bluuebunny/crossref_metadata_2025 huggingface dataset:
On the temporal variability of the transport through Drake Passage
Authors: Meredith Michael P. , Vassie John M. , Heywood Karen J. , Spencer Robert
Published: February 2004

A recently obtained series of bottom pressure measurements from various positions in the Drake Passage (DP) has been used to estimate the transport variability of the Antarctic Circumpolar Current (ACC) under the assumption that the variability is predominantly barotropic and thus proportional to the north‐minus‐south pressure difference. The standard deviations in derived transport range from 5.3 Sv in 1993 to 8.9 Sv in 1990 (10‐day filter; 1 Sv = 106 m3 s−1). All values are less than the 10 Sv (10‐day filter) obtained during the International Southern Ocean Studies (ISOS) program at DP between the mid 1970s and early 1980s [Whitworth and Peterson, 1985]. Although some of this discrepancy is due to differences in procedure in dealing with gaps in the data, our data suggest that the ACC transport can be persistently less variable over several years than was previously thought. In particular, the ISOS data set contains two large‐amplitude changes in pressure difference over timescales less than 2 weeks, while no similar events were observed in the more recent data. Although the more recent pressure gauges were deployed at different positions to those used in ISOS, their reliability as indicators of ACC transport changes has been established by examination of simultaneous pressure measurements within DP.
Basin‐scale variability in the Labrador Sea from TOPEX/POSEIDON and Geosat altimeter data
Authors: Han Guoqi , Ikeda Moto
Published: February 2004

The TOPEX/POSEIDON altimeter data and the Geosat altimeter data from the Exact Repeat Mission (ERM) have been analyzed to show the basin‐scale features of annual sea surface height anomalies in the Labrador Sea. A complex empirical orthogonal function (CEOF) analysis is used to extract spatial and temporal patterns of altimetric sea surface height anomalies. The analysis of TOPEX/POSEIDON data has revealed that the first eigenmode has an annual variation with amplitudes of ∼5 cm, positive in summer and negative in winter. The Geosat data analysis implies similar results but shows only the sea surface height variabilities relative to those of the subtropical gyre circulation and the North Atlantic Current due to an orbit error correction. The steric height anomalies of the climatological monthly‐mean Levitus data and the sea surface height anomalies of a wind‐driven barotropic model are also analyzed using the CEOF technique. The annual cycle in the Levitus data, dominated by thermal expansion, has an amplitude of ∼4 cm and is nearly in phase with the TOPEX/POSEIDON data. The wind‐driven annual signal is approximately in phase with the TOPEX/POSEIDON and Levitus data, but its amplitude is less than ∼1 cm. A correlation analysis suggests that the basin‐scale features deduced from TOPEX/POSEIDON data are dominated by the steric height variability of the Levitus data, supplemented to much less extent by the wind‐driven response of the barotropic model. The Geosat results are found to represent the steric effect only. It is modified by the orbit error correction so much that the phase changes by 180°.
Temporal variability of the Subarctic Front near the Charlie‐Gibbs Fracture Zone
Authors: Belkin Igor M. , Levitus Sydney
Published: February 2004

We document temporal variability of the northernmost branch of the North Atlantic Current, known as the North Subarctic Front (NSAF), for the period 1976–1985. The front's location and temperature‐salinity (T‐S) parameters were determined from 120 repeat transects along three standard sections occupied by Russian ships: (1) ocean weather station (OWS) “C” (52.75°N, 35.5°W) to St. John's, Newfoundland; (2) along 35°W, from 52°N to 36.5°N; and (3) OWS C to Cape St. Vincent, Portugal. The NSAF exhibits significant interannual variability of its location, shifting 200 km to 300 km in a few months. Along the OWS C‐St. John's section, the front reached its southernmost location in 1976–1977, then experienced two 250‐to‐300‐km northward shifts, in 1978–1979 and 1980–1981; all of the three shifts peaked in wintertime. These events nearly coincide with events of the opposite sign along the OWS C‐Cape St. Vincent section (the northernmost location in 1976–1977 and two 200‐km southward shifts, in 1978–1979 and 1980–1981). The NSAF's behavior is thus coherent near the Charlie‐Gibbs Fracture Zone at the 500‐km scale. Frontal time series (associated with the moving front) of T and S at the 200‐m level for the cold side of the NSAF reveal two events: an S200 minimum in the winter of 1976–1977, which might be a manifestation of the “Great Salinity Anomaly” of the 1970s, and a sharp drop of S200 in early 1984, which might signal the arrival of another large salinity anomaly, the “Great Salinity Anomaly” of the 1980s.
Seasonal patterns of surface wind stress and heat flux over the Southern California Bight
Authors: Winant Clinton D. , Dorman Clive E.
Published: February 2004

Patterns of wind stress and heat flux between the atmosphere and the ocean over the Southern California Bight are described based on observations from buoys and ships. During the winter, the wind stress is spatially homogeneous and temporally variable, with strong events corresponding to low‐pressure systems sweeping through the area. During the summer, spatial patterns are more persistent, with large gradients. Inshore of a line running approximately between Point Conception and Ensenada, Mexico, winds are weak. Offshore wind speeds are comparable in magnitude to those found over the continental shelf north of Point Conception. The boundary is the location of maximum wind stress curl, and the spatial resolution afforded by California Cooperative Fisheries Investigation (CalCOFI) observations suggests maximum wind stress curls over 3 times larger than the values proposed by Nelson [1977]. Net heat flux estimates derived from the CalCOFI measurements are somewhat larger than the values proposed by Nelson and Husby [1983], due to differences in latent heat flux estimates. Possible mechanisms responsible for the spring‐summer spatial structure in the wind and the relationship between these gradients and the properties of the underlying ocean are discussed.
Surface and radiative characteristics of the summer Arctic sea ice cover from multisensor satellite observations
Authors: Comiso Josefino C. , Kwok Ron
Published: February 2004

Accurate quantification and characterization of the Arctic summer ice cover are needed for mass balance, heat flux, and modeling studies in the region. A general assessment of the state and basic characteristics of the ice cover can best be done in summer because it is when the perennial component is fully revealed. The main source of summer ice information has been passive microwave and to a lesser degree active microwave data. However, the emissivity and backscatter of sea ice are abnormal and difficult to resolve during this time period, causing large uncertainties in the interpretation of satellite data. In this study we examined the state of the sea ice cover by using special scanning microwave imager (SSM/I), synthetic aperture radar (SAR), and advanced very high resolution radiometer (AVHRR) satellite data synergistically. The surface and radiative characteristics of the summer ice cover were evaluated in the context of three special events: onset of melt, melt ponding, and freeze‐up. These events affect the emissivity and backscatter and may alter the albedo and ice structure. Onset of melt is readily detectable and is shown to migrate rapidly to the north in June. Melt ponding is not directly observable but is postulated to be the main cause of the decreases in brightness temperatures and large discrepancies between the SSM/I and SAR ice concentration results in many areas. In these areas, SAR and AVHRR results show concentrations near 100%, while the SSM/I data were as low as 70%. During freeze‐up the ice signatures are still quite different from those of midwinter ice, but the ice concentrations from SSM/I generally agree well with those from SAR data. Our results show that, generally, the average ice concentration within the pack is usually greater than 90% during the summer, which is substantially larger than that inferred previously from passive microwave data. The use of combined SAR and SSM/I data may also provide melt‐ponding fraction and first‐order estimate of albedo in the Arctic region.
Mean flow generation along a sloping region in a rotating homogeneous fluid
Authors: Zhang Xiuzhang , Boyer Don L. , Pérenne Nicolas , Renouard Dominique P.
Published: February 2004

The mean flow generated by an oscillatory (tidal) current normal to a long sloping region of constant cross section and finite height in a rotating homogeneous fluid is investigated analytically and by laboratory experiments. The geometry is characterized by constant fluid depths hs and hd in the shallow (shelf) and deep regions, respectively, with the sloping region between being smooth and having a characteristic width L. This physical system is characterized by the temporal Rossby number Rot = ω/f, the Ekman number E, the geometric parameters hs/hd and hs/L, and the normalized tidal excursion as/L (or, alternatively, the Rossby number); here ω is the forcing frequency of the oscillatory current, f is the Coriolis parameter, and as is the characteristic tidal excursion. The analysis assumes Rot ∼ O(1), E « 1, hs/L « O(1), (hd − hs)/hd ∼ O(1) (i.e., finite amplitude topography), and as/L ∼ O(1) (i.e., finite amplitude tidal excursions). The analysis is based upon a spectral method [Zimmerman, 1978] and predicts mean flows along the depth contours of the topography with the shallow region on the right, facing downstream, for vertically upward, or northern hemisphere, rotation. The theoretical approaches used by other investigators using different frictional parameterizations for infinitesimal tidal excursions are also presented. The laboratory experiments were conducted in circular rotating test cells of 1.8 and 13.0 m diameters in which continuous annular topographies of constant cross section were installed along the tank peripheries. The various theories were in qualitative support of the laboratory observations with respect to (1) mean flow direction, (2) location of most intense currents, and (3) dependence on the principal system parameters. Owing to the very small velocities associated with the mean flows generated and the inherent errors of the experiments, quantitative comparison could not be conclusive. Predictions from the various theories give reasonable estimates of such quantities as mean Lagrangian displacements and Eulerian surface transports.
Scatterometer data interpretation: Estimation and validation of the transfer function CMOD4
Authors: Stoffelen Ad , Anderson David
Published: February 2004

In this paper we estimate the 18 coefficients of the CMOD4 σ0‐to‐wind transfer function using a maximum likelihood estimation (MLE) method in order to improve the prelaunch function. We show that a MLE method has to be used with caution when dealing with a nonlinear relationship or with measurement errors that depend on the measured values. In the transfer function estimation it is crucial to use the components of the wind, rather than wind speed and direction, to use σ0 in logarithmic units rather than physical ones, and to use well‐sampled input data. In Stoffelen and Anderson [1997a] we showed that the triplets of measured backscatter are very coherent and, when plotted in a three‐dimensional measurement space, they lie on a well‐defined conical surface. Here we propose a strategy for validation of a transfer function, the first step of which is to test the ability of a transfer function to fit this conical surface. We derive an objective measure to compute the average fit of the transfer function surface to the distribution of measured σ0 triplets. The transfer function CMOD4, derived in the first part of this paper, is shown to fit the cone surface to within the observed scatter normal to the cone, i.e., within roughly 0.2 dB, equivalent to a root‐mean‐square wind vector error of ∼0.5 m s−1 The second step in the validation strategy is the verification of retrieved scatterometer winds at each position on the cone surface. Scatterometer winds computed from CMOD4 compare better to the European Centre for Medium‐Range Weather Forecasts model winds than real‐time conventional surface wind data (ship, buoy, or island reports) with the root‐mean‐square wind vector difference typically 3.0 m s−1. This surprising result can be explained by the so‐called representativeness error. We further show that no significant spatial wind error correlation is present in scatterometer data and therefore conclude that the ERS 1 scatterometer provides winds useful for weather forecasting and climate studies.
Internal waves and tides in the western Weddell Sea: Observations from Ice Station Weddell
Authors: Levine Murray D. , Padman Laurie , Muench Robin D. , Morison James H.
Published: February 2004

The upper ocean current and temperature fields in the western Weddell Sea were measured from the drifting pack ice at Ice Station Weddell 1 (ISW) and nearby sites using a vertical profiler and an array of moored sensors in January–June 1992. These data document the structure and variability of the internal gravity wave field and tidal currents in this remote region. The variance of the internal wave continuum (ƒ < frequency < N) at ISW was 0.2–0.6 of the Garrett‐Munk (GM) universal level for the first 60 days, increasing to near GM levels during the final 10 days of the deployment. In contrast, the energy density at site C, 50 km west of ISW and farther up the continental slope, was always near GM levels. Variations may be due to a combination of spatial and temporal gradients of the internal wave field. At ISW, coherence between vertically separated sensors was used to estimate vertical wave number bandwidth. Energy and bandwidth estimates are compared with previous studies in both ice‐covered and temperate oceans. Using our measurements of the internal wave field and existing parameterizations of mixing, we estimate the vertical heat flux from the Warm Deep Water toward the surface. At ISW the upward heat flux due to mixing associated with the internal waves was about 1 W m−2, much less than the 20 W m−2 average flux required to balance the heat budget for the Weddell Gyre. Tidal currents contributed significantly to the total measured horizontal velocity variance. The tides were primarily barotropic and increased toward the west in both the semidiurnal and diurnal frequency bands. It is suggested that the stronger tidal currents to the west, over the shallower water of the upper continental slope, are indirectly responsible for the higher internal wave energy at site C relative to ISW.
A hypothesis concerning the peak in film drop production as a function of bubble size
Authors: Spiel Donald E.
Published: February 2004

A hypothesis is advanced to explain the peak in film drop production as a function of bubble size observed by Blanchard and Syzdek [1988]. Evidence is presented that for bubble diameters less than about 3 mm, bubble films do not disintegrate but roll up, forming a toroidal ring that gains mass as it advances toward the film edge. It is suggested that film drops are created when this rolled‐up mass “hits” the edge of the film and hypothesized that the peak in production arises because of a competition between an increase in the mass of the rolled‐up toroid and a decrease in its speed as bubble size increases. In this scenario the location of the break in the film is an important parameter. Measurements of this break point are provided. The theory provides an estimate of the bubble size dependence of the number and size of the film drops. The predicted number of droplets per bubble is less and the droplet sizes are larger than the values reported by Blanchard and Syzdek. The theory uses one adjustable parameter that fits the onset of film drop production observed by Blanchard and Syzdek and, in so doing, glosses over the complexities of the toroid impact with the film edge. The possibility that some “film” drops are perhaps, in reality, jet drops generated by bubbles created by film drop impacts with the water's surface is advanced.
Near‐boundary mixing above the flanks of a midlatitude seamount
Authors: Toole John M. , Schmitt Raymond W. , Polzin Kurt L. , Kunze Eric
Published: February 2004

Fine‐scale velocity and density profile data with concurrent turbulent velocity and temperature dissipation estimates obtained above the flanks of Fieberling Guyot, a seamount in the eastern North Pacific Ocean, are examined for evidence of near‐bottom boundary mixing. Fine‐scale shear and strain spectral levels were elevated over the flanks of the seamount in a 500‐m‐thick stratified layer above the bottom. The velocity shear was horizontally isotropic, clockwise and counterclockwise‐with‐depth shear spectral levels were comparable, and no significant correlation between shear and strain was observed. Above the steepest bottom slopes near the seamount summit rim, excess vertical strain relative to shear was observed (as compared to the background internal wave field), suggesting the presence of high‐frequency internal waves. These signals may have been the product of wave reflections from the steep flanks of the seamount and/or wave generation from tidal currents flowing over the rough bottom. Associated with the enhanced shears and strains were more frequent occurrences of low 10‐m Richardson number events, increased overturning scales, and larger estimated turbulent eddy diffusivity relative to observations 15 km or more from the seamount. In particular, turbulent diffusivity estimates increased from O(0.1×10−4 m2 s−1) in the ocean interior to 1–5×10−4 m2 s−1 within 500 m vertically (1–3 km horizontally) of the seamount flank. A simple geometric scaling argument suggests that boundary mixing of this intensity has relevance to the large‐scale circulation at abyssal depths where a large fraction of the ocean waters is in close proximity to the bottom.
A laboratory study of the effect of frost flowers on C band radar backscatter from sea ice
Authors: Nghiem S. V. , Martin S. , Perovich D. K. , Kwok R. , Drucker R. , Gow A. J.
Published: February 2004

C band images of Arctic sea ice taken by the ERS 1 synthetic aperture radar show transitory regions of enhanced radar backscatter from young sea ice. Published field observations associate this increase with frost flower growth and the capture of blowing snow by the flowers. To investigate the first part of this phenomenon, we carried out a laboratory experiment on the response of C band radar backscatter to frost flowers growing on the surface of newly formed saline ice. The experiment took place in a 5 m by 7 m by 1.2 m deep saline water pool located in a two‐story indoor refrigerated facility at the Cold Regions Research and Engineering Laboratory. Sodium chloride ice was grown in this pool at an air temperature of −28°C. The frost flowers first appeared on the ice surface as dendrites and then changed to needles as the ice sheet grew thicker and the surface temperatures became colder. The frost flowers reached to a height of 10–15 mm, and beneath each cluster of frost flowers a slush layer formed to a thickness of approximately 4 mm. Far‐field radar measurements of the backscatter from the ice were made at incident angles from 20° to 40° and at approximately 6‐hour intervals throughout the 3‐day period of the experiment. A backscatter minimum occurred early in the flower growth at the time coincident with an abrupt doubling in the ice surface salinity. Once the full flower coverage was achieved, we removed first the crystal flowers and then the slush layer from the ice surface. The results for these cases show that the crystals have little impact on the backscatter, while the underlying slush patches yield a backscatter increase of 3–5 dB over that of bare ice. The laboratory results suggest that this relative backscatter increase of approximately 5 dB can be used as an index to mark the full areal coverage of frost flowers.
Direct velocity measurements in the Samoan Passage
Authors: Rudnick Daniel L.
Published: February 2004

An array of six current meter moorings was deployed in the Samoan Passage (10°S, 170°W) from September 1992 to February 1994 with the goal of determining the northward abyssal transport. The 17‐month mean transport beneath 4000 m was 6.0 Sv (106 m3 s−1). The time series were low passed at a period of 100 hours to remove near‐inertial and tidal variability, and the resulting transport had a standard deviation of 1.5 Sv, a minimum of 1.1 Sv, and a maximum of 10.7 Sv. The dominant low‐frequency variability in transport and area‐averaged temperature was at a period of 30 days, where the temperature led the transport as would be expected if the heat balance were advective. The temporal and spatial variability suggested that the mean transport is known to within a 1 standard deviation error of 0.5 Sv. The along‐passage flow was intensified to the west and toward the bottom as might be expected in a rotating geostrophic system. Three pathways through the sill region were identified on the basis of bathymetry and current direction. The flow was strongly controlled by topography as indicated by the flow's orientation and directional steadiness. The 30‐day variability in transport might be described as a resonance of the northward abyssal flow.
Nitrous oxide in the Bellingshausen Sea and Drake Passage
Authors: Rees A. P. , Owens N. J. P. , Upstill‐Goddard R. C.
Published: February 2004

Concentrations of dissolved and atmospheric nitrous oxide, N2O, were measured in the austral spring of 1992 in the Drake Passage and Bellingshausen Sea as part of the United Kingdom Joint Global Ocean Flux Study expedition to the Southern Ocean. The measured atmospheric mixing ratio was 313±5 parts per billion by volume, in agreement with the hemispherically corrected global mean. In the Drake Passage, surface N2O saturations were generally very close to atmospheric equilibrium, 99.7±3%, although several anomalous points were associated with the presence of frontal and eddy‐like features within the Antarctic Polar Frontal Zone and at the Continental Water Boundary. Further to the south, a series of oceanographic stations and two surface transects along the 85°W meridian between 65.28°S and 70.32°S revealed a transition from undersaturated conditions in open water to oversaturated conditions in the marginal ice zone, in the upper mixed layer (75–100 m). These observations reflect upwelling of Circumpolar Deep Water at approximately 70°S, resulting in the accumulation of N2O under the winter sea ice and its subsequent release to the atmosphere following the ice retreat. Sea‐air N2O fluxes were estimated from the product of the surface N2O anomaly and the modeled gas transfer coefficients of Liss and Merlivat [1986] and Wanninkhof [1992] to find a maximum rate of +3.1 μmol N2O m−2 d−1. North of the upwelling region, Antarctic Surface Water formed from the mixing of surface waters and ice melt was moderately depleted in N2O with respect to the atmosphere, a minimum 90% of saturation. This sink area was estimated to extend between 65.28°S and 69.57°S with a mean sea‐air flux of between −0.6±0.4 and −0.9±0.7 μmol N2O m−2 d−1. The region studied at 85°W (65.28°S to 70.32°S) revealed source and sink areas which were largely determined by the changing physical hydrography, so that overall there was a small net negative flux of between −0.06±0.9 and −0.09±1.4 μmol N2O m−2 d−1.
Electromagnetic fields generated by a three dimensional global ocean circulation
Authors: Tyler Robert H. , Mysak Lawrence A. , Oberhuber Josef M.
Published: February 2004

A simplified form of the motional induction equation is used to calculate the dominant three‐dimensional (3‐D) electromagnetic (EM) fields generated by a specified steady 3‐D global ocean circulation. The EM calculations require, at most, vertical integrations and do not require running a global 3‐D model. Two cases for ocean bottom conductivity are considered: an electrically insulating ocean bottom and a high‐conductance sediment layer. The approximations are discussed, and the solutions are plotted for various depth levels. Many aspects of the dominant ocean‐generated EM fields (particularly the electric currents near the sea surface and the magnetic fields) are shown to be insensitive to ocean bottom conductivity. Other aspects (particularly the horizontal electric field in shallow water) are very sensitive. We perform a global integration to estimate the role of the “nonlocal” electric currents. We find that the importance in including these nonlocal currents when making EM field estimates is the same or less than that for including a model for the bottom conductance. Hence the simple EM estimates from one‐dimensional integrations are not improved in globally integrated models until these models include a realistic model for bottom conductivity.
Buoyancy‐driven plumes in rotating, stratified cross flows: Plume dependence on rotation, turbulent mixing, and cross‐flow strength
Authors: Lavelle J. W.
Published: February 2004

A three‐dimensional time‐dependent convection model is used to describe circulation and property fields in rotating, stratified, and moving fluids near a point source of heat. The study context and, consequently, model scales are those of chronically discharging hydrothermal vent fields found at submarine ridge crests. Hydrothermal plumes having distinctive thermal and chemical anomalies have been observed to rise several hundreds of meters above the deep‐sea floor before being advected away by background cross flows typically of magnitude 1–4 cm s−1. The model is used to study effects of rotation and indicate differences in plumes with respect to variation of subgrid‐scale turbulence intensity and cross‐flow strength. Counterrotating vorticity (ζ) couplets in all three coordinate directions develop in the lower plume stem at startup and follow the plume to the level of neutral buoyancy; for a nonrotational case (Ω=0), ζ patterns resemble those previously found for jets injected into homogenous cross flow. Ambient fluid entrainment into the convecting column is primarily from the upstream side, but deflection of background flow around both sides of the rising column is the root of the relative vorticity (ζz) couplet in the lower plume. Turbulence intensity within the buoyant region of the plume and/or globally controls smoothness and temporal variability of distal nonbuoyant plume distributions, allowing or preventing oscillations of potential temperature, θ, for example, at background buoyancy frequency, N. Over the range of turbulent mixing studied, rise height of plumes did not change appreciably, but breadth of plumes, counterintuitively, increased for decreasing turbulent mixing strength. Increasing cross‐flow strength, U0, bends model plumes such that rise height ∝ U0−0.4. For the two largest values of cross flow, for which R, the ratio of maximum vertical velocity to U0, took values of 2.8 and 1.0, plumes showed evidence of bifurcation.
Inferring coastal upwelling dynamics from a thermistor array
Authors: Kelley Dan E. , Bourque Marie‐Claude
Published: February 2004

Key parameters of coastal upwelling dynamics in the St. Lawrence estuary are inferred from moored thermistor observations, using a data assimilative technique based on a dynamical model that has terms representing upwelling, vertical mixing, and residual heating/advection. The inferred time series of upwelling velocity, w(t), agrees generally with a time series calculated from a more conventional regression calculation based on the upwelling term alone. In each case, w(t) is highly coherent with alongshore wind stress, indicating that coastal upwelling dynamics are predominant in this region. We found data assimilation to be superior to the simpler regression technique in two ways. First, the inferred upwelling time series is more accurate, as gauged by coherence with wind stress. Second, data assimilation allows the additional inference of other dynamical parameters, such as the vertical diffusivity. The thermistor array thus indicates that the dynamics of this region are largely related to Ekman upwelling, with vertical mixing also being important.
A four‐dimensional analysis of the thermal structure in the Gulf of Lion
Authors: Gaillard Fabienne , Desaubies Yves , Send Uwe , Schott Fritz
Published: February 2004

The Theoretical and Experimental Tomography in the Sea Experiment (THETIS 1) took place in the Gulf of Lion to observe the evolution of the temperature field and the process of deep convection during the 1991–1992 winter. The temperature measurements consist of moored sensors, conductivity‐temperature‐depth and expendable bathythermograph surveys, and acoustic tomography. Because of this diverse data set and since the field evolves rather fast, the analysis uses a unified framework, based on estimation theory and implementing a Kaiman filter. The resolution and the errors associated with the model are systematically estimated. Temperature is a good tracer of water masses. The time‐evolving three‐dimensional view of the field resulting from the analysis shows the details of the three classical convection phases: preconditioning, vigourous convection, and relaxation. In all phases, there is strong spatial nonuniformity, with mesoscale activity, short timescales, and sporadic evidence of advective events (surface capping, intrusions of Levantine Intermediate Water (LIW)). Deep convection, reaching 1500 m, was observed in late February; by late April the field had not yet returned to its initial conditions (strong deficit of LIW). Comparison with available atmospheric flux data shows that advection acts to delay the occurence of convection and confirms the essential role of buoyancy fluxes. For this winter, the deep mixing results in an injection of anomalously warm water (ΔT≈0.03°) to a depth of 1500 m, compatible with the deep warming previously reported.
A three‐dimensional model of diurnal and semidiurnal tides on the European shelf
Authors: Davies Alan M. , Kwong Simon C. M. , Flather Roger A.
Published: February 2004

A three‐dimensional hydrodynamic model of the continental shelf is used to examine the sensitivity of computed diurnal (K1 and O1) and semidiurnal (M2, S2, and N2) tidal elevations and currents to changes in eddy viscosity profile, friction coefficient, and water depth. The influence of the tide‐generating potential is also considered. The model uses a standard finite difference grid in the horizontal, with a functional approach in the vertical, thereby giving a continuous tidal current profile from sea surface to seabed. The magnitude of the vertical eddy viscosity depends upon the flow field. Comparisons are made between model‐computed profiles and harmonic analyses of 278 current time series, at a range of locations in varying water depths, with particular emphasis on locations where there are current measurements at a number of points in the vertical. A detailed examination of these comparisons shows that frictional effects in the near‐bed region are particularly important in determining near‐bed shear and that an accurate specification of water depth is critical for tidal current magnitude. The spatial distribution of computed current ellipses shows that the semidiurnal currents are strongest in shallow water, with the diurnal currents exhibiting an intensification in the shelf edge region. The inclusion of the equilibrium tide is particularly important for the correct determination of the magnitude of the K1 tidal current as well as K1 elevations in the North Sea. The detailed comparisons show that the magnitude of the tidal currents is correctly determined to within ±5 cm s−1 at 220 locations, with the model accurately (to within ±2 cm s−1) reproducing the vertical variation (the most sensitive test of a three‐dimensional model) of the tidal current at most locations.
Temperature, accumulation, and ice sheet elevation in central Greenland through the last deglacial transition
Authors: Cuffey Kurt M. , Clow Gary D.
Published: February 2004

We present a combined heat‐ and ice‐flow model, constrained by measurements of temperature in the Greenland Ice Sheet Project 2 (GISP2) borehole and by the GISP2 δ18O record and depth‐age scale, which determines a history of temperature, accumulation rate, and ice sheet elevation for the past 50,000 years in central Greenland. Important results are: that the temperature increase from average glacial to Holocene conditions was large, approximately 15°C, with a 20°C warming from late glacial to Holocene; that the average accumulation rate during the last glacial maximum (between 15 and 30 kyr B. P.) was 5.5 to 7 cm yr−1, approximately 25% of the modern accumulation rate; that long‐term (500–1000 years) averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the Holocene; and that the Greenland Ice Sheet probably thickened during the deglacial transition. The inverse correlation of accumulation rate and temperature in the mid and late Holocene suggests that the Greenland Ice Sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature. The elevation history of the ice sheet is poorly constrained by the model, and independent evidence is needed. We also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins. This was approximately 1900 years for central Greenland during deglaciation.
Nighttime formation of N2O5 inferred from the Halogen Occulatation Experiment sunset/sunrise NOx ratios
Authors: Nevison C. D. , Solomon S. , Russell J. M.
Published: February 2004

The sunset/sunrise NOx ratios measured in 1994 by the Halogen Occultation Experiment (HALOE) onboard the Upper Atmosphere Research Satellite (UARS) peak at up to 2.8 in the tropics around 30 km and decrease to 1 at the stratopause. These ratios are interpreted by using a fully diurnal one‐dimensional model, which considers gas phase reactive nitrogen chemistry, and a two‐dimensional model, which includes a more complete set of chemical reactions. The diurnal cycle of nighttime N2O5 formation followed by daytime photolysis entirely accounts for the observed ratios above about 35 km. In the lower to middle stratosphere, additional reactions involving ClONO2 and HNO3 also appear to contribute to the observed diurnal variations in NOx. The limitations of a simple function of ozone, temperature, and length of night, which estimates diurnal variations in NOx based on gas phase reactive nitrogen chemistry, are tested through comparison to one‐dimensional model results. The function approximates the one‐dimensional model sunset/sunrise NOx ratios well in the lower stratosphere but overestimates them by up to 10% in the upper stratosphere above about 35 km, corresponding to a large overestimate of N2O5 formation. The function is extended to estimate sunrise N2O5 in the middle stratosphere.
Decadal evolution of the Antarctic ozone hole
Authors: Jiang Yibo , Yung Yuk L. , Zurek Richard W.
Published: February 2004

Ozone column amounts obtained by the total ozone mapping spectrometer (TOMS) in the southern polar region are analyzed during late austral winter and spring (days 240–300) for 1980–1991 using area‐mapping techniques and area‐weighted vortex averages. The vortex here is defined using the −50 PVU (1 PVU = 1.0 × 10−6 K kg−1 m2 s−1) contour on the 500 K isentropic surface. The principal results are: (1) there is a distinct change after 1985 in the vortex‐averaged column ozone depletion rate during September and October, the period of maximum ozone loss, and (2) the vortex‐averaged column ozone in late August (day 240) has dropped by 70 Dobson units (DU) in a decade due to the loss in the dark and the dilution effect. The mean ozone depletion rate in the vortex between day 240 and the day of minimum vortex‐averaged ozone is about 1 DU d−1 at the beginning of the decade, increasing to about 1.8 DU d−1 by 1985, and then apparently saturating thereafter. The vortex‐average column ozone during September and October has declined at the rate of 11.3 DU yr−1 (3.8%) from 1980 to 1987 (90 DU over 8 years) and at a smaller rate of 2 DU yr−1 (0.9%) from 1987 to 1991 (10 DU over 5 years, excluding the anomalous year 1988). We interpret the year‐to‐year trend in the ozone depletion rate during the earlier part of the decade as due to the rise of anthropogenic chlorine in the atmosphere. The slower trend at the end of the decade indicates saturation of ozone depletion in the vortex interior, in that chlorine amounts in the mid‐1980s were already sufficiently high to deplete most of the ozone in air within the isolated regions of the lower‐stratospheric polar vortex. In subsequent years, increases in stratospheric chlorine may have enhanced wintertime chemical loss of ozone in the south polar vortex even before major losses during the Antarctic spring.
Interaction of peroxynitric acid with solid H2O ice
Authors: Li Zhuangjie , Friedl Randall R. , Moore Steven B. , Sander Stanley P.
Published: February 2004

The uptake of peroxynitric acid (PNA), HO2NO2 or HNO4, on solid H2O ice at 193 K (−80°C) was studied using a fast flow‐mass spectrometric technique. An uptake coefficient of 0.15 ± 0.10 was measured, where the quoted uncertainty denotes 2 standard deviations. The uptake process did not result in the production of gas phase products. The composition of the condensed phase was investigated using programmed heating (3 K min−1) of the substrate coupled with mass spectrometric detection of desorbed species. Significant quantities of HNO4 and HNO3 desorbed from the substrates at temperatures above 225 K and 246 K, respectively. The desorbed HNO3, which was less than 9% of the desorbed HNO4 and remained unchanged upon incubation of the substrate, was likely due to impurities in the HNO4 samples rather than reaction of HNO4 on the substrate. The onset temperatures for HNO4 desorption increased with increasing H2O to HNO4 ratios, indicating that HNO4, like HNO3, tends to be hydrated in the presence of water. These observations suggest possible mechanisms for removal of HNO4 or repartitioning of total odd nitrogen species in the Earth's upper troposphere and stratosphere.
Estimation of the location of an arbitrarily oriented dipole under single‐point direction finding
Authors: Panyukov Anatoly V.
Published: February 2004

A lightning flash, cloud or cloud‐to‐ground, can be modeled by a time‐varying dipole, whose orientation may differ significantly from vertical, especially for cloud flashes. A direction finder measuring the vertical electric field and the horizontal components of the magnetic field over an infinitely conducting ground can estimate the location of the dipole with an accuracy that depends on the dipole orientation. A vertical dipole gives the best accuracy and a horizontal one the worst. With more than one direction finder, at different sites, the uncertainty for a nonvertical dipole can be removed. The method is based on the use of a pseudodirection angle, which for a vertical dipole coincides with the actual direction, and various time derivatives of the received signal.
Synoptic meteorology and transport during the North Atlantic Regional Experiment (NARE) intensive: Overview
Authors: Merrill John T. , Moody Jennie L.
Published: February 2004

Analyses of meteorological conditions during the North Atlantic Regional Experiment (NARE) intensive period, defined here as August 1 through September 13, 1993, indicate that transport in the study region in Nova Scotia was influenced by several well‐defined synoptic disturbances (weak frontal passages or troughs) and by intervening periods of slowly varying transport conditions. Using trajectory analysis, synoptic charts, and other meteorological products, we have characterized these conditions and indicated the transitions in transport in relation to the synoptic scale meteorological situation. The ideal meteorological scenario for delivering pollution plumes from the U.S. East Coast urban areas over the Gulf of Maine to the Maritime Provinces of Canada is shown to be warm sector flow ahead of an advancing cold front. In addition, for two of these frontal passages we have discussed some of the smaller‐scale effects which appear to have influenced the transport and or the composition of air masses reaching the NARE intensive region. Finally, we compare the conditions actually encountered during the field campaign with our idealized conceptual model of warm sector transport.
Radon 222 simulations as a test of a three‐dimensional regional transport model
Authors: Lin X. , Zaucker F. , Hsie E.‐Y. , Trainer M. , McKeen S. A.
Published: February 2004

Radon 222 distributions during the 1993 North Atlantic Regional Experiment are simulated with the National Oceanic and Atmospheric Administration Aeronomy Laboratory three‐dimensional regional transport model. The model domain covers the western part of the North Atlantic Ocean and part of the eastern United States and Canada. It horizontally elongates 4200 km in the east‐west and north‐south directions and vertically extends from the surface to 50 mbar. The model simulates emissions, transport, and radioactive decay of 222Rn. Advection, cloud transport, mixing in the planetary boundary layer, and diffusion are simulated and the emissions, boundary, and initial conditions are parameterized. The model results are analyzed to study the ability of the model to simulate 222Rn distribution in the troposphere. The analysis indicates that 222Rn simulations near the surface are dominated by emissions. At altitudes from <1 km to ∼6 km the simulations are noticeably influenced by the side boundaries and the influence increases with increasing altitude. Radon 222 simulations in the upper troposphere (z >6 km) are dominated by side boundary conditions and the wind field. The impact of top boundary on 222Rn simulations is negligible provided the top is specified at an altitude well above the tropopause. Cloud mixing is important in the vertical transport of 222Rn from the surface to the free troposphere. Cloud effect is significant in 222Rn distributions at grid columns within or near which there are abundant clouds. The mean effect of cloud mixing cannot be appropriately represented by contrasting large‐scale or long‐term average 222Rn distributions simulated in cases with and without cloud mixing. Surface pressure fluctuation has substantial influence on 222Rn emission and concentration variabilities, but this effect is restricted to altitudes near the surface. The model‐simulated 222Rn concentrations are compared with measurements made onboard the U.S. Department of Energy Gulfstream aircraft based at Halifax, Nova Scotia, to evaluate the quality of the simulations. The comparison indicates that the simulations and observations are usually within a factor of 2 of one another and significantly correlated with each other. The simulations are capable of capturing high 222Rn concentrations observed in continental plumes.
Pb‐212 as a tracer for local influence on air samples at Mauna Loa Observatory, Hawaii
Authors: Whittlestone S. , Schery S. D. , Li Y.
Published: February 2004

Airborne Pb‐212 has been measured at and near the Mauna Loa Observatory (MLO) to evaluate its performance as a tracer to detect periods when air samples have been influenced by passage close to Hawaiian land. This complements radon measurements which do not distinguish between local and distant land contact, and measurements of isoprene which is present in air samples which have contacted flora in daylight. One source of Pb‐212 is decay of thoron from barren recent lava. This source is sufficient to indicate when an air sample from the free troposphere has been close to lava on its way to the observatory. At a height of 3 m above the ground, samples in the clean air wind sector sometimes contained measurable amounts of Pb‐212, particularly when strong temperature gradients and low wind speeds trapped Pb‐212 near the ground. In combination with Rn‐222 and isoprene measurements, Pb‐212 can be very useful for establishing the presence of baseline air which has not interacted with land for several weeks, as well as air which has contacted distant land but not Hawaii.
Atmospheric dimethylsulfide and sulfur species in aerosol and rainwater at a coastal site in New Zealand
Authors: Wylie David J. , de Mora Stephen J.
Published: February 2004

Aspects of the atmospheric speciation of sulfur were investigated at a coastal site on the North Island of New Zealand between December 1990 and November 1992. Gaseous dimethylsulfide (DMS) as well as methanesulfonic acid (MSA) and non‐sea‐salt sulfate (NSSS) in aerosol and rainwater samples were determined. Atmospheric DMS concentrations exhibited seasonal variations in the range 0.25 nmole m−3 to 14.9 nmole m−3, with a median of 3.75 nmole m−3. Also, DMS levels varied diurnally and with meteorological variables such as wind speed and cloud coverage. Aerosol MSA concentrations similarly ranged seasonally from 0.02 to 1.40 nmole m−3 (median 0.36 nmole m−3). Aerosol NSSS concentrations varied from −1.04 to 15.9 nmole m−3, with a median value of 7.92 nmole m−3. MSA and NSSS concentrations in “clean” rainwater samples ranged from 0.03 to 0.31 μeq L−1 (median 0.04 μeq L−1) and 0.83 to 5.44 μeq L−1 (median 1.37 μeq L−1), respectively. The MSA:NSSS molar ratios varied seasonally from 0.44 to 21.4% (median 3.28%) in aerosols of marine character and from 2.38 to 22.3% (median 6.40%) for rainwater samples. Atmospheric DMS and MSA concentrations at this coastal location in New Zealand are comparable to those observed at other remote southern hemisphere sites. In contrast, NSSS concentrations are somewhat elevated, possibly incorporating an unknown contribution from heterogeneous SO2 oxidation. With respect to DMS flux calculations an oceanic emission rate of 83 mg S m−2 yr−1 would be required to account for oxidative losses (70 mg S m−2 yr−1) and leakage out of the marine boundary layer. The sum of wet and dry deposition for MSA and NSSS amounts to 65 mg S m−2 yr−1. The small difference between DMS oxidative losses and total MSA + NSSS deposition likely stems from an underestimate of the wet deposition rate for NSSS.
Computational techniques to incorporate aerosol variability in climate models
Authors: Box Michael A. , Beck Simon , Trautmann Thomas
Published: February 2004

Atmospheric aerosols are among the most variable components of the Earth's atmosphere, which is one of the reasons they have largely been omitted from general circulation models. However, recent work has suggested that the shortwave radiative effects of aerosols can be quite significant, possibly rivaling current greenhouse gas forcing. There is, of course, no time to perform a realistic radiative transfer calculation at each grid point and time step. Instead, one must find a way to parameterize aerosol effects and how they may vary as the aerosol type and loading vary along with the humidity. Radiative perturbation theory is an ideal tool for the development of such parameterizations, as we have recently shown. In this paper, we briefly outline the principles of perturbation theory (the details can be found in the references) and show how it may be applied to this task. Then we examine the specific problem of computing heating rates, which has tested perturbation theory up until now. We show that far more accurate results may be obtained by taking the difference in fluxes either side of the atmospheric layer in question, each calculated using an exponential formulation of perturbation theory.