bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of main tidal constituents from TOPEX altimetry using a Proudman function expansion
Authors: Sanchez Braulio V. , Pavlis Nikolaos K.
Published: February 2004

Tidal models for the main diurnal and semidiurnal constituents have been computed from TOPEX altimeter data and a set of Proudman functions computed numerically in the space defined by the ocean basins. The accurate modeling of the ocean tides is necessary in order to interpret the height measurements of the ocean surface obtained from satellite altimeters. It is also an interesting dynamical problem in its own right. The surface height field due to any tidal constituent can be expanded in terms of the eigenfunctions of the velocity potential (Proudman functions) with coefficients estimated in a least squares sense from a field of discrete data points obtained from altimetry, tide gauges, bottom pressure sensors, etc. The Proudman functions constitute a mass conserving orthogonal basis; their computation does not require any assumption concerning friction or energy dissipation, only a numerical grid expressing the shape of coastline and the bathymetry of the ocean basins. They have the space structure of standing waves and can be identified as the zero‐rotation gravitational normal modes. They have to be evaluated numerically only once for each particular grid resolution. In this investigation the Proudman functions were computed by means of finite differences in spherical coordinates over a 2°×2° grid covering most of the world's oceans for a total of 8608 degrees of freedom. The data field used in this study consists of approximately 15 months of TOPEX altimetry in the form of collinear differences. Results for the major semidiurnal and diurnal constituents (M2,S2, N2, K2, K1, O1, P1, and Q1) have been obtained in terms of corrections to a priori values obtained by fitting Schwiderski's (1980) models. The new models (Goddard Space Flight Center (GSFC94A)) are tested at a set of “ground truth” data points. These tests indicate substantial improvement for most of the constituents as compared with Schwiderski's solutions. Use of GSFC94A results in a 7.8‐cm reduction in the rms overlap difference of 15.5 cm. The GSFC94A model yielded a mean rms sea surface variability of 7.9 cm, compared with the 9.4 cm obtained when using Schwiderski's model.
Observed linear flow resistance in the Öresund due to rotation
Authors: Mattsson Johan
Published: February 2004

The flow through the Öresund is mainly driven by the sea level difference between the Baltic Sea and the Kattegat, suggesting a functional relationship between the sea level difference and the flow rate. In existing hydraulic models, the sea level difference is assumed proportional to the square of the flow rate. While the underlying theory requires an almost constant coefficient of proportionality, observations show that the coefficient is dependent on the flow rate. This effect may be accounted for by adding to the relationship a term that is linear in the flow rate. Theoretical considerations suggest that the linear term may be due to rotational effects in a way similar to flow resistance in the geostrophic control model. Adding nonlinear frictional effects to that model, a simple expression results that combines the nonlinear effects of friction and acceleration and linear effects of rotation. This expression quite accurately describes the observed flow resistance in the Öresund.
Glimpses of a wave boundary layer
Authors: Trowbridge J. H. , Agrawal Y. C.
Published: February 2004

Measurements obtained with a profiling laser‐Doppler velocimeter reveal the vertical structure of velocity in a wave boundary layer over a sand beach during two periods of approximately stationary forcing, each with a duration of several hours. The measurements indicate clearly the reduction in variance and the increase in phase, relative to the overlying flow, that are expected in an oscillatory boundary layer. The measurements also indicate a distortion of the mean alongshore velocity within the wave boundary layer, which has previously been predicted theoretically and observed in laboratory experiments. The thickness and structure of the boundary layer are approximately consistent with computations based on an existing eddy viscosity model, and model‐based best fit estimates of bottom roughness are approximately consistent with existing semiempirical descriptions of wave‐formed sand ripples.
Measuring global mean sea level variations using TOPEX/POSEIDON altimeter data
Authors: Nerem R. S.
Published: February 2004

The variations of global mean sea level are an important indicator of global climate change, and their measurement can provide important information for determining the socioeconomic impact of sea level change on coastal land use. The analysis of historical tide gauge records generally indicates that sea level has risen at a rate of about 2 mm/yr during the last 100 years; however, this estimate is somewhat uncertain due to the effects of regional crustal motion, lack of uniform temporal coverage, and the limited spatial sampling of tide gauges. The prospect of measuring variations in global mean sea level has been assessed using approximately 2.5 years of satellite altimeter data from the TOPEX/POSEIDON (T/P) mission, where synoptic mapping of the geocentric height of the ocean surface is routinely achieved with a point‐to‐point accuracy of better than 5 cm. The global mean sea level variations measured by T/P every 10 days have an rms of 6 mm (4 mm after detrending), some of which is shown to be correlated with sea surface temperature variations. The rate of change of global mean sea level derived from 2.5 years of data is +5.8 mm/yr with a scatter of 0.7 mm/yr. Currently, it is impossible to accurately estimate the error in the measured rate of sea level rise, since little is known about the long‐term behavior of the measurement errors at the millimeter level. In addition, there is evidence from the sea surface temperature record that the measured rate of sea level rise is associated with a relatively short‐term (interannual) variation unrelated to the long‐term signal expected from global warming. Nevertheless, these results suggest that T/P is achieving the necessary repeatability to measure global sea level variations caused by climate change, and a longer time series will significantly improve the sea level rise estimate by averaging measurement error and real sea level variations. A longer time series will also reduce the errors in estimates of the altimeter calibration, providing an important constraint on any long‐term instrument drift. Future research will focus on establishing a realistic error budget for these measurements of global mean sea level, so that they can be put in the proper context with other observations of global climate change.
Comparison of Autonomous Lagrangian Circulation Explorer and fine resolution Antarctic model results in the South Atlantic
Authors: Davis Russ E. , Killworth Peter D. , Blundell Jeffrey R.
Published: February 2004

The motions of eight Autonomous Lagrangian Circulation Explorer (ALACE) floats released near 750 m depth in Drake Passage and followed through the South Atlantic are described and compared with emulations made by advecting model floats through 12 monthly snapshots of velocity from the fine resolution Antarctic model (FRAM). Both ALACEs and FRAM reproduce the major features of the general circulation as follows: strong intermediate depth flow in Drake Passage, bifurcation of the Antarctic Circumpolar Current (ACC) passing over the Falkland Plateau, a strong Falkland Current, its confluence with the Brazil Current, and moderate zonal flow across the South Atlantic. FRAM versus ALACE comparisons are made in both the Eulerian frame and using observed and modeled trajectories. In Drake Passage, where float velocities agree with earlier observations, FRAM velocities are about twice too big. Both FRAM and ALACE velocities are consistent with an O(100 Sv) Falkland Current. In the central South Atlantic the few available float observations indicate the ACC and South Atlantic Current (SAC) to be more localized than in the model. Eddy kinetic energy is much stronger in the observations than in FRAM. Float dispersion in both the model and observations is due primarily to mean shear. Initial RMS particle separation of 100 km grows to nearly 1000 km after 1 year, but most of this is associated with floats that take different paths of the general circulation. The observations indicate that eddy effects are particularly important near the Falkland‐Brazil Current confluence in allowing Antarctic Intermediate Water to transfer from the ACC to the SAC, from which they may enter the subtropical gyre.
An eddy‐resolving model of circulation on the western Gulf of Alaska shelf: 1. Model development and sensitivity analyses
Authors: Hermann A. J. , Stabeno P. J.
Published: February 2004

An eddy‐resolving primitive equation model was used to simulate circulation on the shelf in the western Gulf of Alaska. This paper describes the development and sensitivity analysis of the model, while a companion paper [Stabeno and Hermann, this issue] demonstrates correspondence with measured currents in specific years. The model is initialized with a salinity field derived from local conductivity‐temperature‐depth surveys and forced with an idealized alongshore wind stress pattern. Buoyancy forcing is achieved by freshening the salinity field along a portion of the coastal wall. The model exhibits many of the patterns evident in observations, including mean flows and mesoscale eddies associated with the Alaska Coastal Current (ACC). Sensitivity studies suggest a strong dependence of the barotropic flux of the ACC on local winds and weak dependence on upstream buoyancy forcing. Both wind and buoyancy forcing significantly affect the generation of eddies. Strong northeasterly winds enhance cross‐shelf flow of the ACC to the west of Shelikof Strait, where isobaths veer southward. Strong buoyancy forcing also favors cross‐shelf flow in that region and enhances the formation of eddies.
An eddy‐resolving model of circulation on the western Gulf of Alaska shelf: 2. Comparison of results to oceanographic observations
Authors: Stabeno P. J. , Hermann A. J.
Published: February 2004

Currents generated by an eddy‐resolving, semispectral primitive equation model are compared with those measured by moored current meters (1989 and 1991) and satellite‐tracked drifting buoys (1987) from Shelikof Strait, Alaska. The model reproduced many of the dominant circulation features, including the cross‐sectional spatial structure of the Alaskan Coastal Current, the estuarine inflow at the bottom of the sea valley, and the mean transport. The first‐mode empirical orthogonal functions of the model and of the observed data represent similar spatial structures and were significantly correlated in time. While the model produced eddies (>20‐km diameter) at a similar rate as observed, the timing did not generally match the observations. As a result, correlations between the measured currents and model‐generated currents usually were not significant. Correlations between modeled and measured transport through the sea valley, however, were significant for both years.
Water mass and coastal current variability near Barbados, West Indies
Authors: Stansfield K. L. , Bowman M. J. , Fauria S. J. , Wilson T. C.
Published: February 2004

Moored current meter observations complimented by conductivity‐temperature‐depth (CTD) profiles and shipboard acoustic Doppler current profiler measurements have revealed two distinctly different coastal circulation patterns south of the island of Barbados, West Indies, in the springs of 1990 and 1991. In the first year an abrupt increase in speed and change in direction of the currents south of the island was found to be associated with a large, anticyclonic, horizontally sheared circulation and a marked variation in temperature‐salinity characteristics. The shift in currents was also accompanied by the incursion of a shallow pool of low‐salinity water, a strong internal semidiurnal tidal signal, and a change in the location of larval fish assemblages west of the island. Rapid variations in current speed and direction were also observed in 1991, though less persistent than in the previous year. In this year the island was at one time completely surrounded by a large, shallow pool of low‐salinity water. A comparison was made of the 1990 hydrographic observations with CTD data from the National Oceanographic Data Center archive for the region 0°–20°N, 40°–60°W. This suggests that the variation in hydrographic characteristics was due to the intrusion of a water mass of North Brazil Current origin which displaced the ambient North Atlantic Subtropical Underwater around the island. Several recent studies have demonstrated that mesoscale anticyclonic eddies formed near the retroflection region of the North Brazil Current can be advected toward the eastern Caribbean before being dissipated. It is concluded that the most likely cause of the observed high‐energy event in 1990 was the passage of such an eddy past Barbados.
The warm water inflow into the western tropical Atlantic boundary regime, spring 1994
Authors: Schott Friedrich A. , Stramma Lothar , Fischer Jürgen
Published: February 2004

During March 1994 a survey of the western boundary of the tropical Atlantic, between 10°N and 10°S, was carried out by conductivity‐temperature‐depth and current profiling using shipboard and lowered acoustic Doppler current profilers. In the near‐surface layer, above σΘ = 24.5, the inflow into the boundary regime came dominantly from low latitudes; out of the 14 Sv that crossed the equator in the upper part of the North Brazil Current (NBC), only 2 Sv originated from south of 5°S, while 12 Sv came in from the east at 1°–5°S with the South Equatorial Current (SEC). After crossing the equator near 44°W, only a minor fraction of the near‐surface NBC retroflected eastward, while a net through flow of about 12 Sv above σΘ = 24.5 continued northwestward along the boundary. By contrast, in the isopycnal range σΘ = 24.5–26.8 encompassing the Equatorial Undercurrent (EUC), the source waters of the equatorial circulation were dominantly of higher‐latitude South Atlantic origin. While only 3 Sv of eastern equatorial water entered the region through the SEC at 3°–5°S, there was an inflow of 10 Sv of South Atlantic water in the North Brazil Undercurrent (NBUC) along the South American coast that originated south of 10°S. The transport of 14 Sv arriving at the equator along the boundary in the undercurrent layer was almost entirely retroflected into the EUC with only marginal northern water additions along its path to 35°W. The off‐equatorial undercurrents in the upper thermocline, the South and North Equatorial Undercurrents carried only small transports across 35°W, of 5 Sv and 3 Sv, respectively, dominantly supplied out of SEC recirculation rather than out of the boundary current. Still deeper, three zonal undercurrents were observed: the westward‐flowing Equatorial Intermediate Current (EIC) in the depth range 200–900 m below the EUC, and two off‐equatorial eastward undercurrents, the Northern and Southern Intermediate Countercurrents (NICC, SICC) at 400–1000 m and 1°–3° latitude. In the lower part of the NBUC there was an Antarctic Intermediate Water (AAIW) inflow along the coast of 6 Sv, and there was a clear connection at the AAIW level to the SICC by low salinities and high oxygens and a weaker suggestion also that some supply of the NICC might be through AAIW out of the deep NBUC.
A modeling investigation of the breaking wave roller with application to cross‐shore currents
Authors: Dally William R. , Brown Cheryl A.
Published: February 2004

A mathematical model is developed for the creation and evolution of the aerated region, or “roller,” that appears as a wave breaks and passes through the surf zone. The model, which calculates the roller's cross‐sectional area, is based on a short‐wave averaged energy balance. The vertically integrated energy flux is split between the turbulent motion in the roller and the underlying organized wave motion, and the dissipation of energy is assumed to take place in the shear layer that exists at the interface between the two flow regimes. Calibration of the roller model is done by numerically solving equations for the cross‐shore balances of mass and momentum, with roller contributions included, and then optimizing predictions of depth‐averaged cross‐shore currents. The laboratory data of Hansen and Svendsen [1984] for setup and cross‐shore currents, driven by regular waves breaking on a planar beach, are used to set the roller model's fitting coefficient. The model is then validated utilizing five additional laboratory data sets found in the literature. Results indicate that employing stream function theory in calculating integral properties for the organized wave motion (wave celerity, and mass, momentum, and energy fluxes) significantly improves agreement as compared to results generated using linear wave theory. Using the roller model and stream function theory, root‐mean‐square error for the mean current is typically 19%. The bed stress is found to play a negligible role in the cross‐shore mean momentum balance, relative to the radiation stress, setup, roller momentum flux, and convective acceleration of the current.
Kinematics of a warm‐core dipole ring
Authors: Hooker Stanford B. , Brown James W. , Kirwan A. D. , Lindemann Gloria J. , Mied Richard P.
Published: February 2004

The remote sensing and in situ data used by Hooker and Brown [1994] to establish the dipole identity of warm core ring (WCR) 82‐B is reexamined. It is found that a rotating barotropic modon [Mied et al., 1992] can be constructed with the same dipole rotation rate, center‐to‐center vortex separation distance, and peak anticyclonic vorticity as those of WCR 82‐B. The model‐derived velocity field is used to deform an array of material lines during a rotation period when the dipole is evident in the imagery and agreement between the model and the imagery is good. Specifically, it is observed that at the end of the imaging period, the surface tracer assumes a skewed dipole appearance, in which the line of centers is not perpendicular to the separatrix. Moreover, the separatrix morphology is qualitatively reproduced. Finally, the cyclone assumes an axisymmetric form. An attempt to derive qualitatively similar signatures using only monopole forcing yields results dissimilar from both the advanced very high resolution radiometer imagery and those obtained with the dipole, further confirming the underlying dipole character of WCR 82‐B.
Circulation near submarine canyons: A modeling study
Authors: Klinck John M.
Published: February 2004

Circulation near a submarine canyon is analyzed with a numerical model. Previous theoretical work indicated that stratification controlled the interaction of coastal flow with canyons, specifically, the ratio of canyon width to the internal radius of deformation. A wide canyon was thought to merely steer the flow, while a narrow canyon would create substantial cross‐shelf exchange. Four cases are analyzed considering two directions of alongshore flow and two choices of initial stratification. The weakly stratified case has an internal radius about equal to the canyon width, while the strongly stratified case has one about 3 times the canyon width. The direction of the alongshore flow is shown in this study to be the more important of the two factors. In particular, right‐bounded flow (flow with the coast on the right, looking downstream in the northern hemisphere) leads to shallow downwelling in the canyon and weak exchange across the shelf break, while left‐bounded flow creates upwelling at the head of the canyon and strong exchange between the ocean and shelf. In left‐bounded flow (upwelling), dense water is pumped onto the shelf, even for strong stratification. However, the stratification limits the vertical extent of the topographic influence so that the alongshore flow above the canyon is only weakly affected in the strongly stratified case. With any level of stratification, the surface temperature (density) is not modified at all by the flow interaction with the submarine canyon. The important dynamics involve pressure gradients and Coriolis acceleration and how they interact with the bathymetric gradients but not advection of momentum. Advection of density is clearly important in the upwelling cases. Finally, continued upwelling onto the shelf acts as a drag mechanism and retards the alongshore coastal flow.
Altimetric assimilation with water property conservation
Authors: Cooper Mike , Haines Keith
Published: February 2004

A simple method for assimilating surface pressure data into a 21‐level, eddy‐resolving Cox model in a double‐gyre configuration [see Cox, 1985] is introduced. A conservation principle is used to derive the new water column structure based on rearrangement of the preexisting water masses. This respects the long timescales required to modify water properties on deeper isopycnals, while still allowing for immediate changes in isopycnal geometry and associated currents. Water columns are displaced vertically by an amount which reduces the surface pressure update to zero at the bottom. Current updates are then calculated geostrophically. An identical twin experiment is performed for 1 year with complete surface pressure data assimilated every 9 days. Thermocline temperature and current errors decrease rapidly after a single assimilation of surface pressure. Errors in subthermocline currents and isopycnal potential vorticity (stratification) within the thermocline decrease only after model integration. Deep current (3000 m) RMS errors are, after 1 year, reduced by up to 60%. A mixed layer scheme is added to the simple assimilation procedure to account better for changes in water properties near the surface (where property conservation is less realistic). Assimilation, with both surface pressure and surface temperature data provided, is also described. Surface temperature data have the biggest immediate impact on the circulation where the mixed layer is deep, e.g., in the subtropical gyre or in the far north, although after 1 year it does not contribute much over surface pressure assimilation alone. We speculate that the surface temperature data will contribute more for temporally varying surface boundary conditions, where it influences water properties subducted into the thermocline. This assimilation scheme should be easy to implement in any model framework and can be modified to incorporate an error analysis for use with real data sets.
Seasonal and interannual variability of the North Equatorial Current, the Mindanao Current, and the Kuroshio along the Pacific western boundary
Authors: Qiu Bo , Lukas Roger
Published: February 2004

Along the Philippine coast in the western Pacific, the North Equatorial Current (NEC) bifurcates into the northward flowing Kuroshio and the southward flowing Mindanao Current. Using both the linear, time‐dependent Sverdrup theory and a high‐resolution, nonlinear reduced‐gravity model, this study investigated the changes in the NEC‐Mindanao Current‐Kuroshio (NMK) system induced by large‐scale surface wind forcings. Using the Florida State University monthly wind stress data from 1961 through 1992, we show that the seasonal bifurcation of the NEC occurs at the northernmost position in October and the southernmost position in February. While the meridional migration of the basin‐wide trade wind has a relatively small effect in shifting the bifurcation latitude (by about 100 km), the monsoonal wind along the low‐latitude western Pacific is effective in inducing a large northward excursion of the NEC's bifurcation in the fall season. On the interannual timeseale, the positive wind stress curl of the trade wind tends to intensify and shifts the zero wind stress curl line northward prior to El Niño‐Southern Oscillation (ENSO) events. With a lag of about 1 year this shift induces the bifurcation of the NEC to occur at a higher latitude. During the La Niña years the NEC generally bifurcates at a lower latitude. No significant seasonal fluctuations are found in the transport of the NEC near the Philippine coast. Seasonal changes in the Mindanao Current and the Kuroshio are, however, significant, and their transports tend to fluctuate 180° out of phase, due to the different speeds of the baroclinic Rossby waves at their respective latitudes. The Kuroshio (the Mindanao Current) has a seasonal minimum (maximum) transport in fall when the NEC bifurcates at the seasonally northernmost latitude. The interannual changes in the inflow NEC are largely controlled by the basin‐wide, wind stress curl anomalies. While the quasi‐biennial changes are confined only to the southern limb of the NMK system, signals with ENSO timescales are found to influence the midlatitude, subtropical circulation via the Kuroshio.
Spray droplet modeling: 2. An interactive Eulerian‐Lagrangian model of evaporating spray droplets
Authors: Edson J. B. , Anquetin S. , Mestayer P. G. , Sini J. F.
Published: February 2004

This paper describes an interactive Eulerian‐Lagrangian model of the turbulent transport of evaporating droplets. A k‐ε (where k is turbulent kinetic energy and ε is its rate of dissipation) turbulence closure model is used to accurately simulate stable, near‐neutral, and unstable boundary layers within the large air‐sea interaction tunnel at the Institut de Mécanique Statistique de la Turbulence (IMST), Luminy, France. These results are then used with the Lagrangian model described in part 1 [Edson and Fairall, 1994]. The coupled model is shown to give excellent agreement with droplet dispersion measurements made during the 1988 Couche Limite Unidimensionelle Stationnaire d'Embrums (CLUSE, a French acronym that translates to one‐dimensional stationary droplet boundary layer) campaign. Additionally, this paper describes how the coupled model can now be used to investigate the interaction between the evaporating droplets and the turbulent fields of temperature and humidity. The investigation shows that although the influence of the droplets is small under the conditions simulated at IMST, the potential for substantial modification of the surface energy budget exists for high‐wind conditions over the ocean.
Biogeochemical cycling in the global ocean: 2. New production, Redfield ratios, and remineralization in the organic pump
Authors: Shaffer Gary
Published: February 2004

Analytic solutions for global ocean distributions of phosphate, nitrate, and dissolved oxygen are presented for the high‐latitude exchange/interior diffusion‐advection (HILDA) model. The organic pump, which together with ocean physics controls nutrient and oxygen cycling in the ocean, is characterized in the model by parameters representing element (Redfield) ratios of organic matter exported from the lighted surface layer of the ocean and by parameters representing depth scales of remineralization for each element. Estimates for these biological parameters for the organic pump of the modern ocean are obtained by seeking least mean squares best fits of model solutions to ocean phosphate, nitrate + nitrite, and dissolved oxygen data cast into the HILDA geometry. Values of free parameters characterizing ocean physics in HILDA are adopted from the earlier analysis of ocean temperature and carbon 14 data in part 1 of this series. The best fit to nutrient data yields a depth scale of remineralization of 1060 m for phosphate and 1110 m for nitrate. Estimates of N:P ratios in new production are 14.5 and 15.4 within and outside high latitudes. For the oxygen model, a low‐ to middle‐latitude N:P ratio of 16 and a remineralization scale for nitrate of 1060 were adopted after a discussion of denitrification. The best fit to oxygen data yields a depth scale of remineralization for the carbon + hydrogen fraction of 1370 m and a (C + H):P ratio in new production of 116. These results lead to estimates of −O2:P remineralization ratios of 122, 142, 168, and 199 at 100‐, 1000‐, 2000‐, and 3000‐m depths, respectively. Together with an overall O2:C photosynthetic quotient of 1.4, the best estimate of the C:P ratio in new production is 105, the traditional Redfield value. Sensitivity calculations are carried out to estimate confidence ranges for the best fit parameter values and the dependence of these values on choices of external parameters, such as high‐latitude surface temperature and preformed nutrients. A critical reassessment of recent work on determining Redfield ratios in new production and remineralization ratios is presented. From this reassessment and from the more robust results of this paper, the following conclusions are reached: (1) Total ocean new production is probably at least 5 Gt C yr−1. (2) The best estimate for ocean mean −O2:C:N:P mole ratios in new production is about 148:105:15:1. (3) Significant vertical fractionation of organic matter occurs in the upper kilometer or so of the ocean; nutrients are released in the mean faster than carbon dioxide. Mean remineralization ratios for −O2:P (C:P) increase from about 130 ± 15 (90 ± 15) at the base of the euphotic zone to about 170 ± 10 (125 ± 10) at 1000‐ to 1500‐m depth and remain so deeper down. (4) The organic matter exported out of the surface layer of the high‐latitude oceans is relatively poor in nitrogen but relatively rich in hydrogen.
Upper ocean temperature and salinity sections in the South Atlantic in 1911
Authors: Reverdin Gilles
Published: February 2004

Hydrographic cruises conducted in July–October 1911 by the “Fram,” the “Deutschland,” and the “Möwe” and in January–February 1911 by the “Stephan” provide a survey of the upper South Atlantic subtropical gyre during the cold season and of the equatorial Atlantic. The data accuracy is not as high as in more recent surveys, in particular, the depth of the measurement is not known precisely: the Fram temperature is only good to within 0.10°C accuracy and salinity to within 0.02. The South Atlantic cold season stations often present a 100‐m or thicker surface mixed layer overlaying a thermocline with maximum vertical gradient increasing from the southwest to the northeast. A layer of minimum vertical temperature gradient of 0.01°C m−1 at a temperature close to 13°C is embedded in the upper thermocline between 40°W and 20°W on the southern section of the Fram (30°S to 35°S). This defines a mode water at a density (26.60) close to the surface density of deep mixed layers offshore of the Brazil current found between 34°S and 39°21′S but which is warmer and saltier (14.9°C, 35.70). The 1911 T‐S characteristics in the subtropical South Atlantic thermocline present little zonal variations, with the warmest, saltiest water on isopycnal surfaces found in the southwestern portion of the survey and the coldest, freshest waters near Namibia. The 1911 South Atlantic surface waters are much colder than the average situation in recent decades (average of −0.82°C) and are a little fresher (−0.04) than the hydrographic climatology for that season. In the subtropical gyre, the isopycnals were probably deeper in 1911 than in 1926, except in the southwestern corner, and the slope of the T‐S relationship was larger in 1911 than in 1925–1926, with warmer and saltier waters in the upper thermocline (above γ = 26.6–26.4) and fresher and cooler waters in the lower thermocline. On the other hand, T‐S characteristics in the equatorial thermocline are comparable in 1911 and 1926. Both surveys present water masses which are colder and fresher on isopycnal surfaces than the surveys in the 1960s.
Interannual variability of surface currents in the tropical Pacific during 1987–1993
Authors: Frankignoul Claude , Bonjean Fabrice , Reverdin Gilles
Published: February 2004

Buoy drifts and current meter records between January 1987 and December 1993 are used to investigate the interannual variability of the equatorial Pacific currents at a depth of 15 m. The sampling is coarse until mid‐1988 but more complete afterward, so that the large‐scale features of the anomaly currents can be documented on the seasonal to yearly timescale. Using objective analysis, bimonthly current anomalies are mapped between 20°N and 20°S on a 1° × 5° grid, and the error covariance matrix of the analyzed fields are estimated. The current anomalies are primarily zonal, with largest amplitudes within about 8° from the equator, and they are largely linked to the El Niño‐Southern Oscillation phenomenon. In particular, broad, basin‐wide westward anomaly currents were encountered during the 1988 La Niña, and strong eastward currents persisted from July–August 1991 to January–February 1992, followed by westward currents from May–June to July–August 1992. An empirical orthogonal function (EOF) analysis shows that the first EOF of zonal current anomaly is largely uniform in the equatorial band, while the next two EOFs describe large‐scale currents of opposite sign across the equator and across 160°W, respectively. The EOFs are rather smooth and the errors on the principal component time series relatively small, which indicates that the sampling is adequate to describe the large‐scale, low‐frequency zonal current fluctuations. As the dominant EOFs of meridional current are noisy and the relative errors on the principal components larger, the meridional current fluctuations are not as well captured by the data set. Correlation analysis and a singular value decomposition are used to investigate the influence of advection by the large‐scale, low‐frequency currents on sea surface temperature (SST) anomalies during 1987–1993. Although the data set is noisy and other terms play an important role in the SST anomaly equation, the effect of zonal, and, to a lesser extent, meridional advection is seen in much of the central and eastern equatorial Pacific. The dominant terms are the anomalous zonal advection of mean SST, the mean zonal, and, intermittently, meridional advection of SST anomalies.
The problem of the barotropic mode in deriving pressure from density by using vertical normal modes
Authors: Woodgate R. A. , Killworth P. D.
Published: February 2004

Vertical normal modes provide a decomposition of the vertical structure of oceanic variables. With a view to the assimilation of sea surface altimeter data, we consider the relationship between density and pressure, using vertical normal modes. In theory, these modes form a complete set, and, subject to the normal limitations of the theory, pressure fields can be derived from density fields. In practice, using theoretical and numerical models, we find that results show an extreme sensitivity to the barotropic mode, making the decomposition unusable for many purposes. This work warns against using a density normal mode analysis in regions where the barotropic mode is thought to be important.
Luminous propagation of lightning attachment to CN tower
Authors: Wang Daohong , Kawasaki Zen Ichiro , Yamamoto Kenji , Matsuura Kenji , Chang Jen‐Shin , Janischewskyj W.
Published: February 2004

Using an eight‐channel photodiode system, we resolved in time and space the luminous propagation of lightning attachment to the CN tower. Approximately 60 lightning discharges were recorded during 1991 and 1992. Most of these discharges demonstrated simultaneous, bidirectional propagation starting at several tens of meters above the top of the CN tower. Propagation speeds ranged from 8×10° to 287×106 m/s. The observed bidirectional propagation suggests the presence of space leaders or the reilluminations following these space leaders during the initial stage of development of upward leaders from the CN tower. A few of the bidirectional discharges were found to have the return stroke speeds and these might be deduced to be manifestations of the bidirectional propagation processes of return strokes which occurred after the connection of a downward leader and an upward connection leader. We also recorded portions of seven ordinary dart leader—return stroke sequences. The dart leader speeds ranged from 7×106 to 23×106 m/s; the velocities of the return strokes ranged from 88×106 to 143×106 m/s. These values are in excellent agreement with the previous results by other authors. Positive correlations are found among the return stroke velocity, the leader velocity, and the leader light intensity. However, it also is observed that even in the case of a very tall structure, when a dart leader approaches the CN tower, it is highly possible that no apparent connecting leader is initiated before the ensuing return stroke.
Middle atmosphere version of CCM2 (MACCM2): Annual cycle and interannual variability
Authors: Boville B. A.
Published: February 2004

Aspects of the zonally averaged simulation produced by the middle atmosphere version of the latest generation of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM2) are discussed together with its dependence on horizontal resolution. This model is serving as the basis for several studies of stratospheric transport, chemistry, and dynamics, so the quality of its basic simulation is of considerable interest. It is shown that the model produces a reasonable simulation of the annual cycle of the northern hemisphere and significantly improves upon previous versions of the model. Serious deficiencies exist in the simulation of southern hemisphere winter stratosphere. The polar vortex is much too strong in southern winter, with wind speeds almost double those observed. The strength of the vortex decreases modestly with increasing horizontal resolution, but the vortex is still much too strong at the highest resolution (T106) which can currently be used even for short simulations. This problem is common to other middle atmosphere general circulation models and CCM2 does not represent a significant advance in this respect. The forcing of the mean flow by the Eliassen‐Palm (EP) flux divergence of the resolved waves is much larger during winter in the northern hemisphere stratosphere than in the southern hemisphere. Nevertheless, the interhemispheric difference in the strength of the simulated stratospheric polar vortex requires the effect of the parameterization of stationary gravity waves generated by flow over orography. When this parameterization is removed, the northern vortex in January reaches comparable strength to that of the southern vortex in July. Based on the current simulations and comparisons with parameterizations in two‐dimensional models, it is argued that a parameterization including the effects of nonstationary gravity waves from sources other than flow over orography will be required to obtain accurate simulations of the Antarctic vortex. The alternative of direct simulation of the gravity waves with high‐resolution models will only be feasible for test simulations in the near future.
Hodograph analysis of mesopause region winds observed by three MF radars in the Canadian Prairies
Authors: Hall G. E. , Meek C. E. , Manson A. H.
Published: February 2004

A new method of hodograph analysis is introduced and applied to studies of long period wind oscillations (2 to 10‐hour period) in the upper mesosphere. observed by a triangle of medium frequency radars (500 km spacing) in the Canadian Prairies. This is compared with a method of spatial Fourier analysis which is also employed to study these oscillations. The results of these analyses are shown to be consistent with the behavior of atmospheric gravity waves: in particular, the gravity wave propagation directions are found to vary with season and with latitude. However, the directions do not seem to be consistent with blocking by the mean wind field, probably due to the gravity waves having phase velocities much higher than typical mean winds.
The singular value decomposition method applied to the deduction of the emissions and the isotopic composition of atmospheric methane
Authors: Brown Margaret
Published: February 2004

The determination of the methane emissions distribution from observations of the surface concentration and isotopic composition of atmospheric methane is studied using geophysical inverse methods and a chemical transport model of the atmosphere. First, the latitudinal and temporal distribution of methane emissions is estimated using the observations of the atmospheric concentration of methane at the Earth's surface and the truncated singular value decomposition (TSVD) as a solution technique. The singular value decomposition (SVD) analysis of this inverse methane source problem identifies the spatial and temporal components of the methane emissions distribution resolvable by the observations, quantifies the sensitivity of each component to errors in the observations, and predicts how many components must be retained to produce a modeled surface concentration consistent with the observations. A latitudinal and temporal distribution of the methane emissions distribution is estimated using the truncated singular value decomposition as a solution technique. The sensitivity of this emissions estimate to perturbations in the model parameters is investigated. The SVD analysis is then applied to the deduction of the 13CH4/12CH4 isotopic ratio of the methane emissions from observations of this ratio at the Earth's surface. This analysis identifies the spatial and temporal components of the isotopic ratio of the methane emissions distribution resolvable by the observations, quantifies the sensitivity of each component to errors in the observations, and predicts how many components must be retained to produce a modeled isotopic composition of atmospheric methane consistent with observations. The results show that the solution is sensitive to errors in the observations, with this sensitivity being dominated by errors in the observed atmospheric concentration rather than errors in the observed isotopic ratio. The minimum uncertainty obtainable in the estimate of the 13CH4/12CH4 isotopic ratio of methane emissions within a region is found to be too large to effectively constrain the relative fluxes of the individual methane sources. The feasibility of improving this uncertainty by increasing the spatial density of the 13CH4/12CH4 observational network is investigated.
Atmospheric lifetimes and fates of a series of sesquiterpenes
Authors: Shu Yonghui , Atkinson Roger
Published: February 2004

Using relative rate techniques, rate constants for the gas phase reactions of OH radicals and NO3 radicals with the sesquiterpenes α‐cedrene, α‐copaene, β‐caryophyllene, α‐humulene, and longifolene have been measured at 296±2 K and atmospheric pressure of air. The rate constants obtained (in units of 10−11 cm3 molecule−1 s−1) for the OH radical and NO3 radical reactions, respectively, are α‐cedrene, 6.7±1.4 and 0.82±0.30; α‐copaene, 9.0±1.9 and 1.6±0.6; β‐caryophyllene, 20+5−9 and 1.9±0.8; α‐humulene, 29+7−10 and 3.5±1.3; and longifolene, 4.7±1.0 and 0.068±0.021, where the indicated errors include the estimated overall uncertainties in the rate constants for the reference compounds. These rate constants and those for the corresponding O3 reactions are combined with estimated ambient lower tropospheric concentrations of OH radicals, NO3 radicals, and O3 to allow the lifetimes of these sesquiterpenes with respect to these chemical loss processes to be calculated. The sesquiterpenes studied are all reactive, with calculated overall lifetimes of a few hours or less. In particular, β‐caryophyllene and α‐humulene are highly reactive toward O3 and NO3 radicals, with calculated lifetimes due to these reactions of 1–2 min, and β‐caryophyllene and α‐humulene are therefore not expected to be present in ambient air at measurable concentrations.
Chlorine monoxide in the Antarctic spring vortex: 1. Evolution of midday vertical profiles over McMurdo Station, 1993
Authors: de Zafra R. L. , Reeves J. M. , Shindell D. T.
Published: February 2004

We have obtained a prolonged record of emission spectra from chlorine monoxide in the vicinity of McMurdo Station, Antarctica, during formation of the austral spring ozone hole of 1993. These spectra have been processed to obtain vertical mixing ratio profiles by deconvolution of pressure‐broadened line shapes. The resulting profiles give a detailed evolution for both altitude distribution and mixing ratio of ClO during development of a major ozone hole event. In early September, very strong emission was observed from pressure‐broadened low‐altitude ClO. Deconvolutions show that this came from an unusually thick layer, extending well above 20 km in altitude. This layer decreased steadily in thickness through September, accompanied by a shift of the peak mixing ratio from ∼21 km altitude in early September to ∼17–18 km by the end of the month, indicating an apparent descent rate of order 100 meters per day, although we argue that the true descent rate is probably lower than the apparent rate. A brief, significant decrease in ClO content occured in late September when the inner vortex edge (defined by the magnitude of Ertel's potential vorticity = 5.2·10−5at ∼19–20 km) approached McMurdo, signifying that a strong gradient in ClO exists near the inner vortex edge. A rapid and apparently final deactivation of chlorine in the lower stratosphere was observed to start about October 1–2. The findings of initially large values of ClO well above 20 km are consistent with observation of polar stratospheric cloud formation in this range during the austral winter of 1993, and with observations showing increased ozone depletion above 20 km relative to previous years.
Biogenic sulfur emissions and aerosols over the tropical South Atlantic: 2. One‐dimensional simulation of sulfur chemistry in the marine boundary layer
Authors: Suhre K. , Andreae M. O. , Rosset R.
Published: February 2004

Based on experimental data collected during cruise 15/3 of R/V Meteor in the tropical South Atlantic (19°S), the marine sulfur cycle has been simulated with a one‐dimensional coupled meteorological‐chemical model. The potential of sea‐spray aerosols to act as a sink for SO2 produced from dimethylsulfide (DMS) oxidation has been examined by using a simple humidity and height dependent parameterization for sea salt aerosol distribution. Simulations show significant reduction of total SO2 concentrations to 27–82% of the values found in the absence of this heterogenous process. The sulfur species concentrations calculated using a zero‐dimensional box model agree with excess sulfate measurements in aerosol samples. The DMS, SO2, H2SO4 (sulfates formed by SO2 oxidation in the gas phase), and XSO4 (sulfates formed by SO2 oxidation in sea‐spray aerosols) vertical profiles, their diurnal cycles, and turbulent fluxes have been calculated with a one‐dimensional model, based solely on shipboard chemical measurements and meteorological radiosonde soundings. Sensitivity analysis of the sea salt aerosol parameterization shows strong dependence on the assumed aerosol distribution, thus calling for a more comprehensive approach using a coupled aerosol‐meteorological model.