO2 input from fossil fuel burning. This is coincident with the change we observe in the nature of ENSO and is further evidence that a long‐term change in mixing of upper waters occurred in this region.
A shallow meddy (a smeddy) from the secondary Mediterranean salinity maximum
Authors: Pingree R. D. , Le Cann B.
Published: February 2008

A smeddy, or shallow meddy with temperature and salinity core characteristics of the Secondary Mediterranean Salinity Maximum (SMSM), was found near 36.75°N, 12.5°W in March 1992, west of Cape St. Vincent and some 600 km from the Strait of Gibraltar. A detailed survey defined the temperature, salinity, nitrate, silicate, oxygen, and light transmission structure of the smeddy. The smeddy core had a maximum salinity of 36.483 psu at a depth of 775 m, and a maximum temperature of 13.55°C at a depth of 725 m. The salinity anomaly was 9 standard deviations from the mean at a depth of 600 m. The property distributions suggest that about 0.5 km of a central water column (∼500–1000 m) was traveling with the smeddy, though the dynamic influence extended from the surface to a depth of about ∼1.5 km. The velocity field was derived from dynamic height differences, the gradient equation, a drogued Argos buoy, and acoustic Doppler current profiler (ADCP) measurements. At the core depth of ∼700 m a number of well‐defined structures were found that characterized the horizontal influence of the smeddy. Surrounding an inner core (temperature > ∼ 13.25°C, salinity > ∼36.4 psu) of radius about 7 km, in near solid body rotation (with center period ∼3.7 days), there was a region of maximum azimuthal currents (∼20 cm/s) at a radius of ∼12 km. At a radius of ∼17 km, horizontal gradients of properties reached maximum values. This water mass boundary was thought to define the horizontal extent of the water actually traveling with the smeddy, giving it an aspect ratio of 1.5%. The smeddy was observed to move about 150 km southward over a period of ∼30 days (∼6 cm/s), passing from the Tagus Abyssal Plain to the Horseshoe Abyssal Plain. Remote sensing and other hydrographic data are used to suggest one route whereby water found in the core of the isolated eddy reached the Horseshoe Abyssal Plain.
Zonal gradient of the winter sea level atmospheric pressure at 50°N: An indicator of atmospheric forcing of North Pacific surface conditions
Authors: Seckel Gunter R.
Published: February 2008

The Aleutian Low dominates the winter atmospheric circulation of the midlatitude North Pacific. The long‐term average of the lowest value of the winter (December, January, and February) mean sea level pressure, the AL, is located near 50°N, 177°E. Anomalies in the winter atmospheric circulation are primarily reflected in the magnitude and longitude of the lowest sea level pressure and also in the zonal gradient of sea level pressure at 50°N. West of the AL the zonal gradient is negative, indicating a northerly component of the geostrophic wind. East of the AL the zonal gradient is positive, indicating a southerly component of the geostrophic wind. Because of their pertinence to forcing of near‐surface ocean conditions, the zonal gradients at 50°N for the winters of 1947–1990 have been analyzed, and anomalous distributions have been identified. In this series, two distinct gradient patterns can be recognized. In the first pattern, proceeding eastward from the minimum near 140°E, the gradient rises monotonically to its maximum near 140°W. The magnitudes of the northerly and southerly components of the geostrophic winds are at their maximum at 140°E and 140°W longitudes, respectively. In the second group, again proceeding eastward from the minimum near 140°E, the gradient first rises to a secondary maximum, usually east of the AL and then declines to a secondary minimum before rising again to its maximum value near 130°W. In terms of the geostrophic wind, proceeding eastward from the longitude of the AL, the southerly component first increases then decreases and, in extreme cases, may reverse direction to a northerly component before increasing again to its maximum near the eastern boundary of the ocean. Results of model studies point to a possible linkage between patterns of the zonal gradient and tropical disturbances. To gain an appreciation of how an anomalous atmospheric circulation may affect near‐surface ocean conditions, the characteristic features of the zonal pressure gradient are examined and compared to previous studies. These include the effects on surface currents, on the turbulent flux of heat across the sea surface, and on the precipitation over the northwestern United States, British Columbia, and Alaska. The comparisons show that in the absence of direct observations, the zonal gradient of the mean winter (December, January, and February) sea level pressure at 50°N can be used to infer the consequences of an anomalous atmospheric circulation on ocean conditions in the high midlatitudes of the North Pacific.
Global climatologies of solar radiation budgets at the surface and in the atmosphere from 5 years of ERBE data
Authors: Li Zhanqing , Leighton H. G.
Published: February 2008

As a result of recent satellite‐based observation programs, knowledge of the radiation budget at the top of the atmosphere has improved substantially. In comparison, there has been little improvement in knowledge of the radiation budgets at the surface and in the atmosphere. Based on a simple parameterization, global climatologies of the solar radiation budget at the surface and in the atmosphere are developed from 5 years of Earth Radiation Budget Experiment data and European Centre for Medium Range Weather Forecasts humidity data. Both data sets have global coverage on 2.5° × 2.5° grids. Global distributions of the solar radiation budget at the surface give maximum seasonal values of the net solar radiation for the subtropical oceans of more than 300 W m−2. The maximum seasonal absorption in the atmosphere is about 120 W m −2. Shortwave cloud forcing at the surface and in the atmosphere is also derived. Clouds reduce the seasonally averaged surface net solar radiation by up to 175 W m−2, whereas they increase seasonal net solar radiation in the atmosphere by less than 15 W m−2. The globally and annually averaged net solar radiation budgets in the atmosphere and at surface are 83 and 157 W m−2, respectively. Expressed as percentages of the solar radiation incident at the top of the atmosphere, these values correspond to a globally and annually averaged absorption in the atmosphere and at the surface of 24.3% and 46.0%, respectively, and a planetary albedo of 29.7%.
General circulation model sensitivity experiments with pole‐centered supercontinents
Authors: Crowley Thomas J. , Baum Steven K. , Kim Kwang‐Yul
Published: February 2008

Although high CO2 levels appear to be required to explain most times of ice‐free climates in Earth history, previous calculations with a two‐dimensional energy balance model (EBM) have suggested that the low thermal inertia associated with pole‐centered supercontinents might allow seasonally snow‐free conditions to develop in the absence of high CO2. This conclusion has been challenged by other EBM calculations with lower geographic resolution but more explicit model physics. To address this problem more directly we have undertaken a series of general circulation model (GCM) simulations with a new model (GENESIS) developed at the National Center for Atmospheric Research. For idealized pole‐centered supercontinents we obtain mid summer south polar temperatures of 22°C for the present orbital configuration and 15°C with a modified orbital configuration favorable to cool summers. These results support the two‐dimensional EBM calculations. However, their application to real‐world situations of the Paleozoic is hampered by the fact that due to extremely high sea levels and decreased solar luminosity the best time periods for applying this concept (420–360 Ma) have supercontinent areas too small to yield an ice‐free climate for all orbital configurations. Snow‐free conditions with summer temperatures at the pole of 25°C are simulated for a “hot” orbital configuration for one realistic geography configuration (Carboniferous, 300 Ma). These results indicate that while the basic idea of snow‐free, pole‐centered supercontinents is still theoretically possible, the real‐world application to specific past time periods requires some modifications. Our reframed hypothesis states that for realistic geography and high CO2 levels, pole‐edged supercontinents could have permanent snow cover, but pole‐centered supercontinents would be snow‐free in summer.
Heterogeneous chemistry on Antarctic polar stratospheric clouds: A microphysical estimate of the extent of chemical processing
Authors: Drdla K. , Turco R. P. , Elliott S.
Published: February 2008

A detailed model of polar stratospheric clouds (PSCs), which includes nucleation, condensational growth, and sedimentation processes, has been applied to the study of heterogeneous chemical reactions. For the first time, the extent of chemical processing during a polar winter has been estimated for an idealized air parcel in the Antarctic vortex by calculating in detail the rates of heterogeneous reactions on PSC particles. The resulting active chlorine and NOx concentrations at first sunrise are analyzed with respect to their influence upon the Antarctic ozone hole, using a photochemical model. It is found that the species present at sunrise are primarily influenced by the relative values of the heterogeneous reaction rate constants (and thus the “sticking coefficients”) and the initial gas concentrations. However, the extent of chlorine activation is also influenced by whether N2O5 is removed by reaction with HCl or H2O. The reaction of N2O5 with HCl, which occurs rapidly on type 1 PSCs, activates the chlorine contained in the reservoir species HCl. Hence the presence and surface area of type 1 PSCs early in the winter are crucial in determining ozone depletion.
Aerosol size distributions in the cloudy atmospheric boundary layer of the North Atlantic Ocean
Authors: Hegg Dean A. , Ferek Ronald J. , Hobbs Peter V.
Published: February 2008

Measurements are presented of the size distribution and volatility of particles in the cloudy atmospheric boundary layer of the North Atlantic Ocean. The results suggest that particle nucleation of acid sulfates occurs but is neither spatially nor temporally homogeneous. Such nucleation appears to be inversely correlated with preexisting particle surface area. The few measurements obtained in clouds show a different acid sulfate distribution from those in clear air.
Simulations of the trend and annual cycle in stratospheric CO2
Authors: Hall Timothy M. , Prather Michael J.
Published: February 2008

The distribution and evolution of stratospheric CO2 in response to the observed annual cycle, interannual variations, and long‐term trends in tropospheric CO2 is simulated with the GISS 23 layer stratospheric general circulation model. Carbon dioxide is a tracer of stratospheric transport which has essentially no local sources or sinks but still displays gradients due to the forcing at the surface. Consequently, observations of stratospheric CO2, until recently limited to a few flask samples, but now included as high frequency in situ sampling in aircraft campaigns, provide a test of tracer transport in stratospheric simulations independent of model chemistry. In our model, CO2 enters the stratosphere primarily through the tropical tropopause, where air parcels are effectively labeled in time by their CO2 values (although not uniquely because of the cycles in tropospheric concentration). Parcels of differing ages are subsequently mixed in the stratosphere. Only when the growth is purely linear can the CO2 offset in a parcel relative to the troposphere be interpreted as the average time since stratospheric air was last in contact with the troposphere, i.e., the “age” of the stratosphere. Our model is in qualitative agreement with multiyear averages of balloon soundings at northern mid‐ and high latitudes; the stratosphere at 30 km at mid‐latitudes is about 4 years (6 ppm of CO2) behind the troposphere. We predict significant propagation of the CO2 annual cycle into the lower stratosphere, an effect which must be accounted for when interpreting observations. While the annual cycle is negligible above the lower stratosphere, interannual oscillations, such as those associated with El Ninos, can propagate well into the middle stratosphere as positive offsets from the linear trend lasting significantly longer than their duration in the troposphere.
Charged precipitation measurements before the first lightning flash in a thunderstorm
Authors: Marsh Stephen J. , Marshall Thomas C.
Published: February 2008

On August 15, 1984, we made balloon measurements of the electric field and of the charge and vertical velocity of precipitation particles in a thunderstorm over Langmuir Laboratory. Substantial quantities of positively charged precipitation, comprising a lower positive charge center, were collected before the storm's first lightning flash. Possible origins of the positive charge are discussed. Negatively charged precipitation and positively charged cloud particles found higher in the cloud seem to have generated electrical energy in the way suggested in the typical precipitation theory of thunderstorm electrification.
An intercomparison of instrumentation for tropospheric measurements of dimethyl sulfide: Aircraft results for concentrations at the parts‐per‐trillion level
Authors: Gregory Gerald L. , Warren Linda S. , Davis Douglas D. , Andreae Meinrat O. , Bandy Alan R. , Ferek Ronald J. , Johnson James E. , Saltzman Eric S. , Cooper David J.
Published: February 2008

This paper reports results from NASA's Chemical Instrumentation and Test Evaluation (CITE 3) during which airborne measurements of dimethyl sulfide (DMS) from six instruments were intercompared. Represented by the six instruments are three fundamentally different detection principles (flame photometric, mass spectrometric, and electron capture after fluorination); three collection/preconcentration methods (cryogenic, gold wool absorption, and polymer absorbent); and three types of oxidant scrubbers (solid phase alkaline, aqueous reactor, and cotton). The measurements were made over the Atlantic Ocean in August/September 1989 during flights from NASA's Wallops Flight Center, Virginia, and Natal, Brazil. The majority of the intercomparisons are at DMS mixing ratios <50 pptv. Results show that instrument agreement is of the order of a few pptv for mixing ratios <50 pptv and to within about 15% above 50 pptv. Statistically significant (95% confidence) measurement biases were noted among some of the techniques. However, in all cases, any bias is small and within the accuracy of the measurements and prepared DMS standards. Thus, we conclude that the techniques intercompared during CITE 3 provide equally valid measurements of DMS in the range of a few pptv to 100 pptv (upper range of the intercomparisons).
Relative quantum yield of Q(1D2) following ozone photolysis between 221 and 243.5 nm
Authors: Cooper Ian A. , Neill Patrick J. , Wiesenfeld John R.
Published: February 2008

Relative values of the quantum yield, Φ*(λ), of O(1D2) from the photolysis of ozone in the wavelength region 221–243.5 nm have been measured by observing the weak 630‐nm fluorescence from the spin forbidden O(1D2) → O(3P) transition. The relative quantum yield was determined at 243.5, 235, 231, 228.5, and 221.5 nm, with the value of Φ*(221.5 nm) equal to 0.87 ± 0.04 agreeing with a recent O(3P) resonance fluorescence measurement made at 222 nm. Additionally, the results point to a significantly lower quantum yield at short wavelengths than that currently recommended for stratospheric models. The results of this experiment, when placed in context with previous studies, confirm a trend toward a decreasing quantum yield at shorter wavelengths observed in earlier work.
Tropospheric temperature trends in the Arctic: 1958–1986
Authors: Kahl Jonathan D. W. , Serreze Mark C. , Stone Robert S. , Shiotani Spencer , Kisley Michael , Schnell Russell C.
Published: February 2008

Arctic temperature trends in four tropospheric layers during the period 1958–1986 are examined through analysis of a comprehensive archive of Arctic upper air meteorological data. The goals of the study are to describe the trends in Arctic tropospheric temperatures and to provide verification data for model simulations of Arctic and global climate. This analysis extends the work of previous researchers by examining rawinsonde time series from a much denser distribution of stations than was previously available and by resolving the vertical distribution of tropospheric temperatures as well. Absolute trends of 3°C/30yr or larger were found, with both cooling and warming tendencies observed in all layers. The majority of the trends, however, are not statistically significant. Considerable regional and seasonal variability is observed. Trends at many stations in Eurasia and Greenland are highly sensitive to large positive anomalies during the period 1958–1963, which may be artifacts of the data. On the basis of our analysis, we conclude that greenhouse‐induced warming is not detectable in the Arctic troposphere for the 1958–1986 period.
The influence of local sources on the mineral content of bulk deposition over an altitudinal gradient in the Filabres Range (SE Spain)
Authors: Queralt‐Mitjans Ignasi , Domingo Francisco , Sole‐Benet Albert
Published: February 2008

Mineralogical characteristics of bulk deposition have been determined at seven sites along an altitudinal gradient in the southern face of Filabres Range from 1770 m altitude to sea level, over a period of 8 months, the rainiest in this semiarid area. X ray diffraction and scanning electron microscopy were used for identifying and semiquantifying minerals. The aim was twofold: to ascertain the contribution of mineral particles to both the high pH and alkalinity of bulk deposition, the highest in Europe, and to use mineralogy of bulk deposition as a possible fingerprinting technique to identify the source of insoluble particles. The pH, alkalinity, and electrical conductivity of rainwater increase down the altitudinal gradient as precipitation decreases, indicating a clear increase in alkaline particles, in agreement with the presence of calcite, aragonite, dolomite, and gypsum, which also explains high pH and alkalinity of the bulk deposition. Moreover, pH and conductivity of rainwater are somewhat more related to E‐SE winds than to W‐NW ones. In addition to carbonates and gypsum, seven silicate minerals were found. Some minerals, like feldspars, chlorite, and aragonite, are neither related to site nor to rain events. Some others, like quartz, high‐crystalline illite, paragonite, and kaolinite, are only related to rain events. Finally, a few minerals, like low‐crystalline illite, palygorskite, paragonite, and to some extent calcite and gypsum, are site related. Some minerals have been interpreted to come from local sources, e.g., high‐crystalline illite, gypsum, and paragonite; others, like palygorskite and low‐crystalline illite, smaller in size, may come from distant sources.
Chaotic advection in the stratosphere: Implications for the dispersal of chemically perturbed air from the polar vortex
Authors: Pierce R. Bradley , Fairlie T. Duncan A.
Published: February 2008

The Lagrangian evolution of material lines within the northern hemisphere winter stratospheric vortex is determined using isentropic winds and diabatic heating rates obtained from the NASA Langley Research Center (LaRC) atmospheric circulation model. Transient, subtropical anticyclones lead to deformation of the material lines near the edge of the polar vortex which then rapidly evolve into elongated filaments as material is drawn around the anticyclones. The rate of stretching of the material lines is shown to be exponential, with typical e‐folding times of the order of 4 to 8 days. These results provide evidence for “chaotic advection” near the edge of the stratospheric polar vortex which leads to rapid mixing of vortex air with tropical and midlatitude air. The characteristic timescales of these mixing processes and the extent to which the mixing penetrates the polar vortex have important implications for the dispersal of chemically perturbed air from the polar vortex throughout the northern hemisphere and attendant ozone depletion.
Numerical modeling of the turbulent diffusion and chemistry of NOx, O3, isoprene, and other reactive trace gases in and above a forest canopy
Authors: Gao W. , Wesely M. L. , Doskey P. V.
Published: February 2008

A coupled diffusion‐chemistry model was developed for the turbulent transport of reactive trace gases in and above a forest canopy. The one‐dimensional model was used to study daytime vertical profiles of gaseous concentrations and fluxes and the influences of vertical distributions of solar irradiation and uptake and emission at leaves and the forest floor. The upper boundary of the model was extended to the top of the atmospheric boundary layer to allow adequate coupling at the atmosphere‐forest interface. To study the effects of biogenic nonmethane hydrocarbons, chemical reactions for isoprene and its atmospheric oxidation products were linked with reactions for inorganic species and the oxidation of CO and CH4. Isoprene emission rates at various heights in the canopy were calculated as a function of local radiation, temperature, and leaf density distribution. Photolysis rates for photochemical reactions were allowed to vary with height according to the change in solar irradiation in the canopy. Vertical profiles of O3, NO, NO2, NOx, NOy, OH, HNO3, H2O2, and isoprene concentrations and fluxes simulated for a specified deciduous forest were examined with a single set of measured and computed daytime micrometeorological conditions. Results show that for strongly depositing gases like O3, HNO3, and H2O2, deposition velocities appear to be insensitive to chemistry and to have a profile similar to those predicted for a nonreactive case (simulation without chemistry), although the fluxes are influenced by concentration changes caused by chemistry. Simulated profiles of isoprene concentration and flux agree closely with results for the nonreactive case, largely because of the dominant effects of emission and turbulent mixing. Chemical reactions have the most important influence on profiles of NO, NO2, and NOx concentrations and fluxes. With a small and representative NO emission forced at the forest floor, NO concentration decreases quickly with height near the ground and falls to a minimum value near the middle of the canopy because the chemical transformation of NO is fast while photodecomposition of NO2 is weak inside the canopy. As a result, the NO2 concentration becomes higher inside the canopy than above, and an upward NO2 flux occurs near the canopy top despite NO2 deposition in the canopy. The total flux of NOx near the canopy top appears to be downward because of strong downward NO flux. The flux of NOy above the canopy is almost invariant with height as chemical interchanges create no net effect on the total nitrogen flux, although a large flux divergence caused by dry deposition occurs inside the canopy.
Total ozone variations at Reykjavík since 1957
Authors: Bjarnason Gudmundur G. , Rögnvaldsson Örnólfur E. , Sigfússon Thorsteinn I. , Jakobsson Thor , Thorkelsson Bardi
Published: February 2008

Total ozone measurements using a Dobson spectrophotometer have been performed on a regular basis at Reykjavík (64°08′N, 21°54′W), Iceland, since 1957. The data set for the entire period of observations has been critically examined. Due to problems related to the calibration of the instrument the data record of ozone observations is divided into two periods in the following analysis (1957–1977 and 1977–1990). A statistical model was developed to fit the data and estimate long‐term changes in total ozone. The model includes seasonal variations, solar cycle influences, quasi‐biennial oscillation (QBO) effects, and linear trends. Some variants of the model are applied to investigate to what extent the estimated trends depend on the form of the model. Trend analysis of the revised data reveals a statistically significant linear decrease of 0.11 ± 0.07% per year in the annual total ozone amount during the earlier period and 0.30 ± 0.11% during the latter. The annual total ozone decline since 1977 is caused by a 0.47 ± 0.14% decrease per year during the summer with no significant change during the winter or fall. On an annual basis, ozone varies by 3.5 ± 0.8% over a solar cycle and by 2.1 ± 0.6% over a QBO for the whole observation period. The effect of the 11‐year solar cycle is particularly strong in the data during the early months of the year and in the westerly phase of the QBO. The data also suggest a strong response of total ozone to major solar proton events. Comparisons of the total ozone data made at Reykjavík during the last 13 years with total ozone as measured by the total ozone mapping spectrometer (TOMS) on the Nimbus 7 satellite show an excellent agreement for solar zenith angles less than 80° at times of satellite overpass. Comparisons with results of the zonally averaged TOMS data at this latitude suggest a significant longitudinal dependence of the linear trends in stratospheric ozone. This is in accordance with recent analysis of the satellite data.
Nested simulations of perpetual January climate over the Australian region
Authors: McGregor John L. , Walsh Kevin
Published: February 2008

A limited‐area grid point model is nested at resolutions of 250 km and 125 km within a spectral general circulation model for simulations of perpetual January climate over the Australian region. The spectral model is run for 300 days and provides the lateral boundary conditions for the nested model runs. In these experiments the physical parameterizations are kept as similar as possible. The results show that the limited‐area model significantly improves the simulation of January climate, particularly in regions of steep topography. Some problems are described regarding the integration of the two models, including the generation of spurious precipitation near the boundaries of the nested model and limitations of the soil temperature scheme. Problems relating to the applicability of the technique to tropical domains are also discussed. The simulation in the interior of the domain is good; in particular, the simulation of precipitation is significantly improved compared to that of the general circulation model.
The response of the middle atmosphere to long‐term and short‐term solar variability: A two‐dimensional model
Authors: Brasseur Guy
Published: February 2008

A two‐dimensional chemical‐dynamical‐radiative model of the middle atmosphere is used to investigate the potential changes of temperature, ozone, and other chemical constituents in response to variations in the solar ultraviolet flux, associated with the solar rotation (27 days) and the solar cycle (11 years). The model reproduces satisfactorily the response (amplitude and phase) to the 27‐day forcing of ozone and temperature in the stratosphere but does not properly explain the ozone and temperature responses of opposite sign observed near 70 km altitude. The change in the ozone column abundance associated with the 27‐day solar forcing is estimated to be less than 0.5%. Variations in middle and upper atmospheric ozone concentrations and temperatures induced by solar variations on the 11‐year time scale are not negligible compared to changes produced as a result of human activities over the same period of time. The calculated change in the ozone column abundance from solar minimum to solar maximum conditions is of the order of 1.1–1.3% in the tropics and increases with latitude, especially in winter, to reach up to 1.5–1.7% in the polar regions. There are large unexplained differences between the calculated and observed stratospheric responses. For example, in the photochemically controlled region of the upper stratosphere, the model seems to underestimate the ozone response by more than a factor 2. In addition, the negative temperature and ozone responses observed in the lower stratosphere cannot be reproduced by the model. Potential dynamical feedbacks exist, but reliable data sets covering a period longer than one solar cycle have to become available before this problem can adequately be addressed.
Airborne measurements of sulfur dioxide, dimethyl sulfide, carbon disulfide, and carbonyl sulfide by isotope dilution gas chromatography/mass spectrometry
Authors: Bandy Alan R. , Thornton Donald C. , Driedger Arthur R.
Published: February 2008

A gas chromatograph/mass spectrometer is described for determining atmospheric sulfur dioxide, carbon disulfide, dimethyl sulfide, and carbonyl sulfide from aircraft and ship platforms. Isotopically labelled variants of each analyte were used as internal standards to achieve high precision. The lower limit of detection for each species for an integration time of 3 min was 1 pptv for sulfur dioxide and dimethyl sulfide and 0.2 pptv for carbon disulfide and carbonyl sulfide. All four species were simultaneously determined with a sample frequency of one sample per 6 min or greater. When only one or two species were determined, a frequency of one sample per 4 min was achieved. Because a calibration is included in each sample, no separate calibration sequence was needed. Instrument warmup was only a few minutes. The instrument was very robust in field deployments, requiring little maintenance.
Superthermal ions detected in Venus' dayside ionosheath, ionopause, and magnetic barrier regions
Authors: Grebowsky J. M. , Kasprzak W. T. , Hartle R. E. , Mahajan K. K. , Wagner T. C. G.
Published: February 2008

A comprehensive morphological analysis of the dayside superthermal ion presence in the dayside ionopause region of Venus was made using the Orbiter Ion Mass Spectrometer (OIMS) measurements. Superthermal O+ ions with energies ranging from ∼15 to 90 eV relative to the spacecraft are observed on essentially every orbit of Pioneer Venus that penetrates into the dayside ionosphere. There is a strong inbound‐outbound difference in both the number and intensity of the superthermal ions with most events detected on the inbound crossing of the ionopause. This is consistent with a net flow of plasma along the ionopause away from the subsolar region. The ambient energy (including thermal and/or bulk motions) of the detected ions is comparable to the ram energy of cold ions relative to the moving spacecraft. A comparison of the locations of the superthermal O+ ion occurrences with respect to other parameters shows that the superthermals are a characteristic of the ionopause transition separating the essentially stationary ionosphere plasma from the flowing ionosheath plasma as deduced by Taylor et al. (1980b) from individual examples. These superthermal ions are not often detected within the main ionosphere in the midday region. Except in a few rare instances of changing interplanetary magnetic field (IMF) conditions or at locations near the terminator, they are only present at altitudes within or above the magnetic barrier. The OIMS also detects superthermal H+ ions with an inbound outbound asymmetry in the ionosheath consistent with shocked ionosheath proton flows.
Local time asymmetries in the Venus thermosphere
Authors: Alexander M. J. , Stewart A. I. F. , Solomon S. C. , Boucher S. W.
Published: February 2008

The bright emission at 130 nm from atomic oxygen in the Venus thermosphere, as measured by the Pioneer Venus Orbiter Ultraviolet Spectrometer (PVOUVS), can serve as a diagnostic of Venus thermospheric structure. Images from the PVOUVS at this wavelength reveal unusual large‐scale patterns in the emission which can be interpreted as variations in the O/CO2 ratio in the middle thermosphere. Through analysis of these images, a high‐contrast asymmetry in local time has been revealed which has not previously been observed or predicted by any existing models. The asymmetry can be interpreted as typically a factor of 2 increase in oxygen densities at the nighttime terminator over corresponding day times, and this basic pattern has been present in every image examined, spanning the period 1980–1990. The pattern in local time is not present at equatorial latitudes less than ∼30° but appears in both the northern and southern hemispheres poleward of 30°. This local time asymmetry in O can be understood in the context of the subsolar‐to‐antisolar flow pattern which dominates the thermospheric circulation: gravity waves, originating in the middle atmosphere of Venus where a westward zonal flow pattern dominates, will preferentially enhance eddy mixing in the morning hours if these waves propagate into the thermosphere. Enhanced eddy mixing tends to lower thermospheric O. This same mechanism can also explain zonal asymmetries known to be present in the thermospheric subsolar‐to‐antisolar wind pattern (Alexander, 1992).
Variability of [O I] 6300‐Å emission near Io
Authors: Scherb F. , Smyth W. H.
Published: February 2008

Observations of [O I] 6300‐Å emission near Io have been obtained in 1990, 1991, and 1992 by the National Solar Observatory staff using the solar‐stellar spectrograph on the McMath‐Pierce telescope at Kitt Peak. High‐resolution spectra with a resolving power of about 1.2 × 105 were obtained with an integration time of 10–15 min each. The viewing aperture for the observations was 5.2 arc sec × 5.2 arc sec centered on Io, with spatial resolution limited within this area by seeing conditions. Observations thus far have been reduced to obtain average brightness values over the aperture which range from ∼200 to 1000 R for a number of different Io phase angles and Io system III longitudes. The [O I] 6300‐Å emission brightness exhibits an east‐west asymmetry, where the average intensity for Io phase angles in the west (receding ansa) is 1.5 times brighter than in the east (approaching ansa). Similar east‐west intensity ratios have also been observed for neutrals near Io and ions in the plasma torus for a number of other optical and ultraviolet emission lines which are excited by electron impact. In addition to the east‐west asymmetry, the [O I] 6300‐Å emission brightness exhibits a strong dependence on the Io system III longitude angle, with a maximum value occurring in the range 200° ± 50°. Earlier IUE observations of ultraviolet emission lines of O and S near Io obtained over a number of years have measured the east‐west asymmetry, but the long IUE integration times of ∼7–14 hours masked any detection of system III variability. For the [O I] 6300‐Å emission, the O(1D) state may be excited by electron impact of atomic oxygen and by electron impact dissociation of SO. The molecule SO may be present at the exobase or may be produced above the exobase as the dominant product of SO2 dissociation by electron impact. Preliminary assessment indicates that production of O(1D) by molecular dissociation may be more important. The [O I] 6300‐Å emission may therefore provide a remote signature for monitoring (1) the upward transport rates of molecular species in Io's atmosphere, (2) the relative abundance and time‐variable dissociation of SO2 and/or SO at the exobase and in the corona of Io, and (3) the spatial distribution of these escaping molecular and atomic species and their ion production rates in the planetary magnetosphere.
Coastal geomorphology of the Martian northern plains
Authors: Parker Timothy J. , Gorsline Donn S. , Saunders R. Stephen , Pieri David C. , Schneeberger Dale M.
Published: February 2008

The northern lowland plains comprise approximately one third Mars' surface area. Most outflow channels and many valley networks debouch into the lowlands, yet there is little or no morphologic evidence to suggest that channel cutting continued far into the plains, despite a continued basinward regional topographic gradient. The immediate fate of the water discharged from these channels was dependent on the prevailing paleoclimate at the time of its emplacement. Though current models of the martian paleoclimate suggest that mean annual temperatures were likely below freezing throughout most of martian history, geomorphic evidence suggests that coastal erosion on a scale comparable to that of well‐known terrestrial paleolakes occurred. These landforms can be traced to nearly complete closure of the northern plains and appear to require at least two, and perhaps several, highstands of a sea or ocean with temperatures above freezing at least for geologically brief periods of time. The latest highstand may have been as recent as Early Amazonian time. The elevations and areal extent of these landforms provide independent estimates of the martian water budget that can be compared to prevailing models of martian volatile evolution. Estimated volumes of water and sediment discharged by the major channel systems peripheral to the northern plains can be compared to the volume of the basin based on the available topography. Values for the circum‐Chryse outflow channels alone are sufficient to have produced large bodies of standing water within the basin. These estimates may be comparable to the basin volume contained within the younger, least extensive highstand identified. The earlier, more extensive highstand delineates a basin with a much larger implied volume that may require the presence of a semi‐permanent, possibly ice‐covered ocean in the northern plains prior to major channeling events. The northern plains today probably consist of water‐lain sediments interbedded to considerable depths with flood lavas from the major volcanic centers, with sediment comprising most of the present surface.
Magnetic fields of the outer planets
Authors: Connerney J. E. P.
Published: February 2008

It is difficult to imagine a group of planetary dynamos more diverse than those visited by the Pioneer and Voyager spacecraft. The magnetic field of Jupiter is large in magnitude and has a dipole axis within 10° of its rotation axis, comfortably consistent with the paleomagnetic history of the geodynamo. Saturn's remarkable (zonal harmonic) magnetic field has an axis of symmetry that is indistinguishable from its rotation axis (≪1° angular separation); it is also highly antisymmetric with respect to the equator plane. According to one hypothesis, the spin symmetry may arise from the differential rotation of an electrically conducting and stably stratified layer above the dynamo. The magnetic fields of Uranus and Neptune are very much alike, and equally unlike those of the other known magnetized planets. These two planets are characterized by a large dipole tilts (59° and 47°, respectively) and quadrupole moments (Schmidt‐normalized quadrupole/dipole ratio ≈1.0). These properties may be characteristic of dynamo generation in the relatively poorly conducting “ice” interiors of Uranus and Neptune. Characteristics of these planetary magnetic fields are illustrated using contour maps of the field on the planet's surface and discussed in the context of planetary interiors and dynamo generation.
The obliquity of Mars and “climate friction”
Authors: Rubincam David Parry
Published: February 2008

The mean obliquity of Mars may have increased over geologic time due to climatic changes associated with obliquity oscillations; this mechanism is dubbed “climate friction.” Given here is the mathematical theory of climate friction. Also, a 10 m.y. numerical integration of the equations for a hypothetically large amount of climate friction is performed, for the cases of the obliquity oscillations being (1) a single sinusoid and (2) a sum of three sinusoids. Theory and numerics agree in both cases within about 12% on the size of the secular increase in obliquity, with most of the discrepancy coming from the theory giving obliquity amplitudes ∼10% too large. Further, one possible mechanism of climate friction is investigated here: “ postglacial rebound ” on Mars. The idea is that giant polar caps form when the obliquity is low. These caps slowly squeeze out an equatorial bulge. When the obliquity is high, the caps disappear, but the bulge takes some time to collapse, due to mantle viscosity. Thus the equatorial bulge oscillates but is out of phase with the obliquity oscillations. Like tidal friction, this causes a secular increase in the average obliquity. Using the preferred climate model of François et al. (1990), the total secular increase in the obliquity over the age of the solar system can amount to about 10° if Mars' mantle has an Earthlike effective viscosity of 3 ×1021 Pa s (3 ×1022 P). The secular change is negligible for effective viscosities which differ by more than a factor of 10 from this value. Last, how much climate friction tipped the Earth is presently unknown.
Wrinkle ridges of Arcadia Planitia, Mars
Authors: Plescia J. B.
Published: February 2008

Wrinkle ridges of Arcadia Planitia were examined to determine their morphology, spatial distribution, and the amount of crustal shortening and strain they accommodate. Ridges trend generally northward, but their orientation and distribution are strongly controlled by the relief of the underlying knobby material. Ridges begin or end at inselbergs of older terrain and are associated with buried craters. Arcadia Planitia ridges have an average width of 3425 m and accommodate an average folding shortening of 3 m and a faulting shortening of 55 m; mean total shortening is 57 m. Three east‐west transects were constructed at 20° 25°, and 28°N to estimate regional shortening and strain. Average total shortening across the transects is about 900 m, corresponding to a regional compressive strain of 0.06%. The total shortening and compression across Arcadia Planitia are less than in Lunae Planum. Faults associated with the Arcadia ridges are inferred to have a westward dip compared with an eastward dip for Lunae Planum ridges. The generally levels of compression and symmetric orientation of the ridges suggest a regionally organized stress system. Differences in the shortening and strain between Lunae Planum and Arcadia Planitia may reflect the effects of local lithology or azimuthal variations in the stress field.
Photometric models for Galilean satellite astronomy
Authors: Mallama Anthony
Published: February 2008

Models for the Galilean satellites have been used to determine the distribution of brightness on their surfaces. The results indicate that the distance between a satellite's photocenter and its center‐of‐figure can reach 209 km for Io, 221 km for Europa, 303 km for Ganymede, and 255 km for Callisto. Astrometry of the satellites can achieve much greater precision than this, so a correction for the photocentric offset should be applied. This paper presents tables of photocentric offsets for the Galilean satellites as functions of orbital longitude and solar phase angle. The estimated accuracy of these corrections is 32 km for Io, 45 km for Europa, 49 km for Ganymede, and 40 km for Callisto, but the latter two may be optimistic. The satellite ephemerides derived from astrometry are essential for interpreting observations of variable phenomena on Io, such as volcanic activity inferred from IR observations taken during occultations of the satellite. Accurate ephemerides are also critical to spacecraft navigation.
The variability of the Io plasma torus
Authors: Thomas Nicolas
Published: February 2008

A review of observations of Io's plasma torus with particular reference to time variable phenomena is provided. The observed variations can be separated into six different catagories ranging from short timescale (of order 15 min) oscillations of the entire torus to long‐term, possibly aperiodic, changes in the strength of optical and UV emissions. Much of the evidence for variability can be interpreted in several different ways and is, in some cases, contradictory. Similarly, some qualitative descriptions of the processes leading to observed variations have been discussed in the literature but often alternative explanations are also available and without a more quantitative analysis it is impossible to differentiate between the proposed models. In short, there is a clear need for a more quantitative approach to the modelling of Jovian magnetospheric variability in support of dedicated long‐term observational programs.