bluuebunny/crossref_metadata_2025 huggingface dataset:
Satellite‐tracked drifter measurement of inertial and semidiurnal currents in the northeast Pacific
Authors: Thomson Richard E. , LeBlond Paul H. , Rabinovich Alexander B.
Published: February 2004

We examine current variability in the open northeast Pacific using 3‐hourly sampled trajectories from satellite‐tracked drifters drogued at depths of 15 and 120 m within the wind‐mixed layer and permanent pycnocline, respectively. Findings show the following. (1) Motions were dominated by inertial currents which accounted for 58% (35%) of the total variance measured by the shallow (deep) drifters. Semidiurnal motions accounted for roughly 10% of the variance at both depths. There were detectable currents at twice the inertial frequency (2f ≈ 3 cpd) and negligible diurnal currents. (2) Inertial currents attained root‐mean‐square (rms) speeds of around 50 cm/s in the mixed layer and 10 cm/s in the pycnocline. Speeds in the semidiurnal band were similar (≈10 cm/s) at both depths. (3) Inertial and semidiurnal currents were strongly clockwise rotary, while low‐frequency currents (<1 cpd) were primarily rectilinear. (4) Peak inertial frequencies ωf were at the local inertial frequency ƒ in the mixed layer (ωf/f ≈ 1.0006 ± 0.0022; n =26 estimates) but slightly greater frequencies in the pycnocline (ωf/ƒ ≈ 1.0128 ± 0.0055; n =21). (5) Currents in the inertial and low‐frequency bands were correlated vertically. In the inertial band, maximum horizontal coherency [γ2(ω; Δr) > 0.95] occurred for drifter separations Δr < 75 km. Horizontal coherency diminished for 75 < Δr < 425 km and became marginally significant (γ2 ≈ 0.4) for Δr > 500 km. Semidiurnal currents were uncorrelated beyond several tens of kilometers. (6) Inertial currents appear to be linked to the passage of atmospheric lows, and semidiurnal currents to the passage of remotely generated baroclinic tides. The spectral peak near 2f is indicative of nonlinear inertial wave interactions.
Measured and modeled CFC distribution of lower North Atlantic Deep Water in the Guiana Basin
Authors: Plähn Olaf , Rhein Monika
Published: February 2004

The circulation of the Overflow Lower North Atlantic Deep Water (OLNADW) in the Guiana Basin is studied by the measured and modeled distribution of the chlorofluorocarbon component CFC‐11. The OLNADW is formed in the northern Atlantic and transported south to the equatorial region as the densest part of the Deep Western Boundary Current (DWBC), characterized by CFC and oxygen maxima. Some former studies proposed a northwestward recirculation of the boundary current in the Guiana Basin, because the measured southeastward DWBC transport was larger than the calculated net transport across the entire width of the basin. Despite the variability of the DWBC reported recently, this discrepancy seems to hold. An advective‐diffusive model is presented, which includes recirculation in the northern Guiana Basin as well as horizontal diffusion, and the results are compared with a CFC survey in March 1994. Because of high CFC concentration in the boundary current one would expect to find a high CFC signal in the recirculated water mass near the Mid‐Atlantic Ridge as well. However, this was not observed. Modeled CFC distributions and observations only agree if the presumed recirculation is located in the center of the northern Guiana Basin or if no westward recirculation exists at all. It was found that the strength of the recirculation is about 3±1.5 Sv, which is smaller than previously supposed.
On estimating the atmospheric longwave flux at the ocean surface from ship meteorological reports
Authors: Josey Simon A. , Oakley Daniel , Pascal Robin W.
Published: February 2004

The accuracy to which the atmospheric component of the longwave flux at the ocean surface can be estimated by using simple empirical formulae, with information from standard ship meteorological reports as input, has been tested by comparison with radiometric measurements made on cruises in the midlatitude North Atlantic and the Southern Ocean. Over a wide range of atmospheric conditions the best agreement between the estimated and measured downwelling longwave flux is obtained by using the formula of Clark et al. [1974], which has a mean bias error with respect to our combined data set of −0.7 W/m2. Two other formulae were considered: the Bunker [1976] formula, which is found to overestimate the atmospheric longwave flux at values less than about 350 W/m2, resulting in an overall bias of 9.4 W/m2, and the Bignami et al. [1995] formula, which underestimates the flux over the entire range considered, giving a bias of −26.5W/m2. The difference in performance of the Clark and Bunker formulae is ascribed to the different parameterizations assumed for the effect of clouds and atmospheric humidity on the longwave flux. It is suggested that the Clark formula be used in future climatological studies at midlatitudes.
The West Spitsbergen Current as seen by SOFAR floats during the ARCTEMIZ 88 Experiment: Statistics, differential kinematic properties, and potential vorticity balance
Authors: Richez Claude
Published: February 2004

SOFAR Floats were launched at 300 m depth in the West Spitsbergen Current (WSC) in September 1988. This Lagrangian data set is used to compute statistical parameters; to produce local maps of mean velocity, kinetic energy, and eddy kinetic energy; and to study dispersion and diffusion in the WSC. Despite the limited number of float trajectories, the values obtained for the diffusivities, order 1.0 × 107 cm2 s−1, are close to those found in other regions of the Atlantic Ocean. The Molinari and Kirwan [1975] method is used to compute the differential kinematic properties (divergence, vorticity, shear, and stretching). The conservation of potential vorticity in the region of the Yermak Plateau is not correctly verified; this is mainly due to the jet structure of the WSC and the shape of the cluster, leading to a biased estimate of the relative vorticity. However, the results (based only on five float trajectories over 30 days) are encouraging and not inconsistent with the properties of the West Spitsbergen Current. The existence of a new eastward branch of the WSC, passing just over the top of the Yermak Plateau, is confirmed.
Use of transient features to identify eastward currents in the Southern Ocean
Authors: Hughes C. W. , Jones M. S. , Carnochan S.
Published: February 2004

Sea surface temperatures and sea surface slopes, measured by the ERS‐I satellite, are used to investigate the propagation of mesoscale features in the Southern Ocean. The Antarctic Circumpolar Current (ACC) is clearly identifiable as a region of eastward propagation surrounded by mostly westward propagation. This confirms predictions based on results from the Fine Resolution Antarctic Model, which suggest that the ACC is a region of flow which is supercritical with respect to the propagation of baroclinic Rossby waves. This supercriticality allows the ACC to penetrate deep enough to interact with bottom topography and so may be an important part of the mechanism by which the current achieves a balance of angular momentum. The propagation directions derived from the two independent data sources agree in remarkable detail, and many dynamical features can be identified, including the South Atlantic Current and the Tasman Front. The wavelengths calculated using the two data sets are quite sensitive to details of the processing. While this means they can be said to be “not inconsistent,” the relationship between temperature gradients and slopes demands further investigation. The results on direction of propagation, however, are robust.
Measuring sea surface mean square slope with a 36‐GHz scanning radar altimeter
Authors: Walsh Edward J. , Vandemark Douglas C. , Friehe Carl A. , Burns Sean P. , Khelif Djamal , Swift Robert N. , Scott John F.
Published: February 2004

Values of sea surface mean square slope (mss) computed from the backscattered power falloff with incidence angle measured by an airborne 36‐GHz scanning radar altimeter are presented. When the mss values are plotted against the 10‐m wind speed (extrapolated from the 46‐ to 110‐m aircraft height), for wind speeds up to 7 m s−1 the values scatter about a curve which is scaled from that obtained by Wu [1990] by a factor of 0.85. Wu's result was a reanalysis of the optical measurements of Cox and Munk [1954]. The offset between the microwave and optical curves is reasonable because one would expect the surface to appear smoother to the microwave wavelength than to the optical. This is unlike the mss values inferred from absolute power measurements at nadir by 14‐GHz satellite altimeters, which indicate a higher mss under light wind conditions than the optical measurements indicate. For wind speeds below 7 m s−1 the peak of the backscattered power distribution is observed at nadir, but for higher speeds the peak shifts off‐nadir in the downwind direction.
Simulation of sea ice transport through Fram Strait: Natural variability and sensitivity to forcing
Authors: Harder Markus , Lemke Peter , Hilmer Michael
Published: February 2004

The interannual variability of the sea ice transport through Fram Strait is simulated with a dynamic‐thermodynamic sea ice model. Forcing with daily varying wind fields for the 7‐year period 1986–1992 causes a high variability of sea ice drift on timescales from days to years. Annual means of simulated ice transport through Pram Strait differ up to a factor of 2. Additional sensitivity studies investigate the response of sea ice transports to variations of the prescribed atmospheric and oceanic forcing. Wind speed, ocean current speed, air temperature, and precipitation rate are systematically varied over a wide range. The model predicts an almost linear relation of ice transport with wind speed and ocean current, a strong, nonlinear relation with air temperature, and a rather small sensitivity to changes in precipitation. The results show that the interannual variability of wind forcing causes considerable variations of sea ice export through Fram Strait. The fluxes of freshwater and negative latent heat associated with the sea ice transport can significantly affect the ocean circulation in the Greenland Sea and in the North Atlantic. This shows how variations of the ocean circulation are coupled to the variability of the atmosphere by the mechanism of sea ice advection. To adequately represent these important interactions in the coupled system atmosphere‐cryosphere‐ocean, both the dynamics and the thermodynamics of sea ice must be included in climate models.
Distribution of CO2 species, estimates of net community production, and air‐sea CO2 exchange in the Ross Sea polynya
Authors: Bates Nicholas R. , Hansell Dennis A. , Carlson Craig A. , Gordon Louis I.
Published: February 2004

Measurements of surface total carbon dioxide (TCO2), alkalinity, and calculated pCO2, along with water column nutrients and hydrography, were made on two cruises to the Ross Sea polynya (NBP 94‐6, November‐December 1994 and NBP 95‐8, December 1995 to January 1996). The polynya experiences an intense phytoplankton bloom during a short period of open water conditions from mid‐December to mid‐February each year. Our biogeochemical observations were used to determine the temporal variability of CO2, fluxes of carbon within the ocean, and rates of air‐sea exchange of CO2. Depletions of TCO2, pCO2, and nitrate+nitrite were considerable (∼70–150 μmol kg−1, 80–150 μatm, and 10–20 μmol kg−1, respectively) and associated primarily with biological uptake during Phaeocystis and diatom blooms. Alkalinity was a conservative tracer of salinity and nitrate+nitrite. Surface ρCO2 was undersaturated by ∼50–150 μatm, and air‐sea gas exchange of CO2 during open water conditions was directed from atmosphere to ocean. Observed surface stoichiometric C:N ratios were 6.66: 1 and 6.77:1 for the 2 years, consistent with global “Redfield” ratios, while C:P and N:P ratios were variable (75–141:1, 12–18:1). Estimates of net community production (NCP) rates were made using in situ changes in TCO2 and nitrate+nitrite across repeated transects along 76°30′S. Mean NCP rates across the polynya ranged from 0.86 to 0.98 g C m−2 d−1. These values may be underestimated by 5–25% because of the contribution of atmospheric CO2 to the surface layer through gas exchange. Export of carbon from the surface to depth was at least 55–60% of NCP rates.
Ice thickness in the Arctic Ocean: The statistical reliability of experimental data
Authors: Wadhams Peter
Published: February 2004

The statistical reliability of ice thickness distributions derived from submarine sonar data sets of finite length is examined, focusing on the standard error εH(L) in the mean ice draft H derived from a record of length L km. This is important in assessing the significance or otherwise of quoted temporal or spatial differences in mean ice draft. Estimates for εH(50) are derived from four different submarine surveys, each of which involved repeated sampling of apparently homogeneous ice fields. Values ranged from 7.18 to 15.29% of the mean; for climate‐related comparisons of data sets from different years in the same location a value of 12.75% is recommended, derived from data gathered at the north pole. This value is used to reassess the significance of published data on ice draft variability; quoted variability at the north pole over the 1977–1990 period is found to be not statistically significant, but a difference in area/averaged ice draft over the Eurasian Basin between 1976 and 1987 remains highly significant. Using hitherto unreported 1987 data from the north pole area, the statistical variability of other parameters of the ice cover is examined. The frequency of lead occurrence, the width of leads, the fractional cover of smooth ice, and the frequency of keel occurrence all show standard errors of 22–24% in their mean values, while mean keel draft has a 4% error, all over a 50‐km track length. Finally, unreported 1987 data, for an area extending from the pole to 72°N along the prime meridian and the East Greenland shelf break, are examined in 1° bins, using εH(L) to assess the significance of variations in the mean. A statistically significant decline in mean ice draft is observed, starting at 86%N and continuing to 72%N; a linear regression gives a thinning rate of 0.22 m/deg. The decline within the Arctic Basin itself is tentatively ascribed to a mixture of under‐ice melt and mixing of ice types, since data on young ice occurrence show that divergence did not occur north of Fram Strait.
Flux of particulate organic material from the euphotic zone of oceans: Estimation from phytoplankton biomass
Authors: Legendre Louis
Published: February 2004

A major objective of modem biological oceanography is to quantify, model, and predict the downward flux of biogenic carbon. Previous papers have documented an empirical relationship between the sinking flux of particulate organic material from the euphotic zone (S) and phytoplankton biomass (B), in oceans, and suggested that S could be estimated directly from B. The present paper investigates, using a mathematical model and data from the literature, the relationship between S and B. Calculations show that S is determined mostly by B and also by p (instantaneous coefficient of net phytoplankton production minus euphotic zone heterotrophic community respiration and incorporation in the pelagic food web). The regression S = f(B) accounts for 90% of the variation in S. In remote sensing, B is derived from ocean color, phytoplankton production (P) is derived from B, and S is derived from P. Within that context, the biomass approach could be used as an additional means to constrain the estimates of S.
Role of surface films in ERS SAR signatures of internal waves on the shelf: 1. Short‐period internal waves
Authors: da Silva J. C. B. , Ermakov S. A. , Robinson I. S. , Jeans D. R. G. , Kijashko S. V.
Published: February 2004

Analysis of synthetic aperture radar (SAR) images of internal waves (IWs) on the Iberian shelf and classification of the IW surface signatures is presented. There are three types of IW signatures in the form of bright/dark, dark, and bright bands that correspond to positive/negative, negative, or positive variations of radar backscatter, respectively. Strong positive sign IW signatures occur at very low wind velocities (less than 2m/s). At winds higher than 2 m/s both positive/negative and negative signatures are observed for range‐propagating IWs. For azimuth‐propagating IWs, negative signatures prevail. Two groups of experiments carried out in the frame of the Multidisciplinary Oceanographic Research in the Eastern North Atlantic (MORENA) project on the Iberian shelf in August 1994 are described: (1) IW measurements simultaneous with ERS‐1 SAR overpasses and (2) IW measurements, visual observations of the surface, and sampling of films from the sea surface. IW manifestations at low to moderate winds took the form of slicks locating over IW troughs; at the near‐threshold wind velocity they were of the form of intensified decimeter‐scale waves (antislicks) located over IW crests. Measurements of wave damping due to films collected from the slick and nonslick areas showed the concentration of surfactants and retrieved film elasticity to be higher in the slicks than in the nonslick areas. A theoretical model of the surface wave modulation by IWs is developed to include surface wave straining by the IW current and surface wave damping due to surfactant films. Pressure‐area curves for real marine films are used in the model. The relation between the film and the straining effects on the surface wave spectrum in the IW field is shown to depend on film parameters, surface wave‐length, and wind velocity. The model predicts strong damping of centimeter‐scale waves over IW troughs mainly due to the film effect and intensification and depression of decimeter‐scale waves due to both straining and film effects. Dependencies of the model spectrum variations as a function of wind velocity for range and azimuth propagating IWs are obtained, the negative sign contrast being shown to predominate for azimuth propagating IWs. Model variations of the spectrum of decimeter‐scale waves are shown to be significantly sensitive to surfactants (film pressure), the negative sign contrasts due to films being superimposed on the straining effects, and these are eventually capable of suppressing the positive sign contrasts. It is concluded that the occurrence of the different classes of IW signatures in the SAR images can be explained by the action of surface‐active films.
Relationship between the upper ocean and sea ice during the Antarctic melting season
Authors: Ohshima Kay I. , Yoshida Kazumasa , Shimoda Haruhito , Wakatsuchi Masaaki , Endoh Tatsuo , Fakuchi Mitsuo
Published: February 2004

During the Antarctic ice‐melting season, high‐resolution sea ice data were collected with the video monitoring system aboard the icebreaker Shirase along with the monitoring of temperature and salinity in the upper ocean. On the basis of these data, relationships among sea ice concentration, temperature, and salinity are investigated. In the ice interior region away from the ice‐free ocean, ice concentration is negatively correlated with temperature and positively correlated with salinity for the spatially averaged data, which suggests that the local balances of heat and salt nearly hold in a bulk area. At the ice margin, ice concentration is negatively correlated with both temperature and salinity, suggesting that the local balances are overwhelmed by the effects of ice advection. The expendable bathythermograph profiles at the ice margin also show that a considerable amount of sea ice was advected into the ice‐free ocean and subsequently melted there. It is pointed out that a polynya works as an “ice‐melting factory” in summer; it absorbs solar radiation during the period of opening, and then melts the ice advected there. Prom a heat budget analysis and ocean structure in the melting season, we propose a simple ice‐upper ocean coupled model in which sea ice melts on the bottom and lateral faces with the heat source supplied to the open water area by solar radiation. The relations among ice concentration, temperature, and salinity derived from the model are consistent with the observed relations. The analytic solution for the no lateral melting case shows that the concentration‐temperature relation converges to a certain asymptotic curve with time, which explains that the temperature‐concentration plot shows a similar relation for any region. Dependence of the relations among ice concentration, temperature, and salinity on the spatial scale is also discussed.
Air‐sea exchange of heat in the presence of wind waves and spray
Authors: Makin V. K.
Published: February 2004

Waves extract a considerable part of the surface stress. While breaking, they eject spray into the atmosphere. Spray evaporates and influences a balance of heat and moisture above the waves. A one‐dimensional model of the stratified marine surface boundary layer (MSBL) accounts directly for the impact of waves on the momentum flux and the impact of sea spray on fluxes of heat and moisture. The model is viewed as a higher order parameterization of the MSBL compared to the bulk parameterization. The model is based on the balance equations of momentum, the turbulent kinetic energy and the dissipation rate, heat, and moisture. A general experimental knowledge is used to parameterize the jet droplet concentration above the sea. That is, the surface droplet concentration is proportional to the cube of the friction velocity of the air, and the fast decay of droplet concentration with elevation above waves is parameterized by exponential decay. The exchange coefficients for heat, moisture, and momentum are computed from the wind speed and the sea state. Consistency of the dynamical part of the model is checked against measurements of the drag coefficient. Consistency of the thermodynamical part is checked against measurements of the sensible heat flux for light to moderate winds. The impact of spray is then assessed for stronger winds. It is shown that for a wind speed of about 25 m s−1 and above the impact of sea spray on heat and moisture fluxes becomes significant. The magnitude and the sign of the spray mediated heat and moisture fluxes depend on stratification of the atmosphere. To settle the issue whether or not sea spray plays an important role in exchange of heat and moisture above the sea, simultaneous direct measurements of sensible heat and moisture flux under different stratification conditions at wind speeds of about 25 ms−1 are needed.
VIERS‐1 scatterometer model
Authors: Janssen P. A. E. M. , Wallbrink H. , Calkoen C. J. , van Halsema D. , Oost W. A. , Snoeij P.
Published: February 2004

In this paper a description is given of a physically based theoretical ocean backscatter model (called the VIERS‐1 model) for intermediate incidence angles, and a comparison of its performance against the CMOD4 empirical model is made. The VIERS‐1 scatterometer algorithm is based on a two‐scale composite surface model which includes both specular and Bragg scattering. Its short wave model is based on the energy balance equation and accounts for viscous damping, slicks, dissipation due to whitecapping, and nonlinear three‐ and four‐wave interactions. A number of parameters in the model have been determined by means of laboratory data and analyzed European Centre for Medium‐Range Weather Forecasts (ECMWF) winds. Because of the two‐scale approach the wave number up to which Bragg scattering applies should be determined. This is done by means of laboratory data at X band. In addition, laboratory data of the wave spectrum have been utilized to validate the VIERS‐1 short wave spectrum. An inverse of the algorithm is developed to derive wind speed and direction from the observed (ERS‐1) backscatter and by comparison with ECMWF analyzed winds' three parameters for the short wave spectrum, namely, the Phillips parameter, the directional width of the spectrum, and the wave number boundary between gravity waves and short waves have been obtained. Comparisons between VIERS‐1, C band model, version 4 (CMOD4), and ECMWF analyses are made. VIERS‐1 performs better in the high wind speed range, and this feature is of importance when scatterometer winds are assimilated into an atmospheric model. However, in terms of backscatter rather than wind speed, CMOD4 shows better results. It is suggested that this is caused by the too simple directional distribution of the VIERS‐1 short wave spectrum.
Gravity waves on ice‐covered water
Authors: Keller Joseph B.
Published: February 2004

Gravity waves propagating on the surface of ice‐covered water of finite depth are considered. The ice layer is viewed as a suspension, with an effective viscosity much greater than that of water and a density slightly less than that of water. It is treated as a viscous liquid, and the water beneath it is treated as an inviscid liquid. The linearized motion of gravity waves is analyzed for this two‐layer model, and the dispersion equation is obtained. It is solved numerically for waves of any length. It is also simplified for waves short compared to the layer thickness and for waves long compared to the layer thickness. This equation yields dispersion and strong attenuation, both of which depend upon the effective viscosity of the suspension.
Coupled Ocean‐Atmosphere Response Experiment (COARE): An interim report
Authors: Godfrey J. S. , Houze R. A. , Johnson R. H. , Lukas R. , Redelsperger J.‐L. , Sumi A. , Weller R.
Published: February 2004

This paper is an interim report on results of the Coupled Ocean‐Atmosphere Response Experiment (COARE), conducted from November, 1992, through February, 1993, in the equatorial western Pacific. Early work has emphasized data from single platforms or sensors; more profound, cross‐cutting research, using multiple data sets to address ocean‐atmosphere interaction, is only now yielding results. Three episodes of the intraseasonal oscillation (ISO) occurred during COARE. Atmospheric effects extended well outside the tropics. Each ISO episode was accompanied by numerous deep convective events, loosely organized on lengthscales from 10–10,000 km and timescales from hours to weeks. Similar lengthscales and timescales were evident in all atmospheric variables and in sea surface temperature (SST). We report detailed observations of precipitation and winds from shipborne and airborne radars; atmospheric heat and moisture budgets from sounding arrays; boundary layer properties and fluxes from ships, buoys, and aircraft; ocean heat and freshwater budgets; and numerous ancillary observations. Emphasis has been on achieving accurate heat, momentum, and freshwater fluxes, on documenting the diurnal cycle, on understanding the role of freshwater fluxes in controlling SST, and on obtaining the morphology of typical convective events. Large‐scale context is provided by satellite observations and numerical weather prediction (nwp) output. However, comparison of in‐situ data with nwp products shows systematic discrepancies with winds, scales of convective events, and surface fluxes. Some new parameterizations and early modeling results are reported. Further improvements in modeling the coupled ocean‐atmosphere system are anticipated, based on COARE results.
Path of the North Atlantic Deep Water in the Brazil Basin
Authors: Zangenberg Norbert , Siedler Gerold
Published: February 2004

Recent hydrographic sections and high‐quality historical data sets are used to determine geostrophic currents at subtropical latitudes in the western basin of the South Atlantic. Levels of no motion are determined from water mass information and a mass balance constraint to obtain the transport field of North Atlantic Deep Water (NADW) in this region. The incoming NADW transport of about 20 Sv from the north at 19°S appears to be balanced by only one third of this transport leaving in the south and two thirds leaving to the east or northeast at the Mid‐Atlantic Ridge. A simple model is proposed to determine the cause of the NADW branching. It is shown that potential vorticity preservation in the presence of topographic changes leads to a similar flow pattern as observed, with branching near the Vitória‐Trindade‐Ridge and also an eastward turning of the southward western boundary current at about 28°S, the latitude where a balance of planetary vorticity change and stretching can be expected.
The Columbia River Plume Study: Subtidal variability in the velocity and salinity fields
Authors: Hickey B. M. , Pietrafesa L. J. , Jay D. A. , Boicourt W. C.
Published: February 2004

A comprehensive study of the strongly wind driven midlatitude buoyant plume from the Columbia River, located on the U.S. west coast, demonstrates that the plume has two basic structures during the fall/winter season, namely, a thin (∼5–15 m), strongly stratified plume tending west to northwestward during periods of southward or light northward wind stress and a thicker (∼10–40 m), weakly stratified plume tending northward and hugging the coast during periods of stronger northward stress. The plume and its velocity field respond nearly instantaneously to changes in wind speed or direction, and the wind fluctuations have timescales of 2–10 days. Frictional wind‐driven currents cause the primarily unidirectional flow down the plume axis to veer to the right or left of the axis for northward or southward winds, respectively. Farther downstream, currents turn to parallel rather than cross salinity contours, consistent with a geostrophic balance. In particular, during periods when the plume is separated from the coast, currents tend to flow around the mound of fresher water. At distances exceeding about 20 km from the river mouth, the along‐shelf depth‐averaged flow over the inner to midshelf is linear, and depth‐averaged acceleration is governed to lowest order by the difference between surface and bottom stress alone. In this region, along‐shelf geostrophic buoyancy‐driven currents at ∼5 m (calculated from surface density) and along‐shelf geostrophic wind‐driven currents (computed from a depth‐averaged linear model) are comparable in magnitude (∼10–25 cm s−1).
Evolution of the passive and active microwave signatures of a large sea ice feature during its 2½‐year drift through the Arctic Ocean
Authors: Gohin Francis , Cavanié Alain , Ezraty Robert
Published: February 2004

Although estimation of the total ice concentration from special sensor microwave imagers (SSM/I) has proven to be successful, none of the various algorithms developed to discriminate new and older ice provide satisfying results. While the strong contrast between the emissivity of sea ice and that of open water can be utilized to provide reliable estimators of the total ice concentration, passive microwave characteristics of second‐year and multiyear ice may locally evolve in different ways, even during the cold season. Scatterometers, as the active microwave instrument in wind mode (AMI‐wind) on board the European Remote Sensing Satellites (ERS), provide backscatter data which have a higher sensitivity to the surface topography of ice and a better stability in time, at a resolution compatible with the SSM/I measurements. Here we present the evolutions of the microwave properties of an ice feature appearing along the shores of Novosibirskiye Ostrova (New Siberian Islands) at the end of July 1992 as the ice ages during its 3‐year drift toward the Fram Strait. The track of this well‐defined ice surface is easily followed on the maps of the backscatter coefficient provided by the AMI‐wind during the cold season. In summer, because of melting, the ice undergoes critical changes which alter its microwave signatures and hamper automatic tracking. Moreover, on approaching the Fram Strait the resolution of the scatterometer is not sufficient to capture the complex and rapid transformations of the ice cover. To compensate for this, buoy data obtained from the International Arctic Buoy Program are used, alone during summers or together with satellite data, to build basin‐wide ice displacement fields. These displacement fields, successively applied to each pixel of the ice feature selected, provide a series of Lagrangian observations. During the drift, which ends in May 1995, the active and passive signatures evolve coherently, except for the cold season 1992–1993 when unrealistic multiyear ice concentrations are deduced from the brightness temperatures, which, at that time, are much less stable than the backscatter coefficient over the ice surface tracked, identified as second‐year ice.
A Pacific hydrographic section at 88°W: Water‐property distribution
Authors: Tsuchiya Mizuki , Talley Lynne D.
Published: February 2004

Full‐depth conductivity‐temperature‐depth (CTD)/hydrographic measurements with high horizontal and vertical resolution were made in February‐April 1993 along a line lying at a nominal longitude of 88°W and extending from southern Chile (54°S) to Guatemala (14°N). It crossed five major deep basins (Southeast Pacific, Chile, Peru, Panama, and Guatemala basins) east of the East Pacific Rise. Vertical sections of potential temperature, salinity, potential density, oxygen, silica, phosphate, nitrate, and nitrite are presented to illustrate the structure of the entire water column. Some features of interest found in the sections are described, and an attempt is made to interpret them in terms of the isopycnal property distributions associated with the large‐scale ocean circulation. These features include: various near‐surface waters observed in the tropical and subtropical regions and the fronts that mark the boundaries of these waters; the possible importance of salt fingering to the downward salt transfer from the high‐salinity subtropical water; a shallow thermostad (pycnostad) developed at 16°–18.5°C in the subtropical water; low‐salinity surface water in the subantarctic zone west of southern Chile; large domains of extremely low oxygen in the subpycnocline layer on both sides of the equator and a secondary nitrite maximum associated with a nitrate minimum in these low‐oxygen domains; high‐salinity, low‐oxygen, high‐nutrient subpycnocline water that is carried poleward along the eastern boundary by the Peru‐Chile Undercurrent; the Subantarctic Mode and Antarctic Intermediate waters; middepth isopycnal property extrema observed at the crest of the Sala y Gomez Ridge; influences of the North Pacific and the North Atlantic upon deep waters along the section; and the characteristics and sources of the bottom waters in the five deep basins along the section.
Spaceborne measurements of the upper stratospheric HCL vertical distribution in early 1992 and the trend in total stratospheric chlorine since 1985
Authors: Achard V. , De Mazière M. , Camy‐Peyret C. , Karcher F. , Lippens C.
Published: February 2004

The GRILLE infrared spectrometer was part of the shuttle pay load during the first ATLAS mission in March‐April 1992. This experiment measured the vertical distribution of several important minor constituents in the middle to high atmosphere by solar occultation mid‐infrared absorption spectroscopy. Among the molecules observed is the stratospheric chlorine reservoir species HCl. This paper discusses the vertical profiles measured, including their validation with respect to correlative measurements from ATMOS on board the same mission and from HALOE on board UARS. The most important conclusion drawn from the measured HCl volume mixing ratio of (3.6±0.2) ppbv above 50 km presently measured by GRILLE in comparison with published ATMOS data from 1985 is that the actual GRILLE data confirm the increase of the upper stratospheric total chlorine loading by about 40% with respect to 1985, as reported and/or predicted earlier.
Charge transfer by precipitation between thundercloud and ground
Authors: Soula S. , Chauzy S.
Published: February 2004

Simultaneous measurements of precipitation current, electric field at ground level and 44 m above, and rain rate have been performed during summer experiments in southwestern France. The total precipitation current density was measured with a new instrument consisting of an electrometer connected to a plane conductor surface shielded by a grounded grid. Thus several characteristics of the precipitation current below thunderstorms have been pointed out: (1) The precipitation current can have both polarities, and the polarity can change during the same event. However, it is more often positive, that is, negative raindrops are dominant. (2) The current density reaches 5 nA m−2 in absolute value. (3) The beginning of the detection of a precipitation current generally corresponds to a reverse of the surface electric field or a decrease of the field 44 m above ground. (4) During most of the charged rain periods we observe the mirror image relation between surface field and precipitation; that is, their polarities are opposite. (5) The precipitation seems to carry the charge of the cloud that was dominant in the creation of the field close to the ground before the rain. When this charge reaches the ground, the field decreases at each level and it reverses at the ground. (6) The precipitation current detected at the ground is low compared with that usually measured at cloud base. The Wilson capture across the charge layer above ground seems to be efficient by limiting the precipitation current density. So the total precipitation current at the ground evaluated from our data and for an average thunderstorm is weak; however, because of the Wilson capture, it is not possible to conclude that the charge transfer by precipitation between thundercloud and ground is not important.
Implications of large scale shifts in tropospheric NOx levels in the remote tropical Pacific
Authors: Crawford J. H. , Davis D. D. , Chen G. , Bradshaw J. , Sandholm S. , Kondo Y. , Merrill J. , Liu S. , Browell E. , Gregory G. , Anderson B. , Sachse G. , Barrick J. , Blake D. , Talbot R. , Pueschel R.
Published: February 2004

A major observation recorded during NASA's western Pacific Exploratory Mission (PEM‐West B) was the large shift in tropical NO levels as a function of geographical location. High‐altitude NO levels exceeding 100 pptv were observed during portions of tropical flights 5–8, while values almost never exceeded 20 pptv during tropical flights 9 and 10. The geographical regions encompassing these two flight groupings are here labeled “high” and “low” NOx regimes. A comparison of these two regimes, based on back trajectories and chemical tracers, suggests that air parcels in both were strongly influenced by deep convection. The low NOx regime appears to have been predominantly impacted by marine convection, whereas the high NOx regime shows evidence of having been more influenced by deep convection over a continental land mass. DMSP satellite observations point strongly toward lightning as the major source of NOx in the latter regime. Photochemical ozone formation in the high NOx regime exceeded that for low NOx by factors of 2 to 6, whereas O3 destruction in the low NOx regime exceeded that for high NOx by factors of up to 3. Taking the tropopause height to be 17 km, estimates of the net photochemical effect on the O3 column revealed that the high NOx regime led to a small net production. By contrast, the low NOx regime was shown to destroy O3 at the rate of 3.4% per day. One proposed mechanism for off‐setting this projected large deficit would involve the transport of O3 rich midlatitude air into the tropics. Alternatively, it is suggested that O3 within the tropics may be overall near self‐sustaining with respect to photochemical activity. This scenario would require that some tropical regions, unsampled at the time of PEM‐B, display significant net column O3 production, leading to an overall balanced budget for the “greater” tropical Pacific basin. Details concerning the chemical nature of such regimes are discussed.
Aerosol climate effects: Local radiative forcing and column closure experiments
Authors: Russell P. B. , Kinne S. A. , Bergstrom R. W.
Published: February 2004

In an effort to reduce uncertainties in climate change predictions, experiments are being planned and conducted to measure anthropogenic aerosol properties and effects, including effects on radiative fields. The global average, direct anthropogenic aerosol effect on up welling shortwave fluxes is estimated to be about +1/2 W m−2, whereas errors in flux changes measured with airborne and spaceborne radiometers are 2 to 8 W m−2 or larger. This poses the question of whether flux changes expected in field experiments will be large enough to measure accurately. This paper obtains a new expression for the aerosol‐induced change in upwelling flux, compares it to two‐stream and adding‐doubling (AD) results, and uses all three methods to estimate expected flux changes. The new expression accounts for the solar zenith angle dependences of aerosol transmission and reflection, as well as of surface albedo, all of which can have a strong effect in determining flux changes measured in field experiments. Despite its relative simplicity, the new expression gives results similar to previous two‐stream results. Relative to AD results, it agrees within a few watts per square meter for the intermediate solar elevation angles where the flux changes peak (roughly 10° to 30°), but it has negative errors for higher Sun and positive errors for lower Sun. All three techniques yield aerosol‐induced changes in upwelling flux of +8 to +50 W m−2 for aerosol midvisible optical depths of 0.1 to 0.5. Because such aerosol optical depths occur frequently off the U.S. and European Atlantic coasts in summer, the flux changes they induce should be measurable by airborne, and possibly by spaceborne, radiometers, provided sufficient care is taken in experiment design (including measurements to separate aerosol radiative effects from those of absorbing gases). The expected flux changes are about 15 to 100 times larger than the global average flux change expected for the global average anthropogenic sulfate optical depth of 0.04. Reasons for the larger flux changes include the larger optical depths considered here (factor 2.5 to 12), plus restricting the measurements to cloud‐free, daytime conditions over the ocean (factor 5 to 9).
Behavior of trace gas mixing ratios on a very tall tower in North Carolina
Authors: Hurst Dale F. , Bakwin Peter S. , Myers Richard C. , Elkins James W.
Published: February 2004

We present a 15‐month record of mixing ratios of CO, CH4, N2O, and eight halogenated gases (CCl3F, CCl2F2, CCl2FCClF2, CH3CCl3, CCl4, CHCl3, C2Cl4, and SF6) at a rural site in eastern North Carolina. The data result from hourly gas Chromatographic analyses of air sampled at three heights on a 610‐m‐tall telecommunications tower during November 1994 through January 1996. At night, most of these gases were more abundant near the ground (51 m) than aloft (496 m) because of the buildup of local and regional surface emissions in the shallow nocturnal stable layer. The abundance and variability of trace gases at this continental site were generally higher than those at similar latitude remote locations. Mixing ratios of most gases were well correlated in polluted air masses occasionally advected to the tower. Frequent, strong enhancements in CHCl3 at the lower sampling level(s) indicate a local point source(s) of this gas that is not associated with combustion. Temporal trends of regional background mixing ratios at this continental site are, for the most part, in good agreement with recent trends of remote background mixing ratios in the northern hemisphere.
Heterogeneous interaction of peroxyacetyl nitrate with liquid sulfuric acid
Authors: Zhang Renyi , Leu Ming‐Taun
Published: February 2004

The uptake of peroxyacetyl nitrate (PAN) on liquid sulfuric acid surfaces has been investigated using a fast‐flow reactor coupled to a chemical ionization mass spectrometer. PAN was observed to be reversibly adsorbed on sulfuric acid. From the time‐dependent uptake, Henry's law solubility constant of PAN in sulfuric acid was obtained. The measured solubility constant was found to depend strongly on temperature (199–226 K), but weakly on acid composition (46–72 wt%). On 72 wt% H2SO4, for example, the effective Henry's law solubility constant was determined to be about 5.6×104 and 3.1×103 M atm−1 at 208 and 222 K, respectively. Heterogeneous reactions of PAN with HCl, Cl, ClO, and OClO on liquid sulfuric acid were also investigated and found to be very slow (γ<1×10−4). Atmospheric implications of the present results are discussed.
Atmospheric transport and photochemistry of ozone over central Southern Africa during the Southern Africa Fire‐Atmosphere Research Initiative
Authors: Tyson P. D. , Garstang M. , Thompson A. M. , D'Abreton P. , Diab R. D. , Browell E. V.
Published: February 2004

Vertically integrated back and forward trajectories for the 300–200, 700–500 and surface‐800 hPa levels are calculated using Pretoria as point of origin for the Southern Africa Fire‐Atmosphere Research Initiative (SAFARI) period September‐October 1992. The transport fields are then combined to show both horizontal and vertical transport of air to and from Pretoria at the different levels. Air transport patterns in the vertical are linked to the occurrence of absolutely stable layers which are also evident in the 16 ozonesonde profiles recorded at Pretoria during SAFARI. The coherence of the stratification based on dynamical and ozone analysis permits the use of mean ozone profiles with air volume fluxes to interpret the ozone in terms of photochemistry and transport within stable layers. Extensive recirculation across the meridional plane at Pretoria implies that advection of ozone is slow and that photochemistry is responsible for the observed vertical structure over central southern Africa in September and October 1992. Requisite ozone formation rates are supported by model analysis of ozone and ozone precursors measured from SAFARI and Transport and Atmospheric Research Chemistry near the Equator‐Atlantic aircraft.
Profiles of photosynthetically active radiation, nitrogen and photosynthetic capacity in the boreal forest: Implications for scaling from leaf to canopy
Authors: Dang Qing Lai , Margolis Hank A. , Sy Mikailou , Coyea Marie R. , Collatz G. James , Walthall Charles L.
Published: February 2004

Profiles of photosynthetically active radiation (PAR), leaf nitrogen per unit leaf area (Narea), and photosynthetic capacity (Amax) were measured in an aspen, two jack pine, and two black spruce stands in the BOREAS northern study area. Nareadecreased with decreasing %PAR in each stand, in all conifer stands combined (r=0.52) and in all stands combined (r=0.46). Understory alder had higher Nareafor similar %PAR than did aspen early in the growing season.Amaxdecreased with decreasing Narea, except for the negative correlation between NareaandAmaxduring shoot flush for jack pine. For the middle and late growing season data, NareaandAmaxhadrvalues of 0.51 for all stands combined and 0.60 for all conifer stands combined. For similar Nareathe aspen stand had higherAmaxthan did the conifer stands. Photosynthetic capacity expressed as a percentage ofAmaxat the top of the canopy (%Amax0) decreased with %PAR similarly in all stands, but %Amax0decreased at a much slower rate than did %PAR. To demonstrate the implications of the vertical distribution ofAmax, three different assumptions were used to scale leafAmaxto the canopy (Acan‐max): (1) constantAmaxwith canopy depth, (2)Amaxscaled proportionally to %PAR, and (3) a linear relationship betweenAmaxand cumulative leaf area index derived from our data. The first and third methods resulted in similarAcan‐max; the second was much lower. All methods resulted in linear correlations between normalized difference vegetation indices measured from a helicopter and Acan‐max(r=0.97, 0.93, and 0.97, respectively), but the slope was strongly influenced by the scaling method.