bluuebunny/crossref_metadata_2025 huggingface dataset:
Blocking of the Kuroshio by the continental shelf northeast of Taiwan
Authors: Hsueh Y. , Chern Ching‐Sheng , Wang Joe
Published: February 2008

In an area immediately northeast of Taiwan where in April 1989 an on‐shelf intrusion of the Kuroshio was observed, hydrographic observations from August 1991 indicate a blocking of the Kuroshio at the continental shelf break. It appears that the Kuroshio approaches the shelf break as a near‐bottom current and is deflected. The deflection apparently gives rise to a countercurrent along the shelf break which was observed by a buoy‐mounted acoustic Doppler current profiler deployed within a month of the hydrographic survey. The on‐shelf hydrographic distribution accompanying the deflection exhibits a field of near‐bottom eddies dominated by a particularly well organized large pool downstream and to the right of the approaching Kuroshio but well separated from the Kuroshio. The hydrographic properties of the pool are similar to those of the uplifted Kuroshio subsurface water observed in April 1989, suggesting that it may be the remnant of the Kuroshio subsurface water that intruded during the previous winter. A theory of the blocking is put forth in terms of the approach of a bottom current toward a step rise of the bottom in a two‐layered, reduced‐gravity ocean model in which the thickness of the current exceeds the rise in topography. The theory predicts a complete blocking that features a flow deflection in front of the step and the presence on the step of a preexisting region of high baroclinic pressure, similar to the observation. The theory also predicts approximately correctly the track of the isotherm that bounds the large pool on the side facing the on‐rushing Kuroshio.
Circulation over the shelf and slope off northern California
Authors: Bray N. A. , Greengrove C. L.
Published: February 2008

Hydrographic observations taken during the Northern California Coastal Circulation Study (NCCCS) are analyzed to examine the structure and variability of density and velocity over the shelf and slope, to a distance of 70 km offshore and a depth of 1500 m. Cross‐shore sections with closely spaced stations were occupied between the Oregon border and San Francisco seven times between March 1987 and October 1989. Historical observations from CaICOFI line 60 off Point Reyes are also analyzed to provide better resolution of seasonal and interannual variability in the region than is available from the NCCCS data. Poleward flow dominates the circulation in this easternmost part of the California Current system, much more so than the historical description would indicate. Because there is substantial poleward shear below 500‐m depth, poleward transports in the top 500 m referred to 1500‐m depth are 2 to 5 times the values referred to 500 m. Equatorward flow occurs on average over the shelf, and in an organized fashion offshore. The shelf flows appear to be the local response to wind‐driven upwelling. The offshore equatorward flow is an energetic, narrow feature that persists for several months at a time and has alongshore structure aligned to the shape of the coastline. It can be identified in one of the NCCCS surveys as the same jet described by the 1988 Coastal Transition Zone experiment observations. Empirical Orthogonal Function analysis indicates that seasonal variability in the top 500 m of the water column generally explains less variance (∼20%) than does interannual variability (∼50%). Between 500 and 1500 m, seasonal fluctuations explain a greater proportion of the variance (∼50%). The shape of the coastline, the coastal mountains, and the bathymetry of the shelf and slope have a significant influence on the circulation in this region. There is enhanced upwelling near the two major capes (Mendocino and Point Arena), as well as substantial increases in poleward transports near both. Flow is directed offshore on the north sides of both features and onshore to the south. The equatorward flowing offshore jet is found closer to shore in the embayment between the two capes and farther offshore at the latitude of the capes.
Particle diffusion in a meandering jet
Authors: Dutkiewicz Stephanie , Griffa Annalisa , Olson Donald B.
Published: February 2008

The process of turbulent mixing across an ideal model of a meandering Gulf Stream is studied considering particle motion in two dimensions. The turbulent motion is modeled using a “random flight” model that assumes that the evolution of the turbulent velocity along trajectories is a Markov process, with the velocity at one time step depending linearly on the velocity at the previous step. This turbulent field is superimposed on a meandering jet (similar to the one considered by Bower (1991)) propagating steadily eastward. In Bower's model the particles are constrained to move along streamlines in the translating frame; in our model the turbulent motion allows the particles to cross streamlines, resulting in an exchange between the different regions of the flow. The major exchange occurs between the “jet core” region and the “recirculating” regions moving with the meanders. Particles launched in the jet core tend to be lost from the jet in plumes at the extrema of the meanders and to be entrained in successive recirculation regions. When in the recirculation regions, particles tend to be trapped and homogenized. The exchange between the jet and the “far field” depends only on diffusion mechanisms and is small for the short integration time considered. An application of the kinematic techniques considers the distribution of biological species across the jet. The tendency for “patches” of organisms to develop in the recirculation regions is observed. In a two‐species case, where the species have affinities for the environment on opposite sides of the jet, there is a linear change in species composition across the jet. Patches forming on either side of the jet consist of an admixture of the two species, with the population for the crest or trough environment dominating.
The annual cycle of the western boundary current in the Gulf of Mexico
Authors: Sturges W.
Published: February 2008

It has been known for decades that the mean circulation in the western Gulf of Mexico is a large anticyclone. This flow is driven both by wind stress curl and detached rings from the Loop Current. This paper is an attempt to examine their relative importance by focusing on the annual cycle of the observed flow. A new compilation of ship drift data shows that the flow along the western boundary is strongest in July and weakest in October. It appears physically plausible that the annual variation of this western boundary current is driven by the annual variation in wind curl augmented by Ekman pumping over the western gulf. Because the maximum in wind curl is so close to the western boundary, the phase delay between the annual cycles of curl and current is only approximately 1 month. The magnitudes of the curl and resulting current seem physically plausible. The response is baroclinic and penetrates to ∼1000 m, the bottom of the main thermocline. The change in velocity at the sea surface is consistent with the cross‐stream pressure gradient as determined by the changes in sea level at the coast and dynamic height changes offshore. The annual cycle is relatively large; at the annual minimum the flow is only approximately 25% of the maximum. The data suggest that this flow could be driven by wind curl, but the uncertainty is high. Consequently, the contribution of Loop Current rings turns out to be relatively small. The reason for this is that when rings reach the western wall, the steep bottom topography intersects the sloping fluid contours of the rings. The rings decay slowly by normal deepwater processes until they reach the edge of the continental shelf, and then they decay more rapidly by loss of fluid. Once the rings have reached the western boundary, the decay time scale is found to be only ∼ 70 days.
Modelling the production of dimethylsulfide during a phytoplankton bloom
Authors: Gabric Albert , Murray Nicholas , Stone Lewi , Kohl Manfred
Published: February 2008

Dimethylsulfide (DMS) is an important sulfur‐containing atmospheric trace gas of marine biogenic origin. DMS emitted from the oceans may be a precursor of tropospheric aerosols and cloud condensation nuclei (CCN), thereby affecting the Earth's radiative balance and possibly constituting a negative feedback to global warming, although this hypothesis is still somewhat controversial. The revised conceptual model of the marine pelagic food web gives a central role to planktonic bacteria. Recent experiments have shown that consumption of dissolved DMS by microbial metabolism may be more important than atmospheric exchange in controlling its concentration in surface waters and hence its ventilation to the atmosphere. In this paper we investigate the effect of the marine food web on cycling of dissolved DMS in surface waters during a phytoplankton bloom episode. A nitrogen‐based flow network simulation model has been used to analyze the relative importance of the various biological and chemical processes involved. The model predictions suggest that the concentration of DMS in marine surface waters is indeed governed by bacterial metabolism. Environmental factors that affect the bacterial compartment are thus likely to have a relatively large influence on dissolved DMS concentrations. The ecological succession is particularly sensitive to the ratio of phytoplankton to bacterial nutrient uptake rates as well the interaction between herbivore food chain and the microbial loop. Importantly for the design of field studies, the model predicts that peak DMS concentrations are achieved during the decline of the phytoplankton bloom with a typical time lag between peak DMS and peak phytoplankton biomass of 1 to 2 days. Significantly, the model predicts a relatively high DMS concentration persisting after the phytoplankton bloom due to excretion from large protozoa and zooplankton, which may be an additional explanation for the lack of correlation between DMS and chlorophyll a field measurements. Comparison of the model predictions has been made with tank algal bloom experiments.
High‐frequency internal waves in the strongly sheared currents of the upper equatorial Pacific: Observations and a simple spectral model
Authors: Boyd Timothy J. , Luther Douglas S. , Knox Robert A. , Hendershott Myrl C.
Published: February 2008

Statistics of high‐frequency (0.2–0.5 cph) fluctuations are derived from moored upper ocean measurements of currents and temperatures at four latitudes spanning the equator along 140°W. Some of the more unusual statistics include (1) nonunity ratios of kinetic energy to potential energy; (2) nonunity ratios of zonal to meridional kinetic energy; (3) nonzero current‐temperature coherence amplitudes, with depth‐dependent phases; and (4) high vertical coherence amplitudes, with approximately 180° phases, between current measurements spanning the thermocline. A simple model of shear‐modified internal waves is employed to gain insight into the causes of the latitudinal variability of the statistics. Much of this variability can be attributed to the vertical advection of significantly different mean vertical shears by a spectrum of internal waves. The statistics also suggest that the spectrum of high‐frequency internal waves in the upper equatorial Pacific differs in important ways from canonical spectral models. The statistics are consistent with a model based on vertical modes which neglect advection by the mean flow, provided the energy in the first mode is much less than (about 0.3 times) that in the spectrum described by Garrett and Munk (1972, 1975, 1979) and Munk (1981) and two to four times as much energy propagates eastward as westward. Some of the statistics are inconsistent with the simple internal wave model examined, possibly indicating contamination by mooring motion.
Ekman drift in homogeneous water
Authors: Krauss W.
Published: February 2008

Measurements made with satellite‐tracked buoys drogued in different layers between the sea surface and 30‐m depth under homogeneous winter conditions in the North Sea allow analysis of the Ekman currents under a large variety of wind conditions. The experiment lasted from November 20, 1991, until February 29, 1992. The first 4 weeks of this period, during which the buoys stayed close together, are used to determine the Ekman stresses. The total current field is a superposition of barotropic currents due to sea level variations and Ekman currents. The classical Ekman theory is not able to describe properly the observed deflection of the currents to the right of the wind direction and their decay with depth. This deflection is 10° near the sea surface and increases to approximately 50° in 25‐m depth. The relation between wind stress and the stress field in the interior of the water is given by a tensor, which describes the rotation and the variation of the stress with increasing depth. The concept of eddy viscosity is applicable, if a viscosity tensor is used to relate stress and vertical shear. The viscosity tensor is a function of the vertical coordinate only and is independent from the wind stress. It shows maximum values in 15‐ to 20‐m depth and may be due to Langmuir circulation cells. Further studies are needed to determine the physics of this tensor. Its magnitude in the interior of the mixed layer exceeds 1000 cgs units. Consequently, Ekman currents are weak and may not be the dominant currents within the mixed layer.
Finding the steady state of a general circulation model through data assimilation: Application to the North Atlantic Ocean
Authors: Marotzke Jochem , Wunsch Carl
Published: February 2008

An attempt is made to find a steady state of a general circulation model consistent with error estimates of the North Atlantic hydrography during 1981–1985 and with wind and thermodynamic boundary conditions. The quadratic misfit of the model state from the data is minimized using the general circulation model together with its adjoint. A state which is steady within limits of estimated observational error is found, but no such state is simultaneously consistent with the observed hydrographic and surface flux fields. Model dynamics are able to sharpen gradients which were overly smooth in the mapped data, producing a meridional overturning cell with a maximum value of about 21 Sv. At equilibrium, the model must produce its own water masses and has a strong tendency to go toward wintertime conditions. The consequence is an estimated model surface temperature systematically lower than in the hydrography (which comes from all seasons). The western boundary current thermocline is also both colder and fresher than the mapped hydrography, and the overall meridional heat transport is low (about 0.6 × 1015 W). The results suggest that the concept of a realistic steady state North Atlantic circulation has reached the end of its utility, that models with realistic property fluxes and divergences must have much higher resolution, and that the open‐ocean boundary conditions must be formulated as control variables.
Tidal and atmospheric forcing of the upper ocean in the Gulf of California: 2. Surface heat flux
Authors: Paden Cynthia A. , Winant Clinton D. , Abbott Mark R.
Published: February 2008

Satellite infrared imagery and coastal meteorological data for March 1984 through February 1985 are used to estimate the net annual surface heat flux for the northern Gulf of California. The average annual surface heat flux for the area north of Guaymas and Santa Rosalia is estimated to be +74 W m−2 for the 1984–1985 time period. This is comparable to the +20–50 W m−2 previously obtained from heat and freshwater transport estimates made with hydrographic surveys from different years and months. The spatial distribution of the net surface heat flux shows a net gain of heat over the whole northern gulf. Except for a local maximum near San Esteban Island, the largest heat gain (+110–120 W m−2) occurs in the Ballenas and Salsipuedes channels, where strong tidal mixing produces anomalously cold sea surface temperatures (SSTs) over much of the year. The lowest heat gain occurs in the Guaymas Basin (+40–50 W m−2), where SSTs are consistently warmer. In the relatively shallow northern basin the net surface heat flux is fairly uniform, with a net annual gain of approximately +70 W m−2. A local minimum in heat gain (approximately +60 W m−2) is observed over the shelf in the northwest, where spring and summer surface temperatures are particularly high. A similar minimum in heat gain over the shelf was observed in a separate study in which historical SSTs and 7 years (1979–1986) of meteorological data from Puerto Peñasco were used to estimate the net surface heat flux for the northern basin. In that study, however, the heat fluxes were higher, with a gain of +100 W m−2 over the shelf and +114 W m−2 in the northern basin. These larger values are directly attributable to the higher humidities in the 1979–1986 study compared to the 1984–1985 satellite study. Significant interannual variations in humidity appear to occur in the northern gulf, with relatively high humidities during El Niño years and low humidities during anti‐El Niño years. High humidities reduce evaporation and the associated latent heat loss, promoting a net annual heat gain. In the northern Gulf of California, however, tidal mixing appears to play a key role in the observed gain of heat. Deep mixing in the island region produces a persistent pool of cold water which is mixed horizontally by the large‐scale circulation, lowering surface temperatures over most of the northern gulf. These cold SSTs decrease evaporation by reducing the saturation vapor pressure of the overlying air. As a result, heat loss is substantially reduced, even when humidities are low. By removing heat from the surface, tidal mixing alters the time scale of air‐sea interaction and reduces or possibly even inhibits the formation of deep water masses via convection. Over climatological timescales, it may be tidal mixing that ultimately maintains the estuarinelike circulation in the northern Gulf of California, differentiating it from the Mediterranean and Red seas, which lose heat to the atmosphere.
Diurnal restratification and turbulence in the oceanic surface mixed layer: 2. Modeling
Authors: Brainerd K. E. , Gregg M. C.
Published: February 2008

We find that daytime restratification of the remnant layer is important to modeling both the decay of convective turbulence during the day and convective deepening the following night. Penetrating solar insolation accounted for about 60% of the observed restratification of the remnant layer. Of the other processes supplying the remaining restratification, we believe lateral advection is the most important and sets a limit on the capabilities of one‐dimensional mixed‐layer models. In the morning, with the end of convective forcing, there was an initial period lasting nearly an hour, similar to the convective time scale, during which ε in the remnant layer remained nearly constant. After that, turbulence in the remnant layer could be modeled in accordance with a balance between ε and the storage term for turbulent kinetic energy (TKE). Energy storage is computed using q2 = (εl/C)2/3, where l = 0.84LO, the Ozmidov scale, matches observed overturning scales during most of the decay. Calculating LO for the observed linearly increasing stratification gives a modified exponential form for the decay. A value of C = 0.04 gives the best fit. This decay lasted about 5 hours, until 〈ε〉 reached ≈ 5 × 10−10 W kg−1. Then for the remainder of the restratification period, dissipation and production of TKE due to vertical shear appeared to be approaching a balance.
Self‐consistency between wind stress, wave spectrum, and wind‐induced wave growth for fully rough air‐sea interface
Authors: Caudal Gérard
Published: February 2008

When the flow at the air‐sea interface is aerodynamically rough (i.e., for wind speeds above 7.5 m s−1, typically), then form drag on the roughness elements accounts for virtually all the stress. Assuming such a situation, we seek a self‐consistent solution for the wind stress in the sense that the wind stress entering the models of sea spectrum and wind‐induced growth rate is constrained to be equal to the wind stress obtained through integration of form drag over the wavenumber space. Using the models by Donelan and Pierson (1987) for both fully developed sea spectrum and short‐scale wind‐induced wave growth rate, we find self‐consistent, firmly constrained solutions with roughness lengths in good agreement with Charnock's formulation (namely, z0 proportional to u*2), with a constant of proportionality which is of a magnitude very close to experimental values reported in the literature. Thus it appears that there is consistency between our knowledge of the surface stress, wind input, and spectral shape. In contrast, when the short‐scale wind‐induced growth rate is replaced by the one proposed by Plant (1982), the roughness length z0 is found to become extremely sensitive to small fluctuations of either wind‐induced growth rate or sea‐wave spectral level. A poorly constrained and highly fluctuating rather than deterministic value of Charnock's coefficient would then be expected at full development. Numerous confirmations of Charnock's relationship performed in the field at or near full development tend to support Donelan and Pierson's approach for wind‐induced growth rate at short scales. The model was also tested in versions in which the viscous drag was tentatively parameterized, and the conclusions reported here were qualitatively unaffected.
A theoretical model of ultraviolet light transmission through Antarctic Sea ice
Authors: Perovich D. K.
Published: February 2008

Much of the region of the Earth most affected by stratospheric ozone depletion is covered by a seasonal or perennial sea ice cover, which is the habitat of a productive and extensive sea ice microbial community. To assess the impact of enhanced incident ultraviolet irradiance on this community, a knowledge of the amount of light transmitted through a sea ice cover is necessary. A two‐stream radiative transfer model is used to estimate the penetration of ultraviolet radiation through Antarctic sea ice. Sea ice optical properties were used as proxies to infer scattering and absorption coefficients at ultraviolet wavelengths. Case studies are reported for sea ice in McMurdo Sound and in the Weddell Sea. Values of spectral transmittance are computed as well as integrated transmitted UV‐B, UV‐A, biologically effective irradiance (BEI), and photosynthetically active radiation (PAR). UV‐B light levels under meter‐thick ice are a few percent of incident values. The presence of a snow cover results in a large decrease in transmitted ultraviolet. Snow and ice ameliorate the biological impact of enhanced levels of incident ultraviolet radiation by reducing the BEI relative to the PAR.
Precambrian climate: The effects of land area and Earth's rotation rate
Authors: Jenkins Gregory. S. , Marshall Hal G. , Kuhn W. R.
Published: February 2008

Rotation rate and land area have changed significantly over the Earth's history. The associated climate changes are examined with a general circulation model (GCM) and compared with the current climate state. Unlike previous GCM studies, clouds in the model interact with the radiation field. We show that rotation rate changes can affect mean air temperatures by strongly affecting the cloud field. For example, an Earth day length of 14 hours causes the global mean cloud fraction to decrease 20%. There are also large changes in the wind field; indeed, at some mid‐latitudes the time‐averaged surface wind changes from westerlies to easterlies. If the land masses are removed from the model and with present‐day rotation, the global mean air temperature is 4 K higher than for the present‐day simulation; if in addition the day length is 14 hours, the global mean air temperature is 5.5 K. Thus the early atmosphere may not have contained the very large concentrations of CO2 thought to be necessary to prevent a frozen planet higher.
Atmospheric NO3: 4. Vertical profiles at middle and polar latitudes at sunrise
Authors: Smith J. P. , Solomon S. , Sanders R. W. , Miller H. L. , Perliski L. M. , Keys J. G. , Schmeltekopf A. L.
Published: February 2008

Ground‐based measurements of NO3 absorption in the band near 662 nm were carried out on four occasions using the Moon as a light source during sunrise at both middle and polar latitudes. As the Sun rose, the observed slant column abundance of atmospheric NO3 decreased systematically. The observed time dependent decrease is due to the progression of the solar terminator down through the atmosphere and provides a basis for inferring the vertical profile of NO3 at sunrise. The inferred profiles are sensitive to the adopted NO3 absorption cross sections and photolysis rates, and these sensitivities are investigated. Two sets of measurements made in Colorado during the summer display a large contribution to the total column from the troposphere, while two data sets obtained during the Antarctic spring demonstrate that the column is almost entirely located in the stratosphere there. The NO3 abundances obtained in Antarctica are much smaller than those measured over Colorado and show that NO3 evolves only at altitudes where the temperature is high enough to allow its formation via the reaction of NO2 and O3.
Environmental factors controlling methane emissions from peatlands in northern Minnesota
Authors: Dise Nancy B. , Gorham Eville , Verry Elon S.
Published: February 2008

Controls on methane emission from peatlands in northern Minnesota were investigated by correlation to environmental variables and by field manipulation. From September 1988 through September 1990, methane flux measurements were made at weekly to monthly intervals at six sites in the Marcell Experimental Forest, northern Minnesota (two open bog sites, two forested bog sites, a poor fen, and a fen lagg). Flux was related to water table position and peat temperature with simple correlations at individual sites and multiple regression on all sites together. The effect of water table was also investigated experimentally in “bog corrals” (open‐ended metal enclosures set in the peat) in which water table was artificially raised to the surface in the driest peatland. Temperature largely controlled variation in flux within individual ecosystems at Marcell, but hydrology distinguished between‐site variation. Water table position, peat temperature, and degree of peat humification explained 91% of the variance in log CH4 flux, predicted annual methane emission from individual wetlands successfully, and predicted the change in flux due to the water table manipulation. Raising the water table in the bog corrals by an average of 6 cm in autumn 1989 and 10 cm in summer 1990 increased emission by 2.5x and 2.2x, respectively. Just as expanding the scale of investigation from a single habitat in a wetland to several wetlands necessitates incorporation of additional variables to explain flux (water table, peat characteristics), modeling flux from several wetland regions, if possible, will require the addition of climate parameters.
Climate evolution in the Pliocene and Pleistocene from marine‐sediment records and simulations: Internal variability versus orbital forcing
Authors: Birchfield G. Edward , Ghil Michael
Published: February 2008

Three deep sea δ18O records are compared in the temporal and spectral domain with each other, a simple ice sheet and bedrock climate model simulation and insolation anomalies at 65°N. The model is one‐dimensional; it represents the accumulation and ablation processes of ice, its viscoplastic flow, and the isostatic adjustment of bedrock under it, being forced solely by orbital insolation anomalies. The objective of the comparison is to determine the importance of internal variability arising from feedback between the ice sheets and bedrock through the late Pliocene and Pleistocene in the simulations and in the climate record. It is found that a considerable part of the spectral variability at suborbital frequencies, i.e., below the precessional and obliquity frequencies, as seen in the sediment cores, is simulated by the model. Simulated variability near 100 kyr has its period determined by the difference between two precessional frequencies. The model's internal variability mechanism has a period near 30–50 kyr but is highly damped. Still, it enhances suborbital variability near 100 kyr and 300 kyr and appears to induce nonstationarity of the orbital and suborbital peaks on even longer time scales. Since the insolation forcing exhibits no noticeable peaks at suborbital or lower frequencies, it appears that the nonstationarity in the model simulations as well as the suborbital peaks themselves are due to coupling between model nonlinearities and the orbital forcing. Since the variability at 100 kyr in the deep sea records is correlated with the simulated variability, such a control by the orbital forcing may be also operating in the climate system. The absence from the simulation of a trend toward larger ice sheets and colder climate seen throughout the Pleistocene in the sediment record is possibly due to slow tectonics and CO2 forcing changes not included in the model.
Hemispheric atmospheric variations and oceanographic impacts associated with katabatic surges across the Ross ice shelf, Antarctica
Authors: Bromwich David H. , Carrasco Jorge F. , Liu Zhong , Tzeng Ren‐Yow
Published: February 2008

Numerical simulations and surface‐based observations show that katabatic winds persistently converge toward and blow across the Siple Coast part of West Antarctica onto the Ross Ice Shelf. About 14% of the time during winter (April to August 1988), thermal infrared satellite images reveal the horizontal propagation of this negatively buoyant katabatic airstream for about 1000 km across the ice shelf to its northwestern edge, a trajectory that nearly parallels the Transantarctic Mountains. This takes place when the pressure field supports such airflow, and is caused by synoptic scale cyclones that decay near and/or over Marie Byrd Land. The northwestward propagation of the katabatic winds is accompanied by other changes in the hemispheric long wave pattern. An upper level ridge develops over Wilkes Land, resulting in an enhancement of the split jet in the Pacific Ocean. Then, more frequent and/or intensified synoptic scale cyclones are steered toward Marie Byrd Land where they become nearly stationary to the northeast of the climatological location. The resulting isobaric configuration accelerates the katabatic winds crossing Siple Coast and supports their horizontal propagation across the Ross Ice Shelf. An immediate impact of this katabatic airflow, that crosses from the ice shelf to the Ross Sea, is expansion of the persistent polynya that is present just to the east of Ross Island. This polynya is a conspicuous feature on passive microwave images of Antarctic sea ice and plays a central role in the salt budget of water masses over the Ross Sea continental shelf. The impact of this katabatic airflow upon mesoscale cyclogenesis over the South Pacific Ocean is also discussed.
A two‐dimensional model of ion composition in the stratosphere: 2. Negative ions
Authors: Beig G. , Walters S. , Brasseur G.
Published: February 2008

A two‐dimensional meridional model of negative ions in the stratosphere is presented. It indicates that NO3− and HSO4− core ions with HNO3 and H2SO4 ligands are the most abundant negative ions in this region of the atmosphere. In accordance with observations at 44°N the model predicts a sharp changeover of HSO4− core ions over NO3− core ions at about 32 km. It also predicts a reverse changeover at 42 km above which NO3− core ions again become dominant. For other latitudes no comparison can be made due to the lack of observational data. The negative ion composition appears to be determined by sulfuric acid vapor and nitric acid concentrations. The sensitivity of the ion composition model to the concentration of these acids is discussed. Finally, the meridional distributions of major negative ions are presented. A companion paper on positive ions is presented by Beig et al. (this issue).
Deduction of emissions of source gases using an objective inversion algorithm and a chemical transport model
Authors: Brown Margaret
Published: February 2008

An inversion algorithm, constructed to deduce the emissions of a source gas required to produce a specified surface concentration, is applied to the observed surface concentrations of CFC 11, methylchloroform, and methane, using a two‐dimensional chemical transport model. The information utilized for this deduction process is limited to the measured atmospheric concentration of the source gas, including the associated standard deviations of these measurements. In this way the amount of objective information available in these measurements is assessed. The algorithm is shown to be capable of producing a latitudinal emissions distribution as well as the error bounds on the deduced emission distribution. The “ill posed” nature of this inverse problem is discussed as well as the implications this has on the spatial and temporal resolution at which emissions can be resolved. Finally, a methane emission distribution is deduced which has the expected seasonal variations and consistent with results from other, more subjective, deduction studies.
Structure and seasonal variability of the nighttime mesospheric Fe layer at midlatitudes
Authors: Kane Timothy J. , Gardner Chester S.
Published: February 2008

Lidar measurements of mesospheric Fe were conducted for 325 h during 75 nights at Urbana, Ill. (40°N, 88°W), in fall 1989 and from spring 1991 through summer 1992. The Fe layer abundance and root‐mean‐square (RMS) width have strong annual variations, with minima in summer. The abundance varied from 3.5 × 109 to 25 × 109 cm−2 with a mean of 10.6 × 109 cm−2, and the RMS width varied from 2.3 to 5.3 km with a mean of 3.4 km. The centroid height of the Fe layer has a strong semiannual variation, with minima at the solstices. The centroid varies from 86.0 to 90.3 km and has a mean of 88.1 km. Sporadic Fe (Fes) layers were present about 27% of the total observation time. The Fe measurements are compared with the extensive Na layer observations obtained during the past decade at Urbana and with common volume observations made simultaneously on 24 nights with a Na temperature lidar. The mean Fe column abundance is approximately twice the mean Na column abundance. The Fe layer centroid height is also on average nearly 4 km lower and the RMS width is approximately 24% narrower than the corresponding Na layer parameters. A chemical model of the mesospheric Fe layer is described and compared to various experimental results. The reaction of Fe with O3 to form FeO on the bottom side of the layer and the subsequent reaction of FeO with CO2 to form FeCO3 appear to be the dominant chemical sinks for Fe. The temperature dependency of the latter reaction may explain the annual variation in the column abundance. The lidar observations and the chemical model calculations suggest that the expected cooling of the mesopause region by approximately 10 K due to the doubling of CO2 and other greenhouse gases during the next century may reduce the mean Fe abundance by as much as 45% and the mean Na abundance by 55%.
Emissions of NO from soil at a rural site in central Tennessee
Authors: Valente Ralph J. , Thornton Frank C.
Published: February 2008

Field measurements of soil emissions of NO from a Mountview silt loam soil with three land uses (forest, fertilized pasture, and fertilized corn) were made on a commercial farm during a summer and autumn sampling period. A new automated closed‐chamber sampling system was developed to allow simultaneous measurements on five chambers per 100 m2 plot. Individual chambers with hinged tops, covering 0.3 m2 of soil area were pneumatically operated via data logger control to sample soil NO flux every third hour. Spatial variability in emission rates was high. For each land use type the range from the lowest to the highest emitting chamber was approximately threefold. Land use type significantly affected soil NO emissions. The fertilized pasture had the highest mean emission rate (44.1 ng N m−2 s−1), followed by the fertilized corn (27.0 ng N m−2 s−1), and the forest (8.4 ng N m−2 s−1). NO emission rates and soil nitrate levels at the forest plot were considerably higher than at other forest sites in the region, possibly due to runoff from an adjacent fertilized hayfield. The results of this study, when extrapolated to a regional estimate, suggest that emissions of NO from soils could play a significant role in summertime tropospheric ozone photochemistry in the southeastern United States.
Simultaneous observations of sporadic E, Na, Fe, and Ca+ layers at Urbana, Illinois: Three case studies
Authors: Gardner Chester S. , Kane Timothy J. , Senft Daniel C. , Qian Jun , Papen George C.
Published: February 2008

We describe three sporadic layering events observed in the mesospheric metal layers above Urbana, Illinois, using resonance fluorescence lidars and an MF radar. On June 6–7, 1991, a sporadic Na layer and a prominent temperature inversion layer were observed near 90 km. The formation of this low‐altitude event appears to involve the release of Na from a chemical reservoir or from the surfaces of dust and smoke particles, perhaps facilitated by the high temperatures in the inversion layer. On December 10, 1991, sporadic Fe and Ca+ layers formed near 94 and 95 km, respectively, and on December 17–18, 1991, sporadic E, Fe, Na, and Ca+ layers formed simultaneously near 100 km. The characteristics of these December events suggest that sporadic Fe and high‐altitude sporadic Na layers are formed by the neutralization of Fe+ and Na+ in the associated sporadic E layers.
Comparison of trends in the tropospheric and stratospheric aerosol optical depths in the Antarctic
Authors: Herber A. , Thomason L. W. , Radionov V. F. , Leiterer U.
Published: February 2008

Temporal variations of the aerosol optical depth of the Antarctic troposphere and stratosphere are considered on the basis of long‐term Sun photometer and actinometer measurements which have been made at Mirny and Georg Forster stations since 1956 and 1988, respectively. This data is supplemented by measurements of the stratospheric aerosol optical depth by the satellite‐borne stratospheric aerosol measurement II instrument. These observations indicate that under undisturbed conditions, the stratospheric aerosol optical depth represents approximately 25% of the total atmospheric aerosol optical depth. The aerosol optical depth in the Antarctic is most notably affected by volcanic eruptions, such as El Chichon in 1982 and Mount Pinatubo and Cerro Hudson in 1991, and by the occurrence of polar stratospheric clouds during Antarctic winter and spring. Apart from these episodic events, no long‐term trend in the aerosol optical depth can be discerned from the nearly 40‐year record.
Effect of Mount Pinatubo aerosols on total ozone measurements from backscatter ultraviolet (BUV) experiments
Authors: Bhartia P. K. , Herman J. , McPeters R. D. , Torres O.
Published: February 2008

Error introduced by Mount Pinatubo aerosols in total ozone derived by the backscatter ultraviolet (BUV) technique is described. BUV instruments include the total ozone mapping spectrometer (TOMS) instrument flying on Nimbus 7 and Meteor 3 satellites and solar backscattered ultraviolet (SBUV 2) instruments on NOAA weather satellites. Radiative transfer calculations show that except at very high solar zenith angles, errors in total ozone derived from the aerosol‐contaminated radiances are less than 2% and vary both in magnitude and in sign with angles of observation. At solar zenith angles greater than 75°, total ozone values may be underestimated by as much as 10% if a large concentration of aerosols is present near the ozone density peak. In subsolar latitudes, error in total ozone derived from TOMS as a function of scan angle is very sensitive to the aerosol size distribution parameters. Aerosol parameters derived from these data agree well with in situ measurements.
In situ nonmethane hydrocarbon measurements on SAGA 3
Authors: Donahue Neil M. , Prinn Ronald G.
Published: February 2008

During the third Soviet‐American Gas and Aerosol (SAGA 3) expedition to the central Pacific in February to March 1990 we observed C1‐C5 nonmethane hydrocarbons (NMHCs) both in the atmosphere and in the ocean. Atmospheric NMHCs project strongly on two factors: those NMHCs with lifetimes over 1 week (ethane, ethyne, propane, cyclopropane) display north‐south latitudinal gradients whose magnitudes are proportional to their photochemical loss rates, which is consistent with a northern hemispheric, continental source, while those NMHCs with lifetimes under 1 week (all alkenes and pentane) do not display latitudinal gradients but do vary in common in a more complicated manner, consistent with a heterogeneous, marine source. Data taken from a sea water equilibrator shows that alkene concentrations vary strongly with wind speed, supporting previous conclusions that sea‐air evasion is a major loss term for mixed layer alkenes. A reasonable balance exists between sea‐air fluxes and atmospheric column removal for some alkenes (ethene, propene, 1‐butene, and 1‐pentene), while the remainder are out of balance, with partial pressures insufficient to support atmospheric observations. The total NMHC sea‐air flux is about 0.8 μmole m−2d−1. Both our air and water concentrations fall near the low end of previously observed ranges. It is argued that some previous higher atmospheric observations may reflect systematic problems with canister sampling. During SAGA 3 the role of NMHCs in the remote atmospheric OH budget was secondary though important, accounting (in conjunction with NMHC oxidation products) for 10 to 20% of all OH removal during baseline periods and a substantially larger fraction during episodes of increased NMHC mixing ratios.
Estimates of momentum flux in the mesosphere and lower thermosphere over Adelaide, Australia, from March 1985 to February 1986
Authors: Murphy D. J. , Vincent R. A.
Published: February 2008

In order to balance the momentum budget of the mesosphere, it is necessary to have a source of momentum in that region to drive the observed meridional circulation. Thus measurements of mesospheric momentum flux are fundamental to our understanding of the dynamics of the mesosphere. The first measurements of momentum flux in this region did not adequately take into account the effect of aspect sensitivity on the effective pointing angle of a radar beam and were therefore incorrect by some factor. In this paper a method of determining the pointing direction of a radar beam from the spatial correlation function of the echoes is presented and applied to a data set that spans a year beginning in March 1985. Momentum fluxes during twelve 4‐day periods thoughout the year are calculated using these improved pointing angle calculations and a new outlier rejection scheme. The results, representing a significant expansion of the worldwide mesospheric momentum flux data set, are then presented.
Methane production from global biomass burning
Authors: Hao Wei Min , Ward Darold E.
Published: February 2008

Emissions of methane from various sources of biomass burning are determined quantitatively for tropical, temperate, and boreal regions. About 85% of the total CH4is emitted in the tropical area, which is mainly the result of shifting cultivation, fuelwood use, and deforestation. Methane emissions from biomass burning may have increased by at least 9% during the last decade because of increases in tropical deforestation and the use of fuelwood. Changes in land use practices and population growth in the tropics are possible causes of the increase of atmospheric CH4concentration.
Temperature dependence of the HNO3 UV absorption cross sections
Authors: Burkholder James B. , Talukdar Ranajit K. , Ravishankara A. R. , Solomon Susan
Published: February 2008

The temperature dependence of the HNO3 absorption cross sections between 240 and 360 K over the wavelength range 195 to 350 nm has been measured using a diode array spectrometer. Absorption cross sections were determined using both (1) absolute pressure measurements at 298 K and (2) a dual absorption cell arrangement, in which the absorption spectrum at various temperatures is measured relative to the room temperature absorption spectrum. The HNO3 absorption spectrum showed a temperature dependence which is weak at short wavelengths but stronger at longer wavelengths which are important for photolysis in the lower stratosphere. The 298 K absorption cross sections were found to be larger than the values currently recommended for atmospheric modeling (DeMore et al., 1992). Our absorption cross section data are critically compared with the previous measurements of both room temperature and temperature‐dependent absorption cross sections. Temperature‐dependent absorption cross sections of HNO3 are recommended for use in atmospheric modeling. These temperature dependent HNO3 absorption cross sections were used in a two‐dimensional dynamical‐photochemical model to demonstrate the effects of the revised absorption cross sections on loss rate of HNO3 and the abundance of NO2 in the stratosphere.
North Atlantic air traffic within the lower stratosphere: Cruising times and corresponding emissions
Authors: Hoinka Klaus P. , Reinhardt Manfred E. , Metz Werner
Published: February 2008

This study estimates cruising times and related pollutant emissions (NOx, CO, HC) and H2O of today's aircraft fleet within the troposphere and stratosphere performed for the North Atlantic region in between 45°N, 65°N, 10°W, and 50°W for the years 1989, 1990, and 1991. The tropopause surface distribution is determined through analysis of assimilated data. Both conventional lapse rate and potential vorticity criteria are employed to determine the location of the tropopause surface. These data combined with air traffic statistics are used to evaluate cruising times within the troposphere and stratosphere separately. The study shows an average of about 44% of the cruising time of the aircraft above the North Atlantic flown within the stratosphere. Based on emission indices of aircraft engines, the emission rates of NOx (in mass units of NO2) into the stratosphere and troposphere in the given region result in 0.26 and 0.33 × 10−12 kg m−2 s−1, respectively.
Potential chlorofluorocarbon replacements: OH reaction rate constants between 250 and 315 K and infrared absorption spectra
Authors: Garland Nancy L. , Medhurst Laura J. , Nelson H. H.
Published: February 2008

We measured the rate constant for reactions of the OH radical with several potential chlorofluorocarbon replacements over the temperature range 251–314 K using laser photolysis laser‐induced fluorescence techniques. The compounds studied and Arrhenius parameters determined from fits to the measured rate constants are as follows: CHF2OCHF2 (E 134), k(T) = (5.4 ± 3.5) × 10−13 cm3 s−1 exp [(−3.1 ± 0.4 kcal mol−1)/RT]; CF3CH2CF3 (FC 236fa), k(T) = (2.0 ± 1.0) × 10−14 cm3 s−1 exp [(−1.8 ± 0.3 kcal mol−1)/RT]; CF3CHFCHF2 (FC 236ea), k(T) = (2.0 ± 0.9) × 10−13 cm3 s−1 exp [(−2.0 ± 0.3 kcal mol−1)/RT]; and CF3CF2CH2F (FC 236cb), k(T)= (2.6 ± 1.6) × 10−13 cm3 s−1 exp [(−2.2 ± 0.4 kcal mol−1)/RT]. The measured activation energies (2–3 kcal mol−1) are consistent with a mechanism of H atom abstraction. The tropospheric lifetimes, estimated from the measured OH reaction rates, and measured integrated infrared absorption cross sections over the range 770 to 1430 cm−1 suggest that E 134 and FC 236fa may have significant global warming potential, while FC 236ea and FC 236cb do not.